928 Analyst, October, 1979, Vol, 104, pp. 928-936 Spectrophotometric Determination of Aliphatic lsocyanates in the Occupational Atmosphere Part 1. Determination of Total Isocyanate Concentration R. F. Walker and M. A. Pinches Occupational Medicine and Hygiene Laboratories, Health and Safety Executive, 403-405 Edgware Road, Cracklewood, London, N W2 6LN This paper describes a spectrophotometric method for use in the field for the determination of aliphatic isocyanates and their oligomers in air. The atmosphere being tested is drawn through a mixture of hydrochloric acid and dirriethyl sulphoxide a t a sampling rate of 2 1 min-l for 10 min. Any isocyanates or oligomer present are hydrolysed to the corresponding amine. l-Fluorc1-2,4-dinitrobenzene is added and forms coloured derivatives with the amines.The absorbance of each derivative is measured at approximately 353 nm, and can be directly related to the amount of isocyanate. Concentra- tions down to 0.002 p.p.m. can be determined. Keywords : Aliphatic isocyanate determination ; spectvophotornetry ; l-fluoro- 2,4-dinitrobenzene reagent In general, urethane polymers made from aromatic isocyanates tend to yellow on prolonged exposure to sunlight (possibly because ultraviolet light promotes autoxidation of these polymers to form quinone - imide based products). However, aliphatic isocyanates are used to make urethane polymers with better resistance to ultraviolet-induced discoloration. Consequently, polyurethanes requiring exceptional light stability are largely based on aliphatic isocyanates. Such two-pack polyol-cure paint systems are being used increasingly in the vehicle re-finishing industry.The following aliphatic isocyanates are among the most widely used. This is unsuitable for the production of surface coatings because of its volatility and physiological activity. By treating HMDI with water (3 : 1 molar ratio) it is possible to produce a polyisocyanate of high relative molecular mass, which is no longer volatile and which is used to make polyurethane coatings for air drying and stoving. This compound has two isocyanate groups of different reactivities (the cycloaliphatic group is about ten times less reactive than the aliphatic grouping1), enabling adducts and oligomers to be produced that have a low residual diiso- cyanate monomer content (about 0.7%).These products are used in the production of transparent elastomers, foams, moisture-curing one-pack and polyol-curing two-pack paint systems. Methylene bis(4-cyclohexylisocyanate) (MCHI) . This is used in the manufacture of colourless, light-stable foams and coatings. Trimethylhexamethylene diisocyanate (TMDI). This material consists of a mixture of the 2,2,4- and 2,4,4-isomers. It is used in the production of flexible foams, elastomers and fibres. Like other aliphatic isocyanates, TMDI is mainly used in the form of adducts, condensates or oligomers when used for surface coatings. These diisocyanates indicate the chemical types in the aliphatic isocyanate class which, when combined with polyfunctional polyethers, polyesters or other compounds containing hydroxy groups, are used in the production of polyurethanes.Until recently it was thought that the toxic hazard associated with the application of polyurethane-based surface coatings was a result of monomeric isocyanate vapour in the atmosphere. However, medical evidence2 now suggests that vapour phase inhalation is not the sole method by which isocyanates can enter the respiratory tract, and that inhalation of spray droplets is equally hazardous to health. Therefore, despite the low vapour pressures 1,6-Hexamethylene diisocyanate (HMDI). Isophorone diisocyanate (IPDI). Crown Copyright.WALKER AND PINCHES 929 of the adducts and oligomers gencrally employed in the formation of surface coatings, hazardous concentrations of these polyisocyanates may occur as droplets or as aerosols in industrial atmospheres where the surface coatings are sprayed. The only aliphatic