1868
ANALYTICAL CHEMISTRY, VOL. 51, NO. 11, SEPTEMBER 1979
Sampling of Isocyanates in Air Jiirgen Keller’ Bayer AG, Central Analytical Department, 509 Leverkusen-Bayerwerk, West Germany
R. L. Sandridge’ Mobay Chemical Corporation, Analytical Group, New Martinsville, West Virginia 26 755
cotton is put into the tip of the tube. The tube is then filled to a height of 3 cm with glass powder (type 8330, Schott and Company, Mainz, West Germany), using tapping to lightly compact the filling. About 200 mg of clean surgical cotton is then added to the tube in small portions, packed so as to leave the last 2 cm of the absorption tube unfilled (Figure 1). The absorption medium must be clean. If necessary, materials may be cleaned before use by Soxhlet extraction with ethanol. TLC Analysis. The capability of the TLC plate is important for achieving best results. Pre-coated HPTLC Silica gel plates 60 F 254, catalogue number 5642 (E. Merck, Darmstadt, West Germany) or CHROMAR 7GF plates (Mallinckrodt Chemical Works, St. Louis, Mo.) are suitable. Reagents. Nitro Reagent Absorber Solution. The synthesis of the nitro reagent has been described ( 3 ) and it is available commercially as the hydrochloride salt of the amine (Regis Chemical Co., Morton Grove, Ill.). To prepare the absorber solution, dissolve 120 mg of the nitro reagent hydrochloride in 25 mL of water and precipitate the free amine by adding 13 mL of 2 N Na2C03solution. Extract the free nitro reagent with 50 mL toluene. Dry the toluene with 3 to 5 g anhydrous Na2S04, then filter into a 250-mL volumetric flask. Fill to the mark with toluene. Store the reagent solution in the dark, and do not use the solution after more than three weeks. Toluene. Reagent grade toluene should be passed through a column of alumina to ensure that no trace contaminant will interfere with the analysis. Mobile Phase f o r TLC. Methylene chloride/ethyl acetate/ ethanol/methanol/glacial acetic acid was 90:6:2:1:1 by volume. Titanous Solution. Mix 40 mL of 0.1 N titanium(II1) sulfate solution with 40 mL pyridine and 20 mL of glacial acetic acid. This solution should be prepared fresh daily. Coupling Solution. Dissolve 0.5 g naphthylethylenediamine dihydrochloride in a mixture of 95 mL water and 5 mL of concentrated hydrochloric acid (d = 1.16). Procedure. Air Sampling. Dip the tip of a packed absorber tube into the nitro reagent solution and allow the solution to rise up into the glass powder layer. When the glass powder is wet, withdraw the tip of the tube from the solution and connect the tube to the pump with the tip away from the pump (see Figure 2). Work the pump to draw the air being tested through the tube. On the first stroke of the pump, be sure that the absorption tube is vertical, tip down, so that the nitro reagent solution is spread evenly over the cotton. If the design of the pump requires it, cover the top of the tube with a finger during the return stroke to ensure that the air goes out only the exhaust valve of the pump. When the required volume of air has been drawn through the absorption tube, remove the tube from the pump and proceed with the analysis. TLC Analysis of the Derivatized Isocyanates. Firmly and evenly compress the cotton in the tube with a small metal rod of about 2.5-mm diameter. Place the absorption tube vertically, with its tip downward in contact with the TLC plate. With a syringe, place about 300 pL of cyclohexane in the tube, allowing it to flow down into the tip of the tube. Attach a small surgical rubber tube with a mouthpiece to the top of the absorption tube and very slowly and evenly blow out the cyclohexane. In this manner, the mixing of the true eluate, containing the nitro reagent derivatives, with excess reagent is avoided. Some residual cyclohexane will remain in the tube. A warm air blower can be used to accelerate the drying to keep the starting spot a5 small as possible. One-microliter samples of previously prepared standard solutions are spotted alongside the sample spot (or spots). The standard solutions are prepared by the reaction of the appropriate isocyanates with the nitro reagent solution. They must be kept
