Anal. Chem. 1983, 55, 1409-1414
spiked liver extracts. These results are shown in Figures 3 and 4 for 0.2 ppm levels (1g equiv). Major ions in the spectra of standards are also present in the spectra of samples. Again, the unspiked control tissues showed no major interferences in the CID/MIKE spectra of the (M + H)+ ions ( m / z 311 or 301). We have concluded that, for this degree of sample cleanup, full scan CID/MIKES is capable of confirming sulfas in extracts of swine liver as low as 0.1 ppm. The full scan CID/ MIKE spectra offer considerable specificity for identifications. The use of mult,iple ion detection in conjunction with MIKES is an alternative technique that would increase sensitivity but provide less information with regard to possible interferences. Responses obtained from unspiked control tissue indicate that no interferences are encountered at this degree of cleanup. The simplicity and speed of solid probe introduction make this approach attractive for routine analyses, especially for laboratories that must frequently shift from project to project. Registry No. Sulfamethazine,57-68-1; sulfamerazine,127-79-7; sulfanitran, 122-16-7;sulfabromomethazine, 116-45-0;sulfaethoxypyridazine, 963-14-4; sulfanilamide, 63-74-1; sulfabenzamide, 127-71-9; sulfapyridine, 144-83-2; sulfadiazine, 68-35-9; sulfaquinoxaline, 59-40-5; sulfamethoxypyridazine, 80-35-3; sulfachlorpyridazine, 80-32-0; sulfachlorpyrazine, 1672-91-9; sulfaguanidine, 57-67-0; sulfisoxazole, 127-69-5;sulfadoxine, 2447-57-6; sulfadimethoxine, 122-11-2;sulfathiazole, 72-14-0.
1409
LITERATURE CITED (1) "Code of Federal Regulations"; Title 21, Parts 130-140, 1973. (2) Daun, R. J. J. Assoc. Off. Anal. Chem. 1971, 5 4 , 1277-1282. (3) Goodspeed, D. P.; Slmpson, R. M.; Ashworth, R. B.; Shafer, J. W.; Cook, H. R. J. Assoc. Off. Anal. Chem. 1978, 6 1 , 1050-1053. (4) Spiteller, G.; Kashnitz, R. Monatch. Chem. 1983, 9 4 , 964-980. (5) Roach, J. A. G.; Sphon, J. A.; Hunt, D. F.; Crow, F. W. J. Assoc. Off. Anal. Chem. 1980, 6 3 , 452-459. (6) Garland, W.; Miwa, B.; Weiss, G.; Chen, G.; Saferstein, R.; MacDoriald, A. Anal. Chem. '1980, 52, 842-846. (7) Sphon, J. A. J . Assoc. Off. Anal. Chem. 1978, 6 1 , 1247-12521. (8) Brumley, W. C.; Sphon, J. A. Biomed, Mass Spectrom. 1981, 8 , 390-396. (9) Yost, R. A.; Enke, C. G. Anal. Chem. 1979, 51. 1251A-1256A. (10) McLafferty, F. W. Acc. Chem. Res. 1980, 13, 33-39. (11) Hunt, D. F.; Shabanowitz, J.; Giordani, A. B. Anal. Chem. 1980, 52, 366-390. (12) Kondrat, R. W.; Cooks, R. G. Anal, Chem. 1978, 5 0 , 82A-92A. (13) Henlon, J. D.; Thomson, B. A.; Dawson, P. H. Anal. Chem. 1982, 5 4 , 451-456. (14) Manuel, A. J.; Stoller, W. A. J. Assoc. Off. Anal. Chem. 1981, 6 4 , 794-799. . .. . ... (15) McLuckey, S. A.; Gllsh, G. L.; Cooks, R . G. Int. J. Mass Spectrom. Ion Phvs. 1981. 39. 219-230. (16) Dawson, P. H.; French, J. B.; Buckley, J. A,; Douglas, D. J.; Simmons, D. Org. Mass Spnctrom. 1982, 17, 205-211. (17) Millington, D. S.; Smith, J. A. Org. Mass Spectrom. 1977, 12, 264-265. (18) Malanoski, A. J.; 13arnes, C. J.; Fazio, T. J. Assoc. Off. Anal. Chem. 1981, 6 4 , 1386-1391. (19) Matusik, J. E.; Barnes, C. J.; Newkirk, D. R.; Fazio, T. J. Assoc. Off. Anal. Chem. 1982, 65, 828-834.
