Continuous fluorescence determination of formaldehyde in air

Analytica Chimica Acta 518 (2004) 51–57

Continuous fluorescence determination of formaldehyde in air Kamil Motyka∗ , Pavel Mikuška Department of Environment, Institute of Analytical Chemistry, Academy of Sciences of the Czech Republic, Veveˇr´ı 97, CZ-61142 Brno, Czech Republic Received 28 January 2004; received in revised form 17 May 2004; accepted 17 May 2004

Abstract A flow method for the determination of formaldehyde in air is presented. A cylindrical wet effluent diffusion denuder (CWEDD) is used for continuous collection of formaldehyde from air into distilled-deionized water as denuder liquid. Collected formaldehyde in the denuder concentrate is on-line determined by fluorescence 2,4-pentandione method. The collection efficiency of HCHO is quantitative at the air flow rate of 0.5 l min−1 . The calibration graph is linear in the range 2–300 ␮g m−3 HCHO. The limit of detection is 1.6 ␮g m−3 HCHO. The relative standard deviations for 2 × 10−6 and 1 × 10−5 M HCHO are 2.57 and 1.06%, respectively. Acetaldehyde, acetone, formic acid and acetic acid do not interfere. Delay of the signal is 2.5 min. © 2004 Elsevier B.V. All rights reserved. Keywords: Fluorescence; Formaldehyde; Wet denuder; Air

1. Introduction Formaldehyde is the most widespread carbonyl compound in the atmosphere. Formaldehyde originates from both combustion sources and atmospheric oxidation of hydrocarbons. Formaldehyde and other carbonyl compounds play an important role in the production of photochemical smog [1]. In homes, the most significant sources of formaldehyde are likely to be pressed wood products made by using adhesives that contain urea-formaldehyde resins. Fuel-burning appliances, glues, textiles, and tobacco smoke are other indoor sources of formaldehyde [2]. Formaldehyde has serious toxicological properties. It can cause watery eyes, burning sensations in the eyes and throat, nausea, and difficulty in breathing in some humans exposed at elevated levels (above 0.1 ppm). It has been shown to cause cancer in animals and may cause cancer in humans [3]. Because of the important role of formaldehyde in tropospherical fotochemistry and its toxicological properties, a lot of methods for HCHO measurement have been reported. A few spectroscopic techniques have been developed like differential optical absorption spectroscopy [4], Fourier

∗

Corresponding author. Tel.: +42 0532290165. E-mail address: [email protected] (K. Motyka).

0003-2670/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2004.05.033

transform infrared absorption [5], laser-induced fluorescence spectroscopy [6], and tunable diode laser absorption spectroscopy [7]. Although these techniques are specific, non-destructive and quantitative and allow the continuous detection, the requirement of large, complex, and expensive instrumentation makes these methods not amenable for routine applications. Many chromatographic methods have also been developed for the determination of formaldehyde (and other carbonyls) [8–12]. A chemical derivatization is often used in chromatographic methods for determination of carbonyl compounds [13–16]. The most commonly used derivatization method is based on the reaction of carbonyls with 2,4-dinitrophenylhydrazine (DNPH) to form the corresponding hydrazone derivates, which are then separated by high performance liquid chromatography and determined by UV–vis absorption [17–23]. Carbonyl compounds are, mostly, collected in impingers with DNPH solution [17,18] or onto DNPH-coated solid sorbents [19–23]. However, the method generally requires a long sampling intervals and, moreover, this method (and other chromatography methods) can not be used for continuous analysis. The chromotropic acid method [24,25] and pararosaniline method [26,27] are popular colorimetric methods for the detection of formaldehyde. They provide poor limits of detection and that is why they are not suitable for determination of gaseous HCHO in indoor. More sensitive continuous methods have been

