METHODOLOGY OF MEASUREMENT:

·        Active Sampling:

(I)           The charcoal tubes are available in different sizes and contain varying amount of activated charcoal.  The tubes contain two layers : the sampling layer and the control layer.   The ambient air is sucked through the tubes in a way that first of all, the air flow save the sampling layer.  This results in an enrichment of the relevant substances in the activated charcoal.  Desorption of the absorbed Benzene is done using Carbon Di-Sulphide (CS₂).  The substances desorbed in the CS₂ are analysed by capillary gas chromatography.   A flame ionisation detector (FID) is used for analysis while quantification is performed using the internal/ external standard.

(II)          Thermal desorption tubes are used for absorption of Benzene in place of charcoal tube, for which the sampling methodology is same as used for charcoal tube.   These tubes are directly connected to the thermal disrobers installed with the gas chromatograph.   The thermal desorption techniques offer the advantage of a greatly improved analytical sensitivity, as solvent is not used in this process and the collected sample is not diluted.  In most, cases, analytical recovery is close to 100% and desorption efficiency corrections are not required.  To be suitable for thermal desorption, sorbents must meet exacting specifications that include low contaminant background, high thermal stability and sufficient adsorptive strength to retain components of interest and should also release them quickly when heat is applied.

·        Passive Sampling:

Controlled diffusion with an activated charcoal tube is used to enrich the substances targeted for analysis.  A diffusion sampling system comprises a sampling layer and a diffusion path in front of this layer.  The diffusion path is filled with porous cellulose acetate, to prevent convection currents.  The sample is taken by exposing the tube to ambient air (protected from rain).  During this exposure time, the analytes stream into the activated charcoal due to the concentration gradient between the air and the desorption layer and are absorbed by the charcoal.  Once the sample has been collected, the tubes are taken to the laboratory where desorption is done and the substances dissolved in the CS₂ are analyzed using capillary gas chromatography (GC) equipped with flame ionization detector (FID).

Collection (Trapping) of the Air Borne Benzene and Analysis by Gas Chromatography

Gas Chromatography with mass specific and flame ionisation detector (FID) is used to analyse the highly volatile Hydrocarbons present in air samples.  As these samples contain very low concentration of the substances to be analysed – at level lower than the detection limit of the analytical techniques in question – samples have to be enriched prior to determination.

Adsorption tubes containing various solid adsorption materials, are used for this purpose.  Activated charcoal has highly favourable adsorptive properties, particularly for the acquisition of aromatic hydrocarbons in ambient air.  There are two ways of enriching the sample test substances collected for analysis – Pumped sampling which involves sucking of ambient air through adsorption tubes by means of a pump and Passive sampling in which tubes are exposed to the surrounding air and the substances contained in the air are taken up by diffusion.  Gas chromatography analysis is performed after the enriched components are desorbed thermally or using desorbing solvents.

Samples collected through active or passive sampling technique (sorbent tubes) may be desorbed by conventional solvent (generally Carbon disulphide) or thermally (generally using standard automated thermal desorption apparatus).  Desorbed samples may be analyzed using gas chromatograph (GC) fitted with capillary column and flame ionization detector (FID) or mass spectrometric detector (MSD, generally used for confirmation of analytes – USEPA – TO-17 method).