Abstract :

The environmental fate, global warming effect and human health risk from mono aromatic Volatile Organic Compounds (VOCs) are of major concerns among many consequences of their anthropogenic emission. Present study aims to standardize the methodology for the measurement of BTEX ( i.e. Benzene, Toluene, Ethyl Benzene and Xylene), collected from the major sources using active sampling onto solid sorbent tube . Sampling was done by drawing a specified volume of air using a low volume pump through an Anasorb CSC (coconut shell charcoal, 150 mg) with low air flow rate (0.1-0.2 litre per minute). Samples were chemically desorbed by carbon disulphide and analyzed using gas chromatography-mass spectrometry. Measurement of concentration of the samples was done using  five point external standard calibration curve of BTEX in carbon disulphide (CS2). 

Introduction :

BTEX refers to  a group of  VOC namely, benzene, toluene, ethylbenzene and xylene. The major uses of BTEX are  in the manufacture of  various chemicals, rubber and plastics, in solvents, and in paints and lacquers as solvents . The main sources of BTEX into the environment are the petroleum and chemical industries and other combustion processes . The main anthropogenic sources of BTEX is through emissions from motor vehicles and aircraft and cigarette smoke. BTEX  are also released when natural materials are burned.

BTEX compounds released in the air, bound by their volatile nature, are mostly dispersed in the atmosphere. Although some fractions are distributed to water- bodies, soils and sediments. They react with other air pollutants towards formation of secondary air pollutant  and are involved in the formation of photochemical smog. In presence of oxides of nitrogen and sunlight, BTEX  contribute to form   ground level ozone which can damage crops, building materials, ecosystem and affect human health.

BTEX has considerable health impact potential. Benzene is a known carcinogen (classified as  Group-A by International Agency Research on Cancer) whereas ethyl benzene is known as a probable human carcinogen (classified as Group-B1 by IARC). Apart from their carcinogenic effect long term exposure  to higher concentrations have other health effect such as  – damaging the liver, kidneys, central nervous system and eyes.

The present study tries to standardize the methodology for the measurement of the above VOCs  using personnel sampler onto solid sorbent tubes and few samples had been collected from potential sources and analyzed to establish the method. 

Experimental Detail :

Material & Equipment:

1)      Anasorb CSC (coconut shell charcoal, 150 mg) absorption tube  (SKC Inc., USA)

2)      A portable low flow and constant volume air sampler (SKC Inc., USA)

3)      Carbon Di Sulphide (IR grade, Spectrochem)

4)      Chromatographic grade standard Benzene, Toluene, Ethyl Benzene (Merck) &  Standard m-Xylene, p-xylene and o-xylene (Sigma-Aldrich)

5)      Gas Chromatogram-Mass Spectrometry of Varian, GC- model CP 3800 equipped with mass spectrometer– model Saturn 2200

6)      Capillary Column- VF -5 ms, 30 m X 0.25 mm X 0.25 µm film thickness (Varian).

Sampling :

The method used for the sampling was NIOSH 1501. Air samples were drawn through the sorbent tubes using SKC low flow portable air sampler. The sampling flow rate ranged 0.1-0.2 litre per min and sampling duration 100-150 min.

Transportation & Storage of Sample:

As the pump turned off, the tubes were immediately removed from the Sampler and two open sides were tightly closed with plastic caps. The tubes were securely kept with ice gel in a ice box and immediately sent to the laboratory and refrigerate the tubes at <40 C.

Sample Preparation :

The adsorbent tube was broken and the charcoal was collected in a glass vial. 1 ml CS2 is added to the charcoal and kept it in the same screw capped vial to extract the adsorbed BTEX. The system is allowed  to stand for 30 min with occasional agitation.

Analysis :

1 µl of the desorbed sample is injected in GCMS. The column was programmed at an initial temperature of 350 C for 5 min followed by an increase to 1500C at a rate of 5 0C min-1 and hold for  1 min.

Calibration of Instrument :

External calibration was performed using five concentrations of standard solutions of BTEX in CS2. Liquid phase calibration standards were prepared by standard dilution method from chromatographic grade pure benzene, toluene, ethyl benzene and standard o,m,p – xylene into vials in CS2. Five concentrations ( 0.5 , 1, 3, 5, 10 ppm) were used to prepare the calibration curves. A blank run was performed with the solvent CS2 . To avoid matrix effect laboratory blank test was also performed.

A representative chromatogram of  5 ppm BTEX  in CS2 has been shown in Fig.1.

A calibration curve of p-Xylene using five different standard solution in CS2 is shown in Fig.2. Linear fit in other BTEX components were found good with regression coefficient (R2 >0.98).

Fig 2 : Calibration Curve of p-Xylene

Recovery study :

Recovery study with mix standard solution spiked in blank sorbent tube was done. For Benzene and Toluene the recovery at minimum concentration of 1 ppm hold good (80-85%), whereas for Ethyl benzene and Xylene the recovery at 0.5 ppm were  found good (83-88%). 

Result of Analysis of  few samples :

The threshold limit value (TLV) recommended by the American Conference of Governmental Industrial Hygienists (ACGIH) has been furnished in Table 1 

Table 1

Chemical Name Exposure Limit In Air/ TLV  by ACGIH
  Time Weighted Average (TWA) ppm Short Term Exposure Limit (STEL) ppm
Benzene 0.5 2.5
Toluene 50 50
Ethyl Benzene 100 Not Established
o- Xylene 100 150
m- Xylene 100 150
P – Xylene 100 150

 CPCB limit for Benzene emission in ambient air is 5 mg/m3 (Annual Average).

A no. of samples had been collected from the major contributor sources of BTEX, however three of them were furnished in Table 2. The sampling were done during the month of July and five representative samples  had been collected in different dates and time in each locations.  The average results  of each location are shown in Table 2. 

Table 2

Source Benzene  Toluene  Ethyl benzene p,m-Xylene o-Xylene Unit
Paint Industry 3.7 26.7 2.2 133.8 15.2 ppm
Chemical Industry 29.8 5.1 1.2 38.6 2.5 ppm
Traffic Intersection 21.2 31.3 9.8 26.6 8.5 mg/m3

Conclusion :

The above data shows the concentration of the different component of BTEX are different. Detail studies on BTEX emitted from different sources like different industries and metropolies will indicate the contamination level of BTEX in ambient  air or in workplace atmosphere. The most important part of this study is to identify and determine the concentration of BTEX  from air samples. As soon as we know the amount of BTEX contamination , we can immediately implement an action plan to bring down the existing levels .

The work done under the guidance of Prof.  Barun Gupta the study done by Ms. Sutapa Bhowmik  assisted by Mr. Ranajit Maji & Mr. Pradip Bag.