Sub-millimolar determination of formalin by pulsed amperometric detection

Analytica Chimica Acta 510 (2004) 195–201

Sub-millimolar determination of formalin by pulsed amperometric detection Sirimarn Ngamchana, Werasak Surareungchai∗ School of Bioresources and Technology, King Mongkut’s University of Technology Thonburi, Bangkhuntien-chaitalay Road, Thakam, Bangkok 10150, Thailand Received 5 March 2003; received in revised form 13 January 2004; accepted 19 January 2004

Abstract Formalin, formaldehyde in the presence of methanol, was determined by pulsed amperometric detection (PAD). A triple waveform using Edet = −0.3 V (tdet = 30 ms), Eoxd = +0.8 V (toxd = 200 ms), and Ered = −0.8 V (tred = 350 ms) versus Ag/AgCl was applied at a Au electrode for detection in a flow injection (FI) system. The approach was rapid and yielded a sub-millimolar detection limit (0.0129 mM) with a dynamic range up to 100 mM. A precision of 8.8% R.S.D. at 1.0 mM for two hundred repetitive injections by the FI-PAD was obtained, whereas holding at a constant potential (−0.3 V versus Ag/AgCl) for anodic oxidation of formaldehyde caused the response to decrease dramatically after a few measurements. The method developed was used to analyze the formalin contents of water from rinsed samples of vegetables and fruit and ice-melt from seafood, and the method showed good agreement with the liquid chromatography (LC) method. © 2004 Elsevier B.V. All rights reserved. Keywords: Pulsed amperometric detection; Formalin; Formaldehyde; Flow injection analysis

1. Introduction Formaldehyde is widely used in many industrial processes including wood fixatives, dry cleaning solutions, cosmetics, textiles, and consumer products such as detergents, soaps, and shampoos [1,2]. Also, formaldehyde is found in fruits, vegetables, and biological fluids from human origin [3,4]. Formalin, a generic term describing a solution of formaldehyde in methanol and water, is used in aquaculture as a bath treatment to control external parasitic infections of fish, and in hospitals as a disinfectant and as a fixative for biological samples. In Southeast Asia, there have been reports of the misuse of formalin, by adding it to seafood and vegetables to make them look fresh in local markets [5]. This practice constitutes a serious health risk, since formaldehyde is a carcinogen and can also cause damage to the central nervous system, blindness and respiratory disease [6,7]. Hence, there is a need to monitor formaldehyde levels and ensure they meet current safety regulations. The most common methods of formaldehyde detection involve chemical reaction using various reagents to form colored derivatives, which can be measured spec∗

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0003-2670/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2004.01.018

trophotometrically. The most common color formation reagent used is 2,4-dinitrophenylhydrazine (DNPH) to form hydrazone. However, a disadvantage of this reagent is that it reacts with most aldehydes and ketones [8–11]. A reagent specific to only aldehydes is Nash’s reagent, which undergoes a condensation reaction with ammonia, 2,4-pentanedione and formaldehyde to form yellow 3,5-diacetyl-1,4-dihydrolutidine [12], but these methods are generally time consuming. Liquid chromatography (LC) with either absorption spectroscopy or fluorescence spectroscopy detection has been also used to isolate the derivatives from possible interferences [13–15]. This method shows low limits of detection for formaldehyde, but generates large amounts of waste solvent and cannot easily be adapted to portable, on-site use. A method based on the oxidation of formaldehyde to formate by hydrogen peroxide, followed by ion chromatographic analysis of the resulting formate has been reported, but produced low recoveries of formaldehyde from various collection substrates [16], and again, cannot easily be adapted to on-site use. Electrochemical sensors have also been developed for formaldehyde detection, including polarographic sensor systems based on direct in situ analyte derivatization with (carboxymethyl)trimethyl ammonium chloride hydrazide [17]. The use of solid electrodes (rather than mercury), suggests

