Anal. Chem. 1994,66, 362-367
Determination of Nitrate in Deposited Aerosol Particles by Thermal Decomposition and Chemiluminescence Masatoshl Yamamoto' and Hlroshi Kosaka The Hyogo Prefectural Institute of Environmental Science, 3- 1-27, Yukihiracho, Sumaku, Kobe, 654, Japan Thermal decomposition-chemiluminescence was developed to determine nanogram quantities of aerosol nitrate rapidly. Standard nitrates placed on a stainless steel strip were decomposed by resistance heating of the strip and converted to NO, which was measured with a chemiluminescent NO, analyzer. By adding 1 pLof 0.1 N NaOH and 0.1 mol/L CrO3 prior to heating, ammonium interference was eliminated and about 97-99%NO, conversion from nitrate was achieved. For standard NaN03 and N K N O s a determination range of 5-2000 ng and a detection limit of 1.1 ng of nitrate were obtained. Interferences from the particle nitriteandthe gaseous "0% "02, NO, and NO2 in sampling process were almost negligible. Other inorganic nitratessuch as KNOh Ca(NO&, and Fe(N03)~were examined. Among aqueous extracts of atmospheric aerosols, the nitratesquantifiedusing this method agreed well with the results obtained by means of ion chromatography. The method is effective for measuring the size distribution of a nitrate aerosol in ambient air, when air volumes as small as 30 L are sampled. Nitrate is one of the major inorganic constituents of atmospheric aerosols. Nitrate aerosols, as well as gaseous nitric acid, are important products of the removal pathway of the nitrogen oxide reaction in urban atmospheres. Size distributions of nitrate aerosols measured with a cascade impactor have been reported, and based on those measurements, nitrate formation mechanisms havk been discussed. l4 However, the formation mechanism of the nitrate aerosols are not as clearly understood as those of sulfate^.^-^^ In the size distribution measurements, ion chromatography (IC) has been mostly used for the nitrate analysis, and aerosol sampling times of hours to days were required to collect sufficient samples for analysis. In order to better understand the formation mechanism of the nitrate aerosols in the atmosphere or in a smog chamber, it is necessary to obtain highly timeresolved size distribution data. Therefore, there is a need for an analytical method which can accurately determine nanogram level quantities of aerosol nitrates. One promising technique uses thermal analysis in which the aerosol on a collection substrate is directly heated and the (1) Savoie, D. L.; Prospero, J. M. Geophys. Res. Leff. 1982, 9, 1207-1210. (2) Hobbs, P. V.; Hegg, D. A. Afmos. Enuiron. 1982, 16, 2657-2662. (3) Harrison, R. M.; Pio, C. A. Afmos. Enuiron. 1983, 17, 1733-1738. (4) Cadle, S. H. Afmos. Enuiron. 1985, 19, 181-188. (5) Kadowaki, S . Enuiron. Sei. Technol. 1986, 20, 1249-1253. (6) Milford, J. B.; Davidson, C. I. J . Air. Polluf. Control Assoc. 1987,37, 125-
134.
(7) Wall, S. M.; John, W.;Ondo, J. L. Afmos. Enuiron. 1988, 22, 1649-1656. (8) John W.; Wall, S. M.; Ondo, J. L.; Winklmayr, W. Afmos. Enuiron. 1990, 24A, 2349-2359. (9) Roberts, T. P.; Friedlander, S.K. Enuiron. Sei. Technol. 1976,10,573-580. (10) McMurry, P. H.; Friedlander, S.K. Afmos. Enuiron. 1979, 13, 1635-1651. (11) Hering, S . V.;Friedlander, S. K. Afmos. Enuiron. 1982, 16, 2647-2656.
