Anal. Chem. 1998, 70, 3355-3361
Articles
A Selective Electrochemical Gas Chromatography Detector Based on Axial Ligation Reactions of a Molten Iron Porphyrin Redox Polyether Hybrid Jeffrey W. Long† and Royce W. Murray*
Kenan Laboratories of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27599-3290
An iron tetraphenylporphyrin, with four appended, oligomeric (MW ) 550), poly(ethylene glycol) chains ([Fe(T550PP)Cl]), is an electroactive, room-temperature melt, and undergoes axial coordination with volatile bases, such as pyridines, when they are partitioned at a gas/melt interface into a film of the porphyrin. The Fe(III/II) reaction of the axially coordinated porphyrin can be used amperometrically as a means of detecting the pyridine vapor, as shown in plug flow experiments with open tubular columns. The mechanism of the ECGC detector response is analyzed. This detection scheme is minimally responsive or nonresponsive to gaseous analytes that do not coordinate to the Fe(III) state of the porphyrin. The detector’s selective response to pyridine is linear over a 500-fold range but is not yet very sensitive in comparison to the (nonselective) flame ionization detector. The ECGC detector has utility in detecting weak ligand interactions with the porphyrin (i.e., alcohols), and its response also detects slow dissociation rates of the Fe(II) complex formed in the detection reaction.
of polyether polymer electrolyte containing LiCF3SO3 and an electroactive solute coating a microdisk electrode. Gaseous analytes partitioning into the film plasticized it, altering the diffusion rate of the electroactive solute which was detected amperometrically. Films of hydrous gels offer11,12 another route to improved ECGC detectors. The previous9,10 polymer electrolyte ECGC detector contained no built-in chemistry aimed at selective responses to different analytes, other than that afforded by differences in partition coefficients (Kpart) of volatile analytes into the polyether film. The present report takes a step in the direction of designed selectivity by basing the amperometric detector response explicitly on axial coordination of a metalloporphyrin by the partitioned analyte/ ligand. This approach, preliminarily described in an earlier paper,13 has been limited by the general insolubility of metalloporphyrins in polyether polymer electrolytes. We use here a redox polyether hybrid consisting of an iron tetraphenylporphyrin with four appended, oligomeric, poly(ethylene glycol) chains ([Fe(T550PP)Cl]). In its undiluted state, the porphyrin polyether
Amperometric electrochemical detectors are common in liquid chromatography,1-5 offering advantages of simple design, high sensitivity, and selectivity by choice of applied potential. Applications of amperometric detectors to gas-phase analytes using detectors based on conducting ceramics are also common,6 but their use in conjunction with gas chromatography (GC) has been limited. In early electrochemical gas chromatographic (ECGC) detectors, the analytes in the eluting carrier gas were mixed into a liquid electrolyte solution bathing an electrochemical cell.7,8 Polymer electrolytes and microelectrode technology can simplify the cell design problem, as our laboratory showed9,10 with a film † Present address: Naval Research Laboratory, Code 6170, Surface Chemistry, 4555 Overlook Ave., Washington, DC 20375-5342. (1) Skoog, D. A. Principles of Instrumental Analysis; Saunders College Publ.: Philadelphia, 1985; p 797. (2) Knecht, L. A.; Guthrie, E. D.; Jorgenson, J. W. Anal. Chem. 1984, 56, 479. (3) St. Claire, R. L.; Jorgenson, J. W. J. Chromatogr. Sci. 1985, 23, 186. (4) White, J. G.; St. Claire, R. L.; Jorgenson, J. W. Anal. Chem. 1986, 58, 293. (5) Kissinger, P. T. Anal. Chem. 1977, 49, 447A. (6) Janata, J. Principles of Chemical Sensors; Plenum Press: New York, 1989; pp 209-213. (7) Chrzanowski, W.; Janicki, W.; Staszewski, R. J. Chromatogr. 1984, 292, 19. (8) Blurton, K. F.; Stetter, J. R. J. Chromatogr. 1978, 155, 35. (9) Parcher, J. F.; Barbour, C. J.; Murray, R. W. Anal. Chem. 1989, 61, 584. (10) Barbour, C. J.; Parcher, J. F.; Murray, R. W. Anal. Chem. 1991, 63, 604.
