Environ. Scl. Technol. 1986, 20, 594-596
Interferences in Environmental Analysis of NO by NO plus O3 Detectors: A Rapid Screening Technique Oliver C. Zafiriov” and Mary B. True Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543
+
The NO O3 chemiluminescence detector (NO-CLD) offers a suitable sensitivity, portability, and cost compromise for field studies of NO,. However, its selectivity for NO is not inherent, so empirical validation of this factor in different types of gas samples, particularly those rich in other trace components, is necessary. Here we report a simple, practical, and stringent screening procedure based on determining relative reaction rates of NO and sample with O3to detect interferences. Using the screening technique, we find significant interferences in gases emitted or stripped from some marine environments.
Introduction The NO + O3 chemiluminescence detector (NO-CLD) detects red photons from step 2 of the reaction sequence (I): NO + 0 3 NO2 + 0 2 (14 -+
NO
+0 3
--
NO2*
NO2* + M
-*
NO2*
+0 2
+ hv NO2 + M + heat NO2
(1b) (2) (3)
Although a well-designed detector has a sensitivity approaching 1parts per trillion (ppt) (volume) NO (2),the absolute sensitivity is only on the order of photon per NO. The light-production efficiency for step 1 ( l b j l a branching ratio) is about lo%, and the light collection/ photon counting has -10% efficiency, so the competition between emission and collisional quenching ((2) vs. (3)) results in a response of the order of lo4 photon per NO. Since NOz* has a relatively long radiative lifetime, in principle, molecules may exist that react with ozone to give a red-emitting excited states with 10 times the efficiency of NO ( I ) and an additional lo3 times the response because of a more favorable competition between radiative emission and collisional quenching (steps 2 and 3 above). Despite this fact, extensive tests of such tropospheric air trace constituents as saturated hydrocarbons, ethylene, acetylene, a-pinene, benzene, and related monoaromatjcs, etc. have not revealed any molecules possessing such unfavorable properties; to our knowledge, no response factors greater than 0.1% of that for NO have been measured (3, 4 ) . In clean air the NO-CLD seems nearly immune to interferences. However, in applying the NO-CLD to biogeochemical samples such as seawater (5), fresh waters ( 6 ) ,soils (7,8), or culture media (6, 9),the array of potential trace interferents is more complex than in clean air, and abundances relative to NO may be greater, so the specificity is difficult to assess. For this reason, we previously developed three methods for discriminating NO interferents (11). Unfortunately, they are awkward, are slow, or do not apply to all sample types, and hence are unsatisfactory for field studies. The method described here is simple, is fast, and has revealed that a small fraction of samples from aquatic systems give artifact NO responses. These may be due in part to reduced volatile inorganic and organic sulfur compgunds, which have also recently been reported
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as interferents for NO-CLD systems in smog chamber studies (11).
