Ion Chromatography with Pulsed Amperometric Detection - Advances

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Downloaded by PENNSYLVANIA STATE UNIV on August 11, 2014 | http://pubs.acs.org Publication Date: August 21, 1985 | doi: 10.1021/ba-1985-0210.ch002

Ion Chromatography with Pulsed Amperometric Detection Simultaneous Determination of Formic Acid, Formaldehyde, Acetaldehyde, Propionaldehyde, and Butyraldehyde ROY D . R O C K L I N Dionex Corporation, Sunnyvale, C A 94088-3603 Formaldehyde, acetaldehyde, propionaldehyde, butyraldehyde, and formic acid can be determined in a single analysis by ion chromatography. The aldehydes and formic acid are separated on a fully functionalized cation-exchange resin in the potassium form. They are detected electrochemically by oxidation using pulsed amperometric detection at a platinum electrode. Detection limits range from 1 to 3 ppm. Methanol and ethanol interfere with the analysis.

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A N Y M E T H O D S O F D E T E R M I N I N G F O R M A L D E H Y D E I N AIR involve its S o r p tion on a solid sorbent followed by desorption by a liquid (I). The formaldehyde in the liquid is then assayed. For this method to be reliable, the formaldehyde concentration on the sorbent must be directly proportional to its concentration in the air. The desorption step must be quantitative, and finally, the assay of the solution must be accurate. The methods used today to determine formaldehyde in air suffer from two problems. First, during the time between the sampling and the final assay, formaldehyde will slowly be oxidizing to formic acid. Second, all known assay procedures are subject in some degree to interferences. For formaldehyde, these are usually other aldehydes, alcohols, and phenol (i). The major advantage of a chromatographic assay is the improved selectivity attained when a physical separation is present between the analyte species and the potential interferences. For this reason, two ion chromatographic methods were developed and have been reported previously. In each, the formaldehyde is reacted during the desorption process to produce a stable product. The first method is NIOSH P & C A M 318 (1,2). Formaldehyde is ad-

0065-2393/85/0210/0013$06.00/0 © 1985 American Chemical Society

In Formaldehyde; Turoski, V.; Advances in Chemistry; American Chemical Society: Washington, DC, 1985.


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FORMALDEHYDE: ANALYTICAL CHEMISTRY A N D TOXICOLOGY

sorbed onto activated charcoal and desorbed with a solution containing hydrogen peroxide. The hydrogen peroxide is added to quantitatively convert the formaldehyde to formic acid to solve the oxidation problem. The formic acid is then assayed by ion chromatography. Although this method has worked well, difficulties were reported once. During NIOSH evaluations (3), samples were found to be stable for only a few days, and the quality of the sorbent affected sample stability. In the second method, formaldehyde is adsorbed onto molecular sieves and desorbed with a solution containing bisulfite ion (4). Formaldehyde and bisulfite react to form hydroxymethanesulfonate (HOCH SO^", often called formaldehyde bisulfite), which is assayed by ion chromatography at standard anion conditions. Because the addition product coelutes with sulfite ion, the remaining bisulfite in the solution is oxidized to sulfate by hydrogen peroxide. One disadvantage of this method is that any formaldehyde that oxidizes before the desorption step will be lost. For this reason, a chromatographic method in which both formaldehyde and formic acid can be determined in a single injection has been developed and is reported here. This method requires the formaldehyde and formic acid to be in aqueous solution. For example, they could have been desorbed from a solid sorbent. The species are then determined by ion chromatography by using pulsed amperometric detection. Other aldehydes such as acetaldehyde, propionaldehyde, and butyraldehyde can also be determined. Methanol and other alcohols can interfere with the analysis. 2

Experimental All chromatography was performed on a Dionex system 201 l i ion chromatograph, which consists of a pump, a chromatography module, and a pulsed amperometric detector. A platinum working electrode, a glassy carbon counterelectrode, and a silver-silver chloride (1 M NaCl) reference electrode were used in the amperometric flow-through detector cell. The applied potentials (volts) and pulse durations (milliseconds) were the following: J£l(*l), 0.2(60); E2(*2), 1.3(60); and JE3(*3), -0.3(240). The sample loop volume was 50 yiL. The column was a Dionex HPICE-AS1 with the cation-exchange resin converted to the potassium form. The eluant was 0.10 N H S 0 and 0.05 M K S 0 at a flow rate of 1.0 mL/min. A l l chemicals were reagent grade except the sodium formaldehyde bisulfite, which was technical grade and was obtained from Eastman. Formaldehyde was 37% in water. 2

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Results and Discussion Pulsed Amperometric Detection. In 1981, Hughes et al. introduced pulsed amperometric detection. By using a strongly acidic solution, alcohols, glycols, and formic acid could be detected (5). The same conditions used to detect alcohols and formic acid can also be used to detect aldehydes, and are described next. The most commonly used form of electrochemical detection is single potential amperometry, sometimes called D C amperometry. In this



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Ion Chromatography with Pulsed Amperometric Detection

