Anal. Chem. 1985, 57, 2743-2745 2743

(11) Codding, E. G.; Ingle, J. D., Jr.; Stratton, A. J. Anal. Chem. 1980, 52, 2133. (14) Ingle, J. D., Jr. Notes on Basics on Spectrometric Basics"; Oregon

(12) Grabau, F.; Talmi, Y. "Multichannel Image Detectors"; Y. Talmi, Ed.; American Chemical Society: Washington, DC, 1983; Vol. 2, Chapter 4.

(13) McGeorge, S. W. Ph.D. Thesis, McGill University, Montreal, Quebec,

1985; Chapter 2,;

State University: Corvallis, OR, 1979; Chapter 4.

Received for review May 3, 1985. Accepted June 17, 1985.

Spinning Sample Cup and Adjustable Angle Jet Flame for Molecular Emission Cavity Analysis

S t a n Van Wagenen a n d Qu in tus Fernando*

Department of Chemistry, University of Arizona, Tucson, Arizona 85721

Applications of the hydrogen diffusion flame in molecular emission cavity analysis, MECA, have been published ex- tensively by Belcher and co-workers ( 1 ) and subsequently by Townshend and co-workers (2). In MECA, the emission from an analyte is generated and confined in a small cavity that is positioned,appropriately in a cool hydrogen diffusion flame. The intensity of the emission is monitored as a function of time with the aid of a flame emission spectrophotometer. The concentration of the analyte species that results in the emission is obtained from the peak height or peak area of the emission vs. time plots, and the molecular nature of the analyte species is very often deduced from the time at which the maximum emission intensity is observed. This versatile analytical technique has proved to be useful for a large number of analytical determinations (2) .

The determination of sulfur-containing compounds by the MECA technique has attracted considerable attention espe- cially since it is possible to determine mixtures of compounds containing sulfur in various oxidation states (3). For example, trace levels of sulfate ions in aqueous solutions can be de- termined by the addition of 1-5 pL of the solution into the MECA cavity that consists of a silica cup fitted into the hollow end of a metal rod. The cavity is positioned in a cool hydrogen diffusion flame and the intensity of the band emission from the Sz species generated is measured at 384 nm. Several variables have to be controlled in order to achieve an ac- ceptable precision and maximum sensitivity. The production of Sz species in the cavity is facilitated by the addition of a predetermined amount of H3P04 (3 ,4) . The flow rates of the flame gases must be optimized; the downward tilt of the sample rod when it is positioned in the flame must be identical in every experiment, and the initial and final temperature of the metal rod must not vary from one determination to the next. In addition to these variables, the surface characteristics of the silica cup must not vary in the course of a series of determinations. This is one of the more difficult variables to control because the silica cup must be cleaned periodically to prevent the buildup of residues from a series of determi- nations. Consequently, variations can occur in the physical characteristics as well as the chemical composition of isolated areas on the surface of the silica cup. Another limitation of the silica cup is its size and shape that severely reduces the volume of the sample solution that can be added to the cup. If the sample volume is increased beyond a few microliters, there is a tendency for the sample to flow out of the sample cup and uneven heating of the sample solution in the MECA cavity results in poorly defined emission peaks.

We have minimized these difficulties by (a) modifying the shape of the silica cup, (b) continuously spinning the metal rod and sample cup in the flame while the emission spectrum is recorded, and (c) changing the burner design. These modifications are described below and the results obtained

with the redesigned burner and the modified spinning sample cup, with a series of aqueuos solutions containing nanogram levels of sulfate and sulfite ions, are reported.

EXPERIMENTAL SECTION Emission measurements were made with a modular flame

emission system consisting of a slit and a scanning monochromator (GCA/McPherson Model EU-700-2) coupled with a photomul- tiplier module (GCA/McPherson Model EU-701-30). The output was digitized with the aid of a Keithley picoammeter, and the analog output was displayed on a Linear chart recorder. The sample rod assembly, the burner head, and the components of the emission spectrophotometer were mounted on an optical rail so that the emission generated in the silica sample cup could be lined up with the detection system. Gas flow rates were monitored and controlled with Matheson series R 7630 flowmeters.

