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THIN-LAYER MICELLAR CHROMATOGRAPHY OF COINAGE METAL CATIONS
DISSERTATION S U B M I T T E D IN PARTIAL FULFILMENT OF THE REQUIREMENTS
FOR THE AWARD OF T H E DEGREE OF
dllasiter of ^I)tIos;opl)p IN
Applied Chemistry
BY
YASIR HAMID SIRWAL
DEPARTMENT OF APPLIED CHEMISTRY H. COLLEGE OF ENGINEERING & TECHNOLOGY
ALIGARH M U S L I M UNIVERSITY ALIGARH ( INDIA)
2001
DS3279
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DEPARTMENT OF APPLIED CHEMISTRY
Dr. Ali Mohammad Ph.D., D.Sc. (F.N.A.Sc.)
EDITOR : Chemical & Environmental Research
Faculty of Engineering & Technology, Aligarh Muslim University, Aligarh-202 002, India 564-230 AMU IN (Telex) . 91-571-700920, 700921 (Tel) . 456 (Ext.) . mohammadali4u@rediffmail.com (E-mail)
OEm^^ICJiTfE
Certifiecf that tfie wor^incorporatedin this thesis entitled
"1hin.-Layer 'MiceCfar Chromatography of Coinage MetaC
Cations" Seviig suSmitted 6y 'Mr Tasir Jdamid SirwaC is in
partiaCfudfiCfment of the requirements for the award of the
degree of'Master of (philosophy in Applied Chemistry of Aligarh
Muslim. Vniversity, Aligarh.
The •wor{^em5odiedin the thesis is original and Sonafide
record of the research carried out under my supervision.
(ALI MO^KAMMAO)
SV(FE^ISO(^
Residence : "AL-RAIYAN" Iqra Colony, New Sir Syed Nagar, Aligarh-202002 (INDIA), 9 : 0571-408196
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inspiring guiiiancE. Entl]ufiiafitir support. sgmpatl|Etic attituiiE. f|Ealti]g
anil confitructiUE criticism, ualuafak guidancE ani constant supEruision.
Mu sincErE tijanks ars buE to l^xof. ^M. lKi|an. ffiljairman anii
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ills. 5s'al|tii ilabEsn anb ills. &ufia iiina for tl|Eir gooJi uiist}ES,
constant encouragEniEnt anb cljEErful assistancE during tljis uiork.
gipECial ti|ank arr also hm to mg friEnis Amir. iHE|balj,
iiari (§m. Nanboo. dl|uja. Srfan. Hikmat. Aquil anh ^atfi for tl|Eir
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- 1 -
CONTENTS
Acknowledgements i CHAPTER-!: GENERAL INTRODUCTION I - 40 1.1. Analytical Chemistry 1.2. Chromatography 1.3. Thin-layer Chromatography 1.4. History of TLC 1.5. TLC Methodology 1.6. Principles and Techniques 1.7. Nature of Phase Interactions 1.8. Sample Application 1.9. Development 1.10. Visualization 1.11. Quantitation 1.12. Advantages of TLC 1.13. Latest Developments in TLC 1.14. Combination of TLC with other Analytical Techniques 1.15. Coinage Metals: Copper, Silver and Gold 1.16. Terrestrial Abundance and Distribution 1.17. Biological Aspects of Copper, Silver and Gold 1.18. Aqueous Chemistry and Complexes of Cu (II) 1.19. Compounds of Silver (I) 1.20. Complexes of Au (III) <• Table of literature *l* References CHAPTER - 2: THIN-LAYER MICELLAR CHROMATOGRAPHY
OF COINAGE METAL CATIONS 41 - 67
2.1. 2.2. 2.3. • : •
• : •
• : •
Introduction Experimental Results and Discussion Tables Figures References
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1.1 ANALAYTICAL CHEMISTRY
Analytical chemistry is a branch of chemistry basically concerned with the
determination of the composition of matter. The modem aspects of analytical
chemistry are the identification of a substance, the elucidation of its structure and
quantitative analysis of its composition. The ever-increasing impact of analytical
methods reveals that no other branch of science finds extensive applications as
analytical chemistry for two reasons. It finds numerous applications in various
disciplines of chemistry such as inorganic, organic, physical and biological
chemistry. Secondly, it finds applications in other fields of science such as
environmental, agricultural, biomedical and clinical, solid state and space sciences.
Owing to the great importance of pollution, the environmental chemistry is being
more relevant. Instrumental and non- instrumental methods are being developed for
the analysis of air, water and soil pollutants depending solely on its chemical
analysis, which may be qualitative or quantitative. A quantitative analysis deals with
the methods dealing with the determination of actual amount of a given species
present in sample. Instrumental and non- instrumental methods are used in analytical
chemistry. Instrumental methods are usually faster and more sensitive, whereas non-
instrumental methods, which form the basis of standardization of the instruments, are
considered more accurate. However, it is difficult to draw a clear borderline between
instrumental and non- instrumental analytical methods.
Despite distinct advantages of instrumental methods in many directions, their
wide spread adsorption has not rendered the classical methods obsolete. The non-
instrumental methods being simple, inexpensive and versatile have to be
strengthened. In fact, a close critical comparison of all available analytical methods
demonstrates the truth of well known rule of life that "nobody (here: nothing) is
perfect. The team is always stronger than the individual and nothing can be replaced
completely without loss. Co- operation is best." It is, therefore, concluded that all the
analytical techniques should be developed continuously without discrimination.
Several analytical techniques are available for the analysis of substances but
the choice of a particular method depends upon the simplicity, selectivity, rapidity,
cost effectiveness, sensitivity and ease of application. This is especially important in
the determination of very small quantities of substances. One has to look into the
possibilities of using various separation techniques such as precipitation, distillation,
dialysis, ring- oven technique, ion- exchange, electrophoresis, solvent extraction and
chromatography while handling a new system.
1.2 CHROMATOGRAPHY
According to Keulemans "Chromatography is a physical method of
separation, in which the components to be separated are distributed between two
phases, one of which constituting a stationary bed of large surface area, the other
being a fluid that percolates through or along the stationary phase." The separation of
individual components results primarily due to differences in their affinity for the
stationary and mobile phases. The word "chromatography", derived from the Greek
words "chromatus" and "graphein", meaning "color" and "to write" was used by him.
After the initial work of Tswett, a wide variety of independent techniques that have
little to do with color have come to be called chromatography. From its infancy,
chromatography has grown in prominence and popularity to become a leading
technique of analysis. The salient features of common chromatographic techniques
are summarized in Table 1.1.
Chromatographic Systems
According to the physical arrangement, chromatographic systems can be
divided as planar or column depending on the geometry of the support. The planar
arrangement are represented by paper and thin- layer chromatography. According to
development procedures, the planar systems can be further classifieds as ascendent,
horizontal, descendent and occasionally centrifugal.
The four possible chromatographic systems derived from solid, liquid, and
gaseous phases are:
a. Liquid - Liquid
b. Liquid - Solid
c. Gas - Liquid
d. Gas-Solid
Of these, liquid - liquid and liquid - solid systems constitute "liquid
chromatography (LC)." Column chromatography, paper chromatography and thin-
layer chromatography (TLC) are the variants of LC. Of these, TLC is more efficient
and rapid. However, the high performance liquid chromatography (HPLC), similar in
efficiency to GLC has widened the applicability of LC. As the work being reported
here involves the use of TLC as analytical technique, it is appropriate to summarize
some salient features of this most popular and commonly used separation technique.
1.3 THIN- LAYER CHROMATOGRAPHY
TLC is a subdivision of LC, where the mobile phase (a liquid) migrates
through the stationary phase (a thin layer of porous sorbent on a planar inert surface)
by capillary action. It is a rapid, simple, versatile, reasonably sensitive and
inexpensive analytical tool which is applicable for (a) qualitative (b) quantitative and
(c) preparative analyses.
1.4 HISTORY OF TLC
The history of TLC has been reviewed by Stahl (1), Kirchner (2, 3) and Pelick
et. al. (4). In fact, the beginning of TLC may be attributed to Beyerinck who reported
the separation of sulfuric and hydrochloric acids in the form of rings on thin layers of
gelatin (5). Following the same technique, Wijsman separated enzymes from malt
diastase (6). In 1938, Izmailov and Shraiber separated certain medicinal compounds
on binder free horizontal thin layer of alumina spread over a glass plate (7). As the
development was carried out by placing solvent drops on the glass plate containing
sample and adsorbent, their method was called "drop chromatography." However,
this method could not catch the eyes of scientists until two American chemists,
Minhard and Hall used a mixture of aluminum oxide (adsorbent) and celite (binder)
as a layer on a microscopic slide to separate inorganic ions (8). They called this
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technique as "surface chromatography" and this was the first application of layer
chromatography in the separation of inorganics. Since 1958, when Stahl introduced
the term, "thin- layer chromatography" and standardized procedures, materials and
nomenclature (9, 10), the effectiveness of this technique for separation was realized.
A major breakthrough in the field of TLC came in the early 1960's with the
availability of precoated plates (11). It has recently been realized that modem high
performance thin layer chromatography (HPTLC) initiated in 1975, rivals high
pressure liquid chromatography (HPLC) and gas chromatography (GC) in its ability
to resolve complex mixtures and to provide analyte quantification.
1.5 TLC METHODOLOGY
The complete process of TLC is summarized in Fig. LL Solute identification
in TLC is based on Rp, known as retardation factor which is calculated as:
Distance travelled by the compound from the origin
Distance travelled by the mobile phase from the origin. The Rp values in TLC are dependent upon many variables (nature of the
sorbent, layer thickness, layer-activation, temperature, chamber saturation, and nature
of mobile phase, pH of the medium, development technique, room temperature,
sample size and relative humidity); which must be regulated carefully during the
preparation and evaluation of the chromatography to obtain reproducible results. Rp
values vary between 0.0 (solute remaining at the point of application) and 0.999
(solute migrates up to the solvent front) and have no unit. The differential migration
results because of varying degrees of affinity of components in a mixture, for the
stationary and mobile phases.
1.6 PRINCIPLE AND TECHNIQUES
In TLC, separation of components in a mixture is achieved by optimizing the
experimental conditions. The desired separation can be achieved by proper selection
of adsorbent (stationary phase) and solvent (mobile phase).
