single-column ion chromatography of passive monitors for atmospheric pollution

Analytica Chimica Acta, 217 (1989) 135-147 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

135

SINGLE-COLUMN ION CHROMATOGRAPHY OF PASSIVE MONITORS FOR ATMOSPHERIC POLLUTION

DENIS NOEL*, HELENE ROBERGE and JEAN-JACQUES HECHLER

National Research Council of Canada, Industrial Materials Research Institute, 75 Bd. de Mortagne, Boucheruille, Qukbec J4B 6Y4 (Canada)

(Received 24th April 1988)

SUMMARY

Nitrite, nitrate and sulfate anions deposited on passive monitors were determined by using single-column ion chromatography. These monitors were exposed outdoors for various periods of time in an atmosphere with a very low degree of pollution, and they were used to evaluate the rate of deposition of some corrosive species on metallic materials. Two types of passive monitors were studied, namely sulfation and nitration plates. Comparison of the results obtained by using the classical turbidimetric method and single-column ion chromatography shows that turbidimetry is inappropriate for low sulfate concentrations ( < 2.5 mg/plate) deposited on sulfation plates. Thus, ion chromatography should be used to obtain a representative value of sulfate deposition because it allows the determination of sulfate in amounts as low as 200 pg/plate without further preconcentration. For nitration plates, ion chromatography permits simultaneous determination of nitrite, nitrate and sulfate from the same plate, whereas only nitrite can be determined by the classical calorimetric method. Both gluconate/borate and phthalate eluents were investigated and the results show that phthalate eluent is more appropriate for these plates. It was found that the pH of the sample has an important effect on the area of the sulfate peak. For both eluents, the area increases when the sample pH is lower than 6. This demonstrates the importance of an adequate sample treatment before injection in order to obtain reproducible and accurate values.

Many atmospheric research programs have been initiated to understand the impact of different pollutants in the environment. However, there are few studies on the effect of the pollutants on metallic corrosion. In 1984, a joint US-Canadian atmospheric corrosion program was started in order to evaluate the extent of corrosion of common metals in different environments. In Can- ada, the Experimental Lakes Area (Ontario) was selected as a baseline site because it is known to be relatively unpolluted [ 11. At the same time, similar experiments were done on more polluted sites located in the United States [ 21.

Traditionally, various types of metals were exposed on a particular site for different periods of time in order to measure the weight loss and thus to cal- culate the rate of corrosion. The corrosion of a metal is largely a function of the time of wetness and of the aggressivity of the atmosphere [ 31. The damage

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occurs when corrosive species in the atmosphere are in contact with the metal surface. The deposition process of these corrosive substances is characterized by the concept of deposition velocity which is related to the flux of pollutants per unit area and to the concentration of pollutants. A concentration measurement alone is not enough and the concentration plus the wind velocity must be used to obtain a better approximation of the true deposition [ 3,4].

For metals, atmospheric pollutants such as SO, and NO, are known to be corrosive. The deposition velocity of these species on a given metal cannot easily be evaluated and hence sulfation and nitration plates were used to determine the concentration of adsorbed SO, and NO,. These deposition-velocity-sensitive methods relate much better to corrosion damage than concentration measurements. Sulfation plates are made with a lead peroxide paste whereas nitration plates consist of an inert support impregnated with a triethanolamine solution [ 561. These materials are put in small plastic Petri dishes which are installed in an inverted position on a standard ASTM rack

