Anal. Chem. 1987, 59,2014-2016
2014
to PhCO+, C5H11CO+,McLafferty rearrangement product, and various alkyl group losses from the side chain. At the most positive voltage, a PhCHOH+ ion is also seen; this can be attributed to a product of reduction at the disk, followed by slow migration to the ring and the oxidation to the cation. Decanophenone behaves similarly. Although anisole is electrochemically active in more conventional solvents (In,we observe no radical cation or dimer product for it at any voltage; 1,2-dimethoxybenzene behaves similarly. In contrast, 1,3,5-trimethoxybenzene shows a prominent radical cation signal from +3 to +12 V. A possible problem here is the mass range of the MS-50 in FAB mode; it was calibrated to examine data only down to ca. 90 amu. Products of electrochemical reactions of smaller mass were not observed therefore. Nitrobenzene does not show any distinctive peaks in the negative ion mass spectrum at any EFAB potential, although there is a dependence of probe current on time similar to that described above for bromohexadecane. 1,2- and 1,3-Dinitrobenzene both give radical anion peaks as the base peaks in the spectrum at voltages from +3 to +12 V, with no evidence of higher multiples or fragmentation products. A number of other compounds have been briefly examined. Naphthalene, anthracene, and phenanthrene are very poorly soluble in glycerol. EFAB spectra of these do not show large 1) and (M + 3) cations signals, but there are both (M observed. The latter may be due to some process similar to the reduction/oxidation combination mentioned above for the phenones (17). Triolein, in negative ion mode and the ring reducing, does not show a radical anion (it is unlikely that the electron in such a species would be bound) but does give strong signals for the carboxylate anion C17H33C0;. The major problem with this technique at present is the limited solubility in the glycerol matrix of many of the samples tried. We have emphasized the use of less polar compounds so far, since these are the ones least successful with conventional FAB MS (1-3,11). The high polarity of glycerol, which makes it useful for electrochemistry, also limits the solubility of nonpolar species. We are currently trying a number of other possible matrix materials that should favor the solubility of lipids and aromatic species but still support electrochemistry. In conclusion, we believe that this is a technique that will expand the ability of FAB MS to handle a wide variety of
+
compounds, using a relatively minor instrumental modification. We are continuing our investigations into the design and chemistry of this technique. ACKNOWLEDGMENT We wish to thank the Division of Biochemistry, Walter Reed Army Institute of Research, Washington DC, for use of their mass spectrometry facilities and James Q.Chambers for helpful discussions. LITERATURE C I T E D Barber, M.; Bordoli, R. S.; Sedgwick, R. D.; Tyler, A. N. J. Chem. Soc., Chem. Commun. 1981, 325. Burlingame, A. L.; Dell, A,; Russell, D. H. Anal. Chem. 1982, 5 4 , 363R. Burlingame, A. L.; Whitney, J. 0.; Russell, D. H. Anal. Chem. 1984, 5 6 , 417R. Desiderio, D. M. Anahsis of Neuropeptides by Liquid Chromatography and Mass Spectrometry; Elsevler: N.Y., 1984; p 157. Dell, A.; Morris, H. R.; Egge, H.; von Nicolai, H.; Strecker, G. Carbohydr. Res. 1983, 115, 41. Reinhold, V. N.; Carr, S. A. Mass Spectrom. Rev. 1983, 2, 157. Slowlkowski, D. L.; Schram, K. H. Nucleosides Nucleotides 1985, 4 , 309. Garrison, 8. J. J. Am. Chem. SOC. 1982, 104, 6211. De Pauw, E. Anal. Chem. 1983, 5 5 , 2195. Ion Formation f r o m Organic Solids; Springer Series in Chemical Physics, 25; Benninghoven, A,. Ed.; Springer-Verlag: Berlin, 1983. Dube, G. Org. Mass Spectrom. 1984, 19, 242. Meili, J.; Seibl, J. Int. J. Mass Spectrom. Ion Phys. 1983, 46, 367. Gordon, A. J.; Ford, R. A. Chemist's Companion; Wiley: New York, 1972; p 12. Radin, N.; de Vries, T. Anal. Chem. 1952, 24, 971. Evanoff, J. E.; Harris, W. E. Can. J. Chem. 1978, 56, 574. Pelzer, G.; De Pauw, E.; Dung, D. V.; Marien, J. J. Phys. Chem. 1984, 88, 5065. Hammerich, 0.; Parker, V. D. Adv. Phys. Org. Chem. 1984, 20, 55.
