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Diffusive SamplerDiffusive or passive samplers are known as the cheapest method of monitoring
air quality and can give a good overall picture of average pollutant levels in an area. The
low cost per tube permits sampling at a number of points in the area of interest. Although
results from single point passive samplers are not as precise as those from automatic
point monitors, the accuracy and reproducibility of the measurements has increased over
recent years.
Diffusive samplers are typically clear plastic tubes open, or with a membrane
screen, at one end and a pollutant-absorbing chemical matrix or gel at the closed end. Thediffusion tube collects the pollutant during the exposure period (one week or more), at the
end of which the tube is returned to an analytical laboratory. The time resolution of this
technique is limited, as it can only provide information on integrated average pollutant
concentration over the exposure period. They have been widely used for many years in
personal monitoring and occupational health assessments (Dore and McGinlay, 1997).These kind of devices are very appropriate for gas flux emission measurement,
they do not require another measurement and can be applied over a wide area, because of
their low cost. One disadvantage of these devices are that they still require furtheranalysis in laboratory.
Theoretical understanding of gas movement in diffusive samplers have been
introduced by Palmes, et.al in 1976. For the axial tube type diffusive sampler, the rate of
mass transfer down the concentration gradient induced in the air gap, can be derived fromthe Ficks first law (Bates, et.al, 1997):
where:
dm/dt = rate of mass transfer
d = diffusion coefficient of gas
dc/dx = concentration gradient along the axisIntegration of the above equation gives:
where:mt = mass transferred in time t
dxdCDA
dtdm =
)( 0 at CC
l
DA
t
m=
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Besides the axial tube type sampler described above, to increase the uptake rate of
gases, the radial type sampler has been introduced. Again Ficks first law may be used todescribe the transfer across the still air zone. In cylindrical coordinates the equation
becomes:
where r = radiush = length of the cylinder
dC/dr = concentration gradient along the radius
The integrated form is follows:
Several types of diffusive sampler now commercially available. They can be tubeor badge type, pollutant can be chemically bound by appropriate reagent or physically
adsorbed by inert media. For further analysis the pollutant may be extracted by thermal
desorption or solvent extraction. Various types of diffusive sampler can be seen in
appendix 2.
Adsorbent For Gases
Silica gelSilica gel is one of the synthetic amorphous silicas, It is rigid and forms a
continuous network of spherical particle of colloidal silica. It is synthesised by following
reaction:
Na2SiO3 + 2HCl + nH2O 2NaCl + SiO2.nH2O + H2O
Its properties e.g. surface area, pore volume and strength can be varied by
varying the silica concentration, temperature, pH and activation temperature. Silica gel
(along with activated alumina) is a desirable sorbent for drying because of its high
f d i f ti At l f th t l l
dr
dCrhD
dt
dm2=
a
at
rr
CChD
t
m
0
0
ln
2
=
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- aldehyde
Activated carbonThe surface properties of activated carbon is non-polar or only slightly polar as a
result of the surface oxide groups and inorganic impurities. This properties give the
following advantages:
- can be used to perform separation and purification processes without priorstringent moisture removal.
- it adsorb more non-polar and weakly polar organic molecules than othersorbents do
- the heat of adsorption is generally lower than other sorbents so that strippingof the adsorbed molecules is easier.
It has a large surface area 300-2500 m2/g which is the largest among all sorbents.
For application in liquid phases, it should have pore size near or larger than 30 A,whereas for gas phases the pore diameter in the range from 10 A to 25 A. According to
IUPAC, the pores are subdivided by diameter (d)
- macropores (d>500 A)- mesopores (20 A
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Activated alumina is a dehydrated (or partially) alumina hydrates, both crystalline
and amorphous with high surface area. It has the greatest affinity for water, and is appliedin drying of gases and liquids because of its hydrophilic property and large surface area.
It has been applied as a sorbent for drying gases: Ar, He, H2, alkanes (C1-C3),hydrocarbon, Cl2, HCl, SO2, and NH3. It also may be used for the removal of various
contaminants from gases, such as trace fluorides, cloride, H2S, alcohol and ethers.
