Anal. Chem. 1990, 62, 1656-1661
1656
and
(10) Stetter, Joseph R.; Jurs, P. C.; Rose, Susan L. Anal. Chem. 1986, 58,
pB - ( p
+ A)kC = 0
880-866.
(A4d')
The existence of nonzero solutions implies vanishing of the determinant of the (k-dependent) coefficients A-D in these four homogeneous equations, which leads to the following secular equation in terms of kd: (3p
+ A)(p + A)
COS'
(kd)- ( p + X)'(kd)'
+ p2 = 0
(A7)
Numerical solution of eq A7 provides a countable infinity of solutions in the right-half complex k plane for series approximation to arbitrarily specified forces a t x = 0. The completeness of the double (Re and Im parts of 4) eigenfunction set is indicated by the asymptotic
k,d
- +nx
i In ( n r )
(A81
as can be proven for integer n m . Experimenting with specific values of the ratio ( p / X ) in eq A7 shows that the values Im k are nonzero, implying oscillation in Tij vs x, and that the smallest value of Re k is of the order of dl, implying quick damping in x.
(11) Frye, Gregory C.; Martin, Stephen J.; Ricco, Antonio J. Sens. Mater. 1990, 1-6, 335-357. (12) Ballantine, David S., Jr.; Wohltjen, Hank. Chemical Sensors and Microinstrumentation; ACS symposium Series 403; American Chemical Society: Washington, DC, 1989; pp 222-236. (13) Landau, L. D.; Lifschtz, E. M. Theory of €/asticity; Pergamon Press: London, 1959. (14) Timoshenko, Steven: Goodier, J. N. Theory of Elasticity; McGraw-Hill: New York, 1951. (15) Leipholz, Leyden. Theory of Elssticity: International Publications: London, 1974. (16) Auld, B. A. Acoustic Fieids and Waves in Solids; Wiley-Interscience: New York, 1973; Vol. 2. (17) Zandman, Felix; Redner, S.; Daily, J. W. fhotoelastic Coatings; Iowa State University Press: Ames, 1977. (18) Hauden, D.; Michel, M.: Bardeche, G.; Gagnepain, J.J. Appl. fhys. Lett. 1977, 37 (5),315-317. (19) Brandrup. J.; Immergut. E. H. Polymer Haandbook; Wiley: New York, 1975. (20) Hodgman, C. D., Ed. Handbook of Chemistiy and Physics, 41st ed.; Chemical Rubber Publishing Co.: Cleveland, OH, 1959. (2 1) Sommerfeld, Arnold. Partial Differential Equations in fhysics; Academic Press: New York and London, 1964. (22) Crank, J. The Mathematics of Diffusion: Clarendon Press: Oxford, 1975. (23) Crank, J.. Park. G. S.,Eds. Diffusion in Polymers: Academic Press: New York, 1968. (24) Comyn, J. Polymer Permeability: Elsevier Applied Science: London, 19RS
LITERATURE CITED (1) Wohltjen, Hank; Dessy, R. E. Anal. Chem. 1979, 57, 1458-1475. (2) D'Amico, A.: Palma, A.: Verona, E. Appl. Phys. Lett. 1982, 4 7 , 300-301, (3) WohRjen, Hank. Sens. Actuators 1984, 5 , 307. (4) Snow, Arthur; WohAjen. Hank. Anal. Chem. 1984. 56, 1411-1416. (5) Ricco, A. J.: Martin, S. J.: Zipperian, T. E. Sens. Actuators 1985, 8 , 319-333. (6) Ballantine. David, Jr.; Rose, Susan L.; Grate, Jay W.: WohRjen, Hank. Anal. Chem. 1988, 58, 3058. (7) WohRjen, Hank: Snow, Arthur W.; Barger, William R.; Ballantine, David S. IEEE Trans. UFFC 1987, 34 (2), 172. ( 8 ) Nieuwenhuizen, M. S.;Barendsz, A. W.; Nieukoop, E.: Vellakoop, M. J.: Venema, A. Electron. Lett. 1986, 22, 184-185. (9) Chuang, C. T.; White, R. M.; Bernstein, J. J. I€€€ Nectron Device Lett. 1982, EDL-3, 145-148.