]71. Classical methods for determining pollutants deposited on these passive

monitors are turbidimetry for sulfation plates and calorimetry for nitration plates. Turbidimetric determination involves the precipitation of the barium sulfate after conversion of the insoluble lead sulfate to a soluble lead carbonate [ 51. Reproducible turbidimetric determination is extremely difficult to achieve because many factors can affect the turbidity of the solution. It will be shown that this method is unsuitable for quantifying small amounts of sulfate absorbed on these plates. For the calorimetric method, the determination of nitrite absorbed on a nitration plate is done by complexing N-l- naphthylethylenediamine dihydrochloride (NEDA) with the reaction product of sulfanilamide and nitrite. This method is precise and sensitive but works only for nitrite. However, nitrate and sulfate anions are also present on nitration plates. Hence, ion chromatography represents a suitable alternative to classical methods. Eluent-suppressed ion chromatography (ESIC) has been regularly used to evaluate different anions in atmospheric precipitation or in air [8,9]. This separation technique was also used to determine solid sorbent for personal monitoring [lo]. However, the determination of nitrite by ESIC is subject to error because there is a possibility of oxidation and ion exclusion of nitrite in the suppressor column.

Single-column ion chromatography (SCIC) does not require a suppressor column and this technique is now often used in analysis for contaminants in atmospheric aerosols [ 11,121. It has also been used to determine nitrogen diox- ide in the atmosphere collected on passive monitors [ 131. In this work, a single-column ion chromatographic method was developed to quantify low amounts of different anions absorbed on sulfation and nitration plates exposed to an atmosphere with a low level of pollution. Specific problems were found with this technique applied to the passive monitors and corrections are applied

137

to avoid erroneous results. Data obtained by SCIC are compared with those obtained by classical methods.

EXPERIMENTAL

Instrumentation and chemicals The SCIC system consisted of a Series 3B liquid chromatograph from Per-

kin-Elmer with a Model 7126 automatic universal injector (50-~1 loop) from Rheodyne. The detector was either a Model 430 conductivity detector or a Model 440 fixed-wavelength ultraviolet detector set at 254 nm, both from Waters. Chromatograms were recorded on a Varian DS604 data station. The ion-exchange column was an IC-PAK anion column, 5-cmx4.6-mm i.d., lo- pm particles, from Waters. The stationary phase was a polymethacrylate gel (spherical particles) with quaternary ammonium group ( -C2H3N (CH,- CH,),+ ) with an ion-exchange capacity of 0.06 meq/column [ 141.

All chemicals were ACS certified reagent grade (Fisher Scientific Co.) un- less otherwise stated. Deionized water was purified with the NanoPure II ion- exchange system from Sybron-Barnstead (Boston, MA).

Procedures For anion chromatography, two eluents were used: borate/gluconate and

phthalate. The gluconate/borate eluent was prepared daily by mixing 20 ml of a gluconate/borate buffer concentrate, 10 ml of glycerol solution, 120 ml of acetonitrile (HPLC grade) and 850 ml of deionized water. The mixture was filtered through a 0.22-pm GS filter (Millipore Corp. ) and degassed in an ul- trasonic bath for about 15 min. The gluconate/borate buffer concentrate was prepared by dissolving 16 g of sodium gluconate, 18 g of boric acid, and 25 g of sodium tetraborate decahydrate in 11 of deionized water. The glycerol solution was made by adding 25 ml of glycerol to 75 ml of water. For the phthalate eluent, a stock solution was made by dissolving 15.32 g of potassium hydrogen- phthalate (KHP) in 1 1 of deionized water to give a 75 mM solution. A fresh solution of 0.75 mM KHP was prepared each day and the pH of the eluent was adjusted to 6.5 with 1 M potassium hydroxide. The KHP eluent was filtered and degassed as described previously.

For the sulfation plate, the lead peroxide paste was carefully removed with a spatula and a stream of deionized water; 20 ml of 0.47 M sodium carbonate was added to the mixture of water and PbOz paste. The solution was allowed to stand for 3 h with occasional stirring and then placed in water at 100’ C for 30 min. After this, it was diluted to a final volume of 50 ml and filtered through a Whatman No. 42 paper filter. An aliquot of this solution was diluted five times in deionized water and adjusted to a pH of 6.5 with hydrochloric acid. The final solution was filtered through a 0.45-pm Millex-HV filter before injection on the column.

