Fourier Transform Infrared Detection of Nitramines in Irradiated Amine-NO, Systems Ernest0 C. Tuazon, Arthur M. Winer, Richard A. Graham, Joachim P. Schmid, and James N. Pitts, Jr." Statewide Air Pollution Research Center and Department of Chemistry, University of California, Riverside, Calif. 92521
Atmospheric reactions of dimethylamine (DMA) and diethylamine (DEA) were investigated with a Fourier transform infrared (FT-IR) spectrometer interfaced to an 8-mirror multiple-reflection system (total pathlength of 720 m) in a Teflon cell. Sunlight irradiations of part per million (ppm) concentrations of DMA or DEA in ambient air were conducted at -27 "C and -50% relative humidity in the presence of ppm levels of NO and NO2. For DMA-NO, irradiations an infrared product spectrum was observed, which corresponded to that for an "unknown product" reported by Hanst et al. to be formed upon irradiation of dimethylnitrosamine. Both the product spectrum from the present DMA-NO, irradiations and that of Hanst et al. were in good agreement with infrared data reported for the compound dimethylnitramine by Davies and Jonathan. By analogy with the DMA-NO, study, an infrared product spectrum obtained for irradiated DEA-NO, systems was assigned to gaseous diethylnitramine, and positive identification was made by comparison with the spectrum of an authentic sample of diethylnitramine. These FT-IR observations of nitramines confirm the previous GC-MS identification in this laboratory of nitramines formed during irradiations of DMA, DEA, or triethylamine (TEA) in NO,air systems. The in situ formation of the corresponding nitrosamines was also observed.
Accumulating evidence (1-4) of the ubiquitous occurrence in the environment of N-nitroso compounds, a major class of strong carcinogens, has led to increasing attention concerning their modes of formation and destruction in the atmosphere (5-10). Hanst and coworkers (10)have recently reported results from Fourier transform infrared (FT-IR) studies of gaseous dimethylamine (DMA) in DMA-NO,-HsO-air mixtures. They also irradiated dimethylnitrosamine in air and reported the spectrum of an "unknown product" formed during photolysis. A recent study conducted in this laboratory (11-14) has demonstrated by combined gas chromatography-mass spectrometry (GC-MS) analysis that solar irradiation of sub-ppm concentrations of diethylamine (DEA) and triethylamine (TEA) in the presence of 0.3 ppm NO, (simulated atmospheric conditions) produces diethylnitramine [ (C2H5)z"021 as a major gas-phase product as well as diethylnitrosamine (in the case of TEA) and other species. The present FT-IR study of DMA and DEA had two principal objectives: spectroscopic confirmation of the previous GC-MS identification of diethylnitramine (14) and identification of the unknown compound reported by Hanst et al. (10) from the photolysis of dimethylnitrosamine. Experimental
The experimental facility employed in this study has been described in detail previously (15);only a brief description will be given here. The FT-IR system (shown in Figure 1) consisted of a Digilab Model 296 interferometer with data system and peripherals, which is interfaced to an %mirror, multipass optical system with a 22.5-m basepath. A total pathlength of 720 m and a resolution of 1cm-l were used in these experiments. The optical system was enclosed in a rectangular FEP Teflon cell with dimensions of approximately 0.76 X 0.76 X 954
Environmental Science & Technology
23 m and a total volume of -13 500 L. The measured transmittance of the aged Teflon film was greater than 80% for radiation in the actinic region from 290 to 450 nm. The GC-MS sampling and analysis techniques used here were the same as those employed in the amine study described in the preceding paper (11). DMA (99% purity) was purchased from Matheson, and DEA (99% purity) was obtained from Matheson Coleman & Bell. GC-MS analysis of these samples showed less than 1% oxidized impurities, and infrared analysis of gaseous DMA and DEA samples in a IO-cm cell showed less than 1%ammonia present. Upon introduction of DMA or DEA samples into the long-path cell, however, ammonia bands with intensities corresponding to approximately 5% of the initial amine concentration were observed in the spectra. A plausible explantation is that the ammonia observed was displaced by the amines (DMA and DEA) from ammonium salts (e.g., ammonium nitrate aerosol) deposited on the cell walls during previous air monitoring studies (15) in which very large volumes of ambient air were drawn through the cell. Diethylnitramine was prepared according to the procedure of Chute et al. (16) from diethylammonium nitrate ( 1 7 ) . A fraction with b.p. 32 "C a t 0.2 torr was shown by GC-MS analysis to be 99.5% pure. In a typical