Prepara t ion of Sample
Applied on Chromatop la te By Spotting or S t reaking
Development
Drying of Chromatogram
Detection Visual , UV-scanning, reagent sp ray
C o m p o n e n t Removal (optional)
Documentation
Figure 1.1 The process of thin-layer ch roma tog raphy
1.7 NATURE OF PHASE INTERACTIONS
Some of the important physical and chemical characteristic that determine the
degree of interactions of mobile phase - solute, sorbent- solute, and mobile phase -
sorbent are given in the following paragraphs.
a. Intermolecular Forces: These forces hold neutral molecules
together in the liquid or solid state. These forces are physical,
characterized by low equilibrium and results in good
chromatographic separation.
b. Inductive Forces: These forces exist when a chemical bond has a
permanent electric field associated with it (e.g. C-Cl, C-NO2
groups). Under influence of this field, the electrons of an adjacent
atom, group or molecule are polarized so as to give an induced
dipole moment. This is a major contributing factor in the total
adsorptive energy on alumina.
c. Hydrogen Bonding: It makes a strong contribution in adsorption
energies between solute or solvents having a proton donor group and
a nucleophilic polar surface such as that of alumina or silica gel.
d. Charge Transfers: Charge transfers between components of the
mobile phase and the sorbent can also take place to form a complex
of the type S" A" (where S = solvent or solute, and A = surface site of
sorbent). This is prominent in ion- exchange chromatography.
e. Covalent Bonds: These bonds can be formed between solute and / or
the mobile phase and the sorbent. These are strong forces and results
in poor chromatographic separation.
1. STATIONARY PHASE (ADSORBENT)
A large number of sorbents are available which can be used in TLC. However,
more commonly used sorbents are silica gel, alumina, cellulose and kieselguhr
(diatomaceous earth).
Silica gel is the most frequently used layer material. It is slightly acidic in
nature. At the surface of silica gel the free valencies of the oxygen are connected with
hydrogen (Si- OH, silanol groups) or with other silicon atom (Si- O - Si, siloxane
groups) as shown in Fig.1.2. The silanol groups represent adsorption active surface
centers that are able to interact with solute molecules. The ability of the silanol
groups to react chemically with appropriate reagents is used for controlled surface
modifications. Hence, silica gel is considered as the most favored layer material in
chromatography.
Alumina or aluminum oxide is also generally used as a sorbent. It is basic in
nature and more reactive than silica gel. Kieselguhr is chemically neutral sorbent
consisting of about 90% SiOs along with AI2O3, FcjOs, MgO, NajO, K2O, CaOj and
Ti02 in different proportions representing, the remaining 10% of the kieselguhr
material. It has very low surface activity. Cellulose can be used as a sorbent in TLC
when it is convenient to perfomi a given paper chromatographic separation by TLC
in order to decrease the time required for the separation and increase the sensitivity of
detection.
In addition, the importance of using surface modified sorbents in TLC has
increased. Both hydrophilic and hydrophobic modified sorbents have been used in
recent years. Both commercially available layers as well as laboratory made sorbent
layers have been used by chromatographers. However, more emphasis has been on
precoated HPTLC plates.
II. MOBILE PHASE (SOLVENT SYSTEM)
In liquid Chromatography including TLC, the mobile phase exerts decisive
influence on the separation. Various optimization schemes (windows diagram,
overlapping resolution maps, simplex method and PRISMA model) are proposed for
normal-phase and reversed-phase TLC. A large number of mobile phases have been
reported, of them some are enlisted below:
a) Organic Solvents: The single component mobile phase including acetone,
acetonitrile, benzene. Carbon tetrachloride, chloroform, dioxane, ethanol,
OH
Figure 1.2 Structure of Silica gel
10
ethylacetate, methanol, o-xylene, petroleum ether, toluene, n-octanol,
n-nonane, cyclohexane and binary/ ternary mixtures of alcohols, amines,
ketones, phenols, haloalkanes have been used.
b) Inorganic Solvents: Being non-toxic and non-volatile, solvent systems of
this group have been widely used in TLC of inorganics and
organometallics. This group includes the solution of mineral acids, alkalies
and inorganic salts prepared in double distilled water or water-methanol
mixture.
c) Mixed Solvents: Mixtures of two or more different solvents, most of
which have either a base (NaOH, NH4OH and amine) or an acid (mineral
or carboxylic) as a component are used to develop the TLC plates.
d) Surfactant- Mediated Solvents: Solutions of surfactants (SDS, CTAB, or
Triton X-100) have also been used to lesser extent as mobile phase in TLC.
SURFACTANT - MEDIATED SYSTEMS
These systems contain surfactant as one of the components of the mobile
phase. Surfactants in the aqueous mobile phase can be used in the following ways:
a) As monomer surfactants where the concentration of surfactant in aqueous
mobile phase is restricted to well below the critical micelle concentration
(CMC) of the surfactant. These mobile phases are most suited to separate
ionic species by ion-pair chromatography (IPC). In this technique, a small
concentration of ion-pairing reagent, which has an opposite charge to the
ionic solutes (i.e. cationic surfactant for anionic solutes and anionic
surfactant for cationic solutes), is added to the aqueous mobile phase and
its concentration is kept low to avoid the formation of micelles.
b) As surfactant micelles where the surfactant concentration is kept well
above its CMC value. In such cases, the mobile phase is composed of
surfactant molecules in the form of monomers and aggregates (or
micelles). These mobile phases are very useful for simultaneous
11
separation of ionic and non-ionic compounds by micellar liquid
chromatography (MLC).
c) As microemulsion where surfactant in the presence of water, an oil
(hydrocarbon) and co-surfactant (i.e. medium chain length amine or
alcohol) is used as transparent solution.
Surfactants are long chain amphiphilic organic or organometallic molecules
containing a highly polar (hydrophilic or lipophobic) or "ionic head group" attached
to a non-polar (hydrophobic or lipophilic) hydrocarbon tail of varying chain length.
The "head group" is either cationic (e.g. ammonium or pyridinium ion), anionic (e.g.
hydroxy compounds) or zwitterionic (e.g. amine oxide, carboxylate or sulphonate
betain) and the hydrocarbon tail which may contain at least 8 carbon atoms.
Depending upon the nature of hydrophilic group, surfactant can be classified as
anionic [R-X 'M ^); cationic (R-N '•(CH3)3 X •]; zwitterionic [R-(CH3)2 N ^CHjX "]
and nonionic [R(0CH2 CH2)],r,0H, where R is a long aliphatic hydrocarbon chain,
M^ is a metal ion, X " is a halogen, COO " or SO4 " and m is an integer. A list of some
common surfactants is provided in Table 1.2.
MICELLES
Surfactant (or amphiphilic) molecules comprising of hydrophobic and
hydrophilic moieties tend to exhibit a considerable degree of self organization when
dissolved in aqueous solutions. Above a certain concentration level, termed as critical
micelle concentration (CMC), the surfactant molecules in solutions (water or organic
solvents) aggregate to form micelles. The process of micelle formation is called
"micellization". Micelles do not exist at all concentrations and temperatures. There is
a very small concentration range below which aggregation to micelles is absent and
above which association leads to micelle formation. This narrow concentration range
during which micelle formation occurs is called the CMC. At low concentration i.e.
below CMC and at temperature above the critical micelle concentration (e.g. Kraft
temperature), the surfactant is dispersed in the aqueous media at the molecular level
as a monomer. The average number of monomers per micelle is called the
12
Table 1.2 Typical Surfactants and their CMCs, Aggregation
Numbers, Kraft Point Values*
Surfactant CMC (M) Aggregation Kraft
Number Point
(°C) b
Aqueous (norma!) Anionic Sodium dodecvl sulfate (SDS), g l x l O " ^ 62 9 CH,(CH:)nOS03"Na" Potassium perfluorohcptanoate, 3.0 x 10"' c 25.6 CvFi.COOK' Sodium polvoxvethvlene( 12)dodec\l ether. 2.0 x 10"' 81 <0 CH3(CH:)M(OC'H:CH,:),:OS03"Na""(SDS 12E0)
Cationic Cei\lp\"ridinium chloride.'^ 1.2 x 10" 95 c C:oH;;NX.H5Cr Cet\ I trimethvl ammonium bromide (CTAB). 9 0 x 10" 78 23 CH--(CH:),.N"(CH.OjBr
.N 0 n i 0 n i c Pol\ox\cthvlene(6)dodecanol. 9 .0x10 '^ 400 c CH;,(CH:),,'(0CH;CH;)60H Pol\oxvethvlene(23)dodccanol(Brij-35). l . O x l O ' 40 c CH;(CH:)n(0CH:CH:):30H
Zvr itterionic N-Dodec\l-N,N-dimethvlammonium-3- 3.0 x 10" 55 <0 propane-1-sulfonic acid (SB-12). CH;(CH;)nN'(CH3):(CH:)3S03-N.N-Dimethyl-N- 25x10"" 24 <0 (carboxymethvi)oct\lammonium salt, CgH.,7N"(CH3):CH;COO"(octylbetaine)
Nonaqueous (reversed) Bis(2-ethvlhexvl) sodium sulfosuccinate 6.0 x 10"" c c (AOT).' " Na05SCH(CH2COOCsH,7)COOC8H,7
"values for aqueous solution at 25 C ^Temperature at which the solubility of an ionic surfactant is equal to
the CMC. "Not available or not defined
'^In 0.0175 MNaCl. ^In hexane.
13
aggregation number (N). At 25° C and 1 atm., the CMC is typically less than 20 mM,
with each micelle-consisting of 40-140 monomers. A conventional model of micelles
is that proposed by Hartly (Fig. 1.3) which is very useful for visualization of a
micelle. The various structures formed in aqueous solution on increasing the
concentration of surfactant is illustrated in Fig. 1.4. There are mainly two types of
micelles:
a) Normal Micelles: The molecular organization of surfactant molecules in
aqueous solutions results in the formation of normal micelles. Above CMC.
the surfactant molecules are self aggregated in such a manner that the
hydrophobic moieties (i.e. hydrocarbon tails) are oriented inward forming a
non-polar core and hydrophilic (polar) head groups are outward keeping
themselves in contact with the bulk aqueous phase. Normal aqueous micelles
are generally formed from a singly-chain surfactants and chain-branching
inhibits micellization.
Micelles are considered to be dynamic in nature, with continuous
exchange of surfactant molecules, in and out of the aggregates occurring in
the milliseconds to microseconds range. Thus, individual surfactant molecules
(called monomers) are thought to be distributed throughout the aqueous phase
surrounding the micelles.
b) Reverse Micelles: In contrast to the normal micelles which are formed in
polar (i.e. aqueous media) solvents, reverse micelles are formed in non-polar
solvents like hexane or chloroform and a trace of water where the polar head
groups of the surfactant are directed towards the interior of the aggregate and
the hydrocarbon chains are in contact with the non-polar solvent. Compared
to normal micelles, reverse micelles are more complex and less understood.
Reverse micelles offer the same potential advantages for analysis as do
normal micelles i.e. the ability to solubilize polar species that would be
excluded from normal micelles. An interesting aspect of reverse micelles is
their capability to solubilize water in the interior of micelle structure.