(25) Haward, R. N., Ed. The fhysics of Giassy Polymers; Applied Science Publishers, Ltd.: London, 1973. (26) Grate, Jay W.; Snow, Arthur; Ballantine, David S., Jr.; Wohltjen, Hank; Abraham, Michael H.; McGiII, R. Andrew; Sasson, Pnina. Anal. Chem. 1988, 60, 869-875. (27) Zellers, Edward T.; White, Richard, M.: Wenzel, Stuart W. Sens. Actuators 1988. 74, 35-45.
RECEIVED for review December 11, 1989. Accepted April 23, 1990. Mention of company names or products does not constitute endorsement by either the National Institute for Occupational Safety and Health or the US.Naval Research Laboratory. This work was carried out in part under U.S. Navy Contract NR677-003.
Magnetron Rotating Direct Current Arc Using Graphite Furnace Sample Introduction for Microsolution Analysis David Slinkman and Richard Sacks*
Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109
A direct current (dc) arc plasma In Ar or He at atmospheric pressure Is formed between the tip of a W/Th wire cathode and the end of a coaxlai graphite tube anode. A magnetic field parallel to the electrode axls Is used to generate a motorlike rotatlon of the arc current channel at frequencies In the 2-3 kHr range. Thls results In a diffuse current sheet covering the end of the anode tube. Sample vapor from a graphlte tube furnace Is introduced Into the arc plasma by passlng the vapor through the anode tube. This ensures adequate sample-plasma Interaction and results In detection llmlts generally in the parts per bllllon range. Design features of the arc-furnace system are presented along wlth analytical data for the elements Cu, Mn, Mg, Cr, Zn, Cd, and Fe. The results of t b experiments wring Ar and Ar/He mixtures as the arc plasma gas are compared.
While the high temperatures observed in direct current (dc) arc plasmas suggest that they should be excellent excitation
sources for atomic emission spectroscopy, high continuum background intensity and poor sample penetration are significant problems for solution aerosol samples. In attempts to reduce these problems, current research efforts have focused on developing dc plasma devices that entrain the sample into the hotter regions of the plasma and separate these regions from the analytical observation zone. This may be accomplished through the use of tailored gas flows and electrode positioning. A number of these plasma sources have been described in excellent reviews (1,2). Piepmeier et al. (3-8) recently introduced several multielectrode dc arc devices that use either four or six electrodes to shape the plasma and efficiently entrain the aerosol sample. The six-electrode system uses three anodes and three cathodes in conjunction with quartz tubes that deliver both the Ar plasma gas and the aerosol sample. The plasma has a form similar to an inductively coupled plasma (ICP) in that the sample penetrates the center of the plasma. Since sample emission is viewed above the current-carrying portion of the plasma, the background intensity is also reduced.
0003-2700/90/0362-1656$02.50/00 1990 American Chemical Society
ANALYTICAL CHEMISTRY. VOL. 62, NO. 15. AUGUST 1. 1990
Meyer (9)has also developed a dc plasma device configured to improve the interaction of the plasma with the aerosol sample. His design is based on the commercially available "inverted Y" arc. Rather than keeping the cathode above the two anodes, it was moved so that all three electrodes were in the same plane. The sample is introduced at the bottom of the equilateral triangle where it is completely surrounded by the plasma. A coaxial electrode arc device, which uses a magnetic field normal to the arc electric field to generate a motorlike rotation of the arc current channel, was recently described (lo, 11). At sufficiently high rotational frequencies, the arc current channel loses its discrete characteristics and assumes the form of a relatively uniform current sheet that covers the end of the graphite tube anode. A sample introduced through the anode tube is forced to pass through the current sheet, thus ensuring adequate sampleplasma interaction. However, the current sheet is quite thin, and thus, sample residence time may he inadequate for the desolvation of aqueous aerosol droplets. Preliminary studies using a dry, nearly monodisperse aerosol from a glass-frit nebulizer with a desolvation chamber were very encouraging. However, inadequate sample uptake and memory effects from the glass frit were significant problems. Numerous studies have investigated the use of electrothermal atomizers in conjunction with both ICP (12-18) and dc arc (19,20) devices. In these systems, the electrothermal atomizer is used to desolvate and vaporize the sample for subsequent introduction into the plasma. Thus, the plasma is required only to