Lecture HPLC Student

7/28/2019 Lecture HPLC Student

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TOPIC


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Able to :

1. Explain the fundamental concepts & theories ofseparation techniques in HPLC.

2. Sketch & label the schematic diagrams in HPLC.3. Identify the strength & limitations of each type of HPLC

technique.

4. Suggest & justify the most suitable & efficient separationtechnique to be employed for an analysis.

Course Learning Outcomes


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Introduction Mobile phase is liquid.

Analytes are first dissolved in a solvent / mobile

phase, then passed through column under highpressure.

The separation dependent upon interaction ofthe analytes with mobile & stationary phases.


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HPLC applications4

Analysis ofmixtures of non-volatile or

thermally unstable compounds (thosethat cannot be analyzed by GC)


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To hold the

liquid mobilephase

Topressurizethe liquidmobile

phase

To allowinjection ofthe samplemixture

Columncontainingthe bed ofstationaryphase

To detect thepresence ofcomponents

as they exitthe column

To recordthe detectorsignal


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HPLC

setup
http://upload.wikimedia.org/wikipedia/en/4/4b/Agilent1200HPLC.jpg


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Mobile phase delivery7

Consists of pump to provide the high pressure required.

Equipped with one/more glass reservoirs. Require very clean & pure HPLC grade solvents to prevent

column degradation with impurities & to minimize detectorbackground signals from contaminants (usually UVtransparent).

Solvents must be filtered (to remove dust) through 0.2/0.45m filters & degassed.


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Membrane Filter Holder Assembly


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Degassing (removal of dissolved N2

, O2

, CO2

..) by :

9 Vacuum devices (vacuum applied to headspaceabove solvent).

9 Ultra sonication (high frequency vibration drives

gasses out of solvent).9 Heating (decreases solubility of gases).

9 He sparging. Sparging is a process in whichdissolved gases are swept out of a solvent.


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If solvents are not degassed, gases that are solubleat high pressure (ie. 4,000 psi) become insoluble atlow pressure (ie. at the end of the column or

detector).

Gas bubbles create difficulties with pumps

(destabilize pressure), columns (bad separation) &detectors.

Dissolved O2 absorb UV radiation in the 250-200 nmwavelength range & interfering with UV detection.


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Sample problem 111

Why is high pressure is needed in HPLC?


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Elution12

The mobile phase may comprise of a single solvent or,

Various solvents can be mixed to give mixture with asuitable polarity.

Isocratic elution

The composition of the mobile phase remainsconstant during the separation


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Binary gradient 2 solvents (very common).

Tertiary gradient 3 solvents (quite common).

Quaternary gradient 4 solvents (very rarely).

Gradient elution

Solvent composition of the mobile phase is

changed with time during the separation


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E.g. of a gradient :Starting at 10% methanol & ending at 90% methanolafter 20 minutes.

The two components of the mobile phase are

typically termed "A" & "B";9 A is the "weak" solvent which allows the solute to

elute only slowly.9 B is the "strong" solvent which rapidly elutes the

solutes from the column.


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Solvent A is often water, while B is an organicsolvent miscible with water, such as acetonitrile,methanol, THF or isopropanol.

Organic solvent of the mobile phase is to reducethe retarding strength of the aqueous component.


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Different compounds are eluted by increasing thestrength of the organic solvent.

The sample is injected while a weaker mobile phase isbeing applied to the system.

The strength of the mobile phase is later increased in

increments by raising the organic solvent fraction, whichsubsequently results in elution of retained components.


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Advantages :

9 Decreases the retention of the later-elutingcomponents so that they elute faster, giving

narrower (& taller) peaks for most components.

9 Improves the peak shape for tailed peaks.


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Pumping system18

Pumping system pumps (force) the mobile phase

through the manual injector, column, detector & finallyto the waste container.

The quality of a pump for HPLC is measured by howsteady & reproducible a flow it can produce.

A fluctuating flow rate can create detector noise thatobscures weak signals.


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Requirements for HPLC :9 Pressures up to 6000 psi.9

Pulse free, prevents remixing of solutes.9 Control flow rate from 0.1 to 10 mL/min.


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Reciprocating pump

9 The most used.

9

Mechanical high pressure liquid pumpingachieved by piston, check valves.

