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Anal. Chern. 1983, 55,2097-2099
Detection of Nitrogen and Sulfur Dioxides in the Atmosphere by Atmospheric Pressure Ionization Mass Spectrometry F. M. Benoit
Environmental Health Directorate, Health and Welfare Canada, Tunney's Pasture, Ottawa K I A OL2, Canada
The ionization reactions of sulfur dioxide, [nitrogendioxide, and mixtures of these two gases In air were examined by atmospheric pressure ionization mass spectrometry. The ionization reactions of sulfur dloxide varied with the levels of water and nitrogen dioxide in the sample. The ionization reactions of nitrogen dioxide varied with the concentration of nitrogen dioxide. Accurate quantitative estimations were iimited to the pg/m3 range of concentrations for both dioxides.
Permanent gases often persist in thLe atmosphere until chemical and/or physicdl transformation to less volatile species leads to deposit ion in the condensed environmental media. Upon emission it is assumed that dilution in the atmosphere prior to deposition willl reduce the impact of noxious permanent gases ai, the site of deposition in the aquatic and/or terrestrial media, often located at long distances from the emitting source. The oxides of sulfur and nitrogen are examples of permanent gases that may cause environmental damage either directly or following chemical conversion to other species. Consequently numerous methods (1-6) have been developed to determine their levels in the atmosphere. Most commonly, time weighted average levels are determined over a fixed time period. The techniques of atmospheric pressure ionization/mass spectrometry (API/MEI) are ideally suited for the real time detection of trace gaseous atmospheric contaminants (7). &f'I/MS is able to extract the target connpound directly from the air matrix by ionization in either the positive or negative ion mode. The potential advantages of API/MS are (1)real time analysis, (2) continuous monitoring, (3) sensitivity, and (4) compound specificity. In order to determine the feasibility of monitoring NOz and SOz in the atmosphere by API/MS, the behavior of these gases under API/MS conditions has been studied.
EXPERIMENTAL SECTION Spectra were obtained with a Sciex TAGA 3000 atmospheric pressure ionizatiion mass spectrometer operating in the negative ion mode with air as the source of the reagent ion (Oz-) (8). Atmospheric or synthetic (Ultra Pure 21% oxygen-79% nitrogen, Liquid Carbonic) airs were used to generate the reagent ion plasma. Spectra were acquired by multiple ion monitoring of the following ions: NOz, m/I 16, 17, 32,46, 60, 62, 125; SOz,m/z 32, 64,80,81,96,97,112,113. Quantitative estimations were obtained from analyses of atmospheric air contained in Tedlar bags (LOO L) doped with the appropriate volumes of analytes. Nitrogen dioxide (99.5%) and sulfur dioxide (anhydrous, 99.98%) were obtained from Matheson. RICSULTS AND DISCUSSION Atmospheric vs. Synthetic Air as Reagent Gas. Atmospheric air contains components other than nitrogen and oxygen from which the negative ion mode reagent ions are generated. The presence of carbon dioxide, water, and/or other contaminants can modify the reagent ion plasma and possibly the ionization ]process for the analyte of interest. For this reason the spectra of synthetic (21% oxygen and 79%
