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Anal. Chem. 1981, 53, 1122-1125
dichloromethane, and 2-propanol). Figure 2 shows the reconstructed ion chromatograms for eight ions indicative of the eight components in the standard trace mixture (there is also evidence of impurities or reaction products leading to additional peaks) for the same injection at two different temperatures. The ribbon speed for this analysis was approximately 10 cm/min and the mass spectrometer scans required 6 s. The quantity injected was approximately 1pg for each component, and the identity was confirmed by comparison with elution times for individual standards. The results show that these relatively volatile compounds are primarily desorbed during the f i t ribbon pass through the flash heater. The second pass at a significantly higher temperature shows evidence for only three of the more polar and less volatile components, clearly showing that the bulk of the material was desorbed during the fiit ribbon pass. In contrast, Figure 3 shows a wide range of volatilities for the wood liquefaction products during a similar gradient elution program. In this case, the behavior is illustrated by eight typical reconstructed ion chromatogramsfor a single injection and three ribbon passes through the "flash heater". For this analysis the ribbon speed was approximately 10 cm/min and the mass spectrometerscan speed was 10 s / m . The multiple analpis of the chromatographicallyseparated material clearly provides much additional data on the volatility of the components. The appearance of peaks at the same location at different temperatures may be due to the same compound or different compounds, depending upon their volatilities, and can be resolved by analysis of the mass spectra. As expected, one generally observes that the higher molecular weight components are desorbed at the highest temperature but, as illustrated in Figure 3, the actual situation can be much more complex. The data in Figure 3 were obtained by using the aerosol liquid deposition device; the improved performance may be observed by comparison with Figure 2 which shows the distorted peak shapes which resulted from the use of conventional deposition methods (11). The results in Figures 2 and 3 clearly illustrate the advantages of the ribbon storage technique. As demonstrated, compounds are selectively observed a t certain flash heater temperatures and discriminated against a t higher or lower temperatures. This technique can assist in interpreting complex chromatograms since it provides differential information related to compound volatility. For experiments where mass spectrometer scan requirements are more demanding (high resolution, simultaneous collision induced dissociation
in a triple quadrupole, or increased sampling time), the LC effluent can be deposited on the moving ribbon and the mass spectrometer analysis performed at a different ribbon speed after completion of the liquid deposition, totally decoupling mass spectrometer and HPLC operation. The technique should also be useful in the application of new "soft"ionization techniques which apparently produce ions directly from the solid surface (12). These applications are currently being investigated in our laboratory. ACKNO WLEDGMEN" We thank W. D. Felix for helpful discussions concerning the design, the assistance of A. W. Madsen and E. N. Sullivan in the design and construction, and K. A. Loss and J. E. Burger in the assembly of the LC-MS interface. LITERATURE CITED (1) McFadden, W. H. J . Chrometogr. Sci. 1979, 77, 2. (2) Mclafferty. F. W.; Knutti, R.; Venkataraghaven. R.; Arpbro, P. J.; Dawkins, B. G. Anal. Chem. 1975, 47, 1503-1505. (3) Melera, A. "Chemical and Analytical Applications of an LCMS Interface", Presented at Twenty-Seventh Annual Conference on Mass Spectrometry and Allied Topics, SeatUe, WA, June 3-8, 1979. (4) Yorke, D. A,; Bwns, P.; Millington, D. S. "a New HPLCMS Interface", Presented at Twenty-Seventh Annual Conference on Mass Spectrometry and Allied Topics, Seattle, WA, June 3-8, 1979. (5) McFadden, W. H.; Schwartz, H. L.; Evans, S. J . Chromatogr. 1976. 722, 389-390. (0) Henion. J. D. Anal. Chem. 1978, 50. 1687-1693. (7) Tsuge, S.; Hirata, Y.; Takeuchi, T. Anal. Chem. 1079, 51, 166-109. (8) Christensen, R. G.; Hertz, H. S.; Meiselman, S.; Whlte, E. "LC-MS UsIng Continuous Sample Preconcentration", Presented at Twenty-Seventh Annual Conference on Mass Spectrometry and Allled T o p b , Seattle, WA. June 3-8, 1979. (9) Blekely, C. R.; Adams, M. J.; Vestal, M. L. "LC-MS Interface Uslng Mdecular Beam Techniques", Presented at Twenty-Seventh Annual Conference on Mass Spectrometry and Allied Topics, Seattle,WA, June 3-8, 1979. (10) . . Blaketv. C. R.: Carmodv. J. J.: Vestal. M. L. Anal. Chem. 1960. 52, 1036-.1641. (11) Smith. R. D.; Johnson, A. L. Anal. Chem. 1081, 53, 731. (12) Smith. R. D.; Burger, J. E.; Johnson, A. L., submitted for publlcatbn in Anal. Chem.