14
{
«
MicGl lar core
"_ Hycrocorbon "-T' inter ior
Aqueous ex ter io r
Gouy - Chapman layer
Stern layer
Figure 1.3 Hartley model of a spherical micelle
15
—O Q - ^
ex.
o
< -J t- < o - —o o o
o— —o o o o— —o o o O - —GO O o— —o o o
TO
o ,TO
CO
<u
o
u. o
z ic: o < Q.
_ i < 2 : 0
< UJ X
LU Q 2 — —1 >-0
cr 1x1
<
0
> c ^ 0
0
c5 £ iZ
£ <u
*~~ L-
0
CO u.
o o to
< §
s
G3
O
o
16
From macroscopic perspective, miceliar solutions are homogeneous and
cannot be filtered. However, the unique characteristics of miceliar aggregates stem
from their microscopically non-homogeneous nature i.e., they provide a
microenvironment which is distinctly different from the bulk solvent. The most
important property of micelles is their ability to solubilize substances that are
otherwise insoluble (or sparingly soluble) in water.
1.8 SAMPLE APPLICATION
Sample application is one of the most important steps in the technology
of TLC. Improperly applied samples result in poor chromatograms. Samples are
applied as spots or stripes on the sorbent layer, about 2-3 cm above from the lower
edge of the TLC plate so that only sorbent layer makes contact with the mobile phase
and the sample does not dissolve in the mobile phase. The sample should be
completely dried before placing the plate in the development chamber. Micropipette,
micro syringe, melting point capillaries etc. have been used to apply the sample on
the plates. A number of automatic spotters of varying design are also available in the
market.
L9 DEVELOPMENT
Development in TLC is the process in which the mobile phase moves across
the sorbent layer to effect separation of the sample substances. Ascending
development or linear development is the commonly used mode of development in
TLC in which the mobile phase moves up (ascends) the plate. Any close container
that will hold the plate upright is usable while performing the development and
saturation of chamber apart from other factors. It has been observed that the angle of
development i.e. the angle at which the plate is supported, effects the rate of
developments as well as the shapes of the spots (12). An angle of 75° C is optimum
for development. If a desired separation is not achieved by simple development,
some development options such as (i) multiple development (ii) stepwise
development (iii) continuous development (iv) two- dimensional development
17
(v) circular development and (vi) reverse-phase partition development are available
to achieve the desired separation.
1.10 VISUALIZATION
The methods of visualization (detection used in TLC are of three major types,
(i) physical (ii) chemical and (iii) enzymatic or biological. Among the physical
methods, visualization in UV-light is most common. This method is highly sensitive,
non-destructive, and amenable to the visualization of spots before undertaking
quantitative studies. Chemical methods of detection involve the spraying of
chromatoplates with a suitable reagent, which forms colored compounds with the
separated species. Both selective and non-selective reagents may be chosen for the
location of the separated zone.
1.11 QUANTITATION
The three main approaches associated with quantitation in TLC are (i) visual
estimation (ii) zone-elution (iii) in-situ densitometry. Amongst these, in-situ
densitometr}' is the preferred technique for quantitative TLC. Substances separated
by TLC or HPTLC are quantified by in-situ measurement of adsorbed visible or UV-
light, or emitted florescence upon excitation with UV-light. Adsorption of UV-light
is measured either on regular layers or on layer with incorporated phosphor.
1.12 ADVANTAGES O F TLC
TLC is the most versatile and flexible chromatographic method. It is rapid
because precoated layers are available for use as received, without preparation. It has
highest sample throughout, because up to 30 individual samples and standards can be
applied to a single plate and separated at the same time. The automated sample
applications and developers allow high accuracy and precision in quantification.
There is a wide choice of layers, developers and detection methods. The wide choice
of detection reagents leads to unsurpassed specificity. Less pure samples can be
applied as the layers are normally not reused. Being an "off line" method, different
steps of the procedure are carried out independently.
18
1.13 LATEST DEVELOPMENTS IN TLC
As a result of recent innovations, several new techniques such as high
performance thin- layer chi-omatography (HPTLC), over pressurized thin- layer
chromatography (OPTLC), centrifugal layer chromatography (CLC), reversed-phase
thin layer chromatography (RPTLC), radial chromatography, hot plate
chromatography, bioautoradiography, immunostanning and enzyme inhibition
techniques came into light. HPTLC layers being thinner and made of sorbent with
more uniform particle size are developed for a shorter distance. All these factors lead
to faster separations, reduced zone diffusion, lower detection limits, less solvent
consumption and better separation efficiency.
1.14 COMBINATION OF TLC WITH OTHER ANALYTICAL TECHNIQUES
The careful combination of TLC with other analytical techniques is more
useful to collect information regarding the analysis of a complex sample.
Spectrophotometiy, high-performance liquid chromatography and gas
cluomatography, in conjugation with TLC are the three most widely used techniques.
However, mass/ GC, infrared and thermal analytical techniques in combination with
TLC has also been used. One of the newest techniques used in combination with
TLC is photoaccuoustic spectrometry, which is capable to locate compounds in -situ
on the plate. Issaq and BaiT (13) combined TLC with flameless atomic absorption
spectrometry (FAAS) to identify an inorganic compound in an impure organometallic
complex and to determine the recovery and purity of organometallic samples.
The examples cited above reveal, how the separation method of TLC
complement the analytical methods necessary for the absolute identification of a
substance. TLC provides an excellent purification method for separating a substance
of interest from other contaminants in the sample. Analytical techniques can then be
applied to identify the separated substances.
19
1.15 COINAGE METALS: COPPER, SILVER AND GOLD
Cu, Ag, and Au are collectively known as the 'Coinage Metal' because of
their former usage, these elements were almost certainly the first three metals known
to man. These metals have been used as primitive money long before the introduction
of Au coins in Egypt around 3400 BC.
Cold hammering was used in the late Stone Age to produce plates of gold for
ornamental purposes, and this metal has always been synonymous with beautv,
wealth and power. The coffin of Tutankhamun (a minor pharoh who was only 18
when he died) contained no less than 112kg of gold, and the legendary Aztec and
Inca hoards in Mexico and Peru were a major reasons for the Spanish conquests of
Central and South America in the early sixteenth century. Today the greatest hoard of
gold is the 30,000 tonnes of the bullion (i.e. bars) lying in the Vaults of the US
Federal Reserve Bank in New York and belonging to eighty different nations. By
about 3500 BC copper was obtained from its ores in the Middle East by charcoal
reduction and by 3000 BC the advantage of adding tin in order to produce the harder
bronze was appreciated in India, Mesopotamia, and Greece. This established the
"Bronze Age", and copper has continued to be one of man's most important metals.
The monetary use of silver may well be as old as that of gold but the
abundance of the native metal was probably far less, so that comparable supplies
were not available until a method of winning the metal from its ores was discovered.
It appears that by perhaps 3000 B.C a form of cupellation was in operation in Asia
Minor and its use gradually spread, so that silver coinage was of crucial economic
importance to all subsequent classical Mediterranean civilizations. The name copper
and the symbol Cu are derived from Cyprium (later Cuprum), since it was from
Cyprus that the Romans first obtained their copper metal. The words silver and gold
are Anglo- Saxon in origin but the chemical symbols for these elements (Ag and Au)
are derived from the Latin argentum and aurum, gold.
20
1.16 TERRESTRIAL ABUNDANCE AND DISTRIBUTION
The relative abundances of these three metals in the earth's crust (Cu 68 ppm.
Ag 0.08 ppm, Au 0.004 ppm) are comparable to those of the preceding triad ~ Ni, Pd,
and Pt. Copper is found mainly as the sulfide, oxide, or carbonate, its major ores
being copper pyrites (chalcopyrite), CuFeSa, which is estimated to account for about
50% of all Cu deposits; copper glance (chalcocite), CuaS; cuperite, CU2O, and,
malachite, CU2CO3 (OH) 2- A rarer mineral, CuAlg (PO4) 4(0H) g. 4H2O is the valued
blue gemstone turquoise. Large deposits are found in various parts of North and
South America, and in Africa, Russia and its former alliances. The native copper
found near Lake Superior is extremely pure but the vast majority of current supplies
of copper are obtained from low - grade ores containing only about 1% Cu.
Silver is widely distributed in sulfide ores of which silver glance (argentite).
Ag2S is the most important. Native silver is sometimes associated with these ores as a
result of their chemical reduction, while the action of sah water is probabK
responsible for their conversion into "horn silver", AgCl, which is found in Chile and
New South Wales. The Spanish Americas provided most of world's silver for three
centuries after about 1520, to be succeeded in the nineteenth century by Russia.
Appreciable quantities are now obtained as a byproduct in the production of other
metals such as copper, and the main "Western" producers are Mexico, Peru, Canada,
the USA, and Australia.
Gold too, is widely, distributed both native and in tellurides, and is almost
invariably associated with quartz or pyrite, both in Veins and in alluvial or placer
deposits laid down after the weathering of gold - bearing rocks, ft is also present in
seawater to the extent of around 1x10"- ppm, depending on location. Prior to about
1830, a large proportion of the world's stock of gold was derived from ancient and
South American civilizations (recycling is not a new idea), and the annual output of
new gold was no more than 12 tonnes pa. This supply gradually increased with the
discovery of gold in Siberia followed by "gold rushes in 1849 (California: as a result
2 1
of which the American West was settled), 1851 (New South Wales: within 7 yrs the
population of Australia doubled to 1 million, 1884 (Transvaal), 1896 (Klondike,
North-West Canada), and, finally, 1900 (Nome area of Alaska) as a result which by
1890 world production has risen to 150 tormes pa. It is now 8 times that amount,
-1200 tonnes pa.
Cu is a soft metal and is often used in alloys - brass (for plumbing fixtures)
and bronze (for statues). Also, it is often a minor component in nickel and silver
alloys Tablel.3.
Table 1.3 Important Alloys of Copper:
Alloy
Brass
Bronze
Nickel coins
Sterling Silver
Approximate Composition
77% Cu, 23% Zn
80% Cu, 10% Sn, 10% Zn
75% Ni, 25% Cu
92.5% Ag, 7.5% Cu
Properties
Harder than Cu
Harder than Brass
Corrosion resistant
More durable than pure silver
Au is commonly alloyed with other metals in order to make it harder and
cheaper. The proportion of gold is exposed in carats, a carat being a twenty - fourth
part by weight of the metal so that pure gold is 24 carats. The term is derived from
the name of the small and very uniform seeds of the carob tree, which in antiquity
were used to weight precious metals and stones. The two main uses of gold are in
setting international debts and in the manufacture of jewellery, but other important
uses are in dentistry, the electronics industry (corrosion free contacts), and the aero
space industry (brazing alloys and heat reflection), while in office building it has
been found that a mere 20 pm film on the inside face of windows cuts down heat
losses in winter and reflects unwanted radiation in summer.