experiment conducted on a clear day, ambient air was drawn into the covered cell to serve as matrix air. DMA or DEA vapor in a 5-L bulb was then flushed into the cell with N2 through a Teflon disperser tube running the length of the cell. The calculated initial DMA or DEA concentrations were approximately 4 ppm. Nitric oxide (6 ppm) and nitrogen dioxide (4 ppm) were then introduced via the disperser tube; after allowing 15 min for mixing, the cell was exposed to sunlight for a period of 2 4 h. Initial relative humidities inside the covered cell were 45-50% a t temperatuees of 24-27 "C. The temperature inside the cell during irradiation rose as high as 38 "C. Solar radiation intensities were characteristic of those observed from 1200 to 1700 h during May-June a t a latitude of 33 ON for generally clear skies. Results and Discussion
DMA Experiments. Figure 2 shows spectra for (a) DMA alone, (b) the DMA-NO,-air system after exposure to sunlight for 2 min, and (c) the same mixture after 80 min of irradiation. The characteristic DMA absorptions are centered at 736,929, 1022, and 1157 cm-'. The numerous interfering lines due to the NH3 impurity (see above) were subtracted from each spectrum. After a few minutes of irradiation, an absorption band was observed a t -1308 cm-l; this band became much more intense during the 2-h experiment. Other absorption features shown in Figure 2(c),which became more intense concomitantly with the band a t 1308 cm-l, were those a t 773,985, and 1132 cm-'. The contours of the absorption bands of Figure 2(c), with water interferences removed and absorptions of the unreacted amine subtracted, are presented in Figure 3. In a study of nitramine (NHZN02) (also called nitramide), methylnitramine (CH3NHN02) and dimethylnitramine [(CH&NN02], Davies and Jonathan (18) reported that the characteristic infrared spectral features of this class of compounds are due to the NO2 antisymmetric stretch (-15101600 cm-I), the NO2 symmetric stretch (-1300-1380 cm-l),
0013-936X/78/0912-0954$01,00/0 @ 1978 American Chemical Society
,U-ER'r7c*CT-D
Figure 1. FT-IR spectrometer and km pathlength multiple reflection cell employed in this study
3
(a)
I 1
lCH312NH
FREQUENCY (crn-1)
Figure 2. (a) Spectrum of dimethylamine (3.5 ppm, 720 m). (b) Spectrum of DMA-NO,-air mixture after 2 min of irradiation. (c) Spectrum of dimethylnitramine in DMA-NO,-air mixture after 80 min of irradiation
I (CH3)?N - NO2
800
900
* HoNo--
I000
I100
1200
i
1300
FREQUENCY (cm-1) Flgure 3. Contours of product spectrum from irradiation (80 min) of DMA-NO,-air mixture
and an N-NO2 skeletal bend (-760-785 cm-l). For dimethylnitramine, these correspond to the absorption bands observed (18) in the gas phase at 1562, 1305, and 770 cm-1, respectively. The 770 cm-' absorption was specifically assigned to the NO2 out-of-plane wagging motion by Trinquecoste et al. (19) on the basis of a normal coordinate analysis. A comparison of the four major absorption frequencies observed in our DMA-NO, experiments with the frequencies reported by Davies and Jonathan (18)for dimethylnitramine is presented in Table I. The positions and relative intensities of the bands observed in the present study are in good agreement with the literature values. The dimethylnitramine in these experiments was also identified by GC-MS analysis of samples adsorbed on Tenax cartridges; the mass spectrum was identical to the published reference spectrum (20) of an authentic sample. Hence, the major nitrogenous gaseous product observed in the irradiated dimethylamine-NO,-air system is dimethylnitramine. Table I also lists the approximate positions of the major bands of the "unknown" product spectrum obtained by Hanst et al. ( I O ) (Figure 3 in their article) from photolysis of dimethylnitrosamine in air. We identify this unknown product as dimethylnitramine. The two strongest absorptions at -1560 and -1310 cm-l are due to the antisymmetric and symmetric stretches of the NO2 group. Although it appears only as a weak feature, the absorption at -775 cm-l due to the NO2 wagging motion is also identifiable in the spectrum of Hanst et al. (10). The apparent shift in the positions of the weaker bands a t -1480 and -1000 cm-l can be explained by the overlap of the corresponding dimethylnitramine bands with those of the unphotolyzed dimethylnitrosamine (see Table I). The resolved bands due to the C=O stretch (1746 cm-l) and CH2 scissor (1500 cm-l) of formaldehyde also appear in the spectrum of Hanst et al. (10). DMA Photooxidation Products. After 2 h of irradiation, infrared spectra indicated that only about 10% of the DMA originally introduced into the cell remained in the gas phase. The presence of approximately 0.4 ppm (1500 pg m-3) of dimethylnitramine was calculated from the strength of its 1308 cm-l