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Anal. Chem. 1982, 54. 1572-1575
compounds in the original SRC-11, the development of this gas chromatographic methodology establishes the means to quantitate these individual phenols in the SRC-I1 matrix. It is our belief that reliable quantitative data on individual compounds will be best made in the unseparated coal liquid, because of losses involved during solvent removal and incomplete and variable recovery of sample from chromatographic stationary phases. It should be possible to quantitate these phenolic compounds in the unseparated raw coal liquefaction distillates by employing the GC/MS technique of selected ion monitoring (SIM) and the method of standard additions. This eliminates problems of incomplete recovery during the solvent separation and chemical class fractionation steps. Experiments are currently in progress to attempt this. The use of high-resolution gas chromatography employing fused-silica columns coated with Superox-20M and of combined GC/MS with the same column has led to the detailed characterization of phenolic constituents and identification of 29 phenols from a SRC-I1 middle distillate. These techniques, combined with a knowledge of the retention characteristics of phenols on Superox-ZOM, are useful for the separation and positive identification of individual phenolic compounds present in complex mixtures. The use of this stationary phase should find wide application in separation of phenolic mixtures from a variety of sources. ACKNOWLEDGMENT We wish to acknowledge Takao Hara for fractionating the coal liquid into functional groups and thus providing the phenol fraction. Furthermore, helpful discussions were provided by Dennis Finseth, Richard Sprecher, and Frank Schweighardt. LITERATURE CITED (1) Karr, C., Jr.; Brown, P. M.; Estep, P. A,; Humphrey, G. L. Anal. Chem. 1958, 3 0 , 1413-1416.
(2) Karr, C., Jr.; Brown, P. M.; Estep, P. A.; Humphrey, G. L. fuel 1958, 3 7 , 227-235. (3) Karr, C., Jr.; Estep, P. A.; Hirst, L. L. Anal. Chem. 1960, 3 2 , 463-475. (4) Pihler, H.; Hennenberger, P.; Schwarz, 0. 6f8nnSt.-Chem. 1988, 49, 175-186. ( 5 ) Plchler, H.; Schwarz, G. Bfennst.-Chem. 1969, 5 0 , 72-78. (6) Plchler, H.; Herlan, A. Erdoel Kohle, Erdgas, P8tfOChem. 8rennst.Chem. 1973, 2 6 , 401-407. (7) Buryan, P.; Macak, J.; Nabivach, V. M. J. Chromatogr. 1878, 148, 203-210. ( 8 ) Macak, J.; Buryan, P. Chem. Lis@ 1875, 6 9 , 457-518. (9) Buryan, P.; Macak, J. Sb. Vys. Sk, Chem.-Techno/. Pram, Technol. faliv. 1977. 0 3 4 , 39-84. (IO) Macak, J.; Buryan, P.; Nabivach, V. M. Koks Khlm. 1979, 3 , 29-36. (11) Buryan, P.; Macak, J.; Zachar, P.; Kos, J. Ropa Uhli8 1978, 18, 205-217. (12) Jewell, D. M.;Weber, J. H.; Bunger, J. W.; Plancher, H.;Latham, D. R. Anal. Chem. 1872. 4 4 , 1391-1395. (13) Schiller, J. E.; Mathiasson, D. R. Anal. Chem. 1977, 4 9 , 1225-1228. (14) Coleman, H. J.; Dooley, J. E.; Hirsch, D. E.; Thompson, C. J. Anal. Chem. 1873, 45, 1724-1737. (15) Farcaslu. M. fuel 1977. 56, 9-14. (16) Bartle, K. D.; Matthews. R. S.; Stadelhofer, J. Appl. Spectrosc. 1980, 34, 615-618. (17) Schalbron, J. F.; Hurtubise, R. J.; Silver, H. F. Anal. Chem. 1979, 51, 1426-1433. (18) Guenther, F. R.; Parris, R. M.; Cheder, S. N.; Hilpert, L. R. J. Chromatwf. 1981, 207, 256-261. (19) Zingaro, R. A.; Phillp, C. V.; Anthony, R. G.; Vindiola, A. fuel Process. Technol. 1981, 4 , 169-177. (20) Whlte, C. M.; Schmidt, C. E. frepr. Pap.-Am. Chem. SOC.,Dlv. Fuel Chem. 1978, 2 3 , 134-143. (21) EPRI ProJect Report 410-1; Mobll Research and Development Corp., 1979. (22) Brinkman, D. W.; Whlsman, M. L.; Bowden. J. N. BETClRI 76/23, March 1979. (23) Hara, T.; Jones, L.; LI, N.C.; Tewarl, K. C. fuel 1981, 6 0 , 1143-1148. (24) Whlte, C. M.; Li, N. C. Anal. Chem., preceding paper In this Issue. (25) Tewari, K. C.; Egan, K. M.; Li, N. C. Fuel 1978, 57, 712-716.
