Anal. Chem. 1991, 63, 2109-2114 (30) Hettlch, R. L.; Buchanan, M. V. J . Am. Soc.h s s Spectrom. 1991, 2, 22-28. (31) Hanson, C. D.; Castro, M. E.; Russell, D. H.; Hunt, D. F.; Shabanowltz, J. ms Of L e f (m’r’5000) ~ mmokuks; ACS serles 359 (FTMS); American Chemical Society: Washington, DC, 1987; pp 100-115. (32) Ke&y, E. L.; hnson, C. D.; Castro, M. E.; Russell, D. H. Anel. C t ” . 1988, 8 1 , 2528-2534. (33) Bamberg, M.; Wanczek, K. P. 37th Annual Conferenceon bss Spectrometry and Allied Topics, Miami Beach, FL, 1986; p 456.
2109
(34) Feigl, P.; Schueler, B.; Hlllenkamp, F. Inf. J . Mass Spectrom. Ion Processes 1983, 47, 15-18.
RECEIVED for review February 19,1991. Accepted June 27, 1991. This work is supported by the Welch Foundation (Grant F-1138), the Texas Advanced Technology and Research Program (Grant No. 45151, and the National Science Foundation (Grants CHE9013384) and CHE9057097).
Mechanism of Electrospray Mass Spectrometry. Electrospray as an Electrolysis Cell Arthur T. Blades, Michael G. Ikonomou, and Paul Kebarle*
Chemistry Department, University of Alberta, Edmonton, Alberta, Canada T6G 2G2
If lt Is assumed that the charglng of the unipolar droplet spray In electrospray (ES) is due to a separatlon of posltlve from negative electrolyte ions present in the solution (electrophoretlc charglng), then charge balance requires that a conversion of ions to electrons should occur at the metal-liquid Interface of the ES caplllary, when the caplliary Is the poSnlve electrode. A proof for the occurrence of such an electrochemical process Is provided. When a Zn capMary tip is used, Zn2+ Ions are detected in the sprayed solution. The Zn2+ concentration matches the Zn2+ production expected on the basis of the measured ES current. Slmliar results are obtalned for Fez+ Ions when a stainless steel caplliary is used.
INTRODUCTION Electrospray mass spectrometry is a new technique of extraordinary potential (1-3). The exciting applications of this technique have also created a great deal of interest in the mechanism by which the gas-phase ions required for the mass spectrometric detection are produced. There are two important stages: the production of the charged droplets and the production of gas-phase ions from the charged droplets. The present work addresses mainly the first stage, i.e., the source of charge on the droplets. The electrospray (ES)device as used in ES mass spectrometry (1-4) can deliver a continuous stream of fine droplets that carry charge of the same polarity. Thus, when the ES capillary is at a positive voltage relative to the large planar counter electrode, the droplets are positively charged and a continuous positive current arrives at the counter electrode. This ES current is generally within the range 0.1-1 MA. It depends on the presence of ions in the solution that are due to electrolytes dissolved in the solvent used. The current increases with the concentration of total ionized electrolyte, taken to the -0.35 power (4-7). The mechanism by which the droplets are charged is assumed by some authors (4-9) to be electrophoretic, although this point of view seems not to be accepted by many ES mass spectrometrists. Electrophoretic charging occurs when, due to the imposed electric field, a partial separation of the positive ions from the negative ions present in the solution occurs, which leads to an excess of positive charge on the surface of the liquid at the capillary tip. This excess charge destabilizes the surface and leads to emission of positively charged dro0003-2700/91/0363-2109$02.50/0
plets. The central point here is that the excess ions in the droplets are electrolyte ions that were present in the solution and ion ions created from neutral molecules by processes like field ionization. Considering the requirements for charge balance in such a continuous electric current device and the fact that only electrons can flow through the metal wire supplying the electric potential to the electrodes, one comes to the conclusion that the electrophoretic charge separation mechanism requires that the electrospray process should involve an electrochemical conversion of ions to electrons. In other words, the ES device can be viewed as an electrolytic cell of a somewhat special kind. The cell is special insofar as part of the ion transport does not occur through the solution but through the gas phase. The suggested scheme (8, 9) is illustrated in Figure 1. An essentially conventional electrochemical oxidation reaction should be occurring at the liquid-metal interface of the capillary tip. The actual oxidation reaction(s) will depend on the electrical potential present a t given locations of the metal-liquid interface and on the electrochemical oxidation potential for the given reaction(s). Kinetic factors governing the reaction could also be involved although the kinetic constraints, leading to “overpotentials”, should be small, considering the low currents involved. The net effect of the oxidation reaction at the capillary tip will be the creation of an excess of positive ions over negative ions in the solution and the production of electrons that enter the metal. The excess of positive ions could result from a removal of negative ions from the solution or the production of positive ions. Thus, when NaCl is the electrolyte and wet methanol the solvent, reactions 1 and 2 could be involved in removing negative ions and reaction 3 could be a process 2C1-(,)
= Cl,,,,
40H-(,,) = o~(,,
+ 2e
Eored= 1.36 V
+ 2H,O(,) f 4e
[OH-] =
M
+
(Emd
2H,00, = 02(,) 4H+(aq)+ 4e [H+] = lo-’ M
Eared
(1)
= 0.40 (2)
= 0.81)
Eord = 0.68 (3)
(Er& = 0.27)
producing positive ions. Also given with the reactions are the standard electrochemicalreduction potentials. The tendency for oxidation increases as the reduction potential decreases. The reduction potentials, when the activities of H+ and OH0 1991 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 63, NO. 19, OCTOBER 1, 1991
n .@@I1
Reduction
,"
Oxidation
,
electron{
.
