Central California Ozone Study (CCOS) - Desert Research Institute

C.6 Supplemental Measurements of Volatile Organic Compounds
A.7.1 Collection and Analysis of Hydrocarbons and Oxygenated Species
Canister Sampling
Stainless steel SUMMA-polished canisters of 6-L capacity are customarily employed
for volatile hydrocarbon (C 2 -C 12 ) collection. These canister samples are to be analyzed for
speciated hydrocarbons by a method consistent with EPA Method TO-14, as well as for CO, CO 2 ,
methane, and oxygenated species. Prior to sampling, the canisters are to be cleaned by repeated
evacuation and pressurization with humidified zero air, and certified as described in the EPA
document "Technical Assistance Document for Sampling and Analysis of Ozone Precursors"
(October 1991, EPA/600-8-91/215).
The sampling procedure should essentially follow the pressurized sampling method
described by EPA Methods TO-12 and TO-14 and the EPA document "Technical Assistance
Document for Sampling and Analysis of Ozone Precursors" (October 1991, EPA/600-8-91/215).
A stainless steel Viton pump draws in ambient air from the sampling manifold to fill and
pressurize the sample canisters. A flow control device maintains a constant flow into the canisters
over the desired sample period. This flow rate is preset to fill the canisters to about 1 atm above
ambient pressure at the end of the sampling period (as described by EPA Method TO-14). For
automatic operation, the timer starts and stops the pump at the appropriate time. The timer also
opens the solenoid valve when the pump starts and closes it when the pump stops. The use of the
solenoid manifold valves permits the automatic selection of preloaded canisters. The multipleevent
sampling systems, allowing unattended collection of up to six canister samples are
recommended for this study.
After sampling, an identification tag should be attached to each canister with the canister
serial number, sample number, and sampling location, date, and time recorded on this tag. In
addition a field sampling form and chain-of-custody form should be filled out giving all pertinent
information on the collection of the sample.
GC/FID Analysis
Gas chromatography with flame ionization detector is the established technique for
monitoring volatile hydrocarbons, ozone precursors, in ambient air. An air sample is taken from
the canister and passed through the sample concentration system. This system usually consists of a
freeze-out loop, made from chromatographic-grade stainless steel tubing packed with 60/80 mesh
deactivated glass beads, and a 10-port sampling valve. First, the sample is transferred from the
canister through the loop immersed in liquid oxygen to the volume transfer measurement apparatus.
The C 2 and heavier hydrocarbons are cryogenically trapped inside the loop when air is transferred
to an evacuated flask of known volume. From the difference in pressure inside the flask, the
volume of the air sample can be calculated, based on the ideal gas law. When a sufficient volume
of the air sample has been transferred from the canister to the concentration system, the 10-port
valve is switched to position 2, the liquid oxygen is replaced with boiling water, and the contents
of the trap are injected into a chromatographic column where separation of the C 2 -C 12
C-12