RECEIVED for review February 22, 1982. Accepted April 5, 1982. N.C.L. acknowledges support by the Department of Energy Contract to Duquesne University, No. DE-AC2280PC30252.
Determination of Airborne 1,6=Hexamethylene Diisocyanate by Gas Chromatography G. G. Esposlto* and T. W. Dolzlne U.S. Army Environmental Hygene Agency, Aberdeen Proving Ground, Maryland 2 10 10
A gas-liquid chromatographic procedure (GLC) has been developed for the determlnatlon of hexamethylene dlisocyanate (HDI) in air. HDI is collected In an acidic absorbing solution where it is hydrolyzed to hexamethyienedlamine (HDA). HDA is extracted wlth toluene, derlvatlzed wlth heptafluorobutyrlc anhydride (HFBA), and subsequently analyzed by GLC using an electron capture detector. The method has a detectlon iimlt of 0.050 ng per 2-pL injection which corresponds to 0.53 ppb or 3.75 pg/m3 of HDI in a 40-L alr sample.
Diisocyanates are used to produce polyurethane adhesives, elastomers, rigid and flexible foams, and organic coatings. HDI based coatings have a unique combination of toughness, flexibility, solvent resistance, and light stability which has led to their widespread and continually increasing usage.
Personal exposure to isocyanates can cause irritation of the respiratory tract and may result in chronic impairment of pulmonary function. In 1978, NIOSH published criteria (1) for a recommended standard for six diisocyanates; the TWA limit recommended for HDI is 5 ppb or 35 pg/m3. Isocyanates react readily with compounds containing reactive hydrogens, e.g., acids, bases, water, etc.; this reactivity precludes collection of isocyanates from air without immediate derivatization. Various methods have been published for measuring diisocyanates in air. They all involve simultaneous collection and derivatization of diisocyanates followed by analysis using colorimetry, thin-layer chromatography (TLC), or high-performance liquid chromatography (HPLC). The colorimetric methods (2,3)rely on the formation of intensely colored diazo compounds derived from the reaction of aromatic amines (aromatic diisocyanate hydrolysis products) with a derivatization reagent. Since aliphatic amines do not respond to the diazotization reaction, this technique cannot be
Thls article not subject to U S . Copyright. Publlshed 1982 by the American Chemical Soclety
ANALYTICAL CHEMISTRY, VOL. 54, NO. 9, AUGUST 1982
applied to the determiriation of aliphatic diisocyanates. A procedure for aromatic and aliphatic diisocyanates was developed by Keller et al. (4). This method employs an absorbing solution containing N-(4-nitrobenzyl)-n-propylamine to form urea derivatives of diisocyanates that are subsequently separated by TLC. Even though the method has broad application to diisocyanates, it is tedious, time-consuming, and. lacks adequate sensitivity to meet current governmental standards. A major improvement in diisocyanate measurement was achieved when HPLC (5) was used to separate the isocyanate derivatives used in the TLC procedure. Several modifications of this technique have appeared in the literature; emphasis has been on shortening the method and improving its sensitivity (6-8). Thie problem that we sometimes experience with this technique has been with the instability of the derivatization reagent and derivatives when samples remained in the field without being refrigerated. Since it is difficult to maintain complete control over samples when they are shipped by public transportation, adverse affects caused by temperature fluctuatiorie cannot always be avoided. The proposed method is an extension and modification of the GLC procedure developed by Ebell et al. (9) for the determination of toluene diisocyanate (TDI) in air. Their procedure was significantly shortened by eliminating a solvent evaporation step. Furtlhermore, the sensitivity was greatly enhanced by using HFBA in place of the original derivatization reagent, trifluoroacetic anhydride (TFA). In the new method, HI11 is collected and hydrolyzed in the acidic absorbing solution recommended by Marcali (2)for the collection of TDI. The absorbing solution is made basic with NaOH and the HDA extracted with toluene. An aliquot of toluene is reacted with HFBA and the heptafluorobutyryl (HFB) derivative is anal:yzed by GLC using an ECD detector. Precision and accuracy data of the analytical procedure are presented along with collection efficiency of the sampling procedure. Gas chromatography/mass spectrometry was used to confirm the identity of the HFB derivative.