In previous work we had found that inhomogeneities on the silica surface caused random variations in the emission intensity. This effect can be minimized if the sample solution is spread into a thin film over the entire inside surface of the silica cup during the course of the measurement of the emission intensity. This can be accomplished by adding the solution containing the analyte into the sample cup and spinning the metal rod and sample cup, before it is introduced into the flame, and continuing to spin the sample cup while the emission intensity is recorded. An exploded view of the sample rod assembly and mount is shown in Figure 1. A beveled ring gear, G1, is attached to an axle into which the threaded sample rod containing the silica cup is fitted at A. The axle rotates in the sealed roller bearings, R1 and Rz, that are fitted into two rectangular metal blocks attached to a rectangular plate, P. A hole is drilled in this rectangular plate to accommodate a bearing and an axle through which is passed the shaft, S, of a small electric motor, M. The end of the shaft is attached to a beveled pinion gear, G2, that meshes with the ring gear, G1, on the sample rod axle. The gears remain meshed while the sample rod axle is spinning and while the sample rod axle is being rotated into the appropriate position. This sample rod assembly is fitted as shown in Figure 2 on a mount that rides on an optical rail. The sample rod can be moved on this optical rail toward or away from the slit and the monochromator. The sample rod can also be moved in a direction at right angles to the optical rail. The slot and a setscrew in the plate on which the sample rod assembly is mounted enable the assembly to be moved laterally. The sample rod is continuously cooled with the aid of a copper cooling coil as shown in Figure 2B. The cooling coil is prevented from rotating by a stop that extends from the bottom of plate P.

The silica cup that is fitted into the end of the sample rod is usually cylindrical. If a cylindrical cup containing the analyte solution is spun at about 1500 rpm, there is a tendency for the solution to be lost from the cup. This can be prevented by modifying the shape of the cup. In this work the cup that was used consisted of a hollow silica sphere, 6 mm in diameter, from which a segment, 2 mm in depth, was removed. The resulting sample cup resembled the cylindrical cup with its outer rim turned inward to form a lip. Loss of the analyte solution from a cup spinning at -1500 rpm was prevented by this configuration of the sample cup. A hollow spherical cavity with a 3.5 mm diameter

0003-2700/85/0357-2743$01.50/0 0 1985 American Chemical Society

2744 ANALYTICAL CHEMISTRY, VOL. 57, NO. 13, NOVEMBER 1985

4 Flgure 1. Exploded view of spinning cup assembly: A, sample rod axle; G,, beveled ring gear; GP, beveled pinion gear; R, and R1, sealed roller bearings; P, rectangular plate through whlch the shaft of the electric motor passes; S, shaft of electric motor; M, small electric motor.

SAMPLE

SAMPLE ROD AXLE

OPTICAL R A I L

1 I I 1 LLI

Flgure 2. (A) Vlew of gear assembly and mount for spinning sample cup. (B) Side view of gear assembly and mount for spinning sample cup showing the position of the cooling coil and optical rail.

aperture has been used previously as a receptable for solid SiO,; sulfur dioxide absorbed by the solid Si02 was determined in the spherical cavity by MECA (5).

It was necessary to modify the burner in order to achieve a maximum emission intensity and to confine the emission to the silica cup. The sample rod containing the silica cup at one end is tilted at an angle of about 7 O below the horizontal and intro- duced into the hydrogen diffusion flame. The burner that was used in our previous work was found to be unsatisfactory because the access of the flame gases to the analyte in the MECA cavity was restricted by the smaller opening in the hollow spherical sample cup. The burner head was modified to produce a cool jet-shaped flame that could be directed toward the opening in the cup. A copper tube, -12 mm in diameter, was sealed at one end and a 1.5-mm hole was drilled through the sealed end. A flame in the shape of a jet was produced by passing a mixture of hy- drogen and nitrogen through the copper tube. The angle that the jet flame makes with the vertical was controlled by bending the flexible copper tube. In this work, the copper tube was bent

B I so?

so$-

5 UL!L 0 I2 24 36 0 I2 24

TIM E (SEC)

Flgure 3. S, emission vs. time profiles: (A) conventional sample cup and burner and (B) spinning sample cup and angled jet flame.

at an angle of approximately 30° from the vertical.

RESULTS AND DISCUSSION The performance of the spinning sample cup and jet flame

was evaluted by recording the molecular S2 emission obtained with aqueous solutions of S042- and SO:-. The standard solutions of Na2S04 and NapSOs that were used in this work were 8 X M in EDTA and 0.10 M in H3P04. Five mi- croliters of the standard solution was added to the silica cup, and the spinning silica cup was rotated into the flame. The molecular emission in the silica cup was recorded a t 384 nm. The optimum gas flow rates were 0.7 L/min of H2 and 1.5 L/min of N2. The time elapsed between the introduction of the sample cup into the flame and the observation of the maximum emission intensity (t , value) was 0.4 s for SO?- and 7.5 s for S042-. The emission peaks for Sod2- and SOS2- are shown in Figure 3. The emission peaks obtained previously with a stationary sample cup and conventional burner are included in Figure 3 for comparison.

