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Anal. Chem. 1985,57,2338-2341
Determination of Nitrate in Atmospheric Particulate Matter by Thermal Decomposition and Chemiluminescence Chester W. Spicer,* Darrell W. Joseph,’ and Philip M. Schumacher
Battelle, Columbus Laboratories, 505 King Avenue, Columbus, Ohio 43201
This paper descrlbes a thermal decomposition/chemllumlnescence method for determlnlng nltrate in atmospherlc partlcular matter. Nitrate In the sample Is thermally decomposed to NO,, whlch Is then deterrnlned wlth a comrnerclal chemNumfnescence NO, monitor. The nitrate in a fiiter sample can be deterrnlned dlrectly by heatlng a segment of the filter In a furnace or after extraction of the filter by flash heating the aqueous extract In a sample loop. I n either case, the sample Is decomposed In a nitrogen atmosphere to avoid interference from ammonium. The NO, peak from nitrate decomposltlon can be quantifled by Integrating the chemiluminescence slgnal or by Integrating the gas sample In a Tedlar bag prior to the chemiluminescence measurement. The technique Is straightforward, fast, and sensltlve, and interferences In atmospheric samples are negllglble. A comparlson of the thermal decomposltlon/chemllumlnescence method wlth Ion chromatography uslng fllter samples collected In amblent air showed good agreement over a wide range of concentrations.
Gaseous and particulate nitrates are ubiquitous constituents of the atmosphere and have received considerable attention recently due to their contribution to the acidity of precipitation (1,2) and visibility reduction (3,4). The sampling and analysis of particulate nitrates pose several problems due to interferences by gaseous nitrates (5-9) and interactions on the collection medium resulting in volatilization and loss of the particulate nitrate (9-15). Most laboratories which determine nitrate in atmospheric particulate matter employ colorimetric (16,17) or ion chromatographic methods (18-ZO),although a very sensitive procedure involving electron capture gas chromatography has also been used (21).All of these methods require some pretreatment of the sample. Pretreatment can range from extraction of the filter for ion chromatography, through extraction and chemical treatment to reduce nitrate to nitrite for colorimetry, to extraction and chemical reaction with benzene in acidic solution for the gas chromatographic procedure. All of these pretreatment steps represent potential sources of error in the analysis due to inefficient extraction, nonquantitative reduction or chemical reaction, loss or contamination of sample during handling, or loss of sensitivity due to high reagent blanks. Sample pretreatment also adds considerable time and expense to the analysis. This paper describes a simple thermal decomposition/chemiluminescence method for determining nitrate in atmospheric aerosol samples. Brief investigations by other researchers (22,23) suggested that such a procedure was viable. The method can be used directly on filter samples without pretreatment or used with aqueous extracts of filter collections. The method has the advantages of being extremely sensitive, simple, and very rapid. Any nitrite in the sample will be detected as nitrate, Present address: Oregon Graduate Center, Beaverton, OR 97006.
but this represents a negligible interference for ambient aerosol samples. A detailed description of the development of this analytical method has been reported elsewhere (20).A similar technique has been used by Moskowitz (24) to investigate nitrate particle size distributions in the Los Angeles basin. Cox (25)has described a chemiluminescence method for nitrate and nitrite determination. It has the advantages of the sensitivity of the chemiluminescence principle and can determine nitrite separately from nitrate but requires considerable sample pretreatment in terms of extraction and chemical reaction to evolve nitric oxide.