362 AnaiyticaIChemistry, Vol. 66,No. 3, February 1, 1994
gaseous products from the thermal decomposition are measured by a gas analyzer. This technique needs no sample extraction, which represents a potential source of loss or contamination during handling. Therefore, as long as the nitrates arequantitatively decomposed to NO,, and a sensitive gas analyzer is available, the method has the advantages of being sensitive, accurate, simple, and rapid. Sulfur at nanogram levels in atmospheric aerosol has been measured by thermal analysis. Husar12reported vaporizationflame photometric detection (FPD) for samples on filters. Roberts and Friedlander13 measured sulfur directly from a collection plate of cascade impactor. Roberts and Friedlander's method allowed measurements of sulfur size distribution in an atmospheric aerosol and in a smog chamber aerosol at sample air volumes as small as 60 L. Moscowitz14applied Roberts and Friedlander's method to measuring nitrate size distribution, using a chemiluminescent NO, detector (CLND) instead of FPD for the sulfur analyzer, and the same aerosol sampling techniqueand thermal decomposition apparatus as reported.13 The determination of nanogram levels of nitrate in an atmospheric aerosol by converting the nitrate to NO, gas (NO N02) and detection by CLND has been reported by other investigators. and Yoshizumi et a1.16 determined nitrate in aqueous extracts of aerosols. Braman et al." developed a tungstic acid technique. Spicer et a1.I8 directly heated and analyzed filter samples and aqueous extracts. Moscowitz14 reported that thermal decomposition-chemiluminescence (TDCL) can directly determine the amount of nitrate deposited on a stainless steel impactor collection strip without extraction procedures. In the TDCL reported by Moscowitz, in order to obtain high NO, recovery from the nitrate, the stainless steel plate has to be baked (900 O C 1 h) before aerosol sampling. In our preliminary experiments, however, for standard NH4NO3 placed on the baked strip, the NO, recovery was 32% at most and high NO, recovery as reportedI4 could not be obtained. In addition, since the baked strip tends to adsorb contaminant gases such as NO, during the process of aerosol sampling and nitrate analysis, the method gives a high background, resulting in poor accuracy and low sensitivity.
+
(12) Husar. J. D.; Husar, R. B.; Stubits, P. K. Anal. Chem. 1975,47,2062-2065. (13) Roberts, P. T.; Friedlander, S.K. Afmos. Enuiron. 1976, 10, 403-408. (14) Moscowitz, A. H. Parficle Size Disfriburion of NifrafeAerosols in the Los Angeles Air Basin; EPA-600/3-77-053;1977. (15) Cox, R. D. Anal. Chem. 1980.52, 332-335. (16) Yoshizumi, K., Aoki, K.; Matsuoka, T.; Asakura, S . Anal. Chem. 1985,57, 737-740. (17) Braman, R. S.; Shelley, T.J.; McClenny, W.A. Anal. Chem. 1982,54,358364. (18) Spicer, C. W.; Joseph, D. W.; Schumacher, P. M. Anal. Chem. 1985, 57, 2338-2341.
0003-2700/94/03660362$04.50/0
0 1994 American Chemical Society
7 I
Vent
a
Chemiluminescent
Ig
Stripchart Recorder
I
Electronic
Reaction Cell
l-l
Electric Source
11
Flguro 1. Schematic diagram of nitrate detection system consisting of a thermal reactlon cell, chemiluminescent NO, analyzer, strip chart recorder, and electrlc Integrator.
In this report, we describe a new method using an inert substrate, i.e., a nonbaked stainless steel strip. This method is extremely sensitive and accurate and rapidly determines nanogram levels of nitrates. The method should also be applicableto sizedistribution measurement of a nitrate aerosol in ambient air and in a small-scale (1-2 m3) smog chamber.
EXPERIMENTAL SECTION Apparatus. A schematic diagram of the analytical system is shown in Figure 1. A thermal decompositioncell modified by earlier investigatorsl2J3 consists of an upper and a lower Pyrex glass cell. N2 carrier, at 1.3 L/min, in excess relative to the suction rate of the NO, analyzer, was supplied from a cylinder. The N2 carrier was passed through a stainless steel strip mounted on two stainless steel posts in the lower cell and then into the upper cell connected to a NO, analyzer. The excess gas flowed through a four-way stopcock and was measured with a Rotometer. Teflon tubing was used for all gas lines. The NO, analyzer was a Monitor Labs nitrogen oxides analyzer, Model 8440, and nitric oxide produced by the thermal decomposition reaction of the nitrate placed on the strip was measured. The nitric oxide mode (NO NOz), and the 5 ppm range of the analyzer were set. The chemiluminescentNO, analyzer was calibrated using an N O calibration gas produced by a dilution system (Standard TechnologyInc., SGGU-14). N O (106 ppm) in N2 standard gas, which was referenced to a primary standard of the Chemical Inspection and Testing Institute, was used. A Shimadzu Model CR4A integrator was used to investigate the peak area (pV X s) of the NO, analyzer output. The thermal decomposition was achieved by rapidly heating the stainless steel strip with direct current, using an electric source equipped with a step-down transformer and a timer-controlled switch. A noncontact-type infrared thermometer (Lec Co., Ltd., Model KTL 520B) measured the instantaneous temperature of the heated spot in the strip.