Materials and Methods We modified a previously described (10) evacuatedchamber NO-CLD to permit the measurement of the rate of reaction of O3 with an incoming sample stream. This reaction rate is compared with that of authentic NO. To do this, we mixed a small, constant bleed of ozone from the main supply into the sample stream as it transited an initial reaction chamber (“prereactor”)before the detection chamber. Figure 1shows two suitable arrangements for accomplishing this, with the stippled area representing added components. The instrument sensitivity and the kinetic titration parameters were set utilizing a NO/N2 standard prepared by dynamic dilution of a -2 ppm (volume)Airco standard gas with zero-grade air or 99.998% N2. This modification is simple with the Kley et al. (2) NO-CLD, but careful minimization of dead volumes and reactive surfaces and proper selection of valve orifice size and forechamber volume are necessary for fast, hysteresis-free,reproducible response. These parameters are likely to be substantially different for different instrument designs. They also change as the gas mass flow rate changes in a given detector. Seawater samples were obtained by using specially cleaned 10-L Niskin bottles off Peru and analyzed by stripping the water samples immediately after recovery as described previously (5, IO). Intertidal mudflat gas emission samples were obtained in June 1983 from intertidal/subtidal Narragansett Bay, Rhode Island, mudflats in a joint experiment with Drs. Joel Levine and Iris Anderson of NASA/Langley by withdrawing 50-cm3 gas samples by a gas-tight Teflon/glass syringe from a flux chamber and by analyzing them within 2 h with the modified NO-CLD. The syringes used did not produce or consume NO (&lo%). The Langley group monitored the grow-in of NO signal on site using a Columbia Scientific Model 1600 NO, analyzer (NO-CLD). To prepare for a selectivity test (hereinafter, “T test”, for kinetic titration), the detector is zeroed and calibrated with standard NO/N, or NO/air with valve 3 routing the main O3 flow to the reaction chamber (most instruments have no valve 3 and are always in this configuration) and value 4 closed (no ozone bleed into prereactor). Then valve 4 is opened, and needle valve 2 is adjusted to drop the response to about 50% of the initial value; valve 4 is opened and closed repeatedly to test for response time and reproducibility. This procedure sets the residence time and ozone concentration in the prereactor such that reactions l a and l b proceed to a known extent totaling 50% in the prereactor; in our instrument under usual flow conditions, this reqires only a few percent of the total ozone flow, so that the sensitivity to NO reaching the main reaction chamber is nearly constant. When the chamber walls are clean and the dead volumes between valves 2 and 4 and between 4 and the prereactor are minimized, the response upon changing the valve setting is almost instantaneous-90% in -10 s or less. The percentage decrease in response is also invariant to better than f20%
0013-936X/86/0920-0594$01.50/0
-
0 1986 American Chemical Society
Table I. Some T-Test Results in Environmental Samples signal ppb (volume) (calcd as NO)
sample gas emissions from low
marsh just as tide reaches "hat", 7/16' as above, 7/17' gas emissions from high marsh (with Spartino), 7/16' Salt Pond interface wateP Peru upwelling area eoretop watef
T factor
T ratio f range
interpretation
18
0.7 ( n = 1)
1.46
not only NO
14-55 7-19
0.86 f 0.1 ( n = 3) 0.46 + 0.05 (n = 5)
1.8 1.05
not only NO NO
3 0.680 (n = 2)
0.5,
1.7*
not only NO not only NO
>1.8
0.46
nSamplesof gases emitted from intertidal mudflats of Narragansett Bay, 1/16-7/17/83, daytime. When the T factor was set to 1.00 with dynamically diluted standard, it was 0.91 far NO/+ stored in the syringes used to collect environmental samples. In this experiment, we defined the T factor by the syringe values. Water sample from 3.87 m, near anoxic interface, 9/30/81. 'Clear water drained from above gravity cores in -110 m of water. The overlying water smelled of S compounds and was probably anoxic in situ.
A.
0.
To M c v v m
1.
-
m Gmr Input
BASELINE SIGNAL
DyI
w g
0 1
NL.dlL "0I"L
2
3
4
5
6
7
8
Minotes
On-Ollv~*s Tm).Woy"ol"*
ofN L C U l fu Ttest. ModiRcations are stown in stippled areas. Vahre 1. main O3flow rate adjust. Valve 2, titrator 0, flow rate adjust. Valve 3,main 0, flow path-switching valve (optional). Valve 4, T test on-off valve. FIpm 1. -ton
for humid vs. dry air or NO/N2 vs. NO/air or for [NO] varied in the range 8-450 ppb (volume). An example of the response time and reproducibility is shown in Figure 2. Sample behavior can be compared to that of authentic NO by means of the "T ratio" = background-corrected signal with valve 4 open/background-correctedsignal with valve 4 clwed. The reproducibility of this ratio is better than zt3% over short time periods and better than *lo% over a day. The precision could probably be improved by thennostating the ozonizer and the reaction and detection chambers but is adequate for our applications without these precautions. Signals from gases emitted by environmental samples are tested in an identical manner by opening/closing valve 4. Although the criterion for rejecting samples is partly subjective, to data our samples are bimcdally distributed such that accepting as authentic samples with T factors (defined as T ratio sample/T ratio NO) of 1 0.1 and considering suspect those with ratios outside that range as perturbed by interferents gives very few borderline cases.
