Quality assurance for emissions analysis systems EPA has established a repository of 37 gaseous organic compounds for use in source test performance audits. So far, eight have been evaluated statistically R.K.M. Jayanty Corette B. Parker Clifford F. Decker William F. Gutknecht Research Triangle Institute Research Triangle Park, N.C. 27709 Darryl J. von Lehmden Joseph E. Knoll Environmental Monitoring Systems Laboratory U.S. Environmental Protection Agency Research Triangle Park, N.C. 27711 The application of quality assurance practices is having a significant, positive impact on the quality of environmental data being generated. One such practice is the performance audit, which involves one laboratory providing "unknown" or "blind" samples to another laboratory for analysis. A performance audit conducted simultaneously with a source emission sample analysis provides an assessment of analysis accuracy. This accuracy assessment is important for data use in setting standards and for compliance testing after standards have been promulgated. When it has been demonstrated that specific organic gases are stable enough to be used as performance audit materials in the 5-20ppm concentration range, a performance audit will be included as an integral part of the promulgated test method. Two EPA test methods currently include the requirement for a performance audit during source test analyses—Method 106 for vinyl chloride and Method 110 for benzene
0013-936X/83/0916-0257A$01.50/0
GC calibration. This chemist is obtaining data by the "glass bulb" technique Under a contract to the EPA, the Research Triangle Institute (RTI) is developing an extensive repository of gaseous compounds for use in audits, as designated by EPA project officers. The RTI repository currently contains 37 different compounds selected on the basis of anticipated needs of EPA's Office of Air Quality Planning and Standards. Table 1 lists the compounds in the repository, their concentration ranges, the number of cylinders of each compound, and the cylinder construction material. The audit materials are in two concentration ranges. The low concentration range of 5 to 20 ppm covers the range of possible future emission standards. The high concentration
© 1983 American Chemical Society
range, 50-700 ppm, covers actual source emission levels found prior to the installation of control devices. The balance gas for all gas mixtures is nitrogen. In some cases, a compound that serves as an internal standard is also added to the gas mixture. Other compounds will be added to the repository as needed. The gaseous compounds in Table 1 are purchased in compressed gas cylinders from commercial suppliers. These cylinders, along with an appropriate delivery system, are used directly without dilution in the performance audits. The compressed gas cylinder is especially suitable as an audit material for a number of reasons including simplicity, portability, and Environ. Sci. Technol., Vol. 17, No. 6. 1983
257A
low cost; blind mode of operation (that is, the user cannot readily ascertain the relative or absolute concentration levels); and reliability and ruggedness for interstate shipping.
sponse to audit requests received from the EPA. RTI reanalyzes the contents whenever the cylinders are supplied for use in these audits. These latter anal yses provide values for comparison with the audit results and also allow a better estimate of the stability of the compounds in the cylinders. Instrumentation. Analyses are per formed with a Perkin-Elmer Model 3920 Β gas chromatograph with a flame ionization detector, a PerkinElmer Sigma 4 gas chromatograph with flame ionization and electron capture detectors, and a Tracor Model 560 gas chromatograph with a flame photometric detector. The Tracor in strument has been used principally for measurement of sulfur-containing species. A moderately polar column (Alltech Associates, 10%OV-101 on
Experimental procedures RTI analyzes the contents of the cylinders when they are first received, in order to check the manufacturer's reported values. If the RTI-measured values are not within 10% of the sup plier's values, they are measured by a third party, usually the EPA. The cylinder contents are analyzed several more times over the course of a year to estimate stability. All measurements are made by gas chromatography (GC). Cylinders in the repository are sent to laboratories and field sites in re
Chromosorb WHP) is used for mea surement of most of the hydrocarbons. Other columns used include Durapak η-octane on Porasil C (Waters Asso ciates, Inc., nonpolar),0.1%SP-l000 on Carbopack C (Supelco, Inc., mod erately polar), and 0.4% Carbowax 1500 on Carbopack C (Supelco, Inc., polar). Column and detector temper atures used for each compound are described elsewhere by Jayanty et al. (2). Gaseous samples are injected onto the columns by means of manual gas sampling valves. These valves are equipped with interchangeable sample loops to allow the injection of fixed volumes of gas. Another system per mits the injection of variable sample volumes. The operation of the system is based
TABLE 1
Audit materials currently held in the repository Compound
