S.O.P. No. MLD 058 - Determination of Aromatic and Halogenated Compounds
in Ambient Air
by Capillary Column Gas Chromatography/Mass Spectrometry
This document describes the procedures followed by Monitoring and Laboratory Division (MLD) staff to analyze aromatic and halogenated hydrocarbons by Gas Chromatography with Mass Spectrometry detection, (GC/MS), in ambient air samples collected from the California Toxic Monitoring Network. Staff of the Northern Laboratory Branch (NLB), Organic Laboratory Section (OLS), developed the method. This Standard Operating Procedure (SOP) is based on the U.S. Environmental Protection Agency (EPA) Toxic Organic Compounds in Ambient Air Method TO-15, "Determination of Volatile Organic Compounds (VOCs) In Air Collected In Specially-Prepared Canisters And Analyzed by Gas Chromatography/Mass Spectrometry (GC/MS)", EPA/625/R-96/010b, January 1999. Table 1, page 23, of the SOP lists the Target Compounds and their Chemical Abstract Service (CAS) numbers.
SUMMARY OF METHOD
Ambient air is collected in a SUMMA polished stainless steel canister using a Xontech 910A sampler. The sampling procedure for Toxic 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 Nafionädryer to remove moisture from the gas stream. It is trapped on a cryotrap at -130 degrees centigrade (ºC). At this temperature, the desired components are solidified, while fixed gases, such as nitrogen (N2), oxygen (O2), 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. After purging, the cryotrap is rapidly heated to 200 ºC to transfer/desorb the contents and retrap them on the cryofocuser at -130 ºC. The cryofocuser is rapidly heated up to 200 ºC to inject the sample onto a DB-VRX capillary column.
The sample mixture is separated into individual components by their interaction with the capillary column's stationary phase, using temperature programmed gas chromatography. A Mass Selective Detector (MSD) detects the components eluting from the column. The target analytes, as shown in Table 1, page 23, of the SOP, are subsequently identified and quantified. Identification of a component in a sample is based upon both the retention time and mass spectral matching. The response of one mass fragment, the Primary Quantitation Ion, is used for quantitation.