1.17 BIOLOGICAL ASPECTS OF COPPER, SILVER AND GOLD
Copper is the third most biologically important transition metal after iron and
zinc. About 5 mg are required in the daily human diet. A deficiency of this element
renders the body unable to use ironstones in the liver. There are numerous copper
22
proteins throughout the living world, the most intriguing being the hemocyanins.
These molecules are common oxygen carriers in the invertebrate world: crabs,
lobsters, octopi, scorpions, and snails, all have bright blue blood. At the same time,
an excess of copper is highly poisonous, particularly to fish. This is why copper coins
should never be thrown into fish pools for 'good luck". Humans usually excrete
many excess, but a biochemical (genetic) defect can result in copper accumulation in
the liver, kidneys and brain. This illness, Wilson's disease, can be treated by
administering chelating agents, which complex the metal ion and allow it to be
excreted harmlessly. Both silver and gold have specific medical applications. Silver
ion is a bactericide, and dilute solutions of silver nitrate are placed in the eyes of
newborn babies to prevent infection. Gold compounds, such as the drug auranofin,
are used in the treatment of rheumatoid arthritis.
Some properties of the elements Cu, Ag, and Au that can be directly related to
the d"* s' electronic configuration are summarized in Tablel.4.
1.18 AQUEOUS CHEMISTRY AND COMPLEXES OF COPPER (II)
Most Cu (II) salts dissolve readily in water and give the aqua ion. Addition of
ligands to such aqueous solutions leads to the formation of complexes by successive
displacement of water molecules. With NH3, for example, the species
[Cu (NH3) 2(H20) 5] ", - ' -, [Cu (NH3) 4(H20) 4] ^ are formed in the nonnal way.
Addition of the fifth NH3 can occur in aqueous solution (14), but the sixth occurs
only in liquid ammonia. The reason for this unusual behavior is connected with the
John - Teller effect. Because of it, the Cu (II) ion does not bind the fifth and sixth
ligands strongly (even the H2O). When this intrinsic weak binding of the fifth and
sixth ligands is added to the normally expected decrease in the stepwise formation
constants, the formation constants K5 and Kg are very small indeed. Similarly, it is
found with ethylenediamine that [Cuen (H2O) 4] " and [Cuen2 (H2O) 2] ^ form
readily, but [Cu (en) 3] * are formed only at extremely high concentration. Many
other amine complexes of Cu (II) are known, and all are much more intensely blue
than the aqua ion. This is because the amines produce a strong ligand field, which
23
causes the absorption band to move from the far red to the middle of the red region of
the spectrum, e.g. in the aqua ion the absorption maximum is at -800 nm, whereas in
[Cu (NH3) 4(H20) 2] ^ it is at -600 nm as shown in Fig.1.5.
Table 1.4 Some Properties of the Elements Copper, Silver and Gold:
Property Atomic Number Number of naturally occurring Isotopes Atomic Weight Electronic configuration Electronegativity
Cu 29 2
63.546 (+0.003) [Ar]3d'°4s"^
1.9 Metal radius (12 - coordinate)(pm) 128 Effective ionic radius (6-coordinate)(pm)
Ionization energy (KJmol"')
MP CQ BP (°C) AHfus (KJ mol'') AHvap(KJ mof') A H (nionatomic 2as)(K.J m o l )
Density *''(20°C)(gcm-^) Electrical resistivity at 20°C(|j.ohm cm)
V III II I ^st
2nd i r d
-
54 73 77
745.3 1957.3 3577.6 1083 2570 13.0
307(+6) 337(+6) 8.95
1.673
As 47 2
107.8682 (+3) [Kr]4d'°5s'
1.9 144
-
75 94 115
730.8 2072.6 359.4 961 2155 11.1 258(+6) 284(+4) 10.49
1.59
Au 79 1
196.9665 [Xe]4f''5d'°6s'
2.4 144
57 85 -137
889.9 1973.3 (2895) 1064 2808 12.8 343(+ll) 379(+8) 19.32
2.35 Depends on mechanical history of sample.
In haiide solutions the equilibrium concentrations of the various possible
species depend on the conditions; although CuCls^" has only a low formation
constant, it is precipitated from solutions by large cations of similar charge. Treating
aqueous solution with ligands may isolate many other Cu (II) complexes. When the
ligands are such as to form neutral, water - insoluble complexes, the complexes are
precipitated and can be purified by recrystallization from organic solvents. The
bis- (acetylacetanato) copper (II) complex is another example of this type.
Although as noted previously, addition of CN" normally leads to reduction to
CuCN, in the presence of nitrogen donors like 1,10 - phenanthroline reduction is
inhibited and five - coordinate complexes like [Cuphen2 (CN)]" and [Cuphen2 (CN)2]
are obtained. Multidentate ligands that coordinate through oxygen or nitrogen, such
24
as amino acids, from Cu (II) complexes, often of considerable complexity. The well
known blue solutions formed by the addition of tartarate to Cu solutions (known as
Fehling's solution when basic and when mesotartarate is used) may contain
monomeric, dimeric, or polymeric species at different, pH values. One of the dimers,
Na2[Cu{(+ ) C4O6H2}].5H20, has square coordinate Cu (II), two tartarate bridges, and
a Cu - Cu distance of 2.99A.
10,000 15,000
Frequency (cm" 20,000
Figure 1.5 Absorption spectra of {A) (Cu(H,0),]^- and of the ammines in 2 M ammonium nitrate of 25°C: {B) [Cu(NH3)(H,0),]-; (C) [Cu(NH:,MH,0),]- ; (D) [CuCNHO^H.O),]-- (£)
1.19 COMPOUNDS OF SILVER (I)
The salts AgNOs, AgClOs and AgC104 are water soluble but Ag2S04 and
AgOOCCHs are sparingly soluble. The salts of oxo anions are primarily ionic, but
although the water - insoluble halides AgCl and AgBr have the NaCl structure, there
appears to be appreciable covalent character in the Ag X interactions, whereas in
compounds such as AgCN and AgSCN, which have chain structures as given below,
the bonds are considered to be predominantly covalent.
- A g - C - N - A g - C - N Ag C
25
Silver oxide is more soluble in strongly alkaline solution than in water,
and AgOH and Ag(OH)" 2 are formed. The treatment of water-soluble halides with a
suspension of silver oxide is a useful way of preparing hydroxides, since the silver
halides are insoluble. Analogous to copper and gold, alkali metal silver oxides
contain Ag20''"4 units. The action of hydrogen sulfide on argentous solution gives
black Ag2 S, which is the least soluble in water of all silver compounds. The fluoride
is unique in forming hydrates such as AgF.4H20, which are obtained by crystallizing
solutions of Ag2 O in aqueous HF. The other well-known halides are precipitated by
the addition of X to Ag^ solutions; the color and insolubility in water increase
C l < B r < l .
Many complexes of Ag with nitrogen ligands form readily in aqueous
solution, NH2 being the most important of the nitrogen ligands. Aqueous pyridine
and substituted pyridines form Ag (py) * and Ag (py) 2 ions (15) but when non
aqueous conditions are employed (e.g. CHCI3/ py mixtures) solids containing other
complexes can be obtained. For example, [Ag (py) 4] C104 contains essentially
tetrahedral cations (16). Cationic species such as Ag2X and AgsX are formed when
the silver halides are dissolved in aqueous solution of AgNOs or AgC104
Silver (I) has a relatively low affinity for oxygen donors, although compounds
and complexes containing carboxylate ions, DMSO, DMF, and crown ethers (17) are
known. However, it forms numerous complexes with the donor atoms S, Se, P, and
As. With sulfur the thiosulfate complexes, [Ag(S203)]' and [Ag(S203)2]^' are quite
stable and silver chloride and bromide will dissolve in aqueous thiosulfate, thus
providing a means of "fixing" photographic images. Another important sulfur ligands
for Ag (I) are thiolate anions (18), which give oligomers, (AgSR) „, dithiocarbamate
ions, SCN', thioureas, and thioethers. Silver (I) binds to peptides and proteins with a
preference for the thioether sulfiir atoms and imidazole nitrogen atoms (19).
26
1.20 COMPLEXES OF GOLD (III)
Au (III) like Pt (II) displays predominantly square coordination. Although
there is little evidence for any persisting five- coordinate complexes, there is no
doubt that, as with Pt (II) species, substitution reactions precede via five - coordinate
intermediates. There is no evidence for a simple aqua ion, [Au (H2O) 4] '^ but mixed
chloro - aqua and chloro - hydroxo complexes are formed by hydrolysis of [AUCI4]'.
Complexes of the type [AUX4] ' where X is a halogen or pseudo halogen are well
characterized. By action of BrFs on a mixture of gold and an alkali metal chloride the
M'AUF4 compounds may be obtained. Gold is readily recovered from such solutions
(e.g. by precipitation with SO2). The [AuBr4]" ion is similar to [AuCU]" but [AUI4] ",
obtainable in solid compounds, is subject to the decomposition in solution:
[AUI4]" = [AUI2]" + I2, K = 0.005
Other [AUX4] • complexes are those with X' = SCN (S- bonded), N3' (20), and CN".
The reaction of AUCI3 with ECI4 (E = S, Se and Te) gives compounds that
contain distorted [AuCU] " ions with one CI bridging to the ECls" ion (21). The
cationic complex [Au (NH3) 4] "* is well characterized and numerous others exist, e.g.
[Au py2 CI2] ^ and [Au dien CI] *. Interaction of triphenylposphine complexes of
gold (III) and McsAuPPhs with methyl lithium in ether gives [AuMe4]', which can be
isolated as lithium amine salts and the most stable organogold compounds.
27
Table 1.5 Thin- Layer Chromatographic Studies Carried Out During Last
Twenty Years on Inorganics Including Copper, Silver and Gold:
ION/METAL S.P M.P COMMENT REF.
Forty- nine ions S\ Mi
including W, Ha,
TI, Mo, Re, Sn, Pb,
Cr, Ni, Cd, Zr, Co,
Te, Sc, Sb, Cu, Ag,
Be, Fe, Mg, Th, Y,
and Ge
Forty- eight metals S2 M2
ions
Au, Ir, Ft, Pd, Ir, S3 M3
Ru, and Rh
Development time 35-45 min; layer thickness
0.25 mm; run 17cm; qualitative separation.
