Determination of Sub-Microgram per Cubic Meter Levels of A/-Nitrosodimethylamine in Air R. L. Fisher' and R. W. Reiser Biochemicals Department, Research Division, E. I. du Pont de Nemours & Co., Inc., Experimental Station, Wilmington, Delaware 19898
B. A. Lasoski Industrial Chemicals Department, Research & Development Division, E. I. du Pont de Nemours & Co., Inc., Experimental Station, Wilmington, Delaware 19898
Ambient-temperature caustic impinger traps were used in parallel sampling with cryogenic caustic traps to study the degree of conversion of dimethylamine (DMA) to N-nitrosodimethylamine (DMN) when DMA and NO, were brought together by trapping. Under simulated use conditions where DMA at sub-ppm (0.05) and NO, (0.2 ppm) from light traffic were brought together by trapping, the cryogenic traps produced from one to two orders of magnltude more DMN than the ambient traps. For routinely measuring DMN in air, ambient trapping in caustic followed by combination gas chromatography-mass spectrometry readout has been shown to produce artifact-free results.
Recent literature reports have expressed concern t h a t N-nitrosodimethylamine (DMN) may be a widespread environmental contaminant which may have implications as a human carcinogen (1, 2). T h e necessity for D M N measurement at t h e sub-microgram level in air, water, food, and other materials has been proposed (3-5). Since D M N can form from dimethylamine (DMA) and oxides of nitrogen (NO,), the apparent need to determine D M N in air near areas handling DMA has arisen. One published method ( 3 ) based upon cryogenic trapping of D M N followed by extraction, concentration, and measurement by combination gas chromatography/Thermal Energy Analyzer (GC/TEA) has been reported t o be capable of this analysis. Other publications (2,6) have described the measurement of DMN in air through t h e use of a porous polymer trapping system followed by thermal desorption, separation by GC, and measurement by mass spectrometry. Our primary concern was t o determine D M N actually present in t h e air. For this type of analysis, we felt t h a t artifactual formation of DMN, particularly in t h e sub-microgram per cubic meter range, was a real possibility if t h e two reactants (DMA and NO,) were brought together by cryogenic trapping. Investigation of these aspects (7) led t o a n ambient-temperature caustic containing impinger system for trapping DMN. Following extraction and concentration of D M N from the scrubber solution, measurement is readily achieved by combination GC/MS, GC/TEA, or alkali flame ionization gas chromatography (AFIGC). Upon our invitation (8, 9 ) , this simple, ambient-temperature system has been compared t o the cryogenic and sorbent traps by other investigators who currently have reported their results elsewhere (10-12).
EXPERIMENTAL Apparatus a n d Reagents. Detection and measurement of DMN for this work was by GC/MS and was accomplished by using a Perkin-Elmer Model 990 GC coupled with an all glass system through a jet separator to a Du Pont Model 21-492 Mass
Spectrometer (E. I. du Pont Instrument Products Div., Wilmington, Del.). The GC column was 6 feet X 2 mm i.d. glass packed with 60-80 mesh Tenax GC (Applied Science Laboratories, State College, Pa.). Traps used for the ambient temperature sampling were 500-mL capacity impvnger type (Ace Glass Co., Vineland, N.J., Catalog No. 7537-10). Modifications to the traps were the addition of 18/9 ball and socket ground joints to facilitate coupling in series. Recently 30-mL midget impingers (Lab Glass, Inc., Vineland, N.J., Catalog No. LG.6890) modified with 12/5 ball and socket ground joints have also been used for ambient temperature sampling. KOH was reagent grade (Fisher Scientific, King of Prussia, Pa.). Methylene chloride was glass distilled (Burdick and Jackson, Muskegon, Mich.). DMN was used as supplied (Aldrich Chemical Co., Metuchen, N.J.). DMA was obtained from E. I. du Pont de Nemours & Co., Inc., Belle Plant, W. Va., in a lecture gas bottle. Nitric oxide in Nz (100 ppm, assayed) was supplied by Matheson Co., East Rutherford, N.J. Teflon gas bags (100-L) were obtained from Fluorodynamics, Inc., Newark, Del. Bendix Permissible Air Sampling Pumps were used for gas pumping. Procedure. In general, gas samples for DMN determination were obtained and analyzed in the following manner. The sample was pumped at 2.8-4 L/min through the impinger traps which contained 100 mL of 1 N KOH. One trap was usually sufficient, but two traps in series were used to study trapping efficiency in most cases. When sampling was conducted in areas exposed to bright sunlight, the traps were covered with aluminum foil. The KOH was then extracted with two 100-mL