EXPERIMENTAL SECTION Two commercial chemiluminescence instruments were employed in this investigation. The majority of the work was conducted with a Bendix Model 8101-B chemiluminescence NO/NO, monitor operated in the continuous monitoring mode. The instrument’s heated carbon catalytic converter was employed to reduce any NO2 in the sample stream to NO, although several experiments demonstrated that NO accounted for >90% of the nitrate decomposition products. Use of the converter ensured that even the traces of NOz formed in the decomposition process were measured. For some experiments a Thermo Electron Corp. Model 14-D chemiluminescence NO/NO, monitor was used. The Model 14-D is a dual channel instrument which simultaneously monitors NO and NO,. It employs a heated molybdenum catalytic converter for reduction of NOz to NO. Both instruments were calibrated with low concentrations (0.5-5 ppm) of NO in Np The calibration standards were referenced to a National Bureau of Standards “NO in N2”primary standard. A Hewlett-Packard Model 3370 A integrator was used to integrate and digitize the instrument output. Direct Analysis of Filter Samples. The apparatus employed for the direct analysis of filter samples is shown in Figure 1. An 18 cm by 2 cm Vycor tube surrounded by a resistance furnace heated to 425 “C was used for sample decomposition. The sample pump for the chemiluminescence monitor was used to transfer the NO, resulting from sample decomposition from the furnace to the monitor at a flow rate of 120 mL min-’. At the furnace temperatures necessary for quantitative decomposition of nitrate salts, ammonium ion is oxidized to NO in the presence of 0, and represents an interference. To eliminate the interference due to NH4+,the Vycor furnace tube was connected via a ball joint and Teflon tubing to a gas cylinder of oxygen-free nitrogen. The Vycor tube was connected to the N2source via a tee which was extended and tapered to permit access to the furnace. A continuous flow of excess N2 passed out of the open end of the tee, thereby excluding room air from the furnace. Small segments of actual filter samples or clean filters spiked with test compounds were inserted into the furnace through the open end of the tee. Initial experiments demonstrated that the response time of the chemiluminescence monitor was too slow to respond to the sharp peaks resulting from direct injection of higher concentration nitrate samples, so a 200-mL Pyrex ballast vessel was installed between the tube furnace and the chemiluminescence monitor t o widen the peaks. Analysis of Filter Extracts. The apparatus employed for thermal decomposition and analysis of aqueous filter extracts is shown schematically in Figure 2. The device used for thermal decomposition is a looped 18 cm by 0.30 cm stainless steel tube. The tube is heated rapidly t o 425 “C by passing a 22-A current (at 5 V) through the metal loop. Temperature is controlled by a Variac connected to a step-down transformer. A thermocouple
0003-2700/85/0357-2338$0 1.50/0 0 1985 American Chemical Society
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samples. is positioned at the site of sample injection. Nitrogen, used as the carrier and dilution gas, is passed through the conversion loop and into a Tedlar collection bag. The flow rate is regulated to obtain the desired dilution of the sample. A Swagelok tee fitting is located on one end of the loop with a septum for sample injection. A nonconductive type ferrule is used to ensure that the conversion loop is electricallyisolated from ground. Stainless steel fittings and Teflon tubing are used between the loop outlet and the collection bag to minimize NO, adsorption. Swagelok quick connect fittings are used to facilitate the rapid transfer of the bag contents to the NO, analyzer. Experiments showed that sample volumes in the range 4-20 pL at a N2flow rate of 200 mL min-l yielded the greatest accuracy and reproducibility. The temperature of both the quartz tube and the stainless steel loop was determined with a chromel-alumel thermocouple. Teflon tubing was employed for the gas flow systems. Tedlar (poly(viny1 fluroide)) bags were used for a number of experiments to collect and integrate the gaseous decomposition products. The stability of NO, in these bags was excellent over the short times required for sample collection and analysis. Blanks, consisting of clean filter segments for the direct analysis configuration and distilled deionized water for the liquid analysis system, were run frequently. In both systems the minimum detection limit is controlled by the blank values. Because the signal produced by typical atmospheric samples is much greater than the signal produced by the blanks, we did not investigate further measures for reducing the blank values. All ion chromatography was performed with a D-ION-X Model 10 ion chromatograph operated in the anion mode (19).