258A
No. of cylinders
Low-concentration range Concentration Cylinder range (ppm) construction
a
No. of cylinders
High-concentration range Concentration range (ppm)
Benzene Ethylene
14 4
8-13 5-20
S AI
Propylene Methane/ethane
4
5-20
Al
—
—
—
17 4 6 4 4
Propane
4
5-20
Al
4
Toluene Hydrogen sulfide Meta-xylene Methyl acetate Chloroform
2 4 2 2 2
5-20 5-20 5-20 5-20 5-20
S Al S S S
2 2 2 2 2
300-700 300-700 300-700 300-700 300-700
S Al LS S S
Carbonyl sulfide Methyl mercaptan Hexane 1,2-Dichloroethane Cyclohexane
2 4 2 4
5-20 3-10 20-80 5-20
S Al Al Al
2
100-300
S
—
—
—
—
Methyl ethyl ketone Methanol 1,2-Dichloropropane Trichloroethylene 1,1-Dichloroethylene 1,2-Dibromoethylene
1 1 2 2 2 2
30-80 30-80 5-20 5-20 5-20 5-20
S Al Al Al Al Al
Perchloroethylene Vinyl chloride 1,3-Butadiene Acrylonitrile
2 9 1 2
5-20 5-30 5-30 5-20
S S S LS
Methyl isobutyl ketone Cyclohexanone Methylene chloride Carbon tetrachloride Freon 113
1 2 1 1 1
5-20 5-20 5-20 5-20 5-20
Al Al Al Al Al
Methyl chloroform Ethylene Oxide Propylene oxide Allyl chloride Acrolein
1 1 1 1 1
5-20 5-20 5-20 5-20 5-20
Al Al Al S Al
Chlorobenzene 1 5-20 Carbon disulfide — — " AI = aluminum. S = steel, LS = low-pressure steel
Al Al
Environ. Sci. Technol.. Vol. 17, No. 6, 1983
2 4 1
—
60-400 300-700 3 0 0 0 - 2 0 000 300-700 1000-6000 (methane) 2 0 0 - 7 0 0 (methane) 300-700
Cylinder construction
—
1000-3000 100-600 80-200
—
2 2 2 2
300-700 100-600 100-600 100-600
2
300-700
— —
— —
2
300-700
1
50-100
— — — — —
AI, S Al Al Al Al Al
—
LS Al S
— LS Al Al Al
LS
— — LS Al
— — — —
— — — —
—
—
1 1 1
75-200 75-200 75-200
—
75-200
Al S Al
Al
a
on measurement of pressure differen tials. The sample loop, the volume of which is known, is first evacuated. Then, after the vacuum source is closed off, the sample is introduced into the loop (the final pressure after sample introduction may vary from subambient to superambient). The pressure change with sample intro duction is measured with a very accu rate gauge, and the mass of sample introduced is then calculated accord ing to the ideal gas law. Thus, cali bration curves can be developed easily and quickly with gas from only one cylinder of known concentration. Other cylinders of higher concentra tions can then be made to fall within the bounds of the calibration curve. Standardization and measurement. Several different forms of standards are used for calibration. The preferred standards are National Bureau of Standards (NBS) Standard Reference Materials (SRMs). Unfortunately, SRMs are available for only a few or ganic gases. The SRMs presently being used are methane and propane in nitrogen in compressed gas cylin ders. If the compound to be quantified is not an available SRM but is of the same homologous series (such as alkanes or alkenes), a flame ionization detector is used, and calculations are based on relative response factors, which are related to carbon number.
This procedure is described in the lit erature by Dietz (J). A second calibration method uses commercial permeation tubes. Such tubes are used for vinyl chloride and perchloroethylene. The tube is placed in a temperature-controlled chamber and dry "zero" (hydrocarbon-free) air is passed over the tube at a known flow rate. The resultant gaseous mixture is further diluted if necessary with ad ditional dry hydrocarbon-free air in a glass dilution chamber. The final mixture is then passed into a gas sam pling bulb for GC measurement or is drawn directly into the GC sample injection valve by a small pump downstream. The permeation tubes are not used until their permeation rates, which are determined periodically by measured weight loss, have been well established. A third method of calibration is the "glass bulb" technique in which a known volume of the pure compound, either gas or liquid, is injected into an evacuated glass bulb or flask of known volume. The flask is then returned to atmospheric pressure with a balance gas of choice. If a pure liquid is in jected, total vaporization is assumed and the concentration is determined by applying the ideal gas law. Usually a minimum of 10 μ ι is injected to achieve acceptable volumetric accu racy. If 10 μ ι of the pure liquid results in too high a concentration, a solution
of the analyte and some noninterfering solvent will be injected. Dilutions can be made by using additional bulbs or by pressurizing with a balance gas to a known pressure. Calibrations not performed with NBS SRM cylinder gases or commercial permeation tubes are performed by the glass bulb tech nique. With each of these standards, mul tipoint standard curves are prepared each time a sample is analyzed. Cer tain quality control procedures are followed; for example, the permeation system and the glass bulbs are equili brated with the sample gas before an aliquot is taken for GC measurement. Also, the glass bulbs used are cleaned thoroughly before each use by re peated evacuation. An NBS standard cylinder of methane is analyzed to verify the constancy of the detector •response. Because of condensation effects at elevated concentrations and pressures, several compounds are contained in low-pressure steel cylinders. However, reduced delivery concentrations may still be encountered, especially if these cylinders are exposed to cold temper atures. RTI uses a procedure devel oped by Scott Environmental Tech nology, Inc., that will return the de livery concentration of a low-pressure cylinder to the true value. This proce dure involves slow, mild heating (moderately warm to the touch) and