atomize and excite the sample vapor. In the present study, a conventional graphite tube furnace is combined with the rotating dc arc for the determination of selected trace metals in microvolume aqueous solution samples. The sample vapor or small-particle aerosols prodced by the furnace are easily atomized and excited by the arc plasma. This results in improvements in both the detection limits and the precision. The design features of the rotating arc-tuhe furnace system are presented along with analytical data for several metallic elements. The arc can be operated in either Ar or He, and the effects of the gas composition on the plasma properties and the analytical performance are discussed. EXPERIMENTAL S E C T I O N Rotating Arc Design and Furnace Interface. The rotating BTC is based on the magnetron concept where an external magnetic field, B, normal to the plasma electric field, E, is used to generate and E X B drift motion of plasma electrons in the direction n o d to the plane containing the B and E vectors (21). In a magnetron configuration,this electron drift motion takes the form of closed loops (22, 23). By the use of a cylindrical electrode geometry consisting of a W/Th wire cathode and a coaxial graphite tube anode, a radial electric field is generated in the arc plasma. When this is combined with a coaxial magnetic field, the E X B drift motion is in the azimuthal direction (cylindrical coordinate system), and one end of the arc current channel rotates around the edge of the anode tube. This forms a diffuse, stahle plasma dome that covers the end of the anode cylinder. Figure 1 shows the design of the magnetron arc and the interface to the graphite tube furnace. The construction details are found in Table I. The W/Th wire cathode, C, passes through a machinable alumina ceramic block and into the anode chamber, A. The cathode tip may be positioned anywhere along the anode axis. The plasma, P, forms between the cathode tip and the top, inner edge of the anode cylinder. A ceramic tube is used to insulate the cathode from the lower portion of the anode block. The sample vapor from the furnace is introduced through this ceramic tube in a stream of Ar sample transport gas. Additional Ar or He plasma gas is introduced at the base of the anode block. This gas flow serves to cool the wall of the anode cylinder and reduces the erosion rate of the anode at the point where it contacts the plasma.
I
1657
Y
Flgure 1. Section drawing of The rotating arc plasma device and me graphite furnace atomizer. A. graphne anode DloCk: C. WlTh wire catnode: M. r npshaped ceramic magnet: P. plasma; F. graphne lube lurnaca: Ar. argon or argonlhelium gas in 01s; 1. ceramic interlace tuoe Constr,cton details are found n TaDle 1.
Table I. Conatruction Details of thc Arc-Furnace System rolalmp, arc
anode
graphite cylinder (I.'ltra Carbon Type l F f 4 5 J .5-mm i.d., 8-mm 0.d.. 13 m m long
rarhode W Th 14%) rod. I - m m diam rarhode insulator AI,O, rube. 3.0-mm 1.d.. 5.0.mm i . _ current/voltage gas flows magnet
d ,
25 mm
long 12 A I 7 4 V
4.8 L/min cooling gas (outer flow),1.0
L/min sample vapor transport gas (inner flow) ceramic alloy, 22-mm i.d., 60-mm o.d., 13 mm thick; peak field strength (vertical component), 0.90 kG
furnace
tube gas flow interface
pyrolytic graphite, 5.0-mm i.d., 6.4-mm o.d., 38 mm lone 1.0 L/min, A; or He A1,0, tube, 3.0-mm id., 5.0-mm o.d., 15 mm long; Tygon tube, 3-mm i.d., 9 em long, plus 9-mm i.d., 6.5 em long
A ring-shaped ceramic magnet, M, rests on the anode block and is concentric with the electrodes. The properties of the magnetic are found in Table I. This configuration produces a magnetic field that is parallel to the electrode axis. A 25' segment of the magnet ring was removed so that radiation could he ohserved at the anode surface. A commercial graphite furnace (Instrumentation Laboratory Flameless Atomizer, Model 555) was modified for direct connection to the rotating arc. A 7.5-cm-long ceramic tube was placed in one end of the graphite furnace. The other end of the ceramic tube was connected to the arc assembly by a 16-cm-long Tygon tube. The sample vapor transport gas was introduced into the furnace enclosure and flowed into the open end of the furnace tube to carry the sample vapor through the ceramic interface and into the arc. The arc was powered by a constant-current power supply (Electronic Measurements Inc., Model EMCC) that was continuously adjustable from 0 to 30 A at an open-circuit voltage of 180 V. A spark ignitor was used to start the arc. The gas flows for the arc and the furnace were measured with rotameter-type flowmeters. Optical and Electrical Monitoring. The arc was operated with its electrode axis in the vertical direction (parallel to the spectrometer slits). A horizontal slit placed directly in front of the spectrometer entrance slit was used to obtain spatial resolution in the vertical direction. The plasma was imaged onto the slit assembly by the use of two 50-mm-diameter,500-mmfocal length spherical front-surface mirrors in an over-and-underconfwation. The system had a lateral magnification of 2.0. Both vertical and horizontal spatial resolutions at the plasma were better than 0.1 mm. All spectra were obtained with a 1.0-m Czerny-Turner spectrometer equipped with a 1200 line/mm grating blazed for 300 nm in the first order. Spectrometer slit widths of 60 Irm were used for m a t experiments. The radiation intensity was monitored