I n industrial hygiene and medical studies involving the measurement of isocyanates in air, a need exists to determine concentrations over varying periods of time a t levels down to low ppb ranges. For sampling periods under 10 min, few means of measurement are available which give the required sensitivity. One currently available method is the Sieger TDI Detector Model 7000 in which a chemically impregnated paper tape develops a stain in response to TDI; however, it responds with reduced sensitivity t o other aromatic isocyanates and not at all t o aliphatic isocyanates. A new Model 7005 detector for both aliphatic and aromatic isocyanates has just recently been introduced. The NIOSH versions of t h e Marcali TDI method ( I ) and its modification for MDI ( 2 ) both describe 20-min sampling periods. In addition, these methods are not applicable for aliphatic isocyanates and, since they do not involve a separation step, cannot discriminate among t h e various aromatic isocyanates which are capable of giving a response in t h e test. Thin-layer (3) and high-speed liquid chromatographic (HSLC) methods ( 4 ) based on “nitro reagent” chemistry provide useful means of analysis for aliphatic and aromatic isocyanates. Since a separation step is involved, these methods are essentially specific for each isocyanate, can be used for mixtures of isocyanates, and are free of interferences from amines. However, these methods use conventional impinger collection techniques in which only a n aliquot portion of the sample is used. To achieve desired sensitivities, sampling times of 20 min or longer are typical. A new sampling technique has been developed in which a small-volume air sample is drawn through a tube containing “nitro reagent” (N-4-nitrobenzyl-N-n-propylamine) on a suitable support. The stable urea derivative which is formed is eluted from t h e tube and deposited on a thin-layer plate for subsequent analysis. Since t h e entire sample is utilized, t h e sampling time can be markedly shortened, making measurement possible for time periods of 2 min or less. Using this technique, analysis of “single-breath” samples in the current T L V range for isocyanates becomes possible.
EXPERIMENTAL Apparatus. Air Sampling. A hand-operated precision sampling pump with calibrated volumes up to 100 mL is used to obtain air samples. (Universal Tester from Mine Safety Appliances Company, Pittsburgh, Pa., or sampling pump from Mader-Nukleonik GmbH, Solingen, West Germany, are suitable pumps.) To protect the pump from solvents and chemicals, a tube packed with activated carbon is inserted between the absorption tube and the pump. Toluene may evaporate from the tube during air sampling if more than 0.2 to 0.3 L of air are taken. Because the reaction of nitro reagent and isocyanates takes place only in solution, dip the tip of the absorption tube in pure toluene again in order to rewet the layer. Absorption Tubes. Absorption tubes can be prepared by cutting a 2-mL graduated pipet at the 1.4-mL mark and firepolishing the cut end. The tip of the pipet must be flat so that liquid can be conveniently spotted on the TLC plate. Typical tube dimensions are: length, 135 mm; o.d., 10 mm; i.d., 3 mm. The tubes are packed as follows. A small plug of clean surgical Author to whom foreign correspondence should be addressed. 0003-2700/79/0351-1868$01 .OO/O
C
1979 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 51, NO. 11, SEPTEMBER 1979
1869
Table I. Tube Absorption vs. Impinger Absorption 2
1
isocyanate
3
temp. of isocyanate flask, "C
HDI
2 0" 20" 20 20 40' 40'
60bjC 6OblC 80b,' 80b,' 20
2,4-TDI
tube absorption isocyanate in air, sample TLC value, vol, L ng mg/m3 0.2 0.5 1.0 0.5 0.5 0.2 0.05 0.5 0.5 0.5 0.5
2 0"
5
4
2
5 10
0.5
20
20 20 8ObxC 8Ob9'
7
8
10
9
impinger absorption
sample vol, L
isocyanate absorber spotted solution, on TLC TLC value in air,
0.01 0.01 0.01 0.08 0.08
100 100
1 1
1
100 100
1 1 1
1
1 1
0.01
5 10
0.05 0.1
0.01
400 100 40
2 2
100 100
16
100
30
15
50
100 100
40
100 100 100
1 1 1 1 1 1
50 50
5 5
100
1
1
8
0.08
100
1
1
8
0.08
100
50
100
50
1 1
80 80
40 40
12 35
0.2
20 20
6
1.0
40
5 50
2
200 35 30 30 350
15 15 35
400
40
1.8
0.06
0.07 0.08
0.2
80 15
0.5 1.0
30 100
0.06 0.1
0.5 0.3
18 13
0.075 35 43
40 40
" Smaller porosity frit (40-100 pm) used to control isocyanate flow into the main gas stream. Other runs used 100-160 um frit. All values from 60 and 80 "C runs were obtained by using 1 m L of toluene for elution rather than cyclohexane, because of the lower boiling point of the latter. Also, a 5-pL sample was spotted for the 80 'C HDI sample compared t o 1 pL for the TDI runs. Absorption efficiency tested o n these samples by using two tubes in series. The l m L absorber solutions are concentrated by evaporation from 50 mL.
