Determination of Isocyanates in the Working Atmosphere by Thin-Layer Chromatography Jiirgen Keller Farbenfabriken Bayer GmbH, OC-Produktion, Organic Analytical Laboratory, 509 Leverkusen-Bayerwerk, West Germany
K.
L. Dunlap' and R. L. Sandridge
Mobay Chemical Company, Division of Baychem Corporation, New Martinsville, W. Va. 26 755
With t h e large usage of isocyanate resins in spraying and foaming applications, and t h e growing concern over t h e exposure of workers t o isocyanate vapors, there exists t h e need for a way of determining t h e isocyanate content of industrial working atmospheres. This includes not only free isocyanate monomers but also isocyanate resin aerosols with free isocyanate groups which may be present in spraying applications. Of t h e methods currently available, none can be used t o estimate both t h e isocyanate monomer a n d resin in t h e working atmosphere. T h e colorimetric method described by Ehrlicher a n d Pilz (1, 2) a n d Marcali ( 3 ) is widely used t o determine toluene diisocyanate (TDI) in t h e atmosphere. However, this method is applicable only t o aromatic isocyanates a n d is subject t o interferences by aromatic amines. Some difficulties have been encountered in applying this method t o t h e determination of 4,4'-diphenylmethane diisocyanate (MDI) in air. T h e method of von Eicken ( 4 ) which was later modified by Pilz a n d J o h a n n ( 5 ) can be used t o determine hexamethylene diisocyanate ( H D I ) in air. Several laboratories have reported difficulty with this spectrophotometric method due t o high blanks and lack of reproducibility. Heuser et al. (6) have reported a thin-layer chromatographic method for t h e determination of free H D I monomer in polyurethanes which is based on t h e reaction of t h e isocyanate with N methyl-N- (4-aminobenzy1)amine. In this paper, a thin-layer chromatographic method is presented for t h e analysis of isocyanates in t h e working a t mosphere. T h e procedure is based on t h e determination of t h e ureas formed from t h e reaction of t h e isocyanates with N-4-nitrobenzyl-N-n-propylamine (nitro reagent). 2 OIK*CHI-KH-C,H,
+
OCN-R-KCO
-
C,H7 0 2 N + - c H I - x - C - ~ HI - i , R
II
0 With this method, the concentration of aliphatic and aromatic isocyanate monomers in air may be determined, a n d the concentration of isocyanate resins in t h e working atmosphere may be estimated.
EXPERIMENTAL Synthesis of Nitro Reagent. Dissolve 250 g of 4-nitrobenzyl chloride in 1.2 liters of benzene. Bring the solution to boiling under Author to whom all correspondence should be directed. (1) H. Ehrlicher and W. Pilz, Arbeitsschutz, 1956, 276 (Part I). (2) H. Ehrlicher and W. Pilz, Arbeitsschufz,1957, 7 (Part 11). (3) Kalrnan Marcali, Anal. Chem., 29, 552 (1957). (4) Sigrid von Eicken, Mikrochim. Acta, 1958, 731. ( 5 ) W. Pilz and llse Johann, Mikrochim. Acta, 1970, 351, (6) E. Heuser. W. Reusche, K. Wrabetz, and R . Fauss, Z. Anal. Chern., 257, 119 (1971).
