Anal. Chem. 1983, 55, 1405-1409
\
1.0
\
-
I
L L -
-A
at infinite dilution. The difference between the two pK, values seems reasonable by a comparison with data for the solubility product of lead oxalate. From the results by Klatt (I9),recalculated by Hedstrom et al. (20), pK, of lead oxalate at infinite dilution is found to be about 10.3 by applying an activity coefficient adjustment from I = 0.15 M to I = 0. The value of pK, in 1 M NaC10, reported in ref 20 is 9.02. The difference between the pK, values at I = 0 and I = 1 M would then be 1.3 for lead oxalate. The corresponding difference for cadmium oxalate calculated from our result in 1 M NaNO, and the reported value at infinite dilution is 1.37.
I
'.dl
t
14105
I
LITERATURE CITED Ig[Ox]/M
-3
Johansson, L. Coord. Chem. Rev. 1968, 3 , 293. Shah, A. C.; Ochs, J. F. J. Pharm. Sci. 1974, 6 3 , 110. Tlngstad, J.; Dudzlnskl, J.; Lachman, L.; Shami, E. J. Pharm. Ekl. 1973, 62,1527. Needham, T. E.; Luzzi, L. A.; Mason, W. D. J . Pharm. Sci. 1973, 62, 1860. Danlelsson, R.; Wiltmark, G., unpublished work. Baecklund, P.; Danielsson, R.; Wlkmark, G., unpublished work. Olln, A.; Svanstrom, P. Acta Chem. Scand., Ser. A 1975, A29, 849. Kanemura, Y.; Waitters, J. I. J . Inorg. Nucl. Chem. 1967, 2 9 , 1701. Bonarl, E. Monatsh. Chem. 1975, 106, 451. Schaap, W. B.; McMasters, D. L. J. Am. Chem. SOC. 1961, 193, 4699. McMasters, D. L.; IllRalmondo, J. C.; Jones, L. H.; Lindley, R. P.; Zeltmann, E. W. J. Phys. Chem. 1962, 66, 249. Dhuiey, D. G.; Jahaglrdar, D. V.; Khanolkar, D. D. J. Inorg. N L ~ . Chem. 1975, 3 7 , 2135. Khurana, S . C.; Guipta, C. M. J. Inorg. Nucl. Chem. 1973, 3 5 , 209. Clayton, W. J.; Vosburgh, W. C. J . Am. Chem. SOC.1937, 5 9 , 2414. Vosburgh, W. C.; Beckman, J. F. J. Am. Chem. SOC. 1940, ($2, 1028. Oncescu, T.; Macovschl, M. An. Univ. Bucuresfi Ser. Stint. N a t . , Chlm. 1967, 16, 7'7. Chem. Abstr. 1969, 70, 118674. Ermolenko, V. I.; IErmolenko, G. I. Dopov. Akad. Nauk. Ukr. R ! R , Ser. 8 : Geol., Geoflz., Khlm. 1975, 1 1 , 996. Chem. Abstr. 1976, 8 4 , 127321 Arevalo, A.; Rodriguez Placerez, J. C.; Cabrera Gonzalez, A.; Segura, J. An. Quim. 19741, 70, 824. Chem. Abstr. 1975, 8 3 , 016583. Kian, L. N. Anal. Chem. 1970, 42, 1837. Hedstrom, H.;Olln, A; Svanstrom, P.; Ash, E. J. Inorg. Nucl. Cham. 1977, 39, 1191.
-2
Flgure 3. !Soiubili of cadmium oxalate in oxalate buffers. The drawn curve has been calculated with the constants determined in this study.
1 M KNOBto be log ,f311y= 3.605 and log PzH = 4.589 using a method simililar to the one used by Olin et al. (7). The solubility experiments were performed as six "titrations" producing the experimental points, all shown in Figure 3. The equilibrium constants were obtained by fitting the parameters of the right-hand side of eq 6 to the experimental values of S by the method of least squares. The values of a were calculated from eq 7 by an iterative procedure. In the first cycle the influence of complexation was neglected and in the following cycles values of @, obtained from eq 6 were used. The computations were continued until no further changes in the constants occurred (four cycles). Calculations were made with N = 2, 3, and 4 but a reasonable fit between SexD and Scdcd was only obtained for N = 3. The relative standard deviation of (S,, - Sd&)was 2.6% for all points. This figure may be compared with an estimated accuracy in the analyses of 2% (see also Figure 3). The values of the equilibrium constants are given in Table I. The equilibrium constants obtained in this study are in good agreement with the ones obtained by polarographic and potentiometric measurements at the same ionic strength. The value of the solubility product is considerably larger than that
RECEIVED for review January 26, 1983. Accepted April 14, 1983.
