Basis: The basis of the method described here, which is intended to
obtain an aggregate measure for NMOCs, is, first, to separate methane,
oxygen, and nitrogen from the sample by a cryogenic technique using
liquid argon. Next, the entire sample can then be evaluated in a flame
ionization detector (FID) to obtain a total value for NMOCs. Basis for
commonly known EPA Method TO-12 (Ref. 30, p. TO12-2).
Minimum Detection Level: 10 ppb (Ref. 31), 50 ppb, using propane
standard (0.15ppmvC, using propane standard) (Ref. 32)
Operating Temperature: 0 to 45°C (Ref. 31)
Known Interference: ( See Ref. 30, Project Summary, Table 2, and
Ref. 30, p. TO12-5).
High moisture levels in ambient air cause ice formation in the
cryogenic trap process; can also require adjustment of FID baseline when
calculating NMOC value.
If need arises to determine specific proportions of one or more
species, must change to different process.
If unusual types of organic compounds, outside the VOC category,
are in the sample, the FID measurement loses reliability (precision).
Other gasses, such as oxygen, nitrogen, and methane, if present in
large amounts, and also interfere with this method, and some devices
will specify the limits of the range of these gasses, for example:
CH4, 0.0 to 2500 ppm (Ref. 31).
Notes of Interest:
Simple standard procedures are used, and automated canister equipment
is available to reduce personnel field time.
Adsorbent materials can be used with TO-12, but are not defined as
part of the method. Contaminants and reactions that occur with adsorbents
may be a problem. Storage of samples in the canisters is possible.
Low background noise problem helps improve sensitivity (LDL). The
method suggests that sampling canisters be considered free of
contamination if they contain less than 0.02 ppmC hydrocarbons (Ref. 30,
p. TO12-14).
Total NMOC values are useful as a screening variable for sites or
areas where no other monitoring equipment is installed.
Analysis is much faster and cheaper because GC not used.
State of the art of ambient air quality modeling has not clarified
whether specific (individual) volatile organic compound (VOC) measurements
will be absolutely necessary as input variables for models which predict
ambient air quality, rather than total non-methane organic compounds
(NMOC). Further model development may indicate that NMOC values will be
useful ozone predictors for some conditions.
The term volatile organic compounds generally refers to gaseous
organic compounds that have a vapor pressure greater than 0.15 mm and
generally have a carbon content ranging from C1 through C12 (Ref. 22,
p. 3-107). Reference 22 also describes how the terms nonmethane
organic carbons (NMOCs) and nonmethane hydrocarbons (NMHCs)
have come into use. Interest in a simple method for some large group of
VOCs has led to the current definition of EPA method TO-12 as a method
for all NMOCs, including the NMHC, carbonyl, and polar subgroups.
EPA has defined two separate methods, TO-5 and TO-11A, both of which
use a UV detector, for specifically measuring the carbonyls. Furthermore,
three newer methods, TO-15 (GC/MS), TO-16 (FTIR), and TO-17 (GC/MS),
are intended to separate and to measure every VOC, including those in
polar and non-polar groups. Descriptions of all of these methods are
contained in Reference 30 (Project Summary).
Regarding "lower detection limit": The VOCs are expected to occur
in urban ambient air at concentrations possibly less than 0.1ppbC. This
level, given by Reference 33 ( Section 2.3.1, page 9), is much lower than
the level given in the discussion for TO-12 (5 to 7 ppmC, page TO12-3),
because this very low level refers to levels of individual compounds.
The units ppmC, sometimes given as ppmvC, means that the volume
concentration of the gas is multiplied by the number of carbon atoms in
the molecule.
Instruments - Manufacturer:
Model 51 Flame Ionization Detector (heated total
hydrocarbon analyzer) - Thermo Environmental Instruments Inc. http://www.thermoei.com/
Model 55C Direct Methane, Non-Methane Analyzer (uses combined gas
chromatography and flame ionization detector, can separate methane without
cryogenic preconcentration) - Thermo Environmental Instruments Inc.
http://www.thermoei.com/
Model NGA 2000 McFID Methane, Non-Methane Hydrocarbon Analyzer
(uses combined gas chromatography and flame ionization detector,
cryogenic preconcentration not required) - Rosemount Analytical Inc. http://www.frco.com/proanalytic/