reflux conditions, and add 180 g n-propylamine dropwise over a 30-minute period. Reflux for 5 hours. Concentrate the reaction mixture by stripping off the solvent in a rotary evaporator at 50 "C. Dissolve the residue in 400 ml of water, and slowly add 150 ml of a 45% NaOH solution. Add 500 ml of benzene, and stir the mixture for about 5 minutes. Remove the benzene layer, treat with approximately 5 g of charcoal, and filter. Strip off the solvent and excess n-propylamine in a rotary evaporator. Dissolve the product in about 250 ml of acetone, and add 170 g of concentrated HC1. Evaporate the mixture to dryness at 50 "C in a rotary evaporator. Purify the HCl salt of the nitro reagent by washing the salt with a 1:l mixture of acetone:benzene followed by suction filtration. Repeat this purification step several times. Dry the salt in a vacuum oven at 50 "C. Yield: 60 to 95%, depending on the quality of 4-nitrobenzyl chloride; e.g. 90 to 95% with 99% 4-nitrobenzyl chloride. Reagents. Nitro Reagent Solution. For stability reasons, the nitro reagent is stored as its hydrochloride (mol wt 230.7). Dissolve about 120 mg of nitro reagent hydrochloride in about 25 ml of distilled water; add 13 ml of IN NaOH, whereby the free amine precipitates. Extract the free nitro reagent with 50 ml of benzene (Toluene may also be used as a solvent for the nitro reagent solution.). Dry the benzene layer with NaZS04, and dilute the resulting solution to 250 ml to prepare the 2 X 10-3M solution. Dilute this solution tenfold to obtain the 2 X 10-4M solution which is used as the absorber solution. Store the nitro reagent solution in the dark and do not use the solutions after more than five days storage. Benzene. Reagent grade has been passed through a column of alumina to ensure that no trace contaminant will interfere with the analysis. Titanous Chloride Solution. This solution consists of pyridine: 20% Tic13 so1ution:glacial acetic acid, 40:40:20 by volume. To prevent the oxidation of the titanous chloride, this solution should be deaerated and stored under nitrogen. Apparatus. A TELEMATIC portable air sampler, Type C115, (Bendix, Environmental Science Division) or its equivalent is used in conjuction with midget gas impingers containing the nitroreagent absorber solution to collect the air sample. TLC plates used in the analysis are CHROMAR 7GF plates from Mallickrodt Chemical Works or silica gel plates (layer thickness 0.25 mm) from E. Merck, Darmstadt, West Germany. Procedure. Air Sampling. The required volume of air is drawn through two midget gas impingers a t the rate of 1-2 liters per minute (Two impingers connected in series should be used unless it has been established that the collection efficiency of the first impinger is such that the isocyanates of interest are quantitatively absorbed in the first impinger.). The impingers are connected in series, and each contains 10 ml of 2 X 10-4M nitro reagent solution. After the air sample is taken, the two solutions are combined and carefully evaporated to 1 ml. This is most easily accomplished by evaporating to dryness in a vacuum oven set at 35 "C, and dissolving the residue in 1 ml of benzene. This solution is then submitted for TLC analysis. TLC Analysis of the Ureas of Isocyanates. Ten-microliter aliquots of the solutions to be analyzed are spotted with 10.~1disposable pipets on a thin-layer plate. In addition, ten-microliter aliquots of standard solutions which are to be compared with the samples are spotted on the plate. Before the plate is developed, it is treated with HC1 vapors for 3 minutes by placing the plate in a developing chamber with a small container of concentrated hydrochloric acid. By impregnation with HC1, the amines (including the excess nitro reagent) will be eluted by the initial development while the ureas remain at the starting point. The plate is first developed using a mobile phase of methanol:water, 10:90 by volume.
A N A L Y T I C A L CHEMISTRY, VOL. 46, NO. 12, OCTOBER 1974
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Table I. C o n c e n t r a t i o n Limits for Several Isocyanatesa Isocyanate
HDI TDI MDI DESMODUR N DESMODUR L
Lower limit, mg/m3
0.08 0.08 0.08 0.40
0.50
Upper limit, mg/m3
33
34 50 15 5
a Concentraiion levels are based on a 10-liter air sample and 20 ml of 2 X 10-4Mabsorbing solution.