diiso- cyanates with published threshold limit values (TLVs) are IDPI and hICHI,3 but it is wise t o assume that the atmospheric concentration of all aliphatic diisocyanates should not be allowed to exceed 0.01 p.p.m. The spectrophotometric method of von E i ~ k e n , ~ later modified by Pilz and Johann,5 has been used to determine concentrations of atmospheric HMDI. More recently Dunlap et aZ.697 have developed chromatographic methods for determining concentrations of aliphatic isocyanates.Both methods are based on the reaction of isocyanates with N-4-nitrobenzyl- N-propylamine followed by analysis with thin-layer chromatography6 or high-performance liquid chr~matography.~ The method proposed by von Eicken4 was selected as being suitable for adaptation to a general field-test procedure for the aliphatic isocyanates commonly used in industry.Basically the method consists in hydrolysing any isocyanate present to the corresponding amine, followed by reaction with l-fluoro-2,4-dinitrobenzene to form a 2,4-dinitrophenyl- amine derivative. The absorbance of the derivative is then measured at about 353 nm. Both aromatic and aliphatic isocyanates respond quantitatively to this method. A critical examination of the various steps of the von Eicken method was carried out in order to determine the optimum conditions for the collection and analysis of aliphatic isocyanates and their related oligomers. Experiment a1 Preparation of Standard Atmospheres Dynamic standard atmospheres of a range of aliphatic isocyanates were required in order t o assess the efficiency of the sampling procedure and for application in the development of the analytical method.A double-dilution atomisation apparatus* was used, which generated known concentrations of isocyanate in toluene into a constant-flow stream of dried air. The atmospheres were monitored in duplicate; one sample was analysed by the proposed spectro- photometric method, and the other by a long path length infrared gas analyser (Miran l A , Wilks - Foxboro Analytical). The infrared gas analyser was calibrated in advance by injecting known volumes of each aliphatic isocyanate into a closed-shop calibration system, which could be attached to the analyser when required. The results are summaried in Table I.TABLE I ANALYSIS OF ALIPHATIC ISOCYANATE STANDARD ATMOSPHERES Number Isocyanate of runs HMDI . . 5 8 4 IPDI . . . . 7 6 10 MCHI . . 5 9 4 TMDI . . 11 7 4 Calculated concentration* of isocyanate atmosphere a t 20 "C, p.p.m. 0.048 0.105 0.230 0.045 0.112 0.230 0.049 0.091 0.216 0.043 0.104 0.224 Measured concentration of isocyanate atmosphere, p.p.m. By infrared By spectrophotometry r----J------7 & Standard Standard Mean deviation Mean deviation 0.033 0.01 1 0.032 0.004 0.075 0.009 0.071 0.006 0.171 0.006 0.165 0.007 0.037 0.010 0.039 0.005 0.093 0.005 0.097 0.007 0.191 0.008 0.179 0.004 0.042 0.006 0.048 0.006 0.075 0.005 0.078 0.003 0.178 0.004 0.174 0.006 0.030 0.006 0.034 0.004 0.078 0.010 0.085 0.006 0.169 0.01 1 0.149 0.009 r - * Based on the purity of each isocyanate as determined by HPLC.Q By monitoring the stretching vibration of the N=C=O bond at approximately 4.45 pm and using a cell with a 20-m path length, the lower limit of detection using the infrared930 WALKER AND PINCHES : SPECTROPHOTOMISTRIC DETERMINATION OF Analyst, VoZ.104 method was 0.03 p.p.m. The results given in Table I show that good agreement was obtained between the analyses by the two methods for the range of isocyanate atmospheres examined. The standard deviation for each set of measurements is also given. Once a correlation had been established between the analyses by the infrared and spectrophotometric methods, atmospheres at the 0.01 p.p.m. level could be determined by the proposed spectrophotometric method. The latter method gave excellent straight line graphs over the concentration range 0.002-0.03 p.p.m.The coefficients a and b for the straight-line equations A = a -t bc where .A is the absorbance obtained from an isocyanate concentration c , and the correlation coefficients, y2, are given in Table I1 for this range of concentrations. The results are based on a 20-1 sample. TABLE I1 ALIPHATIC ISOCYANATE CALIBRATION LINES A = a + bc OVER THE CONCENTRATION RANGE 0.002-0.03 p .p.m. Isocyanate a b r2 HMDI . . . . 