Anal. Chem. 1995,67, 2916-2923
Characterization of an Eiectrospray Ion Source as a Controiied=CurrentElectrolytic Ceii Gary J. Van -*el*
and Feimeng Zhou
Chemical and Analytical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 3783 1-6365
An electrospray (ES) ion source is described as a constant
or controlled-current device for which the magnitude of the ES current is controlled by the rate of charged droplet production. Thus, the nature of the electrolytic process that occurs in the metal ES capillary to charge balance the loss of one ion polarity in the charged droplets is shown to be analogous to that of a controlled-current electrolytic (CCE) cell and controlled-current electrolysis carried out in a flow cell. That is, the potential at the metaVsolution interface in the ES capillary, which ultimately determines whether or not a particular species will undergo a redox reaction in the capillary, is a function of both the ES current and the relative redox potentials and concentrations of the various species in the solvent system, including the metal capillary. Furthermore, the extent to which one or more reactions occur is limited both by the ES current and by the flow rate of the sohrent system through the ES capillary. Experimental confirmation of the ES ion source as a CCE cell is made through experiments employing a novel ES ion source in which the effluent from the ES capillary enters the detection cell of a W/visible diode array spectrophotometerprior to the spraying process. This ES setup allowed for the first time the detection of the products of the redox reactions in the ES capillary, while they were still in solution, thereby avoiding experimental complications imposed by the spraying process or by the subsequent mass analysis of the gas-phase ions that might complicate data interpretation. The analytical implications of the operation of the ES ion source as a CCE cell for neutral compound ionization and detection in ES-MS are briefly discussed. Despite the demonstrated utility of electrospray mass spectrometry @SMS)'-'O and a generalized understanding of the various aspects of the overall ES process,'O the details of the individual steps of the ES process remain to be fully elucidated (1) Yamashita, M.; Fenn, J. 8.J. Phys. Chem. 1984,88, 4451-4459. (2) Yamashita, M.; Fenn, J. B. J. Phys. Chem. 1984,88, 4471-4475. (3) Fenn, J. B.; Mann, M.; Meng, C. IC;Wong, S. K.; Whitehouse, C. M. Science 1989,246, 64-71. (4) Fenn. J. B.: Mann, M.: Meng, C. K; Wong, S. K ; Whitehouse, C. M. Mass Spectrom. Rev. 1990,9, 37-71. (5) Smith, R D.; Loo, J. A; Edmonds, C. G.: Barinaga, C. J.; Udseth, H. R. Anal. Chem. 1990,62, 882-899. (6) Huang, E. C.: Wachs, T.; Conboy, J. J; Henion, J. D. Anal. Chem. 1990,62, 713A-725A. (7) Mann, M. Org. Mass Spectrom. 1990,25, 575-587. (8) Fenn, J. B. J. Am. SOC.Mass Spectrom. 1993,4 , 524-535. (9) Ashton, D. S.; Beddell, C. R.; Cooper, D. J.; Green, B. N.; Oliver, R W. A Org. Mass Spectrom. 1993.28, 721-728. (10) Kebarle. P.; Tang, L. Anal. Chem. 1993,65, 972A-986A.
2916 Analytical Chemistry, Vol. 67, No. 17, September 7, 1995
and are under debate. A definitive description of the ES process is desired to better understand the relationship among the various instrumental components and variable parameters of the ES device, and the physical and chemical nature of the solvent systems and analytes as they each relate to the gas-phase ions generated by ES and ultimately observed by the mass spectrometer. Thus, a better understanding of the overall ES process might, for example, be expected to lead to means for improving ESMS performance in terms of analytical figures of merit, such as dynamic range and detection limits, means to expand the range of analytes amenable to analysis, and possibly the means to control or alter the ionic species observed. Addressed in this paper is the nature of the electrochemical phenomenon inherent in the operation of an ES device, which plays an important role in the first step of the ES process, viz., the charging and formation of the ES droplet^.^*-^^ Owing to the electrophoretic charge separation of ions in the solution at the capillary tip due to the imposed electric field, a selective loss of one ion polarity in the droplets 0ccurs.~5-19 This leads to an accumulation (or buildup) of ions of the opposite polarity in the capillary, which must be charge balanced for the ES device to continually operate. Without a charge-balancing process, the buildup of charge in the capillary would create a field in solution counter to the externally applied field, thereby negating the force for ion migration (Le., electrophoretic charge separation), which would lead to the cessation of charged droplet formation. Kebarle and co-workers1'J2demonstrated that the mechanism responsible for charge balance in the ES capillary is electrochemical in nature. Furthermore, they described the ES ion source as "an electrolytic cell of a somewhat special kind. This special nature derives, in their words, from the fact that a portion of charge transport between electrodes in the cell (i.e., the metal ES capillary and the front aperture plate of the mass spectrometer) occurs via the gas phase rather than totally through solution as in a conventional electrolytic ce11.20-22 (11) Ikonomou, M. G.; Blades, A. T.; Kebarle, P. Anal. Chem. 1991,63, 19891998. (12) Blades, A. T.; Ikonomou, M. G.: Kebarle, P. Anal. Chem. 1991,63, 21092114. (13) Van Berkel. G. J.; McLuckey, S. A; Glish, G. L. Anal. Chem. 1992,64, 1586-1593. (14) Xu,X.; Nolan, S. P.; Cole, R. B. Anal. Chem. 1994,66, 119-125. (15) Bailey, A. G. Electrostatic Spraying of Liquids; John Wiley: New York, 1988. (16) F'feifer. R J.; Hendricks, C. D. AlAAJ. 1968,6,496-502. (17) Smith, D. P. H. IEEE Trans. Ind. Appl. 1986,lA-22, 527-535. (18) Hayati, I.; Bailey, A. I.; Tadros, Th. F. J. Colloid Intelface Sci. 1987,117, 205-221. (19) Hayati. I.; Bailey, A. I.: Tadros, Th. F. J. Colloid Inte6ace Sci. 1987,11 7, 222-230. (20) Bard, A J.; Faulkner. L. R Electrochemical Methods; John Wiley & Sons, Inc.: New York, 1980.
