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Anal, Chem. 19a4, 56, 7604-7608
Determination of Polymethylenepolyphenylene Isocyanate in Air by Size Exclusion Chromatography Ronald K. Beasley and J. Michael Warner*
Research Department, Monsanto Agricultural Products Company, 800 North Lindbergh Boulevard, St. Louis, Missouri 63167
A method has been developed for the determination of polymethylenepolyphenylene Isocyanate (PAPI) In air. PAPI Is collected on a glass fiber filter coated with N-(p-nltrobenzyl)-N-npropylamlne as an In situ derlvatlrlng agent. The resulting derivative Is extracted wlth tetrahydrofuran, treated successively with acetyl chloride and methanol, and analyzed by size exclusion llquld chromatography using a UV-vlslble detector at 254 nm. The method was laboratory validated for the range of 2-155 ppb (29-2300 pg m-’) PAPI In air for a 45-L air sample. The method has been used to monitor the level of PAPI In an lndustrlal environment.
Polymethylenepolyphenylene isocyanate (PAPI) is used industrially for the production of polyurethanes and polyisocyanurates. Monsanto Company has maintained a commitment to develop procedures to monitor and control the level of exposure of its employees to chemicals it uses. Potential exposure of workers to aromatic isocyanates is of concern since this is a generally recognized hazardous class of compounds. The toxicity of PAPI has been summarized (I,2) and acute effects include cough, pain on breathing, and increased mucous secretion. Repeated exposure may precipitate allergic reactions resembling hay fever or asthma. However, its potential for chronic effects has not yet been defined. The American Conference of Governmental Industrial Hygienists (ACGIH) has set a threshold limit value (TLV) for diisocyanates at 5 ppb for an 8-h workshift and at 20 ppb for any 15-min sampling period (3). For PAPI these levels correspond to 74 and 290 pg m-3, respectively. The generally low volatility of aromatic isocyanates makes them difficult to analyze by gas chromatography. However, numerous methods for their determination in air exist in the literature. Methylenediphenyl isocyanate (MDI) and toluene diisocyanate (TDI) have been studied extensively. Early work involved colorimetric test papers or midget impingers (4-10)while more recent methods have used derivatizing agents coated on solid sorbents (11,12). These solid-state derivatization techniques offer the advantage of ease of use. In contrast to impingers, the solid sorbent samplers offer greater user mobility, are more lightweight, and have no problems with evaporation or spillage. The isocyanate group reacts readily with many nucleophiles including alcohols and amines which form urethanes and ureas, respectively. This high level of reactivity limits the direct analysis of isocyanates by high-pressure liquid chromatographic (HPLC) methods to a small number of mobile phases and columns. Precolumn, in situ derivatization (chemisorption) makes HPLC analysis more attractive for this application. A number of derivatizing agents for isocyanates may be found in the literature (4-12). Extensive work has been done with N-03nitrobenzy1)-N-n-propylamine(NBPA) as an isocyanate derivatizing agent (8-10,12).The urea formed in this reaction 0003-2700/84/0358-1804$01.50/0
has the advantages of being relatively nonreactive and highly UV active. These features allow HPLC analysis with UV detection.
The analysis of PAPI in air is more complicated than that of other isocyanates because it consists of a mixture of chain lengths. As shown in the equation, chain lengths may vary from two to eight phenylene groups. The average functionality is 2.7 and the nominal molecular weight is 360. Normal- or reverse-phase liquid chromatography of the oligomers present in the NBPA plus PAPI product mix (PAPU) often results in a number of peaks. This complicates the chromatogram and decreases sensitivity (several small peaks are harder to quantitate than one large peak). Thus a goal of this work was to develop a method allowing the coelution of the various PAPU oligomers to facilitate the analytical procedure. This paper reports a procedure for the sampling of PAPI in air as an aerosol and the in situ derivatization with NBPA with subsequent analysis. The sampling method is an expansion of the work of Tucker and Arnold to monitor for MDI and TDI in air (12).Differences from that work include the use of an aerosol standard atmosphere generation system and the development of a procedure for the preparation of more uniform NBPA-treated glass fiber filters. In addition, a method for the destruction of excess NBPA was developed together with a chromatographic technique suitable for the quantitation of polymeric PAPI.