Richard D. Smith* Allen L. Johnson Chemical Methods and Kinetics Section Physical Sciences Department Pacific Northwest Laboratory Richland, Washington 99352
RECEIVED for review December 22,1980. Accepted March 9, 1981. This paper is based on work performed under United Statm Department of Energy Contract DEACM-76RLO 1830.
Determination of Isocyanates in Air by Liquid Chromatography with Fluorescence Detection Sir: Isocyanates are extensively used in the manufacture of polyurethane foams, paints, elastomers, and fibers. With this widespread use, respiratory and allergic reactions have been reported by some workers exposed to these chemical substances. For diisocyanates, NOSH recommended a vapor concentration of 5 ppb as a time weighted average (TWA) concentration for a 10-h workshift, 40-h work week, and 20 ppb as a ceiling concentration for any 10-min sampling period (1). Most of the analytical methods used for determination of isocyanates in air are based on ultraviolet or visible absorption spectrometry (2-10) except for the tape monitors 0003-2700/81/0353-1122$01.25/0
marketed by MDA Scientific, Inc. (11). These techniques are often near their lower limits of detection when used for isocyanate concentrations at or below the 5-ppb range. Thus, utmost care in handling as well as expertise on the part of the analyst is generally required to obtain accurate results. In a contribution paving the way to markedly improved sensitivities, Levine and co-workers (12)introduced a method to determine aliphatic isocyanates in air by fluorescence detection. The isocyanates were converted to stable 1naphthalenemethylamine (NMA) urea derivatives and analyzed using isocratic reverse-phase liquid chromatography. 0 1981 Amerlcan Chemical Society
ANALYTICAL CHEMISTRY, VOL. 53, NO. 7, JUNE 1981
This method was reported to be 50 times more sensitive than the nitro reagent technique; however, these authors encountered unexplainable doublet peaks for air samples from spray tests using hexamethylene diisocyanate-biuret trimer. In our attempt to apply thio technique to a wider range of isocyanates, we found that some NMA derivatives were difficult to dissolve in solvents normdy used in liquid chromatography. To resolve these problems, we prepared a new fluorescenceactive reagent, N-imethyl-l-naphthalenemethylamine (MNMA). We found this reagent to be more versatile and the chromatographic separation significantly improved. We also observed that the use of a quartz photomultiplier window further increased the sensitivity of this technique. A UV monitor could still be used if a fluorescence detector is not available; however, the sensitivity will not be as good. This paper describes our findings to offer an alternative reagent and chromatographic separation for the analysis of isocyanates in air. While this paper was in preparation, a similar method was published by Sango and Zimerson (13). The isocyanates were reacted with N-methyl-l-anthracenemethylamine and the derivatives were separated by using reverse-phase liquid chromatography. Either a fluorescence or a UV monitor was used for detection. EXPERIMENTAL SECTION Preparation of N..Methyl-l-naphthalenemethylamine (MNMA). Reagent grade l-naphthaldehyde, methylamine hydrochloride,and N&HsCN were obtained from Aldrich Chemical Co. and were used without further purification. A modified procedure by Borch and co-workers(14) was used to prepare MNMA. A solution of 3.9 g (25 mmol) of l-naphthaldehyde, 16.7 g (250 mmol) of methylamine hydrochloride, and 16 g (25 