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-nitro- sodimethylamine (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 chromatog- raphy-mass spectrometry readout has been shown to produce artifact-free results.

Recent literature reports have expressed concern tha t N-nitrosodimethylamine (DMN) may be a widespread en- vironmental contaminant which may have implications as a human carcinogen (1, 2). The necessity for DMN mea- surement at the sub-microgram level in air, water, food, and other materials has been proposed (3-5) . Since DMN can form from dimethylamine (DMA) and oxides of nitrogen (NO,), the apparent need to determine DMN 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 chro- matography/Thermal Energy Analyzer (GC/TEA) has been reported to be capable of this analysis. Other publications (2, 6) have described the measurement of DMN in air through the use of a porous polymer trapping system followed by thermal desorption, separation by GC, and measurement by mass spectrometry.

Our primary concern was to determine DMN actually present in the air. For this type of analysis, we felt tha t artifactual formation of DMN, particularly in the sub-mi- crogram per cubic meter range, was a real possibility if the two reactants (DMA and NO,) were brought together by cryogenic trapping. Investigation of these aspects (7) led to an ambient-temperature caustic containing impinger system for trapping DMN. Following extraction and concentration of DMN 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 to the cryogenic and sorbent traps by other in- vestigators who currently have reported their results elsewhere (10-12).

EXPERIMENTAL Apparatus and 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., Wil- mington, 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 an- hydrous 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 op- erated 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 3X 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 An- alyzer 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

ANALYTICAL CHEMISTRY, VOL. 49, NO. 12, OCTOBER 1977 1821

Table I. DMN Recovery Studies Experiment

100 ng of DMN added

20 ng of DMN added

20 ng of DMN in 1 mL

50-ng DMN in 100 L dry N,

300 L air blank (lab air)

(1)

( 2 )

(3)

(4)

(5 )

(6) 20 ng DMN in 1 N KOH, 1 week at room temperature

( 7 ) 20-ng DMN in 1 N KOH, frozen 1 week

to Trap 1

to Trap 1

water in U Tube

% Recovery

90 Trap 1 1 0 Trap 2 80 Trap 1 1 0 Trap 2 90 Trap 1 10 Trap 2 64 Trap 1

(one Trap only) No DMN detected

(<0.01 erg) 100

100

method used for the experiments described herein.

EVALUATION OF PROCEDURE The primary objective of this work was to develop a

procedure for the determination of sub-microgram per cubic meter concentrations of DMN in air containing orders of magnitude higher levels of DMA and NO, without artifactual formation of DMN. Anticipating DMN formation in any trapping system which would trap both precursors of DMN, a procedure was designed which would avoid trapping DMA. By using a caustic scrubbing solution at ambient temperature, most DMA present in the air to be sampled would not be expected to be trapped due to its volatility and insolubility. Using two 500-mL impinger traps in series, each containing 100 mL 1 N KOH, the experiments listed in Table I were conducted.

In experiments 1 and 2, the DMN was added as a dilute solution by microsyringe to the first trap. Air was then drawn through the traps a t a rate of 4 L/min for 1 h. Each trap was then analyzed separately to determine the amount of DMN which was carried over to the second trap. The results showed that once DMN was dissolved in the caustic solution, i t would sparge out very slowly a t laboratory temperature.

The next experiment (3) consisted of drawing air over a 1-mL solution of 20 ng of DMN in water at the rate of 4 L/min until the water had evaporated (150 L of air required). Assuming that the DMN slowly evaporated with the water, the results indicated that the ambient temperature impingers readily removed DMN from a dilute air stream. Experiment 4 showed the overall degree of recovery which could be ex- pected when a minute amount of DMN was placed in a gas bag. In this case, the major loss is attributed to adsorption on the walls of the gas bag. Degradation of DMN in the gas phase by laboratory lighting may also be a factor.

Experiment 5 showed the laboratory air to be free of DMN and experiments 6 and 7 showed DMN to be stable in dilute caustic solution in soft glass for a t least a week in normal

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.

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 a t 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 concen- trations 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 im- mediately 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.

CONCLUSIONS The ambient-temperature caustic scrubber system was

found to be a highly efficient means of removing sub-pg/m3 levels of DMN from air. This system is capable of removing DMN at these low levels in the presence of several orders of magnitude more DMA and NO, without artifactual formation

-- Table 11. DMN Artifact Studies

Reagent Concentration DMN found, p g -- Ambient DMA in air NO in air --

Vol. used ppm (v /v) Vol. used PPm ( v h ) Cold traps traps 0.1 mL (DMA) (Gas syringe) 0.1 mL (NO) (Gas syringe) 0.14 0.02

100 L 0.5 1 0 L 10 10 < 0.01

100 L 100 L

100 L

100 L

0.5 0.5

0.5

0.1

100 L 100 L

100 L

100 L

( 2 7 3 cl-gim’) 0.1 <0.01,0.01 <0.01 1 1 . . .

( 5 ~ g i m ’ ) 0.5 0.03 . . .

(0.15 pglm’) 1 0.01 . . .