TABLE 2
Sample cylinder analysis data: benzene Cylinder No. Cylinder construction
Manufacturer concentration
(ppm) 1
| I
Date (ppm) Day (ppm)
] RTI concentration
/ \
I
% change/month Two std. dev. of % change/month 6
1A AI
Β AI
te AI
ID AI
1P
ΙΟ
s
s
65.4
324
200
117
8.04
7/27/77 (79.0) 136 (74.0) 156 (78.0) 167 (80.0) 630 (77.9) a
-
7/27/77 (374) 136 (337) 156 (350) 167 (355) 402 (331) 433 (343) 969 (358) 1274(348) 1491 (324)
7/27/77 (241) 247 (216) 252(215) 381 (218)
7/27/77 (138) 29 (144) 157(134) 252 (129) 290 (127) 414(127) 1247 (132)
1R S
9.85
4/21/78 (8.37) 4 (8.33) 25 (8.20) 26 (8.34) 56(8.19) 134 (7.81) 434 (8.21) 766 (7.93) 1222 (7.68)
9.89
4/21/78 (9.99) 5 (9.88) 25(10.1) 332 (9.71)
4/21/78 (10.0) 4(10.1) 13 (9.73) 332 (9.77) 1018 (9.46) 1270 (9.64)
0.01
-0.10
- 3.87
-0.12
-0.16
-0.25
-0.10
0.42
0.16
0.66
0.27
0.10
0.26
0.08
AI = aluminum, S = steel •Empty " Criteria for calculation are that at least four measurements have been performed and that at least a one-year span exists between the first and last analysis. Analytical conditions Flame ionization detector. 1 0 % OV-101 on Chromosorb WHP column at 60 °C. Calibration: Reagent-grade benzene liquid is used as a standard. Glass-bulb dilution technique is used for making the series of standards for calibra tion.
Environ. Sci. Technol., Vol. 17, No. 6, 1983
259A
vigorous horizontal rolling of the audit gas cylinder before sampling. Audit gas reanalysis and data treatment. Analysis results collected over time are tabulated and reported in an annual project status report (2). The data included in a typical page of this report are shown in Table 2. This table shows cylinder numbers as as signed by RTI, cylinder construction material, and concentration reported by the manufacturer upon delivery of the cylinder to RTI. The date shown is the date the cylinder contents were first analyzed by RTI. The "day" values are the number of days after the first analysis by RTI, and the values in parentheses are the ppm concentra tions determined on each particular analysis day. The usual analysis fre quency for all audit materials is: • immediately when received from the gas manufacturer, • four weeks after the first RTI analysis, • eight weeks after the first RTI analysis, • one year after the first RTI analysis, and • immediately before any request that would result in the use of the cyl inder gas in a performance audit. The stability of the contents of each cylinder is estimated as percent change in concentration per month. Such an estimate is made only when a mini mum of three analyses have been per
formed on the contents of a particular cylinder. A linear regression analysis of the data is performed, with χ as days and y as ppm, to determine this value. Then, percent change per month is calculated as: % change/month = ψ X 100 X 30 d b
where b = intercept and m = slope. The standard deviation of the per cent change per month is calculated when a minimum of four analyses have been performed. This calculation in volves the use of conventional statisti cal methods. Standard deviation values are doubled and presented at the bot tom of Table 2. It can be argued that a linear model may not be appropriate for all compounds. That is, some compounds may change rapidly soon after they are prepared, and then change only slowly, or not at all after that. The analysis frequency, however, is determined primarily by the request for the audit material for performance audit. Problem compounds. Several of the compounds prepared as gases in cyl inders were found to be unsuitable for use as audit materials. Among these were aniline, cyclohexanone, paradichlorobenzene, and ethylamine, which all have relatively high boiling points. Unacceptable variability in results of analyses of these compounds was noted, which apparently was attrib
utable to losses to the regulators, sample transfer lines, valves, and so on. These problems could be reduced by the use of heated systems, but such systems are impractical for audits. Further work is in progress to deter mine the actual reasons for this ana lytical variability. One additional compound, formaldehyde, was or dered, but the specialty gas manufac turer indicated that cylinders of this compound in the gaseous state could not be prepared because of its insta bility at room temperature. Performance audits
As stated, RTI supplies repository cylinders for audits upon request from the EPA. These requests are some times directed through the EPA from state and local government agencies or from their contractors. However, the contractor must be performing source emission tests on behalf of the EPA or one of the other agencies to qualify for the performance audit. When a request is received, the contents of the cylinders to be shipped are analyzed, the cylinder pressures are measured, and the cylinders are shipped by truck to the audit site. Cylinder gas regulators also are pro vided when requested. General in structions for performance of the audit are included with the cylinders. The audit results are reported to the EPA or the requesting agency audit co