mmol) of N&HaCN in 150 mL of anhydrous methanol was stirred for 72 h at ambient temperature. When the reaction was completed, concentrated hydrochloric acid was added to reach pH =2 and methanol was evaporated on a steam bath. The residue was taken up in 100 mL of water, and the aqueous solution was extracted with two 20-mL portions of ether. The aqueous solution was brought to pH > 10 wit'h potassium hydroxide pellets, saturated with sodium chloride, and extracted with five 15-mL portions of ether. The combined ether extracts were dried with anhydrous sodium sulfate and ether was evaporated on a steam bath to give a crude, yellow oil of MNMA. The oil residue was dissolved in 20 mL of methanol and concentrated hydrochloric acid was added to obtain pH 2 2 . The solvent was removed by evaporation on a steam bath to give 4.2 g (20 mmol, 80%) of crude MNMA hydrochloride. The crude crystals were recrystallizedthree times from absolute ethanol to give 3.0 g (14 mmol, 56%) of fluffy, white crystals of N-methyl-1-naphthalenemethylamine hydrochloride (MNMA HC1) (CI2Hl4C1N,mol w t 207.7.), mp 191-192 "C [lit. (15) mp 190 OC]. Anal. Calcd C, 69.41; H, 6.73; C1,17.19. Found C, 69.31; H, 6.79; C1, 1'7.07. Reagents. MNMA Absorber Solution. Approximately42 mg of MNMA HC1 was diselolvedin 10 mL of distilled water. About 6 drops of 1N HCl was added, and the acidic solution (pH =2) was extracted twice with 10 mL of toluene. The toluene extracts were discarded. To precipitate the free amine, we added 10 mL of 1N NaOH to the acidic aqueous solution. The resulting basic solution was extracted three times with 15 mL of toluene, and the toluene extracts were dried with anhydrous sodium sulfate. The dry toluene extrachi were transferred to a 100-mLvolumetric flask and diluted to the mark to make a 2 mM solution. The solution was diluted 10-fold to obtain a 0.2 mM absorber solution. The prepared solutions were stored in a brown bottle and kept in the refrigerator. If stored for over a month, it is recommended that a chromatogram of the absorber solution be run before use. For phenyl isocyanate analysis, a freshly prepared solution is highly recommended. Isocyanates. The isiocyanates used in this study are commercially available from Mobay Chemical Corp. They are as follows: Mondur P, phenyl isocyanate (PI); Mondur TD-80,80% 2,4- and 20% toluene 2!,6-diisocyanate (TDI); Multrathane M, diphenylmethane 4,4'-diisocyanate (MDI); Mondur HX, 1,6-
1123
hexamethylene diisocyanate (HDI); Desmodur N-75, a high molecular weight biuret of hexamethylene diisocyanate; and isophorone diisocyanate (IPDI). Solvents. UV grade chromatographic solvents of methylene chloride, chloroform, methanol, 2-propanol, and toluene were obtained from Burdick and Jackson Laboratories. Apparatus. Air Sampling. A portable Bendix C115 air sampling pump was used in conjunction with a midget gas impinger (30-mL capacity) containing MNMA absorber solution. This pump b normally equipped with tubing which is not resistant to toluene. Replacement with glass or Teflon connections may avoid problems with leakage or failure. HPLC Equipment. A Water Associates LC system was used for all separations. The system included two Model 6000 highpressure pumps, a U6K Universal septumlessinjector valve, Model 660 solvent programmer, and a Model 440 W absorbancedetector (254 nm). For fluorescence measurements, a Schoeffel FS 970 spectrofluorometer equipped with a quartz photomultiplier window and a SFA 339 wavelength drive assembly was utilized. Signals were recorded on