nitrogen) and atmospheric airs were compared (Figure 1). The spectra differed markedly, particularly in the m/z 60 to 61 region where the peaks m/z 60 (C03-) and m / z 61 (HC03-) were the major peaks in the spectrum of atmospheric air but less important peaks in the spectrum of synthetic air. In addition, a peak a t m/z 77 (HCO,) present in the atmospheric air spectrum was absent in the synthetic air spectrum. The presence of the small m/z 60 and m f z 61 in the synthetic air spectrum was likely due to residual carbon dioxide in the system despite extensive flushing of the system with synthetic air prior to analysis. Furthermore, the spectrum of atmospheric air varied from day to day. The relative heights of the m / z 32, 60, 61, and 77 peaks varied apparently with the levels of humidity and carbon dioxide in the air which could be an important factor in field studies. However, it must be stressed that the relative heights of the m / z 32 and m / z 60 peaks did not reflect the relative concentrations of oxygen and carbon dioxide in the atmospheric air. The m / z 60 peak was enhanced by the high electron affinit,y of COS (EA =5.3 eV) (9) relative to oxygen (EA = 0.45 eV) (10). Thus, a relatively low concentration of C o g in the air produced a relatively strong ion signal at m / z 60. Hence, not only did atmospheric and synthetic airs produce different ion plasmas that could modify the spectra of the analyte gases but also the ion plasma produced from atmospheric air depended upon the concentrations of water and carbon dioxide in the sample at the time of analysis. Nitrogen Dioxide. Nitrogen dioxide produced similar spectra with synthetic and atmospheric airs as reagent gases. No apparent negative interferences from the extraneous species in the atmospheric air ion plasma in the ionization of nitrogen dioxide were observed. However, the ionization process for nitrogen dioxide was observed to vary with increasing concentration of nitrogen dioxide. At low concen-
+ 0,NO2 + NO2NO2
-
4
NO3- + "OB
+0 2 NO3- + NO
NO,-
(1)
-
(2)
",OB-
NO2 + COS--+ NO3- + C 0 2
(3) (4)
trations (ca. pg/m3) reaction 1produced a strong peak at m / z 46 (NOz-) (Figure 2). As the concentration of NOz was increased from 100 pg/m3 to 10 mg/m3 the height of m/z: 32 decreased as the available 0,- reagent ions were depleted by reaction 1. Between the concentrations of 10 mg/m3 and 30 mg/m3 the m / z 32 peak disappeared and the m / z 46 peak decreased in intensity instead of remaining constant as expected if saturation had been reached; two new peaks, one major at m / z 62 (NO3-) and one minor at m / z 125 (HN206-), appeared. At concentrations >15 mg/m3 reaction 2 is proposed as the source of NO3-. This was supported by the observations that (1)NO3- appeared at nitrogen dioxide levels where 0, was absent and NOz- was the dominant ion and (2) the abundance of NO< decreased as that of NO3- increacsed. Furthermore, reaction 2 has also been observed by Fehsenfeld
0003-2700/8~/0355-2097$01.50/0 Publlshed 1983 by the American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 55, NO. 13, NOVEMBER 1983
80
1 7
SYNTHETIC A I R
>
z
40-
w
-
+
z
1'2
w
2 80t6
B
61
so,
ATMOSPHERIC A I R
-
-J
w
LT
30
70
50
LO-
90
90
70
110
m/ z
m/ z Flgure 1. Negative ion API mass spectra of atmospheric and synthetic airs.
Flgure 3. Negative ion API mass spectra of sulfur dioxide with synthetic and atmospheric airs as reagent gas.
6) or hydroxyl (reaction 7) abstraction. No apparent negative effect of the COB- and related ions from the atmospheric air reagent ion plasma on the ionization of sulfur dioxide was observed. In fact COS- and HC03- are reactive with SO2 (reactions 8 and 9) (13) and hence contribute to the ionization of Unlike nitrogen dioxide, the production of the ionized higher oxides of sulfur (SO,-, SO4-, and SO5-) likely occurred by reaction of the sulfur oxide ions with excess molecular oxygen (reactions 10 and 11) rather than by reaction with
- o o l
soz.
