756
Anal. Chem. 1986, 58,756-758
Direct Determination of Tetraethyllead and Tetramethyllead in Gasoline by High-Performance Liquid Chromatography with Electrochemical Detection at Mercury Electrodes A. M. Bond* and N. M. McLachlan
Division
of Chemical and Physical Sciences, Deakin University, Waurn Ponds, Victoria 321 7, Australia
Tetramethyllead and tetraethyllead are highly toxic compounds used as antiknock additivesingasoline. I n this work, direct InJectionof 1- to 1O-gL samples of aviation and motor gasothes onto a C,, reverse-phase HPLC column followed by electrochemical detection at a mercury electrode with acetonitrile (0.05 M tetraethylammonium Perchlorate) as the eluent provides a specific and Sensitive method for determlnatlon of tetraalkyllead compounds via direct calibration. The redox process at mercury electrodes employed for detection involves oxidatlon of the electrode and exchange of alkyl groups. This unique reaction occurs at much lower potentlais than the direct oxidation at solid electrodes. The complex matrix In gasolines precludes the determination of tetraaikyliead compounds using oxidation at a glassy carbon or other Inert solid electrode detectors.
Tetramethyllead (TML) and tetraethyllead (TEL) are widely used antiknock additives in motor and aviation gasoline, although recently in some countries their use for this purpose is being phased out because of their high toxicity. These compounds are now generally regarded as an environmental hazard whose concentration must be closely monitored (1). Most methods for the determination of lead in gasoline involve destruction of, or removal from, the matrix and estimation of btal lead present (2,3).For example, the standard separation method for tetraalkyllead compounds in gasoline involves distillation followed by atomic absorption spectrometry (3). Recently, Messman et al. (4) have shown that different forms of tetravalent lead may be separated by highperformance liquid chromatography (HPLC) and detected by flame atomic absorption spectrometry. Atomic absorption spectrometry, while sensitive, is not a specific detection method enabling independent identification of the different forms of lead. On the other hand, electrochemical methods should have this desirable feature, since the detection potential can be varied as part of the identification procedure. Oxidative electrochemical responses have been reported for tetraalkyllead compounds at Pt electrodes in acetonitrile (5). The electrochemical process was irreversible, and under conditions of linear sweep voltammetry the peak potential for tetramethyllead (+1.80 V) was well separated from tetraethyllead (+1.26 V). At mercury electrodes oxidation processes are also observed in dichloromethane (6,7).The electrode mechanisms for oxidation at mercury specifically involve the electrode and occur a t significantly less positive potentials than at platinum electrodes (6, 7). In line with the above, HPLC coupled with electrochemical detection at both solid and mercury electrodes has been investigated in detail in determine whether a direct injection method can be developed that is simpler, less expensive, and more specific for alkyllead compounds in gasoline than atomic absorption spectrometric detection. Application to a range
of aviation and motor gasolines has been considered. EXPERIMENTAL SECTION Reagents and Chemicals. The acetonitrile used in all experiments was HPLC grade (Mallinkrodt) while tetraethylammonium perchlorate (Et4NC104)used as the electrolyte was electrochemical grade and used as supplied by Southwestern Analytical Chemicals. Tetraethyllead (TEL) (98%)was obtained from the Alfa Chemical Co. Standard solutions of tetraalkyllead antiknock fluid (manufactwed by Octel Chemical Co.) in isooctane and all gasoline samples were kindly donated by the Shell Refinery Co., Victoria, Australia. Tetramethyllead (TML) was obtained from various sources as an 80% solution in toluene. Instrumentation. A. Electrochemical Cells. 