Evaluation procedure in the stability evaluation process, linear regression models are fit to the analysis data collected over time. These models allow data to be pooled so that, for example, all benzene re sults can be tested collectively. Sta bility tests are under way on all 37 compounds in the repository. For eight compounds, sufficient data are available to permit a meaningful statistical evaluation for stability. However, the other 29 compounds have not been analyzed often enough to permit meaningful statistical tests. The first linear regression model used to evaluate the compounds is one that fits a common slope for all cylinders and a distinct intercept for each cyl inder of a particular compound. The model is:
v£> = aj c, + bX+e'.(C)
(1)
where VÇ* is the concentration (in ppm) for compound C for cylinder i at day x; a'C) is the intercept for compound C for cylinder i, with concentration at time zero; b(C) is the slope for
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Environ. Sci. Technol., Vol. 17. No. 6. 1983
compound C, i.e., change in concentration (Δ ppm) with change in time (Δ d); X is time in days; and e'?' is a sta tistical error term. These error terms are assumed to be independent, iden tically distributed, random variables for all cylinders with distribution N(0, σ2). The second model allows a unique slope and intercept for each cylinder i; that is:
Yi? = a!c> + b!c>X+e£>
(2)
C)
where b! is the slope for compound C for cylinder i. The third model allows unique slopes for cylinders within concen tration levels designated as high, me dium, or low, or just high and low. This model is:
Y l ^ a f ' + bf'X+e^
(3)
where Ϋ$χ is the concentration (in ppm) at day χ for compound C for the cylinder i of concentration level j ; bj C) is the slope for compound C for con centration level j ; and e ^ is the error term.
In addition to the above, a model that allows for different slopes for aluminum vs. steel cylinders was in vestigated for benzene. The model is: V l & = a!c» + b S = » X - r e &
(4)
where Υ§1)χ is the concentration (in ppm) for compound C for cylinder i of concentration level i and cylinder material k at day x; bjf ' is the slope for compound C for concentration level j of cylinder material k; and ei§J)x is the error term. The purpose of fitting the collected analytical data to these models is twofold. The most important objective is to determine whether or not the calculated slopes, that is, changes in concentration (ppm) with time (d), are significantly different from zero, using as a statistic the ratio of the slope to its standard error. The second purpose is to determine whether there are dif ferences in stability between con centration ranges or between types of cylinders.
ordinator. To date, 86 performance audits have been completed. Representative audit results are presented in Table 3. As noted previously, high- and low-level concentrations were included in most audits. The average audit result bias, that is, (client-measured concentration — RTI-measured concentration)/ (RTI-measured concentration), for the 86 audits was —0.3% with a standard deviation of ± 10.5%. This low average is somewhat fortuitous in light of the value of the standard deviation. Audit results ranged from —24:8% for a hy drogen sulfide measurement to +32.5% for a toluene measurement. The principal results of these audits have been to assure all concerned that analyses are being performed properly, to uncover problems that could be corrected, and to document the accu racy of the source emission test anal ysis. Stability study The data collected over time from the determinations of cylinder content concentrations are being used to esti mate stabilities of the repository gases. Cylinder gas stability data are impor tant for several reasons. First, audit materials used by the EPA must be stable, or they are unacceptable as audit standards. Second, if organic gases in cylinders are stable, other in vestigators may more readily use cyl inder gases as calibration standards or quality control check samples. Finally, if organic gases in cylinders are stable, future EPA regulations may require performance audits as a means of as sessing the accuracy of analysis data. Fitting models to data. The Tit of the various models to the data for eight compounds was tested. Only benzene was tested with all four models. Though both high and low levels of hydrogen sulfide are included in the repository, only low levels were in cluded in this stability test because of the small number of measurements made of the high levels. The mean square errors resulting from the fit of the data to each of the mathematical models were determined. Statistical tests based on analysis of variance procedures were used to assess the differences between each more com plex model and a simpler one (4). Results of these tests are listed in Table 4. As noted, probability (j>) values