RECEIVED for review March 4, 1983. Accepted April 6, 1983.
Wall-Jet Electrode in Continuous Monitoring Voltammetry Harl Guriaslngham" and Bernard Fleet2 Department of Chemlstry, Imperial College of Science and Technology, London SW7 2AZ, England
The equatlon for the hydrodynamlc boundary layer thlckness Is derived for the wall-jet. From thls equatlon the dlffuslon layer thickness and thence the llmltlng current equatlon lor the wall-Jet electrode Is obtalned. This work shows that the presence of the nozzle body within the boundary layer causes a reductlon In the llmltlng current whlch Is explained In terms of the lows of momentum transfer In the radlal flow of the wall-Jet. A modlfled rlng-dlsk wall-jet cell Is described which has desirable features In regard to the placement of the reference! electrode and the symmetry of the radlal flow of the jet ower the electrode surface.
There has been increasing emphasis in recent years on the need for continuous or automated monitoring in areas such as procens control, environmental monitoring, biomedical screening, and detectors for liquid chromatography. The use of voltammetric techniques in this type of continouous flow mode can be broadly defined by the term hydrodynamic voltammetry (HDV). A number of electrode geometries have gained application in continuous monitoring HDV. These include the tubular, planar, and wall-jet electrodes. The wall-jet electrode is
gaining popularity lbecause of its sensitivity and ease of use. This paper seeks to explain the anomalous behavior of ithe wall-jet electrode a t small inlet-electrode separations. Equations for the boundary-layer and diffusion-layer thickness are derived which provide a theoretical basis for the explanation. THEORY One of the first approaches to the problem of mass tranelfer in electrode processes is due to Nernst (1). Nernst postulated the existence of a motionless thin layer of solution adjacent to the electrode and, also, predicted a linear concentrat ion gradient (of the electroactive species) within this layer. While the theory introducles the important concept of the diffusion layer, it is now knovvn to be an oversimplification in the c,we of both stirred and unstirred solutions. In more recent work, Nernst'n approximate approach has been replaced by more regorous treatments, which take into account the hydrodynamic characteristics of the flowing solution. Equations that have been thus deduced relate the diffusion-layer characteristics to hydrodynamic parametors. One such treatment is centered on solving the three-dimensional equation describing convective diffusion (2).
'Present address: Department of Chemistry, National University of Singapore, Kent Ridge, Singapore 0511. Present address: HSA Reactors Ltd., Fesken Drive, Rexdale, Canada.
0003-2700/83/0355-1409$01.50/00 1983 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 55, NO. 8, JULY 1983
The first group of terms on the right hand side arise from diffusive mass transfer and the second group of terms from mass transfer due to convective processes. The hydrodynamic characteristics of the flowing solution are embodied in the velocity distribution functions V,, V,, and V , and these are often quite complex. The above equation ignores the influence of migration, which is negligible if the electroactive species contributes negligibly to the ionic strength. Solutions to eq 1may be arrived a t by setting appropriate initial and boundary conditions, depending on the particular electrode geometry. However, the mathematical treatment is usually quite difficult and it is only for a few well-defined electrode geometries that a complete solution has been derived. Another route to solving the problem of convective mass transfer is through the analogy between mass transfer and momentum transfer. This approach, described by Levich (3), can be applied to solving the limiting diffusion-current equation for the wall-jet electrode. Matsuda derived an analogous expression for the limiting diffusion current for momentum transfer for axisymetrical flow past a body given by