EXPERIMENTAL SECTION Reagents. The absorbing solution was prepared by mixing 22 mL of glacial acetic acid with 100 mL of distilled water and 35 mL of concentrated HCl; the mixture was diluted to 1L with distilled water. The 40% sodium hydroxide solution was prepared by adding 400 g of sodium hydroxide to 800 mL of distilled water in an ice bath and diluting the mixture to 1L with distilled water. Heptafluorobutyricanhydriide was obtained from Pierce Chemical Co. (Rockford, IL). Hexamethylene diisocyanate and l,&hexanediamine dihydrochloridle were purchmed from Eastman Kodak Co. (Rochester, NY). Deirivatization reactions were carried out in glass culture tubes, 13 X 100 mm, obtained from Fisher Scientific Co. (Silver Spring, MD). Air samples were collected in midget impingers purchased from Ace Glass Inc. (Vineland,,NJ) using a DuPont portable pump, Model No. P-4OOOA (E. I. du Pont de Nemours, Inc., Wilmington, DE). Instrumentation. The chromatography was conducted with a Model 5880 microprocessor-controlled gas chromatograph (Hewlett-Packard, Avondale, PA) equipped with a 63Nielectron capture detector. A 6 ft :r( 4 mm i.d. glass column packed with 3% SE-30 on Gas-Chrom Q,80/100 mesh, was used for the GC separations. GC Operating Conditions. Operating conditions were as follows: detector cell temlperature ("C), 275; injection port temperature ("C), 275; oven temperature ("C), 145; nitrogen flow at exit (cm3/min), 25. Simulated Air Samples. The technique for generating known concentrations of HDI in air was similar to the method described by Graham (8). Two midget impingers were connected in series. A known amount of HDI in methylene chloride was placed in the first impinger and 10 mL of absorbing solution was added to the second. Dry air was pulled through both impingers at a rate of 1L/min for 40 min with a IhPont air sampling pump. The first
( 2 ) NHz-(CH,)~-NHz+2F,C3-~-O-~-C,F,~-F,C3-C-NH.(CHzJ6-NH-E-C3F7+2F7CI-C-O-H O 0 ' A r) 0
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P
Figure 1. Reaction scheme for the hydrolysis and derivation of HDI.
