Anal. Chem. 1995, 67, 3740-3745
Electrocatalytic Oxidation and Amperometric Detection of-Aliphatic and FuranicAldehydes at a MixedmValent Ruthenium Oxide- Ruthenium Cyanide Film on Glassy Carbon Electrodes Tommaso R. I. Cataldi,* Cristiana Campa, and Diego Centonze Dipatfimento di Chimica, Universith degli Studi della Basilicata, Via N. Sauro, 85, 85100 Potenza, Italy
The eledmcatalytic oxidation and amperometric detection of simple and h a n k aldehydes at a mixed-valentruthenium oxide-ruthenium cyanide (mvRuO-RuCN) modified glassy carbon (GC) electrode is described. The oxoruthenium centers cross-linked with ruthenium cyanide are believed to be the electroactive surface species in acidic media. The capability of mvRuO-RuCN films to catalyze the slow electrode reaction of aldehydic compounds is illustrated in cyclic voltammetry and flowing streams amperometric detection. A sensitive quantitation of aliphatic and furanic aldehydes without the need of derivatization steps was achieved by coupling ion exclusion chromatography (IEC) with electrochemical detection, using the mvRuO-RuCN film electrode. The presence of low Ruck and &Ru(CN)6 levels in the mobile phase considerably improves the response stability. Of practical importance is that this modified electrode can be used without detrimental effects at a column temperature of 60 "C. At room temperature, the detection limits of aliphatic aldehydes such as acetaldehyde, propionaldehyde, and butyraldehyde in IEC with amperometric detection (Eappl= +l.O8VvsAg/AgCl) were 1.0,0.8, and 0.8 nmol injected (S/N = 3), respectively. Even though virtually all organic compounds are predicted thermodynamically to be oxidized at potentials available at bare solid electrodes, kinetically inhibited electrochemical processes are very often encountered. Low molecular weight aliphatic aldehydes, inherently difticult to detect due to the lack of a strong chromophore or fluorophore group, are also scarcely electrooxidizable at unmodifled glassy carbon (GC) electrodes. Moreover, at a gold electrode, the oxidation strongly depends on pH with an electrochemical process that is particularly effective only in alkaline sol~tions,l-~ and though the platinum electrode in acidic solutions has been used, it requires pulsed amperometric detection in order to address surface fouling problems! Therefore, the most commonly used method for aldehydes determination is through derivatization with 2,4dinitrophenylhydrazine and subsequent liquid chromatographic separation with ultraviolet photometric dete~tion.~-~ (1) Sibille, S.; Moiroux, J.; Marot, J.-C.; Deycard, S. /.Electroanal. Chem. 1978, 88, 105. (2) Vitt, J. E.; Larew, L. A; Johnson, D. C. Electroanalysis 1990,2, 21. (3) Olivi, P.; Bulhoes, L. 0. S.; Beden, B.; Hahn, F.; Uger, J.-M.; Lamy, C.J. Electroanal. Chem. 1992,330, 583. (4) Rocklin, R D. In Formaldehyde: Analyfical Chemisty and Toxicologv;Turoski, V., Ed.; Advances in Chemistry Series 210; American Chemical Society: Washington, DC, 1985; Chapter 2 , p 13.