+
Reagents. Reagent grade chemicals (Wako Pure Chemical) were used throughout without further purification. Ultrapurified water obtained by passing distilled and ionexchanged water through a water purification system (MilliQTM, Millipore Corp.) was used to clean the strips and prepared reagents. Stainless Steel Strip. Nonbaked and baked strips were compared. A stainless steel film, 0.03 mm thick, type AIS1 No. 304, was used. We could not obtain the same film, type No. 302, used by Moscowitz. The strips were cut from the stainless steel film measuring 5 X 20 mm and two holes, 2.5mm diameter, were punched in the strip at intervals of 12 mm. The nonbaked strip (cleaned strip) was prepared by sonicatingthe strip in hot detergent, washed with ultrapurified water, and dried in a furnace for 1 h at 180 OC. The baked strip, which was only used in the preliminary experiment, was prepared by baking the cleaned strip at 700 OC for 30 min. These baking conditions were milder than that (900 OC 1 h) reported,14 but because these conditions showed the highest NO, recovery of N 100%for NaN03, the surface property of this strip was regarded as being substantially chemically equivalent to that rep0rted.1~ Interference. In order to test NO, adsorption on the stainless steel strip, 37-522 ppb N O and 249-496 ppb NO2 were generated by diluting the standard N O and NO2 with filtered air. These gases were aspirated by a low-pressure impactor (LPI), on which stages the stainless steel strips were set, and the NO, response from CLND was measured for these strips. NO, Recovery. The measured nitrate (ng) was obtained by dividing the measured peakarea (count) by the K (described in the next section), and the recovery (%) was calculated from the measured nitrate and the amount of standard nitrate placed on the strip. Sodium, ammonium, potassium, calcium, and ferric nitrate standard solutions were placed on the strip using a 1 pL Hamilton 7001-NCH syringe. Comparison with IC. IC was performed with a Dionex Qic analyzer, Model QIC-2, operated in the anion mode to compare it with the TDCL method for determining atmospheric aerosol nitrate. The aqueous extracts of the actual aerosol were prepared as follows. Sufficient amounts of aerosol samples for analyzing nitrate by IC were collected with a cascade impactor by means of a sampling time (29-53 h) longer than that required (- 1 h) for the TDCL. The LPI was a singlejet, eight-stage impactor, with dimensions as designed by Hering et al.19 The small tips of about 5 X 5 mm on which the aerosol was deposited were cut from the collection plate. Eight tips and 1 mL of water were placed in a centrifuge tube, and the aerosol samples were dissolved ultrasonically for 10 min. The sample solutions were separated by centrifugation into soluble and an insoluble fractions. The supernatant (water-soluble fraction) was used as the aqueous extract sample. IC needed 500 pL of the extracts; TDCL needed only 10 pL. Atmospheric Aerosol Sampling. The atmospheric aerosol was collected with the LPI preceded by an acid gas denuder. We did not calibrate the cutoff diameter of the impactor. The aerosol collection plates of the stainless steel strip fixed with (19) Hering, S. V.;Flagan, R.C.; Friedlander, S.K.Emiron. Sci. Technol.1987, 12, 667-673.
AnalyticalChemistry, Voi. 66, No. 3, Februaty 1, 1994
303
Table 1. NO, Rocoverler from NaNO, and NHdNOa on a Baked and Nonbaked Strlp (2 nmol of Nttrate) NO, recoverp (% )
method
NaN0.l
NH4N03
baked strip nonbaked strip nonbaked strip + CrOs nonbaked strip + NaOH, CrOs
93 f 6 (10) 54 5 (4) 17 2 (3) 100 f 5 (3)
31 i 6 (10) 51 i 1 (4) 16 1 (3) 98 5 (3)
5
Average
**
*
* standard deviation (number of replicates).
two clips onto a glass strip holder (26-mm diameter) were mounted in the stages of the impactor. The aerosol sampling rate was 1 L/min. The acid gas denuder was a glass tube with an internal diameter of 7 mm and a length of 50 cm and internally coated with Na2CO3 and glycerin.20 The denuder has a theoretical denuding efficiency of 99% for "03; we obtained 98% experimentally.