S0003-2700(98)00155-3 CCC: $15.00 Published on Web 07/03/1998
© 1998 American Chemical Society
Analytical Chemistry, Vol. 70, No. 16, August 15, 1998 3355
Figure 1. Cyclic voltammetry (100 mV/s) showing the Fe(III/II) reaction of a film of [Fe(T550PP)Cl] containing 0.4 M LiClO4 electrolyte, under a flow of dry N2 (solid line) and under a flow of 10% pyridine-saturated N2 (dashed line).
hybrid is (above 10 °C) a highly viscous melt in which LiClO4 electrolyte is soluble, giving an electroactive, ionically conducting melt which can be cast as a thin film onto a microdisk electrode assembly. Voltammetry of the undiluted [Fe(T550PP)Cl] melt phase is reported elsewhere.14,15 The observed effect of pyridine ligation on the molten porphyrin is parallel to that previously known16-18 for dilute solutions of iron tetraphenylporphyrin ([Fe(TPP)Cl]) in noncoordinating solvents and is illustrated by the cyclic voltammetry in Figure 1. As seen, partition of pyridine into the porphyrin melt causes a substantial positive shift of the potential of the Fe(III/II) reduction reaction, owing to the greater relative stability of the pyridine-coordinated Fe(II) form. Figure 1 is taken at room temperature so that Kpart for pyridine partitioning into the film is very large; this and the relatively large partial pressure of pyridine vapor in contact with the porphyrin melt means that enough pyridine partitions into the film to axially coordinate all of the 0.4 M iron porphyrin sites of the undiluted melt. At elevated temperatures and with smaller quantities of pyridine (and other gaseous analytes) as typical in GC, much smaller quantities will partition into the porphyrin melt film, but the qualitative effect remains: part of the [Fe(T550PP)Cl] becomes coordinated and its reduction potential shifts to a more positive value as in Figure 1. It is this partial state of complexation that is relied upon in the present study to detect pyridine and other putative analyte/ligands. The Fe(III/II) voltammetry is converted into a gas detector simply by inserting the microelectrode assembly with [Fe(T550PP)Cl] film into the GC column effluent, from which analytes partition into the film and, if active ligands for the Fe(III) state of (11) Cox, J. A.; Alber, K. S.; Tess, M. E.; Cummings, T. E.; Gorski, W. J. Electroanal. Chem. 1995, 396, 485. (12) Cox, J. A.; Alber, K. S. J. Electrochem. Soc. 1996, 143, L126. (13) Geng, L.; Reed, R. A.; Kim, M.-H.; Wooster, T.; Oliver, B. N. Egekeze, J.; Kennedy, R.; Jorgenson, J. W.; Parcher, J. F.; Murray, R. W. J. Am. Chem. Soc. 1989, 111, 1614. (14) Long, J. W.; Kim, I.-K.; Murray, R. W. J. Am. Chem. Soc. 1997, 119, 11510. (15) Long, J. W.; Murray, R. W., submitted for publication. (16) Kadish, K. M.; Bottomley, L. A. Inorg. Chem. 1980, 19, 832. (17) Bottomley, L. A.; Kadish, K. M. Inorg. Chem. 1981, 20, 1348. (18) Kadish, K. M.; Bottomley, L. A.; Berioz, D. Inorg. Chem. 1978, 17, 1124.
3356 Analytical Chemistry, Vol. 70, No. 16, August 15, 1998
Figure 2. Schematic illustrating the partition of an analyte/ligand, L, into a [Fe(T550PP)Cl] film from a bathing vapor, its diffusion, its subsequent ligation with the Fe(II) and Fe(III) states, and the diffusion of the complexes. The electrode potential E1 causes reduction of only the ligated form of the iron(III) porphyrin (see Figure 1).