RESULTS AND DISCUSSION Chemiluminescent NO, monitors measure the light emitted (600-900nm) when NO in a gas sample reads with excess 03. The detection limit of most commercial instruments is 1-5 ppb by volume of NO. Various catalytic converters are employed to reduce NO2 (and other gaseous nitrogen compounds) to NO for determination; thus the instrument is said to monitor total NO,. For determination of NO, chemiluminescence instruments are fast, sensitive, and free from interference. Therefore, the development and validation of a thermal decomposition/chemiluminescentmethod for atmospheric particulate nitrate must focus primarily on the thermal decomposition step and introduction of the gaseous decomposition products into the chemiluminescence monitor. A preliminary investigation of the feasibility of thermal decomposition/ chemiluminescence for determining nitrate on
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atmospheric filter samples suggested two different approaches and experimental configurations. One approach involves direct analysis of filter samples without pretreatment, while the other is designed to determine nitrate in aqueous filter extracts. Various modifications are possible with either approach. For example, laboratories which lack a spare digital integrator can integrate the sample volumetrically by collecting the gas sample following thermal decomposition in a small Teflon or Tedlar bag. The chemiluminescence monitor is then used to determine the concentration of NO, in the bag. Most of the interference and recovery studies reported here were carried out in the apparatus for direct analysis of filter samples (Figure 1). The comparisons with the ion chromatographic method used the apparatus designed for analyzing aqueous extracts, because the ion chromatograph can easily accommodate aqueous samples. Most of these latter experiments employed volumetric integration, using the apparatus shown in Figure 2. Interferences. The species likely to interfere with the thermal decomposition/chemiluminescencedetermination of nitrate are nitrite, ammonium, and possibly organic nitrogen compounds which can decompose to NO, at the decomposition temperatures wed here. Experiments using filters spiked with NaN03and ",NO3, with air as the carrier gas, demonstrated that ammonium interferes with the nitrate determination. When oxygen-free nitrogen is used as the carrier, however, the interference is almost totally eliminated. Experiments with both NH4Cl and ",NO3 showed that the NH4+ response was less than 4% of the equivalent nitrate response. Amines and other organic nitrogen compounds, which may be present in atmospheric filter samples at very low concentrations, were investigated in several experiments; no significant interference was observed as long as O2 was excluded from the decomposition tube. Interference due to decomposition of nitrite salts is essentially quantitative, as expected. Nitrite interference is not a serious problem in the analysis of ambient filter samples however, because of the low levels of nitrites present in ambient aerosol samples. For example, studies in several US. cities (26) have shown that particulate nitrite averages 1-2% of the particulate nitrate concentration. Thus nitrite interference should be minimal for ambient particulate nitrate analysis. Calibration. In theory, calibration of the chemiluminescent instrument with known concentrations of NO or NO2 should be sufficient if the decomposition step and transfer to the monitor are quantitative. For these studies, the instrument was calibrated with gaseous standards of NO in nitrogen which were referenced to NBS primary standards. T o determine whether the decomposition and transfer steps are quantitative, and also to check the linearity of the system, the response to nitrate was determined using clean filters spiked with aqueous solutions of NaNO,. Within the uncertainty of the measurements, the response to known levels of nitrate agreed with the concentrations of the standards. The theoretical response for this system a t 25 "C and 750 mmHg is
[NOS-] = 5.93(peak area)I.O0 with [NO3-] in nanograms and peak area in volt-seconds. The agreement between the theoretical and measured responses over the concentration range studied indicates that the NO3decomposition and the transfer of the decomposition products to the chemiluminescence monitor are essentially quantitative. Responses of Various Nitrate Salts. The nitrate salts expected to dominate ambient filter collections are ",NO3, NaN03, and, to a lesser extent, KNOBand other alkali or alkaline-earth salts. A series of experiments was conducted to determine the response of the thermal decomposition/
