Envlron. Sci. Technol. 1993, 27, 2849-2854
Gas Standards Containing Halogenated Compounds for Atmospheric Measurements George C. Rhoderick,' Walter L. Zielinski, Jr.,t and Walter R. Miller
Gas Metrology Research Group, Organic Analytical Research Division, Chemical Science and Technology Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 A microgravimetric procedure was previously developed for the preparation of accurate cylinder gas standards for volatile organic compounds in the nanomole/mole mol/mol; parts per billion, ppb) range. The present study evaluated the use of this procedure for preparing such standards in the picomole/mole (10-l2mol/mol; parts per trillion, ppt) range with respect to accuracy and stability. The resulting gas standards were intercompared using gas chromatography with electron capture detection (GCECD). Regression analysis of the GC-ECD responses versus the corresponding gravimetric concentrations showed excellent agreement among groups of the standards for each compound. The standards were evaluated over a period of almost 3 years and were found to exhibit continued stability and accuracy. These primary gas standards were used to accurately characterize the concentration of halogenated volatile organic compounds at the ppt level in two different batches of compressed gas cylinder mixtures prepared commercially. The results showed small but real differences in the concentrations of each compound between different cylinders in a given mixture, demonstrating the difficulty in preparing a batch of cylinders containing an identical mixture a t extremely low concentrations.
Introduction Volatile halogenated organic compounds have been an area of much concern and research over the past 15 years. Currently there is a considerable level of research taking place to assess the chemical composition of ambient air-in particular the urban and industrial environments-and the possible human health exposure risks that these compounds pose. There is a major concern that these compounds will affect the earth's climate. Numerous studies have been conducted worldwide to determine the airborne concentrations of many of these compounds in urban, industrial, rural, and atmospheric environments. To illustrate, measurements have been reported by Edgerton et al. on urban air in nine major U.S.cities (1). The data show average daily concentrations of tetrachloromethane ranging from 100 to 300 pmol/mol (10-l2;parts per trillion, ppt) and from 200 to 1200 pmol/mol for l,l,ltrichloroethane. Other studies and measurement programs are in place in a number of states such as California and New Jersey (2). In other countries such as Russia (3), England (41, and South Africa (5),trace measurements of these compounds have also been carried out in urban and industrial environments. Trends in the concentration of trace gas species and their potential role in climate change have been studied and have been reported (6). t Present address: Division of Drug Analysis, Center for Drug Evaluationand Research,Food and Drug Administration,St. Louis, MO 63101.
This article not subject to US. Copyright.
In order to make accurate and consistent long-term measurements of these species, accurate and stable gas standards are needed for instrument calibration. Permeation devices have been used as calibrant standards and are reported to have less than a 5% deviation in permeation rate for most compounds (7). However, contaminants in the flow control devices used to dilute the permeation rate to the needed concentrations must be determined and closely monitored. Rasmussen and Lovelock (8) have used stainless steel canisters for standards and sampling and have reported the stability of halocarbons over a period of 4 years. Oliver et al. (9)have reported on studies of these compounds in stainless steel canisters and have shown stability over several days. However, their data for 30 days shows increased scatter and suggests stability problems. In contrast, halocarbon mixtures in aluminum cylinders have shown multiyear stability (1012). Based on these factors, we decided to study the feasibility of preparing gas standards, at the picomole/ mole level, in treated aluminum gas cylinders and to determine their long-term stability. Aluminum gas cylinders have an added advantage over the stainless steel canisters of being DOT inspected, more important from an economical standpoint. A single-step microgravimetric procedure for the preparation of volatile organic compounds in a gas matrix was previously developedat the National Institute of Standards and Technology (NIST) (10, 11). But approximately 4.6 pg of organic material is needed to prepare a gas standard at the 200 pmol/mol level in a 30-L aluminum gas cylinder using this one-step procedure. This would result in an uncertainty of k1.5 pg (1u ) or 33% relative. In addition, the task of quantitatively transferring 5 pg of material into an aluminum gas cylinder is not trivial. We found it necessary to prepare primary gravimetric standards at much higher concentrations and use successive dilutions to obtain standards at the picomole/mole level. This procedure has been previously used to prepare gravimetric gas standards of volatile organic compounds at the nanomole/mole parts per billion, ppb) level (12). Studies were undertaken to determine if mixtures containing halogenated organic compounds could be accurately prepared in aluminum gas cylinders at the picomole/molelevel,and whether these mixtures are stable over a 2-4-year period or more. Following this, a study was undertaken to determine if a homogeneous batch of cylinders could be commercially prepared and certified as secondary standards using primary gravimetric standards to determine concentrations, This paper reports the results of the research leading to the development of accurate gravimetric gas standards containing volatile halogenated organic compounds at the picomole/mole level. Also described is the certification of batches of secondary standards, prepared commercially, with total uncertainties (per compound) ranging between 5 and 17% at the 95 % confidence interval.