J o h n E. Bartmess* Department of Chemistry University of Tennessee Knoxville, Tennessee 37996-1600 Lawrence R. Phillips Division of Biochemistry and Biophysics Office of Biologics Research and Review Food and Drug Administration 8800 Rockville Pike Bethesda, Maryland 20892 RECEIVED for review October 23, 1985. Resubmitted October 16, 1986. Accepted April 24, 1987.
AIDS FOR ANALYTICAL CHEMISTS Modified Cell for Stripplng Voltammetry Using the Static Mercury Drop Electrode J o s e p h Wang* a n d T u z h i P e n g
Department of Chemistry, N e w Mexico State University, Las Cruces, New Mexico 88003 Stripping analysis has become increasingly important for highly sensitive measurements of trace electroactive species (1-3). Conventional and adsorptive stripping schemes are now available for measuring low levels of numerous metals and organic compounds. The increasing interest in stripping analysis has been, in part, due to the introduction of the modern static mercury drop electrode (SMDE), the EG&G PAR Model 303, which allows virtually instantaneous automatic production and renewal of hanging mercury drop electrodes (HMDEs) (4). A number of similar devices aiming at automatic generation of HMDEs are now available commercially from different sources. 0003-2700/87/0359-2014$0 1.50/0
While the EG&G PAR Model 303 SMDE represents a major instrumental advance, its cell assembly suffers from a major drawback when stripping applications are concerned. Simply stated, mercury drops-dislodged from the capillary at the end of each stripping cycle-accumulate at the bottom of the cell. The consequence of the mercury presents at the bottom is a gradual decrease in the efficacy of the convective mass transport, employed to facilitate the deposition process (because of increasing resistance to the angular motion of the stirring bar). The small cell dimension (9 mm bottom radius) further enhances the severity of the problem, as only tiny stirring bars can be employed. As the deposition current a t Q 1987 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 59, NO. 15, AUGUST 1, 1987
D
C
i,nA 80
1 inch
2015
I
40
01
0
10
20
30
1
MEASUREMENT NUMBER
Flgure 2. Stripping behavior of the hanging mercury drop electrode under continuous use; successive measurements of 2 X lo-’ M lead using the unmodified (a) and modified (b) cells. Flgure 1. Improved cell for stripping voltammetry at the hanging mercury drop electrode of EG&G PAR Model 303 SMDE: (A) mercury “collector”, (B) stirring bar: (C) worklng electrode; (D) reference electrode; (E) auxiliary electrode.
the HMDE (and hence the resulting stripping peak) depend upon the square root of the solution stirring rate ( I ) , lower and erratic results are obtained. In particular, the reliability of the data obtained in many prolonged experiments is largely reduced, because of the increasing mass of mercury associated with numerous dislodged drops. This represents a perennial problem in common stripping studies, such as the construction of calibration plots, deposition-time dependence, precision, or tedious speciation procedures, like measurements of complexing capacity or pseudopolarograms. These erratic results may be misinterpreted as other effects, e.g., surface saturation or formation of intermetallic compounds. Unfortunately, this important aspect of stripping experiments a t the HMDE has not been addressed. Obviously, only stripping applications of the SMDE assembly-and not polarographic ones (employing quiescent solutions)-suffer from the above shortcomihg. This communication describes a simple and inexpensive modification of the cell to eliminate problems associated with the presence of mercury at the cell bottom. A closed side-tube, branching from the cell bottom, is used to trap the mercury, thus maintaining a “mercury-free” cell bottom for effective and reproducible stirring (Figure 1). Each drop dislodged from the capillary is mechanically pushed by the stirring bar toward the collecting tube. Comparative results presented for the modified and unmodified cells emphasize the superior performance and unique features of the new cell design.