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 12, OCTOBER 1985
Table I. Comparison of Chemiluminescence and Ion Chromatographic Nitrate Determinations compound
Table 11. Replicate Nitrate Analyses amt of NO3-, ppm ion
amt of NO;, wg ion chromatograph chemiluminescence"
NaNO, KNO, KNO, ",NO,
2.97 2.89 1.93 2.80
2.82 f 0.06 2.79 f 0.06 1.86 f 0.04 2.73 f 0.11
" Three to five replicates. chemiluminescence method to various nitrate salts. Ammonium, sodium, and potassium nitrates were studied since they are likely to be found in ambient filter samples. Calcium nitrate was selected because it has a high decomposition temperature and therefore provides a good test of the efficiency of the decomposition step. If calcium nitrate decomposes and is measured under the experimental conditions, then the vast majority of nitrates in ambient samples will also be measured. Analysis of seven samples each of NaN03, NH4N03,and KNOBin the range of 20-2000 ng demonstrated quantitative decomposition and subsequent determination as NO,. Analysis of six samples of Ca(N03)2.4H20covering the range 100-4000 ng showed 70% recovery at the lowest concentrations and >90% recovery at higher levels. Because the nitrate compounds expected to be present in ambient samples are completely recovered, and even a salt with such a high decomposition temperature as Ca(N03)2is largely recovered under the experimental conditions, it can be assumed that nitrate recovery from ambient particulate samples will be nearly quantitative. Accuracy and Precision with Various Nitrates. The thermal decomposition/chemiluminescence procedure was further characterized by preparing aqueous solutions of three different nitrate salts and analyzing these solutions by ion chromatography and chemiluminescence. The apparatus shown in Figure 2 was used for these experiments. The results are shown in Table I. The agreement between the chemiluminescence method and ion chromatography is quite good. Both instruments were calibrated independently. The chemiluminescence results are 3-5% lower than the ion chromatographic data. This small discrepancy could have resulted from small errors in preparing calibration standards for one or the other instrument. The data in Table I confirm that there are no significant differences in the decomposition efficiency for these three salts. The relative standard deviation for the chemiluminescence results varies from 2% for sodium and potassium nitrate samples to 4% for the ",NO3 solution. These deviations are based on three to five replicates for each solution. A second set of comparisons was carried out using NHdN03, KNOB, and an actual ambient filter extract. Solutions of ",NO3 and KNOBwere prepared at 5, 50, and 550 ppm (lg/mL) and analyzed in quadruplicate by both ion chromatography and chemiluminescence. The aqueous extract from a high volume filter collected in Upland, CA, on October 19, 1976, was also analyzed by both methods. The results of these experiments are presented in Table 11. The data in Table I1 show that ion chromatographic results are consistently 3-7% below the prepared sample concentrations. The chemiluminescence results are less consistent, but in four out of six cases the chemiluminescent results are closer to the true concentration. The precision of the ion chromatographic method is better than the chemiluminescence procedure, especially at low concentrations. The relative standard deviation of the ion chromatographic data is better than 1% in all cases but one. The relative standard deviation
chromatosample
method
chemiluminescent method
5 ppm NH4N0, 50 ppm NH4N03 500 ppm NHlN03 5 ppm KNOB 50 ppm KNO, 500 ppm KNO, Los Angeles filter 10/19/76
4.6 f 0.1 46.3 f 0.3 465 f 2 4.6 f 0.0 48.7 f 0.2 478 f 1 132 f 1
5f1 51 k 2 496 f 22 4f1 50 f 1 470 f 14 133 f 5
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by utilizing the differences in decomposition temperatures of the various nitrate salts. The direct analysis apparatus could be modified rather easily to allow a programmed increase in temperature, thereby effecting a separation between the various forms of nitrate present on filter collections.
ACKNOWLEDGMENT The counsel and encouragement of E. Wittgenstein and J. Mulik are gratefully acknowledged. J. Kouyoumjian provided valuable assistance in conducting laboratory experiments. Registry No. NO3-, 14797-55-8. LITERATURE CITED
:on Chromatographic Nitrate, rng/filter Figure 4. Comriarison of chemiluminescent and ion chromatographic methods for ambient filter samples.
these results confirm the utility of the thermal decomposition/chemiluminescence method for ambient particulate nitrate determination.