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the possibility of easily portable detection systems. Enzyme biosensors based on formaldehyde dehydrogenase immobilized pH-sensitive field effect transistor [18] and amperometry using carbon screen-printed electrodes with immobilized formaldehyde dehydrogenase and an osmium redox mediator [19] have been reported. A conductimetric biosensor based on immobilized alcohol oxidase with interdigitated thin planar electrode for determination of formaldehyde has been developed [20]. Though the biosensor method has the advantage of specificity, inherent in enzymes, long-term stability at room temperature can sometimes be poor, and some biosensor methods need the co-enzyme NAD+ for the enzymatic reaction [18,19]. In 1981, Hughes and Johnson [21] first reported the use of pulsed amperometric detection (PAD) at Pt electrodes, for the detection of carbohydrates following LC separation. Since then, this approach has been developed and applied extensively to various organic compounds as a technique for detection, including carbohydrates (recent review in [22]), alcohols [23], amines and organo-sulfur compounds [24–28]. The technique of PAD is normally based on a triple potential waveform that incorporates amperometric measurement together with potentiostatic cleaning and reactivation of the electrode surface. Thus, it reduces passivation problems by regenerating the electrode surface before each measurement. This allows amperometric measurements of many organic species that would otherwise cause loss of electrode activity by fouling. Shi and Johnson [29] have previously applied PAD at an Ag working electrode at Edet +0.10 V versus Ag wire for the determination of formaldehyde in air incorporated with ion-exclusion and ion-exchange chromatography. However, a drawback to this method was the fact that polishing of the Ag electrode was required after 4–6 h of operation. Whereas, oxidation of formaldehyde on platinum surfaces encounters the formation of a stable CO complex that can lead to poor sensitivity [30]. Oxidation of formaldehyde at an Au electrode occurs at negative potentials where the electrode is not covered with oxide and Au shows a high activity for the reaction [31]. Therefore, it is of interest to examine the use of PAD for analysis of formaldehyde in the form of formalin by an unmodified Au electrode. In this work voltammetry of formalin in an alkaline solution is described. The performances of the system based on flow injection (FI) determination were evaluated. This has been done by apply a triple pulse waveform. Effect of methanol and other aldehydes was studied. Also, a comparison with the LC method for the analysis of samples is discussed.

ter and 10% methanol) and methanol (99.9%) were obtained from Aldrich. DNPH was obtained from Fluka. Solutions of NaOH were diluted from a 50% (w/w) stock solution (BDH). The water used was triply-distilled. Solutions were de-aerated by purging with N2 gas. 2.2. Apparatus and procedures A 1.0 mm diameter gold disk (BAS) was used as the working electrode. The electrode surface was polished with 0.3 and 0.015 ␮m alumina slurries and subsequently sonicated in distilled water prior to voltammetric measurement. A platinum wire was used as a counter electrode and the reference was Ag/AgCl. The electrochemical cell contained 5.0 ml of 0.1 M NaOH to which a small volume of analyte solution was added. Voltammetric measurements were performed using a computer-controlled potentiostat (Autolab PGSTAT 10, Eco Chemie). All experiments were carried out at room temperature (25 ± 2 ◦ C). All potentials reported refer to Ag/AgCl. The FIA system used a three-electrode assembly incorporated into a home-made electrochemical flow through cell of volume 1.7 × 10−3 ml. A block diagram of the system is shown in Fig. 1. A solution of 0.1 M NaOH, delivered by a peristaltic pump (EP-1, Bio-Rad), was used as the carrier stream. Samples were injected into the carrier with an injection valve (model 7125, Rheodyne). 2.3. Sample preparation and analysis Samples for the FI-PAD measurement were: (1) seafood such as mackerel, tiger prawn, and squid, (2) vegetables; Chinese cabbage and kale, and (3) fruit, i.e., rambutan from a local fresh market. We have tried to imitate the way the

2. Experimental 2.1. Chemical All reagents were analytical grade and were used as received. Formalin (37%, w/w, formaldehyde dissolved in wa-

Fig. 1. Schematic diagrams of: (A) FI-PAD system and (B) electrochemical flow cell.

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locals apply formalin to food samples. For seafood, the samples were dipped into formalin for a short while and then covered with flake ice. The vegetables and rambutan were sprayed with formalin. After half an hour, solutions from melting ice were collected, as were water samples from the rinsing of the vegetables and fruits. The solutions were then filtered using a Whatman No. 542 filter to remove particles. The filtrate solution was used for the FI measurement. It should be noted that we did not attempt to determine formalin absorbed sample matrices. The same filtrate solutions were also determined by a LC method based on derivatization of the sample with DNPH, as previously described [32]. A 2.0 ml filtrated sample solution was added to 2.0 ml of DNPH reagent (1.4 g DNPH in 100 ml of 2.0 M HCl). The mixture was stirred vigorously for 30 min and then centrifuged (4500 rpm) for 10 min. The residue containing the DNPH derivative of formaldehyde (DNPHF) was dissolved in acetonitrile. Aliquots were analyzed by LC method using an UV detector set at 345 nm (UV-975, Jasco). The column was an octadecyl reversed-phase (C18 ) 250 mm × 4.6 mm column with 5 ␮m silica particles (Ultrasphere, Beckman), used at ambient temperature. The mobile phase was 40% (v/v) acetonitrile (in deionized water) and the flow rate was 1.0 ml min−1 .