0.0025 5.461 5 0.999 2 0.9996 IPDI . . . . -0.0040 6.4109 MCHI . . . . -0.0020 6.9099 0.9995 TMDI . . . . -0.0115 9.6942 0.9995 Oligomer isocyanates with high relative molecular masses were found to be unsuitable for standard atmosphere generation.In these instances solutions of the isocyanate, diluted with dimethyl sulphoxide, were used instead. Selection of Absorbing Solution Previous work4y5 had shown that dimethyl sulphoxide (DMSO) was the only available common organic solvent with a high boiling-point that was suitable for the absorption of aliphatic isocyanates. However, bottles containing DMSO would often freeze during the winter months because of the high freezing-point (18 "C) of this solvent.1° Therefore, absorbing solutions of equal volumes of DMSO and 4.0 M hydrochloric acid were prepared in the laboratory prior to sampling. In addition, no ageing effects were noticed when using this mixture. The original method4 devised for the determination of HMDI in air made use of a mixture of DMSO and 0.1 M hydrochloric acid as the absorbing solution.Subsequent work5 showed that only partial hydrolysis of HMDI was achieved at this acid concentration and suggested 0.8 8 0.7 0.5 0.3 1 I 0.2 1 0.1 ~ H M D I oligomerj Acid concen t r a t ion/M Fig. 1. Effect of hydrochloric acid concentration on the hydro- lysis of 3 x hi HMDI and 2 x M HMDI oligomer.October, 1979 ALIPHATIC ISOCYANATES I N THE OCCUPATIONAL ATMOSPHERE. PART I 931 the use of 1.2 M hydrochloric acid. The present investigation shows that although mono- meric isocyanates are readily hydrolysed by 0.1 M hydrochloric acid at ambient temperatures, oligomeric isocyanates are completely hydrolysed only by acid concentrations in excess of 4.0 M. Fig.1 shows the effect of varying the acid concentration on the extent of hydrolysis for HMDI and its commercial oligomer after 15 min at 25 "C. Thus, 4.0 M was selected as being the optimum hydrochloric acid concentration for the hydrolysis of both aliphatic isocyanates and their oligomers. The efficiency of the original sampling procedure was examined by passing a standard HMDI atmosphere through three impingers in series at a rate of 2 1 min-l; each impinger contained 8 ml of the absorbing solution. HMDI was chosen for this experiment because it has a relatively low boiling-point compared with those of the other aliphatic isocyanates studied. Standard atmosphere concentrations were generally found to be 30% less than their theoretical value because of isocyanate loss on the glass walls of the apparatus.The results are indicated in Table 111. The three-impinger sampling system is assumed to have a collection efficiency of lO0yo; it can be seen that there is 14% isocyanate breakthrough from the first impinger when sampling a 710,~.gm-~ (0.10 p.p.m.) HMDI atmosphere at 2 1 min-I for 20 min. Most field sampling is done in atmospheres with 0.01-0.02 p.p.m. concentrations and hence a single impinger with a collection efficiency of approximately 94% at these concentrations can be used (Table 111). TABLE I11 EFFICIENCY OF SAMPLING PROCEDURE FOR THE COLLECTION OF HMDI IN 4.0 M HYDROCHLORIC ACID AND DMSO (1 + 1 V / V ) Calculated concentration* Number of atmosphere a t of runs 20 "C/pg m-3 3 25.34 2 25.34 2 75.28 2 75.28 2 165.71 3 165.71 3 320.50 2 320.50 2 483.67 3 483.67 2 710.33 3 710.33 3 710.33 * See Table I.Sanipling rate/l min-l 1.0 2.0 1.0 2.0 1 .o 2.0 1.5 2.0 1 .o 2.0 1.0 1.5 2.0 Sampling time/ in in 10 15 15 15 10 15 15 20 20 15 20 20 20 Mean concentration of isocyanate collccted/pg n1r3 Trap 1 16.1 17.9 52.1 56.3 127.5 121.0 243.5 258.3 381.1 369.7 583.2 570.4 525.5 Trap 2 Trap 3 1.5 0.0 1.2 0.0 1.3 0.0 3.2 0.0 6.4 0.0 11.8 0.6 29.3 1.4 22.6 0.8 39.4 1.1 47.7 1.3 38.6 0 . 