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For the nitration plates, the triethanolamine coating was removed with a spatula and a stream of deionized water, and the solution was diluted to 50 ml. After 10 min, it was filtered through a Whatman No. 1 paper filter. Finally, it was filtered through a 0.45pm Millex-HV filter before injection on the column.

Turbidimetric and calorimetric determinations of sulfate and nitrite are standard analytical procedures [5,6] and the methods given by the manufacturer of the plates (Serco Laboratories, Cedar Falls, IA, U.S.A.) were used without modifications. Plates have an absorption surface of 18 cm2.

RESULTS AND DISCUSSION

Sulfation plates GZuconate/borate eluent. The gluconate/borate eluent is recommended com-

mercially for the separation of anions with the IC-PAK column. According to the manufacturer, sensitivity is enhanced with the gluconate/borate eluent in comparison to common organic acid eluents. However, the gluconate/borate eluent can be used only with polymeric matrices such as polystyrene divinyl- benzene copolymer or polymethacrylate gel because its pH is too high for a silica ion-exchange column. At pH 8.5 borate and gluconate react together to form an anionic complex acting as the eluent anion [ 151. For the determination of sulfate absorbed on sulfation plates, the gluconate/borate eluent seems particularly attractive because the difference of pH between the eluent and the solution resulting from the treatment of the plate is smaller than with organic acid eluents. Thus, sample clean-up is reduced. The chemical treatment of sulfation plates involves transforming the insoluble lead sulfate into soluble sodium carbonate and sodium sulfate with a sodium carbonate solution. The final solution is highly basic with a pH of about 12-13.

Figure 1 (a) shows a typical chromatogram of an injection of water with the gluconate/borate eluent. At high sensitivity with a conductivity detector, the noise level is relatively high. Because the passive monitors were exposed in an atmosphere of very low pollution, it is necessary to use high sensitivity to de- tect the small quantities of sulfate without preconcentration. According to Fritz et al. [ 161, noise can result from temperature-dependent equilibria and tends to be more pronounced with eluents that are more strongly retained on the column. However, the gluconate/borate complex is not significantly retained on a polymethacrylate column [ 151. Thus, the noise level with the gluconate/ borate eluent is probably due to the high conductivity of the eluent [ 91, which is about 310 ,uS, as compared to about 185 ,& for a phthalate eluent (0.75 mM, pH 6.5). This noise is an important disadvantage of the gluconate/borate eluent. Different negative peaks are observed in Fig. 1 (a). The first is the injection peak arising from dilution of the eluent [ 171. The first positive peak corre- sponds to the exclusion of cations present in the sample. The second negative peak is a system peak. At this point, it is difficult to determine if this peak is

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b/ i

4 ” “: c c:IIIIIk:’ 0 2 4 6 8 10 0 2 4 6 8 10 12 14 16

Retention time Cmin) Retention time [min)

d

,,I 0 5 10 15 20 0 5 10 15 20

Retention time (min) Retention time hinl

Fig. 1. Chromatograms obtained with the conductivity detector: (a) water; (b) solution from a sulfation plate; (c) solution from a sulfation plate; (d) solution from a sulfation plate (adjusted to pH 6.5 with HCl). Eluent: (a,b) gluconate/borate; (c,d) 0.75 mM KHP, pH 6.5.

caused by an “eluent anion absent effect” [ 181 or by the elution of neutral eluent molecules according to a reversed-phase mechanism [ 191.

