S.O.P. No. MLD 057 - Determination of 1,3-Butadiene and Benzene in Ambient
                                      Air by Capillary Column Gas Chromatography with
                                      Photoionization Detector


This document describes the procedures followed by Monitoring and Laboratory Division (MLD) staff to analyze 1,3-butadiene and benzene by Gas Chromatography (GC) in ambient air samples collected from the California Toxic Monitoring Network. The method was developed by staff of the Northern Laboratory Branch (NLB), Organics Laboratory Section (OLS). This Standard Operating Procedure (SOP), MLD057 Revision 1.0, supersedes SOP MLD051 Revision 3.0 for determining 1,3-butadiene and benzene concentrations in ambient air. The new SOP includes a multi-point calibration procedure for quantitation of butadiene and benzene. Minor changes were also made in the GC relay program, which affects the post-injection system configuration.


Ambient air is collected in a SUMMA polished stainless steel canister using a XonTech 910A sampler. The sampling procedure for Toxics samples is detailed in the Air Resources Board Quality Assurance Manual, Volume II, Appendix Q. All the operational procedures and sampling conditions for each sample are documented in the field. A record of this information is sent back to the OLS along with the sample. Upon receipt, the sample canister pressure is measured with a calibrated external pressure gauge. This information and particulars of the collection are documented in the laboratory. The sample is then analyzed according to the SOP in the laboratory.

An ambient air sample is introduced into the analytical system from a pressurized canister through stainless steel or Teflon tubing with the aid of a mass flow controller (MFC) and a vacuum system. A digital readout attached to the MFC provides a visual indication of the proper sample flow during sampling. Automated sampling of up to 16 canisters can be accomplished using the system's multi-position stream selector valve.

The sample passes through a Nafion dryer to remove moisture from the gas stream. It is trapped on a cryotrap at -150 degrees centigrade (ºC). At this temperature, the desired components are solidified, while fixed gases, such as nitrogen (N2 ), oxygen (O2 ), and carbon dioxide (CO2 ), and methane (CH4) pass through the cryotrap to the vent. The system is purged with ultrapure N2 to flush sample remaining in the tubing or valving on to the cryotrap, and to remove any excess light impurities. The cryotrap is isolated and rapidly heated to 125oC, fol-lowed by injection of the sample onto a DB-VRX capillary column. After a short hold at 125oC, the trap temperature is raised to a final temperature of 190oC.

The sample mixture is separated into individual components by their interaction with the capillary column stationary phase, using temperature programmed gas chromatography. The components eluting from the column are detected by a Photoionization Detector (PID). The target analytes, 1,3-butadiene and benzene, are subsequently identified and quantified. Routine identification of a component in a sample as 1,3-butadiene or benzene is based upon the relative retention time. The initial positive identification of a peak under the method conditions requires the use of a gas chromatograph/mass spectrometer (GC/MS) system.