3.7  Determination of light hydrocarbons in air

3.7.1  Introduction

The EMEP-Workshop on measurements of hydrocarbons/VOC in Lindau 1989 recommended C2 to C5 hydrocarbons to be measured in spot-samples taken twice a week at 10 to 15 sites in Europe. Electropolished stainless steel canisters were recommended for the sampling.

3.7.2  Principle

A cleaned steel canister is flushed with air and then filled to a pressure about 40 psig (2.8 atm). The canister is brought to a laboratory and analysed within a week.

3.7.3  Sampling equipment

The sample cylinders are 1.8 litre “Summa” polished stainless steel canisters manufactured by Prof. R. Rasmussen. The clean air pump is from the same supplier. Both cylinders and pumps were widely used and tested in both USA and Europe. A number of tests and comparisons with other equipment have been undertaken, and are published in several articles and intercalibrations (McClenny et al., 1991; Pate et al., 1992; Westberg et al., 1984; Olivier et al., 1986).

3.7.4  Sampling procedure

See Figure 3.7.1.

  1. Can is evacuated
  2. Remove swagelock-end-cap
  3. Connect purge-tee and tighten
  4. Switch on pump
  5. Pressurize purge-tee
           Vent by opening valve B
           (minimum 10 times)
  6. Open valve A - pressurize can to 15 psig
           Vent by opening valve B
           (minimum 5 times)
  7. Pressurize can to max. pressure (about 40 psig)
  8. Close valve A and switch off pump
  9. Remove purge-tee and replace swagelock

Please use a pencil to fill in the label.

Please do not use force to tighten the green handle of the shut-off valve. Normal use of thumb and forefinger is sufficient!!

3.7.5  Cleaning of canisters before the first use

    (1)     evacuate to a pressure of 10-7 mbar 24 hours, ambient temperature.
    (2)     fill with 10 µl water and purified helium 24 hours, 1 bar
    (3)     evacuate to 1 mbar and fill with helium 5 times
    (4)     humidify with 10 µl water, evacuate to 1 mbar

 

 

 

 

 

 

 

 

 

 

 

 

 

 

  1. Can is evacuated
  2. Remove swagelock-end-cap
  3. Connect purge-tee and tighten
  4. Switch on pump
  5. Pressurize purge-tee
         Vent by opening valve B
         (minimum 10 times)
  6. Open valve A - pressurize can to 15 psig
         Vent by opening valve B
         (minimum 5 times)
  7. Pressurize can to max. pressure (about 40 psig)
  8. Close valve A and switch off pump
  9. Remove purge-tee and replace swagelock

   Please use a pencil to fill in the label.

   Please do not use force to tighten the green handle of the shut-off valve. 
   Normal use of thumb and forefinger is sufficient!!

Figure 3.7.1:   Sampling procedure.

 

After step (1) a one-hour leak-test is performed. The canister shut-off valve is closed and no detectable increase of pressure should occur on the 10-7 mbar scale.

Blank runs of canisters should not show a single signal of more than 2000 µVs (30 ppt ethane or 10 ppt benzene).

From the 200 canisters we bought for the EMEP-program, 7 had a significant high level of C6 hydrocarbons and chlorinated solvents. Those bottles were cleaned with methanol, acetone and water and cleaned as shown above. The evaporation in step (1) is performed with 70 °C instead of ambient temperature. The cleaning of a used bottle is done by 6 to 24 hours evacuation at 10-7 mbar and 50oC. (Turbomolecular pump from Pfeiffer Balzers modified at NILU to allow simultaneous cleaning of 6 bottles).

3.7.6  Commercial supply

Steel canisters:
Prof. R. Rasmussen, Oregon Graduate Center, Biospheric Research Cooperation.

3.7.7  References

McClenny, W.A. et al. (1991) Canister-based method for monitoring toxic VOCs in ambient air. J. Air Waste Manage. Assoc., 41, 1308-1318.

Pate, B. et al. (1992) Temporal stability of polar organic compounds in stainless steel canisters. J. Air Waste Manage. Assoc., 42, 460-46.

Westberg, H. et al. (1984) Analysis of individual hydrocarbon species in ambient atmospheres. In: Identification and analysis of organic pollutants in air. Ed. by L.H. Keith. Woburn, MA, Butterworth. pp. 323-327.

Olivier, K.D. et al. (1986) Sample integrity of trace level volatile organic compounds in ambient air stored in summa polished canisters. Atmos. Environ., 20, 1403-1411.


Last revision: November 2001