Published 1993 by the Amerlcan Chemical Society
Environ. Sci. Technol., Vol. 27, No. 13, 1993 2849
Experimental Section Reagents were obtained at a minimum purity of 99.5 % as stated by the manufacturer. The nitrogen gas diluent was obtained from a commercial supplier with a stated specified purity of 99.9995% . The organic reagents and the nitrogen were analyzed by GC-ECD and GC-MS to confirm purity. New aluminum gas cylinders were obtained commercially (Luxfer,USA LTD) and were used in the preparation of the primary gravimetric standards and the secondary standards. Three sizes of cylinders were used, having internal volumes of 3.4,6, and 30 L. The cylinders were equipped with CGA-350 stainless steel valves. The interiors of the cylinders were cleaned at Luxfer using a caustic etch followed by acid wash procedure for industrial gas cylinders. The cylinders were then treated by Scott Specialty Gases using a chemical vapor deposition process, called Aculife, to make active sites on the inner cylinder walls inert. The stainless steel cylinder valves were also cleaned to minimize organic contamination. Aliquots of liquid organic compounds were carefully weighed by difference in sealed borosilicate glass capillary tubes using an electronicultra-microbalance. The capacity of the balance was 3 g with a sensitivity of 0.1 pg. The weights of the 30-L cylinders and the subsequent amount of diluent gas added were determined using a single-pan electronic floor balance with a 54-kg capacity. The sensitivity of this balance was extended from 1to 0.5 g by using 0.1-g weights and estimating the round-up weight of the balance. The 6-L cylinders were weighed on a tabletop electronic balance with a sensitivity of 0.1 g. In order to weigh the smaller (3.4-L) cylinders, a two-pan balance was used which had a electronic scale readable to 0.001 g. The weights were certified at NIST. Analyses of the gas standards were conducted using a gas chromatograph (GC) equipped with an electroncapture detector (ECD) operatedat 350 "C. Twodifferent groups of mixtures were developed and studied requiring two separate analytical methods. Group 1 mixtures, containing trichloromethane, tetrachloromethane, trichloroethene, and tetrachloroethene, were analyzed under the following conditions. A 60-m by 0.75-mm i.d. borosilicate wide-bore capillary column coated with a 1-pm-thick film of dimethylpolysiloxane phase was used at an isothermal temperature of 40 "C. The nitrogen carrier gas flow rate was 10 mL/min with a 25 mL/min nitrogen gas make-up flow rate into the ECD. A stainless steel gas sample valve equipped with a 2-mL stainless steel sample loop was used to inject the sample onto the column. The sample purge through the sample loop was 30 mL/min and was stopped for atmospheric pressure equilibrium before injection. Group 2 compounds, bromomethane, dichloromethane, 1,2-dichloroethane, l,l,l-trichloroethane, and 1,2-dibromoethane, were analyzed under this set of conditions. A 2.4-m by 3.2-mm 0.d. stainless steel column packed with a 1% loading of polyethylene glycol modified with nitroterephthalic acid on 60/80 mesh graphitized carbon black (176 SP-1000 on 60/80 Carbopack B) was used at an isothermal temperature of 135 "C. The nitrogen carrier gas flow rate was 30 mL/min. The same gas sample valve was used as above, except a 4-mL stainless steel sample loop was used to inject the sample onto the column. Preparation of Primary Gravimetric Standards. The standards were prepared using a microgravimetric 2850