I = knFCoD213p-113(~ ~ [ r a ( x ) 3 ~ ( x d) ] ~1 /)2 ~ (2) ” where ~ ( x =) skin friction, p = solution viscosity, ro(x)is the distance from the axis of the body, x is the direction parallel to the surface of the body, and k is a numerical constant (4). The other terms have their usual meanings. Equation 2 has been used to obtain solutions for a number of electrode geometries. The Wall-Jet Electrode. Various electrode designs and approaches to on-line monitoring have been investigated in the author’s laboratory (5-7). One of the major areas of interest is the design of electrochemical detectors for highperformance liquid chromatography (6). Initial efforts in this area were directed toward the development of miniaturized cells for use with the dropping mercury electrode. However, the utility of this type of detector was found to be limited due to the mechanical instability of the mercury electrode and more especially due to its almost negligible anodic voltage range. The latter factor precludes the use of the DME for electrooxidative studies which are particularly of importance for many organic and biological compounds. The choice of optimum electrode configuration and cell design for HDV requires certain conditions to be fulfilled; these include (i) high sensitivity, (ii) well-defined hydrodynamic characteristics, (iii) low dead-volume, (iv) ease of construction, and (v) ease of maintenance, particularly of constant effective electrode area. The cell design which we have found most satisfactory for a versatile electrochemical flow cell has been based on the wall-jet principle. This term was defined by Glauert (8) to describe the phenomenon when a jet of fluid strikes a wall perpendicularly and then spreads radially over the surface of the wall. Based on Glauert’s findings and by way of eq 2, Yamada and Matsuda (9) described the limiting diffusioncurrent equation in terms of the hydrodynamic parameters of the flowing solution. The equation they derived defines the performance of the wall-jet electrode in terms of the volume flow rate, diameter of the nozzle, and radius of the electrode where a is the inlet diameter, v the kinematic viscosity, and V the flow rate; all other terms have their usual meaning. These authors were able to verify the validity of eq 3 experimentally. Notably, the equation does not include a term to express the separation between nozzle and disk electrode. The assumption is made that the jet of liquid issuing from
the nozzle does not break up before it impinges on the electrode disk. However, Yamada and Matsuda found that measurements should only be made a t inlet-electrode separation greater than a certain empirically determined value. At separations less than this value anomalous results were reported which were particularly noticeable at low flow rates. In the present study, a significant dependence of current on the nozzle-electrode separation was observed, which can be attributed to the effect of the nozzle body within the hydrodynamic boundary layer. Calculation of the Hydrodynamic Boundary-Layer Thickness. When a fluid flows over a surface, a very thin layer is formed adjacent to the surface in which the velocity gradient normal to the surface is very large. This thin layer is termed the hydrodynamic boundary layer. The boundary layer formed when a solution flows over a surface is not strictly defined; it is usually taken to be the region in the immediate vicinity of the surface wherein the major change in velocity gradient occurs. For parallel, laminar flow over a flat plate with zero incidence, the limit of the boundary layer is taken as the point from the plate surface where the velocity approaches 99% of the velocity of the main stream (10). According to Von Karman (11) and Riddiford (12) the hydrodynamic boundary layer for a rotating disk is seen as the layer of liquid dragged by the rotating laminar, the region where most of the radial and tangential velocity components are found. Following this reasoning for the wall-jet, it would be plausible to define the boundary layer to be the layer wherein the major part of the radial velocity profile is found. According to Glauert (8),the radial velocity component is given by the expression
(4) where F is the flux of exterior momentum flux, x is the dimension along the electrode surface having its origin at the center of the electrode (y is the corresponding dimension normal to the surface); 9 is a radial velocity function which is further defined as
=
(
135F)V;
32v3x5
(5)
F is a constant which Glauert showed can be estimated by
F = l/z(typical velocity)(volume flow per radian)2
(6)
Thus for a jet issuing from a nozzle of diameter a cm, with a volume flow rate V cm3/s
(7) the constant k 1 has been found by Yamada and Matsuda to be 0.55 (9). Inspection of the radial velocity profile plotted by using equations defined by Glauert (8) shows that near the end of the profile 9 = 6. This point approximately defines the limit of the boundary layer. Substituting eq 7 in 5 and simplifying, we can obtain the equation describing the boundary-layer thickness in terms of various hydrodynamic parameters.