Zn
I
a .
A
1 electrons C
High Voltage Power supply
-3
Figure 1. Schematic representation of the processes that are assumed to occur in electrospray. The imposed electric field leads to a partial separation of positive from negative electrolyte ions present in the sdution. The spray droplets carry off an excess of positive ions. A conversion of ions to electrons, Le., an electrochemical oxidation, must occur at the liquid-metal interface of the capillary.
are equal to M, where M = moles/liter, are given in brackets for eqs 2 and 3. A reduction reaction should be occurring a t the large counter electrode; see Figure 1. This reaction may be a "conventional" (wet) reaction, assuming that this electrode is kept wet by the arriving droplets, or "unconventional", assuming the arriving charge is mostly due to gas-phase ions. Even then, the ions will be solvated by several solvent molecules ( 4 ) and that may lead to a near conventional reduction reaction. One could try to identify the actual electrochemical reactions occurring a t either of the electrodes. However, the processes occurring a t the metal-liquid capillary interface are of greater interest since they lead to a change of composition of the ions in the sprayed solution. This change can influence the type of ions detected with the mass spectrometer. Therefore, we chose to study the oxidation reactions occurring a t the capillary tip. The electrochemical potentials given in eqs 1-3 indicate that the production of H+ ions would be the most favorable reaction (lowest reduction potential), when water or wet methanol solutions are used. This presupposes also that the metal electrode is nonreactive, as would be the case for platinum. The stainless steel capillaries normally used may be also inert since the surface is "passivated" by a layer of chromium oxides. We chose to force the occurrence of a given oxidation reaction by choosing a metal for the capillary tip that has a very favorable oxidation potential. Furthermore, it is desirable that the dissolution of this metal by the oxidation reaction should lead to distinctive ions not commonly present as impurities. The dissolved metal ions will be carried out by the liquid flow through the capillary tip and enter the sprayed droplets just like any of the other positive and negative ions present in the solution. Therefore, some of the metal ions will then enter the gas phase and will be detected with the mass spectrometer used. Zinc was chosen as the most suitable material. It has a very low reduction potential, see eq 4, and a capillary can be machined out of this material. It was known from previous work
Zqe) = Zn2+(,q)+ 2e [Zn2+] =
-
C
lo*
M
Eored = -0.76 V
(4)
(Erd = -0.94)
(IO) that Zn2+ions present in the solution a t concentrations as low as M can be detected with the mass spectrometer used.
Flgure 2. Spray capillaries used. Both A and B incorporate the same stainless steel capillary. However, in the A arrangement, the Zn tip that was used in capillary C is inserted in the silica tubing solution supply line. The absence of Zn2+ in the spray with capillary A, but its presence with capillary C, indicates that Zn2+ formation is due to an Zn Zn2+ 2e oxidation caused by ES. (a) Epoxy cement, (b) Teflon sleeve, (c) silica tubing, (d) high-vottage connection, (e) stainless steel.
-
+
The test can be not only qualitative, i.e., detection of Zn2+ when a Zn tip is used, but also quantitative. The total electrospray current, which we will call TC, can be measured; see Figure 1. Assuming that the Zn dissolution is the only oxidation reaction that occurs a t the tip, the expected Zn2+ concentration in the solution can be evaluated from the T C and compared with the mass spectrometrically detected Zn2+ intensity.