0003-2700/95/0367-2916$9.00/0 0 1995 American Chemical Society
Kebarle and co-workersl1J2postulated that when the ES capillary is held at a high positive voltage (i.e., positive ion mode), the buildup of negative ions in the capillary might be counter balanced by electrochemical oxidation reactions that result in either the neutralization of the negative ions or the production of positive ions. When the needle is held at a high negative voltage (i.e., negative ion mode), the buildup of positive ions in the capillary might be counter balanced by electrochemicalreduction reactions that result in either the neutralization of the positive ions or the production of negative ions. In fact, Blades et a1.12 observed gas-phase zinc ions when the ES capillary tip was made of zinc metal (i.e,, Zn(,) Zn(,d2+ 2e-) and observed gas-phase iron ions when the ES capillary was made of stainless steel (i.e., Fe(s) Fe(,d*+ 2e-), confirming that oxidation of components of the metal ES capillary could be one of the charge-balancing reactions. We presented evidence that the radical cations o b served in the ES mass spectra of some types of easily oxidized compounds (e.g., metalloporphyrins and polycyclic aromatic hydrocarbons (PAHs))were formed via electrochemical oxidation of these compounds at the metal/solution interface in the ES capillary.13This was the first evidence that solution species could be involved in these redox reactions. More recently, Xu et al.14 presented similar data from an ESMS study of various metallocenes. Clearly, the electrochemical nature of ES has been identified,l1>l2 and the phenomenon has been shown to have potential analytical benefits for neutral compound ionization and subsequent detection by ESMS.13J4 However, the precise electrolytic nature of the device, Le., the experimental parameters that determine the potential at the metal/solution interface in the ES capillary and, therefore, determine the reactions that can or cannot occur, and the factors that determine to what extent specific reactions will occur, have not been thoroughly delineated. As such, the analytical implications of the electrolytic nature of the ES ion source, in reference to ESMS, cannot be fully appraised. In this paper, we show that the ES ion source operates as a controlledcurrent source and the nature of the electrolytic process that takes place in the ES capillary is analogous to controlledcurrent electrolysis carried out in a conventional controlledcurrent electrolytic (CCE) flow cell.2°-22 Confirmation of the ES ion source as a CCE cell is made through experiments employing a novel ES ion source in which the effluent from the ES capillary enters the detection cell of a UV/visible diode array spectrophotometer prior to the spraying process. This ES setup allowed for the first time the study of the products of the redox reactions in the ES capillary, as a function of variable experimental parameters, while these products were still in solution. In this way, experimental complications imposed by the spraying process (e.g., gas-phase ionization, signal suppression, or gas-phase charge-changing reactions) or by the subsequent mass analysis of the gas-phase ions (e.g., m/z discrimination by the atmospheric sampling interface or by the mass spectrometer) that might impede interpretation of the data are avoided. The analytical significance of the operation of the ES ion source as a CCE cell for neutral compound ionization and detection in ESMS is briefly discussed.
-
-
+
+
(21) Stulik, K; Pacakova, V. Electrochemical Measurements in Flowing Liquids; Ellis Honvood. Ltd.: Chichester, West Sussex, England, 1987. (22) Bockris, J. O'M; Reddy, A IC N. Modern Electrochemistry; Plenum Press: New York, 1973; Vol. 1.