Figure l(b) shows a chromatogram obtained with the gluconate/borate eluent for a solution made from a sulfation plate. The solution was diluted five- fold with water before injection to reduce the amount of carbonate in solution because an excess of carbonate hides the sulfate peak. As in Fig. 1 (a), the noise level is high and accurate integration of the sulphate peak is extremely difficult. Thus, the error of measurement with the gluconate/borate eluent is large and this eluent obviously cannot be used in accurate determination of low amounts of sulfate absorbed on passive monitors. The system peak in Fig. 1 (b) is different from the one observed in Fig. 1 (a). With the injection of an alkaline sample, the system peak consists of a positive peak followed by a negative peak. A similar system peak was also observed in certain conditions with organic acid eluent [20]. A possible explanation is that a fraction of neutral eluent molecules gives a positive peak and that the negative peak may be caused by an “anion absent effect” [ 181 caused by perturbation of the complex equilibrium involving gluconate and borate [ 171. Compared to an alkaline sample, injection of an acidic sample gives only a positive system peak. This can be explained by the presence of neutral molecules eluting according to a reversed- phase mechanism [ 191. In both cases, the system peak is well separated from the sulfate peak but the baseline noise is too important to use gluconate/borate eluent with sulfation plates.

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Phthalate eluent. Eluents made with aromatic organic acids such as phthalic and benzoic acids give excellent separations of anions with good detection lim- its [21,22]. Phthalic acid is a powerful eluent which has been relatively well studied [ 23,241 whereas benzoate eluent is not strong enough for efficient elution of divalent anions such as sulfate [ 12,251. For all subsequent work the concentration of potassium hydrogen phthalate (KHP) was kept at 0.75 mM and the pH was maintained at 6.5. Experimental conditions were selected in order to obtain a good separation between nitrite, nitrate and sulfate ions.

Figure 1 (c ) shows a chromatogram for a solution made from a sulfation plate diluted five-fold with water. The baseline noise is considerably reduced with the phthalate eluent in comparison to the gluconate/borate. However, the large difference of pH between the sample (pH 12) and the eluent causes the co- elution of the system peak with the sulfate peak. An increase of the system peak area is often observed with the injection of a sample more basic than the eluent [ 191. With a strongly alkaline sample, a sample treatment is generally required [ 21,261. Figure 1 (d) shows that addition of hydrochloric acid to the sample in order to bring the pH to 6.5 eliminates the system peak.

The presence of the system peak is also related to the buffer capacity of the eluent. An eluent with a stronger buffer capacity will be less affected by the injection of highly alkaline or acidic solution. In a first series of experiments, the concentration of KHP in the eluent was varied between 0.25 mM and 2.0 mM with the pH maintained at 6.5. Neutralized (pH 6.5) and non-neutralized (pH 12) solutions made from sulfation plates were injected on the IC-PAK column. Figures 2(a) and 2 (b) show typical chromatograms obtained with a 1.5 mM KHP eluent. Similar results were obtained for other concentrations. Clearly, the neutralization of the sample eliminates the system peak and gives a single well-defined peak for sulfate. The injection of a non-neutralized sample shows multiple peaks. In addition to the system peak, which is the last peak to elute, there is a splitting of the sulfate peak probably because the equilibrium is perturbed by the strong alkalinity of the injected sample. An increase of the eluent concentration up to 1.5 mM seems to decrease the system peak but does not eliminate it. It is impossible to use higher concentrations of KHP at pH 6.5 because the eluent becomes too strong and elutes the sulfate too rapidly.

In a second series of experiments, the concentration of KHP in the eluent was varied from 1 to 3 mM but the pH of the eluent was adjusted to keep the retention time of the sulfate peak to about 10 min. Figures 2 (c) and 2 (d) show chromatograms obtained with a 3 mM KHP eluent kept at pH 4.4. As previously, it is impossible to determine components on the sulfation plates. The injection of an alkaline sample gives a large system peak eluting immediately after the sulfate peak. Moreover, the sulfate peak is split. In contrast to the eluent kept at pH 6.5, the injection of neutralized sample does not give a well- defined peak. There is a co-elution of sulfate and system peaks. However, it

141

0 2 4 6 .Y 1"

Retention time (min)

c

Da 0 5 10 15 20

Retention time (mini

b

5 0 2 4 6 8 10


d

:‘- 0 5 10 15 20


Fig. 2. Chromatograms of a solution made from a sulfation plate, with KHP as eluent and the conductivity detector; (a) 1.5 mM KHP, pH 6.5, neutralized sample; (b) 1.5 mM KHP, pH 6.5, alkaline sample; (c) 3 mM KHP, pH 4.4, neutralized sample; (d) 3 mM KHP, pH 4.4, alkaline sample.

should be noted that injection of a standard solution of sulfate in water gives a symmetrical peak under these chromatographic conditions.