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technique developed at NIST (10-12). The following is an example of one pathway used to prepare a primary gravimetric gas standard at the picomole/mole level. Briefly, the high-purity organic reagents are transferred into thin-walled glass capillary tubes and weighed. These organic reagents were then transferred into an evacuated preweighed 6-L aluminum gas cylinder by breaking the tubes within a section of Teflon tubing attached to the cylinder valve. The amount of organic reagent used is precalculated so that the concentrations in the final pressurized cylinder range from 200 to 500 nmol/mol. The nitrogen matrix gas is then added to a precalculated pressure, and a final weight is determined. The concentrations are calculated on a molar basis from the weight data. After a thorough mixing of this primary gravimetric gas standard using heat lamps, this freshly prepared standard was compared to existing gravimetric standards by GC/FID and found to be within the uncertainty. Then using this freshly prepared gravimetric gas standard, a nominal amount, 12.5 g, is transferred into a new, preweighed 3.4-L aluminum gas cylinder, which is reweighed after the transfer. The cylinder is pressurized (between 12 000 and 13 000 kPa) with a precalculated amount of nitrogen matrix gas, and a final weight is obtained. The concentrations of the organic compounds are then calculated, on a mol/mol basis, from the weight data and the weight percentages of the organic compounds in the original primary gravimetric standard. This freshly prepared gravimetric gas standard is then intercompared to existing gravimetric gas standards at the same concentration level. After thoroughly mixing this second gravimetric (nmol/ mol level) gas standard, a precalculated amount of this gas mixture, needed to make a gas mixture in the concentration range of 100-300 pmol/mol, is transferred into a new preweighed 3.4-L aluminum gas cylinder that is reweighed after the transfer. A precalculated amount of nitrogen matrix gas is added to the cylinder, and a final weight is obtained. The concentrations of the organic compounds are then calculated, on amolar basis, from the weight data and the weight percentages of the organic compounds in the second nanomole/mole level primary gravimetric standard. Several primary standards were prepared from different gravimetric standards as the starting materials using varying numbers of successive dilutions. The same procedure was used to prepare the primary gravimetric standards for the group 2 compounds. The standards for each group of compounds were analyzed as a set. Additional standards were prepared at different time intervals to determine the stability of these compounds in the aluminum gas cylinders over long periods of time. Preparation of Secondary Standards. A batch of nine treated aluminum gas cylinders containing four compounds, trichloromethane and trichloroethene (both at nominal concentrations of 500 pmol/mol) and tetrachloromethane and tetrachloroethene (both at nominal concentrations of 200 pmol/mol), was obtained from Scott Specialty Gases. A second batch of nine treated aluminum gas cylinders containing five compounds all at picomole/ mole levels (bromomethane at nominal 10 000, dichloromethane at 5000, 1,2-dichloroethane at 2000, l , l , l trichloroethane at 500, and 1,2-dibromoethaneat 200) was also obtained from the same source. Since this paper addresses the preparation and stability of halocarbons at
Table IV. Second-Order Polynomial Regression for Trichloromethane Gravimetric Standards.
Table I. Linear Regression for Tetrachloromethane Gravimetric Standards. standard no. ~138320 ~138311 ~138322 ~138313 ~138305 ~138309
peak area response
gravimetric concnb
predicted concnb
residual (ppt)
325.90 275.95 236.41 158.37 123.41 113.00
293.2 f 6.9 250.0 f 5.0 214.0 f 4.3 143.2 f 2.9 110.0 f 2.2 101.4 f 2.0
294.4 249.1 213.3 142.6 110.9 101.5
+1.2 -0.9 -0.7 -0.6 +0.9 +0.1
Average absolute difference = 0.7;y intercept = -0.3 slope = 0.906;standard error of estimate of y = 0.98. Concentration is in picomole/mole (ppt).