[Fe(T550PP)Cl], shift the Fe(III/II) potential sufficiently positively such that reduction reaction and an amperometric response occurs at the applied microelectrode potential. The microdisk electrode potential E1 that is used is insufficiently negative to reduce the native [Fe(T550PP)Cl] melt but is sufficiently negative to reduce porphyrin ligated by ligands for which the Fe(II) form is relatively much more stable than the Fe(III) form. The factors in the detection process are more completely illustrated in Figure 2. The initial step is in partitioning of the analyte. The values of Kpart and the stability constant for Fe(III) complexation (KFeIII) jointly determine the observed amperometric current for reduction of the complexed porphyrin. The dynamics of the response depend on diffusion coefficients in the equilibration of analyte/ligand between the [Fe(T550PP)Cl] film and the carrier gas stream and on the kinetics of association and dissociation of the analyte/ligand. We will examine the detector response to a variety of analytes. The primary focus is a demonstration of the mechanism of detector response and of its consequent selectivity9 according to analyte ligation properties. The results are both a useful step toward designing gas detector selectivity based on chemical reactivity principles and a demonstration of the usefulness of the detector in the study of porphyrin axial ligation. EXPERIMENTAL SECTION Microelectrodes, Film Preparation, Gas Sensor Configuration. [Fe(T550PP)Cl] was synthesized as described earlier.14 Approximately 25-µm-thick films (based on the amount of porphyrin used) were cast from methanol solutions onto a microelectrode assembly containing a 25-µm-diameter Pt microdisk working electrode and the tips of 0.5-mm Ag and 24-gauge Pt wires, serving, respectively, as quasi-reference and counter electrode. The methanol was evaporated under a stream of N2, the microelectrode assembly with porphyrin film was placed in a
sealed glass cell, and the film was further dried under vacuum and at ∼70 °C for at least 24 h. The microelectrode assembly was later transferred to an electrochemical cell housing in the gas chromatograph. For all measurements, 0.4 M LiClO4 (which is 48:1 ether oxygen:Li ratio) was present as supporting electrolyte and was a component of the casting solution. The electrochemical cell housing in the GC instrument (Shimadzu GC-9A) was simply a Swagelock T-joint, housed within the GC oven, with the microelectrode assembly mounted in the “T”, bathed in the gas stream flowing through the top of the “T” from the GC column and then through an 8-cm-long, 0.22-cm-i.d. open tube to a flame ionization detector (FID). The column in the GC was either a 40-cm open tubular column (0.22-cm i.d.) or a 2-m column (Alltech, 0.22-cm i.d.) packed with poly(dimethylsiloxane) stationary phase. Electrochemical measurements were performed using a locally built system consisting of a highsensitivity potentiostat controlled by an IBM-compatible 486-25 through a Datel 412 12-bit A/D board. The FID signal was collected on a second A/D channel on the Datel board. GC Column with [Fe(T550PP)Cl] Stationary Phase. The partition coefficients of various analytes into the [Fe(T550PP)Cl] phase were estimated from their retention times on a packed GC column using the porphyrin melt as the liquid stationary phase. Column preparation was according to published procedures.19 Chromosorb G/AW-DMCS (6.13 g; Fluka) support and 40 mL of dichloromethane containing 0.244 g of dissolved [Fe(T550PP)Cl] were mixed in a two-neck flask. The mixture was swirled as a stream of Ar was passed over it to evaporate the solvent; after 4 h of drying the coated support was transferred to a fresh container and weighed. The weight percent loading of [Fe(T550PP)Cl] was 3.85%. A 0.8-m length of copper tubing (1/8-in. diameter) was packed, with shaking, with 5.73 g of this coated support, the column ends were sealed with glass wool plugs, and the column was preconditioned in the GC oven at 150 °C for 24 h under 40 mL/min He flow. Retention times for 0.05-µL injected samples of the analytes were detected by FID. The column void volume was determined by the homologous series method19 using injections of methanol, ethanol, propanol, and butanol. Small sample injection volumes were used to avoid overloading the column. RESULTS AND DISCUSSION Partition Coefficients for Various Analytes. As pointed out in Figure 2, the response of the amperometric porphyrin melt ECGC detector should depend intrinsically on the degree to which gaseous analyte/ligand partitions into the melt film. The partition coefficients Kpart were obtained by using [Fe(T550PP)Cl] as the liquid stationary phase of a GC column and measuring the retention times for the various analyte/ligands, based on the relationship
Kpart )
[L]film [L]gas
)
VGFsTC jf(tR - tM)Fs ) 273 w
(1)
where [L]film and [L]gas are, respectively, the concentration of analyte/ligand in the porphyrin film and the contacting gas phase, (19) Conder, J. R.; Young, C. L. Physicochemical Measurement by Gas Chromatography; Wiley: Chichester, 1979; p 595.