EXPERIMENTAL SECTION (a) Electrospray Capillaries and Mass Spectrometric Detection of Zn2+.The electrospray capillaries used in this work are shown in Figure 2. The capillary tip in Figure 2B is conventional, i.e., out of stainless steel, while the tip of the device in Figure 2C is out of zinc. The Zn tip was a machined plug 3 mm long with 1.4-mm 0.d. and 0.3-mm i.d. The 0.d. and i.d. of the stainless tip were the same; however, this tip was an integral part of the stainless capillary; see Figure 2B. The 0.d. of the tips used is larger than is the case in conventional work (1-3). Capillaries with similarly large o.d.'s were used in some of our previous works (4, 6, 7). These capillaries require a higher capillary voltage: 7 kV at a 4-cm distance of the capillary tip from the counter electrode. However, the electric field (E) at the tip is similar to that for the conventional thinner tips, which are operated at -3.5-4 kV, at a 4-cm distance. The thicker tips provide very good sensitivity and signal stability when operated in the multipoint spray mode (4). However, in the present work, the principal reason for the choice was the greater ease in the machining of a wider Zn tip, since Zn capillaries are not available commercially. In the assembly of the Zn-tipped capillary, the flexible silica capillary tubing, which provides a connection to the motor-driven syringe, extends up and into the Zn tip. The silica capillary terminates 1mm from the free end of the Zn where it is bonded with epoxy to the inner Zn wall. Thus, the Zn surface exposed to the flowing solution is only the last 1 mm of the Zn capillary channel and the annular area of the tip itself, which is wetted by the solution. Control experiments were made in order to test whether the Zn2+ion dissolution occurs from the tip only due to action of the electrochemicaloxidation. The Zn tip and attached silica capillary were pulled out from the arrangement shown in Figure 2C and mounted "upstream" in the silica tubing solution supply line as shown in Figure 2A. The Zn tip is now close to the motor-driven syringe but electrically insulated. Also, it is facing the flow of the solution; i.e., the annular area of the tip is now facing the flow. This mounting is not quite equivalent to the mounting in Figure 2C, since the flow patterns and thus the
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ANALYTICAL CHEMISTRY, VOL. 63, NO. 19, OCTOBER 1, 1991
time of exposure of the Zn surface to the solution will be different in the two cases. In all experiments, the solution was supplied from a motordriven syringe at flow rates of 20 ML/min as in previous work (4) and methanol was used aa the solvent. The methanol was purified (deionized) by slow distillation and stored in plastic bottles. The ES capillary was mounted inside the atmospheric pressure source plenum chamber of a SCIEX TAGA 6000E. This instrument samples ions from atmospheric pressure. Mass analysis is obtained with a triple-quadrupole mass spectrometer. The sampled ions can be exposed to collision-induced dissociation by applying accelerating potentials in the gas expansion region immediately past the sampling orifice, which leads to the vacuum chamber housing the mass analysis system. Collision-induced dissociation can be also used with the triple-quadrupole system that is provided. Full details of the ES applications of this apparatus have been published (4, 7, IO). (b) Experiments where the Electrosprayed Solutions and Ions Are Collected on a Platinum Dish Counter Electrode. The aim of these experiments was the determination of the amount of Fe that dissolves from the stainless steel capillary, Figure 2B, when methanol solutions of electrolytes are electrosprayed. The electrosprayed solution was collected in the counter electrode, which was a shallow Pt dish of 3-cm diameter. The spray capillary pointed downwards toward the horizontal Pt dish. In order to collect all the droplets and ion spray, the distance between the stainless capillary tip and the electrode was decreased from 4 to 1 cm and the capillary voltage was reduced from -7 to -5 kV. The dependence of the electric field (E,) on the distance (d) between the capillary tip and the plane electrode and the capillary tip radius (r,) can be estimated, see refs 5, 7 , and 8, with the equation
where V, is the applied voltage to the capillary. This equation shows that with the present rc = 0.7 mm, a change of d from 40 to 10 mm and a simultaneous change of V, from 7 to 5 kV leave E, essentially unchanged. Thus, the spray conditions with this arrangement and the arrangement described in section a should be very similar. In the average run, the spray was collected for 100 min. Two modes of collection were employed: (a) on a dry Pt dish and (b) on a Pt dish containing 2 mL of 5 M HCl. The TC due to the electrospray arriving at the Pt dish was measured in all cases. The solutions obtained from either the dry or the wet collection were subjected to inductivelycoupled plasma atomic emission spectroscopy (ICPAES) determinations. For the runs where the "dry" collection was used, the salt deposit in the dish was dissolved in a few drops of concentrated HCl and the solution was ultimately diluted to 6 mL. For the "wetn collection experiments, the solution was diluted to 6 mL. The ICPAES determinations were performed with a Leco Plasmarray instrument that was calibrated prior and after the runs with standard 0.1 and 1 ppm Fe2+solutions. Test runs in which the workup of the Pt dish was made in the absence of ES-collected solutions were also made and led to minimal Fe readings with ICPAES. The determinations of the ES solutions provide the Fe content in weight per weight solution (ppm). These data were converted to the corresponding Fez+molar concentrations in the electrosprayed solution, by means of the mass conservation equation 56 7[Fe2+]tQ= Fe(ppm)/106 where t (seconds) is the time the spray was collected, Q is the ES capillary flow rate (liters/second), and a Q of 20 pL/min (0.33 X lo* L/s) was used in all experiments. Fifty-six is the atomic weight of Fe, and the factor 6 takes into account that the total solution subjected to analysis was 6 mL.
