Environ. Sci. Technol. 1990,24, 1592-1596
Davis, J. A,; Leckie, J. 0. J . Colloid Interface Sci. 1978, 67, 90-107.
Yates, D. E. Ph.D. Thesis, University of Melbourne, Melbourne, Australia, 1975. Huang, C. P. Ph.D. Thesis, Harvard University, Cambridge, MA, 1975. Kent, D. B.; Kastner, M. Geochim. Cosmochim.Acta 1985, 49, 1123-1136. Anderson, P. R.; Benjamin, M. M. In Chemical Modeling of Aqueous Systems II; ACS Symposium Series 416;
Melchior, D. C., Bassett, R. L., Eds.; American Chemical Society: Washington, DC, 1990;pp 272-281. Received for review January 4, 1990. Accepted June 5, 1990. Funding for this study was provided by the Valle Scandinavian Exchange Program at the University of Washington, the Office of Research and Development of the United States Environmental Protection Agency (Grunt EPA810902-01-0), and the University of Washington Graduate School Research Fund.
Field Evaluation of the Sulfur Chemiluminescence Detector Richard L. Benner and Donald H. Stedman'
Department of Chemistry, University of Denver, Denver, Colorado 80208
A field evaluation of the sulfur chemiluminescence detector (SCD) as a real-time total atmospheric sulfur detector is presented. The SCD was installed in a monitoring trailer along with a flame photometric detector (FPD), fluorescent SO, monitor (Fluor), and a suite of other monitoring instruments. The performance of the analyzers was compared for (1)baseline stability, (2) response stability, (3) interferences, (4) sensitivity, and (5) environmental temperature effects. The SCD exhibited the best baseline stability and sensitivity, but had a drift in sensitivity larger than the other analyzers. The FPD and Fluor both showed interference effects, but none were observed for the SCD. Ambient temperature variations altered both the baseline and sensitivity of the FPD and the sensitivity of the Fluor. The SCD showed no ambient temperature dependence on either the baseline or sensitivity. Introduction The sulfur chemiluminescence detector (SCD) is based on the use of a hydrogen flame to convert sulfur species to SO. The SO is reacted with O3forming an electronically excited state of SO,; 340-nm emission from the SO, is then detected. Sulfur detection by this principle was first reported by Benner and Stedman (I)and was referred to as the universal sulfur detector, but has subsequently been named the SCD. The instrument and laboratory experiments evaluating its performance have been described elsewhere (I). A commercial version of the SCD (Sievers Research, Inc.; SCD Model 350) has also been evaluated as a gas chromatographic (GC) detector by Shearer et al. (2). The GC version of the SCD uses a flame ionization detector (FID) as the hydrogen flame, but operates on the same detection principle as the real-time analyzer. This paper presents results obtained from an evaluation of an SCD under field conditions. The real-time SCD was developed and fabricated at the University of Denver. The SCD along with flame photometric (FPD) and fluorescent SO, (Fluor) analyzers was operated in an air quality monitoring trailer along with several other instruments. The purpose of the study was to evaluate sensitivity, stability, and interferences and quantify the differences between the three analyzers. Since each analyzer relies on a distinctly different detection principle, the effects of interferences and environmental conditions that are unique to any one detection system could be evaluated. The capabilities of real-time sulfur detectors have been stretched to the limit in many studies described in the 1592 Environ. Sci. Technol., Vol. 24, No. 10, 1990
Table I. Instrumentation Employed during the Field Comparison.
instrument
manufacturer
SCD FPD fluorescent SOz relative humidity indoor temperature outdoor temperature COZ a 5-95% of full scale.
University of Denver Meloy Labs Meloy Labs Vaisala Campbell Scientific Campbell Scientific University of Denver
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5 20
literature (3-5). In fact, ambient sulfur mixing ratios are frequently lower than the detection limit of either the FPD or fluorescent monitor (6). It is therefore important that studies which rely on the analysis of low sulfur mixing ratios employ an analyzer that is relatively free of interferences that could invalidate the results. Both C02 and water vapor have been shown to be interferences in the FPD (7,8), and water vapor is an interference in Fluor analyzers (9). The COz mixing ratios also provide a good marker for the status of air quality during the sample period. Since the University of Denver is located in an urban valley, the COz mixing ratio increases dramatically during stagnant meterological conditions. It should be emphasized that the FPD and Fluor analyzers were designed as ambient monitors for compliance with air quality regulations and are not research-grade instruments. The SCD is a research instrument under development at the University of Denver and was designed with the goals of high sensitivity coupled with fast response time. We feel that it is informative to compare these detection techniques in field conditions, however, to better understand the potential usefulness of the SCD. Experimental Section
The instruments listed in Table I were installed in an air quality trailer at the University of Denver. The University is located approximately 7.5 km south-southeast of downtown Denver in a predominately residential area. Data were stored on cassette tape by a Campbell Scientific 21X data acquisition system. Each parameter was sampled once each second and averaged over 2-min intervals before storing to tape. The pneumatic system used in the trailer is shown in Figure 1. The three sulfur instruments sample ambient air separately from a manifold with a 5-cm inside diameter.