RESULTS AND DISCUSSION Calibration of the Instrument. The response of the analytical system (NO, analyzer-integrator) for a known quantity of NO standard was examined. The 106 ppm standard N O in N2 was directly injected into the reaction cell with a syringe through a silicon tube connected to the bottom of the lower cell. A linear curve from 0 to 8 mL of standard NO (equivalent to 0-2121 ng of nitrate) and intersecting at the origin was obtained,with a correlationcoefficient of 0.9997. A K,the constant of the response of the analytical system calculated from the slope of the calibration line, of 3610 (counts/ng of nitrate) agrees within the uncertainty of the measurements with the theoretical value of 3690 counts/ng of nitrate calculated for this system in which the output voltage of CLND was 200 mV vs 1 ppm N O at 25 OC and 750 mmHg. The linearity of the response, and the agreement between the theoretical and measured responses over the concentrations studied, indicated that the transfer of the N O produced in the reaction cell into the analyzer, the NO, measurement, and the peak area counting were essentially quantitative. PreliminaryStudy. The optimum heating temperature was determined by measuring NO, recovery from NaN03. Passing currents for 0.2 s, twice with an interval of 1 s at 1.3 V (- 3 A), gave the highest recovery. Under these conditions, the temperature of the heated spot measured with the infrared thermometer was 280 f 5 OC. Unless otherwise noted, these conditions were used throughout this study. Standard NH4NO3 and NaNO3 solutions, both containing 124 ng (2 nmol) of nitrate, were placed on the two types of strip, and NO, recoveries were examined (Table 1). In the baked strip, a NO, recovery of 93 f 6% for NaN03 was obtained, in agreement with the results of Moscowitz, which were 100%. For NH4NO3, we obtained maximum values of 31 f 6% under any heating conditions. Recoveries of NaN03 and NH4N03 from the nonbaked strip were 54 f 5 and 5 1 f 1%, respectively. In the thermal decomposition of NaN03, Osipovs and Ievins21*22 obtained high nitric dioxide conversion from NaN03
-
~
~~
(20) Appel, B. R.;Winer, A. M.; Tokiwa, Y.; Biermann, H. W. A m o s . Enuiron. 1990, 2 4 4 611-616. (21) Osipovs, L.; Ievins, A. Chem. Absfr. 1959, 53, 9878.
364
Ana!vticalChemistry, Voi. 66,No. 3, February 1, 1994
-
with Fez03 as a catalyst. S ~ g i y a r n reported a~~ that 100% recovery of nitric oxide was obtained by adding powdered Cr2O3 as a catalyst. In our study, on the surface of the strip, some of the metal oxides produced during the baking process may act as catalyst and contribute to high NO, conversion from NaNO3. The thermal decompositionreaction of NH4NO3 produces gaseous products other than N O or N02, such as N2 and N2O. K ~ m m eand r ~ Friedman ~ and Bigeleisen25showed that the N2O produced by the thermal decomposition of pure "4NO3 resulted exclusively from N-N bond formation between ammonium and nitrate ions. It can be considered that, in our study, N2 and/or N20, which cannot be detected by CLND, might have been generated from NH4NO3, resulting in the low recovery of NH4NO3. This suggests that, in nitrate analysis by TDCL, the ammonium of NH4N03 as well as that in an atmosphericaerosol sample may negatively interfere. Therefore we considered it necessary to eliminate ammonium from the sample before its thermal decomposition. In this report, NaOH was added to release ammonium as ammonia gas with a N2 carrier. The effect of addition of NaOH on the baked strip was investigated; 97 f 6% recovery from NH4NO3 was obtained when an alkali-treated baked strip prepared with added NaOH was used, followed by preheating prior to adding nitrate solution. It was found that, for various nitrates, such a method is effectivefor the analysisof nanogram level nitrate in aqueous extracts of aerosol sample. In this report, however, the details are not presented since the baked or alkali-treated strip is not suitable for direct analysis of impactor samples in which it serves as an aerosol collection plate because of the adsorption of interfering gases such as NO, on the surface. In an exposure experiment, the baked strip was placed in room air containing -50 ppb NO2 for 1 h and an increase in the blank value equivalent to 20-50 ng of nitrate was observed. We attempted to use a nonbaked strip with a catalyst instead of a baked strip. In crude