S.O.P. No. MLD 024 - Determination of Total Non Methane Organic Compounds
(NMOC) by Pre concentration Direct Flame Ionization
This document describes the method for the analysis of total Non Methane organic compounds (NMOC) in ambient air. Developed from USEPA 600/4-85/063, this method involves the cryogenic pre concentration of an ambient air sample with subsequent determination of total NMOC concentration represented as a single peak by direct flame ionization detection (FID). The methods provides a rapid analysis of ambient samples (a quick survey of total NMOC levels from various sites) and permits a cross-check with results from the speciated NMOC analyses by FID.
SUMMARY OF METHOD
Ambient air is sampled over a three-hour period and collected in a SUMMA passivated stainless steel canister using a XonTech 910A sampler. All the operational procedures and sampling conditions are documented. A record of this information is sent back to the Northern Laboratory Branch (NLB) Organics Laboratory along with the sample for immediate analysis. Upon arrival into the laboratory, the canister sample must be equilibrated to room temperature for at least one hour before analysis. The integrity of the canister pressure is validated by using a calibrated gauge. All information from the canister is documented in the login protocol, and the canister is analyzed according to the chain of custody within the laboratory.
A 300 cc ambient air sample is introduced into the system from the pressurized
canister through 1/16" stainless steel tubing with the aid of a mass flow
controller and a vacuum system. For low sample volumes, a fixed volume
loop of 10.0 cc can also be used. The digital flow meter readout that is
attached to the gas chromatograph (GC) provides a visual indication of
the proper sample flow during sampling. Automatic sampling of up to 32
sample containers can be accompanied with a multi-position automated sampler
which is controlled by the GC or personal computer (PC with Star Workstation
software). The sample passes through a Nafion dryer to remove moisture
(H2O vapor) from gas streams and is trapped within nickel tubing
(120 uL) filled with silanized glass beads (60/80 mesh) at -168.8oC.
The desired NMOC components are immobilized (solidified), while oxygen
(O2), carbon monoxide (CO), and methane (CH4) pass
through the vent. The injector is purged with N2 followed by
sample heating and injection. The trap is isolated and heated to 190oC
at the rate of 300oC/min. to completely volatilize the trapped
hydrocarbons. The gaseous sample is purged off the trap with nitrogen (N2)
and injected directly into the flame ionization detector (FID). An addtional
1.80 minute hold at 190oC of the isolated trap allows sample
to be completely volatilized (especially the higher boiling components)
before the sample is purged to the detector by N2. Since the
analysis does not utilize a column, all NMOC compounds are represented
as a single peak and are subsequently quantified as total NMOC.