- + + + so,- + - so,+1-+ so,- + - so,+,-
C 0 3 - + SOz HC0330
60 CONCENTRATION
12 0 NO2 l m g l M 3 1
90
et al. (11) in flowing afterglow studies. The origin of the HNz06- ion remains unknown but could be accounted for by reaction 3 which has been reported (11)in flowing afterglow studies. Cabane et al. (12) have reported on the production of HN03 from NO2 in the ion chamber under atmospheric pressure ionization conditions. Hence at higher concentrations, NO2- becomes the reagent ion and could well react yith other species in the sample under analysis. The m/z 60 peak (C03-) was also observed to decrease with increasing NO2 concentration (Figure 2); we suggest that reaction 4, which has also been observed in flowing afterglow studies (13),could account for this observation. Sulfur Dioxide. Sulfur dioxide produced slightly different spectra (Figure 3) with atmospheric and synthetic airs as reagent gases. With synthetic air, sulfur dioxide produced a spectrum with a major peak a t rnlz 64 (SOz-) (reaction 5) corresponding to the molecular ion and a series of smaller peaks at m / z 80 (SO,-), m / z 81 (HS03-), m/z 96 (SO4-), m/z 97 (HS04-),m / z 112 (SO5-), and m/z 113 (HS05-). With atmospheric air as reagent gas, the series of peaks at m / z 81, m / z 97, and m / z 113 were relatively more prominent (Figure 3) and their abundances varied from day to day. We suggest that these ions were formed by reaction of the higher sulfur oxide ions with water in the air by either hydrogen (reaction
-
SO,-
+ HzO
SO,-
+ HZO
s02-
+ OH HSO,+,- + H HS0,-
-+
(5)
(6)
(7)
HS03-
C02 COz
0
0 2
Figure 2. Negative ion API mass spectra of nitrogen dioxide as a function of concentration.
so2 + 02-
SOz
SO,-
0 2
(8)
(9) (10) (11)
excess sulfur dioxide because the higher oxide ions were observed a t low concentrations of sulfur dioxide where interactions were unlikely and the ratio of the higher oxide ion peaks to the sulfur dioxide molecular ion peak did not vary with increasing concentration of sulfur dioxide. Nitrogen Dioxide-Sulfur Dioxide Interaction. A common problem in API/MS at high sample concentrations (ca. parts per million) is interference whereby in a multicomponent system intdraction between neutral and ionized components of the analyte occurs (8) usually under conditions where the limited supply of reagent ions is nearly or completely exhausted. The result is a reduction in the abundance of the ionized component of the analyte to below the level expected in the absence of the interferer. An example of interference was observed in the depletion of m / z 46 in the ionization of nitrogen dioxide at higher nitrogen dioxide concentrations. In this case, the interferer was the analyte itself. Generally, interferences occur in API/MS when the interferring action is favorable energetically and the concentrations of the reacting species are high enough for the reaction to proceed a t a sufficiently rapid rate. Interferences lead to erroneous quantitative estimations but can often be reduced or eliminated by dilution of the sample with clean air. We examined the ionization of mixtures of NOz and SO2 in atmospheric air over a wide range of concentrations and observed interferences when the product of the NO2 and SOz concentrations exceeded 9 (mg/m3)z. The interferences observed were the reduction of peak intensities at m/z 64, mlz 80, and mlz 81 and the increase of peak intensities a t m / z 96, m / z 97, mlz 112, and m / z 113 compared to the spectrum
ANALYTICAL CHEMISTRY, VOL. 55, NO. 13, NOVEMBER 1983
rn/ z
Flgure 4. Negative)ion API mass spectrum of outdoor alr in the Ottawa
area. of SO2 alone. The degree of interference increased with the concentrations of the two gases. In the liimit, at [NO,] = 500 mg/m3 with [SO2]= 140 mg/m3 the peak at m / z 64 (SO2-) was absent. The mechanism of the interference is complex and likely involves charge and/or oxygen transfer reactions 12, 13, and 14. Reactiion 12 is supported by the energetics