EXPERIMENTAL SECTION Reagents. Tetrahydrofuran, toluene, methanol, hexane, and dichloromethane were Matheson, Coleman and Bell (Cincinnati, OH) distilled in glass OmniSolv. Toluene was dried over calcium hydride before use. PAPI-135 (polymethylenepolyphenyleneisocyanate),was obtained from Upjohn Polymer Chemicals (La Porte, TX). TDI was obtained from Eastman Kodak Co. (Rochester, NY) as a 4:l mixture of its 2,4 and 2,6 isomers. Polystyrene, 35 OOO nominal molecular weight (used as internal standard), was obtained from Waters Associates (Milford,MA). Reagent acetyl chloride and purified calcium hydride were obtained from Fisher Chemical Co. (Pittsburgh, PA). 4-Nitro-Npropylbenzylamine hydrochloride (N-(p-nitrobenzy1)-N-npropylamine hydrochloride) was obtained from Aldrich Chemical Co. (Milwaukee, WI). Apparatus. A Perkin-Elmer (Norwalk, CT) Series 3B highpressure liquid chromatographic pump, LC-75 spectrophotometer, LC-75 autocontrol, and Model 420B autosampler were used for all separations. The chromatography was performed on a 25.0 cm X 6.2 mm i.d. Du Pont (Wilmington, DE) Zorbax PSM 60-S size exclusion HPLC column. It was preconditioned by flushing with methanol at 1 mL min-’ for several hours, followed by THF 0 1984 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 56. NO. 9,AUGUST 1884
at 1mL min" until a stable base line was achieved. Occasional regeneration of the column was necessary when retention times started to increase noticeably. Regeneration was accomplished by repeating the preceding procedure and was necessary approximately once a week under constant instrument use, A Hewlett-Packard (Avondale, PA) 3390A recording integrator was used for data collection. Standard Atmospheres. All standard concentrationsof PAPI in air were produced with a TSI, Inc. (St. Paul, MN), Model 3050 Berglund-Liu monodisperse aerosol generator with a Model 3054 aerosol neutralizer and a New England Medical Instrument, Inc. (Medway, MA), accurate infusion pump (syringe pump). Dispersion air on the aerosol generator was set at 1.5 L min-' and dilution air was set at 16.7 L min-' for a total air flow of 18.2 L mi&. A five-port sampling manifold was attached directly to the output of the aerosol neutralizer. With this manifold it was possible to use three ports for aerosol sample collection with treated glass fiber filters, one port for corroborative collection, and one open port to prevent back pressure. Treated Glass Fiber Filters. Approximately 0.10 g of NBPA hydrochloride was dissolved in 15 mL of water. The solution was made basic in dim light (see Results and Discussion section) with 10 mL of 1 M sodium hydroxide solution. The mixture was extracted twice with 15 mL of hexane. The combined hexane solution was dried over sodium sulfate. It was filtered into a foil-wrapped 250-mL round-bottom flask to which was added 50 individually separated Gelman 13-mm A/E glass fiber fdters. The contents of the flask were swirled to assure even wetting of the filters. A nitrogen line was attached to the feed tube of a rotary evaporator so that a 50 mL min-l stream of nitrogen could be blown directly into the flask. The flask containing the filters was attached to the evaporator and slowly turned so that the filters would coat uniformly as the hexane was evaporated at ambient pressure. Evaporation was continued to dryness and the treated filters were stored in a foil-wrapped jar. As a precaution against decomposition, the filters were routinely stored in a refrigerator. However, they were found to be stable at room temperature for at least 2 weeks. Preparation of Urea Derivatives. The NBPA ureas of PAPI and TDI were prepared essentially by the method of Hastings Vogt et al. (13). NBPA Urea of PAPI (PAPU). Approximately 2.1 g of NBPA hydrochloride was dissolved in 50 mL of 1N sodium hydroxide. The resulting cloudy mixture was extracted twice with 25 mL of toluene. To the toluene solution was immediately added 1.0 g of PAPI in 30 mL of toluene. A semisolid brown precipitate formed immediately. The supernatant toluene was decanted leaving the crude PAPU adhering to the sides of the flask. The PAPU was dissolved in minimal dichloromethane and to it was added 75 mL of hexane. A yellow-white precipitate formed and the mixture was allowed to stand for 1h. The supernatant liquid was decanted and the semisolid PAPU was allowed to stand overnight. During this time it formed a brown solid. Experience indicated no special storage precautions