Anal. Chem. 1988, 58, 2949-2954 (19) Feldberg, S.W. I n Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1969; Vol. 3, pp 199-296. (20) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; Wiley: New York, 1980; p 675. (21) Winograd, N.; Blount, H. N.; Kuwana, T. J. Phys. Chem. 1969, 7 3 , 3456. (22) Christie, J. H.; Lauer, G.; Osteryoung, R. A. J. Nectroanal. Chem. 1984, 7 , 60. (23) Bowden. E. F.; Hawkridge. F. M.; Chlebowski, J. F.; Bancroft. E. E.; Thorpe. C.; Blount, H. N. J. Am. Chem. SOC. 1982, 104, 7641. (24) Cohen. D. J.; Hawkridge, F. M.; Blount, H. N.; Hartzell, C. R. I n Charge and Field Effects in Biosystems; Allen, M. J., Usherwood, P. N. R.,
2949
Eds.; Abacus Press: Tonebridge (U.K.), 1984; pp 19-31. (25) Bowden. E. F.; Hawkridge, F. M.;Blount, H. N. I n Comprehensive Treatise of Electrochemistry; Srinivasan, S . , Chizmadzhev, Yu. A,, Bockris, J. O’M., Conway, B. E., Yeager, E. B., Eds.; Plenum: New York, 1985; Vol. 10, pp 297-345.
RECEIVED for review March 13,1986. Accepted July 24,1986. This work was supported by the National Science Foundation (CHE-8208291).
Continuous-Flow Sample Probe for Fast Atom Bombardment Mass Spectrometry Richard M. Caprioli* and Terry Fan T h e Analytical Chemistry Center and Department of Biochemistry and Molecular Biology, University of Texas Medical School, Houston, Texas 77030
John S . Cottrell Kratos Analytical Instruments, Urmston, Manchester, United Kingdom
The deslgn and performance of a sample probe that allows a contlnuowr fbw of sdutlon to be Introduced Into a fast atom bombardment (FAB) Ion source are descrlbed. Samples can be Injected Into a solvent flow that contains water/glycerol (8:2) and dllute buffers. Samples contalnlng 13.5 ng of peptides Injected In 0.5-pL portlons show peaks In the total Ion chromatogram emerging over 30 8, correspondlng to a volume of 2.5 pL. Ion lntensltles recorded with varylng sample amounts show a llnear relatlonshlp from 0.7 to 200 ng. QuantHatlvely, calculatlons of peak areas from repllcate lnJectlonsshow standard deviations of approximately * I O % of the mean. Wlth regard to sensltlvlty, the peptlde substance P (mol wt 1347) at 0.3 ng gave a slgnal-to-nolse ratio of 51. Comparlson of background chemlcal noise between the continuous-flow probe and the standard FAB probe (uslng an 80 % glycerol matrix) showed a slgniflcant Improvement In signal-to-chemlcal nolse using the flow probe. High mass performance Is demonstrated by showlng the resolved molecular Ion regions of Injected samples of oxldlred bovine Insulin B chain (mol wt 3493) and Intact bovlne lnsulln (mol wt 5730). CondHions requlred for the stable operatlon of the probe are discussed.