Equation 8 then describes the hydrodynamic boundarylayer thickness in terms of the hydrodynamic parameters characteristic of the wall-jet electrode. It shows that, in contrast with the rotating disk (where it is independent of the disk radius), the boundary-layer thickness increases rapidly from the center of the electrode. This should also be compared
ANALYTICAL CHEMISTRY, VOL. 55, NO. 8, JULY 1983
with the parallel plate where ab,, the thickness of the layer, is proportional to the inverse 1/2 power of the flow (of the main stream) over the plate. As noted by Yamada and Matsuda (91,the velocity distribution of the wall-jet requires an infinite velocity and zero jet width when x is 0. In the actual case the velocity and jet width are both finite. Consequently, the velocity distribution differs from eq 4 near the center of the electrode. Therefore eq 8 does not hold when x is close to zero. Substituting representative values in eq 8 for an aqueous system, where D = lo4 cm2 s-l, v = cm2 s-l and for a disk radius of’ 1.5 mm and inlet diameter of 0.3 mm, it is found that the boundary-layer thickness at low flow rates (1 to 3 cm3/min) is between 1.5 and 0.7 mm. At higher flow rates bbl reduces to a few tenths of a mameter, which is comparable to the hydrodynamic boundary-layer thickness of the rotating disk. Estimation of the Diffusion-Layer Thickness. For a solution flowing over a solid body, there is a region in the immediate vicinity of the surface where a rapidly changing concentration profile is found. This region is termed the diffusion layer. According to Levich (3) the thickness of the diffusion layer for flow past a flat plate is given by
Where D is the diffusion coefficient and v is the kinematic viscosity, A similar expression was obtained by Levich for the rotating disk, the only difference being a numerical coefficient = 0.5. With these two cases as justification, it would be reasonable to set the diffusion-layer thickness of the wall-jet as 6dl
Substituting for
bd1 :=
8bl
= k2(
f)1J36b1
we have
5.8k2,3/4D1J3a1/2v5/123C5/4V-3/4
(10)
Derivation of the Limiting-Current Equation. From Fick’s first law, the current due to diffusion-controlled electrolysis is given by
i = nFAD( -j-) %J, y=o
where n = number of electrons, F = Faraday’s constant, A = area of‘ electrode, D .- diffusion coefficient, and y is perpendicular to the electrode surface. If 8d1 is3 the diffusion-layer thickness, then eq 11 can be simplified to give an expression for the limiting current given by
.
2Em
=
nFADC bdl
Substituting for
bdl
we have
ili, = k ’ n F D 2 / 3 C a - 1 / 2 ~ - 5 / 1 2 2 3 / 4 ~ / 4
(13)
where
k’ = n1f4/(5.8k2)
(14)
Equation 13 shows the same form as the one evaluated by Yamada and Matsuda (9). The value of k’was determined experimentally by these authors to be 1.38. Substituting in eq 14, we find k2 = 0.17 by which we can estimate the diffusion-layer thickness to be about 2% of the hydrodynamic boundary layer (assuming D = om2 s-l and v = cm2 5-1) *
1411
rim &trode
1
wtld /counter dectrcde
Flgure 1. The modified wall-jet cell.
I
Flgure 2. Experimental system.