Identification of Sulfonamide Drugs in Swine Liver by Collision- Induced Dissociation/Mass Analyzed Ion Kinetic Energy Spectrometry William C.: Brumley," Zhao Mln,' Jean E. Matueik, John A. G. Roach, Charlie J. Barnes, James A. Sphon, and Thomas Fario Division of Chemistry and Physics, Food and Drug Administration, Washington, D.C. 20204
The use of collision-induced dissoclatlon mass analyzed ion kinetic energy spectrometry (CIDIMIKES) for the IdQntlflCation of sulfonamide drug residues in swine liver is described. Routlne tlwe cleanup yields a sample extract that Is directly analyzed with solid probe Introductlon. Drug residues are H)' ion identified by the CID/MIKE spectrum of the (M produced under chemical ionization (isobutane) mass spectrometry. The specificity of the CID/MIKE spectra obtained lor 18 compounds and the sensltlvlty of the technique for residue identlficatlon are discussed and the origin of certaln fragment nons is ciarlfied.
+
V i s i t i n g Scientist from t h e N a t i o n a l I n s t i t u t e of Metrology, Beijing, People's Republic of China.
Sulfonamide drugs (sulfas) are widely used for the prevention and treatment of disease in food-producing animals. The food supply miust be monitored to determine if sulfa residues are present in animal-derived foodstuffs ( I ) . Current regulations specify a maximum limit of 0.1 ppm sulfa residues in animal tissues, with swine liver designated as a target tissue (1).
Methods developed for the analysis of animal tissues for sulfas usually consist of separate determinative and confirmative procedures. Since these compounds are usually highly polar and occasionallly thermally labile or nonvolatile, they are derivatized to facilitate gas chromatography (GC) of the sample. Thus, electron capture (EC)/GC is often used in the determinative step (2,3).
This article not sublect to U.S. Copyright. Published 1983 by the American Chemical Society
1406
ANALYTICAL CHEMISTRY, VOL. 55, NO. 8, JULY 1983
. N ~ ~ - Z - ( $ ' Ten,
H , N ~ ~ - ~ - Q ~ ' "
0 Y
6
*CY,
3
4
7
8
11
Figure 1.
~ N o I - L ~ ~ '
0
2
1
5
n,i-c-e!-;>_q-~-~-w,
B
12
13
Structures of sulfonamide drugs. See Table I for identification.
Mass spectrometry (MS) is usually used for the confiiative step. The behavior of sulfas under electron ionization (EI) was reported 20 years ago by Spiteller and Kashnitz (4). More recently, the positive and negative ion chemical ionization (CI) mass spectra of sulfas were compared in detail to the E1 spectra by Roach et al. (5). Garland et al. (6)successfully used GC/MS multiple ion detection to confirm sulfa residues in tissue extracts. Sphon (7)and Brumley and Sphon (8) reviewed general procedures for the confirmation of residues of veterinary drugs in animal tissues. Determinative and confirmative procedures are ultimately subjected to a limited collaborative study or method trial for validation as acceptable regulatory methods. The recent development of MS/MS techniques (9-11), including collision-induced dissociation mass analyzed ion kinetic energy spectrometry (CID/MIKES) (12),offers the potential for simplifying cleanup, avoiding chromatographic introduction of sample, or decreasing sample analysis time. Henion et al. (13)have described a liquid chromatographic MS/MS technique using a triple quadrupole mass spectrometer for the determination of sulfa drugs in the plasma and urine of race horses. In this paper we describe the application of CID/MIKES to the confirmation of sulfa drug residues in a food matrix using cleaned-up extracts of swine liver. In the course of our investigations, the CID/MIKE spectra of 18 sulfa drugs were obtained and are described. Certain aspects of the fragmentation of sulfa drugs are clarified by use of high-resolution accurate mass measurements. The limits of confirmation, effects of interferences, and specificity are discussed. EXPERIMENTAL SECTION Chemicals. Sulfonamide drug standards were obtained from drug manufacturers. The sulfas had been previously tested for purity by derivatization and EC/GC detection. Solvents (Burdick and Jackson, Muskegon, MI) were distilled in glass. Cleanup of Extracts. Fresh swine liver was obtained locally at a slaughterhouse. The extraction/cleanup procedure is essentially that of Manuel and Steller (14)carried t~ the methylation step. Briefly, it consists of extraction of tissue into chloroformacetone, extraction into aqueous acid, washing with hexane, neutralization and extraction into methylene chloride, and final dissolution into methanol. Mass Spectrometry. CID/MIKE spectra were obtained on a VG ZABdF instrument (8 keV accelerating voltage) by scanning the electric sector voltage using the digital MIKES unit and recording by light beam oscillograph (energy range 2-8.2 keV). Isobutane (99.5%, Matheson, East Rutherford, NJ) was used as reagent gas with an ion source housing pressure of 4 X mbar; emission current 0.50 mA; source temperature 150-200 "C; and helium pressure 2 X lo-' mbar in the collision cell housing region.
