Anal. Chem. 1988, 60, 2454-2460
2454
LITERATURE CITED (1) Mottola, H. A.; Mark, H. B., Jr. Anal. Chem. 1988, 6 0 , 264R-279R.
(2) Mottola, H. A. Kinetics Aspects of Analytical Chemistry; Wlley-Interscience: New York, 1988. (3) Wentzell, P. D.; Crouch, S. R. Anal. Chem. 1986, 58, 2855-2858. (4) Carr, P. W.; Bowers, L. D. Immobillzed Enzymes in Analytlcal and Clinical Chemistry; Wlley: New York, 1980. Carr, P. W. Anal. Chem. 1978, 50, 1602-1607. Mieling, G. E.; Pardue, H. L.; Thompson, J. E.; Smith, R. A. Clin. Chem. (Winston-Salem, N . C . ) 1979, 25(9). 1581-1590. Holler, F. J.; Calhoun. R. K.; McClanahan, S. F. Anal. Chem. 1982. 54, 755-761. Mieling, G. E.; Pardue, H. L. Anal. Chem. 1978, 5 0 , 1611-1618. Engh, S.A.; Holler, F. J. Anal. Chem. 1988, 6 0 . 545-548. Calhoun, R. K.; Holler, F. J. Anal. Chem. 1888, 6 0 , 549-552. Wentzell, P. D.; Crouch, S. R. Anal. Chem. 1988, 58, 2851-2855. Wentzell, P. D.;Crouch, S. R. 13th FACCS Meeting, St. Louis, MO, Oct 3, 1986.
Corcoran, C. A.; Rutan, S. C. Anal. Chem. 1988, 6 0 , 1146-1153. Brown, S. D. Anal. Chlm. Acta 1988, 181, 1-26. Rutan. S. C. J. Chemom. 1987. 1 . 7-18. Kamlnski, P. G.; Bryson, A. E.,Jr.; Schmidt, S. F. I€€€ Trans. Autom. Control 1971, AC-16, 727-735. (17) Gerow, D. D.; Rutan, S. C. Anal. Chim. Acta 1988, 184, 53-64. (18) Fitzpatrick, C. F.; Skoug, J. W.; Weiser, W. E.; Pardue, H. L.; Rutan, S. C., submitted for publication in Anal. Chim. Acta, 1988.
(13) (14) (15) (16)
RECEIVED for review May 6,1988. Accepted August 11,1988. This research was supported in part by the US. Department of Energy (Grant DE-FG05-88ER13833). This support does not constitute an endorsement by the Department of Energy of the views expressed in this article.
Preparation of Accurate Multicomponent Gas Standards of Volatile Toxic Organic Compounds in the Low-Parts-per-Billion Range George C. Rhoderick* and Walter L. Zielinski, Jr. Gas and Particulate Science Division, Center for Analytical Chemistry, National Bureau of Standards, Gaithersburg, Maryland 20899
Methodology Is descrlbed for the mlctogravlmetrk preparatlon and analytical evaiuatlon of accurate, stable multicomponent gas standards In compressed gas cyilnders containing volatile toxic organlc compounds in pure nitrogen at the mid- to iowparts-per-billion (ppb) level. Standard mixtures have been prepared Containing up to nine organic compounds at concentratlons ranging from 1 to 1000 ppb by mole. Current Indications are that the number of organic compounds In a slngle mixture is more limited by analytical capablilty than by the preparation methodology. Over 100 standards, of which several will be discussed in this paper, have been prepared and evaluated for long-term stability and internal consistency. Over 25 different volatile organk compounds spanning three concentration decades have been studled. The sum of preparative and analytical error components of the uncertainty associated with the concentratlon of the organic analytes at the 95% confidence level typically ranges from 3 to 10% relative, depending upon the compound and Its concentration. I ntercomparative analyses of new and previously prepared standards have verifled that such mixtures are stable for several years.
Federal and state environmental monitoring programs aimed at quantifying ambient air levels of volatile toxic organic compounds have been significantly expanded over the last several years. These programs are in response to growing concerns of public exposure to these compounds from industrial emissions, solvent usage, hazardous waste dumpsites, and hazardous waste incineration. The human carcinogenic potential of a number of these compounds has been documented (I). Some of the less volatile aromatic and chlorinated hydrocarbons have been implicated in liver cancer in fish (2). The U S . Environmental Protection Agency has carried out numerous health risk assessments to identify those volatile organic compounds representing the greatest public health
risk (3). At the same time, substantial activities at both the federal and state levels are being focused on formulation of regulatory standards aimed at reducing human exposures. Clearly, reliable measurements of the most hazardous toxic organic compounds in ambient air require the availability of accurate, stable, multicomponent gaseous standards. Recent analytical research has focused on techniques for the preconcentration of air samples containing parts-perbillion levels of volatile organics by using sorbents or cryogenic temperatures. Analyses of the enriched organics have generally been conducted by capillary column gas chromatography coupled to flame-ionization and electron-capture detectors ( 4 ) or a mass spectrometer (5, 6). A variety of methods have been used for the preparation of gaseous calibration standards containing volatile toxic organics. Vejrosta and Novak (7) described a dynamic system for producing parts-per-billion levels of such compounds by two-stage dilution of a pure gas stream saturated with the compound of interest. Other workers reported the use of permeation tubes with gravimetricaly calibrated permeation rates to produce accurate concentrations of single organic species (8, 9). In contrast to dynamic systems for producing calibration standards of volatile toxic organic compounds, a considerable technical effort has been undertaken over the past six years at the National Bureau of Standards (NBS) to develop stable trace level standards of these species in a pure matrix gas in compressed gas cylinders. Several of these have been issued as Standard Reference Materials (SRMs) containing either one (8)or four organic compounds at concentrations of 10 ppm and 250 ppb, by mole. More recently, NBS modified the methodology used for the certification of these SRM's to extend the concentration range to the low-parts-per-billion level for mixtures containing up to five organic compounds (10). The present paper describes research that led to the preparation of standard mixtures containing as many as nine organic compounds at concentrations ranging down to 1ppb.