9 49.7 1.7 83.2 2.1 Choice of Reagent l-Fluoro-2,4-dinitrobenzene was introduced as a quantitative reagent for primary and secondary amines by Sangerll in the determination of the free amino groups in proteins and peptides. Subsequently it has become one of the most important reagents in the analysis of amino acids and peptides.12 In addition to l-fluoro-2,4-dinitrobenzene, l-chloro-2,4- dinitrobenzene, I-cl~loro-3,5-dinitrobenzcne and l-fluoro-3,5-dinitrobenzene were also examined in order to find which gave both a fast reaction rate with aliphatic amines and a high molar absorptivity.l-Fluoro-2,4-dinitroberizene was found to be the most suitable reagent and was used for this work. I t should be noted that l-fluoro-2,4-dinitrobenzene has been shown13 to give positive results in tests for mutagenesis, and should therefore be con- sidered a possible carcinogen. Reaction Conditions In the original method, complete reaction was achieved by heating the reagents to approximately 70 "C. Standard solutions of the aliphatic isocyanates were made up, hydrolysed and treated with 0.1 ml of 1 yo and 1076 l-fluoro-~,~-dinitrc,benzene solutions over a range of temperatures in order to determine the optimum temperature required for complete reaction.These concentrations of l-Auoro-2,4-dinitrobenzene were considered to932 WALKER AND PINCHES : SPECTROPHOTOMETRIC DETERMINATION OF Analyst, T/d. 104 be the two extremes and both produced similar curve profiles. The results are shown in Fig. 2 for 15-min heating periods using a 1% solution. It can be seen that heating the reagents above 80 "C causes some degradation of the coloured products. Below 70 "C a longer period of heating is required for full colour development; 75 "C was chosen as the optimum temper at ure . 0.4 0.3 u C m a 0.2 Ll 6 0.1 0 p' MCH I i 60 70 80 90 Temperature/" C Fig.2. Effect of varying temperature on the reaction between l-fluoro-2,4-dinitro- benzene and aliphatic iso- cyanates. (Isocyanate con- centration was 9 x lop6 M.) 0.8 r 1 0.7 c I C 0.6 0.5 ' 0.4 6 0.3 0.2 0.1 W 2 a 0 0.2 0.4 0.6 0.8 1.0 Reagent concentration, % V/V Fig. 3. Effect of varying the concentration of l-fluoro-2,4-dinitrobenzerie on the absorb- ance a t 353 nm of the diamine derived from HMDI, with varied heating periods. (Iso- cyanate concentration was 1.8 x M.) A, 5 min; B, 15 min; and C, 30 min. A series of experiments was carried out for each aliphatic isocyanate using increasing concentrations of l-fluoro-2,4-dinitrobenzene to find the optimum reagent concentration. The results are shown in Figs. 3-7 for heating periods of 5, 15 and 30 min at 75 "C with 0.1 ml of reagent and a 1.8 x In each instance an increase in the reagent concentration eventually produced a decrease in the heating period required for full colour development of the amine derivative. Heating for periods greater than the optimum time causes a marked degradation in the absorbance of the colour derivatives.I t can be seen that by using 0.1 ml of a 0.3% l-fluoro- 2,4-dinitrobenzene solution, full colour development was obtained within 15 min at 75 "C. M concentration of isocyanate. 0.7 0.6 a, 0.5 0.4 Q 0.3 13 I) 0.2 O.' 0 L1 0.2 0.4 0.6 0.8 1.0 Reagent concentration, % V/V Fig. 4. Effect of varyin,? the concentration of 1 -fluoro-2,4-clinitrobenzene on the absorb- ance a t 352 I i m of the dialnine derived from IPDI, lvith varied heating periods.(Iso- cyanate concentration was 1.8 :* 10F5 M.) A, 5 min; B, 15 min; and C, 30 min.October, 1979 ALIPHATIC ISOCYANATES IN THE OCCUPATIONAL ATMOSPHERE. PART I 933 0.2 0.4 0.6 0.8 1.0 Reagent concentration, % V/V Fig. 5. Effect of varying the concentration of l-fluoro-2,4-dinitrobenzene on the absorb- ance at 355 nm of the diamine derived from MCHI, with varied heating periods. (Iso- cyanate concentration was 1.8 x l O - 5 ~ . ) A, 5 min; B, 15 min; and C, 30 min. This time is considered satisfactory for the purposes of the proposed field test. Although a more concentrated solution required less heating time, the blank solutions produced in this way had an absorbance that was too great for accurate measurement of the sample solutions. 