3. Results and discussion 3.1. Voltammetry of formaldehyde in the presence of methanol Wave (a) in Fig. 2 represents a background voltammogram of the Au electrode in de-aerated 0.1 M NaOH. The small anodic peak at approx. −0.1 V corresponds to the specific adsorption of OH− , and its desorption can be observed at cathodic peak at −0.2 V [33]. The anodic peak at E > +0.3 V is the formation of Au oxide and the cathodic signal (+0.01 V) in the reverse sweep corresponds to desorption of this surface oxide. In the presence of formalin, two distinctive broad peaks were observed for the positive sweep, as illustrated in Fig. 2b. Several reports have shown the oxidation of aldehydes at various types of electrode, involve a two-electron irreversible process producing the corresponding carboxylate [34–36]. In the case of formaldehyde, it has been concluded that in alkaline solutions formaldehyde forms the gem-diol anion by nucleophilic addition of hydroxide to the carbonyl group according to the reaction [37]: HCHO + OH− → CH2 OHO−

(1)

The presence of gem-diol anion in bulk electrolyte has been observed by UV spectroscopy [38], and near the electrode surface during the electrooxidation of HCHO, by electrochemically modulated infrared spectroscopy [31]. The gem-diol, which is apparently the electroactive species

Fig. 2. Voltammetry at an Au electrode in 0.1 M NaOH at a scan rate of 0.1 V s−1 : (a) background voltammogram; (b) voltammogram for increasing concentrations of formalin 10, 20, 30 and 40 mM, referring arrow marked; (c) voltammogram in the presence of 10 mM methanol.

for the oxidation of various aldehydes [37,39,40], is then oxidized at the Au by the following reaction [31]: CH2 OHO− → HCOO− + H+ + Hads + e

(2)

The formate product of the electrooxidation [38,39,41] has been confirmed by GC–MS [42]. Formate is not electroactive and hence cannot undergo further oxidation, even under conditions of extensive electrolysis. In Fig. 2b, the first anodic peak at −0.3 V thus corresponds to oxidation of gem-diol, to produce the formate anion, according to reaction (2). Oxidation thus proceeds in the potential range where the electrode is not covered with oxide. This agrees with a previous report that rapid oxidation of formaldehyde at an Au electrode in 0.1 M NaOH occurred at very negative potential [43]. Also, Van Effen and Evans [40] and Avramov-Ivi´c and Adži´c [44] have found that on

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Au or Ag surfaces, not covered by oxides, the mechanism involves a direct electrochemical process with the oxidation of the gem-diolate. The second broad anodic peak at about +0.3 V (i.e. in the oxide region) was also seen to be dependent on the concentration of formaldehyde. This feature may be attributed to the oxidation of adsorbed hydrogen (Hads ) obtained from reaction (2), as the quantity of Hads will increase with formaldehyde concentration. Although it should be noted that not all the Hads was oxidized to 2H+ , since some bubbles were observed at the working electrode, indicating H2 formation. In the cathodic scan, a large spiked-like current occurred which was similar to previous report [43]. This peak was interpreted due to an instantaneous removal of oxide layer. Methanol is present in formalin as a stabilizer for the formaldehyde. Therefore, the voltammetry of methanol at an equivalent concentration to formaldehyde was examined in the same solution and potential window. As seen in Fig. 2c, at a scan rate of 0.1 V s−1 no electrochemical activity was observed for methanol in this potential range. Avramov-Ivi´c et al. have reported that electrooxidation of methanol in 0.1 M NaOH at Au electrodes occurred only at a very slow scan rate (<0.1 mV s−1 ) and during the reverse sweep at positive potential greater than 0.8 V at the same time of the reduction of oxide formation [45]. 3.2. PAD optimization The optimum value of Edet for PAD can be accurately chosen on the basis of voltammetry data obtained during application of the PAD waveform at a hydrodynamic system [46]. Fig. 3 shows hydrodynamic voltammograms of the oxidation of 10 mM formalin and 100 mM methanol at the Au electrode using PAD. Each point represents a separate experiment using a PAD waveform in which the detec8

i (10-3 A)