VG is the specific retention volume, FS is the stationary phase density (1.12 g/mL for [Fe(T550PP)Cl]), TC is the column temperature (kelvin), w is the mass of stationary phase in the column, and j, f, tR, and tM have their usual chromatographic meaning. The partitioning process presumably involves two processes: dissolution of analyte in the polyether portion of the porphyrin phase and its axial ligation with the iron porphyrin portion. Accordingly, Kpart can also be written
Kpart )
[L]film, free + [FeIII-L]
(2)
[L]gas
where [FeIII-L] is the concentration of coordinated iron(III) porphyrin. Assuming a constant film volume and a 1:1 stoichiometry for the ligation of [Fe(T550PP)Cl], this can be rewritten as
(
Kpart ) K°part 1 +
)
KFeIII[FeIII-Cl] [Cl-]
(3)
where K°part is the analyte partition coefficient in the absence of any ligation, KFeIII is the complex stability constant, and [Cl-] is chloride concentration displaced from the porphyrin (assuming that it is). In other work,15 at large concentrations of pyridine ligand/analyte in room-temperature iron porphyrin melts (as in Figure 1), the results suggested a 2:1 (diaxial) ligation stoichiometry. However, owing to the higher temperature (and much smaller Kpart) used in the GC experiments, the partitioned concentrations are quite dilute in comparison. Figure 3(---) shows examples of voltammetry of porphyrin melt films maintained at 80 °C and bathed in a stream of N2 that has been saturated, at 25 °C, with vapors of three pyridines. The split reduction waves in the upper two voltammograms show that insufficient analyte has partitioned to coordinate all of the porphyrin sites. Given this, and the fact that the amounts of pyridine injected in experiments described below give gas concentrations tiny in comparison to those in Figures 1 and 3, assuming a 1:1 ligation stoichiometry for the pyridines and other, weaker ligands is thought to be an acceptable simplification. Table 1 summarizes the obtained partition data parameters for pyridine and alcohol analytes. For the alcohols, Kpart increases systematically with boiling point, and the results are generally consistent with previous results20 for partitioning of alcohols into high-molecular-weight poly(ethylene oxide)/LiCF3SO3 stationary phases. The pyridines partition more strongly into the porphyrin melt, especially 4-methylpyridine, even though their boiling points are in the same range as the higher alcohols. Pyridines are potent ligands for iron porphyrins16-18 and their larger Kpart values can be understood in terms of the second right-hand term of eq 3. The impact of the KFeIII term is further illustrated by Kpart for 4-methylpyridine being 1.6-fold larger than that for 3-chloropyridine, even though the two pyridines have almost identical boiling points. 4-Methylpyridine is known18 to bind more strongly to iron (20) Barbour, C. J. Ph.D. Dissertation, University of North Carolina, 1987.
Analytical Chemistry, Vol. 70, No. 16, August 15, 1998
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Figure 4. Detector responses to 0.5-µL injection of pyridine onto open tubular column at 80 °C. ECGD detector potential is 0.0 V vs Ag quasi-reference. The signals from ECGD and FID detectors are normalized to their maximum values, for comparison, and the FID time axis is corrected to account for the difference in analyte arrival time. The actual peak current for the ECGD is 1.48 nA. The GC oven temperature is 80 °C, and the He carrier gas flow rate is 40 mL/min.
Figure 3. Voltammograms of a film of [Fe(T550PP)Cl] at 80 °C in a stream of dry N2 (solid line) and in a stream of N2 that had been saturated with vapor of the indicated analyte/ligand at 25 °C (dashed line). (A) Pyridine, 50 mV/s, film contains 1.2 M LiClO4; (B) 4-methylpyridine, film contains 0.4 M LiClO4; (C) 3-chloropyridine 20 mV/s, film contains 0.4M LiClO4. Table 1. Partition Coefficients in [Fe(T550PP)Cl] Melt GC Stationary Phase for Analytes at 80 °C analytea
VG (mL/g)
Kpartb
bpc (°C)
pyridine 4-methylpyridine 3-chloropyridine
990 1690 1060
1430 2440 1537
115 145 148
35 47 91 185 381
48 68 132 271 552
65 78 97 118 137
methanol ethanol propanol butanol pentanol c
a Injections 0.05-µL for each analyte. b Calculated using eq 1. Boiling point at 1 atm.
