Anal. Chem. 1994,66, 1664-1666
Determination of Hexamethylene Diisocyanate in Spray-Painting Operations Using Capillary Zone Electrophoresis Walter E. Rudzlnski' and Llu Pin Department of Chemistry, Southwest Texas State University, San Marcos, Texas 78666 Ron Sutcllffe, Andy Richardson, and Tom Thomas AL/OEA, Brooks Air Force Base, San Antonio, Texas 78235-5501
Capillary zone electrophoresis (CZE) was used for the analysis of hexamethylene diisocyanate (HDI) vapor during spraypainting operations. The use of CZE when compared with high-performance liquid chromatography (HPLC) offers improved resolution of coeluting peaks, comparable limits of detection and speed of analysis, low solvent consumption, and the ability to sample as little as 3 pL of sample. The modified OSHA method 42, which uses capillary zone electrophoresis for the analytical determination of HDI, was validated in a direct comparison with the original OSHA method 42. Field samples were obtained during several spray-paintingoperations. The results obtained using CZE were comparable ( r = 0.998) to those obtained using HPLC. Methods have been developed for the analysis of hexamethylene diisocyanate (HDI) in ambient air during spraypainting operations. These methods depend on sample collection with either a filter or liquid impinger, derivatization of the reactive isocyanate, and then separation using reversedphase high-performance liquid chromatography (HPLC) with either UV, fluorescent, or electrochemical detection. The entire area has been the subject of various OSHA method 42 involves the collection of sample by drawing air through a fiber filter coated with 1-(2-pyridyl)pipera~ine.~ The isocyanatourea that forms is then desorbed and analyzed using HPLC with UV detection. The method has been validated for HDI vapor. We have modified OSHA method 42 for the analysis of HDI by using capillary zone electrophoresis (CZE) for the analysis of the derivatized diisocyanate. CZE may be used as a confirmatory or ancillary method that consumes little solvent and requires as little as 3 p L of sample. CZE also offers baseline resolution of the derivitized HDI within a complicated polyurethane paint matrix; this is useful whenever the HDI component is difficult to isolate.
EXPERIMENTAL SECT1ON The sampling method for HDI was that prescribed in OSHA method 42. Samples were collected at an air flow rate of 1.0 L/min. (1) Melcher, R. G . Anal. Chem. 1983,55.40R-56R. (2)Purnell, C. J.; Walker, R. Andysf 1985,110, 893-905. (3) OSHA method 42: Diisocyanates. In OSHA Merhods Manual; Occupational Safety and Health Administration; Government Printing Office: Washington, DC, 1983; Vol. 42,pp 1-39.
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All spray-painting operations were conducted at Keesler Air Force Base in Biloxi, MS. Sampling was conducted in the area of overspray by positioning the samplers 3 ft above the floor and about 2 ft downdraft from the equipment being painted. Operation 1 involved spray painting of machinery in a paint booth. Operations 2-4 involved spray painting wheels, signs, and aircraft wing parts, respectively, in a spraypainting area with a waterfall air-cleansing system. The area samplers were positioned in front of the waterfall. The Deft polyurethane paint consisted of pigment and hardener in a 1:l ratio. Polyisocyanate represented 30% of the hardener and 40% of the total nonvolatiles in the paint formulation. All HDI standards, and mobile-phase buffers were prepared as described in OSHA method 42. Glass fiber filters were coated with 2.0 mg of 1-(2-pyridyl)piperazine (PPIP) prior to insertion into a 37-mm cassette. A Hewlett-Packard 1090 high-performance liquid chromatograph equipped with an autosampler, diode array UV detector and H P 3396 series 11integrator was used to analyze for HDI. The sample injection volume was set at 20 pL,and the detector wavelength at 254 nm. A Lichrospher 100 RP 18,5-pm(250 X 4cm)columnwasused toseparatetheanalyte. The mobile phase consisted of acetonitrile-0.0 1 M ammonium acetate buffer in a 50:50 (v/v) ratio. The flow rate was set at 1.0 mL/min. A Waters Quanta 4000 CZE system equipped with a Waters 730 data module was used to analyze for HDI. All samples were injected using hydrostatic injection for 8 s. The height differential of the reservoirs was 9.8 cm. The capillary column had an effective separation length of 50.3 cm and an inner diameter of 75 pm. The total column length was 57.8 cm. The operating voltage was set at 20 kV. A 0.01 M sodium phosphate buffer adjusted to a pH of 3.0 was used that produced a background current of 75 pA. Although an 0.1 M acetate buffer was used in preliminary experiments and can be used for this analysis, a 0.01 M phosphate buffer was used throughout. The detector wavelength was set at 254 nm. The column was reconditioned daily with 0.5 M NaOH, and the capillary was purged with buffer for 2 min between each analysis. The preparation of pyridylpiperazine derivatives and standards should be done in a fume hood in order to avoid 0003-2700/94/0366-1664$04.50/0
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exposure to isocyanate and solvent vapors. Isocyanates are known respiratory irritants.