Samples are usually introduced into the ionization chamber of a fast atom bombardment (FAB) mass spectrometer through the use of a direct insertion probe. The sample is first dissolved in glycerol or some other suitable viscous matrix, and then several microliters of the solution are placed on the probe tip. Exposure of this sample to a beam of energetic xenon atoms inside the mass spectrometer source causes surface layers of molecules to be sputtered and the resulting ions are subsequently analyzed (1). Although this type of sample introduction is simple and easy to use, it has several shortcomings. First, it does not easily lend itself to following dynamic processes especially where rapid changes in reactants or products are expected. Since each sample becomes an isolated analysis, the number of samples taken is a matter of speed and/or endurance both in the sampling process and also
in the subsequent analyses. Second, comparisons of ion intensities from sample to sample are difficult and the results uncertain without the use of internal standards. Third, substantial amounts of glycerol (or other matrix material) are required, usually 80-9570, so that the liquid droplet can survive introduction into the vacuum system. This precludes direct sampling of reactions that must proceed in a substantially aqueous environment such as, for example, enzyme reactions. Several investigators have reported work involving the continuous introduction of liquid samples into a FAB source of a mass spectrometer, although these were mostly aimed at on-line HPLC applications. One approach involves use of a moving belt onto which is deposited fractions of the HPLC eluant ( 2 , 3 ) . The belt is then continuously cycled into the source of the mass spectrometer where the sample spots are bombarded. More recently, Ito et al. ( 4 ) reported on the use of a capillary inlet device for the direct connection of a microbore HPLC column to a FAB ionization source. These workers demonstrated the separation and analysis of bile acids using a mobile phase of glycerol/acetonitrile/water (1027:63) at a flow rate of 0.5 wL/min. A stainless-steel mesh frit was used a t the terminus of the capillary in the ion source to disperse the mobile phase and concentrate the solute and glycerol. Other approaches to continuously flowing aqueous solutions into mass spectrometers, although not involving FAl3 ionization sources, include thermospray (5,6)and direct liquid injection (DLI) methods (7, 8). We have constructed and tested a sample introduction probe for use with mass spectrometers equipped with fast atom bombardment sources that permits a continuous flow of solution to be brought directly into the source. The solution can be essentially aqueous, containing as little as 10% glycerol and is brought into the source at a flow rate of about 5 pL/min. Samples may be injected into this flow of solvent or included in the solvent, depending on the application. Dilute buffers, acids, and salts can be used in the aqueous solvent. Total ion current chromatograms obtained with injected samples in the nanogram range produce sharp peaks with little tailing and no significant memory effects. Further, since the spectrometer can operate a t full accelerating po-
0003-2700/86/0358-2949$01.50/00 1986 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 58, NO. 14, DECEMBER 1986
2950
injection overflow
loo-
f
copper tip
high voltage insulator
\
-
=%==-a
0 0 7 5 mm 1.d x 1 m fused silica capillary
______
I
+---
lpump
I jl
! -
(A)
I
q
injection p o r t
---___i,y J ,-
Pump out port
i
f
J
i , injectoi
septum seal
graphite ferrule vacuum seal
50
stainless steel shaft
Flgure 1. Schematic diagram of the continuous-flow probe for fast atom bombardment mass spectrometers.
tential, the probe permits the full sensitivity of the instrument to be utilized, an important factor when making measurements at high mass.
EXPERIMENTAL SECTION Design and Operation of the Continuous-Flow FAB Probe. The probe, shown in Figure 1, consists of a hollow shaft that is capped with an angled tip through which a 0.3-mm hole is drilled. A 0.075-mm-i.d. (0.26 mm 0.d.) X 1-m fused silica capillary (SGE, Victoria, Australia) is passed through the shaft and allowed to protrude no more than 0.2 mm beyond the tip. A vacuum seal consisting of a vespel ferrule and compression nut seals the probe shaft to the capillary at the base of the probe. A pump-out port is provided in the shaft to evacuate the hollow shaft during the rough pumping of the probe in the insertion lock. The capillary is connected to a Rheodyne injection valve: a Model 7520 for injection of submicroliter amounts or a Model 7010 for injection of 5 pL and larger amounts. The continuous-flowsolvent is provided by a suitable pump (an Isco Model pLC 500 syringe pump or Waters Model 590 pump) normally operated at a flow of 5 pL/min. A Kratos MSSORF high-resolution mass spectrometer was used to provide the data presented in this work. The instrument was not specially modified except that the FAB source block was fitted with a heater and thermocouple sensor that are controlled by existing circuits on the operator's console. The source block is generally maintained at 40 OC. Since the source fitted with the continuous flow probe normally operates at a relatively high pressure (about 5 X torr), extreme care was taken t o electrically ground the instrument properly and protect delicate components from the effects of source arcs. The mass spectrometer was operated at full sensitivity (8-keV accelerating potential). The FAB gun is the Ion Tech Model BllNF saddle field source and is operated at 7 keV at a current of 50 pA with xenon gas. Data were taken by use of the Kratos DS90 software with the DG S/280 computer system. Mass spectra were generally obtained by using a wide-range multichannel analyzer program with a magnet scan and the spectrum calibrated with CsI. Voltage scans (varying both the accelerating and electric sector voltages) were used to provide narrow mass scan ranges for the protonated molecular ion regions of bovine insulin and oxidized bovine insulin B chain. Reagents. All chemical supplies were obtained from Sigma Chemical Co. (St. Louis, MO) except where noted. Substance P, des-(Gln)%ubstance P, angiotensin 11, and angiotensin I11 were obtained from Vega Chemical Co. (Phoenix, AZ) and maltoheptaose from Boehringer Mannheim (Indianapolis, IN).
