continuous determination of dimethylsulfide at part-per-trillion concentrations in air by...

Atmospheric Environment Vol. 25A, No. 10, pp. 2155 2160, 1991. 0004~981/91 $3.00+0.00 Printed in Great Britain. © 1991 Pergamon Press plc

CONTINUOUS DETERMINATION OF DIMETHYLSULFIDE AT PART-PER-TRILLION CONCENTRATIONS IN AIR BY

ATMOSPHERIC PRESSURE CHEMICAL IONIZATION MASS SPECTROMETRY*

THOMAS J. KELLY and DONALD V. KENNY Battelle, 505 King Avenue, Columbus, OH 43201-2693, U.S.A.

(First received 10 July 1990 and in final form 24 November 1990)

Abstract--Highly sensitive and specific continuous measurement of dimethylsulfide (DMS) in air has been demonstrated using triple quadrupole mass spectrometry with atmospheric pressure chemical ionization. Detection limits of 2 parts per trillion and 4 parts per trillion by volume in air are achieved for DMS using positive ion detection with benzene charge exchange and hot wire excitation, respectively. Either of these ionization modes provides sensitivity sufficient for continuous direct monitoring of dimethylsulfide in the atmosphere, with time response of approximately 1 s. This capability may be applicable to study the effect of oceanic DMS emissions on global climate. Detection limits in continuous monitoring were also determined for hydrogen sulfide (1 ppbv) and for methyl mercaptan, carbonyl sulfide, and carbon disulfide (~ 10 ppbv).

Key word index: Dimethylsulfide, continuous monitoring, mass spectrometry, atmospheric chemistry.

INTRODUCTION

There is currently a great deal of interest in the possible effect on global climate of dimethylsulfide (CH3SCH3; DMS) and other sulfur compounds emitted from the oceans. That interest stems from the recent hypothesis (Charlson et al., 1987; Mrsz:iros, 1988) that DMS emissions could comprise a negative feedback system for control of global temperatures, which would counteract any global warming caused by CO 2 and other greenhouse gases. Dimethylsulfide is produced by marine microorganisms and emitted to the atmosphere, where it is oxidized to sulfur dioxide (SO2) which ultimately forms sulfate (SO]-) aerosols. Such aerosols are efficient cloud condensation nuclei (CCN). The hypothesis put forth suggests that as the primary sulfur species emitted from the ocean, DMS is the primary contributor to aerosol sulfate in remote marine regions, and thus the primary source of CCN in such regions: According to that hypothesis, any increase in global temperatures could lead to greater marine productivity and increased DMS emissions, which in turn could lead to a greater number density of CCN (and thus of cloud droplets), resulting in an increase in reflectivity of clouds in marine regions. The greater reflectivity of clouds would result in a cooling effect, reducing global temperatures and turning down marine productivity. The validity of this hypothetical control system has been disputed (Schwartz, 1988), perhaps indicating that the system is more complex

* Portions of this work were presented at the 38th Ameri- can Society for Mass Spectrometry Conference on Mass Spectrometry and Allied Topics, Tucson, Arizona, 4-8 June 1990.

than envisioned at present. In any case, understanding of the role of the oceans in global climate must include the potential effects of DMS emissions.

The atmospheric concentrations of DMS in marine regions are typically less than 200 parts per trillion by volume (pptv= 1 x 10-12 by volume) (Barnard et al., 1982; Andreae et al., 1985, 1988; Ferek et al., 1986; Van Valin et al., 1987; Van Valin and Luria, 1988; Ber- resheim et al., 1990; Bates et al., 1990), with a strong decrease in DMS concentration with increasing height above the surface (Ferek et al., 1986; Andreae et al., 1988; Van Valin and Luria, 1988; Berresheim et al., 1990). As a result, measurement of DMS and associ- ated compounds in the atmosphere is a difficult task, usually performed by collection and preconcentration of the sulfur species on filters, adsorbents, metal surfaces, or in canisters or cryogenic traps, with final analysis by flame photometric detection (Barnard et al., 1982; Steudler and Kijowski, 1984; Andreae et al., 1985, 1988; Ferek et al., 1986; Kagel and Farwell, 1986; Van Valin et al., 1987; Van Valin and Luria, 1988; Berresheim et al., 1990; Bates et al., 1990). Gas chro- matographic separation of the sulfur compounds is performed when individual species concentrations are required. However, such methods of collection and analysis are complex and provide poor time resolu- tion. No method has previously been reported for direct analysis of atmospheric DMS in real time.