This article not subject to U S . Copyright. Published 1988 by the American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 60, NO. 22, NOVEMBER 15, 1988
The preparation of a variety of such standards covering over 25 organic compounds and assessments of the preparative accuracy and stability of such mixtures are also described.
EXPERIMENTAL SECTION Apparatus. Preweighed glass capillary tubes, sealed on one end, were used to prepare samples of the pure organic compound. The tubes were sealed after addition of the organic and weighed again on the same ultramicrobalance. The balance has a mechanical tare capacity of up to 2.99 g, an electrical weighing range of 15 mg, and a readability of 0.1 pg. The cylinders used for the standards were weighed before and after the transfer of the organic material and after the addition of the nitrogen balance gas. The cylinders were weighed on a top-loading balance having a capacity of 52 kg with a readability to 1 g. Both of these balances were calibrated with NBS traceable weights. Analyses of the gravimetrically prepared organic gas standards were carried out by use of a gas chromatograph equipped with both electron capture and flame-ionization detectors. Commercially available packed stainless steel and wide-bore borosilicate capillary columns were used to obtain base-line separations of the compounds. Samples were injected onto the column by means of a stainless steel, six-port automatic gas sampling valve equipped with a stainless steel sample loop. Purity analyses of the organic compounds were carried out with a gas chromatograph containing a fused silica capillary column. The column was coupled to a low-resolution mass spectrometer equipped with an interactive data system. Reagents. The organic compounds were obtained from commercial suppliers a t a specified purity of 99.9%. The ultrahigh-purity nitrogen (99.999%) used as the balance gas was obtained from a commercial source. It was qualitatively and quantitatively analyzed for any organic compounds that might be present at trace levels. Gas Cylinders. New aluminum gas cylinders having an internal volume of 30 L and equipped with CGA-350 brass valves were used for the standards. The cylinders were precleaned by a commercial supplier in a manner that excluded contamination with trace hydrocarbons and halocarbons and then treated to deactivate the internal walls. To extract a sample from the cylinder for analysis, a stainless steel manual control valve or a low dead volume regulator was connected to the cylinder. A 3.2 mm 0.d. copper line was connected from the control valve to the gas sampling valve of the gas chromatograph. Gravimetric Procedure for Preparing Gas Standards. The procedure used to directly prepare the low-part-per-billion level gas standards involves the introduction of a pure organic liquid into a preweighed, thin-walled glass capillary tube (ca. 1.6 mm 0.d. by ca. 2.0 cm in length) sealed a t one end. The open end is submerged in a vial containing the liquid organic. A plastic syringe is attached to the vial. The syringe plunger is withdrawn slightly, which creates a slight vacuum in the capillary tube. When the plunger is released the organic liquid enters the capillary tube. The tube is then placed in a centrifuge to transfer the organic liquid to the sealed end. The open end of the capillary is then heat sealed. Once the capillary has equilibrated to room temperature, it is weighed to determine the weight of organic material present. An evacuated, preweighed cylinder is fitted with the appropriate CGA-350 fitting to which a piece of tubing, made from fluorinated ethylene-propylene copolymer, is attached. A capillary tube containing an organic compound is inserted into the tubing. The diameter of the tubing is such that the capillary fits tightly and creates a seal. The cylinder valve is opened slightly and the capillary tube is broken a t the inlet end. Heating the capillary tube with a hot-air gun aids in the
2455
Table I. Organic Compounds Used to Prepare Parts-per-Billion Standards Group 1 chloroform carbon tetrachloride tetrachloroethylene benzene vinyl chloride monomer Group 2 acetonitrile methyl ethyl ketone trichlorofluoromethanea l,l,l-trichloroethane trichloroethylene 1,2-dichloroethane 1,2-dibromoethane dichlorodifluoromethaneb methyl bromide
Group 3
1,1,2-trichloro-1,2,2-triflu~roethane~ 1,2-dichloro-1,1,2,2-tetrafl~oroethane~ vinylidene chloride to1u en e chlorobenzene acetone 1,4-dioxane pyridine Group 4 1,3-butadiene ethylene oxide propylene oxide methylene chloride acrylonitrile o-xylene
a Chlorofluorocarbon (CFC) compound, CFC-11. Chlorofluorocarbon (CFC) compound, CFC-12. Chlorofluorocarbon (CFC) compound, CFC-113. Chlorofluorocarbon (CFC) compound, CFC-114.
vaporization and transfer of the organic material into the cylinder. When it appears that all the material has vaporized, the other end of the capillary is broken and the tube is flushed with high-purity nitrogen. This ensures that any residual material is transferred into the cylinder and then the cylinder valve is closed. This procedure is repeated for any additional organic compounds to be added to the same mixture. In a few cases where the organic compound is a gas a t room temperature, the same procedure as described above is used, except that it is necessary to condense the material in a test tube packed in dry ice. It is then transferred to a preweighed capillary tube in the chilled, liquefied state. Finally, ultrahigh-purity nitrogen is added to the cylinder to a precalculated pressure and the cylinder is then weighed. The concentrations of the organic compounds are calculated from their weights and the weight of the nitrogen, and expressed on a molar basis. This procedure represents a modification of the original procedure developed at NBS to prepare parts-per-million-level standards (8). The precision for producing a five-component standard over a range of 4-150 ppb has been described earlier (10).