EXPERIMENTAL SECTION Sample Preparation. Nickel (ID octaethylporphyrin (Ni"O E P Aldrich, Milwaukee, WI), HPLC-grade methylene chloride and acetonitrile U. T. Baker, Phillipsburg, NJ), and lithium trinuoromethanesulfonate,i.e., lithium trinate VUdrich, 96%purity) were used as received. The various solvent systems and analyte solutions were prepared daily. Distilled, deionized water from a Milli-Q purification system (Millipore Corp., Bedford, MA) was used for preparation of the KCl (Fisher Scientifc, Fairlawn, NJ) solution used in the conductivity measurements. Conductivity Measurements. The conductivity of freshly prepared acetonitrile/methylene chloride solutions (1/1 v/v) containing known amounts of lithium trifiate were measured at 25 "C using a YSI Model 35 conductivity detector (Yellow Springs, OH) with a YSI 3400 series dip cell in a constant-temperature water bath (25 "C). Prior to these measurements, the cell constant (1.09) was determined using a 0.01 N KCl solution. Conductivities of the solutions were measured in the order of increasing electrolyte concentration. After each individual measurement, the cell was washed with an acetonitrile/methylene chloride solution (1/1 v/v) three times prior to the next measurement. The conductivities of these acetonitrile/methylene chloride solutions (0, 0.001, 0.01, 0.10, 1.0, and 9.0 mM lithium trifiate) were measured as 1.4, 3.1, 3.7, 15, 97 and 880 x R-' cm-', respectively. Electrospray Ionization Source. Shown in Figure 1 is a schematic representation of the ES ionization source used in this work to acquire W/visible spectra of the solution exiting the ES capillary. In this setup, a syringe pump (Harvard Apparatus, Inc., Cambridge, MA) and glass syringe were used to deliver the appropriate solution at a constant rate, through Teflon tubing, to a Rheodyne Model 7125 loop injector (using a 5.0- or a 50-pL loop) and then through a short length (-10 cm) of 100-pm4.d. (370pm-0.d.) fused silica capillary (Polymicro Technologies, Inc. Phoenix, AZ). The fused silica capillary was attached to a 13-cmlong, 120-pm4.d. (50Oym-o.d.) dome-tip metal capillary (304 stainless steel, Scientifc Instrument Services, Ringoes, NJ) via a low dead volume PEEK union (254pm through-hole, Upchurch Scientifk, Oak Harbor, WA). The downstream end of the metal capillary exited directly into the detector flow cell (4.5" path length, 5pL volume) of a SoloNetl40 UV/visible diode array detector (DAD)system (Groton Technology, Inc., Concord, MA). The flow cell, constructed of Kel-F, was isolated from ground, allowing a high voltage to be applied to the metal capillary. From the detector flow cell, the solution traveled through an additional length (-30 cm) of 100-pm-id.fused silica capillary before finally being sprayed, with pneumatic nebulization (nitrogen gas, 30 psi backing pressure) ,23 toward a planar electrode (-5" separation of capillary and electrode) that was grounded through a Keithley Model 61OC electrometer (Cleveland, OH) enabling the ES current to be measured. A 13-cmlong metal ES capillary was used in the present case, rather than our normal 6.5cm capillary,23to facilitate tubing connections and application of the high voltage to the capillary. RESULTS AND DISCUSSION
Description of an ES Ion Source as a CCE Cell. As shown in Figure 2a, a conventional CCE cell consists of a controlled(23) Van Berkel, G. J.; Quinofies, M. A; Quirke, J. M. E. Energy Fuels 1993,7, 411-419.
Analytical Chemistty, Vol. 67, No. 17, September 1, 1995
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Current Meter
Nebylizer _I
_.I..-
/,
PEEK Connector
-
S ringe Jump
Sample Loop Figure 1. Schematic representation of the novel ES ion source used to monitor in solution, by means of UVhisible spectroscopy, the extent of redox reactions that occur in the ES capillary as a function of variable experimental parameters.
A I
Current Meter
_______________
I
Controlled-CurrentSource Current Meter Metal Capillary
Controlled-CurrentSource
I
(b)
;%!%Ye
Figure 2. Schematic illustration comparing the basic instrumental components of (a) a controlled-current electrolytic (CCE) cell and (b) an electrospray (ES) ion source.