impinger was heated at 60 OC throughout the sampling period in order to volatilize the HDI. The methylene chloride used to prepare standards was dried over molecular sieve, and all the glassware used to transfer and store HDI was scrupulously dried. Procedure. Air samples were collected at a rate of 1 L/min for 40 min through a midget impinger containing 10 mL of absorbing solution. After sample collection was completed, the impinger stem was gently tapped against the inside of the impinger bottle to recover as much of the impinger solution as possible. The contents of the impinger were transferred to a 25-mL glass stoppered graduated cylinder using enough unused absorber solution to bring the volume back up to 10 mL. Eleven milliliters of the 40% NaOH solution and 6 mL of toluene were added to the sample, and the mixture was shaken vigorously for 2 min. The layers were allowed to separate for at least 15 min. Two milliliters of the toluene layer was pipetted into a vial with a screw cap (care was taken not to include any water). Thirty microliters of HFBA was added to the toluene and the vial placed in a heating block at 55 OC for 1h. The sample was cooled to room temperature and shaken with 1mL of NaOH (0.4 N) for 2 min. The layers were allowed to separate for about 5 min, and precisely 2 pL of the toluene layer was injected into the GC. Peak areas from sample injections were compared to a calibration curve generated from standards containing 1,6-hexanediamine dihydrochloride. If the amount of HDI in a sample exceeded the upper limit of the calibration c w e , the toluene layer was quantitatively diluted with additional toluene and the sample rerun. Calibration Curve. With 1,6-hexanediaminedihydrochloride, standards equivalent to 0.5,1.0,1.5, and 2.0 pg of HDI per 10 mL of absorber solution were prepared. Ten-milliliteraliquots of the forementioned standards were treated according to the directions specified in the procedure. A calibration curve was constructed by plotting detector response against the weight (in pg) of HDI contained in each 10-mL aliquot of standard.
RESULTS AND DISCUSSION Concurrent with assessing the suitability of the TDI procedure for the determination of HDI, two distinct areas of improving the procedure were explored. First, it was decided to modify Ebell's procedure to eliminate the chloroform evaporation step. This part of the procedure significantly increases the length of the analysis and is a potential source of error if samples are inadvertently dried too long. In the new procedure, toluene was substituted for chloroform and the toluene extract was treated directly with the derivatization reagent. The other change made in the method was the substitution of HFBA for TFA. HFBA contains seven fluorine atoms as compared to three fluorines in TFA; thus, HFBA provides a considerable enrichment of fluorine in the derivative. As expected, the ECD detector gave a much better response to the HFBA derivative which amounted to greater than a 50-fold increase in sensitivity. In order to define the structural characteristicaafthe derivative, a reacted sample was subjected to analysis by chemical ionization (methane) mass spectrometry. According to the mass spectral data, HDA combines with 2 mol of HFBA to produce a mole of the diheptafiuorobutyryl derivative. The hydrolysis and derivatization reactions are presented in Figure 1. Since HFBA is readily hydrolyzed by water, reasonable care must be exercised to minimize carry-over of water when aliquots of toluene are taken for derivatization. The amount
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 9, AUGUST 1982
Table 1, Pfecision and Accuracy of Analytical Procedure HDA mean (as HDI),“ recovery,
l a
%
0.7 1.4 2.4
100.4 101.1 99.9
std dev
coeff variation
0.068 0.131 0.124
0.083 0.092 0.044
a Six samples at each level.
Table 11. Results of Collection and Analytical Procedures amt of HDI, gg 1.972
amt of HDI recovered, p g 1.68 1.48 1.97 1.48 1.98 1.78 mean recovery, % std dev coeff variation
recovery, %
85.2 75.1 99.9 75.1 100.4 86.2 87.0 0.223 0.130
I
0
10
5 MINUTES
Flgure 2. Chromatogram of heptafluorobutyryl derivative of HDI.
I
1
I
0
5 MINUTES
10
Flgure 3. Analysls of isocyanate component of urethane palnt.