were necessary for PAPU. yield, 2 g; NMR (60 MHz, Me2SO-d6,integrals represent area ratios) 6 0.83 (m, 3.3), 1.10-1.80 (m, 3.2), 3.33 (broads, 2.0), 3.83 (broads, 2.0), 4.72 (broads, 2.3), 5.77 ( 8 , l.O), 6.75-7.72 (m, 7.2), 7.98-8.48 (m, 2.5). NBPA Urea of TDI. A mixture of the NBPA ureas of 2,4-TDI and 2,6-TDI (2,4-TDU and 2,6-TDU, respectively) was prepared in the same fashion as for PAPU using 1.05 g of a 4:l mixture of these isocyanates, respectively; yield 3.01 g. Air Sampling. Du Pont P4000A personnel pumps were used for sample collection. The collection matrix was an NBPA-treated glass fiber filter (vide supra). The treated filters were held in a Swinnex disk filter holder produced by Millipore Corp. (Bedford. MA). The outlet stem of the filter holder was attached to the personnel pump with Tygon tubing. Pumps were calibrated in a clean atmosphere at approximately 500 mL m i d with the filter and filter holder attached. Sample air was drawn through the filters at this rate for 90 min. Sample Preparation and Analysis. A standard solution in tetrahydrofuran (THF) was prepared containing an accurately known amount of 35K molecular weight polystyrene. The concentration used in this method development was 206 pg mL-'. From this internal standard solution were prepared two other
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solutions, 0.1 mM acetyl chloride (9 pL mL") and 0.4 mM methanol (9 pL mL-l), They were labeled solutions A and B, respectively. After the sample was taken, each glass fiber filter was transferred to a 1-dram developing vial, 500 pL of internal standard solution was added with an Oxford pipet, and the sample was desorbed for 1h in a charcoal developer (vibrator, Occupational Health Associates, Manchester, MO). Next 170 pL of solution A, containing acetyl chloride, was added and the vial was mixed well and allowed to stand for 1min. Then 330 pL of solution B, containing methanol, was added and the vial was again allowed to stand for 1min after thorough mixing. The solution then was filtered through an Acrodisc-CR membrane filter assembly (Gelman Instrument Co., Bedford, MA, 0.45 pm pore size) into an HPLC autosampler vial for analysis, Sample injection size was 50 pL. The UV detector was set at 254 nm which corresponded to A,, for PAPU. No peaks were observed to elute after 10 min. Therefore, autosampler recycle time was set at 11min. An injection of authentic PAPU standard solution containing polystyrene internal standard was made after every three sample analyses. Quantitation was based on the ratio of areas, PAFWpolystyrene, measured by an electronic integrator and using a calibration curve from a series of standards. The standards contained 1.0-103 pg mL-l authentic PAPU (corresponding to 0.61-156 ppb PAPI in air for a 45-L air sample) and 206 pg mL-l polystyrene. Method Validation. The method was validated by the collection, workup, and analysis of six aerosol samples of PAPI at each of five concentration levels from 2 to 155 ppb (29-2300 pg mmS)for a 45-L air sample (29-2300 pg m-3). A colorimetric impinger method was used simultaneously with the treated filter method to corroborate the results (7). Colorimetricdeterminations were performed on a Bausch & Lomb (Rochester,NY) Spectronic 100 spectrophotometer. Laboratory spike recoveries were performed by spiking three series of treated glass fiber filters with aliquots of a 1.14 mg mL-' solution of PAPI in toluene. Spiking was performed with a Hamilton syringe and corresponded to 3.3,6.6, or 33 pg of PAPI. Each filter was worked up and analyzed as described above. Sample filter storage stability studies were performed by collecting nine samples of PAPI in air from the aerosol generator in seta of three samples each. For each set colorimetric impinger corroboration was used. One set of samples was analyzed immediately. The other sets were stored at room temperature in separate aluminum-foil-wrappedjars. One set was analyzed after 24 h and the final set was analyzed after 5 days. In order to study humidity effects on PAPI sample collection and analysis, streams of 96.5% relative humidity (RH) air were generated. This was performed by sparging air from an in-house air line through a saturated solution of potassium sulfate in water. A General Eastman (Watertown, MA) Model 400C % relative humidity/temperature monitor was used to observe the RH of the stream. Humid air was drawn through each filter in two series of three treated