RESULTS Injection of Samples into a Solvent Flow. In order to test the performance of the probe as a microsampling device, 0.5-pL samples of several peptides were injected into the device. The continuously flowing carrier solution consisted of a mixture of water and glycerol (82), containing 0.3% trifluoracetic acid, which was fed into the mass spectrometer at a rate of 5 pL/min. The acid produces maximal sensitivity for a number of compounds, especially peptides. The samples were dissolved in this same solution. Figure 2A shows the btal ion chromatogram resulting from the single injection of 13.5 ng of the peptide substance P (mol wt 1347). The peak emerged over a time of approximately 30 s (seven scans) and
I
TIME lmin)
IW
-1
(B) U1z 1348
€='
I
1300
I400
1200
(C)
I
1400
-
1300
1200
MI2
Flgure 2. (A) Single injection of 13.5 ng of substance P (mol wt 1347) in 0.5 pL of waterlglycerol (8:2) containing 0.3% TFA. The arrow denotes the point of injection of the sample. (B) FAB mass spectrum
at chromatogram peak maximum. (C) Background after the sample peak at about 4 min in the chromatogram. The resolution of the instrument was 1500 (the compressed mass plot in the figure gives an apparent lower resolution).
100
1
20
10
fa
80
100
120
140
amount i n j e c t e d (ng) Figure 3. Linearity of response for the range of 0.7-135 ng of the peptide substance P (mol wt 1347) injected on the probe. The response was calculated from the area of the selected ion chromatograms of the (M + H)+ ions from triplicate injections. The average deviation from the mean is shown as error bars; for response values below 20 ng, this deviation was too small to be shown in the figure. in a total volume of about 2.5 ,uL. Figure 2B shows the FAB mass spectrum at the peak maximum in the chromatogram, and Figure 2C shows the mass spectrum just after the peak (around 4 min in the chromatogram). Only residual traces of the peptide (M H)+ion can be seen in Figure 2C; the (M H)+ ion intensity is less than 0.5% of that of the mass spectrum taken from the chromatogram peak maximum. Thus, at these concentrations, the memory effect of the probe is quite small. Similar results were obtained for a variety of polypeptides injected in this manner, including ribonuclease S peptide, oxidized insulin B chain, angiotensin I1 and 111, and several smaller peptides. Injections of much larger sample volumes such as 5-10 ,uL can be used where it is desirable to maintain signal currents for much longer periods of time. For example, injection of 5 pL of substance P, containing 27 ng/pL of solution, gave
+
+
ANALYTICAL CHEMISTRY, VOL. 58, NO. 14, DECEMBER 1986
2951
I
70\ 60
I
48
I
I
20
20 10
, 0
1,-
1
2
6
4
10
8
Figure 4. Selected ion chromatograms of the molecular ion region ( m l z 1345-1351) from injections of various amounts of substance P. The arrows denote the points of injection of the sample. standard Probe
%I
IBB
28. I0
. 1300
1100
SCAN NUMBER
Figure 5. Reproducibility of muttiple injections of 13.5 ng of substance P on the continuous-flow probe. The selected ion chromatogram of the m l z 1345-1351 region is shown.