Qualitative Separations
Qualitative Separations
22
24
Forty- metals ion S4
Hg, Cu, Cd, and Ag S,
Cu, Hg, Zr, Zn, Cr, S5
Y, Gd, Ho, Nd, La,
Pr, Ce, Pd, Au, Pt,
Co, Ni, Fe, and Bi
Twenty-five Se
Cations
Forty- eight metal S2
ions
Cu, Cd, Zn, Ni, Co, Sy Fe, Pb, Cr, Al, Zr, V, Th, U 0 2 ^ Ag, Se, and VO^
M4
Ms
M.
My
M«
Mg
Qualitative Separations 25
Concentration, separation and detection of 26
microgram amounts of Cu, Ag, Hg, and Cd 26"
salts in fresh water.
Spectrophotometric determination of Pd^ ' after 27
elution from TLC plate, using p-
nitrosodimethylaniline as chromogenic reagent.
Development time 15-25 min, TLC on 28
microscope slides.
Qualitative sepeirations 29
Ascending technique; run 10cm; development time 12-15 mins; 0.25 mm layer, semiquantitive determination of nine - cations on silica gel impregnated with sodium molybdate.
30
28
Pb, Cd, Hg, Te, W,
Mn, Ag, Fe, Be, Ni,
Mg, Pt, Ga, Cu, Ti,
Se, Co, and As
Se, Te, and Au
Bi, Au, Mn, Co,
Zn, Ti, Cu, Ag, Ni,
Pt, Cd, Pb, Hg, and
TI
Noble Metals
Ag, Al, As, Au, Ba,
Be, Bi. Ca, Ce,
Ss
Sg
Sio
Sn
Sl2
Mio
M„
Mi2
-
M B
Mio Ascending technique; run 17 cm; development 31
time 60 - 80 min; 0.25 mm layer; qualitative
separations.
Plates developed in Stahl chamber 32
Ascending technique; run 11 cm; development 33
time 2 hours; quantitative separation of Bi ^
from some ternary and quaternary mixtures of
metals.
Separation of noble metals from commonly 34
present metals.
Qualitative separations on treatment with PEI - 35
cellulose.
Ce ^, Co, Cr, Cu,
Dy, Fe, Ga, Ge,
Hg"", Hg, Ln, La,
Mg, Mn, and Mo^^
Ag, Al, Cu, Th, Si3 Mi4 Semiquantitative determination of Pb, Ag and 36
Mo, Zn, Cd, Pb, Se, Th by spot-area measurement method and
Zr, Ni, W, U02^^, spectrophotometer determination of UO2 ^ after
VO " , Fe, TI, and separation from other metal ions.
Co
Ru, Au, Pd, Pt, Rh, Si4 M15 Qualitative analysis 37
Os, Ir, and Ag no
Fe, Cd, Zn, Cu, Bi, S2 M16 Ascending technique; run 10 cm; development ^°
T F , Ag, Hg, Pb, Se, time 1 5 - 2 0 mins; 0.25 layers; examination of
Mn, U02^*, Ni, Al, effect of sample concentration, element
Zr, Ti, and Th concentration, pH of salt solutions, pH of
mobile phase and presence of anions in the
sample solution on the separation of Cd^* from
Zn^^ and of Cu "' from Ni^^, Co^^ and Cd^^ with
1.0 M HCOONa - 1.0 M KI (9:1).
29
Al, Fe *, Fe "*", Co, S15
Ni, Cu, Zn, Ag, Cd,
Hg, Tl, Pb, and Bi
Ni, Co, Zn, Cd, Ni, S2
Cu, UO2, V "", Fe^^
Fe^^ Ai, Th, Ti,
Mo'", Sc, W, Hg,
V^^ Tl^ Pb, Bi,
and Ag
UO2, Ti, Al, W, Ni, S,6
Fe, Co, Mn, Cu, Bi,
Zn, Cd, Ta, Se, Pb,
Ce, Ag, Th, Zr, Hg,
andTl
Twenty-eight metal Sn
ions including Ru,
Pd, W, Pt, Au, Mo
and UO2
Noble metals (Ru, Sis
Rh, Pd, Pt, Ir, and
Au)
Ag, Cd, Tl, Pb, Bi, S,9
Mg, Hg, Fe, Al, Co, S2
Ni, Zn, and Cu
Noble Metals (Au, Sig
Pd, Pt, Ru Ir, and
Rh)
Au, Se, and Tl S9
Ml7 Ascending technique; run 10 cm; quantitative 39
separation of Ni and Fe, Zn, Cd and Pb.
M, Ascending technique; run 10 cm; 0.25 layer;
loading volume 5 \i\; examination of the effect
of sample pH and the presence of alumina in
the stationary phase on separation of V '*' from 76+ 6+
M21 All noble metals were completely separated.
M22 Much shorter development time for NH4CI-
impregnated silica gel layers; clearer detection
and more compact spot formation on KI-
impregnated silica gel layers; better results
when KBr is used as eluent rather than as an
impregnant, but reverse is true for KI.
M23 Qualitative separations
40
W°" and Mo
Mi9 Ascending technique; run 10-12 cm, qualitative 41
separations.
M20 Quantitative separation of Ru " from other 42
metal ions.
44
45
M24 Simple and rapid separation of Au' ' , Se * and ^°
Tl 4+
30
Au, Pt, Pd, Cr, Mn, S3
Fe, Co, Ni, Cu, Ba,
Al, Bi, Pb, Zn, and
Ag
Cu, Co, Cd, Hg Ni, S20
and Ag
Ni, Cu, Zn, Pd, Cd, S2
Cr, Fe, Ru, Rh, La,
Au, Tl, Zr, Pt, Nb,
Ta, Mn, Ag, Hg,
Co, Mo and W
Cu. Co, Cd, Ni, Ag, S21
and Hg S20
Twenty-one cations Se
Ag, Ni, Cu, Co, Cd, S22
and Hg
M25 Application of method for the analysis of 47
platinum powder and two kinds of Au alloy.
Cu, Mg, Al, Ca, V,
Zn, Ge, Y, Zr, Mo,
Ag, Cd, IN, La, Ce,
Eu, Tb, Tl, Pb, and
Bi
3d metal ions
Twenty-six cations
Au, Ru, Rh, Pd,
Os, andPt
S23
S24
S25
S7
M31
M32
M33
M34
M26 Qualitative separations and determination of
chromatographic parameters as a function of
the concentration of MeOH, NH3, AcOH and
inorganic salts in the mobile phase.
M27 Run 11cm; correlation between Rp values on
impregnated layers developed with DMSO -
THF (1+10) and the atomic numbers of the
metal ions.
M28 Qualitative TLC separation of metal cations.
M29 Separation and identification of 21 cations on
cellulose layer.
M30 Separation of mixtures of inorganic ions on
chitin layers; possible use of chitin and chitosan
layers in wastewater and seawater purifying
systems.
Detection limits and Rp values of fluorescent
cations separated on porous glass sheet were
reported.
Reversed-phase TLC of 3d metal ions.
Qualitative analysis of twenty-six cations on
cellulose layer using six detection reagents.
Rs value of each pair of ions is 1.0 except that
ofRu (III) and Pd (II), limit of de tec t ion is
4.0 [ig.
48
49
50
51
52
53
54
55
56
31
Forty-nine
inorganic ions
526
Til, Fe, Al, Bi, S27
UO2, Cd, Hg, Ni,
Co, Cu, Pb, Ag, and
Ti
Fe, Zn, Cu, Ca, Co, S28
Hg, UO2, VO, Ag,
Mn, and Bi
Thirty-three metal S7
ions
Heavy metal S29
cations
Cu. Pb, Hg. Ag. Tl, S30
Cd, Fe. Bi. Ni. Co
and Zn
M35 Reversed-phase TLC of 49 inorganic ions and 57
their separation from multi-component
mixtures.
M36 Ascending technique; run 10 cm; layer 58
thickness 0.25 mm, limits of detection falls in
the range of 0.22-3.4 jig.
M37 TLC separation and colorimetric determination 59
of SCN" as applied to water and wastewater.
M38 TLC behavior of 33 metal ions was examined 60
and a theoretical model was proposed.
M39 Detection, identification and separation of 61
heavy metals.
M40 TLC separation of Ag^ from other metal cations 62
and colorimetric determination of Ag^ in
synthetically prepared ores.
The metal ions were in their usual valency states.