experiments, a variety of metal (Cr, Mn, Ni, Cu, Fe) oxides were evaluated for NO, recovery of NaNO3, and good reproducibility and high recovery were obtained by adding aqueous CrO3. As shown in Table 1, by adding 0.1 pL of 0.1 N NaOH and 0.1 mol/L CrO3, respectively, for 2 nmol of NaNO3 and NH4N03, 100 f 5 and 98 f 5% recovery were obtained. When only CrO3 was added without NaOH, a brown vapor, which gave noise in the CLND response, was evolved by heating and recoveries of only 17 f 2 and 16 f 1% for NaNO3 and NH4NO3, respectively, were obtained as shown in Table 1 . Generation of the brown vapor persisted until over 0.3 pL of NaOH was added, indicating that the NaOH contributes not only to the removal of ammonium but also aids in activating the C r 0 3 in the thermal decomposition process. The role of C r 0 3in the mechanism of thermal decomposition is unknown. Interferences. In the ambient aerosol, ammonium is the ubiquitous species which contains nitrogen. In TDCL, in addition to the negative interference mentioned above, we obtained data indicating that ammonium was thermally (22) Osipovs, L.; Ievins, A. Chem. Abse. 1959, 53, 14806. (23) Sugiyama, K.; Takahashi, T. Kogyo Kagoku Zosshi 1967, 70, 32-36. (24) Kummer, J. T. J . Am. Chem. SOC.1947,69, 2559. ( 2 5 ) Friedman, L.; Bigeleisen, J. J . Chem. Phys. 1950, 18, 1325-1331.
Tabk 2. NO, Convenlon from (NH4)m4and NH,Cl by TDCL (2 nmol of Wrote) NO, conversiona (%)
method
("I)OSOI
nonbaked strip nonbaked strip + Cr03 nonbaked strip + NaOH, Cr03
0 16*3 2*1
a
Average
"&I 0
16 0
3
standard deviation for three replicates.
Table 4. Measured Nltrate for NO, Adsorbed Stalnies8 StOd Strip
no.
NO (ppb)
NO2 (ppb)
1 2 3
45 f 7 37 f 9 522t 172
249 5 496t9 495t26
sampling vol (L)
mead nitate (rglm9
20 60 63
0.04 0.09 0.02 0.05 0.01 0.05
*
**
1 -
Table 3. NO, Convenlons from NaNOl
nitrite placed on strip (ng) 400 800 a Average
measd nitratea (ng)
NO, conversiona
8.4 2.3 24.2 f 11.3
1.5 0.4 2.2 1.0
(%)
t standard deviation for six replicates. 2000 ng(Nitrate)
oxidized to NO,, resulting in a positive interference. Table 2 shows the response of the analyzer for the thermal decomposition of (NH4)2S04 and NH4Cl. For 2 nmol of ("&SO4 or NH4C1 placed on the nonbaked strip, without NaOH, we calculated that 16%of the ammonium is converted to NO,; Le., ammonium in the atmospheric aerosol may positively interfere in the determination of nitrate by TDCL. However, when NaOH is added prior to the Cr03, NO, conversion of only 0-2% ammonium was observed. The effect on the removal of ammonium by NaOH was examined by IC on an experimental scale 10 times that of TDCL. A 2000-ng aliquot of ammonium from three salts (NH4NO3, ("4)2SO4, NH4Cl) was placed on nonbaked strips, 1-10 pL of 0.1 N NaOH added, and the mixture dried. The residual ammonium was determined by IC; 1 pL of 0.1 N NaOH removes 99.9% of the ammonium in these salts. A 2000-ng quantity of ammonium is equivalent to an ammonium aerosol concentration of 33.3 pg/m3 in every size range at sampling air volumes of 60 L. Particle nitrites, as well as ammonium, are likely to be major species which interfere with nitrate determination by TDCL. We examined NO, conversion from nitrite by using a 400 ppm standard NaN02 solution placed on the strip and measuringNO, by TDCL. Table 3 showsthe measured nitrate and NO, conversions, which are 1.5% for 400 ng and 2.2% for 800 ng of nitrite. These values seem to be much smaller than expected. Further investigation of the thermal decomposition of NaNOz was not conducted. NH4N02, which is most likely to exist in the atmospheric particle, was not tested since it was unavailable as a commercial reagent. However, we can estimate that NO, conversions from NH4N02 will be similar to that for NaN02, by substitution of sodium ion for ammonium in NH4N02 by adding NaOH. Other nitrite salts were not examined, and we consider that the interference due to decompositionof particle nitrite may be almost negligible. In the sampling process, gas-phase "03, "02, NO, and NO2 should be major interferences in TDCL. In order an annular to eliminate acid gas such as HNO3 and "02, denuder is mostly u~ed.~**J0.2"~~ In this report, however, (26) Allegrini, I.; Santis, F. D. CRC Crir. Rev. Anal. Chcm. 1989, 21, 237-257. (27) Eatough, N. L.; et al. Atmos. Enuiron. 1988, 22, 1601-1681. (28) Vossler, T. L.; et al. Atmos. Enuiron. 1988, 22, 1729-1736.