EXPIERIMENTAL SECTION Cell Design and Construction. The working cell design used in the present investigation is somewhat different than the original wall-jet concept and is, in effect, a constricted wall-jet with the working electrode forming a smaller cell volume compared with the earlier design of Yamada and Matsuda (9). Two versions of the wall-jet cell were tried. The first was based on the original design by Fleet and Little (6), though the cell volume in this work was somewhat larger. The cell body was made of Perspex and the working electrode encased in Kel-F (3M, USA) or a Perspex body sealed with epoxy resin. The electrode body was threaded to allow variation of the inlet-electrode separation and electrical contact to the carbon was made by using a silver-loaded epoxy cement to connect a copper lead to the back of the glassy carbon. A Ag/AgCl reference electrode housedl in a removable unit waf3connected to the cell cavity via a ceramic or “Vycor” porous glass frit. This design has been successfully applied t o a variety of on-line applications (5), in particular as a detector for high-performanceliquid chromatography (6, 7) and for metal monitoring via anodic stripping voltammetry (13). The second design was a ring-disk version of the wall-jet cell. In this modification both the disk and the ring were of glassy carbon and the reference electrode was contained in an annulus surrounding the inlet jet. Two solution outlets are used, positioned radial to the electrode. A schematic of the modified cell design is shown in Figure 1. The flow system used in these experiments was a peristaltic pump and included a simple pulse suppression device. This arrangement is shown schematically in Figurc 2. All chemicals were of “Anal&” grade and solutions were freshly prepared for each determination. Instrumentation. Current-potential curves were measured with a polarographic analyzer (Model PAR 174, Princeton Applied Research, Princeton, NJ). A variable-flow peristaltic pump (Model Variopex, LKB Instruments) was used to obtain solution flow rates of 0.2-10 cm3/min. Typical current-voltage curves are shown in Figures 3 and 4,illustrating the effect of flow rate on the limiting current. Experimental measurements were made by using potassium ferrocyanide or hydroquinone as materials in a supporting electrolyte of 0.1 M potassium chloride. Solutions were deoxygenated with purified nitrogen for 20 min before measurement and, in addition, a nitrogen atmosphere was maintained over the test solution reservoir.
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ANALYTICAL CHEMISTRY, VOL. 55, NO. 8, JULY 1983
EN~skg/bgZI1 2
0.8
0.4
0
Flgure 3. Current-voltage curves: 5 X
M hydroquinone, 1 M potassium chloride; ring electrode (modification cell), internal diameter = 7.0 mm, external diameter = 10.0 mm; scan rate = 5 mV/s; inlet-electrode separation = 0.5 mm; inlet diameter = 0.3 mm.
Flgure 5. Varlation of i, with inlet electrode separation: disk electrode (Fleet-Little version), diameter = 3 mm, Inlet diameter = 0.3 mm; 5 mM ferrocyanide, 1 M potasslum chloride.
I
I
E/VvsAq/A$I
1’2
08
04
0
Flgure 4. Current-voltage curves: 5 X M hydroquinone, 1 M potassium chloride; disk electrode (modified cell), electrode diameter = 5 mm; scan rate = 5 mV/s; inlet-electrode separation = 0.5 mm;
inlet diameter = 0.3 mm.
RESULTS AND DISCUSSION In agreement with Yamada and Matsuda, the proportionality between i and V predicted by eq 3 was found to be obeyed only when the inlet-electrode separation, d, was greater than a certain value, which is governed by several factors: the diameter of the electrode, solution flow rate, solution viscosity, and the geometry of the nozzle body. For the disk wall-jet cell of the Fleet and Little design (6), when the inlet-electrode separation, d, was less than 0.5 mm, it was found that the slopes of log i vs. log V plots were less than 0.5 at low flow rates (0.5-2 mL/min); the slope increases to the expected 3/4 value at higher flow rates. At inletelectrode separations greater than 0.5 mm, however, the slope was found to be close to 0.75 at low as well as high flow rates. The anomalous slope at low d can be explained as follows: At low flow rates (that is, large boundary-layer thickness) when the inlet-electrode separation is small, the nozzle is within the hydrodynamic boundary layer. Due to the large radial flow gradients in this layer, significant shearing forces arise between the nozzle surface and the radially flowing solution. There is, consequently, a reduction in the net momentum of the radial stream resulting in a diminishing of the shearing force (skin friction) a t the electrode surface. We could thus