Reported here are the results of a feasibility study which evaluated the capabilities of triple quadrupole mass spectrometry with atmospheric pressure chemical ionization for direct, continuous measurement of sulfur species in the atmosphere. Primary emphasis is on the capabilities established for DMS, which may be useful for study of the role of this compound in

2155

2156 THOMAS J. KELLY and DONALD V. KENNY

atmospheric chemistry; results for other sulfur gases are also noted.

EXPERIMENTAL

Mass spectrometer

The mass spectrometer used in this study is a SCIEX Trace Atmospheric Gas Analyzer (TAGA) 6000E, an atmospheric pressure chemical ionization (APCI) triple quadrupole mass spectrometer. The TAGA has been found capable of a wide variety of fast, sensitive, and specific analyses of trace species in gas mixtures including ambient air. Components of a TAGA that combine to provide these attributes are illustra- ted in Fig. 1. The basic components are the inlet module, ionization source, transfer ion lenses, three quadrupole mass filters, and the detector. The inlet module samples whole air without sample treatment or collection, at typical sample flow rates of 0.5-2.0 ( s 1. The ionization source uses a point- to-plane corona discharge at atmospheric pressure to pro- duce ions of the species of interest. High selectivity for compounds of interest is achieved by ion pair monitoring, i.e. detecting daughter ions selected by the third quadrupole and originating only from a single parent ion mass selected by the first quadrupole. Detailed descriptions of the TAGA in particular, and triple quadrupole mass spectrometry in gen- eral, are provided elsewhere (Dawson et al., 1982a, b). The TAGA is a rugged, mobile instrument and has been used for direct atmospheric monitoring at a variety of field facilities, and aboard an aircraft.

Tests performed

The response of the TAGA to several reduced sulfur compounds was tested, with the aim of identifying and optimizing ionization and detection approaches for these species. The sulfur compounds tested were dimethylsulfide (DMS), hydrogen sulfide (H2S), methyl mercaptan (CHaSH), carbon disulfide (CS2), and carbonyl sulfide (COS). TAGA response to each of the five sulfur species was tested using each of five ionization approaches:

(1) positive ion APCI; (2) negative ion APCI; (3) positive ion APCI with a hot wire inlet; (4) positive ion benzene charge exchange APCI; (5) positive ion APCI with u.v. excitation.

Ionization approaches 1 and 2 are the normal operating modes of the TAGA. In positive ion mode 1 the principal charge carrier is H aO +, produced from water vapor present in the sample air; in negative ion mode 2 the principal charge carrier is 0 2 resulting from atmospheric 0 2 . In addition, three other ionization approaches were tested, because pre- vious published and unpublished work had indicated they might provide high sensitivity for the sulfur species of interest. Approach 3 above used a hot wire (a glowing NiChrome filament) in the sample air stream to enhance the ionization efficiency for the sulfur species (Sunner et al., 1988). Approach 4 involved introduction of benzene vapor into the ionization region, promoting ionization and subsequent charge exchange from benzene to the sulfur species. In approach 5, a u.v. light source was placed in the sample air stream, promoting oxidation of the sulfur compounds, pre- sumably largely by ozone produced in the sample air. This mode allows detection of oxidized products of the sulfur compounds (Hijazi and Debru, 1982), rather than the parent compounds themselves.

Individual compressed gas standards of 50 ppm concentration by volume were used as sources of the gaseous sulfur species (DMS, H2S, COS, CHaSH ). Carbon disulfide was used in the form of the pure liquid. Initial qualitative tests evaluated sensitivity and obtained parent/daughter ion spec- tra. These tests were performed by sampling ambient room air through a charcoal scrubber trap of 5 d volume at 0.7 c ~ s-1 using the TAGA. Background air measurements thus obtained were compared to measurements of a few hundred ppbv of each sulfur compound, produced by metering a low flow of 50 ppmv standard into the sample air flow. In the case of CS2, headspace vapor from a container of the liquid was mixed with the air flow. The concentrations used in these initial tests were much larger than those expected in the ambient atmosphere, and served to estimate relative sensitivity for the several sulfur species.