1. Static Cell. All voltammetric and polarographic experiments in a static cell were performed with an EG&G Princeton Applied Research Model 174 polarographic analyzer at 20 "C in acetonitrile (0.1 M Et4NC104). For polarographic experiments, the working electrode was a conventional dropping mercury electrode. The reference electrode was an Ag/AgCl (saturated LiCl, CH2C12)electrode, separated from the test solution via a salt bridge containing 0.1 M Et4NC104 in acetonitrile. The reversible half-wave potential for oxidation of M ferrocene is +0.38 V vs. this reference electrode in acetonitrile (0.1 M Et4NC104). Platinum wire was used as the auxiliary electrode. 2. Flow-Through Cells. The polarographic detector coupled to the HPLC system described below was an EG&G Model 310 static mercury drop electrode (SMDE) operating in the hanging mercury drop mode with a medium size drop. A new drop was formed prior to each injection and it is recommended that the time between forming a new drop and injection remain constant. The receiving solution was either 0.1 M Et4NC104or 0.1 M NaN03 in distilled water. Unless otherwise specified, the eluent flow rate was 1.0 mL/min. The reference electrode was an aqueous Ag/ AgCl (3 M KCl) electrode and the auxiliary electrode was again platinum wire. For voltammetric detection at a solid electrode, a glassy carbon disk working electrode with an aqueous Ag/AgCl (3 M KC1) reference electrode and a platinum disk auxiliary electrode were used in conjunction with a Metrohm EA 1096 flow-through detector cell. For both polarographic and voltammetric detection in flowthrough cells, constant dc potentialswere applied with a Metrohm VA-Detector E611 potentiostat. B. Chromatography. A Waters M 6000 pump and U6K injector formed the basis of the chromatographic instrumentation. The separation of the tetraalkyllead compounds was achieved on a 15 cm by 3.9 mm i.d. Waters Nova-PAK 5-wm reverse-phase CI8 column used in conjunction with a homemade 2-cm reverse-phase CISguard column. The eluent was acetonitrile containing 0.05 M Et4NC104.In the present work, experiments were conducted in completely nonaqueous media, and excellent separation of TML and TEL was readily achieved. If a wide range of alkyl lead species are present, then H20 (pH 4.0, acetic acid) can be added, as described by Messman and Rains (4). C. Mass Spectrometry. Identification of tetraalkyllead compounds in the column fractions which generated electrochemical responses was undertaken by mass spectrometry using a Finnigan 3000 positive ion mass spectrometer. The mass spectra
0003-2700/86/0358-0756$01.50/00 1986 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 58, NO. 4, APRIL 1986
lTEL
POTENTIAL (VOLTS) 0.60
040
0.20
757
000 TML
I t .
.
.
,
,
,
I
.
,
.
.
.
, -
0 10 0 10 0 TIME/min. Flgure 2. Chromatograms with electrochemical detection (+0.63 V) at a hanglng mercury drop electrode for 5-pL injections of different gasoline samples using acetonitrile (0.05 M Et4NCI0,) as eluent: (a) premium motor gasoline containlng 0.30 g/L Pb (1.5 pg) as TEL, (b) aviation gasoline containing 0.55 g/L Pb (2.7 pg) as TEL, and (c) premium motor gasoline containing 0.84 g/L Pb (4.2 pg) as TEL and 0.32 g/L Pb (1.6 pg) as TML. 10
M tetraFlgure 1. Dc polarograms (drop time 0.5 s) for 3 X methyllead and 5 X M tetraethyllead in acetonitrile (0.1 M Et4NCI0,) at 20 OC.
were compared with authentic samples of tetraalkyllead.
RESULTS AND DISCUSSION
A. Detection at Mercury Electrodes. Acetonitrile is an excellent solvent for both chromatography and electrochemistry. However, no data are available for polarography of alkyllead compounds in acetonitrile. Figure 1shows dc POlarograms for TEL and TML in a static cell using a standard dropping mercury electrode. The limiting currents for each process are extremely well-defined. The second oxidation process for TML is attributable to the formation of dimethylmercury. The polarograms are similar t~ those reported in dichloromethane (7) and the mechanisms therefore are assumed to be the same, i.e. 2R4Pb
+ 2Hg
-
2R3Pb.
2R3Pb.
-
+ 2RHg' + 2e-
R6Pb2,etc.