expect a reduction in the limiting current. The relation between shearing force, represented by the skin friction, and the limiting current is given by the integral expression of eq 2. At higher jet flow rates the hydrodynamic boundary-layer thickness is relatively small (less than 0.5 mm); thus the nozzle body is outside the layer and there is no loss in momentum transfer. The slope of the log i vs. log V plot is, in this case, close to the theoretical value. The variation of the limiting current as the inlet-electrode separation is increased is shown in Figure 5. The general observations are similar to those noted by Yamada and Matsuda. Two distinct trends were observed: at low jet flow rates (3.5 mL/min) no minimum is apparent and ilim increases until it finally reaches a limiting value. These two dissimilar patterns can again be explained in terms of the effect of the nozzle body within the hydrodynamic boundary layer. Considering Glauert’s radial velocity profile, it could be reasoned that the ilimvs. inlet-electrode separation plots of Figure 5 correspond to points on the velocity profile. Then, the maximum of Glauert’s profile corresponds with the minimum of the plots: This is the point at which the radial velocity gradient, with respect to the nozzle-body surface, is a maximum and, hence, the limiting current a minimum. In the second case, at high jet flow rates, the iL, vs. d plot does not appear to reflect Glauert’s profile in that the decrease to a minimum is not observed. This could be because the radial velocity profile at higher jet flow rates is more constricted and, also, due to the scale of the plots shown in Figure 5; hence, ilimappears only to increase with d. On the basis of this reasoning, the point at which the current, iLm,reaches a limiting value should correspond to the boundary-layer thickness. In fact a definite correlation is found as predicted by eq 11. Further studies were made with the modified ring-disk wall-jet electrode. The variation of ilim with increasing jet separation, d , for this electrode gives a clearer indication of the effect of the nozzle body within the boundary layer. Figure 6 shows plots of ilimvs. d obtained with the ring electrode for
ANALYTICAL CHEMISTRY, VOL. 55, NO. 8, JULY 1983
1413
Table 1. Characteristic Reynolds Number for the Free Jet for Different Volume Flow Rates V, mL/min U , cmls Re
i /)A
13@
1.0
23.5
5.0
118.0
70 354
10.0
235.7
7 07
110-
90-
70 -
50-
3.0
L.0
dlrnm
Figure 6. Variation of i,,, with inlet-electrode separation at different flow rates: ring electrode (modified cell), same dlmenslons as Figure 4; 2 mM ferrocyanlde, 1 M potassium chlorlde.
'P
103
A
80
60
.-
20
3.0
40
dirnrn
Flgure 7. Variation of i, with inlet-electrode separation comparison of ring and disk electrodes of modified cell: flow rate = 1.6 mL/min; 2 mM ferrocyanide, 1 M potasslum chloride; (A) disk electrode, (B) ring electrode.
the volume flow rate range 1.05-2.9 mL/min. The plots show more clearly the gradual decrease in ilia with increasing d, reaching a rounded minimum. Unfortunately, the cell construction limited the attainable value of d to a maximum of 5 mm. Thus we were not able to show the effect of removing the nozzle lbody from the boundary layer when we could expect the limiting current to reach a steady state, as observed for the disk ellectrode. For comparison, Figure 7 shows two ili, vs. d plots for the ring and disk electrodes of the modified wall-jet cell. As the radius of the disk is less than that of the outer ring, we should expect to see a smaller boundary-layer thickness for the former. This can in fact be observed in that the limiting current foir the disk appears to reach a steady-state value before that for the ring. Turbulent Flow. In general, when the solution-flow velocity is increased such that its Reynolds number (the dimensionless ratio of intertial to viscous forces) is greater than
a critical value, the solution flow breaks up marking the transition to turbulent flow. The characteristics of turbulent flow are best described as chaotic with a mean flow velocity in a particular direction; the mathematical treatment is expectedly intractable. In the case of the wall-jet, the question as to whether flow characteristics are laminar or turbulent is governed by tlhe stability of the jet as it emerges from the nozzle rather than that of the radially flowing solution after impinging on the surface. The stability of the jet depends on the dimensionless Reynolds number, Re where Re = Ul/v,where U = jet velocity in cm/sec, 1 = nozzle diameter, and v = kinematic viscosity. According to experimental findings, a three-dimensional jet remains laminar up to a critical Reynolds number Rec,, where 25 < Recr < 1000 (14). As the Reynolds number is increased beyond Rec,, the jet becomes more concentrated and less stable, finally breaking up and becoming turbulent. For a nozzle diameter of 0.03 cm and taken to be cm2 s-l, the Reynolds number can be calculated as summarized in Table I. The jet emerging from the nozzle was examined visually by using a permanganate solution as a marker. It was found that the jet remained intact a t least 10 mm from the jet inlet in the flow rate range 1-10 mL/min. No signs of turbuleint eddy formation or spreading of the jet were seen. Thus it could be assumed that the jet is, in fact, laminar.