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developed, based on fluorescence detection of derivates produced by reaction of formaldehyde with nicotinamide adenine dinucleotide in the presence of formaldehyde dehydrogenaze enzyme [28], or with ␤-diketone (Hantzsch reaction) [29–33]. Although the former method with enzyme approach is very sensitive and selective, and has compared well in inter-comparison studies with other methods for HCHO determination, it has some shortcomings. The major problems are with the enzyme, that is expensive, not very stable, and may precipitate out of solution, resulting in reduced response. In case of Hantzsch reaction methods, 2,4-pentandione [29–31] and 1,3-cyclohexandione [33] have been used as reagents. Formaldehyde has been continuously collected in both methods in a diffusion scrubber from analyzed air to water used as a scrubbing liquid. Major advantage of the porous diffusion scrubber is that it provides easy discrimination of particle-bound analytes by selective concentrating molecules of gas-phase origin. Disadvantages of diffusion scrubber are relatively low collection efficiency and clogging of the fine pores of the membrane by particle deposition from either air or aqueous phase. Alternatively, a glass coil scrubber has been used for the collection of gaseous formaldehyde [32]. The glass coil is a simple and rugged device, but, when compared with diffusion scrubber, it does not provide a particularly high air/liquid contact ratio. Using both devices enables the continuous determination of formaldehyde in air. This paper describes the determination of gaseous formaldehyde based on the collection of formaldehyde in a cylindrical wet effluent diffusion denuder (CWEDD) into distilled-deionized water and subsequent fluorescence detection of 3,5-diacetyl-1,4-dihydrolutidin (DDL) in the denuder concentrate formed by reaction of formaldehyde with 2,4-pentandione and ammonium ions.

2. Experimental 2.1. Chemicals All solutions were prepared with distilled-deionized water. Formaldehyde (36–38%, ONEX, Czech Republic), 2,4-pentandione (>99%, Sigma–Aldrich Chemie GmbH, Germany), ammonium acetate (Lachema a.s., Czech Republic), and acetic acid (99.8%, Sigma–Aldrich Chemie GmbH, Germany) were of analytical grade. The reagent solutions were prepared daily. The concentration of formaldehyde was determined by titration of the stock solution using the thiosulfate–iodide method [35]. The source of gaseous formaldehyde was based on the diffusion of formaldehyde from solution through the wall of microporous membrane tube (Gore-Tex TA 001, 1 mm i.d., 0.4 mm wall, 2 ␮m mean pore size, length 5 cm) immersed into 1 × 10−3 M HCHO solution in 250-ml flask into nitrogen stream passing through the microporous tube (50 ml min−1 ). After mixing nitrogen stream with clean air, the formed standard mixture was sampled into the CWEDD. The HCHO solution was maintained at 25 ◦ C. A production of HCHO source was 0.15 ␮g min−1 HCHO. 2.2. Apparatus and procedures The cylindrical wet effluent diffusion denuder consisted of denuder tube (DT), inlet subduction zone (S), and outlet tube (OT) assembled together by two heads (Fig. 1) was used for the continuous preconcentration of ambient HCHO from air into a thin film of distilled-deionized water. The tubes were sealed in the heads by Teflon tape to avoid leaking. The untreated glass tube (8 mm i.d. × 15 cm long) as the inlet subduction zone adjusted the laminar flow of sampled

Fig. 1. A schematic diagram of the measuring system. AC, acceptor; BH, botttom head; D, fluorescence detector; DB, debubbler; DC, diffussion cell; DL, denuder liquid; OR, O-ring; DT, denuder tube; OT, outlet tube; P, air pump; PP, peristaltic pump (numbers represent individual flow rates); R, reagent solution; RC, reaction coil; S, subduction zone; T, thermostating bath; TH, top head; V, flow regulator; W, waste.