Table 11. Linear Regression for Trichloroethene Gravimetric Standards.
standard no. ~138320 ~138322 ~138313 ~138305
peak area response
gravimetric concnb
predicted concnb
residual (ppt)
~138320 ~138322 ~138309 ~138313
866.48 638.63 301.54 200.68
766.1 f 15.3 564.2 f 11.3 263.3 f 5.3 177.6 f 3.6
765.9 564.0 265.3 175.9
-0.2 -0.2 +2.0 -1.7
Average absolute difference = 1.0;y intercept = -1.9;slope = 0.886;standard error of estimate of y = 1.86. Concentration is in picomole/mole (ppt).
gravimetric concnb
predicted concnb
residual (ppt)
647.58 481.40 152.39 95.01
817.2f 16.3 595.4f 11.9 186.8f 3.7 116.6 f 2.3
816.9 596.0 185.5 117.6
-0.3 +0.6 -1.3 +1.0
a Average absolute difference = 0.8; y intercept = 7.5; slope = 1.143;standard error of estimate of y = 1.75. Concentration is in picomole/mole (ppt).
Table V. Second-Order Polynomial Regression for l,l,l-Trichloroethane Gravimetric Standards. standard no.
standard no.
peak area response
~138321 ~138319 ~138308 ~138317
peak height response
gravimetric concnb
predicted concnb
residual (ppt)
58.79 41.01 25.46 22.25
906.4 f 18.1 608.4 f 12.2 366.1 f 7.3 315.9 f 6.3
906.3 608.7 365.1 316.7
-0.1 +0.3 -1.0 +0.8
Average absolute difference = 0.6;y intercept = -0.5; slope = 13.543;standard error of estimate of y = 1.35. Concentration is in picomole/mole (ppt). (I
Table VI. Second-Order Polynomial Regression for 1,2-D)ibromomethaneGravimetric Standards. Table 111. Linear Regression for Tetrachloroethene Gravimetric Standards. standard no. ~138320 ~138313 ~138322 ~138309 ~138305
standard no.
peak area response
gravimetric concnb
predicted concnb
residual (ppt)
690.98 513.97 505.04 230.19 228.97
320.1 f 6.4 239.9 & 4.8 234.3 & 4.7 112.4f 2.2 106.3 f 2.1
320.0 239.2 235.1 109.6 109.0
-0.1 -0.7 +0.8
-2.8 +2.7
a Average absolute difference = 1.8;y intercept = 4.5;slope = 0.457;standard error of estimate of y = 2.35. Concentration is in picomole/mole (ppt).
the 100-500 pmol/mol level, attention will be focused only on those standards prepared within this range.
Results and Discussion When using adilution technique to prepare agas mixture or standard at a particular concentration from one at a higher concentration, there is concern that the organic compounds might be adsorbed on the cylinder walls in the dilution process. Previous studies (12)have shown that, within analytical precision, this problem does not occur with the organic compounds used in these mixtures or with many other organic compounds. Although there is no published data, it is believed throughout the community that, at a minimum, aluminum cylinders must be as dry as possible. Any other added treatments that enhance the stability of organic compounds in aluminum cylinders are a plus. There are other passivation processes besides Aculife which are available including Spectra Seal, a procedure developed by Airco Specialty Gases. The authors have also used Spectra Seal-treated cylinders for organic gas mixtures and have had good stability results. Untreated cylinders have been used to prepare nanomole/ mole (ppb)mixtures, but in several instances compounds, in particular carbon tetrachloride and l,l,l-trichloroethane, have completely disappeared within several days.