Quantitative determination of detection limits in real-time monitoring was performed by preparing concentrations of 2.5 ppbv of individual sulfur compounds, singly or all at once, in a 17.3 m 3 environmental chamber. The chamber was then purged with outdoor ambient air at a rate that approximately halved the concentration each hour. Dilution of the sulfur species was monitored by independent gas chromato- graphic analysis of SF 6 tracer also introduced into the chamber. TAGA measurements were made by monitoring ion current over 1-s intervals for about 1 min on chamber air prior to addition of the sulfur species, and for about 1 min

TAGA ® 6000E TANDEM TRIPLE QUADRUPOLE MASS SPECTROMETER

CAD (Colllslonally Activated Dissociation)

Argon / N2 Gas Membrane Interface Gas / Detector

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sensitivity, selectivity, and speed of measurement.

Continuous determination of dimethylsulfide 2157

approximately each hour during the purging process. Purg- ing of the chamber continued until the detection limit was approached, with the detection limit defined as the concentration of sulfur species producing response equal to three times the standard deviation of the count rate observed on background chamber air (measured before introduction of sulfur compound into the chamber). For COS, CS2, and CH3SH, it was necessary to increase concentrations well above the initial concentration of 2.5 ppbv in order to detect these compounds in the chamber air.

RESULTS AND DISCUSSION

The qualitative tests revealed very high sensitivity for DMS in positive ion mode using both ionization approach 3 (hot wire) and approach 4 (benzene charge exchange). In both modes DMS was measured by monitoring the daughter ion of mass 47 (the major daughter ion formed) originating from fragmentation of the mass 62 parent ion, i.e. the 62/47 parent/ daughter ion pair. Sensitivity for H2S was consider- ably lower than for DMS, and was optimum using negative mode APCI (approach 2 above). Sensitivity for COS, CS2, and CHaSH was much lower than for H2S. Optimum sensitivity for COS and CS2 was observed with ionization approach 3. For CH3SH, optimum sensitivity was observed using ionization approach 5 (u.v. light).

Quantitative tests using the 17.3 m 3 chamber as described above indicated detection limits of approximately 10 ppbv for COS, CS 2, and CH3SH , using the optimum ionization modes for these species noted above. That is, the initial chamber concentration of 2.5 ppbv was not detectable for these species, and additional injections into the chamber were required

to establish a concentration sufficiently detectable above chamber air background. For H2S, a detection limit of 1.0 ppbv was determined in the chamber test. In contrast, the high sensitivity for DMS suggested by the qualitative comparisons was confirmed in the quantitative studies, as shown in Figs 2-4.

Figure 2 shows the TAGA response plot derived from a chamber dilution study lasting about 8 h, in which benzene charge exchange APCI was used. Each symbol represents the TAGA response to the chamber DMS concentration in ion counts per second (ICPS), during an approximately 1-min sampling period on chamber air carried out about once an hour. The data points shown are the average 62/47 ion current based on ten successive 1-s intervals; the _+ 1 standard deviation ranges of the ion current are smaller than the symbols used to plot the data in Fig. 2. The line shown is the linear regression to all the data points. Note that several data points are shown in Fig. 2 at concentrations below 0.10 ppbv (i.e. 100 pptv). Figure 3 is an expansion of the low end of Fig. 2, showing the data points in this range. The vertical error bars represent ___ 1 standard deviation of the ion current data; the error bars on the background data point at zero DMS concentration are the same size as the symbol plotted. The linear regression shown in Fig. 3 has the form

ICPS = 23,480 (___ 915) x DMS(ppbv) + 135 (+ 59).

The standard deviation of the background air count rate is 17.5 ICPS; the ratio of the regression slope to three times the background standard deviation gives a detection limit of 2 pptv DMS. Note that the background count rate shown in Fig. 3 falls somewhat

DMS DETECTION BY BENZENE APCl 6 2 / 4 7 I O N P A I R M O N I T O R I N G

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Fig. 2. Plot of DMS 62/47 parent/daughter ion current, in exponential dilution chamber test, using benzene charge exchange APCI.