Caution. Since these organic compounds are toxic and suspected carcinogens, all procedures involving the use of the pure reagents were performed in an exhaust hood. Before the cylinders were pressurized with high-purity nitrogen, all manifold fittings were checked for leaks. Safety glasses were worn a t all times. RESULTS AND DISCUSSION Over 100 standards have been prepared by the microgravimetric technique. These standards cover 27 organic compounds (Table I) in mixtures containing from 1 to 9 compounds. The reliability of the preparation procedure was assessed by evaluation of four principal factors: (1)preparative accuracy; (2) the total uncertainty that must be ascribed to the gravimetrically calculated concentrations; (3) the analytical consistency among standards prepared a t similar concentrations; and (4) the long-term stability of the concentrations of the organic analytes. Preparative Accuracy. As noted above, the gravimetric procedure described in this paper for preparing parts-perbillion-level standards is a modification of the procedure reported earlier for preparing parts-per-million-level standards
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ANALYTICAL CHEMISTRY, VOL. 60, NO. 22, NOVEMBER 15, 1988
(8). A major modification made in the method was a &fold reduction in the length of the capillary tube, from 100 to ca. 20 mm, used to prepare organics. The preparation of a 10 ppb standard requires the use of only about 1% of the amount of organic used in the original method (8) for preparing a 1 ppm standard. This modification significantly reduced the correction for the amount of air displaced by the organic liquid and its vapor. This correction is trivial in the preparation of parts-per-million-level standards but can be as much as 10% for standards approaching 1ppb. A second modification was to use cylinders with an internal volume of 30 L instead of 6 L, thereby increasing the amount of organic material that could be used by a factor of 5 . To ensure accuracy, any potential sources of bias that might affect the calculated concentrations must be identified and eliminated by appropriate corrections. (a) Calibrated balances and NBS weights were used throughout the procedure. Replicate weighings were used to ensure the accuracy of the values used to calculate the concentrations. (b) The stock organic compounds were analyzed by capillary gas chromatography/mass spectrometry to qualify and quantify impurities. (c) Organic compounds present in the high-purity nitrogen used to prepare the standards were quantified by use of a chromatograph equipped with flame-ionization and electron-capture detectors. (d) The cylinders, cylinder valves, and transfer lines were checked in the same manner by preparing and analyzing “blank” cylinders using the analyzed nitrogen. (e) A small correction was made due to the slight evaporation of glass during the heat sealing of the capillary tubes. The amount of this correction was determined by repetitively preparing “blank” tubes. (f) A correction was made for the weight of air displaced in the capillary tube by the organic liquid and vapor. This was calculated by first determining the volume of the capillary tube. To do this, a series of tubes were prepared, weighed, and then filled with water. The open ends of the tubes were sealed and then weighed. The weight of the water was then converted from grams to microliters by using the density of water. The weight of the empty tube was linearly plotted against the volume of the water to obtain a standard curve. The volume of any tube could then be determined by using the weight of the empty tube to estimate the volume from the calibration line. By use of the volume of the capillary tube, the weight of the organic in the tube, its vapor pressure, and density, the amount of air displaced by the organic compound is calculated. Excellent quantitative transfer of the organic compounds from the sealed capillary tubes to the evacuated cylinders was demonstrated by linear regression plots from multiple standards at different concentrations. The analytical data from sets of standards (gravimetric concentration vs. peak area) showed a high level of internal consistency. Total Uncertainty. The total uncertainty of the preparatory technique was obtained by summing, in quadrature, (a) the sources of error due to gravimetric preparation and (b) the imprecision of analysis. (1) Preparative Uncertainty. The total error in the gravimetric method was estimated by summing, in quadrature, the various sources of potential inaccuracy and weighing imprecisions associated with each organic in the standard. Impurities in the pure organic reagents were determined and in most cases the uncertainty in their absolute purity was estimated not to exceed 0.1% relative. The nitrogen balance gas was analyzed for trace organics by both flame-ionization and electron-capture detectors. This resulted in uncertainties on the order of 1.5% for nonhalogenated compounds and 0.3% for halogenated species a t the 5 ppb level. The total imprecision in the weighing of the capillary tubes (S), was calculated from the relative standard deviation of the means of repeated