1
ea. 13.5 cm
1
T
3 cm
L
Figure 1. Detail of absorption tube SAMPLING
PUMP
ACTIVATED CARBON T U B E
ABSORPTION TUBE
Flgure 2. Air sampling device
in the dark and not used after they are than l-week-old. Solutions containing I, 3, 5, 7, IO, 20, 30, 50, 70, and 100 ng isocyanate/pL nitro reagent solution make a convenient set. Fewer standard solutions may suffice if the expected concentration range is known.
After spott,ing, place the plate in a developing chamber with a small container of concentrated hydrochloric acid for about 3 min to convert amines to their salts and thus immobilize them at the starting point during subsequent development. Develop the plate with the mixed solvent described above; development of approximately 18 cm takes about 15 min. After development, use a warm air blower to dry the plate. Spray the plate lightly and evenly with the titanous solution i o reduce the nitro groups to amino groups. The plate should appear almost transparent when sprayed. Let the plate stand 3-4 min in air, then dry with a cold air stream from a blower. To diazotize the amines, spray the plate lightly with concentrated hydrochloric acid; then expose the plate to nitrous fumes for 2 min in a development chamber. The nitrous vapors can be generated by treating a smaller beaker of sodium nitrite with a few drops of concentrated sulfuric acid. Remove the plate and completely remove the residual nitrous fumes by blowing with cold air; 2 rnin are usually sufficient to accomplish this. Visualize the spots by spraying with the coupling solution until the plate appears quite transparent from being wet. Allow the plate to develop about 2 min in the air, then blow the plate dry with warm air. By comparing the spots from the standard solutions with those from the sample solutions, the amount of isocyanate, in nanograms, which was in the air sample can be found. This may then be readily converted to the concentration of isocyanate in air, expressed as mg/m3 and converted to ppm using the calculations shown helow. Preparation of Test Atmospheres. A procedure similar to that described by Neilson and Booth ( 5 ) was used to generate controllable mixtures of isocyanates. Porous frits rather than glass capillaries were used to control the flow of the isocyanate vapors into the main gas stream. Proper selection of frits and temperatures of the isocyanate flask gave a suitable range of isocyanate concentrations with which to test the new technique.
RESULTS AND DISCUSSION Two isocyanates, hexamethylene diisocyanate (HDI) and 2,4-toluene diisocyanate (2,4-TDI) were used to test the new technique. The isocyanate generator described above was used to Prepare air Samples containing isocyanates a t Several levels through t h e range 0.01 to 40 mg/m3 (-0.001 to 6 ppm). Samples for analysis were taken by t h e new technique and
1870
ANALYTICAL CHEMISTRY, VOL. 51, NO. 11, SEPTEMBER 1979
molecular weight can be defined. The following equations can be used:
Table 11. Limits of Detection of Isocyanates in Air limit of detection (approx.) sampling sample, time, ppm HDI, L s (approx.) mg/m3 TDI ppm MDI 0.1 0.2 0.5 1.0
20-25 45-50 120 140
0.02
0.003
0.002
0.01
0.002
0.001
0.004 0.002
0.0006
0.0004
0.0003
0.0002
concentration in p p m = concentration in m g / m 3 x
24
-
mol wt mol wt H D I = 168.2; T D I = 174.2; M D I = 250.1 molecular volume at 20 " C = 24
by the conventional impinger method. Thin-layer chromatography was selected as the method for analysis, although high speed liquid chromatographic analysis would have been a suitable alternative. The samples were analyzed and the results obtained are shown in Table I. Column 2 shows the conditions selected to generate the required test atmospheres. T h e diluting gas stream was adjusted to 2 L/min. Columns 3-5 give the data for the new tube absorption method. Note that test volumes for the new method were in the range 0.2-1.0 L, whereas test volumes for the conventional impinger technique (column 6) were 100 L. Columns 7-10 complete the necessary information concerning the impinger technique experiments. T o determine how the new tube absorption technique compares with the conventional impinger procedure, the values in column 5 should be compared with those in column LO. The agreement is considered excellent, especially for a procedure based on thin-layer chromatography. The new technique should give values fully equivalent to those of the more conventional impinger method, but should allow satisfactory analysis of samples of 1 L or less. Also, a correspondingly shortened sampling time can be achieved, in the order of 2 min or less. T h e lower limits of detection for the method are based on the ability to detect a 2-ng sample on the TLC plate. Since the whole test volume is concentrated in one spot, the limits of detection which can be expected are described in Table 11. If desired, isocyanate concentrations expressed in mg/m3 can be converted to ppm values for those cases in which the
With higher isocyanate concentrations, it may be necessary to either decrease the sampling volume or collect and dilute the sample from the tube, taking a suitable aliquot for spotting in order to have the proper size sample to be chromatographed. These variations and others which are in accord with good chromatographic technique should be applicable to this method. In conclusion, this technique should provide a new tool for workers in the medical and industrial hygiene fields who deal with short time period sampling of atmospheres containing isocyanates a t very low concentrations. I t is applicable to aliphatic, alicyclic, and aromatic isocyanates, and can be carried out with relatively inexpensive equipment. It is based on previously proven "nitro reagent" chemistry and can be adapted to other means of analysis such as high speed liquid chromatography.