1 .o 0.8 u CJ 0.6 2 a I) 0.4 0.2 0 L 0.2 0.4 0.6 0.8 1 .o Reagent concentration, % V / V Fig.6. Effect of varying the concentration of 1- fluoro-2,4-dinitrobenzene on the absorbance a t 353 nm of the diamine derived from TMDI, with varied heating periods. (Isocyanate concentration was 1.8 x M.) A, 5 min; B, 15 min; and C, 30 min. Determination of Wavelength Absorbance Maxima Aliquots containing 15 pg of the standard isocyanate solutions were dispensed into 8 ml of the absorbing solution by use of an adjustable micropipette (Pipetman, Gilson Commercial Service). The mixtures were agitated and allowed to stand for 10 rnin at room temperature in order to ensure complete hydrolysis of the isocyanates to the corresponding primary amines. Colour derivatives were developed by using the proposed field method and double- extracted with AnalaR 1,1,2-trichloroethane. The absorbances of the derivatives were measured in the range 320-380 nm against a blank solution.The wavelengths of maximum absorbance were found to be approximately 353 nm for the aliphatic isocyanate monomer derivatives and 350 nm for the HMDI oligomer derivative.934 WALKER AND PINCHES : SPECTROPHOTOMETRIC DETERMINATION OF Analyst, VoZ. 104 u 0.2 0.4 0.6 0.8 1 .o Reagent concentration, % VIV Fig. 7. Effect of varying the concentration of 1- fluoro-2,4-dinitrobenzene on the absorbance at 350 nm of the triamine derived from HMDI oligomer, with varied heating periods. (Isocyanate concentration was 1.8 x 1 0 - 5 ~ . ) A, 5niin; B, 15min; and C, 30 min. Molar absorptivities, E , have been calculated for each reaction product (Table IV).It is interesting to note that the molar absorptivity of the HMDI oligomer is considerably higher than that of HMDI itself. Thus, it is unlikely that the central isocyanate grouping of the oligomer (see Fig. 1) is sterically hindered, as is the case for certain secondary amines to this reaction, e.g., diisopropylamine and dicy~lohexylamine.~~ A commercial sample of HMDI oligomer was analysed by use of HPLCg and found to contain 0.82 If 0.05% of free HMDI. A correction factor has therefore been included in the calculation of the molar absorptivity of the oligomer (based on three functional amino groups). TABLE IV MOLAR ABSORPTIVITIES OF THE SECONDARY DIAMINES FORMED BY THE REACTION OF ALIPHATIC DIISOCYANATES WITH 1-FLUORO-2,4-DINITROBENZENE Isocyanate .. HMDI IPDI MCHI TMDI HMDI oligomer f . . . . 33729 f 873 38610 f 649 41.803 f 837 57885 & 1567 45550 3 7171 ~ m a x . l n m * * 353 352 355 353 350 Preparation of Calibration Graphs Calibration graphs for the various aliphatic i:socyanates studied were prepared by using standard isocyanate solutions and adding known aliquots to 8 ml of the absorbing solution. This was considered to be a more reliable method than standard atmosphere generation, where flow fluctuations, albeit minor, and isocyanate adsorption and desorption may occur. Some difficulty was encountered in quantitatively dissolving the necessary amounts of HMDI oligomer (about 200 mg) in DMSO. Satisfactory dissolution was achieved by weighing accurately about 200mg of the oligomer on to a glass slide, which was then placed in a 50-ml glass beaker containing 25 ml of DMSO for approximately 20 min.The solution was then transferred to a 100-ml flask and the procedure repeated, if necessary, in order to remove residual traces of the oligomer from the slide. The solution was made up to volume with DMSO. Aliphatic isocyanate monomer solutions were prepared by dispensing suitable aliquots of the isocyanates directly into 100-ml flasks and making up to 100 ml with DMSO. Occasionally the solutions became opalescent on being made up to volume, but vigorous shaking for 2 or 3 min dissipated any cloudiness. Calibration