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Ambient air was continuously drawn through the manifold at a rate of 11 Lsmin-l. Activation of solenoid valves 1,2, and 3 redirects ambient air through two activated charcoal traps to produce zero (sulfur-free) air. The zero air flows into the manifold and provides a check of the analyzer baseline voltage once every 6 h. After 10 min of sampling zero air, solenoid valve 4 is activated for 20 min, providing 26 ppbv of SO2 to the manifold. Thus, a span gas was sampled by each analyzer to assess the stability of sensitivity to a known SO2mixing ratio. This pneumatic system ensures that pressure fluctuations during the zero/span cycle are minimized. The Meloy FPD, Model SA 285, was modified for this study. The photomultiplier tube (PMT) was connected directly to an external electrometer. This allowed storage of PMT signal current directly on the cassette tape and a calibration equation was applied during data processing. Possible anomalous signal conversions by the internal logarithmic amplifier were thus avoided. The internal pneumatic system was also modified to minimize the length of tubing for the sample gas to pass through. The fluorescent SO2analyzer, Meloy, Model SA 700, was not modified for the study. In all cases care was taken to minimize external tubing and to use only clean FEP Teflon to minimize wall losses (10). Results and Discussion
The field test showed that all three sulfur analyzers worked according to manufacturers specifications. However, there were significant differences in the performance of each analyzer, particularly when sulfur concentrations were very low (ie. 5-10 ppbv). The analyzer performance was evaluated for (1) baseline stability, (2) response stability, (3) interferences, (4)sensitivity, and (5) environmental temperature effects. Baseline Stability. The stability of the baseline signal of a real-time analyzer is an important parameter to determine. A very stable baseline helps to ensure the accuracy of measured mixing ratios and allows for less frequent zero checks. The baseline signal during each zero air sampling period is shown in Figure 2. The drift in zero signals is not well correlated, suggesting that the case of the drift is different for each analyzer. The SCD baseline deviates from the average signal by a maximum of 1.5 ppbv with a standard
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Flgure 2. Drift of the signal while the analyzers are sampling zero air. The analyzers were allowed to sample zero air every 6 h. The lines drawn are to guide the eye. Each data point is a discrete bmin average every 6 h.
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Table 11. Statistics of the Short-Term Noices of Each Analyzer’s Baseline Signal While Sampling Charcoal-ScrubbedAmbient Air parameter
SCD
FPD
Fluor
average standard deviation maximum value minimum value
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deviation of 0.6 ppbv. The FPD had a maximum deviation of 3.5 ppbv (standard deviation of 1.7 ppbv), while the Fluor deviated from the average by as much as 4.7 ppbv (standard deviation of 2.8 ppbv). The maximum change in any two consecutive zeros (6 h apart) is 1.5,3.5,and 5.4 ppbv for the SCD, FPD, and Fluor, respectively. The data for each analyzer were corrected for the zero drift by linear interpolation. The results for the remainder of the analyses are baseline-corrected data. Baseline Noise. The short-term changes (noise) in baseline signal were evaluated by operating the analyzers while supplying charcoal-scrubbed ambient air to the sample manifold for 80 min. Figure 3 shows the baseline during this period. All analyzers were operating for several days prior to the test to avoid startup noise problems. The SCD had a standard deviation of rt0.04 ppbv about the baseline while the FPD and Fluor had standard deviations of h0.25 and f0.69 ppbv, respectively. The results of the test are summarized in Table 11. Environ. Sci. Technoi., Vol. 24, No. 10, 1990
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Response Stability. The sensitivity of each analyzer to a span gas after baseline correction is shown in Figure 4. The SCD demonstrated the largest drift in sensitivity of 1.3 ppbvday-’ followed by the FPD at 0.4 ppbvday-’. The Fluor response did not show a statistically significant sensitivity drift with time, probably due to the large noise level during the span gas measurement. The output of the Fluor analyzer during span gas measurements had a standard deviation of f5.0 ppbv; the SCD and FPD had standard deviations of f0.2 and 2~0.5ppbv, respectively. Interferences. Each analyzer has been carefully evaluated for interference effects and the results are summarized in Table 111. The interferences were evaluated by the effect on baseline signal and sensitivity to the span gas. There are two types of interference that could affect the analyzers. The first is a chemical species, other than the compound of interest (analyte), that causes a signal regardless of whether or not the analyte is also present. The signal is erroneously interpreted as a change in analyte mixing ratio. We refer to this as a “concentration interference“. The second type of interference we call a “sensitivity interference” because the sensitivity to the analyte is changed by the presence of another compound in the sample. The concentration interference can be observed in either the baseline signal or the span signal, but the sensitivity interference can only be observed during sampling of the span gas. A response above the noise level on one analyzer while observing no response on the other two analyzers was an obvious indication of an interference being measured. The Fluor analyzer frequently had a very obvious interference signal as seen in Figure 5. It was suspected that hydrocarbons were responsible, but replacement of the hydrocarbon scrubber did not decrease the interfering signal. It was also thought that the interference might be due to particles in the sample air not removed by the filter. In this case we would expect the interference to be most pronounced during a stagnant meteorological period. Comparison of the Fluor trace and the CO:, trace shows 1594
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Table 111. Results of the Influence Observed in the Baseline and Sensitivity of Each Analyzer for Various Interferences and Environmental Influences
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that the interference was not related to stagnant meteorological conditions during which the worst air pollution episodes occur. From this we conclude that particles were not responsible for the erroneous signal. The Fluor literature states that an interference equivalent to less than 20 ppbv can be expected. The analyzer usually stayed within this expected value except €or short time periods of 10 min or less. The change in the signal that occurs when zero air is introduced to the sample manifold provides a good indication of small interferences experienced by the analyzers. Figure 6 shows the signal change when zero air was introduced to the sample manifold. It can be seen that the FPD signal usually increased when zero air was introduced to the sample manifold. However, since this did not occur every time zero air was sampled, this indicates that pressure changes in the manifold are not responsible for the positive baseline signal changes. The SCD showed the expected decrease in baseline signal during nearly every zero cycle and on a few occasions increased by less then 0.5 ppbv, indicating that sulfur gases were removed to