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ANALYTICAL CHEMISTRY, VOL. 62, NO. 15, AUGUST 1,
1990
with a 1P28 photomultiplier tube. The bias voltage, the load resistance, and the circuit time constant typically had values of -950 V, 1.0 MQ,and 50 ms, respectively. For the transient analyte peaks, the photomultiplier output was digitized by a 12-bit ADC and the wave forms were stored in an IBM PC with a sampling rate of 200 Hz. The plasma rotational frequency was measured photoelectrically by imaging a region near the edge of the plasma onto the spectrometer entrance slit and recording the radiation wave form on a digital storage oscilloscope (Nicolet Model 2090A) using a sampling rate of 0.5 MHz. Magnetic field measurements were made with a Hall-effect gauss meter (Central Scientific, Model 100). Procedures and Materials. The furnace temperature program consisted of a 1.0-min desolvation step at 100 "C followed by a 5.0-s atomization step. No ashing was used since the sample
solutions were pure salts in distilled water. Reagent-grade salts were used to prepare loo0 Fg/mL stock solutions. These solutions were acidified to 1% with either HNOBor HCl. Lower concentrations were prepared daily by serial dilution with distilled, deionized water. The photoelectric signal was digitized and stored at a rate of 200 Hz for a period of about 5 s, beginning with the start of the main furnace heating cycle. For each set of conditions, signal vs time wave forms were averaged from four replicate experiments. Four background wave forms at the analysis wavelength and using distilled water as a blank also were averaged. A net analytical signal wave form then was obtained by a point-by-point subtraction. Peak height measurements were used in most cases. RESULTS AND DISCUSSION Properties of the Rotating Arc. In the absence of the magnetic field, luminous plumes from the cathode tip and a point on the anode ring are observed to merge, thus forming an angular current channel. The point of contact on the anode wanders erratically around the inner edge of the anode. Excessive local heating soon burns holes in the anode edge. In the presence of the magnetic field, the current channel rotates around the inner edge of the anode, and the plasma appears as a diffuse dome with a bright, vertical plume anchored to the cathode tip. The rotational frequency increases with increasing magnetic field strength, with increasing arc current, and with decreasing anode diameter (10). For the conditions used here (see Table I), the rotational frequency in Ar is about 2.2 kHz. At this frequency, the current channel loses its discrete nature, and a relatively constant and symmetric radial current sheet is formed. The rotating arc plasma has three more or less distinct vertical zones. The very bright current sheet is only about 1 mm thick. This region is dominated by very intense and broad Ar ion lines. Radiation from the current sheet shows little or no temporal variation. Directly above the current sheet and extending about 2 mm from the sheet, a high-temperature thermal transport zone is observed. The greatest continuum background and Ar neutral-atom line radiation intensities are observed in this region. In addition, this region shows the greatest temporal variations in radiation intensities during a rotation cycle. Extending for several millimeters above the region of high continuum background intensity is a cooler tail flame region. Greatest anal* line-to-background intensity ratios are found in the tail flame region. Previous studies (11) have shown that a more analytically useful plasma is formed when the cathode tip is withdrawn several millimeters below the end of the anode cylinder. This results in a cone-shaped plasma that resides primarily inside the anode cylinder. This configurational results in a very stable plasma with much of the continuum background intensity associated with processes in the current channel masked by the anode cylinder. In addition, the elongated plasma appears to result in improved sample-plasma interaction. This configuration was extensively evaluated by using