RESULTS AND DISCUSSION (a) Zinc Ions from Zn-Tipped ES. Electrospray mass spectra were obtained when methanol solutions containing M of a given supporting electrolyte such as NaCl, KC1, RbC1, and CsC1, in purified methanol, were sprayed first with
2111
MIZ Figure 3. Partial mass spectra observed with ES. (A) Zn tip capillary, see F we 2C, was used. The ions mlz = 149, 150, and 151 are due to Zn (DMSO), isotopes. (B) Zn tlp mounted In series with silica tubing line of stainless steel capillary: see Figure 2A. No Zn2+(DMSO), is
3
detected.
a conventional stainless steel capillary, see Figure 2B, and then with a Zn-tipped capillary, Figure 2C. The rationale of these experiments was partially explained in the Introduction. Metallic Zn has a very low reduction potential, i.e., a very high thermodynamic tendency toward oxidation; see eqe 1-4. Therefore, it may be expected to undergo oxidation when the capillary tip is made out of Zn. The oxidation reaction will occur if ES operates like an electrolysis cell. The Zn2+formed by the reaction will be swept out by the flow through the capillary, and therefore, one should be able to detect the Zn2+ ions with the mass spectrometer. The concentration of Zn2+expected in the solution can be calculated from the TC; see Figure 1. The relationship is shown in eq 5, where T C is the measured total current (am-
TC = [Zn2+],,2FQ
(5)
peres), [Zn2+Ip, is the predicted concentration of Zn2+ (moles/liter), assuming that the dissolution of Zn is the only oxidation reaction occurring at the metal-liquid interface, F is the charge of 1 mol of electrons (Faraday's constant), and Q is the flow rate (liters/second). In separate runs using solutions containing known concentrations of added ZnC12,where the stainless steel capillary is used, calibrations for the sensitivity of Zn2+detection with the mass spectrometer can be obtained. From the known sensitivity for Zn2+and the observed Zn2+with the Zn-tipped capillary, [Zn2+]obcan be evaluated and compared with the predicted value [Zn2+],, based on eq 5. Two representative mass spectra, one obtained with the stainless steel tip and a solution containing M RbCl and the other with the Zn tip and a solution of M RbC1, are shown in Figure 3. The solutions used also contained M dimethyl sulfoxide (DMSO). In earlier ES work dealing with the detection and properties of doubly charged metal ions including Zn2+,it was found that the doubly charged ions have much lower sensitivities relative to the singly charged (alkali) ions (IO). Furthermore, in the absence of DMSO, the observed Zn2+ ions were hydrates, Zn2+(H20),, which even after declustering by collision-induced dissociation (CID), were still
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ANALYTICAL CHEMISTRY, VOL. 63, NO. 19, OCTOBER 1, 1991
Table I. Determination of Zna+Concentrations for ES Involving Zn-Tipped Capillary
.A 0.20 0.15
!
! e
I
1
0.05
2
4
6
8
10
200000
B
P
.-2
I
150000
100000
I
n
I
++E N
TC, PA” ZnZ+(DMS0)3 Na+(DMSO) K+(DMSO) Rb+(DMSO) Cs+(DMSO) FeZ+(DMS0)3 [ Zn2+l [Znz+lp4
14gC 10lc 117c 163c 21lC 145c
i1 o
0.00 0
-
B
0
0 2
4
6
8
10
[Z~CI~IX 0 6I (moles/Llter)
Figure 4. Calibration plots of runs where ZnCI, is added to electrosprayed solution with stainless steel capillary; see Figure 28. I n A, the TC is shown. I n B the measured Zn2+(DMSO),, m l z = 149, in counts per second is shown for different ZnCI, concentrations. SupM Concentration. (0) NaCI; (+) porting electrolyte used; MCI at KCI. (0)RbCI; (X) CsCI.