9 Pressure surges can be somewhat overcomewith pulse dampeners.

9 Advantages : Provide constant flow rates &independent of solvent viscosity or columnbackpressure.


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Reciprocating pump


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Displacement pump

9 Continuous pressure applied to large volume(syringe pump).

9 Advantage is lack of pressure surges.

9 Disadvantage is price.


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Sample injection system23

Injection port :

9 Device for injection of sample into system.

9 Consists of a stainless steel ring with 6 differentports, 1 to the column.

9 Has interchangeable sample loops, which holda fix volume.


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9 Sample volume is no more than volume

indicated on a loop.9 Samples size ranges from 2-1000 L.

9 Samples introduced manually into the valve witha syringe to fill the sample loop.

9 Sample injection only with flat end needle (toprevent damage to the injection port).

X


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Manual injection port

Sample loop


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Injection valve


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Injection procedure :

1. Place in load position& load sample into loop.

2. Rotate to inject position the content of thesample loop is injected into the column at highpressure.


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Can inject a large number ofsamples automatically.

Inject continuously variablevolume 1 L1 mL.

Small vials with a septum placedin a tray.

Needle penetrates septum &

withdraws the sample.

Valve introduces the sample into

the column.

Autosamplers


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Columns29

1. Guard column

2. Analytical column


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Guard column (precolumn)

310 cm long, placed between the injector & the

analytical column.

Contains the same packing as the analytical column.


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Function : To protect & increase the lifetime of the

analytical column by :

9 Retain debris (eg. pump seal fragments), particulatematter & contaminants that would otherwise get onanalytical column & foul/clog it & changing columnefficiency.

9 Retains highly sorbed compounds that would be

caught on & not be eluted from the analytical column. Must be replaced or regenerated periodically.


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Sample problem 2For the analysis of biological sample using HPLC,a guard column is normally required.


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Analytical column Stainless steel.

10-30 cm long (10,15 & 25 cm).

4-10 mm internal diameter (3.9or 4.6 mm).

3, 5 or 10 m particle size (dp).

40,000-60,000 plates/m.

Eg. 5 m ODS column (250 x 4.5 mm)


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van Deemter equation,

H = A + B/v + Cv

Which of the 3 components is the largestcontribution to H? Consider the following :

9 B/v effects diffusion is 100x less in liquidsthan in the gas & can be neglected.

Band Broadening in LC


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9 Cv effects we will assume that mass transport(M.T.) effects are the largest contribution.

The mobile phase mass transfer coefficient :

CMv = (fM(k)dp2/DM)v

Smaller dp increases the surface area/volume

ratio & thus decreases M.T. in the m.p.


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Reduction of dp from 45 to 6 m results in a tenfoldor more decrease in H


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Sample problem 3Discuss the effect of the particle size on the separation

performance in packed column liquid chromatography.


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Stationary phase Normally, columns are filled with silica gel because its :

9 particle shape (spherical, can be packed morehomogeneously),

9 surface properties,

9 pore size (60-100 range)

structure help to get a good separation.


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Silica structures


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Silica :

9 Is wetted by nearly every potential mobile phase.

9 Is inert to most compounds.

9 Has a high surface activity which can bemodified easily with water & other agents.

9 Can be used to separate a wide variety ofchemical compounds.


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Limitation of silica stationary phase :

X Operating pH range 2-8.

X Dissolves above pH 8. Crosslinked polymericparticles (eg., polystyrene / polymethacrylates) areused for separation of bases.

X Siloxane bond (Si-O-Si) to the stationary phasehydrolyses below pH 2.

These defects will cause changes in tR & loss ofresolution.


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Detector types42

UV absorbance

9 Variable Wavelength UV-Vis detector.9 Diode Array Detector (DAD) : measure absorbance

across a broad spectrum of wavelengths

simultaneously. Fluorescence

Electrochemical

Refractive Index

Mass spectrometric


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HPLC detectors will depend on the nature of thesample.

The most widely used detectors for HPLC are

based on the absorption of ultraviolet or visibleradiation.

UV region : 100-400 nm

Vis region : 400-780 nm


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The detectors often make use of the 254 nm &280 nm lines from a mercury source because

many organic functional groups absorb in theregion.