j 32 j
B
\
A,
0
1
2
3
4
Cathode POS. Below Anode Surfrce (mm)
Figure 2. Relative intensity of the Fe 259.9-nm ion line (a),background intensity (b), and line-to-background intensity ratios (c) as functions of the vertiil location of the cathode tip. A, data from an optical window at the anode surface; B,data from an optical window located 1.5 mm
above the anode surface. I n all cases, the optical path was tangent to the inner wall of the anode. aerosol samples generated by a glass-frit nebulizer equipped with a desolvation chamber. The dry aerosol appears to be deflected off the current sheet and penetrates the plasma near the anode surface. The result is greater analytical line intensities and greater line-to-background intensity ratios for an optical path tangent to the anode wall than for a path along a diameter of the anode. Preliminary studies using the graphite furnace for sample introduction into the rotating arc resulted in nearly symmetric emission peaks typically 1-2 s in width and appearing 1-4 s after the start of the main furnace heating pulse. For all parametric studies described in this section, the peak heights from four replicate samples were averaged. The relative standard deviations for these measurements were in the range 2-4%. Figure 2 shows the effects of both the cathode tip position and the vertical position of the optical path on line intensities (a), background intensities (b), and line-to-background intensity ratios (c) for Fe samples introduced from the graphite furnace. The Fe ion line a t 259.9 nm was monitored. In all cases, 30 ILLof 10 Fg/mL Fe as an aqueous solution of Fe(NO& was used with a 2550 "C furnace temperature. Plots A were obtained at the anode surface, and plots B were obtained a t a distance of 1.5 mm above the anode surface. In all cases, the optical path was tangent to the inner wall of the anode. As the cathode tip is withdrawn into the anode cylinder, the Fe line intensities decrease and reach minimum values for a cathode tip position 3.0 mm below the anode surface. The greater intensities observed a t 4.0 mm probably reflect a more favorable plasma shape with improved sample-plasma
ANALYTICAL CHEMISTRY, VOL. 62, NO. 15, AUGUST 1, 1990
1658
P 32.8 2.8 2.4-
2 0
20
60
40
80
1100
SZ Helium Figure 4. Effects of plasma gas composition of the arc rotation frequency. The total cooling gas flow was 4.8 L/min with volume percent He plotted on the abscissa. The sample transport flow (inner flow) was 1.0 Llmin Ar in ail cases.
O! 0
1
I
I
2
3
O l r i a n c r Above A n o d r , rnrn Figure 3. Relative intensity of the Cu 324.7-nm neutral-atom line (a) and line-to-background intensity ratios (b) as functions of the vertical location of the optical window. A, data obtained for an optical window tangent to the inner wall of the anode; 6,data obtained for an optical window along a diameter of the plasma.
interaction. For measurements made at the cathode surface, the continuum background intensity decreases very strongly as the cathode tip is withdrawn further into the cathode. This is not surprising since the current sheet, which is the region of greatest continuum background intensity, is viewed at the anode surface when the cathode tip and the end of the anode cylinder are coplanar. As the current sheet is pulled into the anode, the continuum intensity decreases dramatically. The largest line-to-background intensity ratios for the Fe ion line are achieved for measurements made at the anode surface and with the cathode tip withdrawn about 4 mm inside the anode cylinder. Note that under these conditions, the line-tobackground intensity ratios are not greatly different for measurements made at the anode surface and for those made 1.5 mm above the anode. Figure 3 shows the effects of the optical path location on analyte emission intensities (a) and line-to-background intensity ratios (b) for Cu samples introduced from the graphite furnace. The Cu neutral-atom line at 324.7 nm was monitored. In all cases, 5 pL of 50 pg/mL Cu as an aqueous solution of Cu(NOJ2 was used with a 2550 OC furnace temperature. Plots A were obtained with an optical path tangent to the inner wall of the anode, and plots B were obtained with an optical path along a diameter of the anode. This is shown in the inset in Figure 3. In all cases, the cathode tip was located 3.0 mm below the anode surface. For all vertical positions, the optical path tangent to the anode wall resulted in greater line intensities. The intensities decrease fairly rapidly with increasing height in the viewing region above the anode cylinder. Thus, the analytically useful tail flame region of the rotating arc plasma is quite short when compared to the tail flame of an ICP.