spread over several n. Since the Zn2+signal was spread over many peaks, it was less easy to detect. When DMSO was used, a much smaller number of Zn2+(DMSO), clusters were observed after CID and this facilitated detection (IO). The ZII~+(DMSO)~ cluster is seen in Figure 3A, and for the quantitative, analytical determinations, we used the most intense peak a t m / z = 149 corresponding to the most abundant 64Znisotope. The mass spectrum shown in Figure 3B is from a control experiment. In this experiment, the stainless steel tipped capillary was used. However, in the silica capillary tubing that transports the solution to the spray capillary, an insert was made containing the Zn tip; see Figure 2A. The Zn tip faces the flow and is electrically insulated from the stainless steel capillary. No Zn2+ions in solution were observed with this arrangement; see mass spectrum in Figure 3B. The possibility that the Zn2+ions observed in the mass spectrum of Figure 3A obtained with the Zn tip spray are due to some corrosive process a t the Zn-liquid interface, which releases Zn2+and negative counterions to the solution, is made very unlikely by the results of the control experiment. A complete redox process, say, due to the presence of dissolved oxygen in the solution, should have released similar concentrations of Zn2+ ions to the solution for both positions of the Zn tip. In order to determine the concentration of Zn2+ions in the solution resulting from the electrospray-caused dissolution a t the Zn tip, a series of calibration runs were made where the stainless steel capillary was used but no Zn metal was present anywhere in the line; see Figure 2B. Solutions containing M supporting electrolyte, M DMSO, and known concentrations of added ZnC12 were electrosprayed, and the observed Z~I*+(DMSO)~ ion (mlz = 149) intensity was measured. The dependence of the m / z = 149 intensity on the molarity of the ZnClz is shown in Figure 4 for the concentration range (1-10)X lo4 M ZnC12. Although there is considerable scatter, a linear relationship between the Zn2+(DMSO), ion intensity and solution concentration is
NaClb
KClb
RbClb
CsClb
0.14 32 2820 430 e e e 2.2 2.2
0.13 24 340 1600 e e 1.0 1.7 1.9
0.11 29 725 e 275 e 0.5 2.0 1.7
0.15 34 e e e 225 e 2.4 2.3
Total ES current; see Figure 2. Supporting electrolyte, mol/L in methanol. Flow rate 20 pL/min. Ion intensities (kilocounts/s) at given m / z detected with mass spectrometer, not corrected for mass-dependent discrimination. d Zn2+ concentration (Fmol/L) in solution based on MS detection of Zn2+(DMS0)3and calibration; see Figure 4. e Intensity was not recorded. f Zn*+concentration (gmol/L) based on measured TC and eq 5.
observed. More experimental determinations were made for the region (1-3) X lo4 M because this concentration falls in the expected range of predicted concentrations, see eq 5, when the Zn tip is used. A summary of the results is provided in Table I. Given are the measured TC’s for four representative experiments obtained with the Zn-tipped capillary, Figure 2C. The supporting electrolytes were NaCl, KC1, RbCl, and CsC1, each used a t M concentration. The observed ZII~+(DMSO)~ intensities and the [Zn2+Ierpconcentrations in solution evaluated from these intensities and the sensitivity calibration runs shown in Figure 4 are given in the table. Also shown are the [Zn2+Ipr,concentrations predicted on the basis of the measured TC’s and eq 5. Comparing [Zn2+leXp and [Zn2+Ip,,one finds agreement within 20%. This is very good agreement considering the difficulties of the measurements. The results in Table I thus strongly support the contention that electrospray operates as an electrolysis cell of a special kind. The need for and the choice of the concentration of the “supporting electrolyte” needs to be explained. ES does not work with a completely deionized methanol solution. A minimum electrolyte concentration corresponding to a conductivity (a) of approximately 3 x IO-’ W cm-’ is required for stable operation. The spray becomes intermittent for a conductivity a = 1 x lo-’ Q-’ cm-’ (9). For the alkali-metal chlorides, which have similar molar conductivities, in methanol (Ao = 100 Q-’ cm2 mol-’), see Landolt-Bornstein (II),this corresponds to total electrolyte concentrations of approximately lo* M. Therefore, in order to maintain a good spray, 1 X 10“ M of a “supporting” electrolyte was used. The name “supporting” is chosen in analogy with the same term used in the electrolysis and electrochemical nomenclatue. It should be noted that the supporting electrolyte in ES should be playing a similar role as in these conventional processes since it provides conductivity without (necessarily) participating in the reactions a t the metal-liquid interface. It was established in earlier ES work ( 4 ) that the mass spectrometrically detected ion signal for a given ionic analyte in solution increases linearly with the solution concentration of the analyte, from the detection limit (- lo4 M) to M. Above this concentration, there is a gradual fall off from proportionality. The linear region a t low analyte concentrations corresponds to the region where the solution concentration of the analyte is much lower than the given, constant, concentration of external (supporting) electrolytes. The external electrolytes might have been added deliberately or may be present due to electrolyte impurities in the methanol used. The concentration of electrolyte impurities in “reagent grade”