Samples were introduced by a solid probe heated independently of the ion source. RESULTS AND DISCUSSION General Features of MIKE Spectra. The CID/MIKE spectra of the sulfas (Figure 1)are given in Table I. Spectra are based upon decomposition of the (M + H)+ion produced under isobutane CIMS. Relative abundances of peaks are representative of typical runs and are based on the most intense fragment ion observed rather than on the main beam, and those ions occurring 1 or 2 amu from the (M H)+ ion are not considered. The spectrum of sulfamethazine provides a reference point for discussion of major fragmentation and is illustrated in Figure 2. Fragment ions are observed that are common to both the E1 and CI mass spectra (5). The ion a t m / z 213 corresponds to (M + H - 66)' or (M H - H2S02)+.The ion a t m / z 213 is also observed in E1 spectra, where it is the base peak (5). The (M + H - 66)+ ion or in some cases the (M + H - 65)+ ion is observed in the spectra of almost all sulfas investigated and therefore constitutes a general feature of the spectra. The most important ion in the spectra of sulfas is m / z 156, which is relatively abundant for each compound studied. The (M H)+ion and m/z 156 constitute a relatively specific indication of the presence of a particular sulfa. The presence of additional ions increases confidence in an identification. High-resolution mass measurement of m / z 156 under E1 conditions (C6H6N102S1: calcd 156.0120, found 156.0119) is consistent with the representation of the ion as shown in structure 1 but does not prove it. This view of the
+
+
+
W
O
m/z
O
2 1'
156
1
fragmentation as arising from the sulfonamide portion of the molecule is also consistent with the negative ion CI spectra, in which m / z 155 or 156 is almost always observed (5), and with the appropriate change in mass of this ion upon compound derivatization (5, 6). In addition, high resolution mass measurements of m/z 140 ( C ~ H B N ~ O ~calcd S ~ : 140.0171, found 140.0159), m / z 108 (C6H6N101: calcd 108.0452, found 108.0446), and m / z 92 (C6H6N1: calcd 92.0501, found 92.0505) confirm that these fragment ions also arise from the sulfonamide portion of the molecule and that the elements of the amine group (N4)are still present. The m/z 140,108, and 92 ions are also frequently observed in the CID/MIKE spectra of sulfas as indicated in Table I.