glass fiber filters for 20 min at approximately 500 mL m i d prior to sample collection from the aerosol generator. After sample collection humid air was drawn through the filters for 70 additional minutes at the same pump rate. One series was worked up and analyzed immediately. The second series was stored at room temperature in a foil-wrappedjar for 5 days prior to workup and analysis. The effects of various air sampling rates were examined. An aerosol stream of approximately 70 ppb was sampled by using treated filters at rates of 200, 1000, and 2000 mL mi&. Simultaneous corroborative sampling at 500 mL m i d with treated filters was used. The distribution of particle sizes produced by the aerosol generator was monitored with a TSI, Inc. (St.Paul, MN), APS-33 aerodynamic particle sizer. The selectivity of the chromatographic determination for three aromatic isocyanates was studied. A standard solution was prepared containing 160 pg mL-* PAPU and 130 pg mL-l of a 4 1 mixture of 2,4- and 2,6-TDU (96 and 24 pg mL-', respectively). A 50-pL aliquot of this solution was analyzed by the same method used for the other sample solutions described in this work. Field Testing. A field test of this method was performed at an industrial site at which PAPI was in use as a raw material and
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 9, AUGUST 1984 A
TIME (mln)
0 2 4 6 8 10 TIME (mln)
Flaure 1. HPLC chromatograms resulting from the desorption of an NBPA-treated glass fiber filter into THF containing 206 kg mL-‘ of polystyrene, as described In the Experimental Section: (a) without treatment wlth acetyl chloride and methanol; (b) treated with acetyl chloride and methanol.
near which many other chemical processes were being performed.
RESULTS AND DISCUSSION The method for PAPI (polymethylenepolyphenylene isocyanate) in air was validated over the range of 2-155 ppb and field tested as described in the Experimental Section above. The results of this work are discussed below. Air Sampling. NBPA (N-(p-nitrobenzy1)-N-n-propylamine) treated glass fiber filters offer an efficient, reliable medium for PAPI aerosol collection and in situ derivatization. A number of in situ derivatization techniques exist in the literature (12,14). However, with the use of a slightly modified rotary evaporator, this work offers an improved method of coating of the filters. NBPA has been reported to be both thermally and photochemically unstable (IO). The treated filters used in this work were found to be usable for at least 2 weeks when stored at room temperature in the dark. As a precaution, however, they were routinely stored at 5 OC during method development. NBPA was chosen over other aromatic amines as the derivatizing agent of choice because of ita steric bulk resulting from the freely rotating propyl group. A bulky derivatizing agent should provide a greater advantage when using size exclusion chromatography. During the development of this method, backup filters were used to monitor for breakthrough. No breakthrough was observed, however, even under conditions of high PAPI concentrations (155 ppb) or high relative humidity (96.5%RH). While the use of a backup filter causes no buildup of back pressure at the sampling train, it is probably not a necessary precaution. Sample Preparation and Analysis. After desorption of the sample filter into THF, the solution was treated with acetyl chloride and then methanol. This procedure had a 2-fold purpose. Excess, unreacted NBPA eluted well after PAPU under the chromatographic conditions of this method. While it was not an interferent, it did tail to such an extent that injections could be made only once every 30-40 min. Acetylation of excess NBPA with acetyl chloride reduced the polarity of the excess derivatizing agent and sharpened the peak sufficiently to allow a total analysis time of less than 10 min. The risk of column damage by irreversible reaction with excess acetyl chloride was eliminated by the addition of a small amount of methanol to form methyl acetate. This esterification generates 1 equiv of hydrogen chloride. The slight increase in sample acidity did not cause observable column damage. Representative chromatograms of samples with and without acetyl chloride/methanol treatment are shown in Figure 1. Comparison of NMR spectra (60 MHz) of PAPU before and after addition of acetyl chloride showed that this sample treatment had no observable effect on the integrity of the analyte.