weighings of empty (s,) and filled (sf)capillary tubes. This imprecision ranges from 0.4% to 1.0% relative depending on the compound. The microgravimetric method described in this work for preparing parts-per-billion standards employs two modifications of the earlier method (8). To illustrate the effect of these changes on weighing error, consider the preparation of a 10 ppb standard of benzene. With a 100-mm capillary and a 6-L cylinder, the amount of benzene required is about 0.021 mg with a weighing uncertainty of 0.005 mg (25%). With a 20-mm capillary and 30-L cylinder, the amount of benzene needed is about 0.105 mg with a weighing uncertainty of 0.001 mg (1%). The final weights must also be corrected for the slight vaporization loss of glass during the heat-sealing process. With blank tubes, the vaporization loss was estimated to be in the order of 0.0013 f 0.0008 mg. This corresponds to 0.4-1% relative uncertainty in the weight of the organic compound. Therefore the weight must be corrected by adding this amount and the imprecision of this weight loss (sv) must be considered in the overall estimation of the weighing imprecision of the organic. The weight of the organic must also be corrected for the displacement of air by the organic liquid and vapor in the capillary tube. The magnitude of this correction depends on the specific organic compound in the tube and the amount. A typical correction would be +0.0015 with an imprecision (sa) of f0.0002 mg. The total imprecision due to weighing is calculated from the equation s, = [se2
+ sf“ + s,2 + s2]1/2
(1)
Organic compounds that are a gas at room temperature (e.g., methyl bromide, vinyl chloride, l,&butadiene) also have a similar error. These species must be liquefied in order to introduce them into a capillary tube. Since such tubes are at dry ice temperature and open to the atmosphere for a short time, more air is present due to the pressure differential. This results in more weight due to the air than at room temperature and this weight must be substracted. This negative weight correction is estimated to be 0.095 f 0.020 mg using “blank” tubes. This consideration was not encountered with the prior method (a), which only addressed organics that are liquids a t room temperature. The error sources discussed above were expressed as a relative percent a t one standard deviation. The total uncertainty associated with gravimetric preparation (sprep)was determined by summing, in quadrature, these errors using the generalized relationship Sprep
= [a2
+ b2 + ... + n23112
(2)
In practice, theoretical preparative uncertainties have ranged from about 0.5 to 3.5% for organics a t the 10 ppb level depending on the organic compound in question. (2) Analytical Imprecision. Replicate injections of a single sample were performed during GC analysis of the standards. The analytical imprecision (s,J was determined from the standard deviation of the mean of the replicate peak areas. The analytical imprecision (lo) often has been found to be greater than the preparative uncertainty, representing a slightly higher contribution to the total uncertainty assigned to the concentrations of the organic analytes in low-partsper-billion standards. Analytical imprecisions have varied from about 0.2 to 15% relative at the 1-15 ppb level depending on the organic compound in question and the sensitivity of detection. The total uncertainty of assigned Concentrations a t the 95% confidence (2a) level was estimated for each organic compound from the equation ubtal
=
2[S2prep
+~~ana1.1~’~
(3)
Data will be provided below to illustrate the total uncertainty
ANALYTICAL CHEMISTRY, VOL. 60, NO. 22, NOVEMBER 15, 1988
lo
E
/”
Figure 1. Linear consistency of microgravimetric standards of benzene ranging from 1 to 10 ppb by mole.
Table 11. Linearity and Stability of Gravimetric Benzene Standards date
concentration. m b bv mole cylinder no. gravimetric measured difference”
4/84 5/84 3/86 3/86 3/86 5/87 8/83 3/83 6/86
AAL-12028 AAL-12008 AAL-15760 AAL-8935 AAL-14276 AAL-19256 AAL-7009 AAL-6554 AAL-17539
~
0.93 2.16 2.99 5.06 7.10 7.21 7.60 9.02 10.98
aAbsolutemean of the differences:
0.85 2.18 2.98 5.14 7.08 7.26 7.80 8.77 11.00 0.08 f 0.09
Table 111. Long-Term Stability of Five-Component Standarda
-0.08 +0.02 -0.01 +0.08 -0.02
+0.05 +0.20 -0.25 +0.02
concentration, ppb by mole cc14 C&l4 C6H6
analysis date
CHC1,
4/83b
3.7
8.2
12.4
9.0
6.5
4/84 7/84 10184 5/85 10185 2/86 10186 7/87
3.8 3.7 3.9 3.8
8.2 8.5 8.2 8.2
12.4 12.2 12.6 12.4
8.7 8.8
6.5 6.5
9.0 8.8
6.4 6.5
3.7
8.0
12.6
Mean std dev
Concentration (PPB)
2457
a
12.7
C7HB
8.9 8.8
3.8
8.2
12.5
8.9
6.5
fO.08
f0.16
f0.17
fO.ll
f0.04
Prepared April
1983.
Compounds: CHCl, = chloroform; CC14
= carbon tetrachloride; c2cl, = tetrachloroethylene; C6H6= benz-
ene; C7H8= toluene. *Gravimetricvalues. All other values are measured values, obtained by analysis against primary gravimetric standards. Table IV. Response Factors for Vinylidene Chloride in Prepared Standards cylinder no.
grav concn, ppb by mole
peak area
response factor
CC-38396 AAL-14277 CC-38391 AAL-14281 A A L - 15749 AAL-15747
154.8 136.3 44.08 23.01 20.77 9.20
2164 3092 1000 519 463 207
14.0 22.7 22.7 22.6 22.3 22.5
ppb.