~138321 ~138319 ~138308 ~138305
peakheight response
gravimetric concnb
predicted concnb
residual (ppt)
44.37 29.63 18.47 11.65
419.4f 8.4 281.6 f 5.6 169.4f 3.4 100.2& 2.0
419.5 281.1 170.2 99.8
+0.1 +0.5 +0.8
-0.4
Average absolute difference = 0.5; y intercept = -24.9;slope = 10.950;standard error of estimate of y = 1.02. Concentration is in picomole/mole (ppt).
Intercomparison of Primary Gravimetric Standards. The ability to prepare internally consistent and accurate standards at the 5-15 nmol/mol range from a set of primary master standards had been determined in previous research (12). Therefore, even though this was our first attempt to prepare standards of these compounds at the picomole/mole level using this dilution technique, nosignificant problems were anticipated. The uncertainty of the concentration for each compound in the primary gravimetric standards was determined from the following variables: (1)weighing imprecisions, (2) uncertainty in the purity of the organic reagents and the nitrogen matrix gas, (3) estimated uncertainty in corrections made for weight loss during sealing of the capillary tubes, (4) displacement of air by the organic liquid and vapor in the capillary tubes used to make the original or “master” primary standards, and (5) cumulative weighing imprecisions for each successive dilution made to obtain the final picomole/mole gravimetric standards. Considering these factors, the total uncertainty in preparing the picomole/mole standards was estimated to be f2.0% (95% confidence interval) for each of the organic compounds. It should be noted that half of this uncertainty is due to determining the amount of the organic compounds present in the “pure” nitrogen balance gas, which contained trace levels of a number of the compounds. The primary gravimetric standards at the hundreds of pmol/mol level for the group 1 compounds were interEnviron. Sci. Technol., Vol. 27, No. 13, 1993
2851
~~~~~
~
Table VII. Stability of Tetrachloromethane in Gravimetric Standards standard no. ~138320 ~138317 ~138311 ~138322 ~138313 ~138305 ~138309
gravimetric concna 293.2 f 6.9 266.8 f 5.3 250.0 f 5.0 214.0 f 4.3 143.2 & 2.9 110.0 i 2.2 101.4 f 2.0 y intercept
294.4
294.4 264.0 250.9 214.8 143.2 109.8 101.6 -0.5 0.941 1.48
nmb
slope std error of y estimate a
predicted concnn Apr 1988 Oct 1990
249.1 213.3 142.6 110.9 101.5 -0.9 0.906 0.98
Concentration is in picomole/mole (ppt). Not measured.
Table VIII. Stability of l,l,l-Trichloroethane in Gravimetric Standards standard no. ~138321 ~138319 ~138308 ~138317 ~138313 ~138305
gravimetric concnn 906.4 f 18.1 608.4 f 12.2 366.1 & 7.3 315.9 f 6.3 130.7 f 2.6 107.3 f 2.1 y intercept
slope std error of y estimate a
predicted concnn Aug 1988 Jan 1991 906.3 607.7 365.1 316.7
906.3 608.6 366.3
nmb nmb
126.8 111.0 -0.5 0.941 1.48
-0.5 13.543 1.35
nmb
Concentration is in picomole/mole (put). Not measured.
Table IX. Measured Concentrations of Compounds in Group 1 Samples
sample no. 1BSb 2 3 4 5 6 7 8 9
trichloromethane 455 487 490 497 527 464 503 498 490 490 21 4.3
concentrationn tetrachloro- trichloromethane ethene 190 203 206 207 218 190 209 198 202 203 9 4.4
458 489 496 503 526 458 506 493 487 491 22 4.5
tetrachloroethene 190 205 203 231 217 190 210 202 201 205 13 6.3
mean std dev % stddev 0 Concentration is in picomole/mole (ppt). Batch standard.