current source and a cell that houses a working electrode and a counter electrode.20 The controlled-current source can be a modest voltage supply (e.g., a 3WV battery) and a variable resistor of high resistance (e.g., 300 kQ), the cell might be a conventional electrochemical cellzoor possibly a flow-through device,21and the electrodes might be made of any one of several conducting materials in various shapes and dimensions. The solution within, or flowing through, the cell typically consists of the electroactive analyte(s) and a supporting electrolyte at higher concentration (typically -0.05-0.1 M) dissolved in a suitable solvent. The role of the electrolyte ions is to transport charge, i.e., conduct current, between the electrodes in solution. The output of the controlledcurrent source determines the magnitude of the current flowing through the cell, i.e., the cell current, ic (typically a few milliamperes). Under the influence of the potential gradient between the two electrodes in solution, electrolyte cations migrate toward the negatively charged electrode, or cathode, while electrolyte 2918 Analytical Chemistry, Vol. 67, No. 17, September 1, 1995
anions migrate toward the positively charged electrode, or anode. Transfer of electrons at the electrode/solution interface to complete the electrical circuit involves oxidation and reduction of one or more of the various species in the solution (e.g., the analyte(s), solvent, electrolyte, and/or contaminants) at the anode and cathode, respectively. This current is termed the faradaic current, i ~and , is equal in magnitude to ic. Note that these redox reactions, in addition to completing the electrical circuit, charge balance the buildup of excess ions of one polarity at the respective electrodes (i-e.,electrode polarization) due to the electrophoretic charge separation of the electrolyte ions in solution.22 The ES ion source illustrated in Figure 2b is also a controlledcurrent electrolytic device, albeit of a different type. The ES system consists of two electrodes and a high-voltage supply that can output about 3-5 kV. The metal ES capillary needle (usually stainless steel) and the atmospheric sampling aperture plate of the mass spectrometer (also stainless steel) serve as the working and counter electrodes, respectively, which are immersed in the surrounding atmosphere. Under typical ESMS operating conditions, a solution containing the analyte(s) of interest (usually ionic) is pumped through the working electrode and sprayed toward the counter electrode. The addition of electrolyte to the solution, other than the analyte (or small amounts of acids or bases to ionize the analyte(s)), is typically avoided as they can suppress the formation of gas-phase analyte i 0 n s . 2 ~ However, 1~~ some number of ions, either the analytes, contaminants, or deliberately added electrolytes, must be present in the solution or the ES device will not function.11 This is because the electrophoretic charge separation of these ions in solution is required for charged droplet f ~ r m a t i o n . ~ ~Specifically, -'~ disruption of the liquid surface and production of the charged droplets in this electrostatic sprayer results from the electric stress at the liquid/air interface due to the imposed electric field.I5 Heifer and Hendricks,16Smith," and (24) Van Berkel, G. J.; Zhou, F.Anal. Chem. 1994,66,3408-3415. (25) Fuhrhop, J.-H.; Mauzerall, D. J Am. Chem. Soc. 1969,91, 4174-4181 (26) Tang, L.; Kebarle, P. Anal. Chem. 1991,63,2709-2715.
Hayati et al.,18J9among others, recognized that this electric stress and droplet formation resulted from the separation of positive ions from negative ions in solution. That is, under the influence of the applied electric field, ions of the same polarity as the voltage applied to the ES capillary migrate from the bulk liquid toward the liquid at the capillary tip, while ions of the opposite polarity migrate in the opposite direction back into the capillary. When the buildup of an excess of ions of one polarity at the surface of the liquid reaches the point that Coulombic forces are sufficient to overcome the surface tension of the liquid, droplets enriched in one ion polarity are emitted from the capillary. Thus, the charged droplet formation process is the controlled-current source in this device with the "cell current", i.e., the ES current, Z'ES, being equal to the product of the rate at which charged droplets are formed and the average number of charges per droplet. As discussed below, several operational parameters, including the electric field at the capillary tip, the composition of the solution, and the solution flow rate, can affect the droplet production process and, therefore, affect the magnitude of ~ E S . In a fashion analogous to the conventional CCE cell, the buildup of one polarity of ion at the working electrode (in this case the metal ES capillary owing to the selective loss of one ion polarity in the charged droplets) must be charge balanced. This charge-balancing process involves electrochemical oxidation/ reduction of the metal capillary12 and/or one or more of the species in the ~ o l u t i o n . ~Specifically, ~.~~ an oxidation reaction occurs in the ES capillary (the working electrode) in positive ion mode while a reduction reaction occurs in negative ion mode. Thus, the magnitude of the current due to these redox reactions, Le., faradaic current, i ~ is , equal to Z'ES. Oxidation/reduction of some species at the counter electrode (Le., the front aperture plate of the mass spectrometer) must occur to complete the electrical circuit. Assuming the ES ion source is a CCE cell as implied in the above description, the nature of the electrolysis process that occurs in the ES capillary should be that characteristic of a CCE cell and controlled-current electrolysis carried out in a flow ce11.2°-22 Thus, in direct analogy to controlled-current electrolysis, the potential at the metal/solution interface in the ES capillary (Le., the potential at the working electrode/solution interface, EEIS),which determines which redox reactions can occur, is expected to be a function of both the magnitude of the ES current, i ~ sand , the respective concentrations and redox potentials of all species in solution. The value of EEISfor a given magnitude of Z'ES should be that necessary to oxidize sufficient species in the ES capillary so as to maintain that current, Le., 1 ' ~ s= i ~ . This relationship is expressed by Faraday's law shown in eq 1,where
nj is the number of electrons involved in the oxidation of one molecule of species j , Aj is the concentration of species j oxidized (mol/L), F is the Faraday constant (9.648 x lo4 C/mol), and Y is the solution flow rate through the ES capillary (I.,/The $. individual species should oxidize in order of their increasing halfwave potentials until the required current is supplied. Furthermore, the extent of any reaction involving a solution species should be controlled both by the rate at which the species flows through the electrode for a given magnitude of i ~ s(eq 1) and by the rate
t
EC+/C
iFi
-
IF2
iEs (PA)
iF3
Figure 3. Schematic illustration, based on the operation of an ES ion source as a CCE cell, showing the expected interdependence of the potential at the electrodekolution interface, Ews, in the ES capillary as a function of the ES current, iES, and as a function of the composition of the electroactive species in the solution. Solid line: three electroactive species, viz., A, 6,and C, with electrode potentials €A+/A < EB+IB < &-IC, respectively, are present in the solution at equal concentration. Dashed line: only the electroactive species C is present in the solution.