a broad flat-topped tailing peak in the total ion chromatogram and produced intense (M + H)+ ion currents for about 5 min. Linearity of Response. In order to determine the linearity of response of the probe, solutions containing 0.7, 1.4,6.8,13.5, 67.5, 135, and 675 ng of the peptide substance PlO.5 /.LLof water/glycerol(82, containing 0.3% TFA) were injected into the probe. By use of measurements of the areas of selected ion chromatograms of the (M H)+ ions, the response was found to be linear in the range of approximately 0.7-140 ng as shown in Figure 3. Above approximately 200 ng, a positive deviation from linearity was observed as a result of overloading the probe surface. In this case, several minutes were required for the solvent to sweep the samples outside the area on the tip from which ions can be focused into the mass spectrometer. Figure 4 compares the selected ion chromatogram peak profiles of the (M + H)+ion region for injections of 0.7, 13.5, and 657 ng of the peptide. Reproducibility. Figure 5 shows the selected ion chromatograms of the (M + H)+ions for injections of 10 replicate samples of the peptide substance P, each containing 13.5 ng (injected in 0.5 pL). Measurements of peak heights show a standard deviation of &42% of the mean with a range of 86-156% of the mean. However, areas calculated for each
+
lBB
Continuou8 Flow Proba M i l 931
I M l z 1046
4
J M i z 1220
IO I
I.,
1300
.
At. 1100
% 900
Mi1
Figure 6. Comparison of mass spectra taken from (top)the standard FAB probe and (bottom) the continuous-flow probe from a sample containing angiotensin 111 (mol wt 930), angiotensin I1 (mol wt 1045), and des-(Gin)*-substance P (mol wt 1219). Experimental details are given in the text. The peak profiles above the m l z 1220 ion show the actual resolution of the scan (the compressed plot gives an apparent lower resolution). Approximately 20 nmol of each peptide was
used for each sample. of these peaks showed a standard deviation of f10.5% from the mean with a range of 86-125% of the mean. The variation in peaks widths indicates that flow conditions probably change
2952
ANALYTICAL CHEMISTRY, VOL. 58, NO. 14, DECEMBER 1986
x
118
Y
/
/
/ /
conthmm Flow Probe
1 loo
MI2 Figure 7. Expanded background regions from the mass spectra shown in Figure 6.
somewhat throughout the run and give rise to slightly different peak profiles. Some of the variability can be attributed to irreproducibility involved in the injection of submicroliter volumes of sample. Sensitivity. The sensitivity of the mass spectrometer for samples introduced via the probe was tested with several peptides. The peptide substance P gave a S I N level of approximately 5:l at 340 pg (252 fmol) injected onto the probe. For most samples, improvement in signal to background chemical noise is seen using the continuous flow probe relative to the standard FAB probe where samples are dissolved in 80-90% glycerol. Figure 6 shows the mass spectrum of a mixture of peptides taken with both the standard probe and the continuous-flow probe. The compounds employed were angiotensin 11, angiotensin 111, and des-(Gln)6-substanceP. Approximately 20 nmol of each peptide was used in each sample. In the case of the flow probe, the sample was injected in 0.5 pL of solution (which becomes diluted to about 2.5 pL on exit from the capillary), while for the standard probe the sample was dispersed in a total volume of 2 pL (containing 80% glycerol and 0.3% TFA on the tip). Generally, the two spectra are quite similar, although there appears to be considerably greater background chemical noise in the case of the standard probe. A portion of each spectrum is detailed in Figure 7 , showing quite clearly the increase in the signal-tochemical noise obtained from the flow probe (Figure 7B) relative to the standard probe (Figure 7A). Of great significance in this regard is the fact that the background "matrix" ions, producing a peak at every mass, are greatly reduced in