*
illustrated on - 6 ppb (volume)of NOIN, standard. The projected base line (for no NO addition) is shown at the bottom. The overshoot is due to relief of pressure buik up in the valve 2-valve Figure 2. T test 4
dead volume.
Results and Discussion Detection of Interferences. We have applied the T test to over 100 samples of gases stripped from seawater in oxygen minimum regions of the ocean, from the oxicsuboxic gradient of Salt Pond, a local brackish system with anoxic bottom water (12), and from cultures of nitrifying and denitrifying marine and terrestrial bacteria. In almost all cases, we found no significant deviation of the sample T ratio from that of authentic NO (Tfactor = 1.0 f 0.1). In practice, in the field when the T factor is near 1, we assume there is no interference. However, when possible, it is desirable t o further verify this assumption (see methods in ref 10 and discussion in the next section). However, in a few cases, highly anomalom T fadors were observed two samples taken near the bottom in shallow water off the coast of Peru and several samples in the suboxic (possible anoxic) region of Salt Pond. We also found strong interferences in some samples from a gas emissions "hat" in the shallow intertidal zone of Narragansett Bay, RI. Table I illustrates the major instances of interferences we have observed to date. At the mudflat Environ. Sci. Technoi., Vol. 20. No. 6, 1986 595
site, two locations only meters apart gave signals similar in magnitude, but one seems unreliable, while the other is not. All of these rejected signals come from environments in which complete anoxia and the microbial production of volatile organic and inorganic compounds, including S compounds, are particularly likely. The Peru sample smelled of hydrogen sulfide and the Salt Pond suboxicanoxic interface contains HzS and dimethyl sulfide (12). While sulfur compounds are likely interferents in environmental work since they generate excited sulfur dioxide on reaction with ozone, in part via heterogeneous reactions (13), we reiterate that i t is not feasible to catalog exhaustively potential interferents. However, Grosjean and Harrison (11)recently reported that CH3SH, (CH,),S, and (CH&H2)2Sall show an NO interference in the NO-CLD, and HzSproduces a response in our detector corresponding to a T factor of 1.7. However, as reduced sulfur compounds also give strong hysteresis effects, we have not studied them extensively and do not know if they are the usually major interferents in environmental samples. The rapid response of the T test permits it to be applied, with slightly reduced accuracy, to peaks stripped from solution with half-widths of only -60 s, so it is practical to use routinely on water samples. However, samples giving anomalous T factors often also give much slower response recovery when valve 4 is reclosed. Presumably they contain polar ozone-reactive compounds that adsorb in the system. Chemical Basis and Generality of the TTest. The evidence above shows the possibility of NO-CLD detector interferents in environmental samples, as does other recent work (11)establishing the need for specificity verification. We believe that the proposed screening test is capable of detecting all such interferences, but this assumption is not easy to evaluate. The T test is thus basically a screening procedure for flagging samples requiring more sophisticated investigation. The criterion for discrimination is a measurement of the relative rate of reaction of NO with ozone and of the unknown mixture with ozone, made under identical conditions in a flow reactor and detected by red chemiluminescence emission. Thus, the criterion is general if the rate of reaction of NO with O3 is substantially different from that of other substances. At least for homogeneous gas-phase processes, this assumption appears to be correct. O3 reacts with a variety of unstable species-atoms and radicals-at similar rates to that of NO; relative rates of 25 “C are approximately the following: NO = 1;H, 0, F, C1, and Br atoms 50-5000; radicals such as OH, HOz, NOz 0.05-2. However, such species will be absent from samples from aqueous environments and will not survive transport into the instrument if present. Among more stable compounds, the known relative rates are much lower: olefins 10-4-0.05, toluene and SOz “slow” (14-1 6). Since electron-rich olefins such as 2,3-dimethyl-2-butene react with O3 considerably more slowly than NO, even more electron-rich, sterically less hindered compounds would seem to be required to approach the T factors we observe if only gas-phase reactions are important. Sulfur compounds such as H2S,mercaptans, and mono- and disulfides may well have the chemical (11) and biogeochemical (12) properties required to generate the responses observed in the environments where interferences have been found, and they interfere in commercial NO-CLDs. They are one of the more likely interferents to be en-
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Environ. Sci. Technol., Vol. 20, No. 6 , 1986
countered in environmental samples. However, in practice heterogeneous reactions may contribute to the effects seen in the T test, in which case the chemistry cannot be clearly specified.