The modifications that have been described above were designed to increase the reproducibility of the emission peaks in MECA. With the spinning cup and angled jet flame we were able to determine 6-640 ppm sulfur as sulfate by measuring the emission peak areas. The relative standard deviations of these determinations varied from 2% at the lower concentrations to 1 % at the higher concentrations. This is a significant improvement over the reproducibilities that have been reported in previous work (6).

We have observed that the rotation of the spinning sample cup into the jet flame is not the best method to use for the determination of components with low t , values. While the spinning sample cup is being lowered into the flame, the sample is continuously volatilized and, as a consequence, the reproducibility of the molecular emission is affected. Figure 3 shows that the emission peaks for S032- and SO4'- are completely resolved. The initial loss of volatile compounds from the S032-, however, affects the determination of SO3'-, especially at low parts-per-million concentrations. The relative standard deviation for the determination of low concentrations of SO?- is about 20%, which is a factor of 10 greater than that for the determination of Sot-. The ideal method that should be used for volatile samples is to first position the spinning sample cup in line with the slit and monochromator and then to move, with the aid of a mechanical device, the appropriately angled jet flame into a fixed position with respect to the MECA cavity.

LITERATURE CITED (1) Belcher, R.; Bogdanskl, S. L.; Townshend, A,; Anal. Chim. Acta 1973,

67, 1-16.

Anal. Chem. 1985, 57, 2745-2748 2745

(2) Burguera, M.; Bogdanskl, S. L.; Townshend, A. CRC Crit. Rev. Anal. Chem. 1880, 1 1 , 185-246.

(3) Belcher, R.: Boadanskl, S. L.; Knowles, D. J. Townshend, A. Ana/.

( 5 ) Koumltzls, A. Anal. Chim. Acta 1877, 88, 303-311. (6) Belcher, R.; Bogdanskl, S. L.; Knowles, D. J.; Townshend, A. Anal.

Chlm. Acta 1975, 79, 292-295. .

(4) Bogdanskl, S. L.; Townshend, A,; Blanco, P. T. Anal. Chim. Acta Chim. Acta 187% 77, 53-63.

1881, 131, 297-301. RECEIVED for review June 10,1985. Accepted July 22,1985.

Electrochemlcal High-Performance Liquid Chromatographic Detection in Aqueous and Nonaqueous Eluents without Supporting Electrolyte

Gary W. Schieffer

Analytical Chemistry Department, Norwich Eaton Pharmaceuticals, Inc.,’ Norwich, New York 13815

Because of its selectivity and sensitivity, electrochemical detection has been increasingly used in high-performance liquid chromatography (HPLC). For reasons of conductivity, the great majority of work has been restricted to reversed- phase or ion-exchange systems employing aqueous/organic modifier eluents with added supporting electrolyte salt. However, for reasons of simplicity, ease of extracting the analyte peak by solvent evaporation, or employment of the selectivity advantages of normal-phase separations, the ability to use less-conducting mobile phases (especially those without added supporting electrolyte) might be desirable.

Using a conventional small-volume wall jet cell, Gunas- ingham and Fleet (I) were able to detect nanogram quantities of phenols using a normal-phase eluent of 85:15 hexane/ ethanol containing a t least 0.01 M tetrabutylammonium fluoroborate as supporting electrolyte. Using a large-volume wall jet cell, Gunasingham, Tay, and Ang (2) were able to use a similar eluent system free of supporting electrolyte by making the supporting electrolyte addition postcolumn. However, nearly a 10-fold loss in sensitivity resulted.

Recently Evans and Kaaret (3) described a stationary electrochemical cell in which a screen electrode was impressed on a Nafion 117 ion-exchange membrane. The electrode side faced a supporting electrolyte-free aqueous or nonaqueous solvent containing the electroactive species, while the back side faced an aqueous electrolyte solution containing counter and reference electrodes. The transport of charge by ionic conduction occurred through the “solid polymer electrolyte” membrane, allowing electrochemical experiments to be per- formed without adding supporting electrolyte to the sample solution.