porphyrins than does 3-chloropyridine. In contrast, alcohols bind porphyrins relatively weakly.21 Detection of Pyridines. As outlined in the introduction, analytes capable of ligating with the iron(III) porphyrin and positively shifting its reduction potential can be detected amperometrically by applying a potential of 0.0 V which, according to Figure 3, is too positive to reduce [FeIII(T550PP)Cl] but is sufficiently negative to reduce ligated iron(III) porphyrin. Figure (21) Brault, D.; Rougee, M. Biochemistry 1974, 13, 4591.
3358 Analytical Chemistry, Vol. 70, No. 16, August 15, 1998
4 gives an example of the porphyrin ECGC detector amperometrically detecting a pyridine vapor plug injected through an open tubular column (OTC; see Experimental Section). The good correlation between the ECGC and FID detector (normalized) responses is obvious. The FID response is slightly broader on the trailing edge of the peak, which may at least in part be due to the longer time for longitudinal diffusion at the FID detector position or to poor flow hydrodynamics in the electrochemical T-cell. Importantly, the amperometric and FID peaks match nearly exactly on their leading edges, indicating that equilibration of pyridine vapor with the [Fe(T550PP)Cl] film is relatively rapid on this time scale. Figure 5A compares amperometric responses for the three pyridine analyte/ligands. Their responses are similar but different in magnitude, which is analyzed later. Detection of Alcohols. Relatively little is known about the ligation of iron porphyrins by alcohols. One study has reported spectroscopic evidence for alcohol ligation with an iron porphyrin.21 Figure 5B shows amperometric responses of the ECGC detector to vapor plugs of two lower alcohols injected through an OTC. Ligation to [FeIII(T550PP)Cl] is strongly suggested by the appearance of the well-defined current peaks. While the response is >10-fold smaller than for the pyridines (Figure 5A), detection of weak ligation in this manner should be reliably sensitive (relative to voltammetry), for the same reasons that amperometric detection in flow injection analysis has higher concentration sensitivity than potential-sweep voltammetries. Detection of Other Ligands. Dimethyl sulfoxide (DMSO) and dimethylformamide (DMF) are good solvents for metalloporphyrins, which is suggestive of ligating action, which is consistent with voltammetric properties of iron porphyrins in these solvents.17 The amperometric ECGC response to DMF is shown in Figure 5C; that for DMSO is qualitatively similar. The severe peak tailing may reflect the low volatility of these analytes at 80 °C (the boiling points for DMSO and DMF are 189 and 153 °C, respectively). The source of the dip in current on the leading edge of the peak is suspected to be a double-layer charging effect arising from adsorption of the DMF analyte at the porphyrin/electrode
Table 2. Summary of Results for the Electrochemical Detection of Analyte/Ligands (for 0.5-µL Injections at 80 °C with E ) 0.0 V vs Ag QRE) analyte/ligand
Ipeaka
Qpeakb
Qpeak/ MXc
Qpeak/ MXKpartd
pyridine 4-methylpyridine 3-chloropyridine methanol ethanol pentanol dimethyl sulfoxide dimethylformamide
1.5 × 10-9 1.75 × 10-9 2.2 × 10-10 1.8 × 10-11 2.6 × 10-11 6.4 × 10-11 9.6 × 10-11 2.8 × 10-11
4.8 × 10-9 1.1 × 10-8 2.1 × 10-9 3.5 × 10-11 5.0 × 10-11 2.45 × 10-10 2.6 × 10-9
7.8 × 10-4 2.2 × 10-3 3.9 × 10-4 2.9 × 10-6 5.9 × 10-6 1.1 × 10-5 3.7 × 10-4
5.4 × 10-7 9.4 × 10-7 2.5 × 10-7 5.4 × 10-8 8.7 × 10-8 2.0 × 10-8
a Peak current, in amperes. b Peak integral, in coulombs. The unusual peak shape for dimethylformamide precluded an accurate measurement. c MX, moles injected. d Kpart, gas chromatographically measured. Values unavailable for dimethyl sulfoxide and dimethylformamide.