The complexity of the matrix in the case of sulfation plates suggests that the concentration of KHP in the eluent must be kept relatively low for a pH higher than 6.0. Moreover, the increase of the KHP concentration increases the conductivity of the eluent, and thus decreases the sensitivity. The sample must be diluted five-fold to reduce the amount of carbonate and neutralized with hydrochloric acid to eliminate the system peak.

Detection. Comparison of indirect ultraviolet spectrophotometry detection with conductivity detection showed that indirect detection (254 nm) is more sensitive than conductimetric detection. The slopes of the calibration curves obtained with the phthalate eluent for the sulfate ion are respectively 9780 and 28 800 for conductivity and ultraviolet detection; the ratio of the two slopes is 2.9 indicating that the latter detector is three-fold more sensitive. With ultraviolet detection, the limit of detection for sulfate is about 100 pg 1-l or 5 ng for an injection of 50 ~1. The slope of the calibration curve obtained with the gluconate/borate eluent and a conductivity detector is about 15 000, which indi- cates that this eluent increases the sensitivity by a factor of about 1.5 compared to the phthalate eluent.

Effect of the samplepH on the peak area. With organic acid eluent, the pH of the eluent has a significant effect on the peak response [ 16,271. Acidic eluents

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B

n ‘0 3 3.0 - x

I 2

a 2.0- 2

1.25 - n

0.0-J I I I I 1.00~ I I I I

2.0 40 6.0 8.0 10.0 12.0 2.0 4.0 6.0 8.0 10.0 12.0

PH PH

Fig. 3. Effect of the sample pH on the sulfate peak area: (A) for a phthalate eluent (0.75 mM, pH 6.5); (B) for a gluconate/borate eluent. Detection: (m) conductivity; (0) indirect ultraviolet.

decrease the sensitivity because they are less ionized. However, there is little information on the effect of the pH of the sample on the peak response. It has been shown only that the pH of the sample has an influence on the system peak. Figure 3A shows the effect of the sample pH on the sulfate peak area for a phthalate eluent (0.75 mM, pH 6.5) with conductivity and indirect ultraviolet detection. The sample injected on the column is a standard 10 mg 1-l solution of sulfate adjusted to different values of pH. The value obtained at pH 13 is not reported on the graph because the peak area was very high. Surpris- ingly, the peak area for sulfate increases when the pH is < 6. Figure 3B shows that the same effect is observed with the gluconate/borate eluent. For both eluents, the peak area remains constant if the pH of the sample is kept between 6 and 12. Figure 4 illustrates the variation of the retention time of the sulfate peak as a function of the pH of the sample. At extreme pH such as pH 2 or pH 13, there is a slight increase in the retention. This behavior is explained by equilibria perturbed by the high concentrations of H+ or OH-. However, this Figure demonstrates that the increase in the peak area cannot be explained by a sudden variation in the retention time.

A similar experiment was done with a phthalate eluent having a higher concentration of KHP (1.5 mM, pH 6.5). Figure 5 illustrates the effect of the sample pH on the sulfate and nitrate peak areas. For sulfate, the same behavior as in Fig. 3 is observed. However, for nitrate, this effect is less pronounced, but there is a significant increase at about pH 2. Injection of solutions without sulfate but adjusted to pH 2 or 13 with HCl or KOH did not give a system peak eluting in the region of the sulfate peak. Thus, the increase of the peak area

8.0L-___ ? 2.0 4.0 6.0 80 10.0 12.0 14.0 2.0 4.0 6.0 8.0 10.0

PH PH

Fig. 4. Variation of the retention time of the sulfate peak as a function of the pH of the sample with 0.75 mM KHP, pH 6.5 as eluent. Detection: ( n ) conductivity; ( 0 ) indirect ultraviolet.