-
-
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ANALYTICAL CHEMISTRY, VOL. 63, NO. 19, OCTOBER 1, 1991
Table 11. Data for Calibration of Zn2+Sensitivity in ESa NaC1' l b
MtDMSOc 2400 NatDMSOc 2400 130 K+DMSOc Fe2+(DMS0)3c 49 Zn2t(DMS0)3c 10 TCd 0.9
RbClf
KClf
26
36
l b
2b
3b
l b
3000 3000 490 68 15 0.15
2500 2500 120 79 55 0.16
1130 2130 1130 80 5 0.16
1240 1800 1240 92 18 0.17
1440 1200 1440 80 51 0.17
295 g 410 50 9 0.15
CSCY
2b
36
l b
2b
3b 250 g g 82 45 0.16
275
290
265
320
g
g
g
270 85 30 0.17
g 50 13 0.13
g g
340 56 19 0.16
60 42 0.14
OThe Zn2+ sensitivity determinations were made with the use of prepared solutions of ZnC1, in methanol. The solutions contained also M of an external (supporting) electrolyte,MCl = NaCl, KCl, RbCl, CsC1. One of the above electrolytes was used in a given series of runs with known variable ZnClz concentration. Stainless steel capillary, Figure 2B, was used. Concentration of ZnCl (pmol/L) in methanol solution. Mass spectrometricallydetermined ion intensity (kilocounts/s). Not corrected for mass-dependent discrimination of quadrupole. dTotal ES current (PA). eG1ass vials used to store solutions. fPlastic vials used to store solutions. gIon intensity was not recorded. methanol is generally close to 5 X lo4 M; see refs 4 and 7). The slope of the ion signal versus concentration plot, i.e., the analyte sensitivity, is approximately inversely propertional to the given constant concentration of the external electrolyte (4,121. A mechanistic interpretation of these relationships on the basis of competitive "evaporation" of analyte and external ions from the electrosprayed droplets is given elsewhere
(12). The use of supporting electrolyte in the present work thus has two functions: (a) it supports the ES process; (b) when used at constant concentration, it provides a linear region for the ionic analyte (Zn2+)signal. The relatively large scatter observed in the Zn sensitivity calibration plot, Figure 4, is due to difficulties in maintaining the concentration of the external electrolyte constant. For example, if the prepared solutions (see Experimental Section) were stored even for a short time in Pyrex containers, the amount of external electrolyte gradually increased. A set of experiments where the solutions were kept in plastic vials is included in the data shown in Figure 4 and Table 11. Uncontrolled amounts of external electrolytes could be inadvertently introduced also by handling of the silica tubing near the connecting ends, see Figure 2, presumably due to deposition of sodium and potassium salts from the finger tips of the operator. Examples of the mass spectrometrically detected presence of external sodium and potassium ions are given in Table 11. Thus, the Zn calibration experiments with M KCl as supporting electrolyte indicate that -0.5 X M NaCl was also present. Similar sodium contamination is observed also for the RbCl and CsCl supporting electrolyte experiments; see Table 11. Potassium contamination corresponding to 2 X lo4 M is also observed. The contamination observed with the plastic vials is somewhat less, see Table 11, but still significant. We chose to use M concentrations of the external electrolyte for the Zn tip experiments, because lower concentrations would have been associated with even larger fluctuations of the electrolyte concentration due to accidental introduction of electrolyte impurities. A choice of higher nominal concentrations, say lo4 M, would have reduced much more the fluctuation due to accidental impurities; however, the sensitivity for Zn2+ion detection would have been close to 10 times lower (4, 12). (b) Fe Ions from the Stainless Steel Capillary and Some General Observations. The presence of Fe2+(DMSO), ions was observed in the experiments where the stainless steel tip was used. The intensities of Fe2+(DMS0)3at m / z = 145 observed in the Zn sensitivity calibration runs are given in Table 11. Significantly, essentially no Fe2+(DMS0)3is observed with the Zn-tipped capillary; see Table I. A calibration for the sensitivity of Fez+under ES conditions was not made. Such a calibration would have suffered the complication that the stainless steel capillary releases Fez+. A truly inert metal
-
Table 111. Fe2+Concentrations from ES with Stainless Steel Capillary, Determined with the ICPAES Technique expt 1 2 3d 4 5 6d 7 8 9 10 1l e
salt, Ma ZnCl,, ZnCl,,
io-' io-'
ZnC12, io-'
KBr, KBr, KBr, KBr, lo4 KBr, lo4 KBr, lo-' NaN03, lo-' KBr, lo-'
TC, pA
[Fez+],,db
0.5 0.55 0.53 0.24 0.22 0.41 0.21 0.57 0.54 0.45 0.60
7.6 8.2 7.9 3.6 3.3 6.1 3.1 8.6 8.1 6.8 9.0
8.9 8.8 9.3 5.8 2.9 10.7 2.7 0.1 1.4 3.3 7.6
a External (supporting) electrolyte concentration in methanol (mol/L). Predicted concentration (pmol/L) evaluated from TC with eq 5. Concentration (pmol/L) based on collected electrospray in Pt dish electrode and analysis of resulting solution with ICPAES. The ppm values obtained with ICPAES were converted to [Fez+];see Experimental Section part b. dPt dish contained 2 mL of 5 M HC1. 'ES capillary was a t 4-cm distance from Pt electrode, V , = I kV.