Molecular sieve carbonMolecular sieve carbon (MSC) is less hydrophilic than zeolites, so can be used
more efficient in separation process involving wet-gas streams. It has been used for the
production of nitrogen from air.
In the preparation and sieving properties study, three approaches were taken:a. Carbonization of polymers such as poly vinylidene chloride (PVDC); saran
(90/10 mixture of vinylidene chloride and vinyl chloride); and cellulose, sugar
and coconut shell.
b. Slightly carbonizing coals, especially anthracites
c. Coating of the pore mouth of the commercial activated carbon with acarbonised or coked thermosetting polymer.
Direct Reading Colorimetry Detector TubeColorimetry detector tube or direct reading diffusive sampler is a simple device
that capable to analyse the airborne contaminant quantitatively based on coloration of
indicating layer. When the molecule of airborne contaminant reach the reagent layer they
react chemically with the filling material causing a colorimetric stain. The residual stainlength is a measure of the dosage of the contaminant to be measured, i.e. the product of
concentration and time.
Molecules of air contaminants migrate into a diffusive sampler according to the
laws of gaseous diffusion. When the sampler is exposed in air a concentration gradient isdeveloped between the ambient air and the air inside the sampler.This phenomenon can
be described by Ficks first law of diffusion as described previously.
Because of a gas molecule react with the impregnated chemicals, there are
virtually no free gas molecule in the air inside the sampler. The full concentrationdifference is thus available as a driving force for diffusive sampling. However the
diffusion pathway for the gas molecule becomes longer as the stain gets longer. It ismean, according to Ficks law, that the mass flow decreases with increasing stain length.
The increase of the stain length (dl) can be formulated by
d
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2/1
2
=
ikctDl
The validity of above relationship can be demonstrated by the calibration curve of the
direct-reading diffusive sampler using scale figures for the dosage of the contaminant to
be measured (May, 1989).
Due to the air movement only depending on diffusion of gases, then a passivedetector tube can be imagined as long term sampler. The problem associated with this
kind of device is the flow rate of gas can be too low for the reagent system used becauseside reactions can occur on long contact of the gas with the indicating layer.
For satisfactory function, the reagent must fullfil the following requirements
((Leichnitz, 1976):1. the indication color must be a measure of the absolute amount (mass) of gas
(product of concentration and air sample volume must be constant).
2. the absorption capacity of the reagent must not be affected by humidity changesand must remain constant during the measuring process.
3. The rate of reaction of the gas with the reagent system must be much higher thanthe rate of any side reaction with may take place on the surface of the reagentcarrier In many detector tubes, the calibration curve can be represented by the
following equation:
where:
l = length of the discolorationm = mass of gas which has reacted in the tube,
A = constant with the dimension of reciprocal time
C = sorption capacity of the indicating preparation
v = flow by volume of air sample in the tube.
Indicating Layer Material for Detector TubeThe supporting material for the indicating layer in the tube may be silica gel,
alumina, ground glass, pumice, or resin (Jungreis, 1985). The indicating layer or thereagent system comprises of one or more chemical compound which can produce a
colour change when react with target contaminant.
)1( /1vA
eA
v
C
ml
+=
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Ozone (O3)
The indicating layer contains indigo compound which changes colour from blue tocolourless due to oxidation by ozone.
O3 + Indigo (blue) Isatine (colourless)
Chlorine and nitrogen dioxide also discharge the color of indigo when present in
concentration higher than 5 ppm.
Sulphur dioxide (SO2)The indicating reaction for measurement of sulphur dioxide in the various detector tube
may involve formation of the extremely stable anion complex [Hg(SO3)2]2-
. Theindicating reagent is composed of disodium tetra chloromercuriate and methyl red.
Hydrochloric acid liberated during the reaction changes the indicator color.