portions of methylene chloride which were filtered in succession through 10 g of anhydrous sodium sulfate. The combined extracts were then concentrated by evaporation under a vigorous stream of nitrogen. Water condensation was prevented by mild heating with a warm hot plate. At no time was the glassware allowed to become warm to the touch. Final evaporation was to 1 mL in a small (5-mL) conical bottom vial. When the cryogenic traps were used for sampling, the literature procedure ( 3 ) was followed. For GC/MS analyses, the chromatographic column was operated isothermally at 150 "C a t a flow rate of 20 mL/min He. The glass-lined GC inlet was at 200 "C and the GC/MS line was maintained at 250 "C. Generally, 3-pL sample volumes were injected. Using single ion monitoring at m / e = 74, the smallest detectable level of DMN at 3 X noise level was 1 X lo-" g. Low voltage ionization (20 eV) was used to reduce formation of fragment ions. Currently, gas chromatography with a Thermal Energy Analyzer Detector and alkali flame ionization detector are also being used for DMN analyses. The GC column for the TEA procedure is a 5-ft X '/4-in. 0.d. stainless steel column packed with 10% Carbowax 20 M on 80/100 mesh Chromosorb W. A column temperature of 140 "C with a flow rate of 50 mL/min Ar elutes DMN in 4 min. The minimum detectable amount of DMN is approximately 5 X IO-" g under these conditions. For AFIGC, the column used is 20-ft X l/a-in.stainless steel containing 2070 FFAP on 80/100 mesh Anakrom ABS. Operating conditions are 170 "C isothermally at a flow rate of 30 mL/min He. The smallest detectable level of DMN with the detector optimized is 0.2 X lo-" g. These optional chromatographic systems are capable of producing the same results as the GC/MS system which was the A N A L Y T I C A L CHEMISTRY, VOL. 49, NO. 12, OCTOBER 1977
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laboratory light levels. Later experiments, however, showed that bright sunlight through Pyrex glass destroyed up to 80% of added DMN in 1 N caustic during a 1-h exposure. For this reason, all outdoor sampling must be done with the impingers protected from sunlight by aluminum foil or other light blocking method.
Table I. DMN Recovery Studies Experiment (1)
(2)
(3) (4)
% Recovery
1 0 0 ng of DMN added to Trap 1 20 ng of DMN added to Trap 1 20 ng of DMN in 1 mL water in U Tube 50-ng DMN in 100 L dry N,
90 Trap 1 0 Trap 80 Trap 1 0 Trap 90 Trap
1 2 1 2 1 1 0 Trap 2 6 4 Trap 1
COMPARISON OF DMN TRAPPING SYSTEMS Other experiments were conducted with the caustic trapping system and a parallel system using cryogenic traps as described in the literature ( 3 ) to assess differences in ability to trap sub-pg/m3 levels of DMN in the presence of higher levels of DMA and NO,. Table I1 lists the results obtained. DNA was passed through the traps first, followed by NO. Pumping rates were all at the rate of 1.8 L/min. DMN concentrations were calculated on the basis of the total volume of gas being sampled. Analyzed (100 ppm) NO in N2 was used for dilution with lab air to produce the level of NO listed in the table. However, since oxidation reactions of NO are unavoidable in a system such as this, NO, was considered to be the species present. The results of these experiments showed that when DMA and NO, are brought together, some reaction to produce DMN will occur. Even in the ambient traps, a large surge of DMA followed by NO, before the temporarily dissolved DMA can be purged out of the trap, can produce a small artifact as noted in the first experiment listed. However, with low concentrations of DMA and NO,, the amount of DMN found is negligibly small. Ambient temperature trap experiments were not conducted in the latter three experiments since the previous two gave no evidence that DMN would form at these reagent concentrations. The significance of this laboratory demonstrated artifactual DMN formation to the sub-pg/m3 determination of DMN in an environment where DMA and NO, would be expected was then assessed. For these experiments, we purposely chose a sampling site near a highway where NO, levels were expected to be high. Both cryogenic and ambient temperature traps were preloaded with DMA in air a t the pg/m3 level by pumping 100 L of air from a gas bag containing DMA through the traps. Air (100 L) from the traffic area was then immediately sampled a t a rate of 1.8 L/min during a period of light traffic flow and during a period of heavy flow. NO, N02, and NO, levels were concurrently measured with a NO, analyzer. Results are listed in Table 111. DMN concentrations reported are calculated on the basis of a 200-L total sample.
(one Trap only)
( 5 ) 300 L air blank (lab air)
No DMN detected (