RESULTS AND DISCUSSION
1
HDI
TDI
MDI
N
L
Figure 1. Scheme of chromatogram HDI = 1,6-hexamethylene diisocyanate; TDI = 2,4- and 2,6-toluene diisocyanate; MDI = 4,4'diphenylmethane diisocyanate; N = DESMODURN, Mobay Chemical Company; L = DESMODUR L, Mobay Chemical Company. A = Front for first development; B = Excess nitro reagent; C = Front for second development: D = Start
The length of development is approximately 18 cm, which will require about 1.25 hours. After the initial development, the TLC plate is dried for exactly 10 minutes in an oven set at 100 O C . The conditions of this drying step should be followed closely since deviations from these conditions will result in changes of the R/ values for the ureas. The plate is developed a second time using a mobile phase of ethyl acetate: acetone:hexane, 10:10:30 by volume to elute the ureas. After the second development, the plate is dried and sprayed with the titanous chloride solution to reduce the nitro groups to amino groups. The plate is dried and treated wtih HC1 vapors. The amine hydrochlorides on the plate are then diazotized with nitrous fumes by placing the plate in a developing chamber with a container of NaN02. Several drops of HC1 are added to the container, and the plate is removed from the chamber after a period of 2 minutes. A stream of air is passed over the plate for 2 minutes to remove any residual nitrous fumes, and then the plate is visualized by spraying it with a 0.5% aqueous N - I-naphthylethylenediaminesolution. By comparing the spots of the sample solutions with those of the standards, the concentration of isocyanate in the solution can be determined. This may then be related t o the concentration of isocyanate in air by the volume of air sampled. Preparation of Standard Solutions. Standard solutions are prepared as needed by adding a weighed amount of the isocyanate of interest to the nitro reagent solution. These standard solutions should be stored in the dark and should not be used if they are more than 5 days old. Normally, the 2 X 10-3M nitro reagent solution is used to make a stock solution which is then diluted to obtain standard solutions in the concentration range of interest. For example, stock solutions for diisocyanates such as TDI, MDI, or HDI would be prepared by weighing out 10 mg of the diisocyanate to the nearest 0.1 mg and diluting to 100 ml with the 2 X 10-:'M nitro reagent solution. The stock solution would then be diluted to give standards in the concentration range of interest.
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T h i s method can be applied t o both aliphatic a n d aromatic isocyanates whether they are in t h e form of vapors or resin aerosols in t h e working atmosphere. T h e method offers both a qualitative identification of isocyanates by R, values a n d quantitative determination of t h e amount of these isocyanates. T h e scheme of a developed thin-layer chromatogram is given in Figure 1. Upper a n d lower concentration levels which may be determined by t h e method a r e given in Table I for several of t h e more common isocyanates. These levels are based on taking a 10-liter air sample a n d using 20 ml of t h e 2 X 10-4M nitro reagent absorbing solution. T h e lower limit may be decreased by taking a larger air sample, a n d t h e upper limit may be increased by increasing t h e amount of nitro reagent or by decreasing t h e volume of t h e air sample. For quantitative determinations of t h e isocyanates, a visual comparison of t h e sample and standards may be used; however, t h e use of t h e scanning densitometer or other scanner designed for t h e evaluation of TLC plates would eliminate any analyst bias in t h e interpretation of t h e results a n d improve t h e precision of th'e method. With t h e use of a scanner, a n absolute calibration or a calibration based on a n internal standard could be used, t h u s avoiding t h e necessity of preparing standard solutions for comparison every five days. An important concern in t h e analysis of isocyanates is t h e likelihood of undesirable reactions of t h e collected isocyanate. T h i s possibility is eliminated by t h e method since t h e isocyanate is reacted with t h e nitro reagent t o form a stable derivative. T h e only limitation of t h e method is t h a t it cannot be applied to atmospheres which decompose t h e reagent by oxidation or reduction. In conclusion, this paper presents a highly sensitive TLC method t o determine isocyanate levels in t h e working a t mosphere for most types of isocyanates of current industrial interest.
RECEIVEDfor review March 14, 1974. Accepted J u n e 7, 1974.
ANALYTICAL CHEMISTRY, VOL. 46, NO. 12, OCTOBER 1974