compared using the GC/ECD under appropriate conditions described in the Experimental Section. The data were plotted as GC peak area response versus gravimetric concentration for each compound and were fitted using linear regression. The predicted concentrations for each of the standards were then calculated from the regression, using their respective GC response. The results of this intercomparison for tetrachloromethane are given in Table I. The differences between the gravimetric and predicted concentrations ranged from 0.1 to 1.2 pmol/mol, with the average of the residuals being 0.7 pmol/mol. These data show excellent agreement between the standards, with the average residual being well within the 2 % uncertainty on an individual gravimetric standard. The correlation coefficient for this regression was 0.99988 (slope = 0.9059; y intercept = -0.9; standard error of estimate ofy = 0.9835). 2852
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Tables 11-IV show the results for trichloroethene and tetrachloroethene, both using linear regression, and trichloromethane, using a quadratic regression, to obtain the best fit to the data. The data demonstrate excellent agreement between the standards for each compound with average absolute residuals of less than 2 pmol/mol. (All the standards may not have the four compounds in them or may not have been used because the concentration may not have been the range of interest.) The averageresiduals from the regression for each compound were well within the 2 % uncertainty estimated from the method of preparation. The standards for the group 2 compounds were also intercompared using GC/ECD under the conditions described earlier. The data were plotted as GC peak height response versus gravimetric concentration for each compound (peak height provided better precision, probably due to the use of the packed column). Table V shows the data for l,l,l-trichloroethane (quadratic regression resulted in the best fit of the data). The differences between the measured and gravimetric concentrations ranged from 0.1 to 1.0 pmol/mol, with an average residual of 0.6 pmol/ mol, which was within the 2% uncertainty of the gravimetric concentrations. The correlation coefficient for this regression was 0.99999 (slope = 13.542;y intercept = -0.5; standard error of estimate of y = 1.35). Table VI shows the results for 1,2-dibromoethane (quadratic regression). The data show the same type of results as for previous compounds. The average residual of 0.5 pmol/mol is again well within the uncertainty of the gravimetric standards. Similar results were obtained for the remaining three compounds in this mixture, showing excellent agreement between the standards. These results show our ability to accurately prepare a set of primary gravimetric gas standards of halogenated organic compounds at the picomole/mole level using the microgravimetric and dilution techniques. However, it is also important to know the long-term (Le., multiyear) stability of these compounds in gas mixtures contained in properly treated aluminum cylinders. Such standards could provide reliable baseline measurements of urban, rural, or other air environments and could be used to assure the continuity of and reliability of long-term data records. Therefore, these mixtures were checked over extended periods of time using the same analytical conditions. Table VI1 shows stability data for tetrachloromethane in group 1. The measured concentrations agree well between analyzed dates. Another standard was added to the set for the October 1990 analysis and it fit well, showing that the compound had remained stable in the original group of standards. The regression data for each analytical date are given and show that they intercept, slope, and standard error of the estimate of y agree well. These data show that tetrachloromethane has remained stable in these primary gravimetric gas standards within the limits of uncertainty for over 2 years. The other compounds in this group 1mixture have also demonstrated stability for over 2 years. Stability data are shown in Table VI11 for l , l , l trichloroethane in the group 2 mixture. There is good agreement between the August 1988 and January 1991 data for each standard. Standard no. 4 (X138317) was not included in the January 1991 analysis due to very low gas pressure in the cylinder, but two other standards were added. The difference in the slope is due to the fact that
Table X. Measured Concentrations of Compounds in Group 2 Samples concentrationsa sampleno. 1BSb 2
3 4 5 6 7 8 9 mean std dev % std dev (1
bromomethane
dichloromethane
1,2-dichloroethane
10200 10900 10910 10930 10830 11230 11140 10930 10760 10870 290 2.7
5300 5280 5290 5350 5240 5370 5350 5350 5420 5330 55 1.0
1890 1930 2040 2070 1870 2100 2120 2100 2070 2020 97 4.8
l,l,l-trichloroethane 500 504 504 509 505 513 513 506 502 506 5 1.0
l,2-dibromoethane 176 192 213 215 184 202 226 210 198 202 16 7.9
Concentration is in picomole/mole (ppt). b Batch standard.