Conclusion We have shown that significant interferences plague NO-CLD analyses of some environmental samples. Sample screening by a determination of the reaction rate with O3is a convenient and stringent criterion for differentiating interferences due to stable molecules. However, it cannot quantify NO in the presence of interferences and may possibly give false indications of interference. While it does not solve the interference problem, it can flag samples requiring further analysis to validate results. NO-CLD data obtained without some such assurance, especially in marine-reducing environments, should be interpreted with caution. Acknowledgments M. McFarland contributed useful discussion of the specificity problem and suggested a related approach to the one we present. We thank Iris Anderson, Joel Levine, and Edwin F. Shaw of the NASAlLangiey Research Center for sharing their Narragansett Bay results. Iris Anderson, Cindy Lee, and Stuart Wakeham provided helpful discussion. Registry No. NO, 10102-43-9;NO,, 11104-93-1;03, 10028-15-6; HzO, 7732-18-5.
Literature Cited (1) Fontijn, A.; Sabadell, A. J.; Ronco, R. J. Anal. Chem. 1970, 42, 575-579. (2) Drummond, J. W.; Volz, A.; Ehhalt, D. H. J. Atmos. Chem. 1985, 2, 287-306. (3) McFarland, M.; E. I. du Pont, de Nemours & Co., Wilmington, De, personal communication, 1983. (4) Kelly, T., Brookhaven National Laboratory, Upton, NY, personal communication, 1985. (5) Zafiriou, 0. C.; McFarland, M. J. Geophys.Res. 1981,86, 3173-3182. (6) Levine, J. S.; Augustsson, T. R.; Anderson, I. C.; Hoell, J. M.; Jr. Atmos. Environ. 1984, 18, 1797-1804. (7) Galbally, I. E.; Roy, C. R. Nature (London) 1978, 275, 734-735. (8) Delany, A. C.; Davies, T. C. Atmos. Enuiron. 1983, 17, 1391-1394. (9) Lipschultz, F.; Zafiriou, 0. C.; Wofsy, S. C.; McElroy, M. B.; Valois, F. W.; Watson, S. W. Nature (London)1981,294, 641-643. (10) Zafiriou, 0. C.; McFarland, M. Anal. Chem. 1980, 52, 1662-1667. (11) Grosjean, D.; Harrison, J. Enuiron. Sci. Technol. 1985,19, 862-865. (12) Wakeham, S. G.; Howes, B. L.; Dacey, J. W. H. Nature (London)310, 770-772. (13) Kelly, T. J.; Gaffney, J. S.; Phillips, M. F.; Tanner, R. L. Anal. Chem. 1983,55, 135-138. (14) Logan, J. A.; Prather, M. J.; Wofsy, S. C.; McElroy, M. B. J. Geophys. Res. 1981,86, 7210-7254. (15) U.S. Department of Commerce NBS Spec. Publ. (U.S.) 1979, NO. 557. (16) Wagner, H. G.; Zellner, R. Angew. Chem., Int. Ed. Engl. 1979, 18, 663-673.
Received for review June 25,1985. Revised manuscript received December 30, 1985. Accepted January 8,1986. This work was funded by NSF Grants OCE83-15614 and OCE83-00022. This is Woods Hole OceanographicInstitution ContributionNo. 5966.
