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EPA Method TO-14

Compendium of Methods for the Determination of Toxic Organic Compounds in Ambient Air - Compendium Method TO-14A Determination Of Volatile Organic Compounds (VOCs) In Ambient Air Using Specially Prepared Canisters With Subsequent Analysis By Gas Chromatography

Full EPA Method TO-14 in PDF Format

METHOD TO-14

Compendium Method TO-14A Determination Of Volatile Organic Compounds (VOCs) In Ambient Air Using Specially Prepared Canisters With Subsequent Analysis By Gas Chromatography

1. Scope

1.1 This document describes a procedure for sampling and analysis of volatile organic compounds (VOCs) in ambient air. The method was originally based on collection of whole air samples in SUMMA® passivated stainless steel canisters, but has now been generalized to other specially prepared canisters (see Section 7.1.1.2). The VOCs are separated by gas chromatography and measured by a mass spectrometer or by multidetector techniques. This method presents procedures for sampling into canisters to final pressures both above and below atmospheric pressure (respectively referred to as pressurized and subatmospheric pressure sampling).

1.2 This method is applicable to specific VOCs that have been tested and determined to be stable when stored in pressurized and sub-atmospheric pressure canisters. Numerous compounds, many of which are chlorinated VOCs, have been successfully tested for storage stability in pressurized canisters (1-3). However, minimal documentation is currently available demonstrating stability of VOCs in subatmospheric pressure canisters.

1.3 The Compendium Method TO-14A target list is shown in Table 1. These compounds have been successfully stored in canisters and measured at the parts per billion by volume (ppbv) level. This method applies under most conditions encountered in sampling of ambient air into canisters. However, the composition of a gas mixture in a canister, under unique or unusual conditions, will change so that the sample is known not to be a true representation of the ambient air from which it was taken. For example, low humidity conditions in the sample may lead to losses of certain VOCs on the canister walls, losses that would not happen if the humidity were higher. If the canister is pressurized, then condensation of water from high humidity samples may cause fractional losses of water-soluble compounds. Since the canister surface area is limited, all gases are in competition for the available active sites. Hence an absolute storage stability cannot be assigned to a specific gas. Fortunately, under conditions of normal usage for sampling ambient air, most VOCs can be recovered from canisters near their original concentrations after storage times of up to thirty days.

2. Summary of Method

2.1 Both subatmospheric pressure and pressurized sampling modes typically use an initially evacuated canister and pump-ventilated sample line during sample collection. Pressurized sampling requires an additional pump to provide positive pressure to the sample canister. A sample of ambient air is drawn through a sampling train comprised of components that regulate the rate and duration of sampling into a pre-evacuated specially prepared passivated canister.

2.2 After the air sample is collected, the canister valve is closed, an identification tag is attached to the canister, a chain-of-custody (COC) form completed, and the canister is transported to a predetermined laboratory for analysis.

2.3 Upon receipt at the laboratory, the canister tag data is recorded, the COC completed, and the canister is attached to the analytical system. During analysis, water vapor is reduced in the gas stream by a Nafion® dryer (if applicable), and the VOCs are then concentrated by collection in a cryogenically-cooled trap. The cryogen is then removed and the temperature of the trap is raised. The VOCs originally collected in the trap are revolatilized, separated on a GC column, then detected by one or more detectors for identification and quantitation.