capillary, such as a platinum capillary, was not available to us. Experiments with platinum-iridium capillary tubing, which is available commercially, will be undertaken in a future investigation. Assuming that the Fez+sensitivity is similar to that for Zn2+, the observed intensities indicate Fez+ concentrations in solution in the lo4 M range. The concentration is similar to that observed with the Zn tip, and this suggests that the iron dissolution is due to the ES-induced oxidation reaction. Thus, the passivation of the iron by the chromium oxides in the stainless steel appears to be insufficient for ES conditions. Fez+ions were observed not only with the wider tip stainless steel capillary used in the present work but also with thin stainless steel capillaries as used in conventional work (1-3). Furthermore, the detected Fe2+(DMS0I3intensities were somewhat smaller but of similar magnitude to those given in Table 11. This means that the Fe dissolution from the stainless steel capillaries is not specific only to the wide tip variety. Additional tests were made in order to establish whether the stainless steel capillary releases Fe2+due to an oxidation caused by the electrophoretic ES process. These tests relied on collecting the electrosprayed solution and ions in a platinum dish counter electrode. For full details, see the Experimental Section. The collected solution, after a known spray time (- 100 min), was worked up and subjected to Fe determination by ICPAES. The Fez+concentrations in the ES solution could be evaluated from the ICPAES results. These Fez+ concentrations can be compared with [Fe2+Ip, obtained from eq 5 and the measured T C to the Pt dish counter electrode. The data obtained are shown in Table 111.
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ANALYTICAL CHEMISTRY, VOL. 63, NO. 19, OCTOBER 1, 1991
Inspection of the table shows that the two sets of Fe concentrations are in fair quantitative agreement. Only two of the experiments (nos. 8 and 9) show much lower [Fe2+], than [Fe2+],,. The reason for the failure in these experiments is not known. The ICPAES instrument seemed to perform reliably, and it is likely that the collected solutions did not contain the predicted amount of Fe. The results in Table I11 lend strong support to the premise that ES behaves like an electrolysis cell and that the stainless steel capillary is not inert so that the oxidation reaction occurring at the metal-ligand interface is almost exclusively the oxidation of Fe to Fez+. Considering the results for the stainless steel capillary, it was of interest to establish whether other metals that resist oxidation might also produce ions in solution under ES conditions. A silver tip similar to the Zn tip was mounted in the spray capillary; see Figure 2C. When solutions with M NaN03 as the supporting electrolyte and M DMSO in methanol were electrosprayed, ions corresponding to Ag+(DMSO) and Ag+(DMS0)2 were observed with the mass spectrometer. The intensity of the most abundant Ag+ ion, Ag+(DMSO), was -3000 counts/s. A sensitivity calibration was not made but the low intensity indicates that the dissolved Ag+ accounts for only a small fraction of the ion concentration expected from the ES caused oxidation. The standard reduction potential of Ag is +0.8 V. This reduces to +0.45 V assuming that the Ag+ concentration is lo4 M. This process is thermodynamically less favorable than the H+ production from water; see reaction 3, which has a reduction potential of -0.33 V for an H+ concentration of 10-6 M. This suggests that the major oxidation reaction that occurs with this electrode is the production of H+ ions. More direct evidence based on mass spectrometric detection of H+ ions is difficult to obtain, since protonated methanol is generally observed in ES mass spectra. Furthermore, some of the protonated methanol is converted to NH,+ by the gas-phase proton transfer from CH30H2+to NH3 gas impurity present in atmospheric air. I t is known to practitioners of ES that liquids can be electrosprayed from the tip of capillaries of nonconducting materials such as silica or Teflon where the connection to the high voltage can be several centimeters upstream. Therefore, the question arises, where does the electrochemical conversion from ions to electrons occur in the absence of an electric gas discharge? While we have no answer supported by experiments for this case, we assume that the electrochemical process will occur at the place where the metal that carries the potential is in contact with the solution, even if this point of contact is several centimeters upstream of the capillary tip. The actual electrochemical reaction will depend on the conditions and need not involve the dissolution of metal ions into the solution. Creation of H+ ions or removal of negative ions, see eqs 1-3, could be involved. The reactions will be driven by a potential difference at the metal-liquid interface, which
is equal to the absolute oxidation potential, which is no more than -2 V. Such a potential drop can be easily maintained by the escape of positively charged liquid at the tip and the relatively low conductivity of the solution. The experiments described in this work indicate that ES mass spectrometry might be an analytical method suitable for corrosion studies. Thus, with ES mass spectrometry, one is capable of detecting singly charged metal ions down to lo4 M levels and doubly charged ions a t lo* M levels. Furthermore, the corrosion processes resulting say due to the addition of molecular oxygen and hydrogen ions to the solution could be followed in real time.