ligated porphyrin, [FeIIIL], generated by analyte partitioning, i.e., Figure 5. ECGD responses (oven and detector temperature 80 °C, 0.0 V electrode potential, 40 mL/min He flow rate) to various analytes: (A) 0.5-µL injections of pyridine, 4-methylpyridine, and 3-chloropyridine; (B) 1-µL injections of methanol and ethanol; (C) 0.5µL injection of dimethylformamide; and (D) 0.5-µL injections of cyclohexane and dimethoxyethane.
interface and a possibly associated shift in potential of zero charge. The quantity of charge passed in the current dip is consistent with this interpretation, which would require only a small negative shift in EPZC for the anodic dip to occur. Signals for Noncoordinating Analytes. Analytes that are classically “noncoordinating”, such as cyclohexane and dimethoxyethane, typically produced current responses (Figure 5D, note the current scale) near the signal/noise limit. While sometimes strangely shaped, these responses are very small in comparison to those for known analyte/ligands. Similar signals are observed for such analytes as chloroform, tetrahydrofuran, benzene, ethyl acetate, and dioxane. Some or all of these responses are probably double-layer charging effects, but this was not investigated further. Comparison of ECGC Responses to Various Analytes. The amperometric responses for the ligating analytes tested are given in Table 2 in terms of peak current, Ipeak, and the total charge under the amperometric peak, Qpeak, for 0.5-µL injections of each analyte. The peak charge Qpeak was normalized for the number of moles injected (i.e., Qpeak/MX) and for the amount of analyte partitioning into the porphyrin melt film (i.e., Qpeak/MXKpart). The latter responses are, for the pyridines, ∼1 order of magnitude larger than for the alcohols, and for both classes of analytes there are ∼4-fold differences within each class. If the ECGC amperometric response were determined simply by the extent of partitioning alone (i.e., is entirely a diffusion/plasticization effect like that earlier reported9,10), the Qpeak/MXKpart factor would be expected to be roughly the same for all compounds. The existence of differences among compounds reflects how the ECGC response depends on the extent of analyte/ligation of the iron porphyrin, for which we give an approximate analysis next. Following the scheme of Figure 2, the amperometric ECGC currents should be proportional to the concentration of analyte/
I ) Z[FeIIIL]
(4)
where the proportionality constant Z contains such parameters as the electrode dimension, porphyrin diffusion coefficient, and whether the applied electrode potential is entirely on the plateau of the porphyrin reduction wave or not. The [FeIIIL] concentration can be expressed by combining eqs 2 and 3 and substituting into eq 4
KFeIII[FeIIICl]Kpart[L]gas I)Z [Cl ](1 - KFeIII[FeIIICl])
(5)
which defines the current for each concentration [L]gas of analyte/ ligand in the gas phase, assuming rapid equilibration between the bathing gas and the [Fe(T550PP)Cl] film. Integration of eq 5 over the sample peak and rearrangement gives
Qpeak KFeIII[FeIIICl] ) Z′ Mx Kpart [Cl ](1 + K [FeIIICl])
(6)
FeIII
This equation shows that if the ligated porphyrin stability constant were sufficiently large, so that KFeIII[FeIIICl] . 1, all analyte/ligands would yield the same amperometric response. In fact, the opposite is true, since (for small sample sizes) [FeIIICl]∼0.4 M and (at 25 °C)15 KFeIII ∼0.03, so relative values of the parameter Qpeak/MXKpart, such as those in Table 2, should approximately describe the relative coordinating abilities of each analyte for ligation of the iron(III) porphyrin. Previous electrochemical studies of dilute solutions of iron porphyrins have shown that, among pyridine derivatives, the tendency for porphyrin ligation is related to the pyridine pKA.16 More basic pyridines complex the iron(III) porphyrin more strongly. These observations are consistent with those in Table 2, where the normalized Qpeak/MXKpart responses decrease from 4-methylpyridine (pKA ) 5.98) to pyridine (pKA ) 5.28) to 3-chloropyridine (pKA ) 2.81). That the normalized Qpeak/MXKpart responses for the alcohols in Table 2 are 1 order of magnitude Analytical Chemistry, Vol. 70, No. 16, August 15, 1998
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Table 3. Temperature Dependence of ECGC Signals temp ( °C)
Qpeak (×109)a
Kpartb
Qpeak/Kpart (×1012)
70 80 90 100 110
6.2 4.9 3.4 2.6 1.8
1880 1430 1022 790 525
3.3 3.4 3.3 3.3 3.4
a Peak integral in coulombs for detection of 0.5 µL of pyridine with E1 ) 0.0 V. b Determined from eq 1 as in Table 1.