3740 Analytical Chemistry, Vol. 67, No. 20, October 15, 1995
A simple and sensitive analytical method, avoiding the need for troublesome derivatization reactions, is still a major research goal for aliphatic aldehydes quantitation with constant-potential electrochemical (EC) detection. An interesting strategy for their amperometric monitoring in flowing solutions is to employ modfied electrodes with a catalytically active surface. One of the most promising and valuable methods of electrode modification uses electropolymerization to produce an adherent, conducting, and electroactive film.s Kuleszag showed that coatings of mixedvalent ruthenium oxide cross-linked with ruthenium cyanide (mvRu0-RuCN) exhibit considerable electrocatalytic activity in the oxidation of methanol. This inorganic film is believed to contain mixtures of oxo- and cyano-bridged ruthenium centers in mixed oxidation states.1°-13 Kulesza et al.14 later described a composite film of cobalt tetra(P4olyl)porphyrin and mvRu0RuCN, which shows good electrocatalytic activity toward propionaldehyde. As we have demonstrated previously, the mvRuORuCN sensing electrode can be used as an electrocatalytic amperometric sensor in liquid chromatography and flow injection analysis of primary aliphatic alcohols in acidic media.15 The modified electrode surface exhibited excellent stability under vigorous hydrodynamic conditions, provided that low levels of the electroplating solution were present in the mobile phase.16 This operation mode provides great potential for amperometric monitoring of flowing streams, even if relatively high analyte concentrations are inje~ted.1~ In this work, the action of the mvRuO-RuCN is further pursued for the detection of simple aliphatic aldehydes and furanic compounds such as 2-furaldehyde (2-F), 5(hydroxymethyl)-2furaldehyde (SHMF), and 5methyl-2-furaldehyde (5MF). These (5) Grosjean, D.; Fung, IC Anal. Chem. 1982,54, 1221. (6) Takami, IC: Kuwata, K.; Sugimae, A; Nakamoto, M. Anal. Chem. 1985, 57, 243. (7) Ogawa, I.; Fritz, J. S. /. Chromatogr. 1985,329, 81. (8) Murray, R. W. In Molecular Design of Electrode Suyfaces; Murray, R. W., Ed.; Techniques of Chemistry Series XXII; John Wiley & Sons, Inc.: Chichester, UY 1992. (9) Kulesza, P. J. /. Electroanal. Chem. 1987,220, 295. (10) Cox, J. A.; Kulesza, P. J. Anal. Chem. 1984,56, 1021. (11) Cox, J. A; Gray, T. Electroanalysis 1990,2, 107. (12) Cox, J. A.; Jaworski, R IC; Kulesza, P. J. Electroanalysis 1991,3, 869. (13) Kulesza, P. J.; Bandoch, M . J. Electroanal. Chem. 1992,323, 131. (14) Kulesza, P. J.; Mlodnicka, T.; Haber, J. J. Electroanal. Chem. 1988,257, 167. (15) Cataldi, T. R. I.; Centonze, D.; Guenieri, A Anal. Chem. 1995,67, 101. (16) Cataldi, T. R. I.; Centonze, D.; Desimoni, E.; Forastiero, V. Anal. Chim.Acta 1995,310, 257. (17) Cataldi, T. R. I.; Centonze, D.; Desimoni, E. Food Chem., in press. 0003-2700/95/0367-3740$9.00/0 0 1995 American Chemical Society
latter substances are relevant compounds in food control and processing.1*-22For most practical applications,furanic aldehydes can be conveniently detected by liquid chromatography at 280 nm. The electrochemical oxidation of,these solutes, as well as aliphatic aldehydes, at carbon electrodes is very poor. Therefore, the scope of their inclusion in this work was to veriry the electrochemicalcapability of mvRu0 -RuCN-modified electrodes to promote the oxidation of a wider range of scarcely electroactive aldehydic compounds. EXPERIMENTAL SECTION Reagents and Samples. Acetaldehyde (99%), propionalde hyde (97%), isobutyraldehyde (99+%), butyraldehyde (99%), valeraldehyde (97%), Smethyhrfural (99%), 2-furaldehyde (99%), S(hydroxymethy1)fral (W),r u t h e n i u m 0 chloride hydrate, and potassium hexacyanoruthenate(11) hydrate (Aldrich Chemical Co.) were used as received. Other chemicals employed were of analytical grade and were used without further purification. Solutions were prepared in deionized and doubly distilled water. Electrode Modification. Several conducting substrates such as F't, Au, and glassy carbon (GC) can be coated with mvRu0RuCN inorganic films in mineral acid solutions without the need for alkali metal ions.15 GC electrodes were always employed throughout this work. Before each modification, the electrode surface was wet polished with 0.05 pm a-alumina powder, rinsed with a stream of deionized water, and sonicated for a few minutes. Electrochemical deposition can be easily accomplished in a solution containing 0.5-2 mM RuC13 and &RU(cN)6 in dilute HzS04 (e.g., 10-25 mM), either by holding a polished