CONCLUSIONS The assumed electrophoretic ion separation mechanism for electrospray requires that a conventional electrochemical oxidation reaction be occurring at the liquid-metal interface of the ES capillary tip. The experiments in which a special Zn tip is used provide qualitative and quantitative evidence that such a reaction occurs. Very good evidence is provided also, which shows that the stainless steel capillary is not inert and that the oxidation, when this capillary is used, corresponds to the reaction Fe(8)= Fez+ + 2e. Since the ion spray method (2) also produces unipolar charged droplets, it can also be viewed as an electrolytic cell of a special kind. LITERATURE CITED Whitehouse, C. M.; Dreyer, R. N.; Yamashita, M.; Fenn, J. 8. Anal. Chem. 1985. 57, 675. Wong, S. F.; Meng, C. K.; Fenn, J. B. J . Phys. Chem. 1988, 9 2 , 546. Fenn, J. B.; Mann, M.; Meng, C. K.; Wong, S. F. Mass Spectrom. Rev. 1990, 9 , 37. Bruins, A. P.; Covey, T. R.; Henion, J. D. Anal. Chem. 1987, 59, 2642. Covey, T. R.; Bonner, R. F.; Shushan, B. I.; Henion, J. D. Rapid c o ” U n . Mass Spectrom. 1988, 2 , 249. Hung, E. C.; Henion, J. D. J . Am. SOC. Mass Spectrom. 1990. 1 , 158. Olivares, J. A.; Nguyen, J. A.; Yonker, C. R.; Smith, R. D. Anal. Chem. 1987, 5 9 , 1230. Smith, R. D.; Olivares, J. A.; Nguyen, N. T.; Udseth, H. R. Anal. Chem. 1988. 60, 436. Udseth. H. R.; Loo, J. A.; Smith, R. D. Anal. Chem. 1988, 60, 1948. Ikonomou, M. G.; Blades. A. T.; Kebarle, P. Anal. Chem. 1990, 6 2 , 957. Pfeifer, R. J.; Hendricks. C. D. AIAA J . 1988, 6 , 496. Juhasz, P.; Ikonomou, M. G.; Blades, A. T.; Kebarle, P. In Melh0d.S and Mchenisms for Producing Ions from Large Mlecules; Standing, K. E., Ens. W., Eds.; Plenum Press: New York, 1991. Ikonomou, M. G.; Blades, A. T.; Kebarle, P. Anal. Chem., in press. Smith. D. P. H. IEEE Trans. Ind. Appl. 1988, IA-22, 527. Hayati, I.; Bailey, A. I.; Tadros, T. F. J . CollOM Interface Scl. 1987, 117, 205, 222. Jayaweera, P.; Blades, A. T.; Ikonomou, M. G.; Kebarle, P. J . Am. Chem. Soc. 1990, 172, 2452. Blades, A. T.; Jayaweera, P.; Ikonomou, M. G.; Kebarie, P. J . Chem. Fhys. 1990. 9 2 , 5900. Blades, A. T.; Jayaweera, P.; Ikonomou, M. G.; Kebarle, P. Int. J . Mass Spectrom. Ion Processes 1990. 101, 352; 1990, 102, 251. Landott-Biirnstein Zahlenwme und FunkHOnen, 2nd ed.; Springer Verbg: Berlin, 1960; Voi. 7. pp 366. 533, 651. Tang, L.; Kebarie, P. Unpublished work.
RECEIVED for review April 1, 1991. Accepted June 7, 1991. This work was supported by the Canadian Natural Sciences and Engineering Research Council and the Premiers Council, Ontario Technology Fund.