S.P.: Stationar>' Phase
M.P. : Mobile Phase
REF. : Reference
32
Tablel.6 LIST OF STATIONARY PHASES USED:
Si Cellulose phosphate (Whahnan P41, U.K) and cellulose phosphate +
microcrystalline cellulose (3:1)
52 Silica gel G
53 Ecteola - cellulose
54 Dowex 1 or 50 mixed with cellulose (Avicle SF)
85 Ti (IV) antimonate (H^ form)
Se Cellulose
S7 Silica gel
Sg Cellulose phosphate in H^ form (Whatman P41, U.K)
S9 Alumina
Sio Hydrous Zirconium oxide
Si 1 Semi cr}'stalline Sn phosphate ion-exchanger + Silica gel G
S12 PEI cellulose
Si3 Silica gel impregnated with 0.5M NH4CI and a saturated aqueous solution of
barium nitrate
Si4 Tin pyrophosphate and silica gel containing sodium CM cellulose as binder
Si5 Silica gel impregnated with O.IM aqueous sodium nitrite, sodium molybdate
and potassium dihydrogen ortho phosphate
516 Silica gel loaded with different concentrations of TBA
517 Binder free Zr (IV) antimonate in the H" form, Silica gel G and mixture of
Zr (IV) antimonate and silica gel G (1:1)
33
Si8 Microcrystalline cellulose modijfied with silica gel G
519 Silica gel impregnated with 0.1 - l.OM aqueous solutions of NaCl, NH4CI,
KBr, or KI
520 Chitosan
821 Chitin, chitosan and their derivatives
522 Chitin
523 Porous glass sheets
524 Silica gel coated with high molecular weight amines (Primine JM-T,
Amberlite LA-1, Alumina 36, Aliquat 336)
525 Microcrystalline cellulose
526 Silica gel impregnated with amberlite LA-2
527 Silica gel and alumina impregnated with 0.1-1.0% LiCl
528 Cellulose microcrystalline/cellulose + kieselguhr (4:1, 3: 2, 1:1); Kieselguhr
529 Silica gel, silica gel mixed with Sn'*" arsenosilicate and impregnated with
tributyl amine
530 Microcrystalline cellulose, alumina and their mixtures
34
Table 1.7 LIST OF MOBILE PHASE USED:
Ml Aqueous acetic acid (0.1 - 3.0 M) and mixtures of acetic acid and ammonium
acetate solutions
M2 Dimethyl sulphoxide- HCl (1 - 6 M) (1+9, 3+7, 5+5, 7+3 and 9+1, v/v)
M3 Different concentrations of HCl and aqueous chloride solutions of Li, Mg, Na,
Ca, and Sr
M4 HO Ac - HCl
M5 Distilled water, aq. sodium chloride (0.1 - 1%)
Mg HNO3 (0.001-1.0 M),DMF-0.1M HNO3 (1+0, 4+1, 1+4, 4+6, 6+4)
M7 Binary solvent systems
Mg Oxalic acid - oxalate systems
M9 Demineralized water, mixtures of sodium formate and formic acid solutions in
different ratios
M,o 0.5 M H2S04 - acetone or methanol (1+0, 4+1, 3+2, and 2+3)
M]i Aqueous solutions of inorganic acids (HCIO4, HCl, HBr, HNOs H2SO4 and
H3PO4), their mixtures and some of their salts
M12 Aqueous HCl and HNO3 at different pH levels and their mixtures with organic
solvents in different ratios
M,3 0.01 - l . O M HCl
Mi4 Aqueous solutions of HCOOH, HCOONa, CH3COOH, CH3C00Na,
CH3COOH - CHsCOONa (1+1) and HCOOH - HCOONa (1+1)
Ml5 Acids (nitric, tartaric, citric, perchloric, formic), bases [NH4OH, (CH3)3N],
neutral compounds (NH4CI, NH4NO3, ACONH4) or mixture of these with
organic solvents (EtOH, MeOH, n - PrOH, Me2C0)
35
M,6 Aqueous HCOONa (10"\ 10'. 0.1, and 5.0M), 0.1 M HCOONa - O.IM
HCOOH (1+9, 3+7, 1 + 1, 7+3 and 9+1), 1.0 M HCOONa-1.0 MNaCl (1 + 1,
1+2, 2+1, 1+9 and 9+1) and mixtures of 0.1 or 1.0 M HCOONa and 0.1 or
1.0 M NaCl, KBr, KI or KBrOs in different ratios
Mi7 Mixtures of aqueous 1.0 M formic acid and alkaline salt solutions
Mi8 Ternary mixtures of TBA, fonnic acid and ammonium acetate in different
ratios
Mi9 Aqueous formic acid (10""' - 2.0 M), aqueous sodium formate (10' - 5 M) and
mixtures of 1.0 M formic acid and 1.0 M sodium formate (1 + 1, 4+6, 6+4, 2+8,
and 8+2)
M20 HNO3 (10"-- 1.0 M), DMSO - 0.1 M HNO3 (1+0, 4+1, 3+2, 1+4), dioxane-
O.IM HNO3 (1+0, 4+1, 3+2. 2+3. 1+4)
M21 n - Butanol - HCl - acetone (100-1 + 100)
M22 Demineralized water, O.IM formic acid; l.OM KI, KBr or NaCl; O.IM formic
acid-0.1 MKIorKBr(l+9), 1.0 M formic acid - l.OM HBr (1+9 and 9+1),
l.OM fon-nic and -1.0 M NaCl, NH4CI, KBr or KI (1+9, 3+7, 1+1, 7+3 and
9+1)
M23 n - BuOH - IBMK - P^" - HCl (140 + 100 + 15 +18) and (140 +100+15 + 1)
M24 Aqueous solutions of HCl, HBr, H2SO4, HNO3, H3PO4 and various organic
acids
36
M25 Mixtures of 2.5 M HCI, 2.5 M NaCl and 0.6% hydrogen peroxide in different
ratios
M26 l.OM inorganic salt solutions in aqueous methanol
M27 DMSO - l.OM HNO3 (1+1); DMSO - THF (1+10); n - butanol - acetone -
HNO3 (6+6+1); diisopropyl ether systems
M28 Several aqueous mobile phases
M29 Acetylacetone - acetone - cone. HCI (5:5:1)
M30 Aqueous methanolic solutions (1+1, 3+1, v/v) of ammonia or AcOH with
ammonium nitrate, or sometimes ammonium acetate
M31 n - Butanol - benzene - IM PfN03 - IM HCI (75+69+4+2, v/v) or acetone -
3M HCI (99+1, v/v)
M32 Aqueous solutions of succinic acid
M33 Numerous solvent systems
M34 Sulfuric acid and sulfuric acid - ammonium sulfate media
M35 HCOOH (1.0 M); HCOONa (l.OM) and their mixture
M36 NH4OH (l.OM)-acetone (1:9, 3:7, 1:1,7:3,9:1)
M37 Methylisobutylketone - formic acid mixtures
Msg MeOH containing aqueous mobile phases
M39 Aqueous solutions of ammonia, acetic acid, sodium acetate and sodium nitrate
37
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40
t * J haBt^P
2.1 INTRODUCTION
Mutual Separation of copper (29CU), silver (47Ag) and gold (79AU), the
elements of IB group of the periodic table is analytically important because of their
similarities in chemical properties. These metals with general configuration (n-1) d'°
ns have a tendency to form complex salts in which the metal may be complex cation
or complex anion. Copper is associated with silver in copper glance (Cu, Ag) 2 S ore
and hence its separation is needed to get pure silver from the ore. On the other hand,
the presence of silver ions has been found to decrease the rate of adsorption of Au" ^
on activated carbon from thiourea solution (I).
Thus, the presence of one metal in small quantities offers deleterious effects
on performance of other metals. Because of industrial, commercial and medicinal
importance of these metals, several analytical techniques such as ion-exchange
chromatography (2,3,4), potentiometry (5,6), capillary zone electrophoresis (7),
solvent extraction (8,9), single sweep oscillopolarography (10), ion pair - reversed-
phase HPLC (II), size - exclusion chromatography (12), titrimetry (13,14), reversed-
phase column chromatography (15,16), reflectance spectroscopy (17), flame or
graphite furnace atomic absorption spectrometry (18,19), laser excited atomic
fluorescence spectroscopy (20), neutron activation analysis, ion paired (21, 22) and
foam plastic (23) chromatography etc. have been used in addition to hyphenated -
techniques e.g. TLC- spectrophotometry (24), solvent extraction - AAS/FAAS (25,
26), ICP-AES and ICP - MS (27,28) and ion-exchange chromatography-photometry
(29), for the separation and determination of Au''' ,Cu " ,and Ag" from a variety of
matrices. Ion chromatography of Au-cyanide complexes and problems arised during
analysis of gold by titrimetric and spectrophotometric methods has been reviewed
recently (30, 31). ;
Recently, new materials including chelate forming plastics containing amino-
thiourea (32), VS-II anion exchange fibre (33), ion-exchange resin loaded with
41
bismuthiol (34), silica gel bound thia-crowns (35), chemically modified silica gel
with p-dimethylaminobenzylidenerhodanine (36), chitosan treated with
dithiocarbamates (37) and nanofilter membrane (38) have been developed and
utilized by several workers for preconcentration and separation of gold, silver, and
copper.
Amongst analytical techniques used, thin layer chromatography (TLC) being
inexpensive and versatile is still enjoying popularity among analytical scientists
specially working in India, China, Japan and European Countries. As a result, TLC
systems comprising of ECTEOLA- cellulose and HCl + NaCl +H2O for separation of
gold, platinum and palladium from associated base metals (39); chitin and aqueous
buffer solutions for separation of Cu~ and Ag^ (40), alumina and aqueous solutions
of both organic and inorganic acids as well as some sodium salts for rapid separation
of Au'" trom Te''^ and Se ^ (41,42) and silica gel containing sodium or ammonium
acetate and toluene for separation of Cu"" complex from transition metal complexes
(43) have been reported.
Analytical techniques listed above have been successfully applied to separate
Au"" either from Cu or Ag but the work on mutual separation of these metal ions
from their three-component mixtures is lacking. As far as, we are aware, not a single
reference is available on the separation of co-existing Au " , Cu " and Ag^ by TLC
using surfactant mediated mobile phase (or micellar system). During our previous
study (44) on micellar thin layer chromatography of heavy metal cations, we realized
that micellar mobile phases have unique separation capabilities to provide unusual
selectivity, enhanced detection sensitivity and faster analysis. Efficiency of micellar
systems in the separation of cations (45, 46,) and anions (47, 48) has been reported
and reviewed by Okada (49). Surprisingly, very little work appears to have been done
on the use of micellar mobile phases in TLC of inorganics (50, 51), none of these
studies examined the separation of metal cations. It was therefore decided to identify
novel micellar mobile phase systems to achieve highly selective separations of metal
42
cations. As a result, simultaneous separation of Au , Cu , and Ag^ from their
mixtures has been achieved on silica layer using buffered anionic micellar eluent
with added amino acids. The proposed TLC method is selective and rapid with
development time averaging 5 min.
2.2 EXPERIMENTAL
Apparatus
A TLC applicator (Toshniwal, India) was used for preparation of 20 x 3.5 cm
glass plates. The chromatography was performed in 24 x 6 cm glass jars. A glass
sprayer was used to spray reagents on the plate to detect the spot.
Chemical and Reagents
Silica gel 'G' (Merck, India); Sodium dodecyl sulphate (BDH, India);
L - arginine, L-histidine and DL- phenylalanine, phenols, amines and anions (CDH,
India); and L-tryptophan (Loba - chemie, India), were used. All reagents were of
Analar Reagent grade.
Metal - Cations Studied
Ve\ Cu'\ Ni'\ Co^^ UO,'^ VO^^ Cd^^ Zn2\ Ag^ P b ^ Tf, Bi^^ Hg^^
A P , T i ^ a n d A u ' ^
Materials Used in Separation Tests
1. Gold-plated printed-circuit board (GPCB) (Au, Ni, Cu) from Toyama
Electric, Bangalore.
2. Silver mirror scrap (SMS) (Ag, Cu) and Silver mirror spent solution
(SMSS) (Ag, Cu), both from Ship Mirror Industries, Bangalore.
3. High-copper dental amalgam (HCDA) (Ag, Hg, Cu, Zn, Sn) from the
Dental College, A.M.U., Aligarh.
4. Sterling silver scrap (SS) (Ag, Cu), (synthetic).
43
Test Solutions
1.0% aqueous solutions of following salts were used as test solution:
a. Nitrates of Cd^^ Zn^^ Pb^^ Tf, Bi^^ Al^^ and Ag^
b. Chlorides of Ni^^ Co^^ Fe^^ Hg^^ Ti^^ and Au^^
c. Sulfates ofCu^^VO^^ and UOz "".
All the solutions were prepared in demineralized water with a specific
conductivity (K = 2xl0'^ohm"* at 25°C). The solutions of nitrates of lead, silver and
bismuth, and the chloride of mercury also contained small quantities of
corresponding acid to limit the extent of hydrolysis.
The solutions (1%) of various anions were prepared in demineralized water.
The solutions (1%) of various amines and phenols were prepared in methanol.