of concentrations of organic compounds in several different types of multicomponent standards. Preparative Consistency. One measure of the reliability of any method for preparing standards is the degree with which a set of standards show agreement. Figure 1shows a linear regression of gas chromatographic peak height vs concentration for nine standards of benzene. Peak areas are generally used but in this case peak heights were more reproducible due to another compound in the standards that eluted on the very tail end of the benzene. The standards were prepared over a 4-year period and range in concentration from 1 to 11 ppb. The line fit resulted in a high linear correspondence (slope, 0,9999; intercept, 0.002; regression correlation coefficient ( r ) ,0.9993). Stability. Most of the organic compounds that have been evaluated have been found to exhibit long-term stability. To assess stability, a set of standards must be prepared and analytically intercompared by GC analysis. The set of standards is analyzed on several different days. The concentratons of the compounds in the standards are predicted from linear regression. These results give a base line of concentrations of each organic compound in each standard. At appropriate time intervals (3,6, or 12 months for example) new standards are prepared and introduced into the set. If the set continues to show good internal agreement, then stability has been demonstrated. Any consistent change in the concentration of an organic compound in a particular standard suggests instability with that standard. The stability of benzene standards prepared over a 4-year period is shown in Table 11. Table I11 shows the stability of low-parts-perbillion-level organic compounds in a five-component standard over a 3-year period. Most of the organic compounds listed in Table I have been found to exhibit similar stabilities. Notable exceptions include pyridine, ethylene and propylene oxides, and acrylonitrile.
Linear regression analyses of multiple standards of a given organic compound are useful for verifying the accuracy of newly prepared standards, as well as for assessing long-term stability. Another method that can be used to determine the same is the response factor (peak area divided by gravimetric concentration). To illustrate, Table IV shows response factors for a series of standards containing vinylidene chloride. The significant difference noted for the response factor for cylinder CC-38396 indicated that some vinylidene chloride may have been lost in its transfer to the cylinder. Hence, this method provides a quality control check on preparative accuracy. This is reflected by the fact that the constancy of the response factors for the remaining five standards illustrates the precision of the gravimetric procedure. Development of G r o u p 1-4 Standards. A variety of gravimetric standards have been prepared for EPA group 1-4 volatile organic compounds (Table I). These “groups” consist of organic compounds that are considered to be toxic and hazardous to the population and the environment. The compounds selected for each group were chosen by EPA with technical assistance from NBS. Development of these groups as standards proceeded a t about one group per year. A number of these standards have been provided to EPA a t nominal concentrations of 10 and 150 ppb to serve as quality assurance standards in its hazardous waste incineration audit program (3, 11). Original analyses of the five-component group 1 standards were performed with a GC containing a flame-ionization detector (FID) and a gas sampling valve equipped with a 10-mL gas sample loop. A 6 m by 3 mm stainless steel column packed with a 10% loading of poly(ethy1ene glycol) modified with nitroterephthalic acid on 100-120 mesh support was used at a column temperature of 80 “C. The analysis of a nominal 10 ppb standard is shown in Figure 2. Later analyses were done with an electron-capture detector (ECD) to measure the halocarbons. Table V demonstrates the long-term stability
2458
ANALYTICAL CHEMISTRY, VOL. 60, NO. 22, NOVEMBER 15, 1988
Table VI. Gravimetric and Measured Concentrations for a Typical Group 2 Standard
3
concentrations,”ppb by mole measured measured grav concn (8/84) (2/86)
compound 1
1
14.0 f 0.6 11.5 f 0.5 15.4 f 0.6 26.9 f 1.0 13.7 f 0.2 10.4 f 0.2 11.9 f 0.2 8.9 f 0.2 18.2 f 0.2
acetonitrile methyl ethyl ketone CFC-llb CFC-12‘ bromomethane l,l,l-trichloroethane trichloroethylene 1,2-dichloroethane 1,2-dibromoethane
,
0
a
4
12
Minutes
Figure 2. Isothermal gas chromatographic analysis of a five-component standard prepared at a nominal concentration of 10 ppb: (1) vinyl chloride, (2) carbon tetrachloride, (3)benzene, (4)chloroform, (5) tetrachloroethylene. Table V. Gravimetric and Measured Concentrations for a Typical Group 1 Standard
compound vinyl chloride chloroform carbon tetrachloride benzene tetrachloroethylene
concentration,”ppb by mole measured measured grav concn (11/1983) (9/1985) 16 f 1
10 f 1 19 f 1 17 f 1
23 f 1
15 f 1 9f1 19 f 1 17 f 1 22 f 1
16.1 f 1.0 9.6 f 0.2 19.3 f 1.0 16.8 f 0.5 22.8 f 0.2