of mass transfer of the species to the electrode surface. Thus, whether or not a particular reaction takes place, and the extent to which that reaction takes place, should be governed by the magnitude of i ~ s by , the respective concentrations and redox potentials of all the species in the system, and by the solution flow rate. Some of these characteristics are made more apparent by reference to the EEISversus Z'ES plots in Figure 3. The solid line curve represents a situation for positive ion mode ES in which three electroactive species are in the solution at equal concentration, viz., A, B, and C, with electrode potentials of EA-IA< EB-/B < Ec-/c, respectively. As the magnitude of Z'ES increases, these electroactive species are oxidized in the order of increasing electrode potential to maintain this current (Le., the easiest to oxidize species is oxidized first and so on.). Thus, the total faradaic current, ia, where n = 1, 2, or 3, is equal to the sum of the faradaic currents, FA, ~ F B and , ~ F C ,resulting from oxidation of 1 the respective individual species A, B, and C. In this case i ~ = Z'FA, Z F ~= FA ~ F B ,and i ~ 3= Z'FA Z'FB Z'FC. Note that, as i ~ s increases, the magnitude of EE/Sincreases so that a sufficient amount of species can be oxidized. This diagram also demonstrates that changing the composition of electroactive species in solution can alter the magnitude of EEISfor a given Z'ES. For example, with species A, B, and C present in the solution, and with an Z'ES corresponding to ~ F I ,only A is oxidized and EEISis equal to EA+/A.However, if only species C is present in the solution, EEISwill increase to EC-/Cfor the same current since no species other than C can be more easily oxidized to maintain the required current. Experimental Verification of the Operation of an ES Ion Source as a CCE Cell. To ver@ the CCE cell nature of an ES ion source, experiments were performed that allowed the extent of a particular redox reaction in the ES capillary to be determined as a function of (i) the magnitude of Z'ES, (ii) the solution flow rate through the ES capillary, and (iii) the composition of electroactive species in the electrosprayed solution. The instrumental setup shown in Figure 1 was used to detect in solution, by means of W/visible spectroscopy, the products of the redox reactions in the ES capillary as they exited the capillary. A similar ES setup, without the diode array detector flow cell in-line, was shown in a previous study to be a viable ES ionization source for ESMS.I3
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Analytical Chemistry, Vol. 67,No. 17, September 1, 1995
2919
NickelGI) octaethylporphyrin (NiIIOEP) was chosen as the test compound for these studies on the basis of a favorable potential for formation of the radical cation (Ell2 = 0.73 V versus SCE),13 a relatively long lifetime of the radical cation in solution, and a distinctive UV/visible spectrum for the neutral compound and the radical Although not discussed here, similar results were obtained using rubrene (Ell2 = 0.82 V versus SCE)I3 as the test analyte. (i) Effect of the Magnitude of the ES Current, ~ E S on, the Extent of Ni*IOEP Oxidation. As mentioned above, the magnitude of ~ E Sis determined by the product of the rate at which charged droplets can be formed and the average number of charges per droplet. Thus, altering the magnitude of i ~ s ,and therefore the extent of electrolysis in the ES capillary, requires that the rate of droplet production and/or the average number of charges per droplet be altered. Reference to the modified form of the theoretically derived Hendricks equation,"J6 shown in eq 2, indicates the means to accomplish this task. This equation
expresses the functional dependance of ~ E Son several variable experimental parameters. The term H i s a constant, the value of which depends on the dielectric constant and surface tension of the solvent, vf is the volumetric flow rate through the ES capillary, US is the specific conductivity of the solution, and EESis the imposed electric field at the capillary tip. The value of US can be expressed in terms of the limiting molar conductivity of the electrolyte, Amo, and the concentration of the electrolyte, CE. The value of EESis a function of the voltage, VES,applied to the ES capillary, the outer radius of the capillary, YES, and the distance of the capillary tip from the counter electrode, d. On the basis of their experiments, Kebarle and c o - w o r k e r ~ have ~ ~ ~reported ~ ~ , ~ ~the. ~Hendricks ~ equation (eq 2) to be largely valid, with the values of the exponents in the equation to be Y x 0.5, E x 0.5, and n x 0.2-0.3. The small values of these exponents indicate only a weak dependence of ~ E Son of, EES,and US. Moreover, this assignment seems to indicate independence of Y , E , and n and the respective values of o f , EES, and us. However, the original work and equation derived by Reifer and Hendricks16 indicates that these paramefers are not expected to be independent. Furthermore, Kostiainen and Bruinsz8 have recently found the value of n to vary with solvent we have found composition. In this work and in other that the values of Y , E , and n are all dependent on the respective values of V I , EES,and US and that the values of v , E , and n can be signikantly different from those reported by Kebarle et al.11,12,26,27 In any case, under the present set of conditions with a k e d solvent composition, we found that the magnitude of E'ES, and therefore the extent of electrolysis in the ES capillary (i,e., Z'ES = i~), could be altered most easily and to the greatest extent by changing the conductivity of the solution, through addition of electrolyte to the solvent system, and by changing the voltage applied to the ES capillary. With high concentrations of electrolyte added to the (27) Tang, L.; Kebarle, P. Anal. Chem. 1993,65, 3654-3668.