CONCLUSIONS This report describes a simple, rapid, and sensitive method for determining nitrate in atmospheric particulate matter or aqueous extracts. The method is based on thermal decomposition of nitrate to gaseous NO,, which is then determined by a commercial chemiluminescence monitor. The nitrate salts expected to be present in atmospheric samples are quantitatively converted to NO, at 425 "C. The quantitative recovery of atmospheric nitrate compounds and lack of significant interferences were demonstrated by comparing the chemiluminescence method with ion chromatography results for actual ambient filter samples covering a wide range of nitrate concentrations. Two modes of operation were investigated. For greatest sensitivity, the nitrate sample can be decomposed by rapid heating to 425 "C and the decomposition products drawn into a nitrogen oxides chemiluminescence monitor. The chemiluminescence response is integrated, the peak area being directly related to sample concentration. For somewhat less sensitive but simpler and less expensive analyses, the decomposition products can be integrated by collection in a Tedlar bag, after which the NO, concentration in the bag is determined by chemiluminescence. No electronic integration apparatus is required in this mode. The detection limit of the former mode of operation is better than 10 ng of NO3- in a 15-pL sample. The latter procedure can detect less than 50 ng of NO3- in a 15-pL sample. The apparatus can be assembled in 1-2 days and operated by a laboratory technician. It should be possible to derive information on the nature of the nitrate species present in atmospheric aerosol collections
(1) Galloway, J. N.;Llkens, G. E. Atmos. Environ. 1981, 1 5 , 1081. (2) Durham, J. L.; Overton, J. H.; Aneja, V. P. Atmos. Environ. 1981, 15, 1059. (3) Blumenthal, D. L.; Richards, L. W.; Macias, E. S.; Bergstrom, R. W.; Wilson, W. E.; Bhardwaja, P. S. Atmos. Environ. 1981, 15, 1955. (4) Appel, B. R.; Tokiwa, Y.; Hsu, J.; Kothny, E. L.; Hahn, E. Atmos. Environ ., in press. (5) Spicer, C. W.; Schumacher, P. M. Atmos. Environ. 1977, 11, 873-876. (6) Spicer, C. W.; Schumacher, P. M. Atmos. Environ. 1979, 1 3 , 543-552. (7) Appei, B. R.; Wail, S. M.; Tokiwa, Y.; Haik, M. Atmos. Environ. 1979, 73,319-325. (8) Witz, S.; MacPhee, R. D. J. Air. Poiiut. Control Assoc. 1977, 27, 239-241. (9) Appel, B. R.; Tokiwa, Y.; Haik, M. Atmos. Environ. 1981, 15, 263-289. (10) Harker, A. B.; Richards, L. W.; Clark, W. E. Atmos. Environ. 1977, 7 7 , 87-91. (11) Appel, B. R.; Tokiwa, Y.; Haik, M.; Kothny, E. L. Atmos. Environ. 1984, 18, 409-416. (12) Splcer, C. W.; Howes, J. E., Jr.; Bishop, T. A,; Arnold, L. H.; Stevens, R. K.; et al. Atmos. Environ. 1982, 76. 1467-1500. (13) Appei, B. R.; Tokiwa, Y. Atmos. Environ. 1981, 75, 1087-1088. (14) Forrest, J.; Spandau, D. J.; Tanner, R. L.; Newman, L. Amos. Envir O n . 1982, 16, 1473-1485. (15) Pierson, W. R.; Brachaczek, W. W.; Korniski, T. J.; Truex, T. J.; Butler, J. W. J. Air. Pollut. Control Assoc. 1980, 3 0 , 30-34. (18) US. Environmental Protection Agency, Methods for the Chemical Analysis of Water and Wastes, EPA-625-116-74-003, 1974. (17) Standard Methods for the Examlnation of Water and Wastewater, 14th ed., Washington, DC. (18) Smaii, H.; Stevens, T. S.; Bauman, W. C. Anal. Chem. 1975, 4 7 , 1801. (19) Mulik, J.; Puckett, R.; Williams, D.; Sawicki, E. Anal. Lett. 1976, 9 , 653-663. (20) Spicer, C. W.; Schumacher, P. M.; Kouyoumjian, J. A.; Joseph, D. W. "Sampling and Analytical Methodology for Atmospheric Particulate Nitrates", EPA1600-2-78I067, 1978. (21) Tesch, J. W.; Rehg, W. R.; Slevers, R. E. J. Cbromatogr. 1976, 126, 743-755. . . - . - -. Coutant, R. C., private communication, 1976. Stevens, R. K., private communication, 1976. Moskowitz, A. H. "Particle Size Distribution of Nitrate Aerosols in the Los Angeles Air Basin", EPA-60013-77-053, 1977. Cox, R. D. Anal. Chem. 1980, 52, 332-335. Spicer, C. W., Atmos. Environ. 1977, 7 7 , 1089-1095.
RECEIVED for review April 11,1985. Accepted June 3,1985. This research was supported by the Environmental Sciences Research Laboratory of the U.S. Environmental Protection Agency under Contract No. 68-02-2213 to Battelle's Columbus Laboratories. This article has not been subjected to Agency review and therefore does not necessarily reflect the views of the Agency. No official endorsement should be inferred. Mention of trade names or commercial products does not constitute endorsement or recommendation for use.