1 1 1 1 1 1 0 2 4 6 8 1 0 TIME (rnin)
Figure 2. HPLC chromatogram resulting from the injection of 50 &L of a sample resulting from the collection of 11 kg of PAPI (17 ppb for a 45-L air sample) on an NBPAtreated glass fiber filter, as described in the Experlmental Section.
The second reason for the acetyl chloride/methanol treatment is that repeated injection of untreated samples caused a noticeable increase of analyte retention time. This phenomenon has been observed by others (12). A possible explanation is the retention of NBPA, changing the column’s separation characteristics with time. Frequent purge washings with methanol returned the column’s separation performance to its original state. Acetyl chloride/methanol sample treatment reduced this requirement to approximately one wash per week even with heavy instrument use. A size exclusion HPLC column was used because it separates components on the basis of the molecular size. The nominal molecular weight of PAPU is approximately 880 (based on a PAPI nominal molecular weight of 360 and an isocyanate functionality of 2.7). It consists of approximately 50% MDU and 50% higher oligomers. Acetyl NBPA, methyl acetate, and methanol have significantly smaller molecular dimensions than do the oligomers comprising PAPU. These smaller molecules eluted late enough in the chromatogram that they did not interfere with the quantitation. The internal standard, 35K molecular weight polystyrene, is sufficiently large in its nominal molecular dimensions that it eluted essentially with the solvent front, well before PAPU. An example of these separations is shown in Figure 2. Two shoulders on the leading edge of the PAPU peak are evident, corresponding to partial resolution of the higher weight oligomers. However, these shoulders caused no problem with the integration of the peak of interest. There was a major advantage to having all PAPU oligomers elute as one major peak. A single large peak simplified area integration and permitted lower detection limits than did several smaller peaks. The Zorbax PSM 6 0 4 column met these needs: a single PAPU peak was obtained with a high degree of resolution from the other components. No interferences were encountered during analysis of laboratory-generated samples or blanks. Method Validation. Laboratory validation for this method was performed over the range of 20-2300 kg m-3 (2-155 ppb based on a 45-L air sample). Data were analyzed by NIOSH-type statistical testing (15-17). The pooled coefficient of variation (relative standard deviation) was 0.067. The average sample recovery for the five validation levels was 103% relative to the colorimetric impinger corroborative method. Linear regression analysis of PAPI in air by the treated filter method vs. the impinger method produced an excellent correlation (R2 = 0.995) with a slope of 0.97 and a y intercept of 0.24 ppb. Results at these five levels are summarized in Table I. An example of a chromatogram which resulted from the sampling of a 17 ppb PAPI in air stream used in laboratory validation is reproduced in Figure 2. Standard atmospheres of PAPI were produced as aerosols
ANALYTICAL CHEMISTRY, VOL. 56, NO. 9, AUGUST 1984 ~
Table I. Results for Validation of PAPI in Air Method PAPI in air PAPI by by reference filter method,a method,b ppb ppb 2.0 5.0 16 67 155
1.8 4.6 16 76 150
RSDC
90 92 100 113 97
0.088 0.098 0.061 0.038 0.035
Based on a 45-L air sample. Value was determined by colorimetric impinger method used for corroboration ( 7). Average of six samples. Measurements were tested for outliers by Grubb's test ( 1 6 , 1 7 ) at the 95% confidence level. Pooling the individual RSDs gives the result of 0.067 with a X z value from Bartlett's test ( 1 6 , 18) of 8.15. Table 11. Storage Stability of PAPI on Treated Filters after Samplinga
day 0 1
5
PAPI in air, ppb filter reference method method 5.0 (0.6) 4.4(0.1) 6.2 (0.3)