ANALYTICAL CHEMISTRY, VOL. 55, NO. 8, JULY 1983
1407
The ion at m / z 124 of sulfamethazine is labeled as the (AHz)+ firagment in Table I, and it or the (AH)" ion is characteristically found in the CID/MIKE spectra of most sulfas. Its analogue in the E1 spectrum of sulfamethazine occurs at m J z 123. High-resolution mass measurement of mJz 123 under E1 conditions (C6H9N3: calcd 123.0796, found 123.0792) substantiates the view that this fragment ion arises from the pyrimidyl portion of the molecule and involves a hydrogen rearrangment in its formation. This observation is also consistent with the shift in mass observed for this fragment ion when derivatives of sulfas are considered (5,6). Thus, mJz 124 in the CI spectra undoubtedly results from protonation and hydrogen rearrangement. The last general fragmentation considered results in the ion at m/;r 186 in the CID/MIKE spectrum of sulfamethazine. This ion, designated AS, undoubtedly arises as shown in structure 2, which summarizes the major fragmentation pro149
:
279
205
92 186(AS)
156 12Z(A)
2
cesses. This ion is frequently present in the sulfa spectra presented, but it is absent or of very low abundance in CI and E1 spectra (5). Its presence for diagnostic purposes seems unnecessary since enough specificity is afforded by the (M + H)+ ion1 together with the fragments m / z 156 and (AH2)+. Other Ions. Additional fragment ions are observed in the CIDJMIKE spectra of the sulfas. Losses of 15,16,17, or 18 amu from the (M H)+ ion are encountered. Most often, fragment ions are specific for a given compound and may be associated with particular functionality present in the molecule. For example, loss of 42 amu from the (M + H)+ ion of sulfanitran is observed and undoubtedly involves elements of the acetyl group on N4,whereas m/z 105 is observed in the spectrum of sulfabenzamide and presumably represents the benzoyl group. Frequently, the additional functionality is part of the A moiety, and a and fl cleavage to a ring system are observed from both the (M H)' and (AH# ions. For example, 31 amu is lost from the (A4 H)+ and (AHz)+ ions of sulfadimethoxine, affording m / z 280 and 125. On the other hand, the (M + H)+ and (AH2)+ions of sulfaethoxypyridazine each exhibit a loss of 28 amu, resulting in m / z 267 and 112. The presence of halogen in the A portion of the molecule does not lead to relatively abundant fragmentation involving loss of the halogen atom, as may be seen from the spectra of sulfabromomethazine, sulfachloropyridazine, and sulfachloropyrazine. Therefore, in the absence of isotope peaks, the presence of halogen can only be inferred indirectly via the mass of the ions. Isomer Differences. Isomer effects in the CIDJMIKE spectra are observed in the pairs sulfadoxine and sulfadimethoxine and sulfachloropyridazine and sulfachloropyrazine. In the former pair, the primary difference involves the relative abundance of mJz 140, which is substantially more intense in the spectrum of sulfadoxine than in that of sulfadimethoxine. There are also some differences in fragmentation. For example, m / z 296 and 279 of sulfadoxine differ from mJz 294 and 280 of' sulfadimethoxine. The domination of both spectra by m / z 156, which can arise from both the sulfonamide and the (AH2)+fragment, is apparent. In the latter pair of conipounds, the spectra again exhibit an abundant ion at mJz 156 and more subtle differences in the relative abundances of m/z 167, 250, 249, 239, and 140. Isomer distinctions may con-
+
+
+
c
_---L-----_.
3
4
I _
5
6
7
0
heV
+
Flgure 2. CID/MIKE spectrum of (M H)+ ion of sulfamethazine: (A) standard, (B) 0.12 ppm in swine llver extract (1 g equiv),(C) same as B with effect of dibutyl1 phthalate impurity.
ceivably be enhanced by proper selection of activation energy, although translational energies in the kiloelectron volt range are not expected to be as sensitive to structural differences as those in the 90% at the 0.1 ppm level (18,19). These extracts still contain a large number of coextractives, which can interfere in the multiresidue determinations when rellatively nonspecific detection such as colorimetric determination and EC/GC is used. Thus, the issues regarding specificity and the role of controls and interferences remain pertinent in these analyses. Figure 2A illustrates the CID/MIKE spectrum of a sulfamethazine standard land Figure 2B the spectrum obtained for a 0.12 ppm spiked liver extract (1 g equiv). The sensitivity appears to be just adequate at this level for a full scan determination. Major ions are present at appropriate relative abundances, and interfering ions appear minimal. However,
ANALYTICAL CHEMISTRY, VOL. 55,
1408
Table I. CID/MIKE Spectra of the (M
NO. 8,
JULY 1983
+ H)+Ion of Sulfonamide Drugs % relative abundance ( R A )
(MtH66)' m/z (%RA)
no.a
mol wt
sulfamethazine
1
278
213(35)
124 (41)
186 (100)
sulfamerazine sulfanitran
2 3
264 335
199 (28) 270(-)
109 ( 7 0 ) b 139 (14)
172 (35) 201 (-)
sulfabromomethazine sulfaethoxypyridazine
4 5
356 294
291 (-) 230(19)d
202 ( 8 6 ) 140 (57)
264 (100) 201 (14)e
sulfanilamide
6
172
107 (-)
sulfabenzamide
7
276
211 (1)
sulfapyridine
8
249
184(47)
95 (33)
157 (-)
sulfadiazine
9
250
185(19)
96 (11)
158 (-)
sulfaquinoxaline
10
300
235(35)
145 ( 4 5 ) b
208 ( 9 )
sulfamethoxypyridazine
11
280
215(28)
126 (38)
188 (-)
sulfachloropyridazine
12
284
219(15)
130 ( 2 1 )
1 9 1 (5)e
sulfachloropyrazine
13
284
219(11)
130 (15)
1 9 1 (5)e
sulfaguanidine
14
214
149(3)
60(6)
122(3)
sulfisoxazole
15
267
202(1)
113 ( 8 )
175 (-)
sulfadoxine
16
310
245(15)
156 (100)
218 ( 7 )
sulfadimethoxine
17
310
245(16)
156 (100)
218 ( 6 )
sulfathiazole
18
255
190(3)
100 ( l l ) & 163 ( 3 )
compound
e
a See Figure 1 for structures. Corresponds to (AS - H)+.