2SO2 + Na2[HgCl2] + 2H2O Na2[Hg(SO3)2] + 4 HCl
The second principle that can be used for sulphur dioxide detection is reaction of free
iodine with sulphur dioxide (decolorizing the blue iodine-starch complex)
SO2 + I2 + H2O SO42-
+ 2I-+ 4H
+
Hydrogen sulphide become serious interferen if use the detector tube without a cupric-containing precleansing layer.
Reactions in Colorimetric Detector TubeSeveral types of chemical reaction that used to generate colour change are:
Oxidation-reduction
Acid-base
Substitution-addition and others
Oxidation-reduction
Example of this type is CO detector tube that created by using reagent a mixtureof iodine pentoxide and fuming sulphuric acid. The iodine is reduced and the carbonmonoxide is oxidised:
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SeO2
I2O5 + H2 S2O7
I2+ Ox.Pr White/Brown-green CO, many organicsI2O5 + H2 S2O7 I2+ Ox.Pr White/Brown-green CO, many organics
I2O5 + H2 SO4 I2+ Ox.Pr White/Brown-green Aromatic hydrocarbon
Cr2O7+H2SO4 Cr3+
+ Ox.Pr Orange/Green Alyphatic Hydrocarbon
Cr2O7+H3PO4 Cr3+
+ Ox.Pr Orange/Green Alcohol
CO + K2Pd(SO3)2 CO2+SO2+K2SO3+Pd Yellow/Green CO specific
R1C=CR2 + Mo6+
Mo3+ + Ox.Pr(splitting of double bond)
Yellow/blue Unsaturated hydrocarbon
Acid-Base Reactions
Many detector tubes use acid-base indicators impregnated on a neutral or buffered
substrate to measure gaseous substance that have acidic or basic properties. Such
compounds are acetic acid, ammonia and amines, carbon dioxide, hydrazine, hydrogenchloride, hydrogen fluoride, nitric acid and sulphur dioxide. As can be seen, these
reactions are not specific, other acids and bases will interfere, particularly strong acids
and bases.
Acid-base indicator used in detector tubeGas/vapour Indicator Colour change
Acetic acid Phenol red Pink/white
Ammonia Bromophenol blue or thymol blue Orange/blue green;
Lavender/pale yellow
Carbon dioxide Thymol blue Blue/pale yellow
Hydrazine Bromophenol blue Yellow/blue
Hydrogen chloride Bromophenol blue or congo red Blue/white; pink/blue
Sulphur dioxide Phenol red or bromocresol green Pink/white; blue/yellow
Other types of reactions
These reactions included addition, substitution or mixtures of these. Some typesare fairly specific for one compound. For example, the substitution reaction of carbonylcompounds with dinitrophenyl hydrazine, addition reaction of chlorine with
tetraphenylbenzindine. The reaction can be simply one stage reaction or two stages ormore.
For some gases, because of their stability, it is not possible to obtain directly a colour
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Reaction Colour change Specificity
Substitution
H2S + PbAc2 PbS + 2Hac White/black Specific
R1R2C=O + H2NNHC6H3(NO2)2
R1R2C=NNHC6H3(NO2)2 + H2O
Dinitrophenylhydrazine hydrazone
Pale-yellow/
yellow
Specific for carbonyls
Addition
Cl2 + TPB Cl-TPB-Cl
Tetraphenylbenzidine quinoidimonium saltWhite/blue Fairly specific
Combination
2C6H6 + CH2O C6H5CH2C6H5 + H2O
+2H2SO4
3H2O + SO2 + C6H5CH= =O
paraquinoid compound
White/brownSpecific for aromatics (or
aldehydes)
Using strong oxidant
Chlorinated hydrocarbons + KMnO4 +
H2SO4(conc) Cl2
Cl2 + TPB Cl-TPB-Cl
White/blue Nonspecific
Using heat to break down
compoundCCl4 + heat Cl2
CH3CN + heat NO2
CF2Cl2 + heat Cl2
White/blue nonspecific
From the literature review, the first task identified was to prepare a sensitivedetector system for nitrogen dioxide (NO2).