of HFBA added to the samples provides a sufficient excess of reagent to drive the reaction to completion, even if the 2 mL of toluene used is water saturated. Once formed, the derivatives are stable for at least 24 h when refrigerated. The excess reagent is easily removed by washing the toluene with base. The analytical results were linear over the concentration range described in the calibration section. The regression equation for the calibration was Y = 3.13X + 0.00 with a correlation coefficient of r = 0.999. These weights of HDI were selected so as to encompass the weight (1.4 pg) of HDI that would be contained in a 40-L air sample at a concentration of 35 pg/m3. The linearity of the method, outside the range mentioned, was not investigated. HDI was analyzed according to the new procedure producing the chromatogram shown in Figure 2. As can be seen,
the HDI derivative had a retention time of 4.28 min; other peaks were produced by the reagent.?,. To investigate potential interferences that might result from urethane paints, a commercial HDI prepolymer was hydrolyzed in absorbing solution, derivatized, and chromatographed. As shown in Figure 3, the components in the isocyanate prepolymer do not appear to pose a problem with the analysis. Furthermore, when the HDI peak waa compared to a calibration c w e and back-calculated to its concentration in the isocyanate prepolymer solution, the calculated value of 0.46% compared favorably with the anticipated value of about 0.4-0.5%. When the corresponding amines of TDI and diphenylmethane diisocyanate (MDI) were subjected to the analytical procedure, the TDI isomers emerged with the HDI peak; the MDI derivative required over 60 min to elute. The operational parameters described in this paper were optimized for the analysis and determination of HDI; therefore, no attempt was made to generate data for TDI and MDI. These two isocyanates were chromatographed because of their industrial importance as urethane precursors; they may be the subject of a future investigation. Table I lists precision and accuracy data for the analysis of three sets of six samples that were spiked with levels of 1,6-hexanediamine dihydrochloride equivalent to half, one, and two times the current OSHA standard (5 ppb) of HDI in a 40-L air sample. The samples were prepared in 10 mL of absorber solution and analyzed according to the conditions specified in the procedure. As can be seen, mean recoveries for the 0.7,1.4, and 2.4 pg samples were 100.4%, 101.1%, and 99.9%, respectively. Standard deviation and coefficient of variation are also presented. Precision and accuracy data on the recovery of six simulated air samples containing 7 ppb of HDI are shown in Table 11. The mean recovery of 87% is lower than recoveries obtained on spiked samples (Table I). Accordingly, the precision was not quite as good. This decrease in precision and accuracy could possibly be attributed to the inability to generate known, reproducible concentration of HDI in air and not totally due to the overall efficiency of the method. Others (4-6,8) have encountered similar difficulties in their attempts to validate diisocyanate collection procedures.
LITERATURE CITED (1) NIOSH “Criteria for a Recommended Standard for Occupatlonal Exposures to Dllsocyanates”; U.S. Department of Health, Educatlon and
Anal. Chem. 1982, 5 4 , 1575-1578 Welfare: U.S. Goverlnment Printing Offlce, Washlngton, DC, 1978, Publlcatlon No. 76-21!;. (2) Marcali, K. Anal. Chem. 1957, 2 9 , 552. (3) Grim, K. E.; Linch, A. I_. Am. Ind. Hyg. Assoc. J . 1964, 25, 285. (4) Keller, J.; Dunlap, K. I..; Sandrldge, R. L. Anal. Chem. 1974, 46, 1845. (5) Dunlap, K. L.; Sandridge, R. L.; Keller, J. Anal. chem. 1978, 48,497. (6) Levine, S. P.; Hoggatt, J.; Chladek. E.; Jungclaus, G.; Gerlock, J. L. Anal. Chem. 1979, 51, 1106. (7) Bagon, G. A.; Purnell, C:. J. J . Chrornatogr. 1980, 790, 175. (8) Graham, J. D. J. Chrornatogr. Scl. 1980, 78, 384.
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(9) Ebell, G. F.; Fleming, D. E.; Genovese, J. H.; Taylor, G. A. Ann. Occup. Hyg. 1980, 2 3 , 185.
RECEIVED for review March 2, 1982. Accepted M~~ 3, 1982. The opinions or assertions contained herein are the private views of the authors and are not to be construed as reflecting the views of the Department of the Army of the Department of Defense.
Theory of Digital Alternating Current Polarographic Techniques J. E. Anderson and 14. M. Bond* Division of Chemical and Physical Sciences, Deakin Universiv, Waurn Ponds, Vlctoria 32 17, Australia
I n the technique of dllgitai ac polarography, a digital step function sine wave is applied to the cell rather than an analog sine wave. By modification of existing theory for step functional changes in potential, a theory for reversible systems may be obtained. The theory enables the effect of the number of steps in the digital sine wave to be examined and results are compared with conventional ac theory. The theory lndlcates the number a l steps in the digital sine wave Is Important In determining the magnitude of the alternating current. Phase angle relationships are affected by the number of steps as well as the point in the time at which current measurements are made during the potentlai steps. Experlmental data for both frindamentai and second harmonic responses confirm the thieoretlcal predictions.