2.4 The analytical strategy for Compendium Method TO-14A involves using a high-resolution gas chromatograph (GC) coupled to one or more appropriate GC detectors. Historically, detectors for a GC have been divided into two groups: non-specific detectors and specific detectors. The non-specific detectors include, but are not limited to, the nitrogen-phosphorus detector (NPD), the flame ionization detector (FID), the electron capture detector (ECD) and the photo-ionization detector (PID). The specific detectors include the linear quadrupole mass spectrometer (MS) operating in either the select ion monitoring (SIM) mode or the SCAN mode,
or the ion trap detector (see Compendium Method TO-15). The use of these detectors or a combination of these detectors as part of the analytical scheme is determined by the required specificity and sensitivity of the application. While the non-specific detectors are less expensive per analysis and in some cases far more sensitive than the specific detectors, they vary in specificity and sensitivity for a specific class of compounds. For instance, if multiple halogenated compounds are targeted, an ECD is usually chosen; if only compounds containing nitrogen or phosphorus are of interest, a NPD can be used; or, if a variety of hydrocarbon compounds are sought, the broad response of the FID or PID is appropriate. In each of these cases, however, the specific identification of the compound within the class is determined only by its retention time, which can be subject to shifts or to interference from other non-targeted compounds. When misidentification occurs, the error is generally a result of a cluttered chromatogram, making peak assignment difficult. In particular, the more volatile organics (chloroethanes, ethyltoluenes, dichlorobenzenes, and various freons) exhibit less well defined chromatographic peaks, leading to possible misidentification when using nonspecific detectors. Quantitative comparisons indicate that the FID is more subject to error than the ECD because the ECD is a much more selective detector and exhibits a stronger response. Identification errors, however, can be reduced by: (a) employing simultaneous detection by different detectors or (b) correlating retention times from different GC columns for confirmation. In either case, interferences on the non-specific detectors can still cause error in identifying compounds of a complex sample. The non-specific detector system (GC/NPD/FID/ECD/PID), however, has been used for approximate quantitation of relatively clean samples. The non-specific detector system can provide a "snapshot" of the constituents in the sample, allowing determination of:

— Extent of misidentification due to overlapping peaks.
— Determination of whether VOCs are within or not within concentration range, thus requiring further analysis by specific detectors (GC/MS/SCAN/SIM) (i.e., if too concentrated, the sample is further diluted).
— Provide data as to the existence of unexpected peaks which require identification by specific detectors.

On the other hand, the use of specific detectors (MS coupled to a GC) allows positive compound identification, thus lending itself to more specificity than the multidetector GC. Operating in the SIM mode, the MS can readily approach the same sensitivity as the multidetector system, but its flexibility is limited. For SIM operation the MS is programmed to acquire data for a limited number of targeted compounds. In the SCAN mode, however, the MS becomes a universal detector, often detecting compounds which are not detected by the multidetector approach. The GS/MS/SCAN will provide positive identification, while the GC/MS/SIM procedure provides quantitation of a restricted list of VOCs, on a preselected target compound list (TCL). If the MS is based upon a standard ion trap design, only a scanning mode is used (note however, that the Select Ion Storage (SIS) mode of the ion trap has features of the SIM mode). See Compendium Method TO-15 for further explanation and applicability of the ion-trap to the analysis of VOCs from specially prepared canisters. The analyst often must decide whether to use specific or non-specific detectors by considering such factors as project objectives, desired detection limits, equipment availability, cost and personnel capability in developing an analytic strategy. A list of some of the advantages and disadvantages associated with non-specific and specific detectors may assist the analyst in the decision-making process.

Non-specific Multidetector Analytical System
Advantages Disadvantages

Advantages
! Somewhat lower equipment cost than GC/MS
! Less sample volume required for analysis
! More sensitive
- ECD may be 1000 times more sensitive than GC/MS

Disadvantages
! Multiple detectors to calibrate
! Compound identification not positive
! Lengthy data interpretation (1 hour each for analysis and data reduction)
! Interference(s) from co-eluting compound(s)
! Cannot identify unknown compounds
- outside range of calibration
- without standards
! Does not differentiate targeted compounds from interfering compounds Specific Detector Analytical System

GC/MS/SIM

Advantages
! positive compound identification
! greater sensitivity than GC/MS/SCAN
! less operator interpretation than for multidetector GC
! can resolve co-eluting peaks
! more specific than the multidetector GC

Disadvantages
! cannot identify nonspecified compounds (ions)
! somewhat greater equipment cost than multidetector GC
! greater sample volume required than for multidetector GC
! universality of detector sacrified to achieve enhancement in sensitivity GC/MS/SCAN

Advantages
! positive compound identification
! can identify all compounds
! less operator interpretation
! can resolve co-eluting peaks

Disadvantages
! lower sensitivity than GC/MS/SIM
! greater sample volume required than for multidetector GC
! somewhat greater equipment cost than multidetector GC