When the cathode tip is positioned 3.0 mm below the anode surface, the highly luminous current sheet is almost entirely masked by the anode cylinder, and the continuum background intensity in the tail flame region is greatly reduced. In addition, the continuum intensity shows a much weaker dependence on the vertical position of the optical path. While the analyte line intensities generally are greater for the optical path tangent to the anode wall, the continuum background intensities also are greater. For the Cu neutral-atom line, the result is that somewhat greater line-to-background intensity ratios are observed for most vertical positions using the optical path along the anode diameter. For some of the elements investigated, the larger line-to-background intensity ratios were observed for the optical path tangent to the anode wall. For neutral-atom analyte lines, the greatest line-to-background intensity ratios are usually obtained in the range 2-4 mm above the anode surface. However, for ion lines, the ratios may be largest for vertical positions closer to the anode surface. For all subsequent studies, the optical path was chosen tangent to the inner wall of the anode, and radiation was integrated over a vertical region extending from the anode surface to about 3.5 mm above the surface. The use of He and Ar/He mixtures was evaluated as a means of reducing the continuum background intensity and to provide more efficient excitation for some elements. A stable rotating plasma was achieved at all Ar/He ratios. Preliminary studies using pure He for both the cooling gas (outer flow in Figure 1)and the sample transport gas resulted in shifts to lower peak intensities, greater peak widths, and later peak times relative to pure Ar. Shot-to-shot reproducibility also was poorer in He. In all cases, the cooling gas flow rate was 4.8 L/min, and the inner flow rate (sample transport gas) was 1.0 L/min. When He was used for the cooling gas only, with 1.0 L/min Ar through the furnace for vapor transport, the sample peak shapes and reproducibility were not significantly different from those when pure Ar was used as the cooling gas. Figure 4 shows the effect of He on the rotational frequency of the plasma. For this experiment, the total flow of cooling gas was 4.8 L/min, with the volume percent He plotted on the abscissa of Figure 3. The inner flow was always 1.0 L/min pure Ar. In the range &30% He, only a small increase in frequency is observed with increasing percent He. For larger amounts of He, the increase is more dramatic. The E X B drift motion of plasma electrons is significantly degraded by collisional drag effects (IO,21). This degradation should be smaller in He because of its lower molecular weight and its smaller collisional cross section for electrons. The frequency shift illustrated in Figure 3 is clearly audible. In addition, the sound becomes more staccato with increasing He despite the higher frequency. This suggests that the
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ANALYTICAL CHEMISTRY, VOL. 62, NO. 15, AUGUST 1, 1990
Table 11. Compounds and Furnace Temperatures for Analytical Curves element
compd
furnace temp, "C
cu Cr Zn Cd Fe Mg Mn
Cu(N03h CrCl, Zn(NOd2 Cd(N03h Fe(NOd2 MgC12 MnCL
2550 2550 2040 1700 2550 2040 2380
Table 111. Analytical Data for the Arc-Furnace System
element
wavelength, nm
slope (Ar/He)
cu Cr Zn Cd Fe Fe Mg Mn
(I) 324.7 (I) 357.8 (I) 213.8 (I) 228.8 (I) 372.0 (11) 259.9 (11) 279.5 (11) 259.3
0.89 1.22/1.21 0.97 1.01 1.31 1.23/1.05 0.97/1.03 0.92
14
.-
1
2 o J
I
0
I
20
.
.
.
.
40 60 X Helium
.
.
80
.
corr coeff detection limit, PLg/mL (Ar/He) (Ar/He) 0.991 0.999/0.995 0.999 0.999 0.998 0.995/0.999 0.999/0.994 0.999
0.006 0.044/0.048 0.015 0.055 0.55 0.26/0.051 0.007/0.009 0.020
_1
LOO
Effects of plasma gas composition on analyte line intensities (a),background intensities (b), and line-to-background intensity ratios (c). A, Mg 279.5-nm ion line; B, Mg 285.2-nm neutral-atom line. The total cooling gas flow was 4.8 Llmin with volume percent He plotted on the abscissa. The sample transport flow (inner flow) was 1.0 L/min Ar in all cases. Figure 5.