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method, a single potential is applied to the working electrode of a flowthrough cell, and the resulting current is continuously monitored. Pulsed amperometric detection is a new form of electrochemical detection used to detect those species that cause the electrode to become poisoned when they are oxidized. Pulsed amperometric detection uses a repeating sequence of three potentials. The analyte molecules are oxidized at the first potential (El), and the current is measured. The potential is then stepped to a more positive value ( £ 2 ) , and an oxide layer is formed on the electrode surface. The third potential (£3) is a negative potential at which the oxide layer is reduced to produce the bare metal. This sequence is repeated several times per second, and one point on the chromatogram is acquired during each cycle. The advantage of pulsed amperometric detection is the electrode cleaning provided by the alternating positive and negative pulsing. When only a single potential is used, peak heights from a series of injections will quickly decrease as the electrode becomes coated by the products of the oxidation reaction. Information regarding the choice of the three applied potentials is obtained from electrochemical experiments such as cyclic voltammetry and rotated-disk voltammetry. Steady state cyclic voltammetry for formaldehyde in acid on a platinum electrode is shown in Figure 1. The curve is very similar to that of formic acid, shown in Figure 2 and in Reference 5. Hughes et al. have developed a theory for the mechanism of alcohol and formic acid oxidation that probably applies to aldehydes. They conclude that analyte molecules are adsorbed onto the platinum electrode surf ace at negative potentials and oxidized (probably to carbon dioxide and water for formic acid, formaldehyde, and methanol) on the positive going scan. When platinum oxide is formed on the electrode surface at approximately 0.7 V , the analyte oxidation reaction stops. Rotated-disk voltammetry with formic acid and alcohols shows that the decrease in current is caused by platinum oxide blocking the electrode, and is not just the decreasing current normally seen with diffusion control in potential-scan voltammetry. This situation probably occurs with aldehydes. At potentials beyond 1.0 V , oxidation continues. After the reversal of the potential-scan direction, the current will actually reverse from cathodic to anodic, reaching a peak at approximately 0.2-0.3 V . This result is caused by the resumption of the analyte oxidation reaction as platinum oxide is reduced (E = 0.44) to produce a bare and, therefore, more active surface. The cyclic voltammetry of propionaldehyde (Figure 3) is quite different. Little deviation from the background current occurs except when the potential is positive of 0.5 V , where oxidation (probably to propionic acid) takes place. Because propionaldehyde can be oxidized when the electrode is coated with platinum oxide, the reaction mechanism is probably different from that of the single carbon molecules. The lack of current between 0 and 0.5 V implies that propionaldehyde cannot be detected if £ 1 is set bep



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E vs. Ag/AgCI Figure 1. Steady state cyclic voltammetry of 20 mM formaldehyde in 0.1 N H S0 and 0.05 M K2SO4 at a platinum working electrode. Sweep rate is 200 mV/s. Analyte is indicated by the solid line; background current is indicated by the dashed line. 2

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tween 0 and 0.5 V . However, this situation is not the case. The difference between the pulsed amperometric detection and the cyclic voltammetry could be caused by the large difference in time scale for the two experiments. The cyclic voltammetry of acetaldehyde and butyraldehyde are similar to that of propionaldehyde. Information from cyclic voltammetry is now used to choose the three applied potentials. The potentials and pulse durations used are illustrated in Figure 4. Of the three potentials, £ 1 is the most important because the current is measured at this potential. £ 1 is chosen to be not only on a large oxidation peak for the analyte, but also at a potential where little, if any, faradaic background current occurs. On a platinum electrode in an acidic solution, this region is between the hydrogen adsorption and reduction reactions that are negative of 0 V and the platinum oxide formation that begins at approximately 0.5 V. Then, £ 2 is set to 1.3 V , a value chosen to be within 0.1 V of the positive potential limit, and £ 3 is set to - 0.3 V , within 0.1 V of the negative limit. The optimum value for £ 1 is determined by making a series of injections while varying £ 1 . A plot is then made of peak



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0.5 E vs. Ag/AgCI Figure 2. Steady state cyclic voltammetry of 20 mMformic acid. Conditions are the same as in Figure 1.

0.5 E vs. Ag/AgCI Figure 3. Steady state cyclic voltammetry of 40 mM propionaldehyde. Conditions are the same as in Figure 1.

heights versus £ 1 . This plot for formaldehyde, acetaldehyde, propionaldehyde, and formic acid is shown in Figure 5. The background current is also shown with a minimum at 0.1 V . For formaldehyde and formic acid, one would expect from the cyclic voltammetry that maximum peak heights would result if £ 1 was set near 0.35 V , which is the peak current for the first oxidation reaction. Actual optimum potentials are lower: 0.2 V for formaldehyde and 0.1 V for formic acid. A possible explanation is that adsorbed molecules are known to decrease the rate of the oxide formation In Formaldehyde; Turoski, V.; Advances in Chemistry; American Chemical Society: Washington, DC, 1985.

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Ion Chromatography with Pulsed Amperometric Detection - Advances

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