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are 1-s 62/47 ion count rates for (a) 17 pptv DMS, and (b) background chamber air.

Continuous determination of dimethylsulfide 2159

below the regression line. The background count rate was measured at the start of the study, before introduction of DMS into the chamber, and nearly 8 hours before measurement of the lowest data point in Fig. 3. Thus the slight offset may simply indicate an increase in DMS concentration of about 2 pptv in the outdoor ambient air used to purge the chamber, over the course of the day. Such a DMS concentration may be reasonable for Columbus, Ohio, in late fall.

Examples of the continuous ion current data from the TAGA are shown in Fig. 4, which presents the raw data from two sampling periods in the study shown in Figs 2 and 3. Figure 4a shows the ion current data taken at 1-s intervals over the course of a minute or so in sampling DMS at 17 pptv in the chamber; this data point is shown in Fig. 3. Figure 4b shows a similar trace of the background air ion current, also shown in Fig. 3, taken about 6 h before that of Fig. 4a. In both cases, the period of chamber air sampling is flanked by periods of sampling clean air. These data illustrate the sensitivity, stability, and speed of response of the TAGA to DMS.

In a corresponding chamber dilution study using ionization approach 3 (hot wire), a DMS detection limit of 4 pptv was found. It is noteworthy that the parent ion observed with the hot wire approach was re~z=62, rather than the m / z = 6 3 (i.e. M + H ) ion previously suggested for DMS using this approach (Sunner et al., 1988). The same detection limits were obtained as well with both ionization modes for DMS in the presence of a mixture of the other four sulfur species, each at a concentration equal to that for DMS. This specificity for DMS is consistent with the inherent selectivity of the TAGA. Dimethyldisulfide (CH3SSCH3) was not tested in this study, but will not interfere in DMS measurement because of the large difference in parent ion masses. Similarly, dimethyl peroxide (CH3OOCH3) in ambient air will not interfere in DMS detection, because although it has a parent mass of 62, it does not readily form a mass 47 fragment.

In addition to high specificity and sensitivity for DMS, the TAGA provides advantages over existing methods of measurement. Measurements with the TAGA require no sample pre-treatment or collection, and data are obtained continuously rather than inter- mittently. Calibration of the TAGA is easily performed by standard addition of small flows of standard mixtures to the large sample flow (Spicer et al., 1988). This procedure assures comparability of calib- rations to the sampled air, and can be performed at any time with minimal interruption of data collection. TAGA time response is more than adequate for atmospheric studies involving airborne sampling, such as might be required in measuring DMS in marine areas. In that regard, the TAGA has been used previously aboard an aircraft for simultaneous continuous monitoring of several trace atmospheric species. We also plan to evaluate whether TAGA time response is sufficient for direct measurement of DMS in

eddy correlation flux measurements, aimed at deter- mining the flux of DMS to the atmosphere. A dis- advantage of the TAGA is the lower sensitivity found for the other sulfur gases tested, relative to that for DMS. This result indicates that simultaneous measurement of several reduced sulfur gases at ambient atmospheric levels may not be feasible. However, this limitation may not be critical in marine environments where DMS is the predominant reduced sulfur species. We note also that the TAGA detection limit for SO2 is about 1 ppbv, but specificity for SO2 is relatively poor due to the lack of characteristic fragment ions from this compound.

CONCLUSIONS

Triple quadrupole mass spectrometry with atmospheric pressure chemical ionization has been found to be an extremely sensitive and specific method for rapid, direct, continuous measurement of dimethylsulfide in air. Detection limits of 2 pptv and 4 pptv DMS are found when using APCI with benzene charge exchange and with hot wire ionization en- hancement, respectively. This capability for continuous measurement of dimethylsulfide may be applicable to study of the role of DMS in atmospheric chemistry and global climate.

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Charlson R. J., Lovelock J. E., Andreae M. O. and Warren S. G. (1987) Oceanic phytoplankton, atmospheric sulphur, cloud albedo and climate: a geophysiological feedback. Nature 326, 655-661.

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Ferek R. J., Chatfield R. B. and Andreae M. O. (1986) Vertical distribution of dimethylsulfide in the marine atmosphere. Nature 320, 514-516.

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