a different sample size was used for the January 1991 analysis, which resulted in a different peak response. These data show that l,l,l-trichloroethane has remained stable in the primary gravimetric gas standards for over 2 years. The other compounds in this group also demonstrated stability for over 2 years. Certification of CommerciallyPrepared Secondary Standards. The two batches of cylinders representing the group 1and 2 compounds were received from a specialty gas vendor. A cylinder (no. 1) from the group 1 lot was chosen to serve as the “batch standard”. This batch standard at the picomole/mole level was compared with the NIST primary gravimetric standards, and the concentration for each organic compound in this commercially prepared mixture was determined from the regression of the NIST gravimetric standards concentrations. The analyses were performed on several different days according to a statistical plan. All of the other commercial cylinder mixtures in the batch were intercompared to the batch standard. The GC responses for the samples were normalized to the batch standard for each compound. After all analyses were completed and concentrations calculated, the data were tabulated for each compound. Table IX shows the results for the concentration of each of the organic compounds in the batch standard and each sample in the batch. The relative standard deviation for an individual compound for all the samples analyzed for all compounds studied ranged from 4.3 to 6.3%. This degree of homogeneity for such low concentrations is considered to be very good by NIST. A cylinder (no. 1)was selected from group 2 to serve as this group’s batch standard. The same procedure used for the group 1 batch was applied to the group 2 batch. After all analyses were completed and concentrations calculated, the data were tabulated. Table X shows the results for the samples in this batch. The relative standard deviation for an individual compound for all samples analyzed ranged from 1.0 to 7.9% for all compounds studied. These results are also considered to be very good considering the concentration level. The batch standards were retained at NIST for future reference and stability study. These batch standards have been analyzed against primary gravimetric standards since the initial study. Table XI shows data that demonstrate the stability of these compounds for both group 1 and group 2. The concentrations have not changed within the stated uncertainty in these secondary gas standards for over 2 years.
Table XI. Stability of Batch Standards for Groups 1 and 2
compound
Group 1: Sample 1 (BS)” measured concnsb Apr 1988 Jan 1991 total uncertainty
trichloromethane tetrachloromethane trichloroethylene tetrachloroethylene
455 190 458 190
457 188 462 188
130 110 120 120
Group 2: Sample 1 (BS)“ measured concnsb Aug 1988 Jan 1991
compound l,l,l-trichloroethane 1,2-dibromoethane 1,2-dichloroethane bromomethane dichloromethane a
500 176 1890 10200 5300
498 188 1890 9995 5270
total uncertainty 120 130 1300 11000 1600
Batch standard. b Concentration is in picomole/mole (ppt).
Table XII. Certified Concentrations of Compounds in Group 1 Samples
sample no.
trichloromethane
certified concentration” tetrachloro- trichloro- tetrachloromethane ethene ethene
2 490 200 490 3 490 210 500 4 500 210 500 5 530 220 530 6 460 190 460 7 500 210 510 8 500 200 490 9 490 200 490 uncertainty 130 110 120 Concentration is in picomole/mole (ppt).
210 200 230 220 190 210 200 200 *20
(1
Certified Concentrations and Total Uncertainty of Secondary Standards. Although the concentrations for an individual compound agreed well between the samples in the batch for bothgroup 1andgroup 2, it was determined that samples within the batch were not identical and that a smaller uncertainty could be assigned if each sample was individually certified. Tables XI1 and XI11 show the certified concentrations and total uncertainties (955% confidence) for groups 1 and 2, respectively. The total uncertainty was calculated by doubling the quadrature summation of (a) the uncertainty in the primary graviEnvlron. Sd. Technol., Vol. 27, No. 13, 1993 2853
Table XIII. Certified Concentrations of Compounds in Group 2 Samples sample no.
bromomethane
dichloromethane
11000 11000 11000 11000 11000 11OOO 11000 11000
5300 5300 5400 5200 5400 5400 5400 5400
uncertainty = flOOO " Concentration is in picomole/mole (ppt).