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air. Inner wall of the denuder tube (borosilicate glass tube, 8 mm i.d. × 50 cm) was specially treated to provide wettable surface. The inner wall was etched by KOH solution. An excess of the etching solution was then removed from the denuder tube and silica gel particles (Aerosil 380, Degussa, Germany) were dispersed onto the inner wall surface to create a uniform film of silica particles. The denuder tube was heated for 1 min at 700 ◦ C and then slowly cooled to laboratory temperature [34]. The denuder liquid was fed to the denuder tube through the porous PTFE O-ring (OR) (Porex Technologies, Fairburn, GA) located between the outlet tube (untreated glass tube 8 mm i.d. × 9 cm) and the denuder tube in the top head (TH). The flow rate of denuder liquid was 336 ␮l min−1 . A compact film of the denuder liquid flew down continuously onto surface of inner wall, and the analyte concentrate was aspirated at the bottom of the denuder tube through O-ring between the end of the denuder tube and the inlet subduction zone at the bottom head (BH). The air passed through the CWEDD in the direction opposite to the denuder liquid flow at air flow rate of 0.5 l min−1 . The CWEDD was mounted vertically. The denuder concentrate leaving the CWEDD entered a debubbler (DB) that consisted of a vertically mounted glass tube (4 mm i.d.) and a PTFE head placed at the top of tube. Two PTFE tubes (0.5 mm i.d.) were placed in the head. Through one of them, an inlet tube with longer end, the denuder concentrate flew into the debubbler and through the second tube the overflow of denuder concentrate was aspirated to the waste. At the bottom the tube was tapered and into tapered part a PTFE tube (0.5 mm i.d., 1.6 mm o.d.) was inserted to sample the bubble-free HCHO concentrate for subsequent on-line analysis. A schematic diagram of employed detection flow system is represented in Fig. 1. Two 4-channel peristaltic pumps (Ismatec, model ISM 852, Switzerland, PP) were used for delivery of the reagent solution (R), denuder liquid (DL), acceptor (distilled-deionized water, AC), denuder concentrate (DC) and for the aspiration of the overflow of denuder concentrate from the debubbler. The optimum flow rates of the reagent solution and the denuder concentrate were 205 and 267 ␮l min−1 , respectively. The denuder concentrate containing collected formaldehyde was merged in the PTFE tee with a reagent solution (2 M ammonium acetate, 0.25 M acetic acid and 0.01 M 2,4-pentandione). The reaction mixture flow was then pumped through a stainless steel capillary coil (0.5 mm i.d. × 40 cm long, RC) maintained at 95 ◦ C (±1 ◦ C) in the thermostating bath (T). The 40 s reaction time in the coil was sufficient for nearly quantitative conversion of HCHO to diacetyl-1,4-dihydrolutidine derivate. The solution leaving the heated reaction coil entered a diffusion cell (perspex, length, width, and depth of a channel are 40, 2, and 0.5 mm, respectively; DC), where bubbles formed in the coil were eliminated. The reaction mixture flew through a lower channel and the acceptor (distilled-deionized water) flows through an upper channel. Both liquid streams were separated by Teflon microporous membrane (Fluoro-

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pore, type of filter FH 0.5 ␮m Milipore) mounted in horizontal position. The bubbles passed from the reaction mixture through the microporous membrane to distilled-deionized water which cocurrently flew at opposite side of the membrane. The bubble-free solution was pumped to the fluorometer (Kratos FS 950 Fluoromat, excitation filter FSA 400; lamp FSA 113; emission filter FSA 429; D) for the detection of the DDL derivate. A fused silica capillary (i.d. 530 ␮m, untreated, Supelco) with removed protective layer of polyimide was used as the detection cell in the fluorometer. The resistance capillary placed at the outlet from fluorometer eliminated releasing of bubbles inside the detector. An analytical signal was recorded on the chart recorder.

3. Results and discussion 3.1. HCHO detection in denuder concentrate The flow system with fluorescence detection was chosen for HCHO determination in wet denuder concentrate. In comparison with other principles of detection, fluorescence allowed fast and sensitive HCHO detection. Nevertheless, the flow system had to be optimized for actual conditions. The effect of the composition of the reagent solution, the denuder concentrate flow rate and the length of the reaction capillary coil on the sensitivity of HCHO detection in the solution was investigated. At the beginning, we used concentrations of reagents adapted from literature [32]. We found out that the reagent solution containing 0.01 M 2,4-pentandione, 0.16 M acetic acid and 6 M ammonium acetate plugged, quickly, pores of the microporous teflon tube (pore size 0.2 ␮m, tube length 5 cm) placed ahead of the fluorescence detector that was employed for the elimination of bubbles. The bubbles formed at the heated reaction coil were not removed efficiently even after lengthing of the microporous tube (15 cm). Ammonium acetate that was contained at relatively high concentration probably caused the plugging of pores of the microporous tube due to its crystalization inside pore. Employing of different composition of reagent solution with smaller ammonium acetate concentration (i.e. 0.1 M 2,4-pentandione, 0.25 M acetic acid and 2 M ammonium acetate) [31] lengthened measurement period without plugging of pores to 2–3 h. However, after this time interval bubbles entered into the fluorescence detector again. In addition, the application of this reagent solution [31] caused a decrease in detector signal to about 30% of the signal, by using the first reagent solution [32]. Therefore, the effect of the concentration of 2,4-pentandione on the signal was investigated (Fig. 2). With decreasing concentration of 2,4-pentadione the signal increases, after passing a maximum at 0.01 M 2,4-pentadione, the signal again decreased. The signal at 0.01 M 2,4-pentadione reached almost the same value as with the first reagent solution. To extend measurement time without bubbles, the diffusion

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Fig. 2. Effect of the 2,4-pentandione concentration on the signal. 1 × 10−5 M HCHO; 2 M ammonium acetate; 0.25 M acetic acid; denuder concentrate flow rate is 264 ␮l min−1 ; reagent solution flow rate is 205 ␮l min−1 ; the reaction time is 30 s.