plasma has a more oscillatory character in He. The greater diffusivity and thermal conductivity of He may promote more rapid cooling after passage of the current channel. This would produce a more discrete rotating current channel and would result in a more oscillatory character of the plasma. Figure 5 shows the effects of percent He on line intensities (a), background intensities (b), and line-to-background intensity ratios (c) for Mg samples introduced from the graphite furnace. In all cases, 20 pL of 2.0 pg/mL Mg as an aqueous solution of MgC12 was used with a 2040 "C furnace temperature. Plots A are for the Mg 279.5-nm ion line, and plots B are for the Mg 285.2-nm neutral-atom line. The element Mg was chosen because of its relatively low ionization potential, which results in relatively sensitive ion lines as well as neutral-atom lines. For both lines, the intensity values show a range of about a factor of 2 over the entire range of percent He. Both lines show local minima at 50% He. The background intensities at the two wavelengths show relatively similar changes with percent He. The background intensity decreases quite rapidly from 0% to 50% He and much more slowly from 50% to 100% He. The line-to-background intensity ratios increase with increasing He, but the improvement is much more significant for the ion line. Analytical Performance of the Arc-Furnace System. Analytical curves were constructed for the elements Cu, Mn, Mg, Cr, Zn, Cd, and Fe in aqueous solutions over the concentration range typically from 0.5 to about 10 pg/mL. The experimental conditions and the furnace temperatures used for these curves are given in Table 11. The optimal furnace temperatures were empirically determined for each compound. Figure 6 shows the emission signals from four replicate samples at each of three concentrations for the 213.8-nm Zn neutral-atom line. For each trace, a 30-bL sample of Zn(NO& was used. Signals A, B, and C are for Zn concentrations of 10, 5.0, and 2.0 pg/mL, respectively. The peaks widths a t half-height are typically less than 1 s. In all cases, over 90% of the peak area is observed in a time window f l . O s from the
w 1.0 5
Reproducibility and peak shape study for the Zn 213.8-nm neutral-atom line for Zn(NO,), samples vaporized in the graphAe tube furnaceand detected in the rotating arc plasma. A, 10 wg/mL Zn; B, Figure 6.
5.0 pg/mL Zn; C, 2.0 pg/mL Zn.
peak maximum. This suggests that the transport of the sample vapor from the furnace to the arc plasma is reasonably efficient. The relative standard deviations for peak height measurement at the three concentrations are 1.6%, 2.0%,and 3.3 %, respectively. These values are typical of values obtained for all elements and spectral lines investigated. The relative standard deviations for peak area measurements usually were not significantly different from those for the peak height measurements. Table 111summarizes the analytical data from this study. Most data were obtained by using pure Ar as the cooling gas. For the Cr neutral-atom line and the Fe and Mg ion lines, a direct comparison was made between the Ar and He cooling gases, and the values to the right of the slashes (/) are for He. The slopes of the analytical curves from log-log plots are in the range 0.89-1.31 with a median value of 1.03. The effect of the He is negligible for the Cr and Mg lines, but the Fe ion line slope in He is significantly closer to unity. All correlation coefficients for linear regression on the log-log plots are greater than 0.991, and 6 of the 11 reported values are 0.999 or greater. Note that since the concentration values were preselected, the values of the correlation coefficients in Table I11 may provide a distorted estimate of the correlation. Again, the results of the experiments using the He and Ar cooling gases are comparable. The detection limits were estimated by extrapolating the analytical curves to give the analyte concentration required to produce a net emission signal equal to 3 times the root mean
Anal. Chem. 1990, 62, 1661-1665
square noise in the average wave form from four distilled-water blanks. The detection limits for the magnetron arc-graphite furnace system are very good when compared to most reported dc arc values. Note that the Fe, the detection limits for the ion line in Ar are more than a factor of 2 lower than for the neutral-atom line. With He as the cooling gas, the Fe ion line detection limit is more than an order of magnitude lower than the value for the neutral-atom line with the Ar cooling gas. This preliminary investigation suggests that the combination of the magnetron rotating arc and an electrothermal atomizer may be very attractive for the determination of some trace metallic elements in microsolution samples. The significant improvements in the powers of detection and precision observed with the graphite tube furnace relative to previous studies with a glass-frit nebulizer (11) suggest that the magnetron arc may be an excellent atomization and excitation source, but it may be less useful as a means of vaporizing the sample particles produced by nebulization techniques. This is probably the result of the relatively small vertical dimensions of the plasma and the correspondingly short sample residence times. The magnetron rotating arc is very simple and easy to construct and operate. The gas flow requirements are relatively modest. A single anode and cathode typically can be used for about 100 shots before erosion by the plasma requires electrode replacement. A single, compromise set of operating conditions can be used for several elements with detection limits in the 0.005-0.5 wg/mL range and with relative standard deviations typically in the 1-370range. Since the heart of the atomizer and the heart of the rotating arc are both graphite cylinders of comparable dimensions and properties, it should be possible to combine the two functions in a single device. A prototype system is under construction.