Buffer Solutions
S. No.
1
2
4
5
Composition
0.04 M Boric acid-0.04 M phosphoric
acid
0.02 M Boric acid-0.04 M phosphoric
acid-0.24 M NaOH
0.04 M Boric acid-0.04 M phosphoric
acid- 0.24 M NaOH
0.04 M Boric acid-0.04 M phosphoric
acid- 0.24 M NaOH
0.04 M Boric acid-0.04 M phosphoric
acid -0.24 M NaOH
Volume Ratio
50:50
50:50:8
50:50:10
50:50:14
50:50:60
pH
2.3
3.4
5.7
7.0
11.9
Detection Reagents: For the detection of various cations, the following reagents were
used:
a. 8x10'^% (w/v) Dithizone in carbon tetrachloride for Cd , Zn , Ag , 2+ T.,+ r»:3+ 2+ ?h'\ Tf, Br, and Hg"".
44
x2+ b. Aqueous Potassium Ferrocyanide for Fe^ , Cu " , U02^^, VO'^, and
4+ Ti
c. Dimethylglyoxime for Ni^ , and Co^ . i 3 +
3+
d. Aluminon for Al , and
e. Yellow ammonium sulphide for Au "".
Stationary Phase: Silica gel 'G'
Mobile Phases: The following Solvent systems were used as mobile phases
Symbol
M,
M2
M3
M4
M5
Me
M7
Ms
^49
Composition
0.001, 0.005, 0.01 and 0.05M aqueous SDS
0.001, 0.005, 0.01 and 0.05M-buffered SDS (pH 2.3)
0.001, 0.005, 0.01 and 0.05M-buffered SDS (pH 3.4, 5.7, 7.0 and
11.9).
O.OIM SDS (pH 2.3) + O.OIM L-arginine (1:9, 3:7, 5:5, 7:3 and 9:1)
O.OIM SDS (pH 2.3) + O.OIM DL phenylalanine (1:9, 3:7, 5:5, 7:3
and 9:1)
O.OIM SDS (pH 2.3) + O.OIM L-tryptophan (1:9, 3:7, 5:5, 7:3 and
9:1)
O.OIM SDS (pH 2.3) + O.OIM L-histidine (1:9, 3:7, 5:5, 7:3 and 9:1).
0.001, 0.005, or 0.05M SDS (pH 2.3) + O.OIM L-arginine (9: 1).
O.OIM SDS (pH 2.3) + 0.00IM L-arginine, DL-phenylalanine,
L-tryptophan or L-histidine (1:9, 9:1)
Preparation of TLC Plates
Silica gel plates were prepared by mixing the adsorbent with double distilled
water in 1:3 ratios by weight. The resultant slurry was mechanically shaken for
10 min, after which it was applied to well-cleaned glass plates with the help of TLC
applicator to give a layer of approximately 0.25 mm thickness. The plates were air
45
dried at room temperature and then heated at 100 + 5 C for 1 h to activate them.
After activation, the plates were stored in a dessicator.
Preparation of Test Material
Peeling'. Peeling of silver mirror scrap (specimen surface area 19 cm ) was
performed with concentrated formic acid (90%, w/w) in a glass beaker. The acid was
heated at 110°C and the material was added into it. On completion of peeling (within
1 min.), the solution was separated and the peeled material was used as the 'source
material' for silver.
Leaching: Leaching of silver mirror scrap (specimen surface area 19 cm ) was
performed with 50% nitric acid in a glass beaker. On completion of leaching (within
1 min), the solution was separated from the leached residue and used for silver
separation. Similar leaching procedure was applied on other silver-containing
material (0.153 g sterling silver scrap and 0.25g high-copper dental amalgam.
Leaching of gold - plated printed- circuit board (specimen surface area 72.42
cm^ containing 16 large pins and 14 small pins) was performed with aqua -regia in a
glass beaker. On completion of leaching, the solution was separated from the residue
and used for gold separation.
Chromatography
About 10 JJ.L of test solution was applied using a micropipette about 2.0 cm
above the lower edge of the chromatoplates. The spots were dried, and the plates
were developed in glass jars containing the mobile phase using a one-dimensional
ascending technique. Before developing the plates, the glass jars that contained the
mobile phase were covered with a lid for about 20 min so that the glass jars would
get pre-saturated with the mobile phase vapors. The mobile phase (solvent) was
allowed to migrate up to 10 cm from the starting line in all cases. After development,
the plates were dried again and the cations were visualized as colored spots by
spraying with appropriate detection reagents. The cations were identified on the basis
46
of their Rp values, calculated from RL (Rp of leading front) and RT (RF of trailing
front) for each spot.
Separation
The test solution (0.0 ImL) of copper, silver and gold mixture was spotted on
TLC plate coated with silica gel 'G' and the chromatography was performed using
O.OIM SDS(pH2.3) + 0.01M L-tryptophan (1:9) or O.OIM SDS (pH 2.3) + O.OIM
L-histidine (1:9) as mobile phase. The resolved spots for these metal cations were
observed on TLC plates after spraying chromogenic reagents. The Rp values of Au^ ,
Cu^^ and Ag" in their mixture were found to vary marginally from their individual Rp
values.
Interference
For investigating the interference of inorganic anions, amines, and phenols on
the Rp values of Au^ , Cu " , and Ag" , an aliquot (O.OlmL) of impurity solution was
spotted along with the mixture (O.OlmL) of Au"" , Cu^ , and Ag" and chromatography
was performed as described above. The spots were detected and the Rp values of
separated metal ions were determined.
Limit of Detection
The identification limits of various cations were determined by spotting
different amounts of cationic solutions on the chromatoplates. The plates were
developed and the spots were detected as described above. The method was repeated
with successive lowering of the amount of cation until spots could no longer be
detected. The minimum amount of cation that could be detected was taken as the
limit of detection.
47
Applications
(i) Chromatography of Unspiked Materials
Chromatography of unspiked dental amalgam and printed circuit board was
done using O.OIM SDS (pH 2.3) + O.OIM L-histidine (1:9) and the spots of Ag^,
Hg^ , Zn " , Cu '* and Au' ' were detected and the Rp value of each cation was
determined.
(ii) Chromatography of Spiked Materials
The spiked samples of PCB, dental amalgam, silver mirror scrap, silver mirror
spent solutions, and sterling silver was prepared as follows:
(a) One ml of PCB/ dental amalgam solution was mixed with ImL of silver
test solution (1%). The chromatography was done using 0.0ImL solution of this
mixture for spotting.
(b) One ml of gold solution was added to 1.0 mL of each SMC, SMSS or SS
solution and 0.0ImL of the mixture was used for chromatographic separation of
Au^^- Cu" - Ag^. The spots were identified by their respective Rp values.
2.3 RESULTS AND DISCUSSION
The results of present study have been summarized in Figures 2.1- 2.3 and
Tables 2.1- 2.6. The unique features of this study are:
(1) Selection of micellar systems composed of anionic surfactant, sodium dodecyl
sulphate (SDS) as mobile phase, which have a fraction of negative charge and
tend to attract positively charged species including metal cations.
(2) Utilization of polar (arginine and histidine) and non-polar (phenylalanine and
tryptophan) amino acids as additives.
(3) Realization of mutual separation of Au^ , Cu^^ and Ag^, from their mixtures
and the investigation of effect of phenols, cations and anions on the separation
of coexisting Au "* , Cu " , and Ag^ ions.
48
(4) Application of proposed method to the analysis of several real and synthetic
samples for the presence of gold, silver and/ or copper.
Effect of Concentration andpH of SDS Solution
Results obtained with different concentrations of unbuffered aqueous SDS
(Ml) and buffered SDS (M2 and M3) solutions reveal the following trends.
(i) Metal ions such as K\^\ Ti^^ VO^^ Fe^^ Cu^^ Zn^^ Pb^^ Bi ^ and UOz ^
show either no mobility (Rp = 0.0) or very little mobility (Rp » 0.05) at all
concentration levels as well as over entire pH range of SDS solutions. A
slightly higher mobility (Rp = 0.3) in the case of Zn^^ was observed when
0.05M SDS solution of pH 2.3 was used as mobile phase.
(ii) Ni""", Co "" and T\* produce badly tailed spots (RL-RT > 0.3) almost with all
the mobile phases used with the exception of O.OIM SDS (pH 2.3).
Compared to buffered SDS solutions (pH 3.4, 5.7, 7.0 and 11.9), higher
mobility for these cations was realized with SDS solution (pH 2.3).
(iii) Au^^ as well-formed spot, always migrates with solvent front (Rp > 0.9)
regardless the concentration or pH of SDS solution and hence it can be
selectively separated from binary mixtures of other metal ions.
(iv) The mobility of certain metals (Cu^*, Zn " , Ag" , Bi' " and Al ^) was increased
marginally on substitution of 0.001 or O.OOSM-buffered SDS (pH 2.3) with
0.01 or0.05MSDS(pH2.3).
(v) The development time for 10 cm ascent was typically short i.e. 10-12 min for
all mobile phase systems used.
The Rp data of metal ions obtained with buffered SDS (pH 2.3) at different
concentrations (M2) levels are compared and presented in Figure 2.1. It is clear from
this figure that the mobility of metal ions is slightly influenced by the concentration
of SDS in the mobile phase.
49
Effect of Added Amino Acids
As micellar SDS solutions (buffered as well as non-buffered) were found to
resolve only limited number of two component mixtures of metal cations, it was
decided to improve the separation efficiency of SDS mobile phase system. Several
workers have reported (52-56) the improvement in chromatographic efficiency of
micellar systems as a result of alteration in micellar properties in the presence of
organic and inorganic additives. The additives tested so for include alcohols, diols,
dipolar aprotic solvents (DMSO, dioxane), alkylnitriles, alkanes, urea, NaCl, and
acetone etc. It is surprising that not a single reference is available on the use of
amino acids as additives in micellar TLC of inorganics. Amino acids being
amphiphilic substances are supposed to provide unique selectivities in the separation
of metal cations. Amino acids were chosen as additives for the following reasons.
(i) Amino acids contain both amino (-NH2) and carboxylic acid (R-COOH)
functional groups providing nitrogen and oxygen as electron donor centers to
coordinate with metals (57).
(ii) Amino acids form complexes with Cu ^ (58) and the presence of ionic
surfactant in the medium influences the binding of Cu^^ with certain amino
acids (59).
Cu^^ and Ag" have greater tendency to form coordination compounds with
nitrogen than that with oxygen (60).
(iii) In solution, amino acids exist in the following protonic equilibrium:
R-COOH <^R-COO-+H^
R-N^H3C:>R-NH2 + H^
Thus, the pH depended protonated (R-COOH and R-N^Hs) and proton
acceptor (R-COO^ and R-NH2) groups may influence the migration trend by
modifying the properties of micellar mobile phase.