(28) Kostiainen, R.; Bruins, A P. Rapid Commun. Mass Spectrom. 1994,8, 549-
558. (29) Zhou, F.; Van Berkel, G. J., unpublished data 2920
Analytical Chemisty, Vol. 67, No. 17, September 1, 1995
t
I.o-
A
r
0.0 300
7
37d.-
t
, ! A
LA-, 440 510 580 650 Wavelength (nm)
Figure 4. UVhisible spectra of NiIlOEP recorded during five separate flow injection experiments (50yL injections)which show the effects of varying the conductivity of the solution, and therefore, varying iES, on the extent of NVOEP oxidation. The voltage applied to the ES capillary (5 kV), the solution flow rate through the system
(20pUmin, acetonitrile/methylene chloride (1/1v/v)), and the concentration of NiIlOEP in the injected samples (5.0 ,uM, acetonitrile/ methylene chloride (1/1 v/v)) were kept constant. The concentrations of the electrolyte lithium triflate added to the NiIlOEP solutions and the corresponding Solution conductivity, US, and values of iES measured at the apex of the eluting peaks were as follows: 0.0 mM, l .4 x 10-8 R-l cm-l, 1.2x A; 0.01 mM, 3.7x R-' cm-l, 2.0 x 10+ A; 0.1 mM, 15 x R-' cm-l, 1.0 x A; 1.0mM, 97 x S2-l cm-I, 6.4 x A; and 9.0 mM, 880 x R-l cm-l, 2.7 x A. The direction of the arrows on the spectra indicates the change in the absorption peaks with increasing solution conductivity.
solutions (e.g., 1.0-9.0 mM), the effect of flow rate on current was much less significant (see below). (a) Effect of Vatying the Solution Conductivity, US = AmoC~ . The effect of varying the ES solution conductivity, US, on the magnitude of ~ E Sand on the extent of oxidation of NiilOEP to its radical cation was determined on the basis of five separate flow injection experiments in which the voltage applied to the ES capillary, VES, the solution flow rate through the system, uf, and the concentration of Ni"0EP in the injected samples were kept constant, but the concentration of electrolyte in the sample solutions, CE,was increased. Varying CEallows for a much greater change in the conductivity of the solutions than does changing electrolytes since the values of 1," typically vary by much less than 1 order of magnitude among electrolytes in a given solvent system.26 However, CE can be changed over at least 4 orders of magnitude, resulting in an equivalent change in solution conductivity. We found, as expected, that as the concentration of electrolyte was increased, the conductivity of the solutions increased proportionally, resulting in the expected increase in 1'~s. In this case, we found from the slope of the log-log plot of ~ E S versus CE that ~ E S= C E ~where , n x 0.7. The five W/visible spectra of NiIIOEP shown in Figure 4 that were obtained during these experiments demonstrate that as ~ E S increased, the amount of porphyrin oxidized also increased. The spectrum recorded when no electrolyte was added to the system is that of the neutral porphyrin (Amm = 386, 510, and 548 nm), while that recorded when the electrolyte concentration was 9.0 mM is that of the porphyrin radical cation (Amw = 370 nm).24,25 In analogy to a CCE cell, one can assume that porphyrin oxidation is not observed at magnitudes of Z'ES lower than 1.0 x A (CE < 0.1 mM) because these lower current levels can be maintained by oxidation of easily oxidized contaminants in the solvent system and/or by oxidation of iron in the ES capillary. Therefore, the
magnitude of EEISnever reaches a value close to that necessary to oxidize the porphyrin @.e.,0.73 V). However, as the concentraS tion of electrolyte is increased and the magnitude of ~ E increases, the supply of easily oxidized species in the solution is exhausted and the magnitude of EEISmust increase to the value at which the porphyrin can be oxidized to maintain the faradaic current. Moreover, the extent of the porphyrin oxidation, with all other conditions fixed, is limited by the magnitude of Z’ES. The current limited nature of porphyrin electrolysis observed in these data is analogous to the situation illustrated in Figure 3 where species A (a contaminant) and B (Ni”0EP) are present in the solution and the value of iB equals iF1. As iB increases from i ~ to 1 in, because of the increase in solution conductivity, the amount of A in the solution is insufficient to maintain all of ~ E Sand the magnitude of EE/Smust increase to the level necessary to oxidize the porphyrin (Le., EB+/B= 0.73 V). As the magnitude of ~ E Sincreases even further, say to the midpoint between in and im, all the porphyrin in the sample is oxidized to maintain the required faradaic current. It is worth noting that the data in Figure 4 represent the first direct, solution-phase proof that electrolysis of solution species can occur in the ES Capillary. Other studies of this phenomenon13J4relied on the gas-phase detection of the electrolysis products. As a result, the findings of those studies were subject to speculation regarding the possible role of other solution-phase processes (e.g., chemical redox reactions) or gas-phase processes (e.g., corona discharge ionization) in the formation of the ions observed. Closer examination