Table 111. Storage Stability of Samples Collected under High Relative Humidity Conditionsa PAPI in air, ppb filter reference method method
%
recovery
5.0 5.2 5 .I
%
filters of reference 100 85 109
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~~
day
5.2 (0.3) 4.1 (0.1)
0 5
% PAPI filters of reference
4.9 5.0
106 82
Sample filters were stored in the dark at room temperature and analyzed as described in the Experimental Section. Average of three samples, based on a 45-L air sample. Values in parentheses represent standard deviations. Values were determined by the colorimetric impinger method (7). All corroborative analyses were performed at day 0. Table IV. Pump Rate Effect pump rate, mL/min 200 1000 2000
ppba test ppbb std sampling rate sampling rate 70 66 83
68 73 83
a Concentration of PAPI in air determined at pump Conrate indicated (average of two determinations). centration of PAPI in air determined simultaneously at a pump rate of 500 mL/min (average of two determinations).
a Sample filters were stored in the dark at room temperature and analyzed as described in the Average of three samples, based Experimental Section. on a 45-L air sample. Values in parentheses represent standard deviations, Values were determined by the colorimetric impinger method (7). All corroborative analyses were performed at day 0.
using a Berglund-Liu monodisperse aerosol generator. The low volatility of PAPI (vapor pressure of 3 X lo4 torr at 25 "C,ref 13) makes it unlikely that it would be present in the air in significant amounts as a vapor. Thus, any airborne potential hazard would be in the form of an aerosol. It is generally recognized that the upper limit of respirable particle size is approximately 10 pm in diameter (18). Aerodynamic particle size measurements of the aerosols used in the method validation (2,17, and 155 ppb) were determined to be 1-8 pm in diameter and were thus in the respirable range. Storage for up to 5 days of samples collected on NBPAtreated glass fiber filters had a minimal effect on the PAPU analysis. Results of a storage study at the 5 ppb level are summarized in Table 11. Average recovery (relative to the corroborative method) after 1 day was 85% and after 5 days was 109%. These differences from the corroborative method are not statistically significant. It is important to note that the effects of high humidity (96.5% RH) and of a combination high RH/5 day storage were negligible. This was critical to allow the application of this method to actual in-plant conditions. Average recovery (relative to the corroborative method) for the high RH/no storage conditions was 106%, and for the high RH/5 day conditions it was 82%. These data are summarized in Table 111. No adverse effects on the determination of PAPI in air were observed by varying the sampling rates (200,1000, and 2000 mL min-') relative to the 500 mL min-l standard rate used in this work. The data from these studies are summarized in Table IV. Average recoveries as PAPU of the 3.3,6.6, and 33 pg spikes of PAPI on treated filters were 112,98, and 94%, respectively. These spike levels were equivalent to 5,10, and 50 ppb PAPI in 45-L air samples.
0 2 4 6 810 TIME (mln)
Figure 3. HPLC chromatogram resulting from the injection of 50 pL of a solution containing 160 pg mL-' PAPU, 96 pg mL-' P,CTDU, and 24 pg mL-' 2,6-TDU.
A chromatogram illustrating the separation found on this HPLC system for PAPU, 2,4-TDU, and 2,6-TDU is shown in Figure 3. While resolution was not complete between PAPU and 2,4-TDU and between 2,4- and 2,6-TDU, their retention times were sufficiently different that they could be at least qualitatively determined. Field Testing. Field test samples collected at an industrial site at which PAPI was in use as a raw material showed that its concentrations were below detectable levels (