CONCLUSIONS The foregoing discussion defines the hydrodynamic performance of the wall-jet cell and makes some conclusions for optimizing its experimental operation. In order that the maximum efficiency may be obtained from the electrode, the body of the jet nozzle should be located well clear of the hydrodynamic boundary layer. This minimum distance between nozzle and electrode surface is given by eq ll which also defines the boundary-layer thickness. Alternatively, the nozzle should be conilcally shaped such that it remains outside the boundary layer even at small inlet-electrode separations or low flow rates. Similar considerations should be borne in mind when designing other electrochemical reactor cells in that the counterelectrode, anode, separator, or diaphragim should not be located so close to the working electrode that the boundary layer is disturbed. The implication of these studies to the practical utilization of the wall-jet electrode in HDV is clear. In the design of wall-jet detectors for high-pressure liquid chromatography and anodic stripping vokammetry, it might be anticipated that increasing the inlet-electrode separation might cause loss of resolution due to break up of the jet. However, as the preceding studies show, the jet remains intact up to quite large separations. Furthermore, the effective cell volume is only of the order of the boundary layer and not the geometric cell volume. This is a characteristic feature of the wall-jet cell which is not present in other designs. In other geometries, attempts to minimize cell volume have resulted in interference with the boundary layer, reducing the overall sensitivity, and causing undesirable side effects such as band spreading to occur. The present study has attempted to review the principal factors involved in the design of electrochemical detectors based on hydrodynamic voltammetry. It is concluded that
Anal. Chem. 1983, 55, 1414-1417
1414
the wall-jet design is probably the most effective design so far reported, in that it offers well-defined hydrodynamic performance characteristics and high sensitivity to flow rate and is both mechanically and electrochemically stable, The cell is simple to construct and contains no moving parts, and the component electrodes are demountable for ease of maintenance. Registry No. Carbon, 7440-44-0.
LITERATURE CITED (1) Nernst, W., Z . Phys. Chem. 1904, 47, 52. (2) Delahay, P. “New Instrumental Methods in Electrochemlstry”; Intersclence: New York, 1954. (3) Levich, V. G. “Physicochemical Hydrodynarnlcs”; Prentlce-Hall: Englewood Cllffs, NJ, 1962. (4) Matsuda, H. J. Electroanal. Chem. lg68, 76, 153.
(5) Fleet, 8.; Gunasingham, H.; Berger, T. A.; Das Gupta, S.; de Damia, G.: Llttle. C. J. “Phvslcochemlcal Hydrodynamics”; Advance Pub.: . . London, 1977; Vol. 1; p 373. Fleet, B.; Llttle, C. J. J. Chromatogr. Sci. 1974, 12, 747. Gunaslngham, H.; Fleet, B. J. Chromatogr. 1983, 261, 43. Glauert, M. B. J. Fluid Mech. 1956, 1 , 625. Yamada, J.; Matsuda, H. J. Electroanal. Chem. 1973, 44, 169. Schllchting, H. “Boundary Layer Theory”; McGraw-Hill: New York, 1968; p 130. Von Karman, T. 2.Angew. Math. Mech. 1921, 1 , 233. Riddiford, A. C. Adv. Electrochem. Electrochem. Eng. 1966, 4 , 47. Berger, T. A. Ph.D. Thesls, London University, 1975. Birchoff, G.; Zarruntoneilo, G. H. “Jets Wakes and Cavities”; AcademIC Press: New York, 1957. Hanekamp, H. B.; van Niewkerk, H. J. Anal. Chim. Acta 1980, 121, 13.
RECEIVED for review January 18, 1983. Accepted April 14, 1983.