a 10-mV Omniscribe recorder. A Whatman Partisil PXS-1025 PAC column was used. Procedure. Air Sampling. A midget impinger was filled with 15 mL of the absorber solution and a known volume of air was drawn through the impinger at a rate of 1 L/min. Afterward, the solvent was evaporated with a stream of dry nitrogen. The remaining trace of toluene was removed by subjecting the residue to 27 mmHg pressure at 45-50 OC for 10 min or more. The residue was dissolved in 1 mL of methylene chloride and analyzed by liquid chromatography. For personal monitoring, precautions should be taken to avoid excessive inhalation of toluene vapors from the pumps and impingers. Preparation of Standard Solutions. About 10-20 mg of pure isocyanate was weighed out exactly in a 25-mL volumetric flask and fiied to the mark with dry toluene. An aliquot of this solution was added to 15 mL of the absorber solution. The resulting solution was allowed to stand for 30 min and worked up like the air samples for LC analyses. HPLC Analysis of Urea Derivatives of Isocyanates. A 30-min linear program which started with 0% B and ended with 50% B was employed to separate the excess MNMA reagent from the urea derivatives. A is pure chloroform with 1% ethanol as preservative while B consists of 10% methanol and 90% A. A flow rate of 2.0 mL/min and chart rate of 0.25 in./min were used during the analysis. For aromatic isocyanates, a program delay of 3-4 min was utilized to separate PI(MNMA) peak from the impurity (see Figure 3). For fluorescence measurements, the excitation wavelength was set at 226 nm and a 300-nm cutoff emission filter was used. A time constant of 6 s and an attenuation of 0.1-0.2 pA were utilized. The injection volume was 20-50 WL. For absorbance measurements, attenuation was set at 32 and the injection volume was 90-200 pL. Treatment of Data. Peak areas for the urea derivatives of isocyanates in the chromatogramswere determined manually or electronically. Calibration curves for each of the isocyanates of interest were prepared by plotting peak area vs. isocyanate concentration of the standard solutions. The concentration of isocyanates in the sample solutions was determined from their respective areas and calibration curves. The obtained concentration was related to the concentration of isocyanate in the atmosphere by the volume of air sampled.
RESULTS AND DISCUSSION Aliphatic and aromatic isocyanates react readily with Nmethyl-l-naphthalenemethylamine (MNMA) t o form fluorescence-active urea derivatives (eq 1). Excess MNMA CH,NHCH,
was separated from isocyanate urea derivatives by liquid
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ANALYTICAL CHEMISTRY, VOL. 53, NO. 7, JUNE
1981
(MNMA), 21 nq
TDI (MNMA), 104 nq
JL 0
4
S
12 16 20 24 E L U T I O N T I M E , min.
28
30
Flgure 1. Fluorescence chromatogram of N-methyl-l-naphthalenemethylamine derivative of Desmodur N based paint spray system.
I
I
I
4
S
12
I
I
I
16 20 24 E L U T I O N T I M E , min.
I
28
30
Figure 5. Fluorescence chromatogram of N-methyl-l-naphthalenemethylamine derivatives of some aromatic isocyanates.
Table I. Detection Limit for Several Isocyanatesa detection limit isocyanate mg/m3 ppb (v/v) IPDI 0.004 0.44 HDI 0.001 0.1t ... HDI-BT 0.010 PI 0.001 0.20 0.00 5 0.49 MDI TDI 0.015 0.42 Detection levels are based on a 1-L air sample and a 50-pL injection volume. The molecular weight of Desmodur N (HDI-BT) is not well-defined, therefore it is unsuitable to express the concentration in ppb (v/v).
I 0
I 4
8
12
16
20
24
28
30
ELUTION TIME, min.
