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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