STANDARD OPERATING PROCEDURE FOR THE DETERMINATION OF OXYGENATED HYDROCARBONS (OHCs) 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 oxygenated 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 following U.S. Environmental Protection Agency (EPA) method:
Toxic Organic Compounds in Ambient Air Compendium 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.This SOP, and its predecessor, MLD050, “Standard Operating Procedure for the Determination of Ambient Air Oxygenated Hydrocarbons (OHCs) Using Summa Canisters and Gas Chromatographic Analysis,” can be used for several oxygenated hydrocarbons. Table 1, page 27, shows a list of compounds that can be detected by this method. The only compound with data being reported using this SOP is methyl tert-butyl ether.
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 desired components of the sample are trapped on an adsorbent trap at 10 degrees centigrade (ºC), while fixed gases, such as nitrogen (N2), oxygen (O2), carbon dioxide (CO2) pass through the adsorbent trap to the vent. Following the sample stream, the adsorbent trap is purged with dry, ultrapure N2 to flush sample remaining in the tubing or valving onto the adsorbent trap. This step also allows additional minimally adsorbed non-target compounds, like methane (CH4) and especially water, to pass through the adsorbent trap to vent. After purging, the adsorbent trap is rapidly heated to 200 ºC to desorb and reconcentrate the contents onto a cryofocuser at -130 ºC. The cryofocuser is rapidly heated to 200 ºC to desorb the trapped components onto a DB-VRX capillary column.
The trapped 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 analytes 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.