graphs, over the concentration range 0-20 pg, were prepared for all of the isocyanates studied, by using the optimum conditions determined for each stage of the procedure.A linear response was exhibited between the concentration of each isocyanate and the absorbance of its tliamine. Table V shows the absorbances,October, 1979 ALIPHATIC ISOCYANATES IN THE OCCUPATIONAL ATMOSPHERE. PART I 935 TABLE V ABSORBANCE VALUES PRODUCED BY THE PROPOSED FIELD METHOD Isocyanate ~ HMDI IPDI MCHI TMDI HMDI oligomer L - f - 7 r - - - \ r - - - - - - ~ 7- Amount of Standard Standard Standard Standard Standard isocyanatelyg Mean deviation Mean deviation Mean deviation Mean deviation Mean deviation 5 0.208 0.09 0.170 0.06 0.155 0.04 0.265 0.06 0.051 0.03 10 0.392 0.08 0.349 0.08 0.324 0.07 0.564 0.05 0.213% 0.02 15 0.595 0.05 0.531 0.09 0.485 0.07 0.829 0.04 0.255 0.09 20 0.803 0.05 0.697 0.10 0.648 0.09 1.117 0.11 0.330 0.07 measured on a Pye Unicam SP6-500 spectrophotometer at the appropriate wavelengths (see Determination of Wavelength Absorbance Maxima) with a 10-mm path length quartz cell.Five determinations were made at each concentration. Interferences N-Ethylmorpholine, NN-dimethylcyclohexylamine and other tertiary amines commonly used as catalysts in the polyurethane industry do not interfere in the proposed field method. Primary amines, if present, will interfere and the determination of aliphatic iso- cyanates in the presence of primary amines, together with data from field trials, will be the subject of Part I1 of this paper. Proposed Field Method for the Determination of Aliphatic Isocyanates in Air Apparatus 1-cm path length quartz cell.inlet tube and a flat-bottomed receiver. 2 1 min-1. ture to 100 "C. Spectrophotometer. Impingers. Sampling pump. Heating block. Mechanical stirrer. Fisons Whirlimixer. A Pye Unicam SP6-500 spectrophotometer equipped with a 4-ml, All-glass midget impingers of the Greenberg - Smith type, with a tapered jet A sampling pump capable of drawing air through the apparatus at A heating block (Techne, Model DB-3) with a range from ambient tempera- Reagents All the reagents were of analytical-reagent grade unless otherwise stated. Absorbing solution. Standard sodium hydroxide solutions, 0.1 and 0.4 M. Sodium borate bufer solution. Dilute 36 ml of concentrated hydrochloric acid (sp. gr. 1.19 at 20 "C) to 100 ml with distilled water. Mix 50 ml of the resulting solution with 50 ml of DMSO. Dissolve 30.0 g of boric acid in 700 ml of distilled water.Add slowly, while stirring, 4.0 M sodium hydroxide solution until the pH is 8.8, as indicated by a pH meter. Pipette 0.3 ml of 1 -fluoro-2,4-dini t ro benzene into 5ml of DMSO in a 10-ml flask. Prepare a fresh solution daily. Suitable amounts of the aliphatic isocyanates to be moni- tored are dissolved in DMSO and the solutions made up to 100 ml. For example, 35 pl of HMDI (Bayer AG) are dissolved in 100 ml of DMSO (see Preparation of calibration graphs). Freshly prepared solutions were used. 1 - Fluoro-2,4-dinitro benzene solution. Dilute the solution to volume with DMSO. Standard isocyanate solutions. 1 , l ,Z-Trichloroetlzane. AnalaR-grade material was used. Procedure impinger receivers.and a second to which 10 p1 of the appropriate standard isocyanate solution are added. standard is included in the test procedure as a check on the calibration graphs. In an uncontaminated atmosphere pipette 8.0 ml of the absorbing solution into the midget- Set aside one impinger so that its contents act as the blank solution The Insert the936 WALKER AND PINCHES inlet tubes and position the impingers at the sampling site in a vertical position (the position was found to be especially important when monitoring aliphatic isocyanates in paint aerosols). Alternatively, a midget impinger can be attached to the worker as close to his breathing zone as possible, usually to his lapel. Attach the pump to the impinger and draw a sample of the atmosphere being tested through