‘Uncertainties following the concentrations are at the 95% confidence level. of a typical group 1 standard at the nominal 10 ppb concentration level. It also illustrates that equivalent results can be obtained by using either a FID (11/83 data) or an ECD (9/85 data) for the analysis of chloroform, carbon tetrachloride, and tetrachloroethylene. The ECD has a low sensitivity to benzene and vinyl chloride and these were not detected. The number of compounds that can be quantitatively transferred into an evacuated 30-L cylinder by the gravimetric procedure is limited by the total evacuated volume available. Considerable care was required to prepare group 2 standards containing nine compounds. It is estimated that 12 individual compounds are about the limit that can be reliably introduced into a 30-L cylinder. Alternative approaches for preparing mixtures exceeding this limit (e.g., cross-blending of several mixtures) are being investigated and will be described in a subsequent paper. A number of different columns were evaluated for the analysis of group 2 standards, with no one column found capable of separating all nine compounds. Consequently, three different sets of conditions were used. A 6 m by 3 mm stainless steel column packed with 60/80 mesh of ethylvinylbenzene-divinylbenzene copolymer and a FID were used to analyze the acetonitrile and methyl ethyl ketone. An ECD and two columns, a 6 m by 3 mm stainless steel packed with 1% poly(ethy1ene glycol) modified with nitroterephthalic acid on 60-80 mesh graphitized carbon black and a 4.6 m by 3 mm stainless steel packed with a loading of 20% methyl silicone on 80-100 mesh support, were used separately for the analysis of the halogenated species. It was found that the column packed with vinylbenzene-divinylbenzene copolymer could provide base-line separations for all nine compounds. However, optimum peak shapes for quantitation were not achieved and the analysis time was excessively long. Table VI shows preparative and analytical results for a typical group 2 standard. It illustrates the good agreement between gravimetric and measured concentrations for all except dichlorodifluoromethane (CFC-12). This suggests that some loss of the CFC-12 may have occurred in its transfer to the
14.1 f 2.1 11.4 f 2.3 14.7 f 0.8
8.9 f 1.0 13.7 f 0.3 10.4 f 0.2 12.0 f 0.4 8.9 f 0.4 18.6 f 0.5
14.6 f 2.1 12.4 f 2.3 15.4 f 1.0 8.8 f 1.0 13.6 f 0.5 10.2 f 0.5 11.6 f 0.5 8.7 f 0.5 18.8 f 1.0
‘Uncertainties following the concentrations are at the 95% confidence level. Trichlorofluoromethane. e Dichlorodifluoromethane. Table VII. Gravimetric and Measured Concentrations for a Typical Group 3 Standard concentration, ppb by mole grav concn measured concn
compound CFC-113‘ CFC-14‘ vinylidene chloride acetone
15.7 f 0.6 18.4 f 1.0 20.8 f 0.6 46.8 f 1.0 10.8 f 0.4 42.4 f 1.0 11.2 f 0.4
toluene
1,4-dioxane chlorobenzene
15.4 f 0.9 19.2 f 1.8 20.7 f 1.1 46.8 f 7.0 10.8 f 0.9 42.9 f 6.0 11.2 f 0.7
Uncertainties following the concentrations are at the 95% confidence level. 1,1,2-Trichloro-1,2,2-trifluoroethane. 1,2-Di-
‘
chloro-l,1,2,2-tetrafluoroethane. 5
0
, 4
8
12
1
,
16
Minutes
Flgwe 3. Gas chromatographic analysis of an eight-component group 3 standard illustrating peak tailing of polar compounds and loss of pyridine: (1) CFC-114, (2)CFC-113, (3)vinylidene chloride, (4)acetone, (5) toluene, (6) 1,4-dioxane, (7) pyridine, (8) chlorobenzene.
cylinder. A number of additional standards have been prepared in which quantitative transfer of CFC-12 was effected, as confirmed by intercomparative analyses. The preparation of the eight-component group 3 standards presented no unusual difficulties. However, two separate problems were encountered in analyses of these standards. The first was excessive peak tailing for acetone and 1,4-dioxane (Figure 3). This problem resulted in inconsistent peak integrations, poor detection capability, and a high analytical uncertainty (Table VII) in the nominal 10 ppb range. This problem was reduced by increasing the concentration of acetone and 1,4-dioxane in subsequent standards and by replacing the 10-mL stainless steel gas sampling loop with a 3-mL loop made of fluorinated ethylenepropylene copolymer. The second problem was associated with irreversible sorption of pyridine within the cylinder, leading to unpredictable analytical results. Pyridine was detected in one standard on the first sample injection immediately following its prepara-
ANALYTICAL CHEMISTRY, VOL. 60, NO. 22, NOVEMBER 15, 1988
Table VIII. Stability Analysis of a Typical Group 4 Standard measd concn,” ppb by mole component 1,3-butadiene ethylene oxide propylene oxide methylene chloride acrylonitrile o-xylene
11/86 11.8 f 17.2 f 14.7 & 15.8 f 12.7 f 14.6 f
4/87 11.8 f 1.0 15.2 f 3.0 15.2 f 3.0 15.8 f 1.5 12.3 f 0.8 14.6 f 1.5
1.0
3.0 2.9 1.5 0.8 1.5
Concentrations determined by analysis against primary gravimetric standards. Uncertainties following the concentrations are at the 95% confidence level. tion. However, it was not detected thereafter. In a number of other standards pyridine was not detected at all. Pyridine has been detected in some standards but in very few cases. This problem was attributed to the polarity of pyridine, which may reflect a limitation of the method for other polar compounds. The other compounds in this group showed excellent behavior and results. The data in Table VI1 were obtained by using FID analysis for all compounds. The ECD was also used to obtain data on the halocarbon compounds. The data in Table VI1 demonstrate the consistency of results obtained from different detectors. With the exception of pyridine, analyses over time indicated that the remaining seven compounds in group 3 are stable in gas mixture at low-partsper-billion levels. The six compounds designated by EPA for group 4 standards included one compound of questionable stability [ethylene oxide (12)],a structurally related compound having suspected instability (propylene oxide), a compound that readily forms peroxides in contact with air [1,3-butadiene (13)], and a compound that had exhibited instability in gas mixtures prepared several years ago at NBS (acrylonitrile). In addition, two of the compounds, 1,3-butadiene and ethylene oxide, are gases at room temperature and had to be chilled to their liquid state for preparation. Analytical results for a typical group 4 mixture are shown in Table VIII. Analyses of a number of these standards indicated that ethylene oxide exhibited instability. Anomalous behavior was observed for propylene oxide. Analyses of several standards either did not detect propylene oxide or showed a decrease in its response factor over several days. The decline in the response factor with time