LITERATURE CITED (1) "NIOSH Manual of Analytical Methods", 2nd ed.,Vol. 1; P&CAM No. 141 "2,4-Toluenediisocyanate (TDI) in Air"; U.S. Dept. of Health, Education and Welfare: Cincinnati, Ohio, 1977. (2) "NIOSH Manual of Analytical Methods", 2nd ed.,Vol. 1; P&CAM No. 142 " p ,p-Diphenylmethanediisocyanate(MDI) in Air"; U.S. Dept. of Health, Education and Welfare: Cincinnati, Ohio, 1977. (3) Keller, Jurgen; Dunlap, K. L.; Sandridge, R. L. Anal. Chem. 1974, 4 6 , 1845. (4) Dunlap, K . L.;Sandridge, R. L.; Keller, Jurgen Anal. Chem. 1976, 4 8 , 497.
(5) Neilson, Arthur: Booth, K. S. Am. Ind. Hyg. J . , 1975, 36, 169.
RECEIVED for review April 11, 1979. Accepted May 18, 1979. This publication describes work funded by the International Isocyanate Institute, Incorporated.
Determination of Sulfide in Pyritic Soils and Minerals with a Sulfide Ion Electrode Darwin L. Sorensen,' Walter A. Kneib,2 and Donald
B. Porcella'
Utah Water Research Laboratory, Utah State University, Logan, Utah 84322
The determination of sulfide in geological materials is often done by leaching naturally occurring sulfate from the sample, oxidizing the sulfide to sulfate, and measuring it gravimetrically. This method is time consuming and carries inaccuracies caused by coprecipitation interferences with gravimetric determination of sulfate. These problems have led authors such as Maxwell ( 1 ) and Jeffery ( 2 ) to recommend methods for sulfide determination which rely on the release of sulfide ion. Murthy et al. ( 3 , 4 )have developed a method for determining sulfide which generates hydrogen sulfide from mineral sulfides in the presence of hydriodic acid and mercury catalyst. The H2S is precipitated as CdS and analyzed by iodimetric titration. The titration procedure is time consuming and is not sensitive enough to measure very low levels of sulfide with good precision. Morie (5) measured H2S in Present address: Department of Microbiology, Colorado State University, Fort Collins, Colo. 80523. 2Presentaddress: 5114 Aspen Ave., NE,Albuquerque, N.M. 87110. 0003-2700/79/0351-1870$01.00/0
cigarette smoke by scrubbing the smoke through a sulfide antioxidant buffer and measuring the sulfide concentration in the buffer with a sulfide ion electrode. We developed a sensitive and precise method for soil and mineral sulfide by combining the sulfide ion electrode measurement, using Cd as a titrant, with a modification of the H2S generation/ precipitation technique.
EXPERIMENTAL Apparatus. The apparatus of Murthy e t al. (3)was modified to include a cold trap between the reaction flask and the first gas scrubbing bottle (Figure 1). This trap was maintained at -30 to -35 "C with a Thermoelectrics Unlimited, Inc. Stir Kool Model SK 12 cooling plate. The cold trap removed mercury vapor (mercury "poisons" the sulfide electrode) and other potential contaminants which are carried over from the reaction flask. Nitrogen was used as the carrier gas (hydrogen may be used) and a magnetic stirrer caused mixing in the reaction flask. A heating mantle held between 110 and 115 "C was used to warm the reaction flask. 1979 American Chemical Society
