S.O.P. No. MLD 050 - Determination of Ambient Air Oxygenated Hydrocarbons
                                      (OHCs) Using SUMMA Canister Sampling and
                                      Gas Chromatographic Analysis


This document describes the procedure for determination of oxygenated compounds from ambient air samples. This method was developed by the Northern Laboratory Branch (NLB), Organics section. For this revision, only three ethers are addressed. They are methyl tert-butyl ether (MTBE), ethyl tert-butyl ether (ETBE), and tert-amyl methyl ether (TAME). All are possible oxygen additives to the Federal and State newly reformulated gasoline for the reduction of carbon monoxide auto emission.


Ambient air is collected in SUMMA passivated stainless steel canisters using a Xontech 910A sampler. A detailed sampling procedure can be found in the MLD Quality Assurance Manual, Volume II, Appendix Q and in the U.S. EPA Method TO14.

A 150 cc ambient air sample is drawn from the pressurized canister by the house vacuum with the aid of a mass flow controller to a cryogenic sample trap. The trap is packed with silanized glass beads and cooled to a sub ambient temperature (-150oC). A digital flow meter readout, attached to the Gas Chromatogram (GC), provides a visual indication of the proper sample flow during sampling. The concentrated sample in the trap is thermally desorbed and transferred to a gas chromatograph. Subsequent separation is accomplished with a pre column (DB-WAX) and an analytical column (DB-1) joined by a column switching valve. Thepre column does the preliminary separation as well as the moisture management. After all three ethers are eluted out from the pre column onto the analytical column, water and all other late elution compounds are switched to vent. The target components eluting from the analytical column are then detected by a flame ionization detector (FID). The desired peaks are identified by their characteristic retention times, and quantitated in parts per billion by volume (ppb v) by comparison to an external standard.