GC electrode at a constant potential of 1.05 V or by cycling the potential between -0.2 and 1.1V vs SCE. Figure 1shows a set of cyclic voltammograms (CVs) obtained when the GC electrode was cycled continuously at 50 mV s-l. The CVs are characteristic of accumulating electroactive species on the electrodesurface with increasing surface coverage with time. The modification proce dure was effected in freshly prepared and not deaereated solutions. Film deposition by controlled potential consistentlyyields results similar to those obtained from films prepared by potential cycling. Apparatus. Cyclic voltammetry was performed with an Amel Instruments (Milan, Italy) Model System 5OO0, equipped with a Linseis X-Y recorder, Model LY 18100. Electrochemicalexperiments were made at room temperature using a conventional singlecompartment glass cell. The GC disk electrode (3" diameter) used in cyclic voltammetry was purchased from Amel, Model 493. Amperometric detection was performed with an EG&G Princeton Applied Research (FAR)Model 400 electre chemical detector and a flow-through thin-layer electrochemical cell with a single GC working electrode (MP 1305) moditled with mvRuO-RuCN films, a Ag/AgCl reference electrode, and a stainless steel auxiliary electrode. The detector time constant was set at 1s. The output signal was recorded on an X-t Amel Model 868 recorder. All measurements were performed by applying the desired operating potential and allowing the transient current to decay before the amperometric monitoring. (18) Jeuring, H. P.; Kuppers, F. J. E. M. J. Assoc. OfiAnal. Chem. 1980,63, 1215. (19) Porretta, S.; Sandei, L. Food Chem. 1991,39,51. (20) Kim, H.-J.;Richardson, M. J. Chromafogr. 1992,593,153. (21) Blanco Gomis, D.; Gutierrez Alvarez, M. D.; Sopefia Naredo, L;Mangas Alonso, J. J. Chromatographia 1991,32,45. (22) Villal6n Mir, M.; Qhesada Granados, J.; Upez G.; De La Serrana, H.; Ibpez Martinez, M.C. J. Liq. Chromatogr. 1992,15, 513.
1.0
0.8
0.6
0.4
0.2
0.0
-0.2
POTENTIAL (V) vs. SCE Figure I. CVs recorded continuously at a glassy carbon electrode (12.5 mm2)between -0.2 and 1.1 V vs SCE in 25 mM H2S04 solution containing 1 mM RuCl3 and 1 mM K~Ru(CN)G.Scan rate, 50 mV S-'.
The chromatographic system consisted of a Hewlett Packard 1050 pump equipped with an on-line degasser system and a Rheodyne (Berkeley, CA) Model 7125 injector. A Timberline
Instruments Inc. Model H-500 (Boulder, CO) was used as column heater. Chromatographic separations were effected with an Aminex HPX-87-H @io-Rad Labs, Richmond, CA) cationexchange resin column, 300 mm x 7.8 mm i.d. (4pm particle size), using diluted solutions of Hi304 as the eluent spiked with low concentrations of RuC13 and &RU(cN)6. RESULTS AND DISCUSSION The deposit of mvRuO-RuCN on GC electrodes is attributed to the growth of a compact and conducting film with extended Ru-0-Ru and Ru-CN-Ru bridging in the structure. Indeed, it seems that the growth of rutheniumcontaining inorganic films is propitiated by the ability of Ru to form oxo and cyano bridges in acidic This modified electrode prepared in dilute sulfuric acid solutions exhibits three well-defked redox transitions (see Figure 1) with anodic peak potentials at +0.05, f0.78, and +1.00 V vs SCE, which probably correspond to Ru(II/III)-oxo, cyanoruthenate QI/III), and Ru (III/IV) -oxo centers, respectively? Similar electrochemicalbehavior was observed in buffered solutions at pH 2.9J In particular, the more positive oxidation peak was assigned to ruthenium species in the ruthenium oxide portion of the inorganic film, and the involvement of these species in several oxygen-transfer oxidations of organic compounds was sugge~ted.'~-~~
Electrocatalytic Oxidation of Aliphatic and F d c Aldehydes. To assess the potential electrocatalytic activity of this electroactive inorganic film toward the oxidation of aldehydic compounds, the voltammetric behavior was initially investigated. (23) Cotton, F. A; Wilkinson, G. Aduanced Inorganic Chemktry, 5th ed.; John Wiley & Sons, Inc.: New York, 1988; pp 878-890, 253-255.
Analytical Chemistty, Vol. 67, No. 20, October 15, 1995
3741
,A - a b
B a
I
1.2
c
C V I
1
1
I
1.0
0.8
0.6
0.4
1.2
1.0
0.8
0.6
0.4
POTENTIAL (V) vs SCE
POTENTIAL (V) vs SCE
Figure 2. (A) CVs at a bare GC electrode between +0.4 and +1.20 V (a) in the blank supporting electrolyte, 25 mM H2S04, and (b) upon addition of 0.93 M propionaldehyde. (B) Electrocatalytic oxidation at a GC/mvRuO-RuCN film electrode (0.4-1.15 V) in the absence (a) and the presence of (b) 5 and (c) 10 mM propionaldehyde. Scan rate, 2 mV s-’.