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This should nearly eliminate any sample loss during transport. In addition, narrower, taller peaks should be produced. This may reduce the detection limits. In addition, the elimination of the transport tube may allow the use of pure He as the transport and cooling gas. This may result in improved detection limits for some elements. LITERATURE CITED Keirs, C. D.; Vickers, T. J. Appl. Spectrosc. 1977, 3 7 , 273. Greenfiekl, S.;McGeachin, H. M. D.; Smith, P. B. Talent8 1975, 22, 1. Mattoon, T. R.; Piepmeier, E. H. Anal. Chem. 1983, 5 5 , 1045. Masters, R. A.; Plepmeier, E. H. Spectrochim. Acta 1985, 408, 85. Lee, G. H.; Shields, J. P.; Piepmeier, E. H. Spectrochim. Acta 1988, 438, 1273. Lee, G. H.; Piepmeier, E. H. Spectrochim. Acta 1985, 408, 1485. Shields, J. P.; Lee, G. H.; Piepmeier, E. H.Appl. Speclrosc. 1988, 42, 684. Shields, J. P.; Piepmeier, E. H. Appl. Spectrosc. 1988, 42, 693. Meyer, G. A. Spectrochlm. Acta 1987, 428, 333. Slinkman, D.; Sacks, R. Appl. Spectrosc. 1990, 4 4 , 76. Slinkman, D.; Sacks, R. Appl. Spectrosc. 1990, 44. 83. Matusiewicz, H.; Barnes, R. M. Appl. Spectrosc. 1984, 3 8 , 745. Matuslewicz, H.; Barnes, R. M. Spectrochim. Acta 1965, 408, 29. Ng, K. C.; Caruso, J. A. Appl. Spectrosc. 1985, 408, 719. Swaiden, H. M.; Christian, G. 0. Anal. Chem. 1984, 5 6 , 120. Aziz, A.; Broekaert, J. A. C.; Leis, F. Spectrochim. Acta 1982, 3 7 8 , 369. Tikkanen, M. W.; Niemczyk, T. M. Anal. Chem. 1986, 5 8 , 366. Kumamaru, T.; Okamoto, Y.; Matsuo, H. Appl. Spectrosc. 1987, 4 7 , 918. Elliott, W. G.; Matusiewicz, H.; Barnes, R. M. Anal. Chem. 1986, 5 8 , 1264. Greene, B.; Mitchell, P. G.; Sneddon, J. Spectrosc. Lett. 1988, 79, 101. Chen, F. F. Introduction to Plasma Physics: Plenum Press: New York, 1974. Vossen, J. L., Cuomo, J. J., Ed. Thin Film Processes; Academic Press: New York, 1978. Thornton, J. A. J. Vac. Sci. Techno/. 1978, 75, 171.
RECEIVED for review February 9, 1990. Accepted April 16, 1990.
Direct Monitoring of Enzyme-Catalyzed Reactions via Laser-Based Polarimetry Patrick D. Rice and Donald R. Bobbitt*
Department of Chemistry and Biochemistry, University of Arkansas, Fayetteville, Arkansas 72701
A new method Is described In which a laser-based polarimetric detectlon system Is used to monitor enzyme-catalyzed reacllons. This method provides dlrect detectlon of the reaction progress based on the Inherent chirality of elther the substrate or product. The technique Is tested on the glucose oxidase catalyzed conversion of B-D-giucose to Dgluconic acld. The abillty of this system to negate the large background signals produced by optically active substrates Is examined, and a mass Ilmltofdetectlon for glucose oxldase of 34 fmol Is demonstrated. Results obtained with the polarimetric technlque are compared to those from an accepted spectrophotometrk method, and the technique is shown to be as accurate wlth an LOD 15 times lower. The theoretical relatlonship between the polarimetric response and various experimental parameters will be developed and verified.
*To whom correspondence should be addressed.
INTRODUCTION The use of kinetic methods of analysis has expanded greatly in the past quarter century, drawing upon recent advances in computer technology for data acquisition and manipulation. Techniques such as continuous-flow monitoring ( I ) , electrode kinetics ( 2 ) ,and elution chromatography (3)are a few of the many analytical areas where kinetic methods play a major role. One important application of kinetic methods involves the monitoring of enzyme-catalyzed reactions. The use of enzyme-based determinations is widespread, particularly in medical applications, and because of the routine use of automated clinical techniques, kinetic methods of analysis now outnumber both equilibrium-based and direct instrumental measurements ( 4 ) . The inherent selectivity of enzyme-catalyzed reactions makes these techniques an important area of study in the field of chemical analysis. However, in many enzymatic reactions the properties of the substrate or product are such that de-
0003-2700/90/0362-1661$02.50/0 0 1990 American Chemical Society