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Appendix 1
Various types of diffusive samplers
a. Thermal desorption passive samplerThis sampler is usually used for organic vapour and un-reactive pollutants. The sampler can be radial or axial type. The filling material for
trapping the gases may be molecular sieve, activated charcoal, graphitized carbon black etc.
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b. Solvent desorption passive sampler.This type of sampler commonly uses for inorganic gases: NO, NO2, NOx , SO2, O3, NH3 etc. Reagent used depends on the target gases. The
support material can be paper, glass fibre, etc.
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Appendix 3
Various type colorimetric detector tube
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Appendix 4
Diffusive sampler for greenhouse gases
NITROGEN DIOXIDE PASSIVE SAMPLER (NO2)No Type Dimension Chemistry of active material Method of analysis Ref
1 Badge (Ferm) D: 24 Whatman filter paper coated with a solution of NaOH and NaI in
methanol
Spectrophotometry (540nm: nitrite)
after mixing with diazotizing
reagent.
8
2 Badge (Willems) D: 28 Whatman GF-A glass fibre filter impregnated with a solution of
triethanolamine-acetone.
FIA_ Spectrophotometry (540nm:
nitrite) after mixing with
diazotizing reagent (Saltzman
method)
9
3 Badge (Ogawa) D: unknown Impregnated-cellulose fibre filter (unknown chemicals) FIA-Spectrofotometry (545nm:
nitrite)
10
NITROUS OXIDE PASSIVE SAMPLERNo Type Dimension Chemistry of active material Method of analysis Ref
1 Badge (RS
Landauer)
Unknown Molecular sieve Thermal desorption and than
analysed by IR
11
2 Tube with stainless
screen
L:55; D:4 Molecular sieve 5A (60/80 mesh, Supelco) in glass tube Partially desorption in vial,
measurement head space by GC
with EC detector
12
SULPHUR DIOXIDE PASSIVE SAMPLERNo Type Dimension Chemistry of active material Method of analysis Ref
1 Badge (Toyo Roshi
Kaisha)
Unknown Filter paper impregnated with tetraethanolamine (TEA) SO2 absorbed was oxidized to SO4
by H2O2; SO4 was analyzed
spectrophotome-trically (662)
13
2 Badge (Ferm) D: 24 Whatman filter paper coated with a solution of NaOH (dissolved in Ion chromatography 8
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small water) in methanol
3 Badge Unknown Potassium Carbonate Ion chromatography 14,154 Badge (Ogawa) D: unknown Impregnated-cellulose fibre filter (unknown chemicals) Ion Chromatography 10
OZONE PASSIVE SAMPLERNo Type Dimension Chemistry of active material Method of analysis Ref
1 Diffusion tube in
protective box
L:74; D:10 Glass fibre filter coated with a solution of 1,2-di(4-pyridyl)ethylene
(6%m/v) in acetic acid (75%), ethylene glycol (15%); reaction withmethylbenzothiazoline hydrazone (MBTH)
Spectrophotometry (442 nm) 16
2 Indigo papers under
shelter
D: 50 Indigo on filter paper buffered to pH 2.2; extraction of isatin with
ethanol
Spectrophotometry (408 nm) 16
3 Diffusion tube L:240 D:30 Indigo on filter paper unbuffered; extraction of isatin with ethanol Spectrophotometry (408 nm) 16
4 Badge under shelter
(poor correlation)
D:50 Potassium Iodide on filter paper; Ion Chromatography 16
5 Petridisk under
shelter (poor
correlation)
D:50 Potassium Iodide-starch solution on filter paper; Spectrophotometry (575 nm)16
6 Badge sampler
(Ogawa & Co)
Filter coated with a nitrite-based solution Ion Chromatography 17
7 Badge sampler
(CSIRO Australia)