of the data in Figure 4 reveals that only a small percentage of the total faradaic current, i ~can , actually be attributed to oxidization of the porphyrin. For example, on the basis of these data and similar data recorded using porphyrin concentrations of 1.0 and 10 pM (data not shown), it was determined that an iB of -6 x A was sufficient to fully oxidize a porphyrin concentration of -3-5 pM. A calculation based on Faraday’s law (eq 1) indicates that oxidation of 5.0 pM of porphyrin at a flow rate of 20 pL/min would require -1.6 x A. This corresponds to only 27%of the measured iB (-6 x lo-’ A). There are at least three possible explanations that may contribute fully, or in part, to this observation. First, at this flow rate, mass transfer of the porphyrin to the electrode/solution interface (mainly via molecular diffusion in this case) may be insufficient to supply more of the faradaic current (see (ii) below). Second, as the concentration of the electrolyte is increased in the solution, the concentration of easily oxidizable contaminants probably also increases. Thus, although ~ E Sincreases as the electrolyte concentration increases, a substantial fraction of the faradaic current might be due to oxidation of Contaminants added to the sample with the electrolyte. And third, as Kebarle and co-workers11J2 have demonstrated, oxidation of the iron in a stainless steel ES capillary can be part of the faradaic current. Using a solvent system comprised of methan~l/lO-~ M NaCl, they found that the resulting Z’ES (-0.5 x A) could be totally accounted for by oxidation of the ES capillary (i.e., Fe(s) Fe(ad2++2e-, E” = -0.65 V versus SCEZ0).In the present case, because a substantial fraction of the capillary surface may undergo dissolution (corrosion) due to the oxidation reaction, only a fraction of the total capillary area may be amenable to oxidation of the porphyrin or other solution species. As such, as ~ E Sincreases, the fraction of the porphyrin in the solution that is oxidized increases, but the percentage of ~ E that S is used to oxidize the porphyrin remains low and constant.
-
1.Ol
t,
0.2 0.0 300
370
440
510
580
650
Wavelength (nm)
Figure 5. UV/visible spectra of NiiiOEP recorded during an experiment in which a 5.0 pM solution of NiilOEP (acetonitrile/methylene chloride (1/1 v/v)) containing 1.OmM lithium triflate as the electrolyte (97x R-‘ cm-’) was continuously infused through the system (20pUmin) and the voltage applied to the ES capillary, VES,was varied. The magnitude of VESand the corresponding magnitude of ks were as follows: 0 kV, 0 A; 1.O kV, 2.7 x A; 2.0kV, 1.45 x A; 3.0 kV, 2.75 x A; 4.0 kV, 4.4 x A; 4.5 kV, 5.3 x A; and 5.0 kV, 6.0 x io-’ A. The direction of the arrows on the spectra indicates the change in the absorption peaks with increasing capillary voltage.
Data obtained using a platinum capillary in place of the stainless steel ES capillary in the setup shown in Figure 1 support this a r g ~ m e n t . 2 ~ 3We ~ ~ found that under the same experimental conditions used to obtain the data in Figure 4, the fraction of porphyrin oxidized at a given iEs was at least 50%greater when the platinum capillary was used. The platinum capillary is more difficult to oxidize (Le., Pt(s) Pt(aq12+ 2e-, E” = 0.96 V versus %Ez0), and therefore, a higher fraction of total faradaic current can be supplied by oxidation of solution species. (6) Effect of Varying the Voltage Applied to the ES Capillary, Vm. The effect of varying the magnitude of VESon the magnitude of i ~ s and , on the extent of oxidation of NiIIOEP, was determined in an experiment in which a 5.0 pM solution of NiIIOEP containing 1.0 mM electrolyte was continuously infused through the ES capillary and VESwas varied from 0 to 5.0 kV. As the magnitude of VB increased, the magnitude of iEs increased as expected. In this case, we found from the slope of the log-log plot of iEs versus EESthat iEs = E E S where ~ , n = 1.6. The seven UV/visible spectra of NiIIOEP shown in Figure 5 demonstrate that, as Z’ES increased, in response to an increase of VES,the amount of porphyrin oxidized increased as expected. (i) Effect of Solution Flow Rate through the ES Capillary, uf,on the Extent of NPOEP Oxidation. The effect of varying vf on the extent of NiIIOEP oxidation was determined in an experiment in which a 5.0 pM solution of NiIIOEP containing 1.0 mM electrolyte was continuously infused through the ES capillary (VES= 5.0 kV) at values of vf from 10 to 80 pL/min. As mentioned above, with relatively high concentrations of electrolyte in the solvent system, the magnitude of iEs was affected little by changes in of. In this case, as flow rate was increased by a factor of 8, Z’ES increased by a factor of only 1.08 from 6.5 x to 6.9 x 10+ A (Le., ~ E S= vf”, where Y 0.03). Although the magnitude of iEs changed little with flow rate, the W/visible spectra of NPIOEP shown in Figure 6 demonstrate that the fraction of porphyrin oxidized changed dramatically from
-
+
(30) Van Berkel, G . J.; Zhou, F.. submitted for publication in Anal. Chem.