Figure 6. Comparison of normalized voltammetric and ECGD measurements of the ligation of [Fe(T550PP)Cl] by (A) pyridine, (B) 4-methylpyridine, and (C) 3-chloropyridine. The solid lines represent the voltammetric wave (20 mV/s) for the reduction of the coordinated form of the porphyrin with the [Fe(T550PP)Cl] film equilibrated under vapor flows of the respective ligands (see Figure 5). Normalized ECGD signals for the detection (0.5-µL injections) of the respective pyridines are shown in terms of peak current, Ipeak (b), and peak integrals, Qpeak (O). All signals are normalized to their maximum values. In each case, the [Fe(T550PP)Cl] film is maintained at 80 °C.
smaller than for the pyridines is also consistent with the relatively weak interactions of alcohols with porphyrins as reported before.21 Effect of ECGC Detector Electrode Potential. The electrode potential is a crucial parameter in determining the magnitude of the amperometric responses. In Figure 1, the operating potential (vertical dashed line) is set so that ligated porphyrin is reduced at a diffusion-controlled rate. A shift to a more positive electrode potential would presumably drive the porphyrin reduction at less than diffusion-controlled rates and yield smaller amperometric responses and, at sufficiently positive values, none at all. Figure 6 presents the potential dependence of ECGC peak currents Ipeak (b) and of the amperometric peak integrals Qpeak (O), both normalized to their maximum values, for detection of 0.5-µL injections of the three indicated pyridines. It is evident that the amperometric responses vanish at more positive detector potentials; at +0.3 V only small background perturbations are 3360 Analytical Chemistry, Vol. 70, No. 16, August 15, 1998
observed. For all three pyridines, the responses increase sharply as potentials more negative than +0.10 V are used. By reference to Figures 1 and 3, the native [Fe(T550PP)Cl] melt is not reduced at such positive potentials. The potential dependencies of both peak current and charge for each pyridine are also very similar to those (‚‚‚) of the cyclic voltammograms in Figure 3 taken for porphyrin films equilibrated with their vapors at the same temperature. Over the investigated potential range, just as in the voltammograms of Figure 3, the current and charge potential dependencies for pyridine achieve a plateau, that for 4-methylpyridine nearly does so, and that for 3-chloropyridine displays no evident plateau. These observations provide compelling evidence that reductions of ligated porphyrin are indeed responsible for the amperometric responses of the ECGC detector, as presented in the scheme of Figure 2. ECGC Detector Dependency on GC Oven Temperature and Sample Size. The effect of temperature on the amperometric ECGC responses was examined since the detector was housed within the GC oven. (In practical GC operation, the oven temperature would be varied.) Peak charges Qpeak for a fixed sample size of pyridine were measured from 70 to 110 °C using the OTC column, as were partition coefficients Kpart for pyridine into the porphyrin melt (using eq 1 as described above). Table 3 shows that the ratio of Qpeak and Kpart is constant over the temperature range studied. This means that the experimental temperature dependencies of coordination of [Fe(T550PP)Cl] by pyridine and how much pyridine partitions into the porphyrin film are both adequately represented by variations in Kpart (eq 3). While the amperometric ECGC detector temperature dependency can be explained as above, its sensitivity to temperature would be awkward in, for example, a programmed-temperature GC experiment. In any such use, the detector should be housed in a separate constant-temperature enclosure. Other important gas detector response characteristics are sensitivity and response linearity. These were investigated for the amperometric ECGD detector (at 0.0 V and 80 °C) for varied pyridine sample volumes, using in this case a 2-m column containing poly(dimethylsiloxane) as stationary phase. Pyridine volumes smaller than 0.5 µL were injected by dilution with ethyl acetate; the two were easily resolved by the column. Excellent linearity of Qpeak versus sample size (R2 ) 0.9984) was found for pyridine injection volumes ranging from 0.010 to 5 µL. Under these conditions the limit of detection for pyridine is 0.01 µL (∼10 µg). This is not a very good detection limit compared to that of the FID detector (∼10-4 µg), but the tradeoff is the advantage of selectivity afforded by the axial ligation chemistry of the porphyrin melt.