EXPERIMENTAL SECTION Apparatus. The EG&G PAR Model GO057 cell, used in conjunction with the SMDE (Model 303A), was modified as follows. A glass tube (15 mm long, 2.5 mm i.d., 5 mm 0.d.) was branched from the cell bottom; the tube was closed in its outer end (see Figure 1 for schematic). The tube joined the bottom of the cell at ca. 15O to facilitate the collection of mercury drops. The cell was placed on an EG&G PAR Model 305 stirrer. A Sargent-Welch Model S-76507-60BTeflon-coated stirring bar (8 mm long, 1.5 mm i.d.), placed at the center of the cell bottom, provides reproducible mass transport during the deposition period, as well as pushing dislodged mercury drops toward their trap. A medium-size HMDE, with a surface area of 0.015 cm2,was used as working electrode. The sample cell was fitted with a Ag/AgCl reference electrode and a platinum wire auxiliary electrode. Voltammograms were obtained with the EG&G PAR Model 264A stripping analyzer, with the EG&G PAR Model 0073 X-Y recorder collecting the experimental data. Reagents. All solutions were prepared from analytical-grade M, of chemicals and deionized water. Stock solutions, 1 X various metal nitrates, were prepared and stored in polyethylene bottles. Stock solutions, 5 x IO-* M, of oxytetracycline (Sigma) were prepared daily. Supporting electrolytes for conventional
and adsorptive stripping measurements were 0.05 M acetate buffer (pH 4.5) and 0.025 M boric acid solution (adjusted to pH 5.5 with sodium hydroxide),respectively. The untreated river water sample (Rio Grande) was analyzed within 12 h of sampling. Procedure. A 10-mL volume of the supporting electrolyte solution was added to the cell and degassed with nitrogen for 8 min. The deposition potential was applied at the electrode for a selected time. The stirring was then stopped and after 15 s the voltammogram was recorded by applying a linear scan (50 mV/s) in the positive direction (for conventional stripping analysis) or in the negative direction (for adsorptive stripping voltammetry). A differential pulse stripping waveform (10 mV/s scan rate, 50 mV amplitude) was employed for the metal speciation experiments.
RESULTS AND DISCUSSION The modified cell was tested and compared to the unmodified one under various experimental conditions, commonly employed in stripping analysis. Figure 2 compares the stability of the response for 30 repetitive measurements of 2 X lo-’ M lead using the unmodified (a) and modified (b) cells (1min deposition at -1.0 V, with 400 rpm stirring). In both cases, a highly stable response is observed over the first 20 measurements. However, subsequent measurements using the unmodified cell exhibit a gradual decrease in the lead peak (up to 19% a t the 30th measurement; relative standard deviation over the entire series, 3.5%). In contrast, no loss of stability is observed for analogous measurements a t the modified cell (relative standard deviation of 1.6% ). Apparently, the first 20 drops collected at the bottom of the unmodified cell have only negligible effect on the efficacy of the stirring motion. However, with larger volumes of mercury present, convective mass transport becomes sluggish, with the stirring bar unable to effectively overcome the large mercury weight. A temporary stoppage of the stirring motion may also be observed in the presence of a large mass of mercury. At the modified cell, the dislodged mercury drops are being “collected” a t the closed sidearm, and the stirring motion is not interrupted. The present design allows collection of over a hundred ”medium-size”drops, more than enough in common stripping experiments. Obviously, the stability profile shown in Figure 2a is dependent on the size of the mercury drop employed. While “medium-size” drops were employed for collecting these data, the use of “large-size”drops would result in earlier degradation of the stability at the unmodified cell. The implications of the data of Figure 2 upon prolonged stripping experiments are obvious. Many such experiments %onsumen large numbers (20-50) of mercury drops; the presence of mercury in the bottom of conventional cells can thus strongly affect the reliability of the data. For example, Figure 3 compares ten-point calibration experiments for lead (A) and oxytetracycline (B) using the conventional (a) and modified (b) voltammetric cells. Anodic stripping and adsorptive stripping procedures were employed to quantitate the metal and the drug, respectively. Measurements a t each
2616
ANALYTICAL CHEMISTRY, VOL. 59, NO. 15, AUGUST 1, 1987
cu
Conc.,io-'~ IBI 2
0
4
,
I
20
10
0
conc.,lo-'M IO
0
20 r-
1,nA
--
500
0
rb