cell, instead of microporous tube, was used for removing of bubbles from the reagent stream. The diffusion cells were currently used in various flow injection analysis techniques for separation of gases [36]. The diffusion cell consisted of two channels separated with microporous membrane. The reagent stream with bubbles passed through lower channel of the cell and bubbles diffused through the membrane into the stream of distilled-deionized water flowing at the other

side of the membrane through the upper channel of the cell. The diffusion cell was capable of removing, efficiently, bubbles for several days. The final composition of reagent solution was 0.01 M 2,4-pentandione, 0.25 M acetic acid and 2 M ammonium acetate. During optimization of liquid flow rates, the reagent solutions flow rate was kept at constant level, 205 ␮l min−1 , and only flow rate of the sample (i.e. the denuder concentrate)

Fig. 3. Effects of the sample flow rate and the denuder liquid flow rate on the signal. (䊉) sample flow rate (1 × 10−5 M HCHO); (䊊) denuder liquid flow rate (94 ␮g m−3 HCHO; air flow rate is 0.5 l min−1 ); 2 M ammonium acetate; 0.25 M acetic acid; 0.01 M 2,4-pentandione; reagent solution flow rate is 205 ␮l min−1 ; the reaction time is 30 s.

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Fig. 4. Dependence of the signal on the reaction time. 1 × 10−5 M HCHO; 2 M ammonium acetate; 0.25 M acetic acid; 0.01 M 2,4-pentandione; denuder concentrate flow rate is 264 ␮l min−1 ; reagent solution flow rate is 205 ␮l min−1 .

was optimized. If distilled-deionized water was used instead of denuder concentrate in sample line, the baseline of the system was measured. The effect of the sample flow rate on the signal was shown in Fig. 3. With an increase in the sample flow rate the concentration of DDL formed by Hantzsch raction firstly increased and consenquently the signal increased too. The signal reached a maximum at 267 ␮l min−1 and above this value the signal decreased because of the dilution of reagents. The reaction time, the time of DDL synthesis, is determined by the length of the reaction coil (i.d. 0.5 mm) at the constant flow rates of the sample and the reagent solution. The relationship between the signal and the reaction time is shown in Fig. 4. First, the signal increased with increasing reaction time, after 40 s reached a plateau and then it was practically constant. The reaction time of 40 s corresponding to the length of the reaction coil of 40 cm was sufficient for the quantitative conversion of the collected HCHO to the DDL derivate. The sample flow rate of 267 ␮l min−1 and reaction time of 40 s were chosen for further experiments. 3.2. Preconcentration of formaldehyde in the CWEDD The preconcentration in the cylindrical wet effluent diffusion denuder was used for continuous collection of formaldehyde from air into stream of distilled-deionized water. Usage of CWEDD eliminates problems with membrane clogging and, furthermore, provides much higher collection efficiency than diffusion scrubber. Because the hydration of HCHO is catalyzed by H+ [37], a diluted so-

lution of sulphuric acid (0.05 M) besides distilled-deionized water was tested as the denuder liquid. The collection efficiency was calculated as a ratio of the concentration of the formaldehyde collected in the denuder liquid in the CWEDD to the total concentration of formaldehyde found out by the collection of formaldehyde in two impingers in series with distilled-deionized water. The collection efficiencies of HCHO at air flow rate of 1.1 l min−1 for distilled-deionized water, 0.01 M H2 SO4 , 0.05 M H2 SO4 and 0.1 M H2 SO4 were 86.1, 94.3, 95.6 and 90.8%, respectively. From investigated liquids 0.05 M H2 SO4 seemed to be the most efficient collection medium of HCHO. Although the collection of HCHO was not quantitative, at used air flow rate it is far much higher than that at diffusion scrubbers. The collection efficiency of HCHO for distilled-deionized water and sulphuric acid (0.05 M) used as denuder liquid as a function of the air flow rate is shown in Fig. 5. For both liquids, with decreasing air flow rate, the collection efficiencies increased and for flow rate of 0.5 l min−1 , the collection efficiency was 100% for both distilled-deionized water and 0.05 M H2 SO4 . The solid line curve in Fig. 5 describes theoretical collection of the HCHO, calculated according to the Gormley–Kennedy equation. The difference between experimental data and data by Gormley–Kennedy equation results probably from a non-performance of an assumption of “perfect” sorption, for which this equation is derived. Fig. 3 shows the effect of the denuder liquid (distilled-deionized water) flow rate on the signal. With an increase of the distilled-deionized flow rate the concentration of HCHO in the denuder concentrate decreased. For further measurements, we have selected