intensity while those related to sample emerging from the capillary remain intense. For example, the ion at m / z 1282 in Figure 7 corresponds to (M + 63Cu)+for des-(Gln)%ubstance P, and the ion at m/z 1234 corresponds to (M 63Cu3)+ for angiotensin 11. Similarly, in the lower mass region of Figure 7 , the ion at m / z 1108 corresponds to (M + 63Cu)+for angiotensin 11. Results of calculations of isotope patterns from elemental compositions fit well for these ions. Of course, these ions are of relatively low intensity (less than 5 % ) in comparison to the (M + H)+ ions for the peptides and arise from bombardment of the copper tip and sample, as described earlier (9). These data presented in the figure show an advantage of the continuous-flow probe in reducing background chemical noise, thereby improving the observation of sample-related ions. This reduced background effect is presumed to be due to the ejection of ions from the capillary tip and surrounding surface from a solution containing much less glycerol than that obtained from the standard probe. The effect is seen with other compounds as well for example, the FAB mass spectrum obtained from an injection of a sample of the oligosaccharide maltoheptaose (mol wt 1152) gave similar results. High-Mass Performance. The mass spectra of bovine insulin (mol wt 5730) and oxidized bovine insulin B chain (mol w t 3493) were obtained to determine the performance of the flow probe with high-mass samples. Figure 8A shows the narrow-range voltage scan for the (M + H)+ ion of oxidized insulin B chain. The sample solution containing 1.5 Fg/pL of peptide (8:l:l aqueous 50 mM Tris buffer, pH 7.2/
+
ANALYTICAL CHEMISTRY, VOL. 58, NO. 14, DECEMBER 1986
(A)
Y
2953
culations of flow dynamics through capillaries. On the other hand, too high a flow rate will lead to spattering of the liquid surface, again giving unsteady ion currents. The pressure in the source is quite high, approximately 5 x 10" torr for the system described here. Nevertheless, under stable operating conditions, this pressure varies surprisingly little. With respect to the use of salts and buffers, relatively dilute buffers (e.g., 5 mM) seem not to present a problem. On the other hand, use of concentrated nonvolatile buffers can eventually lead to high-voltage breakdown if they are used for extended periods of time. When the conditions for stability are achieved, the probe tip contains a wet viscous surface but no definable droplet. Usually, this condition requires 15-30 min to develop on initial insertion of the probe. I t is speculated that when the solution emerges from the capillary tip after the probe is stabilized, ions are produced and analyzed from this highly aqueous area of the tip. This zone is pushed outward by oncoming solution, and as the zone migrates toward the edge, water and glycerol are evaporated until there remains only a very viscous matrix a t the edge. Fortunately, ions seem not to be collected from this outer region. The buildup of viscous material with time is remarkably small; the probe is often operated 3-4 h without being removed from the instrument with as many as 50-60 injections of samples containing nanomole amounts of compounds during this time without significant deterioration of performance.
DISCUSSION M/Z
Figure 8. Mass spectra of the molecular ion regions of high molecular weight samples injected onto the continuous-flow probe: (A) 1.5 pg/pL of oxidized bovine insulin B chain (mol wt 3493),the injected volume was 0.5 pL in water/glycerol/thioglycerol (8:l:l), the spectrum represents 60 signal-averaged voltage scans at a resolution of 3000;(B) bovine insulin, the injected volume was 0.5 pL of a solution containing 2 pg/pL in water/glycerol/thioglycerol(8: 1: l), the spectrum represents 20 signal-averaged voltage scans at a resolution of 5000.