(1)
TML
+200
1.60
1.20
0.80
0.40
POTENTIAL(VOLTS)
(2)
MacCrehan and Durst (8)developed HPLC techniques with electrochemical detection of alkylmetal cations using the reduction processes for these compounds and found that the development of large background currents due to the presence of oxygen is a constant difficulty. Under chromatographic conditions, the use of a dc potential of +0.63 V for oxidative amperometric detection is ideal since the presence of oxygen will not interfere as would be the case if using reduction processes. The major concern with using amperometric detection at positive potentials is possible effects arising from the chemically complex matrix of gasoline. Figure 2 shows chromatograms for three different aviation and motor gasoline samples containing TEL injected directly onto a reverse-phase CIS column with acetonitrile (0.05 M Et4NC104)eluent. Included in Figure 2c is the response for gasoline spiked with TML. Excellent separation of the tetraalkyllead compounds is observed with reverse-phase CIS chromatography as reported by Messman (4).Despite the presence of numerous chemicals in gasoline, no other peaks are observed in the chromatograms. The mercury electrode acts as a very specific detector for alkyllead compounds. Collections of the fractions containing the electroactive compounds were confirmed to contain TML and TEL by mass spectral analysis. Calibration curves prepared with standard compounds in isooctane were linear over the concentration range of 5 x 10-7 M to 5 X M for a 10-pL injection (2.4 ng to 24 pg) and passsed through the origin. Reference of gasoline samples to a calibration curve prepared in the above manner gave data
Flgure 3. Oxidative cyclic voltammograms (scan rate 100 mV s-') on a glassy carbon electrode for 1.2 X M tetramethyllead and 1.7 X M tetraethyllead in acetonitrile (0.1 M Et4NCI04)at 20 OC.
typically 10% too low by comparison to the manufacturer's specifications or to the method of standard addition. This indicates that the gasoline matrix slightly suppresses the lead response relative to isooctane. Calibration curves prepared in lead-free gasoline eliminate the matrix problems and give rise to reliable data. Calibration curves prepared in this manner for the direct determination of TEL in different aviation and motor gasoline grades were all found to be linear with concentration and had the same slope. Experiments with deliberately spiked samples show 100% recovery of TML and TEL. The direct determination of tetraalkyllead compounds by HPLC with detection at mercury electrodes proved to be extremely simple. Precision was found to be *3%, which is essentially the precision of manual injection. Determinations also agree with the manufacturers specified values to within this precision. The limit of detection for TML in all classes of gasoline examined is about 2 mg/L (6 X lo4 M) for 10 pL (20 ng) injections and a signal to noise ratio of 2:l (faradaic current to background ratio, peak to peak, at the time of elution). The limit of detection for TEL is slightly lower than for TML because of greater base line stability. Nominally lead-free motor gasoline may contain tetraalkyllead compounds. The method proposed in this work is sufficiently sensitive to certify gasoline as lead-free at the legislated levels (e.g., currently less than 12 mg/L in Australia). U.S.A. levels
758
Anal. Chem. 1986, 58, 758-761
9.0
3.0
6.0
0.0
TIME (MIN.)
Flgure 4. Chromatogram of a l-kL injection of motor gasoline with electrochemical detection (+ 1-90 V) at a glassy carbon electrode using 90: 10 acetonitrile-water (0.05 M Et,NCIO,) as the eluent.
have recently been summarized in a report in ref 9. B. Detection at Glassy Carbon Electrodes. Figure 3 shows cyclic voltammograms for TEL and TML at a glassy carbon electrode in a static cell containing acetonitrile (0.1 M Et4NC104). The oxidation processes are well separated in acetonitrile. Dc amperometric detection at +1.40 V should be specific for TEL while detection at +1.60 V should be suitable for both TEL and TML when coupled to HPLC. Kochi et al. proposed the following general mechanism for tetravalent alkylmetal compounds (5):
R4M RdM+ R.
-
--
R4M++ e-
(3)
RSM+ + Re
(4)
R+ + e-
(5)
In this work, R3M+was identified as a product of the oxidation process using cyclic voltammetry and scanning immediately after oxidation to negative potential regions and noting a reduction peak consistent with that expected for trialkyllead compounds (approximately -1.2 V for R,Pb+).