Figure 7. ECGD and FID responses to eight-component mixture eluting from a 2-m column (i.d. ) 0.22 cm) with poly(dimethylsiloxane) stationary phase. The electrode potential for the ECGD is 0.0 V, its temperature is 80 °C, and carrier gas flow rate is 40 mL/min.
There are, however, potential avenues to increase sensitivity of the ECGC detector; major differences in electrochemical response according to technique employed are well-known. Examples under consideration include the use of microband electrodes (which can give much larger current responses than microdisks),22 ac modulation of the electrode potential, and holding the electrochemical detector at a somewhat lower temperature to enhance Kpart. Also, the axial ligation chemistry of metalloporphyrins is a rich subject, and possibilities exist for axial ligation to turn on (or off) electrocatalytic processes that exhibit, by their nature, magnified current responses. Illustrated Selectivity of Electrochemical GC Detector. A simple gas chromatographic separation was carried out on a 2-m poly(dimethylsiloxane) column, to illustrate the selectivity of response of the amperometric ECGC detector. Figure 7 shows a chromatogram of a mixture of ligating and nonligating analytes. With the exception of a small response to methanol, no response is seen from the putatively nonligating analytes at the current sensitivity shown. The relative sizes of the amperometric response to the three pyridines are qualitatively in the proportions expected from Figure 5A. (22) Porat, Z.; Crooker, J. C.; Zhang, Y.; Le Mest, Y.; Murray, R. W. Anal. Chem. 1997, 69, 5073-5081.
Figure 7 shows an additional feature of the ECGC detector behavior, a dip on the trailing edges of the pyridine peaks, below the current baseline to an oxidation current that then slowly decays away. This feature was not evident in the preceding experiments, which used a short OTC column. The small peak widths in the latter resulted in exposing the porphyrin film to the gaseous analytes for much shorter periods of time. In Figure 7, the peaks are broader, and consequently, during the peak passage over the porphyrin film, pyridine-coordinated [FeIII(T550PP)Cl] is formed and undergoes reduction for a longer period of time, building up a more substantial layer of reduction product [FeII(T550PP)Py] around the microdisk electrode. This layer of reduced complex persists even though the main pyridine vapor peak has passed by the detector, because the Fe(II) complex only very slowly dissociates its coordinated pyridine. The result is an oxidation current that only slowly decays, as the diffusion layer of [FeII(T550PP)Py] becomes oxidized by slow dissociation of pyridine and subsequent oxidation of [FeII(T550PP)]. That is, the oxidation currents following the peaks for the three pyridines in Figure 7 all reflect slow ligand dissociation rates of analyte/ligand from the Fe(II) form of the porphyrin. This characteristic is more fully explored elsewhere.15 The sensitivity of the ECGC detector response to the dynamics of ligand dissociation is Figure 7 is an undesirable aspect of the ECGC detector behavior as configured in these experiments. The oxidation current dip following an amperometric peak distorts measuring the charge under it and under later-eluting peaks, such as the 3-chloropyridine case. Obviously any scheme that builds in strong chemical selectivity of binding in a selective detector carries with it the potential difficulty of irreversible or slow release of binding of analyte. However, the irreversible effect (the oxidation current) in Figure 7 involves the product of the detection reaction (i.e., [FeII(T550PP)Py] and [FeII(T550PP)]), rather than the initially detected ligated porphyrin, which is the more labile Fe(III) state. It may be possible by altering the electrochemical mode of observation of formation of coordinated iron porphyrin to avoid or minimize this secondary response, and such schemes will be considered in further ECGC detector experiments. ACKNOWLEDGMENT This research was supported in part by grants from the Department of Energy Division of Basic Sciences and the National Science Foundation.
Received for review February 10, 1998. Accepted May 27, 1998. AC980155D
Analytical Chemistry, Vol. 70, No. 16, August 15, 1998
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