band in these spectra. This estimate was based on an absorptivity of 37 cm-l atm-l ( a = In (Zo/I)/pL),the value measured a t 1288 cm-1 for the NO2 symmetric stretch of an authentic sample of diethylnitramine (cf. below). Approximately the same absorptivities are expected for the bands of this vibrational mode for both dimethylnitramine and diethylnitramine since the NOz group is in the same type of molecular environment. A concentration-time profile for dimethylnitramine from the FT-IR spectra is shown in Figure 4a. Gas chromatographic estimates of dimethylnitramine concentrations, based on the area of the dimethylnitramine peak in the mass chromatogram, are comparable to those determined by the FT-IR measurements. Present data do not permit assessment of what fraction of the products was formed from heterogeneous reactions on the surfaces of the cell. When the absorptions of unreacted dimethylamine were subtracted from the spectrum recorded after 2 min of irradiation, a band centered at 1015 cm-l was observed. Its contour agreed with that of the strongest band of dimethylnitrosamine (21,22),and its intensity relative to that of the 1308 cm-1 band of dimethylnitramine would indicate that the concentrations of the two compounds were comparable. No quantitative information could be obtained for dimethylnitrosamine in later spectra (after further irradiation) due to the overlap of the broad 980 cm-' band of dimethylnitramine with this 1015 cm-' band. Spectra taken during the course of the irradiation showed that the contour of the 1015 cm-l band changed with time. Volume 12, Number 8, August 1978
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Table 1. Major IR Absorption Frequencies (cm-I) of (CH3)2NN02and (CH3)2NN0in Region Below 1600 cm-' a (CH3)2"02 Davies 8 Jonathan
*
This work
1562 f 7 vs 1475 sh 1462 m
f
1305 f 7 vs 1132 w 980 m
1308 vs 1132 w 985 m
770 m 612 m
f
Unknown product, Hans1 et ai.d
-1560 s -1480 m-s
(CH3)zNNOe
1487 s
f
773 w-m
-1310 s -1000 m
1414 w-m 1295 s 1138 w 1015 s 846 m
w; I
I
I
-775 w
f
a s = strong, m = medium, w = weak, sh = shoulder, v = very. Ref. 18. Uncertainty in tha band centers is 1 5 cm-'. Ref. 70. Refs. 21 and 22. 'Not observed in present work due to interference by HZO and/or COz absorptions.
0
I
HOURS OF
2
0
2 3 IRRADIATION
HOURS OF IRRADIATION
Figure 4. Time dependence of (a) dimethylnitramine and (b) diethylnitramine concentrations (determined by FT-IR analysis) during sunlight irradiations of DMA and DEA, respectively, in NO,-air mixtures. See text for absorptivities employed
A peak a t 1024 cm-l (marked with an asterisk in Figure 3), which appeared only as a weak shoulder in the initial spectrum taken after 2 min of irradiation, became more prominent as the irradiation proceeded. Two smaller unassigned peaks a t -895 and -1150 cm-' were present in the later spectra in addition to the 1024 cm-l band. These three bands are tenLCZH&NH t =O tatively assigned to tetramethylhydrazine [ ( C H ~ ) Z N N ( C H ~ ) ~ ] , a product possibly formed by a displacement reaction between the dimethylamino radical and dimethylnitramine or dimethylnitrosamine, or possibly by the reaction of two dimethylamino radicals. The strongest absorption bands of tetramethylhydrazine (below 1300 cm-l) have been reported ( 2 3 , 2 4 )a t 1151,1026,and 899 cm-', in good agreement with the bands observed in this study. The presence of nitrous acid (HONO) during the experiment was indicated by the absorption bands a t 791 cm-1 (trans-HONO) and 853 cm-1 (cis-HONO). As shown by Figure 2c, approximately 0.1 ppm HONO is present after 80 min of irradiation. Other products indicated by our infrared spectra included carbon monoxide and formaldehyde. Small amounts of di700 800 900 1000 1100 I200 '300 methylformamide were identified by GC-MS in agreement FREQUENCY ( c d l with results obtained in the companion amine study (1I ) . A Figure 5. (a) Spectrum of diethylamine (2.6 ppm, 720 m). (b)Spectrum band a t 828 cm-' appeared in both DMA and DEA experiof DEA-NO,-air mixture after 5 min of irradiation. (c) Spectrum of diments and is tentatively assigned to a nitrate aerosol, probably ethylnitramine in DEA-NO,-air mixture after 95 rnin of irradiation either a dialkyl nitrate (11) or ammonium nitrate (possibly produced by the oxidation of ammonium nitrite formed in an NH3 HONO reaction). No 0 3 or peroxyacetyl nitrate (PAN) was observed during these runs since the concentrations of NO By analogy to the DMA-NO, case, diethylnitramine is the and amine remained large enough to inhibit formation of these major product expected from irradiation of the DEA and NO, secondary products (25). In these experiments significant mixtures and the intense absorption observed at -1290 cm-1 can readily be assigned to the NO2 symmetric