SO2-
+ NO2
+ NO3SO3- + NOz SOz
--
.--*
+ NOzSO4- + NO SO4- + NO
SO2
(12)
(13) (1.4)
of the reaction: NO2 has a higher electron affinity than SO2 (hEA(NOZ- SO,) = 1.3 eV (13). The increase in m / z 96 peak was only observed at NOz concentration where NO3- ion formation occurred, hence reaction 13 is proposed to account for SO, ion formation. Reaction 14, also reported in flowing afterglow experiments (11),would account for the decrease in SO3- levels. The complexity of the !302/N02interaction was indicated by the peak at m / z 142 (N02.S04-)at higher oxide concentrations. Quantitative Estimations. As with dl ionization reactions under API/MS conditions, quantitation is subject to the saturation effect that occurs when the limited supply of reagent ions is depleted by reaction with the analyte. Generally, saturation occurs in the low parts per million range of analyte concentrations but varies from compound to compound. In addition, interference reactions at higher analyte concentrations may deplete the abundance of the ion of interest. For example the interactions of sulfur dioxide and water and of nitrogen dioxide and sulfur dioxide and the formation of NO< at higher concentrations of NOz in the
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spectrum of nitrogen dioxide all deplete the abundance of the ion of interest such that accurate quantitative estimations are difficult under these conditions. Both the above difficulties can be overcome by sample dilution. Generally, accurate quantitative estimates could be obtained at the pg/m3 (part per billion) or lower level where the ionic response to analyte concentration is linear. Fortunately, lower concentrations are achievable by dilution with cylinder air. One problem with sample dilution, however, is that components of lower concentration may be diluted to levels below the limit of detection (low parts per trillion levels). Hence caution must be used in quantification of nitrogen dioxide and sulfur dioxide levels in the atmosphere by API/MS. For example, the absence of m/z 46 in the spectrim of an air sample could be interpreted as either the absence of nitrogen dioxide in the sample (level below limit of detection of ca. 1 pg/m3) or a concentration in excess of ca. 500 mg/m3. Further, the presence of both nitrogen dioxide and sulfur dioxide in an air sample would require careful examination at Concentrations where the threshold for interference is exceeded. A typical spectrum (Figure 4) of outdoor air sampled in the Ottawa area exhibited small peaks at m / z 46, 64, 80, 81,96,97,112,and 113., from which the concentrationsof NOz and SO, were estimated at 24 pg/m3 and 2 pg/m3, respectively. Registry No. Nitrogen dioxide, 10102-44-0; sulfur dioxide, 7446-09-5.
LITERATURE CITED Hollowell, G. D.; Gee, G. Y.; McLaughlln, R. D. Anal. Chem. 1973, 45, 63A.
Dasgupta, P. K.; DeCesare, K.; Velrey, J. C. Anal. Chem. 1980, 5 2 , 1912.
Bruckensteln, S.; Tucker, K. A.; Glfford, P. R. Anal. Chem. 1980, 5 2 , 2396.
Vinjamoori, S . ; Ling, C. S . Anal. Chem. 1981, 53, 1689. Maeda, Y.; Aokl, K.; Munemorl, M. Anal. Chem. 1980, 5 2 , 307. Adema, E. H. Anal. Chem. 1979, 5 7 , 1002. Mitsul, Y.; Kambara, H.; Kojima, M.; Tomlta, H.; Katoh, K.; Satoh, K. Anal. Chem. 1983, 5 5 , 477. Benolt, F. M.; Davldson, W. R.; Lovett, A. M.;Nacson, S.; Ngo, A. Anal. Chem. 1983, 5 5 , 805. Hiller, J. F.; Vetal, M. L. J. Chem. Phys. 1980, 72, 4713. Tiernan, T. 0.; Hughes, 8.M.; Llfshltz, C. J. Chem. fhys. 1971, 5 5 , 5692.
Fehsenfeld, F. C.; Howard, C. J.; Schmeltekopf, A. L. J. Chem. Phys. 1975,'63, 2835.
Cabane, M.; Playe, P. J. Aerosol Sci. 1980, 1 7 , 475. Hughes, B. M.; Llfshltz, C.; Tiernan, T. 0. J. Chem. f h y s . 1973, 5 9 , 3162.
RECEIVED for review May 6, 1983. Accepted July 27, 1983.