A 200 i,
_lid
, r a
100 -
100
-
0
U
1 *
-0.4
A
1
-0.8
1
l
l
concentration level were performed in triplicate. Both analytes exhibit a highly linear response a t the modified cell (slopes, 283 (A,b) and 296 (B,b) nA/M; correlation coefficients, 0.999 (A and B,b)). In contrast, nonlinear calibration curves, with a curvature a t high concentrations, are observed a t the conventional cell (correlation coefficients, 0.989 (A,a) and 0.959 (B,a)). Such deviations from linearity may be misinterpreted as other factors, such as full surface coverage (in the case of adsorptive stripping voltammetry) or amalgam saturation and formation of intermetallic compounds (in conventional stripping measurements). Many other typical stripping experiments can be strongly affected by the growing mass of mercury at the cell bottom. For example, the dependence of peak current on deposition time was evaluated for 1 X lo-' M lead (other conditions, as in Figure 2). With the conventional cell, the resulting plot exhibited a curvature for periods longer than 5 min (correlation coefficient, 0.989). Such curvature disappeared using the modified cell; the latter exhibited a highly linear time dependence (with correlation coefficient of 1.000). Similar improvements are expected for many other prolonged parametric experiments, such as those examining the effect of the preconcentration potential or, in the case of the chelate adsorption scheme, the effect of the ligand concentration. Trace metal speciation studies are of the most important application of stripping analysis (1-3). Some of the common stripping/speciation schemes involve prolonged experiments using the same solution. The progress of such experiments is accompanied by a continuous accumulation of mercury a t the cell bottom. The consequence of the resulting erratic data would be unreliable information regarding the distribution of trace metals. Figure 4 compares such pseudopolarographic
d -
/
-1.2,-0.4 E,
Figure 3. Calibration plots for lead (A) and oxytetracycline (B) using conventional (anodic)and adsorptive (cathodic),respectively, stripping voltammetry, with the unmodified (a)and modified (b) cells: deposition, 1 min at -1.0 (A) and -0.6 (b) V; supporting electrolyte (B), boric acid-sodium hydroxide (pH 5.5). Other conditions are as in Figure 2. Measurements in each level were performed in triplicate.
i
I -
1
I
-0.8
I
_-1.2
v
Figure 4. Use of conventional (a) and modified (b) cells for metal speciation work: (A) pseudopolarograms for a 5 X M lead solution: (B) complexing capacity titrations of a river water sample (9 mL of river water -t- 1 mL of acetate buffer). Deposition was for 60 (A) and 90 (B) s. Deposition potential (6)was -0.7 V. Pseudopobrograms were obtained by increasing the deposition potential from -0.4 to -1.3 V (0)and then repeating the same sequence (0);the complexing capacity was obtained by measuring the copper peak in each level
in triplicate. (A) and complexing capacity (B) experiments, as obtained with the conventional (a) and modified (b) cells (differential pulse waveform with 10 mV/s scan rate and 50 mV amplitude). The improved reliability, resulting from the use of the new cell is obvious. Note, for example, the increased dispersion of the data points, at each copper level, as the titrimetric procedure progresses at the conventional cell (B,a). In contrast, the modified cell exhibits an excellent agreement between the individual points over the entire titration procedure (B,b). Note also the difference between the two pseudopolarograms recorded, repetitively (in the same solution) using the conventional cell (A,a). In contrast, an identical response is observed for the early and late polarograms a t the modified cell (A,b). In conclusion, the new cell design permits collection of much more data using the same solution, e.g., carrying out several different experiments, thus saving important analytical time (replacing solutions, cleaning cell, deaeration, etc.). Users of existing cell should bear in mind possible erratic results and consider possible changes in mass transport when interpreting their data.
LITERATURE CITED (1) Wang, J. Stripplng Analysk: Principles, Instrumentation, and Appllcatlons ; VCH Publishers: Deerfield BeachlWeinhelrn. 1985. (2) Florence, T. M. J . Nectraanal. Chem. 1084, 188. 207. (3) Nbrnberg, H. W. Anal. Chim. Acta 1084, 184, 1. (4) Peterson, W. M. Am. Lab. (Fairfield, Conn.) 1070, 72, 69.
RECEIVED for review February 18,1987. Accepted April 22, 1987. This work was generously funded by National Institutes of Health (Grant No. GM 30913-04) and the American Heart Association.