Flgure 3. (A) CVs at a bare GC electrode between +0.4 and fl.10 V (a) in the blank supporting electrolyte, 25 mM H3P04, and (b) upon the addition of 2 mM 2-furaldehyde. (B) Electrocatalytic oxidation at a GC/mvRuO-RuCN film electrode in the absence (a) and the presence of (b) 1 and (c) 2 mM 2-furaldehyde. Scan rate, 5 mV s-’.
Figure 2B shows typical cyclic voltammograms (CVs) obtained with a GC/mvRuO-RuCN-modified electrode and recorded at 2 mV in a 25 mM H2S04 solution containing 5 and 10 mM propionaldehyde (curves b and c, respectively). Curve a was recorded in the blank supporting electrolyte. Acetaldehyde and butyraldehyde exhibited similar qualitative voltammetric behavior. Under identical conditions, aldehydes are not effectively oxidized at bare GC electrodes and require much higher concentrations to produce an observable voltammetric response. Curve b in Figure 2A was obtained at a GC electrode in the presence of 0.93 M propionaldehyde. Formaldehyde and longer chain aliphatic aldehydes were not examined because reagent grade solutions of the former contain an elevated percentage of methanol as a preservative, while the latter are sparingly soluble in aqueous solutions. Upon propionaldehyde additions, the catalytic oxidation is evidenced by a strong enhancement of the anodic current that increases on increasing propionaldehyde concentration (voltammograms b and c). This behavior implies a catalytic mechanism that involves the cycling of the catalyst between oxidation states. The anodic peak potential in the presence of propionaldehyde occurs at about $1.05 V, a value that is more positive (about 80100 mv) than the third anodic peak of the mvRuO-RuCN film electrode. At higher scan rates, the CVs exhibited drawn out oxidation waves instead of peak-shaped profiles, likely due to kinetic limitations of the electrocatalytic process. It has been shown that the mvRuO-RuCN film electrode exhibits better activity and above all good stability in a moderately acid en~ir0nment.l~ For most of the experiments, a 10 or 25 mM H2S04 solution was used as supporting electrolyte/mobile phase (see next section). However, the effect of some mineral acid solutions in the pH range 1.5-2.5 was also evaluated. Apparently, the electrochemical behavior of the mvRuO-RuCN iilm electrode and the catalytic oxidation of aldehydic compounds is similar in 3742 Analytical Chemistry, Vol. 67, No. 20, October 15, 1995
dilute solutions of H2SO4, H3P04, HCl, and HC104. Indeed, the overvoltage for the oxidation process is also much lower for furanic aldehydes. Figure 3 displays CVs recorded at 5 mV s1 in 25 mM H3P04 for 2-furaldehydeat the mvRuO-RuCN-modified electrode (B) and at an unmodified GC electrode (A). As mentioned earlier, also in this case, no oxidation peak was observed at the bare GC electrode (curves a and b in Figure 3A are indistinguishable), On the contrary, curves b and c of Figure 3B, which refer to cyclic voltammograms obtained upon the addition of 1.0 and 2.0 mM 2-furaldehyde, respectively, exhibit a large increase in the anodic current with peak potential at +1.0 V. The catalytic oxidation currents observed were proportional to the concentration of 2-furaldehyde. It follows that also in this case the inorganic film participates catalyticallywithin the oxidation mechanism. Apparently, the electrocatalytic oxidation occurs through the high-valent oxoruthenium species formed in the potential region of the third oxidation wave of the modified electrode. As outlined earlier in the oxidation of aliphatic alcohol^,'^ the specific catalytic activity of the mvRu0-RuCNmodified electrode is presumably related to the formation of oxoruthenium(W) or higher oxidation states of ruthenium. Very recently, R u O has also been suggested as the active oxidant in the catalytic process at the m o d ~ e delectrode surface.24 Electrochemical Detection in Flow-ThroughAnalysis of Aldehydic Compounds. On the basis of the voltammetric results, an electrocatalytic sensor seems very attractive for greatly enhancing the amperometric detection of underivatized aliphatic aldehydes. However, before we illustrate the applications of the mvRuO-RuCN film electrode in chromatographic separations with EC detection, some features of the flow injection response will be given. Hydrodynamic voltammetry under flowing stream operations, which possesses a much higher current sensitivity (24) Gorski. G.; Cox, J. A Anal. Chem. 1994, 66, 2771.