was greatest for ethylene oxide. These results could also mean that there is a problem with the transfer of these compounds to a gas cylinder. The inconsistencies observed for ethylene and propylene oxide were consistent with prior expectations of their instability. A lower degree of instability was observed for acrylonitrile. In contrast, it was found that l,&butadiene has appeared to be stable for at least 6 months. Development of Standards at 1 ppb. The greatest challenge to date was encountered in an attempt to gravimetrically prepare two different four-component standards at 1ppb. These two standards included an aliphatic mixture containing vinyl chloride, chloroform, carbon tetrachloride, and tetrachloroethylene. The other mixture was aromatic containing benzene, toluene, chlorobenzene, and bromobenzene. The preparation of these mixtures was extremely difficult due to the requirement of accurately weighing very small amounts of each organic compound, 0.010-0.025 mg. Analysis of the aromatic standards was found to be the most difficult, requiring operation of the FID close to the detection limit of the compounds. Vinyl chloride also was analyzed by use of the FID due to its low ECD response. In contrast, the remaining three halogenated organic compounds in the aliphatic standards were easily analyzed by using an ECD. In spite of the preparative and analytical difficulties, excellent agreement was found between the measured and gravimetric data for the two types of mixtures (Table IX). Repeated calibration over time confirmed that such standards have excellent long-term stability at concentrations as low as 1 ppb. Limitations and Future Directions. Obvious limitations include losses due to sorption of polar compounds and chemical instability of certain compounds. Other materials, such as stainless steel, have not been used but are being considered for containers to determine if the absorption and stability problems for compounds such as pyridine and ethylene oxide can be improved. However, the methodology described in this paper should be directly applicable for the preparation of accurate and stable multicomponent mixtures at the low-parts-per-billion level for many other volatile organic compounds. Several technical barriers must be overcome to significantly extend the number of compounds in lowparts-per-billion mixtures. A modified approach to the preparation method must be developed to markedly increase the number of organic compounds that can be introduced into a single mixture. Such a method is currently under study. In addition, a method for enhancing detection sensitivity at concentrations approaching 1ppb must be established. Also,
Table IX. Gravimetric and Measured Concentrations for 1 ppb Standards A. Aromatic Standard
concentration,”ppb by mole measured compound
grav concn
6/14/84
benzene
0.93 f 0.07
toluene chlorobenzene
0.88 f 0.07 1.42 f 0.06 1.22 f 0.08
0.98 f 0.07 0.88 f 0.07 1.48 f 0.06 1.20 f 0.08
bromobenzene
10/10/84 1.00 f 0.91 f 1.38 f 1.18 f
0.07 0.07 0.06 0.08
5/1/85 0.91 f 0.07 0.90 f 0.07 1.42 f 0.06 1.22 f 0.08
10/16/85 0.96 0.86 1.45 1.16
f 0.07 f 0.07 f 0.06 f 0.08
B. Aliphatic Standard concentration.‘ Dab bv mole measured
compound vinyl chloride
grav concn
11/29/84
0.90 f 0.08
0.98 f 0.08
1.09 f 0.04 1.08 f 0.04 chloroform 1.15 f 0.03 1.18 f 0.03 carbon tetrachloride 1.85 0.04 1.85 f 0.04 tetrachloroethylene a Uncertainties following the concentrations are at the 95% confidence level.
*
2459
5120185 0.94 f 1.10 f 1.16 f 1.87 f
0.08 0.04
0.03 0.04
10/15/85 0.96 f 1.06 f 1.15 f 1.84 f
0.08 0.04
0.03 0.04
Anal. Chem. 1988, 60. 2460-2464
2460
chromatographic conditions must be optimized to separate compounds in highly complex gas mixtures. We recently have given attention to each of these areas. Current investigations of the cross-blending of gravimetrically prepared mixtures for the development of accurate complex multicomponent lowparts-per-billion standards has shown a high degree of promise. Cryogenic preconcentration and prefocusing techniques are being used to enhance detection capabilities. Experiments are in progress using programmed temperature techniques for packed and capillary GC analyses to enhance the separation of compounds in complex mixtures. In addition, plans currently are under way to certify an 18-component mixture of volatile toxic organic compounds a t the 5 ppb level as a new SRM. In summary, the methodology described in this report confirms that the preparation of accurate and stable multicomponent standards of volatile toxic organic compounds at the low-parts-per-billion level is readily feasible.
ACKNOWLEDGMENT The authors wish to acknowledge Darryl von Lehmden and Howard Crist of the U.S. Environmental Protection Agency’s Environmental Monitoring Systems Laboratory for their strong support of this work. Registry No. Vinyl chloride, 75-01-4; chloroform, 67-66-3; carbon tetrachloride, 56-23-5; benzene, 71-43-2; tetrachloroethylene, 127-18-4; acetonitrile, 75-05-8; methyl ethyl ketone, 78-93-3;bromomethane, 74-83-9; l,l,l-trichloroethane, 71-55-6; trichloroethylene, 79-01-6; 1,2-dichloroethane, 107-06-2;1,2-dibromoethane, 106-93-4; trichlorofluoromethane, 75-69-4; dichlorodifluoromethane, 75-71-8; vinylidene chloride, 75-35-4;
acetone, 67-64-1;toluene, 108-88-3;l,l-dioxane, 123-91-1;chlorobenzene, 108-90-7; 1,3-butadiene, 106-99-0; 1,1,2-trichloro1,2,2-trifluoroethane, 76-13-1; 1,2-dichloro-1,1,2,2-tetrafluoroethane, 76-14-2;ethylene oxide, 75-21-8; propylene oxide, 75-56-9; methylene chloride, 75-09-2; acrylonitrile, 107-13-1; o-xylene, 95-47-6.