Figure 2. Fluorescence chromatogram of N-methyl-l-naphthalenemethylamine derlvatlves of some allphatlc isocyanates.
chromatography. The use of a fluorescence detector has significantly improved the detection limit for isocyanates. Sensitivity is increased at least 25 times for aromatic isocyanates and over 100 times for aliphatic isocyanates compared to that of the nitroreagent technique. Maximum sensitivity was obtained by using 226 nm for excitation and a 300-nm cutoff emission filter. Using this new reagent, we have eliminated several difficulties that we encountered with 1-naphthalenemethylamine (NMA). First, the unexplained doublet peaks observed with the biuret-trimer of hexamethylene diisocyanate contained in paint sprays were eliminated (see Figure 1). Second, fast deterioration of the column was not observed. Whatman Partisil 10-PAC column was used continuously for 6 months without any sign of deterioration. Third, the urea derivatives of MNMA were more soluble in organic solvents than those of NMA. Fourth, we obtained a better separation and a more stable base line using this chromatographictechnique. Lastly, the use of a quartz photomultiplier window has doubled the
sensitivity for aliphatic isocyanates. Figure 2 shows a typical chromatogram of MNMA derivatives of aliphatic isocyanates. The cis and trans isomers of IPDI(MNMA)2 were completely resolved into two peaks; hence the areas were combined to obtain an improved detection limit. The peak at 1.6 min came from an impurity in the fluorescence reagent. This was probably an oxidation product since ita intensity increased with time and exposure to light. In the preparation of the absorbing medium, it is imperativeto extract these oxidation produdslimpurities with toluene before precipitation of the amine. If this does not suffice, a recrystallization of the hydrochloride with absolute ethanol should be done. To retard deterioration of MNMA, the stock solution should be kept in a brown bottle and stored in a refrigerator. MNMA solutions could last for a t least 3 months when stored properly. However, we recommend running a chromatogram of the absorber solution before each use, especially if it is to be used for field sampling. A typical chromatogram for aromatic isocyanate derivatives of MNMA is shown in Figure 3. A program delay of 3-4 min was utilized to obtain a better separation between PI(MNMA) and the impurity. The 2,4 and 2,6 isomers of TDI(MNMA)2 were completely separated; thus the areas were combined for better quantitative determinations. The detection limits for several isocyanates are given in Table I. Note that these values are expressed for a 1-L air sample, demonstrating the utility of the method for analyzing “single breath” samples. The short sampling times needed for adequate sensitivity will allow better characterization of
-
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Anal. Chem. 1981, 53, 1125-1128
the chromatographic separation should remove interferences due to airborne trace contaminants, except in the rare case wherein the contaminant has an elution time identical with the derivative. Due to the strong UV absorption of MNMA, a UV detector (254 nm) could be used with subsequent decrease in sensitivity. A UV chromatogram of MNMA derivatives of aliphatic isocyanates is shown in Figure 4.
HO
LITERATURE CITED
0
4
8
12 16 20 24 ELUTION T I M E , m i n .
28
30
Figure 4. Ultraviolet chromatogram of N-methyl-l-naphthalenemethylamine derivatives of some aliphatic isocyanates.