Usually set the UV at 254 nm. This will detect any to * transitions. So, it would detect anything

with the benzene ring.


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Based on electronic transitions withinmolecules.

Most common type of detector for LC.

Fixed wavelength, Hg lamp 254 nm ( => *).

UV-Vis absorbance detector


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A diagram of the UV detector


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The detectorsensorconsists of a small cylindrical

cell (normally 2.0 to 10 l in volume) through which,the eluent from the column flows.

UV light from a UV lamp (usually a Hg dischargelamp) emitting UV light mostly at 254 nm.


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Variable wavelength, selectable for specificwavelengths in the visible region (eg.deuterium lamp for use down to 190-400 nm).

Still limited to single wavelengths.


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Analyte must absorb in the visible region (haschromophore).

You might wonder why the solventsused don't absorb UV light. They do!

But different compounds absorb moststrongly in different parts of the UVspectrum.


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Methanol, for eg., absorbs at wavelengths below205 nm & water below 190 nm.

If you were using a methanol-water mixture as

the solvent, you would therefore have to use awavelength greater than 205 nm to avoid falsereadings from the solvent.


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Measure absorbance across a broad spectrum ofwavelengths simultaneously.

Allows for the recording of the entire spectrum of eachsolute as it passed through the diode array detector.

Utilizes a deuterium or xenon lamp that emits light overthe UV spectrum range.

UV-Vis absorbance detectorDIODE ARRAY (DAD)


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Sample problem 4

A sample mixture containing acetophenone, phenol, 1-hydroxyphenol & methyl benzene was analyzed usingHPLC with a mobile phase system of a ACN/H2O 80/20 &

a UV detector set at wavelength 420 nm.Quantitative analysis was done using external standard

method. Discuss the reason for each of the followingproblems encountered & suggest an approach toovercome the problem.


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i. Very low detection signals were obtained.


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ii. After doing 5 replicate measurements, the peakareas obtained vary considerably.


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Based on the n, index of refraction. How much the

light is bent in a substance is unique for all liquids &relative to their density.

Advantages :9 universal, insensitive to flow, economical, robust.

Disadvantages :X not sensitive, highly temperature dependent

Refractive Index detector


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Fluorescence :

The most sensitive, 10-1000 times higher than thatof UV detector for strong UV absorbing materials.

Electrochemical :Based on the measurements of the current resulting

from oxidation/reduction reaction of the analyte at asuitable electrode.

Other detectors


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Conductivity :

The conductivity of the column effluent iscontinuously measured & the appearance of the

analyte in the cell is indicated by a change inconductivity.


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Mass spectrometry (MS):

Examining a compound (in a gas phase), that will providethe structural information of each compounds match fromthe library of mass spectra of known compounds storedon the computer & thus can identify the compound.

Measures the mass-to-charge-ratio (m/z) of charged

analytes.


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Eg. mass spectrum for benzene


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It is not a simple matter to connect the MS to LC,because

9 LC operates in the liquid phase whereas MS is agas phase method.

9 Analytes are non volatile & may be thermally labile

but must be presented in gaseous form.

In order to allow separated components of a mixture to

be passed sequentially from the LC into the MS, aninterface is needed.


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Sample problem 5Identification of compound is more confirmative by using

mass spectrometer as a detector compare to UV detector.


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Sample problem 663

Explain the advantage of using MS detector.


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Performance of HPLC detectors64

HPLC detector LOD

Absorbance 10 pgFluorescence 10 fg

Electrochemical 100 pgRefractive index 1 ng

Conductivity 100 pg 1 ngMass spectrometry < 1 pg


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Chemical derivatization of a sample in HPLC isundertaken to convert a non-UV/fluorescenceresponding analyte into one that is.

The derivatization step may enhance the separation.

Sample derivatization


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Types of liquid chromatography66

1. Partition chromatography

(separation based on polarity)

2. Adsorption chromatography

3. Size exclusion chromatography(separation based on molecular size)

4. Ion exchange chromatography(separation based on charge)


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Partition chromatography Is a chromatographic technique in which solute are

separated based on their partition between a liquid

mobile phase & a liquid stationary phase coated ona solid support.


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The most widely used type of HPLC.