The technique of digild ac polarography has recently been developed (I) as a means of simplifying ac polarographic techniques from an instrumental point of view. The technique has some of the characteristics of digital fast Fourier transform (FFT)ac polarography but is more closely related to the analog techniques. The basis of digital 8ic polarography is the use of microprocessor-based instrumentation to simulate the electronic components found in conventional ac instrumentation. The simulation begins with the use of a digital sine wave consisting of a fixed number of potential steps (e.g., 36) and the current is measured a t a fixed point in time for each potential step. Although in digital ac polarography the data are clearly measured in the time domain, it was found that the collection of data could be considered to be taken once every 10" in the frequency domain (once per step; 36 steps). Once the data are collected, software can be used to manipulate the data and simulate the high pass filter input and phase-sensitive detector commonly used in ac polarography. The subsequent digital ac polarograms which can be generated (either total ac current, phase-sensitive fundamental, or second harmonic) show the approximate phase relationships between the applied potential and the faradaic and capacitance components predicted from conventional sinusoidal theory (1). Furthermore, the ac peak shapes were as theoretically predicted (for reversible systems) and the peak currents showed a linear increase as a function of the square root of t h e angular frequency ( I ) . Reported here is the modification of general voltammetric theory for any stepwise potential wave form (2)to comply with the conditions existing in the digital ac technique. The theory 0003-2700/82/0354-1575$01.25/0
yields satisfactory results when compared with the digital ac experiments (for a reversible system) and can be used to examine the effect of the number of potential steps used to define the digital sine wave. The digital ac theory and experiment are compared with conventional ac theory which can take into account sphericity, amalgam formation, and slow electron transfer.
EXPERIMENTAL SECTION Instrumentation. The microprocessor-based instrumentation used to perform the digital ac experiments was essentially the same as described previously ( I ) . Further details of this system, based on a Motorola 6800 D2 microprocessor "kit", are also available elsewhere (3,4). The only modification made from ref 1was the substitution of a 1-MHz clock for the 614.4-kHz clock normally available with the 6800 processor. This clock was changed to increase the upper ac frequency previously (I) attainable. In the present work, data from the digital ac experiments were converted to decimal code and sent to a DEC 20/50 computer system via a RS 232 interface on the microprocessor. This procedure facilitated experiment-theory correlations. All computer programs associated with the theoretical work were written in FORTRAN and used with the DEC-20 computer. Computer programs are available on request. Data from experiment and theory were plotted on a Tektronix 4662 digital plotter. A static mercury drop electrode (SMDE), Model 303, from EG&G Princeton Applied Research Corp., Princeton, NJ, was used as the working electrode in the digital ac experiments. A platinum auxiliary electrode and a Ag/AgCl (saturated KC1) reference electrode completed the three-electrode potentiostat system. Reagents. Analytical reagent grade chemicals were used throughout as was distilled water. Prior to the experiments all solutions were degassed for at least 5 min with high-purity nitrogen saturated with water. Polarograms were recorded at ambient temperatures of 21 A 1"C. Electrode areas were obtained from the weighing of 400 mercury drops collected during the actual experiments.
RESULTS AND DISCUSSION The general equation for voltammetry with step-functional potential changes given by Rifkin and Evans (2) was modified so as to apply to digital ac polarography a t the constant electrode area SMDE. This equation applies to planar stationary electrodes, and it was assumed that sphericity does not significantly affect the ac current for reversible systems. This was confirmed by using a digital simulation program for the SMDE which showed that the spherical correction terms are usually small. The theory for digital ac polarography for a reversible process based on the above assumption is given by 1-5. 0 1982 American Chemical Society