While the electrochemical resolution is excellent and chromatograms for standard mixtures are well-defined, chromatograms for gasoline samples using acetonitrile as the eluent contain numerous peaks (almost one broad band) that obscure the analytical responses expected for tetraalkyllead compounds. Figure 4 demonstrates that numerous peaks are observed in chromatograms of lead-free gasoline even when water is added to improve the resolution of gasoline constituents. Under the same conditions no background responses are observed at a mercury electrode. Further, the need to apply very positive potentials has deleterious effects on the long-term stability of the glassy carbon electrode and no analytically usable responses are observed for TEL or TML in gasoline samples where well-defined responses are found with the mercury electrode as a detector. There are many constituents of gasoline containing organic functional groups that can be oxidized at glassy carbon electrodes at the positive potentials required for the oxidation of tetraalkyllead compounds. The much lower positive potentials required at mercury electrodes and the highly specific nature of the electrode response involving direct interaction with mercury alleviate this problem, clearly making this the electrode material of choice. Registry No. TEL, 78-00-2; TML, 75-74-1.
LITERATURE CITED (1) Grandjean. P.; Nielsen, T. Resldue Rev. I97g, 72, 97-146. (2) Frigerio, I. J.; McCormick, M. J.; Symons, R. K. Anal. Chlm. Acta 1982, 143, 261-264, and references cited therein. (3) ASTM, Standards, Part 24, D1949, "Separation of Tetraethyllead and Tetramethyllead In Gasollne".
(4) Messman, J. D.; Rains, T. C. Anal. Chem. 1981, 5 3 , 1632-1636. (5) Kochi, J. K.; Kllnger, R. J. J . Am. Chem. Soc. 1980, 102, 4790-4796, and references cited therein. (6) Bond, A. M.; McLachlan, N. M. J . Electroanal. Chem. 1985, 182, 367-382. (7) Bond, A. M.; McLachlan, N. M. J. Electroanal. Chem. 1985, 194, 37-48. (8) MacCrehan, W. A. Anal. Chem. 1981, 53, 74-77, and references clted therein.
(9) Anderson, E. Chem. Eng. News 1985, April 8, 17-18.
RECEIVED for review August 26, 1985. Accepted October 22, 1985.
Pump/Probe Thermal Lens Spectrometry with Oppositely Propagated Beams for Liquid Chromatography Yen Yang,* Steven C. Hall, and Marijo S. De La Cruz Department of Chemistry, Loyola University of Chicago, Chicago, Illinois 60626 A new slngle-laser/dual-beam thermal lens photometer Is developed for liquid chromatographic detection. The pump beam propagates In a dlrectlon opposite to the probe beam. By use of appropriate beam splltters, this optical scheme enables effective spatial separation of the pump/probe without uslng a relectlon optic or losing the beam colllnearity. Wlth a 10-mW He-Cd laser operatlng at 442 nm, the system Is shown to have a noise level of 3 X lo-' absorbance root mean square with a 1-s time constant and a 1 mL/mln mobile-phase flow rate. The signal-to-noise ratlo Is also demonstrated to be superior to that of other single-laser/lock-in detection systems.
Since the advent of thermal lens spectrometry, there have 0003-2700/86/0358-0758$01.50/0
been significant advances in developing it into a practical analytical technique (1-16). A recent development is in the area of new and simple experimental arrangements. Two distinct CW laser-based approaches have evolved, both of which are based on the utilization of a single laser and lock-in detection. The first approach involves a single-beam technique in which the thermal lens strength is extracted from the optical signals through lock-in detection referenced at the second harmonic modulation frequency ( I 7). The second approach is a dual-beam method in which both the pump and probe beams are derived from the same laser source. Separation of the two beams is accomplished by taking advantage of their differences in polarization (18,19). Both arrangements appear to be experimentally simple and provide a fast, real-time response that is well-suited for liquid chromatographic applications. 0 1986 American Chemical Society