Subdivided into :

1. Liquid-liquid partition chromatography.2. Liquid-bonded phase chromatography.


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1. Liquid-liquid partition chromatography

9 The stationary phase is a solvent that is held inplace by adsorption on the surface of packing

particles.

9 The liquid coat might itself be soluble in the mobile

phase which will cause the column to bleed.

9 To prevent loss of stationary phase, liquid-

bonded-phase partition chromatography is used.


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2. Liquid-bonded-phase partition chromatography

9 The stationary phase is an organic species thatis attached to the surface of the packing particles

by chemical bonds.


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Most bonded phase packings are prepared by

reaction of an organochlorosilane with the OHgroups formed on the surface of silica particles byhydrolysis in hot, dilute hydrochloric acid.

The product is an organosiloxane.

Bonded phase packings


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Bondedstationaryphase

Alkyl /substituted

alkyl group


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The reaction for one such SiOH to produce C18 :


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R Structure Polarity

OctadecylOctyl

Phenyl

(CH2)17CH3 (C18)(CH2)7CH3 (C8)

(CH2)3C6H5

NON

POLAR

AminoCyano

Diol

(CH2)3NH2(CH2)3CN

(CH2)2OCH2CH(OH)CH2(OH)

POLAR

Many different polarities forthe bonded stationary phaseare available


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REVERSED PHASE

NORMAL PHASE


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It is not possible to bond all thesurface silanol (Si-OH) groups.

Unreacted silanols are capableofadsorbing polar molecules.


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Can be reduced by

end-capping process :

Silanols are reacted with asilylating agent e.g.trimethylchlorosilane.


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2 types of partition chromatography aredistinguishable based on the relative polarities

of the mobile & stationary phases.

1. Normal Phase

2. Reversed Phase


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The stationary phase is a bonded siloxane with a polarfunctional group (e.g. cyano < diol < amino ELUTED FIRST.

Polar molecules- retained by stationary phase--> ELUTED LATER.


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Example :


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i. Discuss the relative polarity of the compounds in the

above analysis.

Sample problem 7


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Partition HPLC : Reversed phase (RP) The polarity of stationary phase is reversed (non-polar;

C18 > C8 > phenyl groups).

The mobile phase is polar(H2

O, methanol, acetonitrile,THF orusually is a mixture of H2O with one of theorganic solvents).

The organic solvent is called the modifier.


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Metanol is used for acidic compounds. Acetonitrile for basic compounds.

Tetrahydrofuran for those with large dipoles.

Eluent strength is increased by adding a lesspolar solvent.

A less polar solvent has a higher eluent strength


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Hydrophobic groups (C18, C8, phenyl group) haveto be chemically bonded on to matrix.

Octadecylsilane (ODS or C18H37 or C18 chain)widely used.

Gives non-polar (hydrophobic) interaction with

sample molecules & eluents.


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CH3 CH2COOCH3

CH3 CH2COOCH3

Silica-C18 (ODS)

Hydrophobic interaction


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In RP chromatography,

Polar sample molecules - interact least with

stationary phase --> ELUTED FIRST.

Non-polar- retained by stationary phase --> ELUTED LATER.

These phases retain non polar comps inpreference to polar comps


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9 The most polar component is eluted first; increasingthe polarity of the mobile phase increases the elutiontime.

9 The affinity of the hydrophobic analyte for thehydrophobic stationary phase stronger relative to the

now more hydrophilic (polar) mobile phase.

9 Can decrease tR by adding more organic solvent tothe eluent.


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A

B

C

0 0 05 5 5 10 (min)

A

AB

B

C

C

ACN / H2O(70/30) (60/40) (50/50)

HighLow Polarity of Mobile phase

Column : Finepak SIL C18

Retention behavior in RP HPLC

< <


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In normal phase chromatography :

9 The least polar component is eluted first ;increasing the polarity of the mobile phase

decreases the elution time.

Solvents


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Solvents polarity93

water > methanol > acetonitrile > ethanol >

propanol = ethyl acetate > tetrahydrofuran > butanol> chloroform > dichloromethane > n-hexane >

cyclohexane

Polar solvents

Non-polar Solvents

Polarities of organic functional groups


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Polarities of organic functional groups

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Hydrocarbons < olefins < aromatic hydrocarbons

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