glycerol/thioglycerol) in 0.5 p L was injected onto the flow probe using a continuous flow of an aqueous solution containing 20% glycerol and 0.2% TFA. The spectrum shown in the figure is the sum of 60 scans with the instrument resolution set a t about 3000. Figure 8B shows the (M H)+ion region for bovine insulin from 0.5 pL injected onto the flow probe of a sample containing 2 gg/pL of protein in 8:l:l water/glycerol/thioglycerol with 0.2% TFA. The continuous-flow solution had this same composition. The spectrum is the sum of 20 scans taken using a voltage scan with an instrument resoluton of about 5000. This spectrum shows a signal-to-chemical noise response of approximately 1O:l. A spectrum taken under the same conditions using the standard FAB probe where the sample is dissolved in a thioglycerol matrix gave a signal-to-chemical noise response of approximately 4:l. Operation and Stability. The stable operation of the probe can be defined simply as the condition where a constant ion current is obtained from ions sputtered from the tip under the flow condition described. Such stable operation can be achieved in a mass spectrometer when the rate of evaporation of the solvent from the probe surface and the pumping speed of the source of the mass spectrometer are in balance. The latter is generally not easily controlled, and so it is more practical to control the rate of evaporation. In the system described, this may be altered within limits by changing either the flow rate of liquid to the tip, the total vapor pressure of the solution, or the amount of heat applied to the tip. Too slow a flow and insufficient heat will lead to freezing within the capillary tip and will give rise to unsteady ion currents, as described by Arpino and co-workers (10) from their cal-
+
The continuous-flow FAB probe can be used effectively for several types of applications. It provides an easy and fast method for the analysis of compounds in aqueous solution. For dilute solutions of sample, 0.5-pL injections may be made every 2 min without showing a significant memory effect. Small volumes of solutions containing only water as solvent may also be used when the continuous-flow buffer contains some glycerol. Also, solutions may be sampled from outside the mass spectrometer on a continuous basis using a suitable pump. A combination of several factors are responsible for the performance of the instrument equipped with this continuous-flow FAB probe. These include the maintenance of a relatively high solvent flow rate, the unobstructed flow of the sample into and out of the region on the probe tip from which ions are collected, and the dynamic balance of the input solvent flow and its evaporation. Ito et al. ( 4 ) emphasize in their report that the function of the frit in their device is to vaporize the volatile portion of the mobile phase, leaving the solute and glycerol matrix on the frit surface to be bombarded by the atom beam. In addition, they report that flow rates of about 0.5 gL/min are required in order to maintain the production of stable ion currents. We have found that with the continuous-flowFAB probe described in the current paper, conditions or physical restrictions that impede solvent flow tend to result in greater memory effects and accumulation of glycerol on the probe tip, which increases chemical background noise and, generally, give less satisfactory performance. One of the major disadvantages of the use of the continuous-flow probe involves the high pressures under which the ion source must operate. For magnetic instruments that operate at high accelerating voltages, this may lead to highvoltage breakdown especially when the probe is unstable. Also for this reason, high salt concentrations are to be avoided in samples or solvent. Since the stable operation of the probe is a dynamic one, any change in one of the major parameters can lead to unstable operation, such as drastically changing flow rates as the result of clogging of the capillary with undissolved material or precipitation or crystallization of a sample in the capillary.
Anal. Chem. 1986, 58,2954-2961
2954
The s k p response of the mass spectrometer equipped with the probe with samples contained in relatively small volumes suggests that it may be of utility for other applications such as combined HPLC/mass spectrometry. Although this was demonstrated by Ito et al. ( 4 ) ,the higher flow rates used in the present work may provide much greater general utility for such an interface. Although flow rates of about 5 pL/min can be used with microbore HPLC, the device could also be of use for large-bore applications where stream splitting is acceptable.
LITERATURE CITED (1) Barber, M; Bordoli. R. S.; Sedgwick, R. D.; Tyler, A. N. J. Cbem. SOC Cbem. Commun. I981, 325-327. (2) Stroh, J. G.; Cook, J. C.; Milberg, R. M.; Brayton, L.; Kihara, T.; Huang, 2.; Rhinehart. K. L. Anal. Chem. 1985, 5 7 , 985-991. .)
(3) Dobberstein, P.; Karte. E.; Meyerhoff, G.: Pesch, R. I n t . J. Mass Spectrom. Ion Pbys. 1983, 4 6 , 185-188. (4) Ito, Y.; Takeuchi, T.; Ishi. D.; Goto, M. J. Cbromatogr. 1985, 346, 16 1- 166. (5) Biakeiy, C. R.; Carmody. J. J.: Vestal, M. L. Anal. Cbem. 1984, 56, 1236-1239. (6) Pilosov, D.; Kim, H. Y.; Dykes, D. F.; Vestal, M. L. Anal. Chem. 1984, 5 6 , 1236-1239. (7) Arpino, P. J.; Bounine. V. P.; Dedieu, M.; Guiochon, G. J. Cbromatogr. 1983, 271 43-48, (8)Covey, T.: Henion, J. D. Anal. Cbem. 1983, 55, 2275-2279. (9) Martin, S. A.; Costeiio, C. E.; Biemann, K. Anal. Cbem. 1982 5 4 , 2362-2368. (10) Arpino, P. J.; Krien, P.; Vajta, S.: Devant, G. J. Chromatogr. 1981, 203,117-130. ~
RECEIVED for review March 24, 1986. Accepted July 25, 1986. Support for this work by NSF Grant PCM-8404230 and NIH Grant RR-01720 is gratefully acknowledged.