m/z 156
(AH,)' (AS)+ m/z (% RA) m/z ( % R A )
1 8 (-1
Corresponds to (AH)'.
263 (16), 1 7 1 (4), 140 (6), 108 (9), 96 ( 3 ) , 9 2 ( 7 ) 248 (9), 1 4 0 (14),92 (32) 318 (20), 294 (48), 198 (loo), 181 (ll),134 (33), 1 2 1 (6), 108 (9), 92 (13) 342 (98). 279 (23), 267 (30), 251 (l), 214 (l), 186 (7), 172 (2), 124 (lo), 1 1 2 (21), 108 (23), 92 (24),
140(4): 124 (l), 108 ( 8 ) , 92 (16), 75 (11, 65 ( 5 )
80 (1)
122 ( 2 )
other m/z (% R A )
1 8 4 (-)
260 (1);140'(5), 108 (9), 105 (9), 92 (91577 ( 2 ) 233 (9), 140 (18),108 (27), 9 3 (22) 234 ( 8 ) , 140 ( 8 ) , 108 (14), 92 (11) 284 (9), 220 (5), 1 9 1 (3), 118 (131,108 (81992 ( 5 ) 265 (8), 140 (9), 108 (18), 92 (17) 267 (5), 249 (2), 184 (5), 140 (9), 1 0 8 (12), 102 (6), 92 (14) 267 (14), 250 (6), 184 (4), 140 (12), 108 (lo), 102 ( 8 ) , 92 (10) 198 (4), 1 7 3 (4), 140 (lo), 1 3 3 (3), 108 (1613 92 (131, 80 (21, 65 (3) 251 (71,140 (91,108 ( 5 ) , 9 2 (6),86 (2) . 296 (12), 279 (2), 230 (7), 216 (5), 214 (4), 1 4 0 (29), 114 ( 8 ) , 108 ( 8 ) , 92 ( 7 ) 294 (4), 280 (ll), 230 (4), 214 (31,140 (91,125 (111,108 . .. (12), 92 (19) 239 (l), 140 (9), 108 ( l o ) , 92 (10)
Corresponds to (198 - 42)+.
Corresponds to (M t H - 65y.
ll A
A 3
4
5
6
7
8 k.V
+
-
~ 3
-/ 4
i 5
6
L 7
8 heV
+
Figure 3. CID/MIKE spectrum of (M H)+ Ion of sulfadlmethoxine: (A) standard, (B) 0.2 ppm in swine liver extract (1 g equiv).
Flgure 4. CID/MIKE spectrum of (M H)+ ion of sulfaquinoxaline: (A) standard, (B) 0.2 ppm In swine liver extract (1 g equiv).
we found that inadvertent postcleanup contamination, presumably by dibutyl phthalate (M, = 278), could pose an interference as it results in intense ions a t m / z 205 and 149 superimposed on the spectrum of sulfamethazine (Figure 2C). The validity of an identification is doubtful in cases where additional ion responses are observed and cannot be accounted
for. Unspiked extracts showed no significant contributions in the CID/MIKE spectrum of m / z 279. This indicates that this level of cleanup is adequate for the solid probe determination. T o explore the generality of this approach, we also investigated sulfadimethoxine and sulfaquinoxaline in cleaned-up
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-layerthickness 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