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Fig. 5. Dependence of the collection efficiency of HCHO on air flow rate. Solid line, theoretical curve; (䊊) 0.05 M sulphuric acid; (䊉) distilled-deionized water; HCHO concentration 100 ␮g m−3 ; denuder liquid flow rate 267 ␮l min−1 .

distilled-deionized water as the denuder liquid and air flow rate 0.5 l min−1 . 3.3. Interferences The effect of various potential interferents concurrently presented in the ambient air on the determination of formaldehyde by the proposed method was investigated. Especially, the effect of other carbonyl compounds and products of their oxidation proceeded in the troposphere was tested. We compared signals for equimolar concentrations (1 × 10−5 M) of HCHO and interferents (acetaldehyde, propionaldehyde, acetone, formic acid and acetic acid) in water and no signal was observed. Moreover, investigated compounds did not interfere even at ten thousand higher concentration than HCHO concentration (1 × 10−6 M HCHO). This result shows that the fluorescence detection is specific for formaldehyde. 3.4. Analytical parameters A calibration graph for formaldehyde in aqueous phase was linear in the concentration range 1 × 10−7 −1 × 10−5 M HCHO (y = 54535600x + 7.0745; R2 = 0.997). A calibration graph for formaldehyde in gaseous phase was measured employing the source of gaseous formaldehyde. The calibration graph for formaldehyde in gaseous phase was linear in the concentration range 2–300 ␮g m−3 HCHO (y = 0.1705x + 2.4543; R2 = 0.994). The detection limit of gaseous formaldehyde depended on the detection limit of formaldehyde in solution (denuder concentrate) and conditions of the collection of formalde-

hyde in the CWEDD. Under the optimum conditions (summarized in Table 1), the detection limit of formaldehyde in solution for a signal-to-noise ratio of 3 was 1 × 10−7 mol l−1 (3 ␮g l−1 HCHO). The detection limit of formaldehyde in air was 1.6 ␮g m−3 for the air flow rate of 0.5 l min−1 and the denuder liquid flow rate of 267 ␮l min−1 . The relative standard deviations for 15 repeated measurements of 2 × 10−6 M and 1 × 10−5 M HCHO in solution were 2.57 and 1.06%, respectively. A delay of the signal, the time between an entry of HCHO into the CWEDD and the detector signal at the output, was 2.5 min. A resolution time was 1 s. 3.5. Analysis of indoor air (real samples) The proposed method was applied for the determination of formaldehyde and indoor air. Air at the office, at the Institute of Analytical Chemistry, was analyzed. Two months before the analysis the office was equipped with new pressed wood furniture. An elevated concentration of 27.2 ± 0.4 ␮g m−3 HCHO was found. Found concentration did not exceed a hygienic limit (60 ␮g m−3 ) valid in the Czech Republic (regulation no. 6/2003). A record of the continuous analysis is Table 1 Conditions for the determination of formaldehyde Reagent solution composition

0.01 M 2,4-Pentandione 0.25 M Acetic acid 2 M Ammonium acetate

Denuder concentrate flow rate Reagent solution flow rate Denuder liquid Absorption liquid flow rate Air flow rate

267 ␮l min−1 205 ␮l min−1 Distilled-deionized water 336 ␮l min−1 0.5 l min−1

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Fig. 6. Record of the analysis of air at the office with new press wood furniture. HCHO concentration is 27.2 ± 0.4 ␮g m−3 ; air flow rate is 0.5 l min−1 ; denuder liquid flow (distilled-deionized water) rate is 336 ␮l min−1 .

shown in Fig. 6. First, the baseline was measured, after the sampling of analyzed air to the CWEDD the signal rose and reached a plateau (with rise and fall times, 10–90% and 90–10%, of 1.5 min) and then it was constant. The measurement was repeated. The method was also applied for determination of formaldehyde in indoor urban air.

4. Conclusion A new method for the determination of formaldehyde in air based on the preconcentration of formaldehyde in the cylindrical wet effluent diffusion denuder is described. Distilled-deionized water was chosen as the denuder liquid because of simplicity of procedure. The method provides continuous and fast determination of formaldehyde in indoor or urban polluted air. In comparison with the method using diffusion scrubber, employment of wet denuder eliminates difficulties with plugging of membrane pores observed in diffusion scrubbers.

Acknowledgements This work was supported by a grant no. IAA4031105 from the Grant Agency of Academy of Sciences of the Czech Republic, a grant no. 526/03/1182 from the Grant Agency of the Czech Republic.

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