the absorbing solution at a flow-rate of 2 1 min-1 for at least 10 min.Do not attach rubber or poly(viny1 chloride) tubing to the air inlet of the impinger as the tubing will adsorb isocyanate vapour or spray and this adsorption causes low results. Disconnect the impinger from the pump, remove the impinger and its contents to an uncontaminated atmosphere, and allow them to stand for 10 min to ensure complete hydrolysis of the isocyanate collected. Next lift the inlet tube clear of the liquid in the impinger receiver tube and expel the liquid from the inlet tube with a blow-ball. Neutralise the solution in the impinger with 4.0 ml of 4.0 M sodium hydroxide solution (checking the pH with indicator paper). By US(: of a pipette, add 1.0 ml of sodium borate buffer solution and mix.Then add 0.1 ml of 0.3% l-fluoro-2,4-dinitrobenzene solution and again mix. Heat the mixture for 15 min at 75 “C on a heating block (taking care not to heat it above 75 “C). Below 70 “C a longer period of heating is required for full colour development. Remove the impinger receiver tubes from the heating block and allow their contents to cool for 5 min. Next add 4.0 ml of 0.1 M sodium hydroxide solution and mix. Pipette 3.0 ml of 1,1,2-trichloroethane into each receiver tube and mix the contents on a Whirlimixer for 30 s, then transfer the liquid to a 25-ml separating funnel, allowing the two phases to separate, and filter the lower organic layer through a folded sheet of 7-cm Whatman No. 2 filter-paper into a 5-ml flask.Return the aqueous layer back into the receiver tube and repeat the extraction with 2.0 ml of 1,1,2-trichloroethane. Combine the second extract with the first, and make up the solution to 5 ml with 1,1,2-trichloroethane. Finally, shake the flask. The absorbance is measured a t the appropriate wavelength (see Table IV), using the blank solution to zero the spectro- photometer. Determine the amount of isocyanate by reference to the appropriate calibra- tion graph. The solution is stable for at least 24 h. Conclusion and and 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. A spectrophotometric method for use in the field has been developed for the determination of low concentrations of atmospheric aliphatic isocyanates. The limits of detection are significantly below the current threshold limit value. The proposed method has been satisfactorily assessed under field conditions at a number of car re-finishing industries. The equipment necessary to carry out the analysis is portable requires only an electrical power supply to operate the spectrophotometer, heating block mechanical stirrer. A determination can be completed in about 1 h. References “Isophorone Diisocyanate IPDI,” Product Information Sheet 22-ME-871-6, Veba-Chemie AG, Bunge, W., Ehrlicher, H., and Kimmerle, G., Zentbl. Arbeitsmed. Arbeitsschutz Prophylaxe, 1977, 4, Health and Safety Executive, “Threshold Limit Values for 1977,” Guidance Note EH15/77, HM von Eicken, S., Mikrochinz. Acta, 1958, 731. Pilz, W., and Johann, I . , Mikrochim. Acta, 1970, 351. Keller, J . , Dunlap, K. L., and Sandridge, R. I>., Analyt. Chern., 1974, 46, 1845. Dunlap, K. Id., Sandridge, I<. L., and Keller, J . , Analyt. Chem., 1976, 48, 497, Meddle, D. \V., and Wood, K., Cheiny I n d . , 1968, 1635. Cox, G. U., and Sugden, K., Analytica Chiw.. Acta, 1977, 91, 365. Weast, R. C., Editor, “Handbook of Chemistry and Physics,” Fifty-third Edition, CRC Press, Cleveland, Ohio, 1973, p. C-501. Sanger, F., Biochem. J . , 1945, 39, 507. Pataki, C . , “Thin Layer Chrornatography in Aminoacid and Peptide Chemistry,” W. de Gruyter, Purchase, I. F. H., Longstaff, E., Ashby, J., Styles, J . A., Anderson, D., Lefevre, P. A., and West- ?kTcIntire, F. C., Clements, W. M., and Sproull, M., Analyt. Chew.., 1953, 25, 1758. Gelsenkirchen, Germany, 1971. 1. Stationery Office, London, 1978. Berlin, 1966. wood, I;. I<., R r . J . Cancer, 1978, 37, 873. Received February 19th, 1979 Accepted A p r i l 19th, 1979