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8/2/2019 Analysis of Benzene
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Analysis of Benzene
Envirotech Center for Research & DevelopmentNew Delhi
Harsh K. Dixit
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Principle
The ambient air is sucked through the activatedcharcoal tube using a low flow sampler used forcollection of Banzene in a way that results in anenrichment of the relevant substances in the activated
charcoal.Desorption of the adsorbed benzene is done by using
carbon disulphide (CS2)
The substances desorbed in the CS2 are analyzed by
capillary gas chromatography. A flame ionizationdetector (FID) is used for analysis while quantification isperformed using the standard.
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Requirements
Low flow sampler
Activated charcoal tube
Gas chromatography with FID
Fused silica capillary columns
Carbon disulphide (CS2) Purity >99.9% (GLC)
UltrasonicatorCarrier Gas - Helium or Nitrogen Purity >99.9%
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Sampling Procedure
A sample is collected by opening a tube at two ends,connecting it to a sampler Airborne chemicals are trapped ontothe surface of the sorbent.
Two tubes are used in series to take care of breakthroughcompatible to the thermal desorber.
Keep the tube in a vertical position during sampling toprevent the possibility of channeling that can lead to undersampling.
Sampling flow rate is around 50ml/m.
A sampling component may breakthrough from the backend of tube, if excessive flow rates are used. Sample is tobe discarded, if the breakthrough is observed more than10%.
If analyzed concentration in backup section is more than
10% of front section, sample needs to be discarded.
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Analytical Procedure
Sample collected through sorbent tubes are extracted ordesorbed by 5ml of CS2 using ultrasonication for 15 min.
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Gas flow
Nitrogen: 30ml/min (FID make up + column)
(Column flow ~1ml/min)
Hydrogen: 30ml/min
Air: 300ml/min
Capillary Column 624Coating: cyanopropyl phenyl polysiloxane
Length: 30 meter
Internal Diameter: 0.25mm
Film thickness: 1.4m
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Temperature programming
Injection port: 250C
FID: 300C
Column/Oven: 50C (hold for 3 min)
Ramp @ 10C/min
140C (for 1 min)
Ramp @ 20C/min
240C (for 1 min)
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Retention Time
Injection volume: 5 l
Total run time: 19.5 min
Split: 10
Benzene: 6.8 min, Search window: 1 sec, 3%
Toluene: 9.18min, Search Window: 1 sec, 3%
Xylene: 11.37 min, Search window: 1 sec, 3%
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Calculation
Concentration (g/m3) =
at ambient condition
WhereC = amount of benzene found injection sample volumefrom standard curve, in g/l
VT = total volume of the sample extracted in ml
V2 = volume of sample extract injected in GC, in lV3 = volume of air sucked through the tube, in m3
Blank value is to be subtracted from the amount ofbenzene found in the sample.
C x VT x 103V2 x V3
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Calculation
Concentration (g/m3) =
at STP
Where
C = concentration at ambient condition, in g/m3
T = temperature of the ambient air, in C
P = atmospheric pressure, in kPa
Volume of air sampled (m3) =
S = sampling rate, in ml/min
t = sampling time, in min
C x 101.3 (273 +T)273 x P
S x t106