6

4

3.3. Flow injection determination characteristics

2

0

-2 -0.55

tion potential (Edet ) is shown on the x-axis, while the potentials for cleaning (Eoxd ) and reactivation (Ered ) were kept constant at +0.8 and −0.8 V, respectively. The Edet value was varied from −0.5 to +0.1 V in 100 mV increments. The faradaic current arises from formaldehyde oxidation as described earlier and increases with more positive potentials to a peak value at −0.3 V. From here it drops, as the Au surface becomes oxidized and eventually inactive, as seen in Fig. 3. On the basis of the result, the optimum value of Edet was chosen at −0.3 V. It should be noted that methanol at 0.1 M did not reveal any PAD response, which confirms that formaldehyde can be detected in the presence of methanol, i.e. formalin measurement can be interpreted as formaldehyde measurement. To get reproducible signals over time, the electrode should be pulsed sufficiently large and positive to remove adsorbed species and then backed to negative potential to removing the resulting oxide, providing reactivation of the electrode surface. Since, choice of potentials for cleaning (Eoxd ) and reactivation (Ered ) in PAD waveform can be traditionally based on the i–E response from cyclic voltammetry. As shown in Fig. 2, sufficient large potentials for Eoxd and, Ered , were selected at +0.8 and −0.8 V, respectively. Optimal timing parameters for PAD cannot possibly be deduced from the CV response. Time periods are instead chosen from chronoamperometry by observing the time required for double layer charging to decay to a minimal value [47]. Therefore, time periods for application of Edet , Eoxd and Ered , can be determined on the basis of chronoamperometry. From chronoamperometric curves following the step −0.8 to −0.3 V, it was observed that the current from double-layer charging decayed to zero after 25 ms. Therefore, any value for tdet > 25 ms was satisfactory. For toxd and tred , the same process was followed, using chronoamperometric curves obtained when stepping from −0.3 to +0.8 V and +0.8 to −0.8 V. The time for cleaning the adsorbed/passivated species and eliminating oxide formation was again deduced from when the currents became zero, as seen at >100 and > 310 ms, respectively. On the basis of the results, Eoxd and Ered were applied for 200 and 350 ms as they were sufficient to assure for the efficient oxidative cleaning of the electrode surface and to dissolve the surface oxide formed, respectively.

-0.50

-0.45

-0.40

-0.35

-0.30

-0.25

-0.20

E/V Fig. 3. FI hydrodynamic voltammogram of 1.0 mM formalin (䊉) and 1.0 M methanol (䉲) in 0.1 M NaOH at an Au electrode. Conditions given in the text. Error bars give ±sample S.D. (n = 3).

FI peaks were obtained to 1.0 mM formalin in 0.1 M NaOH, at a flow rate 1.2 ml s−1 and an injection volume of 20 ␮l. The potentials Edet = −0.3 V (tdet = 30 ms), Eoxd = +0.8 V (toxd = 200 ms), and Ered = −0.8 V (tred = 350 ms), were sufficient to regenerate and maintain electrode activity. The result is shown in Fig. 4 for 200 repetitive injections. The average current to 1.0 mM formalin was 3.23 × 10−5 A, with a standard deviation of 2.8 × 10−6 (n = 200). There was no significant change in the current over this time. Whereas, amperometric measurements

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Table 1 PAD response of formalin and added-aldehydes

7

10

-5

i (10 A)

Substance

i (10-6A)

6

8

4

1.0 mM +1.0 mM +5.0 mM +1.0 mM +5.0 mM +1.0 mM +5.0 mM

3

1 0

5

10

15

20

25

Number of measurment

4

a

2

20

40

60

80 100 120 140 Number of measurement

160

180

formalin glutaraldehyde glutaraldehyde acetaldehyde acetaldehyde propionaldehyde propionaldehyde

2.07 6.80 18.60 5.26 10.23 5.36 10.79

200

Fig. 4. PAD response of 1.0 mM formalin, conditions given in the text. Inset: amperometric response of 1.0 mM formalin at −0.3 V.

at a fixed potential of −0.3 V caused fouling of the electrode surface as is evident in the continuous diminution of the electrode response over the first measurements (inset in Fig. 4). FI-PAD determinations of formalin at varying concentrations, were performed using the procedure described earlier. The peak current at Edet scaled linearly with formalin concentration up to 10 mM, as shown in Fig. 5. The sensitivity of the response in the linear region was 2.0 × 10−5 A mM−1 (r 2 = 0.9968). The limit of detection (LOD) was calculated through the definition following IUPAC recommendation (2t1−α,ν σ/b, where t1−α is the critical value from the t-distribution at degree of freedom ν, σ the standard deviation of baseline and b the sensitivity obtained from the corresponding calibration curve) [48]. The critical t value was