stretch of diamounts of dimethylamine, dimethylnitramine, and dimethylnitrosamine may have been lost to the walls or removed ethylnitramine. Pure diethylnitramine was prepared for from the gas phase due to aerosol formation. positive identification, and the vapor-phase infrared spectrum DEA Experiments. Irradiations of DEA and NO, were of the authentic sample is shown in Figure 7. This spectrum carried out in the same manner as those described above for was recorded a t the same resolution (1 cm-') and pathlength (720 m) used in the photooxidation experiments and, to our DMA. Figure 5 shows spectra for (a) DEA alone, (b) the knowledge, is the first infrared spectrum of gaseous diethylDEA-NO,-air system after 5 min of irradiation, and (c) the same mixture after 95 min of irradiation. Figure 6 shows the nitramine reported in the literature. GC-MS analysis also contours of absorption bands of Figure 5c with the absorption showed that diethylnitramine was the major gas-phase of unreacted DEA subtracted and water interferences reproduct formed in this experiment and in the related amine moved. As in the case for DMA runs, no significant amounts study (11). of 0 3 or PAN were formed during these irradiations due to the Comparison of Figure 7 with Figure 6 shows that the major high concentration of NO and amine present, but carbon band envelopes of diethylnitramine are present in the product spectrum. The distortion of the band contours in the region monoxide and formaldehyde formation were observed. l
+
956
Environmental Science 8. Technology
,
I
,
l
#
of the C-N and N-N bond stretches (900-1100 cm-l) in the product spectrum is due to the absorption of a residual amount of unphotolyzed diethylnitrosamine, which was formed during the early stage of the reaction. The strong and relatively broad N-N stretching band of diethylnitrosamine in the vapor phase was reported (26,27) to be a t 1047 cm-l. The two weaker bands of diethylnitramine a t 766 and 823
0
800
530
I000
I200
I100
1300
FREQUENCY (cm-1)
Figure 6. Contours of product spectrum from irradiation (95 min) of DEA-NO,-air mixture
I288
1
I 01 703
8CC
500
lOC0
I00
I200
FREQUENCY
2800
1300
2900
3000
1cm-l)
Figure 7. Infrared spectrum of diethylnitramine vapor at 25 ppm, 720 m, 1 cm-I resolution)
OC
(0.48
cm-l in the authentic sample spectrum are barely discernible in the product spectrum because they overlap with the Pbranches of the trans-HONO and cis-HONO absorptions. Table I1 summarizes the major absorption bands of gaseous diethylnitramine that are accessible in atmospheric systems with infrared spectrometers utilizing pathlengths comparable to those employed in this study. The strong absorption at 1500-1600 cm-1 due to the NO2 antisymmetric stretch could not be observed due to severe H20 interference. Also listed in Table I1 are the published infrared frequencies for gaseous diethylnitrosamine (26,27). The three bands listed at 1218, 1323, and 1353 cm-1 are of comparable intensities (26). Since the 1218 cm-1 band, which is located at a relatively clear part of the spectrum, did not measurably appear in the product spectra, except during the first few minutes of the reaction, the diethylnitrosamine absorption at 1323 cm-l should cause no significant interference in estimating the diethylnitramine concentration from its 1288 cm-l absorption. The absorptivity of the 1288 cm-l band of gaseous diethylnitramine was measured to be 37 cm-' atm-l. From this value it was calculated that approximately 0.5 ppm (2400 pg/m3) of diethylnitramine was formed during a 3-h irradiation period. A time-concentration profile for diethylnitramine from the FT-IR measurements is shown in Figure 4b. Estimates of Detection Limits. It is of interest to establish the detection limits attainable for (CH&NNO, (CH&NN02, (CzH&NNO, and (C2H5)2NN02in ambient air with the km pathlength FT-IR spectrometer employed in this study. A range of absorptivities (7-21 cm-l atm-I) for the 1020 cm-' band of dimethylnitrosamine was calculated from a spectrum published by Hanst et al. (10) in which they estimated the dimethylnitrosamine vapor concentration to be between 1and 3 ppm. Assuming a 1%absorption in the ratioed spectrum and the absence of interfering species, detection limits of 5-15 ppb are then estimated for dimethylnitrosamine. This range of values is also expected to be applicable to diethylnitrosamine. For dimethylnitramine and diethylnitramine, a detection limit of approximately 8 ppb was calculated on the basis of a 3% absorption in the ratioed spectrum. The larger absorption needed for detection of the nitramines is due to the interference of strong water lines in the 1300 cm-1 region for the long pathlengths employed. Thus, the calculated detection limits are significantly higher than those afforded (