The present work explores the feasibility of employing a similar electrochemical cell with a solid polymer electrolyte as an HPLC detector. The main advantage of such a detector might be the ability to operate in reversed-phase and nor- mal-phase eluents without addition of supporting electrolyte, postcolumn addition of reagents, or a significant decrease in signal-to-noise ratio (increase in detection limit).

EXPERIMENTAL SECTION Electrochemical Cell. The cell, used initially without mod-

ification, has been described previously (Figure 2 of ref 4). It consisted basically of crushed reticulated vitreous carbon (RVC) (Fluorocarbon Co., Anaheim, CA) packed in a 2.2-cm by 2.8- mm4.d. tube made from a cation-exchange membrane (0.25 mm thick, Ndion XR-170, Du Pont, Inc., Wilmington, DE). The tube was placed in a Plexiglas chamber containing 0.1 M potassium chloride and a platinum counter electrode and a silver-silver chloride reference electrode. After the initial experiments, the cell was transformed into an annular porous electrode by centering

‘ A Procter & Gamble Company.

0003-2700/85/0357-2745$0 1.50/0

a 2.2-cm by 1.6-mm-0.d. piece of microbore Teflon tubing in the membrane tube and packing the crushed RVC around it, BS shown in Figure 1. The 0.3-mm4.d. channel of the Teflon tube was allowed to become at least partially plugged with RVC particles so that only a small fraction of sample solution avoided working electrode contact.

Electrochemical Instrumentation. An EG&G Princeton Applied Research (PAR, Princeton, NJ) Model 174A polaro- graphic analyzer was used to apply potentials to the crushed RVC cell, The 10-V output of the analyzer was converted to 1 V with a voltage divider and fed to a computing integrator (Model 3357, Hewlett-Packard, Avondale, PA) through an analog-to-digital (A/D) converter (Hewlett-Packard Model 18652A). The output of the computing integrator in pV s was converted to coulombs through knowledge of the analyzer’s current range setting, the exact resistance values for the voltage divider, and a calculated correction factor for imprecise A/D converter components (5). The value of 1.075 X lo-’’ C/(pV s) obtained for a 10-pA full-scale current range can only be considered approximate since the in- jector loop volume (vide infra) and analyzer current-to-voltage amplifier were not accurately calibrated as in the previous work (5). Peak base lines were drawn manually with a BASIC program. All chromatograms shown were obtained with the potential of the crushed RVC electrode set in the limiting current/charge region as determined from hydrodynamic voltammograms. Chromatograms were retrieved with a Hewlett-Packard Model 7470A plotter.

HPLC Equipment. The HPLC was equipped with a dual- piston reciprocating pump (Laboratory Data Control Model 111, Riviera, Beach, FL) without a pulse dampener and a Rheodyne Model 7120 injector (Rheodyne, Inc., Berkeley, CA) with a nominal 100-pL loop.

Reagents, Columns, and Eluents. For a-tocopherol (Sigma Chemical Co., St. Louis, MO), a 4.6-mm by 25-cm reversed-phase column Containing 5-pm CIS packing (Zorbax ODs, Du Pont Instruments, Wilmington, DE) was used with a pure methanol eluent both with and without 0.05 M sodium perchlorate sup- porting electrolyte. The flow rate was 2.0 mL/min. A 4.6-mm by 25-cm normal-phase column containing 5-pm silica packing (Partisil5, Whatman, Inc., Clifton, NJ) was also used for a-to- copherol with a 93:7 hexane/l-propanol eluent flowing at 1.0 mL/min. The eluent contained no supporting electrolyte. For hydroquinone (Eastman Kodak Co., Rochester, NY)f the re- versed-phase column was used at 1.0 mL/min with a 30:70 methanol/water eluent both with and without 0.05 M potassium nitrate supporting electrolyte. For propyl p-hydroxybenzoate (Eastman Kodak), the normal-phase column was used with an 85:15 hexane/ethanol eluent at 1.0 mL/min with no supporting electrolyte. Flow injection analysis was also done on this com- pound using the 30:’70 methanol/water eluent with 0.05 M po- tassium nitrate. All analytes were dissolved in the appropriate eluent.

RESULTS AND DISCUSSION Reversed Phase. Hydrodynamic voltammograms for the

oxidation of 408 ng of a-tocopherol in methanol and of 43 ng

0 1985 American Chemical Society