3.4. Sample analysis As mentioned in the Introduction it is of interest to evaluate the FI-PAD method for the mis-use of formalin in food samples, i.e., seafood, vegetables and fruits. Solutions of ice-melt from the seafood and rinsings from the vegetables and fruit were analyzed, as described in Section 2. It should be noted that the samples used were tested in their initial state

5

i (10-4 A)

i (10-4 A)

2.0

4 3 2 1 0 0

10

20

30

40

50

60

Formalin / mM

1.5

1.0

0.5

0.0 0

2

4

0.22 0.32 0.56 0.25 0.43 0.33 0.39

Expressed as mean ± S.D. of triplicates.

3.0

2.5

± ± ± ± ± ± ±

2.262 for 9 degrees of freedom in an interval of confidence of 95%, resulting that the estimate LOD was 0.0129 mM. The effect of other aldehydes, namely glutaraldehyde, acetaldehyde and propionaldehyde, was also investigated. All three compounds gave an increase in response when added to the formalin solution, as listed in Table 1. The aldehydes tested normally exist in the atmosphere occurring from combustion of wood, automotive exhaust fumes, and coal refining and waste incinerators [49]. Therefore, some considerations of sample types will need to be made when determining formaldehyde/formalin concentrations by the FI-PAD. Otherwise, the resolution of this analytical problem should involve a chromatographic separation step.

0 0

Current response (×10−5 A)a

5

2

6

199

6

8

10

12

Formalin / mM Fig. 5. Linear response of formalin. Error bars give ±sample S.D. (n = 3). Inset: response curve of formalin.

200

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Table 2 Comparison of results of the FI-PAD with LC analysis Sample

Concentration of formalin (mM) FI-PAD

LC

Tiger prawn Squid Mackerel Kale Chinese cabbage Rambutan

0.41 0.68 0.56 0.48 0.74 0.63

0.43 0.62 0.47 0.31 0.94 0.59

(4.39) (2.65) (2.74) (4.32) (2.45) (3.22)

pollutant examination, although the formaldehyde would need to be transferred from air to an aqueous phase, and this would have to be combined with an air-sampling device. Possible applications can also be extended to matrices such as cosmetic products and biological extracts. (0.63) (0.73) (1.24) (0.17) (1.87) (0.12)

R.S.D. of the results (%) in parenthesis.

and no formalin was found prior to the application of formalin in the lab. The results were compared with LC analysis of the same samples. The results obtained are summarized in Table 2. The precision of the FI-PAD determinations was satisfactory, with relative standard deviation (R.S.D.) values from triplicate analysis of the samples within 5%. The LC analysis gave better precision, with R.S.D. within 2%. The paired t-test was used to evaluate the accuracy of the FI-PAD with the LC analysis. The mean difference, x¯ d and the standard deviation of the difference, sd , between the two methods were −0.0233 and 0.126, respectively. The absolute √ t was found to be 0.453 (t = x¯ d n/sd ). Therefore, the calculated t value obtained was much smaller than the critical t value of 2.571 for 5 degrees of freedom (n = 6) and the significance level = 0.05. This indicated that no significant difference between the method and the HPLC analysis was found for mean values of the formalin concentration.

4. Conclusions This work presents a rapid, sensitive and reproducible PAD method for the FI analysis of formalin. The approach does not require either chemical reaction of the sample solution to form a colored complex, or derivatization with a specific reagent. Despite the presence of methanol in commercial formalin solutions, methanol did not interfere with the response. Because of the negative potential of −0.3 V versus Ag/AgCl used for the oxidation of formaldehyde, interferences would be unlikely except for other aldehydes. This is in contrast to the previous use of PAD for formaldehyde determination, with an Ag electrode (Edet +0.10 V versus Ag wire), which reported interference from sugar [30]. We have demonstrated the applicability of the method toward samples of solutions from rinsed vegetables and fruit, and ice-melt from seafood, that showed good agreement with results obtained from the LC method. The method developed can be used to determine formalin at concentrations down to 0.01 mM. This limit of detection is low enough for the regulations of the Occupational Safety and Health Administration (OSHA), which specify formaldehyde concentrations from 0.1 to 1.0 mM [50]. Hence, the FI-PAD method reported here could also possibly be applied to air

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