CORRESPONDENCE Sample Introduction into the Inductively Coupled Plasma by a Radio Frequency Arc Sir: The inductively coupled plasma (ICP) has over the past decade developed into a powerful tool for multielement analysis by atomic emission spectrometry (AES) (1-3). Despite ita widespread use, ICP-AES remains limited in routine applications by sample introduction methods that require large volumes of aqueous solutions. Considerable impetus has existed and continues for the development of ICP sample introduction techniques capable of accepting either solid samples or very small quantities of solutions. Previous efforts in this direction have encompassed a wide variety of techniques, including direct aspiration of powders into the plasma (4-8), spark or arc sampling (9-11), laser vaporization (12,13), aspiration of microliter quantities of solution (14, 15),electrothermal atomization (16-20), and direct insertion of a graphite-cup sample holder into the base of the ICP discharge (21-24).
In this preliminary communication we describe a new method for the analysis of solids and microsamples by ICPAES. The new method parallels the direct insertion methods mentioned above; however instead of the sample being transported to the plasma, the plasma is brought to the sample. Initial experiments described here indicate that the method is capable of sampling a wide range of elements in solid form and can be applied to solution microsampling as well.
EXPERIMENTAL SECTION The new technique is based on the discovery that a grounded conductor, placed below the sample tube of a modified ICP torch, can attract a stable arc filament from the base of the ICP discharge. The torch and sample holder assembly used in this preliminary study is illustrated in Figure 1. The quartz torch differs from conventional ICP torches in that its overall length has been reduced to 8.5 cm and that the 4-mm i.d. central sample tube is flared at the base to form a small bell jar. The sample gas is introduced tangentially into this bell jar, which is clamped with a nylon ring to a water-cooled copper base. A viton O-ring provides a gastight seal between the bell jar and the base. A boron-nitride shield isolates the base thermally and electrically from the arc. Several different sample holders and configurations
were used, each of which will be described in detail in later sections. Power was supplied to the plasma by a 27.12-MHz, 2.5-kW rf generator (Model HFP-2500 D with Model APCS-1 power control and AMN-2500E impedance matcher, Plasma-Therm Inc., Kresson, NJ). A conventional three-turn load coil was used. Argon flow rates were 18.0 L/min coolant gas, 0.9 L/min plasma gas, and 0.9 L/min sample gas. In early studies the radio frequency (rf) arc was ignited simultaneously with the ICP by turning on all gas flows and applying a high-voltage pulse from a Tesla coil to a metal ring placed around the torch between the base of the plasma tube and the bell jar. For later experiments the plasma and the arc were ignited independently; the plasma was lit with the sample gas turned off and electrical contact between the copper base and ground broken by a solenoid-actuated, highvoltage switch. Once the plasma had stabilized, the arc could be lit by turning on the sample gas, grounding the base, and striking the Tesla coil a second time. If struck to a thermally stable electrode, the arc has no apparent effect on the ICP discharge itself except to lower the power delivered to the fireball. A rough measure of the amount of power being siphoned from the plasma by the arc was obtained by measuring the continuum intensity at 300 nm with the arc on at a power of 1.5 kW. The plasma was then relit with the arc off and the power was reduced until the continuum intensity at 300 nm matched that measured earlier with the arc on. The match occurred after a reduction in rf power of 140 W. The arc was further characterized by measuring with an ac current probe (Tektronix Model No. 6022) and oscilloscope the current passing through the lead connecting the copper base to ground. The oscilloscope trace revealed the current to be nearly sinusoidal at 27 MHz and to vary with applied rf power as shown in Figure 2. The difference in Figure 2 between the current measured with a tungsten-pin electrode and that passed by a copper-cylinder sample is probably due t o thermionic emission and the difference in temperature reached by the two electrodes. The 0.8-mm tungsten pin had poor thermal contact with the copper heat sink and glowed white hot. In contrast, the 6-mm copper cylinder remained relatively cool. Thermal emission of electrons from the tungsten would lower the impedance of the arc channel and raise the current. A surprising result of the current measurements was the discovery of a substantial current that is present even when the arc is not lit (cf. Figure 2). This current presumably results from capacitive coupling between the
0003-2700/83/0355-1414$01.50/00 1983 Amerlcan Chemical Society