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RESULTS AND DISCUSSION Working standards for HDI were prepared within the range 0.3-4.0 pg/mL. A typical CZE calibration curve showed a good correlation between detector response and concentration ( r = 0,999) . The analytical sensitivity was 4 X lO3/(pg/ mL), with an estimated detection limit (defined as 2 times the standard deviation, s) of 0.2 pg/mL. Migration times for the derivatized HDI in nine different field samples were reproducible (8.38 min, s = 0.03). The HDI was identified by comparing its migration time with the migration time of authentic standards under variable conditions of pH, ionic strength, buffering medium, and applied voltage. An HPLC calibration curve also showed a good correlation between detector response and concentration with a sensitivity of 6 X 104/(pg/mL), and a detection limit of 0.2 pg/mL at a sample injection volume of 20 pL. The average retention time for 15 chromatographic runs based on nine different field samples was 3.42 min, s = 0.04, which represents a relative standard deviation of 1.2%. All chromatographic runs employed a 50:50 (v/v) 0.01 M ammonium acetate bufferacetonitrile mobile phase. In a comparison of CZE and HPLC, the limit of detection is the same, and the time reproducibility is comparable. Overall, the ability of CZE to analyze for diisocyanate in spray-painting operations is comparable to HPLC except that the CZE method offers 250 000 theoretical plates (calculated according to the approach outlined by Jorgenson and LukacsS and based on the original concept suggested by Giddings6 ) as compared with the HPLC approach, which provides 2500. Figure l a shows the electropherogram, and Figure l b the chromatogram for field sample 314. The HDI peak in the electropherogram is clearly resolved from two other peaks, while the chromatogram contains many, closely spaced peaks, one of which interferes with the HDI peak. The HPLC integrated area for HDI for sample 314 (run in triplicate) varied between 113 000 and 236 000 area counts (corresponding to HDI concentrations between 0.3 and 0.7 pg/ mL). The CZE integrated area (run in duplicate) was reproducible and yielded a concentration of 0.3 pg/mL. CZE clearly can give better results than HPLC when the sample is poorly resolved. For all field samples analyzed by CZE, the HDI peak was baseline-resolved from other polyurethane paint components, but for two samples analyzed by HPLC, components in the paint formulation interfered in the analysis. We attribute the “clean” separation of HDI using CZE to two primary factors. At a pH of 3, most of the components of the paint do not migrate all the way to thedetector4 because of attenuated electroosmotic flow. Since the uncharged components in the paint (for example, pigment, resin, and solvents) do not reach the end of the capillary within the analysis time, the HDI derivative is easily resolved from any comigrating positively charged interferences (for example, excess acetylated polypyridylpiperazine). The second factor (4) Lukacs, K. D.;Jorgenson, J. W. HRC CC, J . High Resolut. Chromatogr. Chromatogr. Commun. 1985,8, 407. ( 5 ) Jorgenson, J. W.; Lukacs, K. D. Anal. Chem. 1981, 53, 1298-1302. (6) Giddings, J. C. Sep. Sci. 1969, 4, 181-189.
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Flgure 1. Separation of the polypyridylpiperazine derivative of HDI (sample 314, 0.3 pglmL HDI) using CZE (a) and HPLC (b). All run conditions are as reported in the ExperimentalSection. The migration time for the HDI derivative in the electropherogram is 8.37 min, whereas the retention time in the chromatogram is 3.42 min. Both peaks are denoted with an asterisk. Table 1. Isocyanate Analysis at Keesler AFB
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362 363 313 315 336 388
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HDI amt (pg) HPLC CZE 0.39 0.39
1.00 0.67 0.57
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0.47 0.47 1.08 0.64 0.57 0.30
air concn (mg/m3) 0.012 0.015 0.014 0.059 0.026 0.037
that improves the separation and accurate quantitation of the HDI derivative is the large number of theoretical plates (250 000). Table 1 shows the resultsof analysis for actual field samples. The field samples were obtained over 4 days during several spray-painting operations. The results obtained for six of nine field samples have been used in order to compare the methods. The results from three samples were not used: Sample 314 exhibited poor precision for the HPLC analysis with up to lOO%variability in the integrated area (vide supra). Samples 316 and 335 had HDI concentrations which were below the limit of quantitation. The pooled standard deviation based on the six field samples which were compared in Table 1 was 0.02 pg ( N = 15) for HPLC and 0.04 pg ( N = 10) for CZE. In all cases the results AnalyticalChemistty, Vol. 66,No. IO, May 15, 1994
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were comparable as measured by a plot of CZE response as a function of HPLC response. The r value was equal to 0.998 and the ratio of responses (slope) was 0.98. The calculated air concentrations ranged from 0.014 to0.059 mg/m3. These values are about half to twice the threshhold limit value (TLV) of 0.034 mg/m3 established for HDI in ambient air.7 The results illustrate that the CZE approach is a reliable alternative to HPLC for the analysis of HDI. Further, since the CZE approach is based on a separation mode completely different from that used in reversed-phase HPLC, the results from a CZE method can be used to confirm the identity of a diisocyanate. Further work is now in progress in order to assess the viability of other derivitizing agents for diisocyanate, their (7) American Council of Governmental IndustrialHygienists: T L Y s Threshhold Limit Values and Biological Exposure Indicesfor 1986-1 987, Cincinati, OH, ACGIH 1987.
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compatibility with the CZE approach, and their applicability to the analysis of total polyisocyanates.
ACKNOWLEDGMENT We thank Major Bruce Dahlquist, Keesler AFB, and Sargeant Don Johnson, Brooks AFB, for collecting the diisocyanate samples during spray-painting operations. We also thank the Air Force Office of Scientific Research, Bolling Air Force Base, Washington, DC, for its financial support.
Received for review September 7, 1993. Accepted February 9, 1994.'
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Abstract published in Advance ACS Abslracts, 'April 1, 1994.