Fundamental Factors in the Polarographic Measurement of Ion Transfer at the Aqueous/Organic Solution Interface Sorin Kihara,* Mitsuko Suzuki, Kohji Maeda, Kaoru Ogura, Shigeo Umetani, and Masakazu Matsui T h e Institute for Chemical Research, Kyoto University, Uji, Kyoto 611, Japan Zenko Yoshida Japan Atomic Energy Research Institute, Tokai, Ibaraki 319-11, Japan
I n order to elaborate on polarography for Ion transfer at the aqueouslorganlc solutlon Interface as a quantitatlve method in analytical chemistry, fundamental factors In polarography were reviewed systematkally. A polarographic cell and a clrcUn for the compensation of the ohmk drop were proposed. The characterlstksof the aqueous solution dropplng electrode were summarlzed In cormedlon wlth the slze of the capillary and kinds of both supporting electrolytes and olgank solvents. The potential windows of the residual currents and the halfwave potentlals for Ion transfers were Investigated by use of various supportlng electrolytes and many organlc solvents whose dlelectric constants are between 5 and 36. The limiting currents were proportlonal to the concentratlon of such monovalent ions as Cs', tetramethylammonium Ion, CIO,-, IO,-, I-, and Br-, Reo,-, and BF,- In the concentratlon range between and M In aqueous solution when 1,2-dlchloroethane was used as the organic solution.
The voltammetry for the ion transfer at the interface of two immiscible electrolyte solutions, VITIES, has become recognized as a powerful method in understanding the dynamic feature of the ion transfer because of the unmatched advantage that the transfer free energy and the amount of ions transferred can be measured simultaneously ( 1 , 2). The ion transfer at aqueous (w) and organic (org) solution interfaces has been reported in connection with alkaline metal (3-5), alkaline-earth metal (6, 7),tetraalkylammonium (8,9), and heavy halide and oxyacid (10, 11) ions using VITIES without or with such strong complexing agents as ionophores in the organic phase.
Since most of these ions are not easily reduced or oxidized in solutions, ordinary redox voltammetry cannot be applied and the electrochemical determination of them had been limited to potentiometry with ion selective electrodes, ISE. Potentiometry, however, is not appropriate for precise determination because the potential in the method depends merely on the logarithm of the concentration of the objective ion, Cion. Therefore, VITIES, in which the limiting current is proportional to C,,, (B), is expected to be an epoch-making electroanalytical method for the precise determination of ions. Among many methodological investigations on VITIES (for review, see, e.g., ref 12 and 13), polarography, PITIES, with the electrolyte solution dropping electrode, EDE, proposed by Koryta et al. (8, 14) is considered to be most promising to get the quantitative and reproducible results, because the continuous renewal of the solution drop greatly decreases the contamination of the electrode surface which is one of the difficulties in VITIES. In the present paper, fundamental factors concerning the measurement of the ion transfer by PITIES are investigated in behalf of the wide application of polarography to the field of analytical chemistry. The characteristics of the currentscan polarography employed in this work were described previously ( 15).
EXPERIMENTAL SECTION Chemicals. In order to prepare crystalviolet tetraphenylborate, CV+.TPhB-, and tetraphenylarsonium dipicrylaminate, TPhAs+.DPA-,which are the supporting electrolytes in the organic medium, a methanol solution of CV+-CI-or TPhAsC.C1-was mixed with a methanol solution of Na+.TPhE- or Na+.DPA-, respectively. After filtration, the precipitate of CV+.TPhB-was dissolved with 1,2-dichloroethane,DCE, and then recrystallized by pouring the DCE solution into methanol. The precipitate of TPhAs'BDPA-
0003-2700/86/0358-2954$01SO/O0 1986 American Chemical Society