D:25 Impregnated filter (unidentified) Ion Chromatography 18
AMMONIA PASSIVE SAMPLERNo Type Dimension Chemistry of active material Method of analysis Ref
1 Open Tube D: 10.9 L:71.2 Unknown FIA- Conductometric detection 19
2 Tube with a
stainless steel grid
in the inlet
D: 10.9 L:71.2 Unknown FIA- Conductometric detection 19
3 Tube with per-
meable membra-ne
in the inlet
D:10.9 L:35.6 Unknown FIA- Conductometric detection19
4 Badge, with a D:28 Tartaric acid applied to a glass fibre filter disc FIA- Conductometric detection 19, 20
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Teflon filter
5 Badge, with amembrane inlet
Unknown Impregnated filter with citric acid FIA/Indophenol 21
6 Badge with teflonfilter (5um)
Unknown Stainless steel grid with phosporic acid Ion chromatography21
7 Badge with
membran inlet
Unknown Impregnated filter with phosporic acid Indophenol21
Badge with teflon
filter
Unknown Impregnated glass surface with Phosphoric acid Ion Chromatography 21
8 Palmes type with
membran
Unknown Stainless steel grid with sulphuric acid Conductance 21
References:
1. K Leichnitz,Ann.Occup. Hyg., 19, 1976, pp 159-1612. ED Palmes, AF Gunnison, J DiMattion, and C Tomczyk,AIHA Journal, 37, 1976, pp 570-5773. E Jungreis, Spot Test Analysis: Clinical, Environmental, Forensic and Geochemical Applications, John Wiley & Sons, New York, 19854. CJ Dore and J McGinlay, In: Air Pollution in The United Kingdom, edited by G Davison and CN Hewitt, Royal Society of Chemistry, 19975. WJ May,Ann.Occup.Hyg., 33, 1, 1989, pp 69-786. M Bates, N Gonzales-Flesca, V Cocheo and R Sokhi,Analyst, 122, 1997, pp1481-14847. DA Skoog, DM West and FJ Holler, Fundamental of Analytical Chemistry, 7nd Ed, 1996, Saunders College Publishing, London8. GP Ayers, MD Keywood, R Gillett, PC Manins, H Malfroy and T Bardsley, Atmospheric Environment, 1998, 32, 20, pp 3587-35929. AH Gustafsson, R Lindahl, JO Levin and D Karlsson, J. Environ. Monit, 1999, 1, pp 349-35210.Ogawa & Co., NO, NO2,Nox and SO2 sampling protocol using Ogawa sampler, 4
thEd., 1998
11.OSHA,Nitrous oxide in workplace atmosphere (passive monitor), 1994, OSHA Salt Lake Technical Center, Salt Lake City USA12.S Kumagai and S Koda,Am. Ind. Hyg. Ass. J, 1999, 60, 4, pp 458-46213.Y Yang, XX Zhang, T Korenaga, K Higuchi, Talanta, 1997, 45, pp 445-450
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14. M Ferm,A sensitive diffusional sample, 1991, IVL-Report B-1020, Gothenburg
15. M Hangartner,Diffusive sampling as an alternative approach for developing country, World Conggres on Air Pollution in DevelopingCountries, 1996, Costa Rica
16. M Hangartner, M Kirchner and H Werner,Analyst, 1996, 121, pp 1269-127217. Anonymous, Protocol for Ozone Measurement Using The Ozone Passive Sampler Badge, 1994, Rev.2, Harvard School of Public Health,
Dept. of Environmental Health, USA
18.CSRIO Website,Atmospheric Research, http://www.csrio.edu.au19.Th R Thijsse, JH Duyzer, HLM Verhagen, GP Wyers, A Wayers and JJ Mols,Atmospheric Environment, 1998, 32, 3, pp 333-337
20.JJ Willems, P Hofschreuder,In Air Pollution Research Report 37, Eds. I Allegrini, A Febo, and C Perrino, 1990, pp 113-11821.M Kirchner, S Braeutigam, M Ferm, M Haas, M Hangartner, P Hofschreuder, A Kasper-Giebl, H Rommelt, J Stiedner, W Terzer, L Thoni,H Werner, and R Zimmerling, J Environ. Monit, 1999, 1, pp 259-265
22.RT Yang, Gas separation by adsorption processes, Imperial College Press, 1997 London