Fig. 5. Effect of the sample pH on the peak area for a 1.5 mM phthalate eluent at pH 6.5: (W) sulfate; (0 ) nitrate. Detection: conductivity.

cannot be explained by the co-elution of a system peak as is sometimes observed with other experimental conditions [28]. This behavior can be explained by using equations which describe the detector signal. The difference of response for the conductimetric detector between the eluent and the solute is given by the following equation [ 91:

dG=G, -Gn = =Cs {[(Lx+ +&-)I, - (As+ +L,-)IsI,]/10-3K} (1)

where G, A, I, C, and K are respectively the conductance (PUS), the equivalent conductance, the degree of ionization, the concentration and the specific conductance; the subscripts S, B and E refer respectively to the sample, back- ground and eluent. This equation shows that the sensitivity decreases as the amount of dissociation of the eluent increases. Thus, the sensitivity increases if the eluent becomes more acidic.

The acidity constant for the species HSO,- (pK,,= 1.94) implies that at pH lower than 4, the presence of HS04- begins to become significant. For exam- ple, at pH 4.0, the proportions of HSO,- and SO,‘- are respectively 0.8% and 99.2%. At pH 3.0, these proportions become 7.7% and 92.3% and at pH 2.0, there is 45.5% of HS04- and 54.5% of SOd2-. The increase of the peak area observed in Fig. 3 is explained by the following mechanism. When an acidic sample is injected, both HSO,- and SOd2- are present. As the eluent is neutral (pH 6.5), the HSOI- species is rapidly converted to SOd2-. This dissociation

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creates an excess of H+ which reacts with the eluent anion, E-, to displace the equilibrium towards the non-ionized species, HE. As the dissociation of the eluent is decreased, the sensitivity of the system is slightly increased according to Eqn. I. For nitrate, this effect is less pronounced because nitric acid (pK,= 1.44) is a stronger acid than HSO,-. In this case, the increase of sensitivity starts at more acidic pH as shown in Fig. 5.

The variation of the degree of ionization of the eluent can also explain the increase in sensitivity observed with the absorbance detector. The absorbance signal measured by the detector during the sample elution is given by the following equation [ 91:

As= [~E(CEIE-CS)+~HECE(l-IE)+~SCS]l (2)

where E is the molar absorptivity and 1 the path length of the detector cell. Thus, if ts and tus are different, a decrease of the degree of ionization of the eluent with the injection of an acidic sample of sulfate can slightly increase the sensitivity of the system. As observed in Fig. 3, this effect is less pronounced than with the conductivity detector. Hence, it is extremely important to control the pH of the sample in order to determine accurately the amount of sulfate.

Contamination. Contamination of the solution by containers or filters is often neglected. The possibility of contamination with nitrite, nitrate and sulfate was investigated with various types of container materials, namely low-density polyethylene, polycarbonate and glass. Containers were rinsed with pure water before their use for the sample solution. No contamination was observed when rinsed materials were used. There was also no contamination for unrinsed polyethylene and polycarbonate but there was a small contamination of about 0.5 mg I-’ sulfate for the unrinsed glass. Thus, low-density polyethylene containers or polycarbonate containers were used.

Before injection of the sample on the chromatographic column, the solution is filtered through a Millex-HV filter. This filter is made of poly(vinylidene fluoride) (Durapore), so it cannot be used in analyses for fluoride. In contrast to the finding of Baltensperger and Hertz [ 111, a sulfate contamination of about 0.5 ml 1-l was found when the filter was used without previous rinsing with water, but there was no contamination in sulfate from rinsed filters. For nitrite and nitrate, no contamination was found either for the rinsed or unrinsed filter. Thus, it is very important to clean the filter before filtration of the sample in which sulfate is to be determined.