-.-
ACETALDEHYDE
--4 g
1.25
-A-
PROPIONALDEHYDE
-0-
BUTYRALDEHYDE
1.00
-0-
5-HMF
5-HMF SMF
0.75
W
0.50
t
0
0.25
1.0 pA
0.00 ,
0.8
1.0
0.9
(VI vs.
POTENTIAL
.
I
1.1
1.2
Ag/AgCI
Figure 4. Hydrodynamic voltammograms of 2.0 mM acetaldehyde, 2.4 mM propionaldehyde,2.0 mM butyraldehyde, and 0.3 mM 5-HMF in ion exclusion chromatography at a GC/mvRuO-RuCN-modified electrode. Column, Aminex HPX-87-H, 300 mm x 7.8 mm i.d.; isocratic elution at room temperature with 10 mM H2S04 containing 5 p M RuCIa and 5 p M K~Ru(CN)~; flow rate, 0.8 mL min-l; loop, 20 pL; thin-layer electrochemical cell. The mvRUO-RuCN film was grown at constant applied potential for 15 min in a solution containing 1 mM RuCI3 and 1 mM bRU(CN)6 in 25 mM sulfuric acid.
2
40 nA
1
4
20
t
1 3
40
60
T I M E , min Figure 6. Chromatographicseparation and amperometric detection of furanic aldehydes at a mvRuO-RuCN CME. Sample mixture injected: 26 nmol of 5-HMF, 24 nmol of 2-F, and 26 nmol of 5-MF. Detection potential, +1.05 V; column temperature, 60 "C; back pressure, 520 psi; loop, 20 pL. Other experimental conditions were the same as those described in Figure 5.
io
I
20
30
40
-
50
T I M E, min Figure 5. Ion exclusion chromatographic separation with EC detection of aliphatic aldehydes at a mvRuO-RuCN CME. Detection potential, +1.08 V vs Ag/AgCI; loop, 50 pL. Sample solution injected: (1) 18 nmol of acetaldehyde, (2) 21 nmol of propionaldehyde, (3) 15 nmol of isobutyraldehyde, (4) 14 nmol of butyraldehyde, (5)10 nmol of valeraldehyde, and (6) 0.75 nmol of 2-F. Temperature, ambient; column back pressure, 814 psi. Other conditions as in Figure 4.
than CV, was used to characterize the electrochemical oxidation of simple aldehydes. The GC/mvRuO-RuCN film electrode in a thin-layer electrochemical cell maintained its catalytic stability under constant-potential operation. The repeatability of response, expressed as the relative standard deviation (RSD)of the peak height for a set of 20 consecutive injections using a test solution containing 20 nmol (2 mM x 10 pL) of acetaldehyde, was about f0.m at an applied potential of +1.08 V, indicating good shortterm stability. The examination of the peak profile in flow injection yielded a throughput of -40 injections/h at a flow rate of 1.0 mL min-l. Very recently, it was successfully demonstrated that the presence of low concentrations of the electroplating mixture in
the carrier streams provides a constant response to ethanol injections over an &h working time when the mvRuO-RuCN film on the GC electrode is continuously used as amperometric sensor.16 Consequently, also in this case, a carrier electrolyte of 25 mM HzS04 containing RuC13 and &RU(cN)6 at equimolar low levels (Le., 5 pM) was delivered through the electrochemicalcell. Figure 4 shows the hydrodynamic voltammograms (HDVs) obtained for acetaldehyde, propionaldehyde,butyraldehyde, and 5HMF by liquid chromatographic amperometric detection with the mvRuO-RuCN-modified electrode. The experiments were performed with an Aminex HE-87-H ion exclusion column in the H+ form using 10 mM HzS04 spiked with RuC13 and &RU(CN)s at equimolar concentrations (5 pM) as the mobile phase. After each change in applied potential, 5-15 min of stabilization time was allowed prior to injection of the sample. Each point in the hydrodynamicvoltammetric curves represents an average of two or three separate injections. The mvRuO-RuCN-modified eiectrode exhibited a peak-shaped HDV, with a maximum response at 1.07-1.08 V vs Ag/AgCl for all aliphatic and furanic aldehydes examined. The data were collected over a period of about 9 h with the flow rate set at 0.8 mWmin. These findings demonstrate that the mvRuO-RuCN-modified electrode is suitable for amperometric detection of aldehydic compounds. Moreover, the stability of response was greatly improved with the use of an appropriate mobile phase/supporting electrolyte. While the ion exclusion column imposes the use of dilute solutions of sulfuric Analytical Chemistry, Vol. 67, No. 20, October 15, 1995