50
(iv) The aromatic R-groups of phenylalanine and tryptophan are hydrophobic, a
property that has important consequences for the ordering of water molecules
in the mobile phase.
In the present study, the mobile phase systems consisting of variable
concentrations of amino acids (L-arginine, DL-phenylalanine, L-histidine or L-
tryptophan) and buffered SDS (pH 2.3) were formulated by adding required volumes
of O.OIM amino acids into SDS solution (0.001-0.05M) in volume ratio of 1:1, 3:7,
7:3, 9:1 and 1:9 keeping total volume constant in each case. The chromatography
performed with these mobile phase systems (M4 - M9) reveals the following trends:
(a) The solvent systems [M4-M7 (3:7, 5:5, 7:3)] containing amino acids (arginine,
phenylalanine, tryptophan or histidine) at concentration levels of 30 - 70%,
produce tailed spots (RL-RT> 0.3) for all the metal ions except Al" , Pb" and
Fe"' which stayed at the point of application and Au^^ that moved with the
solvent front. Thus, the presence of amino acids in SDS containing mobile
phases causes tailing in the spots of metal cations probably due to competitive
interactions among cations, charged amino acid and anionic SDS. Since the
experiments were performed at pH 2.3, the amino acids in this study are
supposed to bear a net partial positive charge.
(b) In case of mobile phases composed of either 10% SDS and 90%) amino acids
[M4-M7, (1:9)] or 10% amino acids and 90% SDS [M4-M7, (1:9)], number of
cations producing tailed spots was decreased as compared to mobile phases
discussed above in (a). The resuhs are compared in Table 2.1. Compared to
mobile phases containing 90%) SDS (O.OIM, pH 2.3) plus 10%) amino acids
(O.OIM), better separation possibilities were with those containing 10% SDS
(2.3 pH) plus 90%o amino acids.
(c) Against our hope, arginine (aliphatic amino acid with side chain containing
basic group) which has been utilized satisfactorily for resolution of amino acid
enantiomers via ligand exchange TLC (61) was found unsuitable for
separating metal cations. Similarly, phenylalanine (a non-cyclic aromatic
51
amino acid) was found ineffective to produce satisfactory results. However,
heterocyclic aromatic amino acids such as tryptophan and histidine were
found capable to provide better-resolved spots of metal cations. On the basis
of results presented in Table 2.1, the chromatographic performance (or
separation efficiency) of amino acids was in the following decreasing order:
L-Histidine > L-Tryptophan > DL-Phenylalanine > L-Arginine
Thus, the micellar solvent systems containing heterocyclic amino acids
like L-tryptophan or L- histidine proved superior to solvents containing aromatic or
aliphatic amino acids like phenylalanine or arginine for separation of metal cations.
(d) The solvent system consisting of O.OlM-buffered SDS (pH 2.3) and O.OIM
histidine (M7) or tryptophan (Mg) in 1:9 ratios was found most suitable for
separating co-existing Au "*", Cu^ , and Ag" ions (Table- 2.2).
(e) When chromatography was performed using mobile phases (Mg) obtained by
mixing O.OOIM, 0.005 or 0.05M SDS (pH 2.3) with O.OIM amino acid
(L-arginine) in 9:1 ratio, less tailed spots for Ni , Co , slightly higher
mobility for CvT* and T^ and increased compactness for Zn " and Cd * was
noticed with the increase in SDS concentration of mobile phase. However, the
mobility of Fe^^ Pb^^, Bi^^ Hg^ , AV\ Ti^\ UOi^^ or VO^* (Rp = 0.0) and
Au" " (Rp^ 0.9) remained unchanged over the entire SDS concentration range.
(f) When O.OIM amino acid in the mobile phases M4-M7 was substituted with
O.OOIM, little increase in mobility of Cu^^ Fe "", Ni " , Co " , Zn^^ and Tf was
noticed with mobile phases containing L-arginine or DL-phenylalanine.
Whereas opposite trend (i.e. decrease in the mobility) for these metal ions was
realized with L -tryptophan containing mobile phase. The mobility of all other
metal ions was found to remain unchanged. In the case of L-histidine, mobility
of all metal ions remains almost the same irrespective of the concentration of
histidine (0.01 or O.OOIM).
In order to provide a clearer picture about the variation of Rp values of
the metal ions as a function of concentration of amino acids in the mobile phase, a
52
representative plot of Rp Vs volume fraction of 0.01 M L-histidine was constructed
(Figure 2.2). The curves shown in (Figure 2.2) pass through maxima and minima
exhibiting the variation of Rp values (or mobility) of metal cations without any
regular pattern. This situation is probably arised due to the formation of occasional
tailed spots in certain cases as a result of multiple interactions.
In Figure 2.3, the A Rp [Rp in Me (1:9) and Rp in M7 (1:9)] was plotted against
metal cations to show the net effect of change in microenvironment of micellar
mobile phase on the mobility of metal ions. It is clear from this figure that metal ions
either migrate faster (positive A Rp value) in tryptophan (5 or 6 membered
heterocyclic amino acid) containing micellar system or show the same mobility (A Rp
values + 0.05) compared to their mobility in histidine (5 membered heterocyclic
amino acid) containing micellar system. Thus, an enhanced mobility of cations is
possible with micellar systems containing amino acid with large non-polar side chain
(e.g. tryptophan) compared to the micellar systems having amino acid with polar side
chain (e.g. histidine).
Separations
Separation of metal ions obtained on silica layer with micellar systems in the
presence and absence of amino acids has been listed in Table- 2.2. To widen the
applicability, the separation of Au " , Cu " , and Ag" was examined in the presence of
organic (phenols, urea, thiourea) and inorganic (cationic and anionic) impurities.
The results presented in Table 2.3 and 2.4 indicate that all the impurities are
ineffective in bringing about the change in the mobility of Au " and Ag^. Conversely,
the mobility of Cu "" is influenced by the impurities. The variation in Rp value of
Cu was from 0.30 (o-nitro phenol impurity) to 0.60 (thiourea impurity). However,
the simultaneous separation of Au " , Cu "", and Ag" from their mixtures was always
possible. The poorest separation was in the presence of K2S2O8 and ammonium
oxalate because of the formation of tailed spots of Cu " .
53
From the data provided in Table 2.3, it seems that the position of substituent
groups in the benzene ring control the mobility of Cu " . For example, the order of
increase in Rp value of Cu " (given in parenthesis) in the presence of o-, m -, and p-
isomers of nitro phenols and cresols was:
o-nitro phenol (0.3) < m - nitro phenol (0.50) < p-nitro phenol (0.53) and
p-cresol (0.45) < m - cresol (0.53) < o-cresol (0.55).
This reverse order in the mobility of Cu " can be attributed to the opposite
effects of NO2 (an electron withdrawing group) and CH3 (an electron releasing
group) attached to benzene ring. The A I^values (difference in Rp values of resolved
spots from binary mixtures of metal cations) obtained for Au'^^-Ag^, Au^^-Cu^" and
Cu~^- Ag' pairs on silica gel layer were 0.95, 0.30 and 0.65 respectively with 0.01 M
SDS and 0.01 M L-tr}'ptophan (1:9) eluent and 0.95, 0.42 and 0.53 respectively with
0.0IM SDS + 0.0IM L-histidine (1:9) eluent. From these data, it can be safely
concluded that SDS-tr\'ptophan system is better for resolving Cu^*- Ag^ (A Rp =
0.65) whereas SDS-histidine system is more useful for resolving Au' - Cu^^ mixture
(A Rf = 0.42). The detection limit ()ag) given in parenthesis for Cu^^ (6.6), Ag* (1-6),
Fe^^ (0.76). Zn^^ (0.022), Cd^ (0.038), and Hg"^ (0.029) is highly sensitive to detect
heavy metals at trace levels. Zn * and Hg " down to 0.02 and 0.03 (p-g) levels
respectively can be easily detected on TLC plates.
The data presented in Table 2.5 clearly demonstrate that the individual solvent
s\'stems are not of much practical utility for separation purposes. However, buffered
SDS (pH 2.3) in combination of amino acids (histidine/tryptophan), as discussed
above has enormous analytical potentialities to facilitate analytically important
separations.
Application: The proposed method was successfully applied for identification and
separation of Au''", Cu'", Ag^, Ni "" and Hg "" in a variety of matrices. The results are
summarized in Table- 2.6.
54
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56
Table 2.3 Effect of Certain Organic and Inorganic Additives on the
Separation of Au^\ Cu^^ and Ag^ Developed with O.OIM SDS (2.3 pH)
+ O.OIM L-histidine (1: 9):
1% aqueous solution of
different impurity
Urea
Thiourea
NaNOs
NaN02
Na M0O4
NH4SCN
NaH2P04
KIO3
KIO4
KI
K4[Fe(CN)6]
K3[Fe(CN)6]
KSCN
K2S2O8
Ammonium Oxalate
Au^"
0.94
0.95
0.92
0.93
0.94
0.94
0.93
0.92
0.95
0.95
0.93
0.95
0.94
0.92
0.95
Separations (RF)
Cu^^
0.56
0.6
0.44
0.52
0.44
0.55
0.58
0.5
0.44
0.55
0.52
0.52
0.55
0.52T
0.52T
Ag^
0.05
0.05
0.0
0.0
0.0
0.05
0.0
0.05
0.05
0.05
0.0
0.05
0.05
0.0
0.0
T= Tailed spot (RL - RT>0.3)
57
Table 2.4 Separation of Au^\ Cu^^ and Ag^ Ions from Their Mixture in
the Presence of Phenolic Compounds on Silica Layer Developed with
O.OIM SDS (2.3 pH) + O.OIM L-histidine (1: 9):
Phenolic compounds as
impurity
Phenol
Phloroglucinol
Pyrogallol
m-Nitro phenol
o-Nitro phenol
p-Nitro phenol
Vaniline
Pyrocatechol
m-Hydroxyacetophenone
Gallic acid
Orcinol
Picric acid
Hydroquinone
Resorcinol
o-Cresol
m-Cresol
p-Cresol
Au^^
0.93
0.95
0.94
0.96
0.96
0.95
0.95
0.95
0.95
0.95
0.93
0.96
0.95
0.93
0.96
0.95
0.92
Separations (RF)
Cu^^
0.55
0.56
0.51
0.50
0.30
0.53
0.57
0.49
0.52
0.56
0.35
0.59
0.55
0.53
0.55
0.53
0.45
Ag^
0.05
0.06
0.0
0.0
0.0
0.05
0.05
0.0
0.05
0.05
0.0
0.05
0.05
0.00
0.05
0.0
0.05
58
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