Comparison of turbidimetry and ion chromatography. Turbidimetry is affected by many experimental factors and it is very difficult to obtain reproducible results. For the sulfation plates exposed in an environment of low pollution, the quantity of sulfate deposited can be as low as 200 lug/plate and the calibration curve for this range of concentration is not linear. Table 1 shows a comparison of the results obtained by using turbidimetry and ion chromatography. For a standard solution of sodium sulfate, both methods give the same result

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TABLE 1

Comparison of turbidimetry and single-column ion chromatography for sulfation plates (exposure time, 30 days)

Sample Turbidimetry SCIC

Control (mg 1-l)

Sulfation plates (pg/plates) Unpolluted area Unpolluted area City area City area

145 144

2000 250 1800 400 2000 800 2800 2300

0 2 4 6 8 10 12 14

Retention time (mini

Fig. 6. Chromatogram of a solution made from a nitration plate, with 0.75 mM KHP, pH 6.5, as eluent and conductivity detection.

of 145 mg 1-l. However, for the sulfation plates, the turbidimetric method gave high values for sulfate. Values lower than 2.5 mg/plate cannot be determined by the turbidimetric method. By comparison, concentrations of sulfate deposited on passive monitors in a city are generally higher than 3.0 mg/plate, in which case turbidimetry is an adequate method. However, ion chromatography is faster than turbidimetry and can be easily automated for routine determinations.

Nitration plates Nitration plates were examined with the phthalate eluent (0.75 mM, pH

6.5 ). In this case, there is no specific problem with the chromatographic technique. Figure 6 shows a typical chromatogram of a solution made from a nitration plate. It is not necessary to dilute the sample as for sulfation plates because

146

TABLE 2

Comparison of calorimetry and single-column ion chromatography for nitration plates (exposure time, 30 days)

Nitration plate

Amount (&plate)

Colorimetry

Nitrite

SCIC

Nitrite Nitrate Sulfate

s-1 105 109 114 264 s-2 144 149 219 280

there is no carbonate in solution. The sample is simply filtered and injected on the column. The main advantage of the chromatographic technique is that several anions can be determined simultaneously in one experiment. For the nitration plates, this is advantageous because there is absorption of nitrate and sulfate in addition to nitrite when they are exposed to the atmosphere.

Comparison of calorimetry and ion chromatography. Table 2 shows the comparison of the results obtained using the calorimetric method and ion chromatography. For the determination of nitrite, both methods give the same value. However, SCIC also gives nitrate and sulfate. Hence, ion chromatography is preferable because more information can be obtained from the same experiment. The amounts of sulfate found on the nitration plates are different from those measured with sulfation plates. A study is currently underway in order to verify whether nitration plates can be used to evaluate sulfate concentrations with an appropriate calibration.

The authors thank Andre Pilon and Jean Boulanger for their technical assistance.

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J.G. Tarter (Ed.), Ion Chromatography (Chromatographic Science Series, Vol. 37), M. Dek- ker, New York, 1987. D.V. Vinjamoori and C. Ling, Anal. Chem., 53 (1981) 1689. U. Baltensperger and J. Hertz, J. Chromatogr., 324 (1985) 153. M.J. Will&on and A.G. Clarke, Anal. Chem., 56 (1984) 1037. Y. Nishikawa, K. Taguchi, Y. Tsujino and K. Kuwata, J. Chromatogr., 370 (1986) 121. J.A. Glatz, Ph.D. Thesis, The American University, Washington, DC, 1985. G. Schmuckler, A.L. Jagoe, J.E. Girard and P.E. Buell, J. Chromatogr., 356 (1986) 413. J.S. Fritz, D.L. Du Val and R.E. Barron, Anal. Chem., 56 (1984) 1177. C. Erkelens, H.A.H. Billiet, L. De Galan and E.W.B. De Leer, J. Chromatogr., 404 (1987) 67.

single-column ion chromatography of passive monitors for atmospheric pollution

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