3743
Table I. Quantitative Parameters of Aldehydic Compounds in IEC' with Amperometrlc Detection at a GC/ mvRuO-RuCN Film Electrode Evaluated at Different Temperatures i OlA) = a
compound
capacity factor (&')
bf
+ bc (mM)* f
LODd (nmol)
0.03 f 0.05 0.05 f 0.05 0.05 f 0.07 0.04 f 0.07 0.01 f 0.03 0.01 f 0.01
0.9983 0.9979 0.9973 0.9961 0.9982 0.9999
1.0 0.8 0.8 0.8 1.1 0.05
0.01 f 0.02 0.04 f 0.02 0.01 f 0.03 -0.00 f 0.04 0.00 f 0.01 -0.01 f 0.01
0.9998 0.9994 0.9994 0.9989 0.9996 0.9999
1.6 1.5 1.6 1.6
a f t9&
T = 22 f 2 "C
acetaldehyde propionaldehyde isobutyraldehyde butyraldehyde valeraldehyde 2-furaldehyde
1.55 2.18 2.98 3.39 5.80
9.20
0.20 f 0.01 0.25 f 0.01 0.35 f 0.03 0.32 f 0.03 0.24 f 0.02 6.91 f 0.06
acetaldehyde propionaldehyde isobutyraldehyde butyraldehyde valeraldehyde 2-furaldehyde
1.71 2.37 3.17 3.48 5.53 6.03
0.20 f 0.01 0.20 f 0.01 0.32 f 0.01 0.33 f 0.02 0.23 f 0.01 8.2 f 0.1
T=60fl"C
2.0 0.04
Column, Aminex HPX-87-H, 300 mm x 7.8 mm i.d.; eluent, 10 mM HzS04, 5 pM RuC13, and 5 pM &Ru(CN)6; flow rate, 0.8 mL min-'; loop, 50 pL; thin-layer electrochemicalcell with a GC/mvRuO-RuCN CME applied potential, +LO8 V as Ag/AgC1; the same deposit was used at room temperature and'at 60 "C. The mvRuO-RuCN film was grown electrochemicallyfor 15 min at +LO5 Vvs SCE in 25 mM H&04 solution containing 1 mM RuC13 and 1 mM &Ru(cN)6. Regression lines calculated by linear least-squares analysis using six or seven data points spaced over the linear range; t is taken at the 95%confidence level. Correlation coefficient. Limit of detection determined for S/N = 3 from the lowest injected
concentration.
acid as the mobile phase, the electrode stability of the rutheniumbased electrode was considerably enhanced in the presence of micromolar concentrations of the electroplating salts. It is noteworthy that low Ru(III) and RU(cN)s4- levels in the mobile phase did not influence the retention of compounds and, above all, column selectivity and lifetime. Indeed, no detrimentaleffects on the column's performance were observed even after intensive use lasting about 3 months. For example, no differences were observed in the retention times, capacity factors, and selectivity of aliphatic aldehydes during the operations with this mobile phase. The presence of Ruck do not affect the column's performance probably because, as mentioned earlier, the moderately acid media tend to form and stabilize ruthenium oxo species.23 A valuable technique for the separation of undissociated compounds such as weak acids, alcohols, aldehydes, ect. is through ionexclusion chromatography (IEC) using a cationexchange resin in the H+ form with an acidic e l ~ e n t . 2The ~~~~ application of the ruthenium-based electrode as an amperometric sensor in IEC with EC detection of aliphatic aldehydes is illustrated in Figure 5. The chromatogram was obtained for a sample mixture containing acetaldehyde,propionaldehyde, isobutyraldehyde, butyraldehyde, valeraldehyde, and Zfuraldehyde. Taking into account the HDVs, an operating potential of +1.08 V was applied. The capacity factors (k? and limits of detection GOD) are summarized in Table 1. Linear plots of peak current vs concentration were obtained for concentrations up to 5-10 mM. The LODs of aliphatic aldehydes, evaluated at room temperature as the concentration that yields a signal 3 times the background noise from the lowest injected concentration, range from 0.8 to 1.1nmol. The linear dynamic ranges, expressed as the ratio of the upper limits and the LODs, span 2-3 orders of magnitude. The correlation coefficients (r) of linear least-squares fitting of the calibration curves were in the range of 0.9973-0.9999. As (25) Pecina, R;Bonn, G.; Burtscher, E.; Bobleter, 0.1. Chromatogr. 1984,287, 245. (26) Fritz, J. S. J. Chromatogr. 1991,546, 111.