peak exposures. Also, the small sample volumes required put the technique in the region where precision hand sampling pumps can be effectively employed. Methylene chloride could be used instead of chloroform. Solvent B also consists of 10% methanol. The same program was usedhut shortened to 20 min. The retention time of the compounds is almost double that observed with chloroform. For analysis of aliphatic isocyanates, MDI, and TDI, either p-tert-butylphenol or p-methoxyphenol is suitable as an internal standard. With chloroform, the retention time is -3.5 min. The internal standard should be added after the isocyanates have complletely reacted with the fluorescence reagent. In any fluorescence measurement, potential problems due to quenching must be considered; however, it is assumed that
(1) “Criteria for a Recommended Standard-Occupational Exposure to DE. Isocyanates”; Publication No. 78-215 Department of Health, Education and Welfare (NIOSH): Washington, DC, 1978. (2) Marcall, K. Anal. Chem. 1957, 29, 552. (3) Grim, K. E.; Llnch, A. L. Am. Ind. Hyg. Assoc. J . 1964, 25, 285-290. (4) Keller, J.; Dunlap, K. L.; Sandridge, R. L. Anal. Chem. 1974, 46, 1845-1846. (5) Keller, J.; Dunlap, K. L.; Sandridge, R. L. Anal. Chem. 1976, 48, 497-499. (6) Hastlngs, C. R.; KO, C. Y.; Ryan, 1. A. J. Chromafogr. 1977, 134, 451-458. (7) Sango, C. J. Lip. Chmmfogr. 1979, 2(6), 763-774. (8) Hardy, H.; Walker, R. F. Analyst (London) 1979, 104, 890-891. (9) Walker, R. F.; Pinches, M. A. Analyst (London) 1979, 104, 928-936. (10) Keller, J.; Sandridge, R. L. Anal. Chem. 1979, 51, 1868-1870. (1 1) MDA Scientific, Inc., Park Ridge, IL. (12) Levlne, S. P.; Hoggatt, J. H.; Chladek, E.; Jungclaus, Q.; Qerlock, J. L. Anal. Chem. 1979, 51, 1106-1109. (13) Sango, C.; Zlmerson, E. J . Llq. Chromafogr. 1980, 3(7), 971-990. (14) Borch, R. F.; Bernsteln, M. D.; Durst, H. D. J. Am. Chem. Soc. 1971, 93, 2897-2904. (15) Hutton, R. C.; Salem, S. A.; Stephen, W. E. J. Chem. Soc. A 1966, 1573. ’Present address: Bayer, A.Q., Central Analytical Department, 509 Leverkusen-Bayerwerk, West Qerrnany (to whom correspondence from Europe should be addressed).
Laurie H. Kormos* Robert L. Sandridge Jurgen Keller’ Mobay Chemical Corporation New Martinsville, West Virginia 26155
RECEIVEDfor review December 4, 1980. Accepted March 4, 1981.
Refractive Index Gradients in Stopped-Flow and Temperature-Jump Kinetics andl Liquid Chromatography Sir: Some time ago, Gibson (I, 2) observed an anomalous response following the injection of water into a warmer or colder stainless steel cuvette of a stopped-flowapparatus. The apparent absorbance ‘ofthe sample changed rapidly after the flow stopped and then slowly returned to its original level. Gibson’s qualitative explanation is that a transient radial temperature gradient is induced in the liquid by heat flow through the cell w&. The associated refractive index gradient acts as a lens which somehow affects the apparent absorbance of the liquid. This effect has also been reported by Miller and Gordon (3)for 1,2-dichloroethane injected into a Kel-F cell. Similar behavior is obeierved with temperature-jumpapparatus after the application of a pulse of energy. Some effects observed with liquid chromatography (LC)detectors can also be ascribed to tempemture-induced refractive index gradients. Included are changes in apparent absorbance that are observed when the temperature of the flowing liquid is varied, when the nominal flow rate is changed, or when transient changes 0003-2700/81/0353-1125$01.25/0
in flow rate occur. The purpose of this note is to present a somewhat more quantitative explanation of the phenomenon and to describe an optical means of minimizing the effect. Dual wavelength detector systems have been used to compensate for some optical disturbances in both LC and kinetics apparatus (4). These systems tend to be cumbersome, and they are not completely effective because of the wavelength dependence of the effects (5). Moreover, it is recognized that reduction of optical disturbances does not eliminate the effect of temperature errors in reaction rate determinations as discussed by Chattopadhyay and Coetzee (6).
TEMPERATURE DISTRIBUTION Standard methods are available (7)to solve the heat flow differential equation for the temperature distribution in a cylinder. The cylinder has a radius a, is infinite in length, has initially a uniform temperature TO, and at time T = 0 is placed in contact with a reservoir of temperature T,. The 0 1981 American Chemical Soclety