I
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
because of a low air flow rate of 1 L/min, we used a single cylindrical denuder coated with Na2C0329*30 and obtained a high HNO3 collection efficiency of 98%. It is known that NazCO3 can effectively denude both "03 and HN02;2629 therefore, interference from these gases can be minimized. If NO, (NO, NOz) in atmospheric air is adsorbed on the aerosol collection plate in the sampling process, these gases might be possible interference species in TDCL. NO, adsorption on the stainless steel strip was examined (Table 4). In these three cases, average measured nitrate are less than 0.04 pg/m3, which is the same order as that of the detection limit described below. Thus, these results show that NO, adsorbed on the aerosol collection substrate is not significant. Recovery of N a N O 3 and N H 4 N O 3 . To determine the optimum conditions for maximum NO, recovery, the relationship between the quantity of C r 0 3and NO, recovery was investigated. As shown in Figure 2, in the range of 0.8-1.5 mL of 0.1 mol/L Cr03, 93-103% recoveries were obtained from 200 and 2000ng of nitrate in NaNO3 solutions. Samples of 1.0-1.3 p L of 0.1 mol/L CrO3 were optimal and were used in the following experiments. The background nitrate value in purified water was measured. Purified water, 1.OpL, and 1.O pL of NaOH and CrO3 were placed on a nonbaked strip and then 10 replicates of 1.8 f 0.4 ng of nitrate were measured. All solutions were freshly prepared, but these background signals could not be reduced. Since NaNO3 and NH4N03 have been identified as predominant inorganicnitrate salts in ambient aerosols,31-38 ~~
(29) Possanzini, M.; Febo, A.; Liberti, A. Armos. Enuiron. 1983, 27,2605-2610. (30) Ferm, M.; Sjcdin, A. Armos. Enuiron. 1985, 19, 979-983. (31) Blanco, A. J.; McIntyre, R. G. Armos. Emiron. 1972, 6, 557-562. (32) Schuetzlc, D.; Crittenden,A. L.; Charlson, R. J.J. Air Pollur. Conrrol Assm. 1973,23,70&709. (33) Gordon, R. J.; Bryan, R. J. Enuiron. Sei. Technol. 1973, 7, 645-647.
Analyticsl Chemlstty, Vol. 66, No. 3, February 1, 1994
365
Table 5. NO, R e c o v ~ w from K W , Ca(Wk, and F o ( N 4 h nitrate nitrate added (ng) NO, recove@ ( % )
io3
9.5 91 1.3 96 f 6.1
loo
0
96 & 12.0 96 & 5.0 96 5.7
al
*
P .-c Z
TI
a
102
Average & standard deviation for three replicates.
L
VI
z
I 10'
Nitrate in Standard ng Figure 3. Calibration curve for NaN03 and NH,NO,.