3744 Analytical Chemistry, Vol. 67,No. 20,October 15, 1995
may be seen from the chromatogram, the peaks are fairly symmetric in shape with an asymmetry value of 1.20, 1.15, and 1.22 estimated for isobutyraldehyde,valerialdehyde,and 2-furaldehyde, respectively. When 10 mM H3P04was used as mobile phase, a pronounced tailed response was observed for all compounds. Effect of Column Temperature. The effect of temperature on the column separation and amperometric detection was evaluated by performing determinations of aliphatic and furanic aldehydes at 60 "C. The retention times of furanic compounds are greatly influenced by column temperature; thus, the next step was to verify whether the mvRuO-RuCN-modified electrode may be influenced by higher temperatures compared to room temperature. A chromatographic separation, carried out at the column temperature of 60 "C for a sample mixture containing 5HMF, 2-F, and SMF, is shown in Figure 6. Increasing column temperature is accompanied by a noticeable decrease in retention times of furanic aldehydes, while aliphatic aldehydes are much less influenced by temperature changes (see Table 1). The capacity factors evaluated for SHMF, 2-F, and 5MF were respectively 5.2, 9.2, and 14.4 at room temperature and 3.9, 6.0, and 9.7 at 60 "C. Accordingly, the increase in temperature also has an impact on peak resolution by increasing the number of theoretical plates. For example, the number of plates ( S D ) evaluated at the two temperatures were 4900 f 300 and 7800 4~500, at 22 and 60 "C, respectively. These data compared very well with those reported by Pecina et A systematic investigation of the ruthenium oxidebased m o a e d electrode at 60 "C column temperature was made. Temperature does not have a significant effect on sensor response, as shown by the data in Table 1. The slopes of the calibration graphs were almost unchanged for aliphatic aldehydes, but the background noise increased by a factor of 3, Le., 5 nA compared to 1.8 nA at 22 "C. Therefore, the S/N ratio decreased, leading to slightly higher LODs for the aldehydes tested. It is worthwhile mentioning that the same deposit was used for both runs, at 22 and 60 OC, on two consecutive days. Finally, the stability of the
mvRuO-RuCN was also evaluated at 60 "C. A series of four repeated chromatographic injections of a furanic aldehyde mjxture (1.2 mM) yielded RSDs of 1.7%,1.5%,and 2.1%for 2-F, 5HMF, and 5MF, respectively. These data were collected during a 4 h time, with the last injection producing peak heights almost identical to those exhibited by the first injection. CONCLUSIONS The present study demonstrates that a chemically m o d ~ e d electrode based on the mvRuO-RuCN inorganic film is capable of electrocatalyticallyoxidizing aliphatic and furanic aldehydes in acidic media. This modified electrode surface can be used as an amperometric sensor in flow-through analysis of aldehydic compounds with excellent sensitivity. It should be emphasized that not only does this modified electrode provide a simple and sensitive sensor for aliphatic aldehydes, but also column temper-
ature can be employed to some extent for controlling the selectivity in ion exclusion chromatographic separations. The reliability and stability of mvRuO-RuCN films offer a valid alternative to W or fluorescence detection of aliphatic aldehydes, where derivahtion steps are needed. ACKNOWLEDQMENT Financial support from the Italian National Research Council (C.N.R, Rome) and Minister0 dell'UniversitA e della Ricerca Scientifica e Tecnologica (M.U.RS.T., Rome) is gratefully acknowledged. Received for review April 3, 1995. Accepted July 25, 1995.a AC950330F @Abstractpublished in Advance ACS Abstracts, September 1, 1995.
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