The analytical finish for the measurement chosen by the analyst should provide a definitive identification and a precise quantitation of volatile organics. In a large part, the actual approach to these two objectives is subject to equipment availability. Figure 1 indicates some of the favorite options that are used in Compendium Method TO-14A. The GC/MS/SCAN option uses a capillary column GC coupled to a MS operated in a scanning mode and supported by spectral library search routines. This option offers the nearest approximation to unambiguous identification and covers a wide range of compounds as defined by the completeness of the spectral library. GC/MS/SIM mode is limited to a set of target compounds which are user defined and is more sensitive than GC/MS/SCAN by virtue of the longer dwell times at the restricted number of m/z values. Both these techniques, but especially the GC/MS/SIM option, can use a supplemental general nonspecific detector to verify/identify the presence of VOCs. Finally the option labelled GC-multidetector system uses a combination of retention time and multiple general detector verification to identify compounds. However, interference due to nearly identical retention times can affect system quantitation when using this option. Due to low concentrations of toxic VOCs encountered in urban air (typically less than 25 ppbv and the majority below 10 ppbv) along with their complicated chromatographs, Compendium Method TO-14A strongly recommends the specific detectors (GC/MS/SCAN/SIM) for positive identification and for primary quantitation to ensure that high-quality ambient data is acquired. For the experienced analyst whose analytical system is limited to the non-specific detectors, Section 10.3 does provide guidelines and example chromatograms showing typical retention times and calibration response factors, and utilizing the nonspecific detectors (GC/FID/ECD/PID) analytical system as the primary quantitative technique. Compendium Method TO-15 is now available as a guidance document containing additional advice on the monitoring of VOCs. Method TO-15 contains information on alternative water management systems, has a more complete quality control section, shows performance criteria that any monitoring technique must achieve for acceptance, and provides guidance specifically directed at compound identification by mass spectrometry.

3. Significance

3.1 The availability of reliable, accurate and precise monitoring methods for toxic VOCs is a primary need for state and local agencies addressing daily monitoring requirements related to odor complaints, fugitive emissions, and trend monitoring. VOCs enter the atmosphere from a variety of sources, including petroleum refineries, synthetic organic chemical plants, natural gas processing plants, biogenic sources, and automobile exhaust. Many of these VOCs are toxic so that their determination in ambient air is necessary to assess human health impacts.

3.2 The canister-based monitoring method for VOCs has proven to be a viable and widely used approach that is based on research and evaluation performed since the early 1980s. This activity has involved the testing of sample stability of VOCs in canisters and the design of time-integrative samplers. the development of procedures for analysis of samples in canisters, including the procedure for VOC preconcentration from whole air, the treatment of water vapor in the sample, and the selection of an appropriate analytical finish has been accomplished. The canister-based method was initially summarized by EPA as Method TO-14 in the First Supplement to the Compendium of Methods for the Determination of Toxic Organic Compounds in Ambient Air. The present document updates the original Compendium Method TO-14 with correction of time-sensitive information and other minor changes as deemed appropriate.

3.3 The canister-based method is now a widely used alternative to the solid sorbent-based methods. The method has sub-ppbv detection limits for samples of typically 300-500 mL of whole air and duplicate and replicate precisions under 20 percent as determined in field tests. Audit bias values average within the range of ±10 percent. These performance parameters are generally adequate for monitoring at the 10-5 lifetime exposure risk levels for many VOCs.

3.4 Collection of ambient air samples in canisters provides a number of advantages: (1) convenient integration of ambient samples over a specific time period (e.g., 24 hours); (2) remote sampling and central analysis; (3) ease of storing and shipping samples; (4) unattended sample collection; (5) analysis of samples from multiple sites with one analytical system; (6) collection of sufficient sample volume to allow assessment of measurement precision and/or analysis of samples by several analytical systems; and (7) storage stability for many VOCs over periods of up to 30 days. To realize these advantages, care must be exercised in selection, cleaning, and handling sample canisters and sampling apparatus to avoid losses or contamination.

3.5 Interior surfaces of canisters are treated by any of a number of passivation processes, one of which is SUMMA polishing as identified in the original Compendium Method TO-14. Other specially prepared canisters are also available (see Section 7.1.1.2).

3.6 The canister-based method for monitoring VOCs is the alternative to the solid sorbent-based method described in conventional methods such as the Compendium Methods TO-1 and TO-2, and in the new Compendium Method TO-17 that describes the use multisorbent packings including the use of new carbon-based sorbents. It also is an alternative to on-site analysis in those cases where integrity of samples during storage and transport has been established.

Full EPA Method TO-14 in PDF Format

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