their calibration, determination range, and detection limits were mainly examined. The range of 0-2000 ng of nitrate equivalentwas calibrated with four or five replicates. Standard NaN03 and NH4N03 (0-2 pL of 0-1000 ppm) and 1 pL of 0.1 N NaOH were placed on strip and dried under N2, then 0.1 mol/L CrO3 was added and dried again under Nz, and the strip was mounted in a reaction cell and heated. The background-corrected average measured nitrates are plotted vs the quantity of nitrate placed on thestripin Figure 3; theerror bars represent standard deviation (SD). For NaNO3 and NH4NO3, least-squares fit of the data between 1 and 2000 ng resulted in correlation coefficients of 0.997 for NaN03, 0.998 for NH4N03, and 0.999 for NaNO3 NH4NO3 and slopes of 0.98 for NaN03, 0.97 for NH4NO3, and 0.99 for NaNO3 + NH4NO3. Defining the range that showing less than 10% of the CV as the determination range gives a range of 5-2000 ng for both nitrates, with 2.3-10.4% CV, in this method. The lowest detection limit (3SD of blank) is 1.1 ng of nitrate. At a rate of atmospheric air sampling for LPI of 1 L/min, in a 1-h sampling period the determination range would be 0.08-33.3 pg/m3 and the detection limit would be 0.02 pg/m3 for each impactor stage. Other nitrates such as those of calcium, potassium, and ferric salts are likely to be found in the ambient aerosol. Thus, the recoveries for Ca(NO&, KNO3, and Fe(N03)3 containing 400 and 1000 ng of nitrates were examined (Table 5 ) . High recoveries of over 96% were obtained for these three nitrates. The recovery experiments for NaN03, NH4NO3, and other nitrates showed that TDCL can determine nitrates across a very wide range of concentration and with high sensitivity. Comparison of TDCL and IC. TDCL was compared with IC using aqueous extracts of actual aerosol samples collected
+
(34) Grosjean, D.; Friedlander, S. K. J. J . Air Pollur. Conrrol Assoc. 1975, 25, 1038-1044. (35) OBrien, R. J.; Holmes, J. R.; Bockian, A. H . Enuiron. Sci. Techno/. 1975, 9,568-576. (36) OBrien, R. J.; Crabtree, J. H.; Holmes, J. R.; Hoggan, M. C.; Bockian, A. H.Emiron. Sci. Technol. 1975, 9, 577-582. (37) Harker,A. B.;Richard,L.W.;Clark, W. E. Armos. Enuirm. 1977,11,87-91. (38) Kadowaki, S. Armos. Ewiron. 1977, 11,671475.
366 Ana~lcalChemistty,Vol. 66, No. 3, February 1, 1994
Ion Chromatqmphic Nitrate uglml Flgure 4. Comparison of the nitrate quantities determined by TDCL of an
3-
--
Total Nitrate 5.95 r d m : '
~
-
cw 2 -3
-
-
-
% -w
-a --
\
5
-
-
l -1
-
--
...... - ....... .............. ..................................................... 0
1
,
I
I
I
I
I
F w e 5. Nitrate size distributionin an atmosphericaerosol. Samples were collected on 30 August 1993 at Kobe, by LPI preceded by an acid denuder for 43 min at a flow rate of 1 L/m. The dashed line indicates the determination limit.
by LPI. Six samples were taken at this laboratory at Suma, Kobe, from 27 February to 9 May 1990. The range of concentrationmeasuredby IC was0.32-2.95 ppmina 0.5-pL extract and by TDCL was 4.5-55.7 ng of nitrate in 10 pL of extract. The results are compared in Figure 4; the correlation coefficient is 0.994, the slope is 1.004, and the TDCL nitrate intercept is 0.02 pg/mL. Over these nitrate concentrations, the agreement between TDCL and IC is reasonable, and the results confirm the utility of TDCL for ambient aerosol nitrate determination.
Measurements of the Size Distribution of Aerosol Nitrate. The distribution of the aerosol nitrate in the Kobe atmosphere was measured by TDCL. As an example, the nitrate distribution taken by the LPI for 43 min at a flow rate of 1 L/min on 30 August 1993 is shown in Figure 5. It can be seen that there is a trimodal size distribution (two modes in the submicrometer range of 0.075-0.1and 0.5-1.0 pm and one in the range of 2-8 pm). The nitrate distribution (two modes in the submicrometer range) is similar to those reported by Wall et al.' and John et ala,*except for a small difference in the range of the smallest mode. In this experiment, no adhesive coating was used on the collection plate to eliminate particle bounce. However, in a crude experiment using a silicon grease-coated strip, we obtained excellent recoveries and reproducibility for standard
NaNO3 and NH4NO3. Further investigations into issues of the sampling process such as impactor collection efficiency are in progress.
CONCLUSIONS The TDCL technique, which determines nitrate levels in a very wide concentration range with high sensitivity and accuracy, has been developed. Combination of this technique and aerosol sampling by LPI makes it possible to obtain highly time-resolved sizedistribution data for ambient nitrateaerosol. Received for review May 18, 1993. 1993.'
Accepted November 5, ~
~
~~~~~
Abstract published in Aduancc ACS Abstracts, December 15, 1993.
AnalyticaiChemistry, Vol. 66, No. 3, February 1, 1994
967