LITERATURE CITED (1) IARC Monographs on the Evaluation of Carcinogenic Risk of Chemicals to Man, International Agency for Research on Cancer, World Health Organization, Geneva, Switzerland, 1972, Vol. 1, pp 53-65, and 1974, Voi. 7, pp 203-216, 291-305. (2) Mailns, D. C.; McCain, B. B.; Brown, D. W.; Myers, M. S.; Krahn, M. M.; Chan, S.-L. Environ. Scl. Technol. 1987, 2 1 , 765-770. (3) von Lehmden, D. J. Conference on Recent Developments in Monitoring Methods for Toxics in the Atmosphere, Boulder, CO, July 1987. (4) Termonla, M.; Alaerts, G. J. Chromatogr. 1985, 328, 367-371. ( 5 ) Arnts, R. R. J. Chromatogr. 1085, 329, 399-405. (6) Scott, D. R. Anal. Chem. 1986, 58,881-890. (7) Vejrosta, J.; Novak, J. J. Chromatogr. 1979, 175, 261-267. ( 8 ) Schmidt, W. P.; Rook, H. L. Anal. Chem. 1983, 55,290-294. (9) Viil-Madjar, C.; Parey, F.: Excoffier, J.-L.; Bekassy, S. J. Chromet w r . 1981, 203, 247-26 1. (10) Rhoderick. G. C.; Cuthreli, W. F.; Zielinski. W. L., Jr. I n Transactions, APCA IASQC Specialty Conference on Quality Assurance In Air Pollution Measurements; Johnson, T. R., Penkala. S. J., Eds.; Air Pollution Control Association: Pittsburgh, PA, 1985; pp 239-246. (11) Jayanty. R. Conference on Recent Developments in Monitoring Methods for Toxics in the Atmosphere, Boulder, CO, July 1987. (12) Gas Encycbpaedia; L’Air Liquide, Divlsion Scientifique, Elsevier: Amsterdam, 1976; pp 501-508. (13) Braker, W.; Mossman, A. L. Matheson Gas Data Book; Matheson Gas Products: East Rutherford, NJ, 1971; pp 63-68.
RECEIVED for review March 29, 1988. Accepted August 19, 1988. This work was supported in part under Interagency Agreement DW-13932187-01-0 with the U.S. EPA.
Submicrosecond Measurements with Cyclic Voltammetry David 0. Wipf and R. Mark Wightman*
Department of Chemistry, Indiana University, Bloomington, Indiana 47405
Cycllc voltammograms at very fast scan rates can be distorted by the time constant of the electrochemical cell and the current transducer. The double layer capcitance can be lowered by working with small (6 Mm diameter), disk-shaped electrodes. The distortion caused by the Instrumentationcan be removed by deconvolutlon of the ampltfler response function as illustrated In thls work by the recovery of undlstorted voltammograms recorded with purposely added Hlering. The remaining sources of distortion, ohmlc drop and the nonlinearity of the potential scan of the working electrode, can be accounted for by digital simulation. This is illustrated for the reduction of 9-fluorenone at a scan rate of 1 000 000 V s-‘. Digital simulation of the expected result incorporating finite electron transfer kinetics is in good agreement wlth the measured, deconvduted data. I n additlon, this technique can be used to obtain the rate of rapid chemlcai reactions, which follow electron transfer. This is illustratedfor the radkai anlon of 2-chloroqulndlne, which has a haH-life In acetonitrile of 1 M.
-
Disk-shaped electrodes of micrometer dimensions allow fast voltammetric measurements because of the reduced ohmic
* Author to whom correspondence should be addressed.
drop and cell time constant associated with these electrodes (1-5). Such measurements are useful to determine the rates of rapid processes associated with heterogeneous electron transfer (3-8). In a previous paper, we demonstrated that the scan rate employed in the cyclic voltammetry technique could be extended to values as high as 500 kV s-l (9). These voltammograms were, however, distorted by the combined effects of ohmic drop, cell time constant, and instrument response. Information was recovered from the distorted voltammograms by comparison of the experimental data with simulated data that had been convoluted with the factors responsible for distortion. In this paper we present an alternate approach. The effect of the cell time constant can be decreased by the use of an even smaller electrode and the instrument distortion can be removed by deconvolution of the experimental data. Deconvolution methods have been used previously in electrochemistry to remove the contributions of heterogeneous charge transfer contributions (10) or diffusion (11,12). Recently Fletcher used deconvolution methods to analyze the number of crystals growing as a function of time at ensembles of microelectrodes (13). In this communication, we will show that experimental data can be deconvoluted directly from the major source of distortion at fast scan rates-the low-pass filter action of the measurement instrumentation. As will be shown voltammograms with useful information have been obtained up to 1000000 V s-l. Voltammograms at this scan rate con-
0003-2700/88/0360-2460$01.50/00 1988 American Chemical Society