*600
metric standards, (b) the imprecision in intercomparing the batch standard with the primary gravimetric standards, and (c) the imprecision in intercomparing the batch standard with the eight cylinder mixtures in the batch. The resulting total uncertainties ranged from 4 to 17 % . Conclusions The results of this study show that primary gravimetric gas standards of halogenated organic compounds can be precisely and accurately prepared at the picomole/mole (ppt) range with uncertainties of less than 2% (95% confidence interval). Although the uncertainty in the gravimetric standards is low, it is felt that lower uncertainties can be achieved. This can be accomplished by improving the determination of the trace impurities in the matrix gas by cryogenic preconcentration of the matrix gas sample prior to GC analysis for improved sensitivity or by scrubbing the organic impurities out of the matrix gas. The success in preparing accurate and stable multicomponent primary gravimetric gas standards of halogenated organic compounds provided the basis for the development and certification of secondary gas standards that were commercially prepared. The estimated total uncertainty range of 10-30 pmol/mol for the individual components in the secondary standards is acceptable, considering these low concentration levels. This technique can be used to develop future NIST Standard Reference Materials (SRMs) of these compounds at the picomole/ mole level. The results obtained in these studies indicate that it would be technically feasible to develop such standards for mixtures at the low picomole/mole level. New SRMs of this type would more closely approximate levels in environments where these compounds are of concern to human health and the climate.
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certified concentrations" 1,2-dichloroethane l,l,l-trichloroethane 1900 2000 2100 1900 2100 2100 2100 2100 k300
500 500 510
510 510 510 510 500
*20
l,2-dibromoethane 190 210 220 180 200 230 210 200 f30
Acknowledgments The authors wish to acknowledge the California Air Resources Board for partial support of this work. Certain commercial equipment, instruments, or materials are identified in this paper in order to adequately specify the experimental procedure. Such identification does not imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose. Literature Cited (1) Edgerton, S. A.; Holdren, M. W.; Smith, D. L.; Shah, J. J. JAPCA 1989,39 (5))729-732. (2) Harkov, R.; Kebbekus, B.; Bozzelli, J. W.; Lioy, P., Daisey, J. Sci. Total Environ. 1984, 38, 259-214. (3) Ioffe, B. V.; Isidorov, V. A.; Zenkevich, I. G. J . Chromatogr. 1977, 142, 787-795. (4) Thornburn, S.; Colenutt, B. A. Int. J . Environ. Stud. 1979, 13 (4)) 265-271. ( 5 ) Louw, C. W.; Richards, J. F. S. Afr. J . Sei. 1977, 73 (8)) 240-245. (6) Ramanathan, V.; Cicerone, R. J.; Singh, H. B.; Kiehl, J. T. J . Geophys. Res. 1985, D90,5547-5566. (7) Noij, T.; Fabian, P.; Borchers, R.; Cramers, C.; Rijks, J. Chrornatographia 1988,26, 149-156. (8) Rasmussen, R. A.; Lovelock, J. E. J . Geophys. Res. 1983, 88, 8369-8378. (9) Oliver, K. D.; Pleil, J. D.; McClenny, W. A. Atmos. Environ. 1986,20 (7)) 1403-1411. (10) Schmidt, W. P.; Rook, H. L. Anal. Chem. 1983,55,290-294. (11) Rhoderick, G. C.; Zielinski,W. L., Jr. Anal. Chem. 1988,60, 2454-2466. (12) Rhoderick, G. C. Fresenius J . Anal. Chem. 1991,341,524531.
Received for review March 25, 1993. Revised manuscript received May 17, 1993. Accepted August 18, 1993." Abstract published in Advance ACS Abstracts, October 1,1993.