(0.05 clg/m3)

1822 ANALYTICAL CHEMISTRY, VOL. 49, NO. 12, OCTOBER 1977

Table 111. DMN Artifact Studies under Possible Environmental Conditions

Reagent concentration

DMA in sir NO,, VOl. PPm PPm used (viv) (viv) 100 L 0.05 0 . 2 a

100 L 0.10 0 . 2 a

100 L 0.00 (Blank) 0.12a

100 L 0.05 3 b

100 L 0.10 36

100 L O.OO(Blank) 3b

DMN found, ccg Ambi-

ent Cold traps traps

0.06 < O . O l C

(0.3 pg/m3 in air) 0.05

(0.25 pg/m3 in air)

(<0 .05 pg/m3 in air)

(0.75 y g / m 3 in air)

(1.4 p g / m 3 in air)

(<0 .05 pg/m3 in air)

<0.01 <0.01

0.15 <0.01

0.28 . . . <0.01 <0.01

a Average value light traffic; excursions > 2 found. Average value heavy traffic; excursions > 1 0 found. Limit of detection.

of DMN. When cryogenic trapping was used in parallel sampling situations, artifactual formation of DMN occurred. The level of artifactual formation of DMN found would be reason for suspecting the validity of the results obtained by cryogenic trapping if highly accurate sub-bg/m3 analyses of DMN in air are to be considered relevant and necessary.

We believe that ambient temperature traps should be used to collect DMN in any area where DMA and NO, are expected

to be present together and yet accurate DMN analysis is needed. If the relatively large ambient traps are objectionable because of size, smaller midget impingers can be substituted. Additional studies (not reported herein) have shown the same degree of recovery as the larger traps using two traps in series, each containing 10 mL of 1 N KOH, a t a flow rate of 1.8 L/min.

Additional research in the area of‘ DMN artifact formation by Fine et al. (10, 12), implicates ozone to be involved in the formation of DMN from DMA and NO,. Although there was no reason to expect ozone levels to be higher than the trace level normally encountered in a suburban eastern location, we have no data on the specific level actually present a t the time of this study.

LITERATURE CITED W. Lijinski and S. S. Epstein, Nature (London), 225, 21 (1970). E. D. Pellizzari et al., Biomed. Mass Spectrom., 3. 196 (1976). D. H. Fine et al., Bull. Environ. Contam. Toxicol.. 15, 739 (1976). D. H. Fine, D. P. Rounbehler, and P. E. Oettinger, Anal. Cbim. Acta, 78, 383 (1975). D. H. Fine, D. P. Rounbehler, F. Huffman, A. W. Garrison, N. L. Wolfe, and S. S. Epstein, Bull. Environ. Contam. Toxicol., 14, 404 (1975). E D. Pellizzari, et al., Anal. Lett., 9 , 579 (1976). E. A. Lasoski, Dimethylnitrosamine (DMN) Sampling Program, presented to EPA, Research Triangle Park, N.C., January 6. 1976. B. A. Lasoski, Letter to K. Krost of USA Environmental Protection Agency, February 12, 1976. E. A. Lasoski, Personal Communication to D. P. Rounbehler, April 6, 1976. D. H. Fine, D. P. Rounbehler, E. Sawic:ki, and K. Krost, Environ. Sci. Techno/., 11, 577 (1977). D. H. Fine et al., Eviron. Sci. Tecbnol., 11, 581 (1977). D. H. Fine et al., APCA Speciality Book, in press.

RECEIVED for review April 4, 1977. Accepted July 5 , 1977.

Performance of a Nitrogen Dioxide Permeation Device

E. E. Hughes,* H. L. Rook, and E. R. Deardorff

Analytical Chemistry Division, National Bureau of Standards, Washington, 0. C. 20234

J. H. Margeson and R. G. Fuerst

Environmental Protection Agency, Research Triangle Park, North Carolina 277 1 7

An in-depth study of the performance of a nitrogen dioxide permeation device, developed at NBS, has been conducted in cooperation with researchers at EPA. The study detailed conditions which would affect permeation rate and stabillty. Parameters such as temperature, humidity, nitrogen dioxide purity, and calibration procedures were investigated and their effect on rate characteristics was determined. Also, studies were conducted into the temperature memory effects so as to define the minimum temperature equilibration time necessary to obtain stable, reproducible rates. The results of this study have helped define the care necessary to use nitrogen dioxide permeation devices as primary gas standards.

The use of permeation devices to generate calibration gas mixtures is a fairly widely used technique (1-7). Permeation, or effusion, of a compound through a membrane will occur a t a constant rate if the ratio of the concentration of the compound on either side of the membrane is constant. The concentration on one side of the membrane may be controlled

by exposing it to a gas-liquid system held a t constant tem- perature while the concentration om the other side of the membrane is held effectively a t zero by sweeping the per- meating species away in a stream of air or other carrier gas. If the temperature of the membrane and gas-liquid system is maintained constant and if the flow of carrier gas is constant, then a constant concentration of the permeating species will result in the carrier gas. In practice, a porous and inert tube, usually F E P Teflon (fluorinated ethylene propylene co- polymer) is partially filled with the pure liquid of the com- pound of interest and is then sealed. The tube is maintained a t constant temperature while passing a measured flow of carrier gas over it. The concentration of the compound in the carrier gas stream can be calculated from the measured rate of permeation of the compound out of the tube and from the measured flow rate. The permeation rate is determined by periodically weighing the device at measured time intervals.

Permeation devices are particularly useful for generating low concentrations of gas mixtures o f reactive compounds which cannot ordinarily be prepared and stored a t elevated pressure in gas cylinders. Permeation devices do not offer any

ANALYTICAL CHEMISTRY, VOL. 49, NO. 12, OCTOBER 1977 1823