Analytical Chemistry, Vol. 67, No. 17, September 1, 1995
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0.0 300
370
440
300
0
I
I
I
370
I
440
1
1
510
I
- - - - - - - - -. 1
I
1
580
650
580
650
Wavelength (nm)
Wavelength (nm) Figure 6. UV/visible spectra of NiIlOEP obtained in an experiment in which a 5.0 pM solution of NiIlOEP (acetonitrile/methylenechloride (1/1 v/v)) containing 1.0 mM lithium triflate as the electrolyte (97 x C2-l cm-I) was continuously infused through the ES capillary (VES= 5 kV) at different flow rates, vf. Spectra were recorded at solution flow rates of 10,15,20, 25, 30,35, 40, 50, 60, 70, and 80 A at lOpU pUmin. The magnitude of iES increased from 6.5 x min to 6.9 x A at 80 pUmin. The direction of the arrows on the spectra indicates the change in absorbance peaks with increasing flow rate.
essentially 100%at 10 pL/min to near oo/o at 80 pWmin. This observation is largely a result of the increased flow rate of porphyrin through the capillary without the requisite increase in Z'ES necessary to maintain the same extent of analyte oxidation (eq 1). It is also possible that this change in the proportion of porphyrin oxidized is due, in part, to the limited rate of porphyrin mass transfer to the metal/solution interface in the ES capillary. Assuming that convective forces are not great in the capillary, as might be expected for flow rates of 10-80 pL/min,2I molecular diffusion becomes the main means of mass-transport for neutral species. When the residence time of the species in the capillary (i.e., the electrolysis time) becomes small relative to the diffusion time, t ~the , extent of the reaction is limited. Assuming in this case a typical diffusion coefficent,2O D, of 5 x cm2/s and a diffusion distance corresponding to the internal radius of the capillary, d~ = 63.5 x cm, the diffusion time t~ can be estimated using eq 3 to be -8 s. Assuming oxidation of the
tD = dD2/D
r
(3)
porphyrin can take place along the total length of the 1km-long metal capillary, a maximum electrolysis time is calculated to be 9.9 s at a flow rate of 10 pL/min. In contrast, the maximum electrolysis time is only 1.2 s at a flow rate of 80 pL/min, which is much less than the 8 s required for analyte diffusion from the solution to the capillary surface. Thus, the lowest flow rates through the ES capillary allow for the longest electrolysis time and the greatest extent of reaction. Note that although the extent of porphyrin oxidation decreases with increasing uf,other species in the solution must be oxidized to maintain (and slightly increase) ~ E as S the flow rate increases. Thus, one can speculate that, as vf increases, the magnitude of EEISincreases to values higher than that necessary to oxidize the porphyrin. Most probably, this portion of the faradaic current can be attributed to oxidation of species that are more accessible to the metal/solution interface (e.g., electrolytes and solvents). (iii) Effect of the Composition of Electroactive Species in the Solution on the Extent of NilIOEP Oxidation. As dis2922 Analytical Chemistry, Vol. 67, No. 17, September 1 , 1995
0.8
I 300
;i 370
440
510
Wavelength (nm)
0.8
d
I1
Wavelength (nm) Figure 7. UV/visible spectra acquired in four separate flow injection experiments (5.0-pL injections) in which the voltage applied to the ES capillary (5 kV), the flow rate through the system (20pUmin), the concentration of electrolyte in the carrier and sample solutions (acetonitrile/methylenechloride (l/l v/v), 1 .O mM lithium triflate), and the concentration of NiIlOEP in the sample solutions (1 1 p M ) were kept constant, with various amounts of other electroactive species added to the samples: (a) no other electroactive species added, (b) 30 pM ferrocene added, and (c) 28 pM anthracene added. The spectra represented by the solid lines are those of the neutral porphyrin recorded during the respective flow injection experiments when no high voltage was applied to the capillary. The spectra represented by the dotted lines are those obtained with the high voltage turned on.
cussed earlier, one characteristic of a CCE cell is that the magnitude of EEISis not fixed for a given cell current, but is dependent upon the redox potentials and concentrations of the respective electroactive species in the solution. Thus, if the ES source is a CCE cell, it should be possible to alter the extent of any particular redox reaction by altering the composition of electroactive species in the solution. The effect of varying the composition of electroactive species in the solution on the extent of Ni'IOEP oxidation was determined on the basis of four separate flow injection experiments in which VES, V I , and CE in the carrier and sample solutions, and the
concentration of NiIIOEP (11 pM) were kept constant, but different In order to electrochemically ionize a particular analyte in an amounts of two other electroactive species, either ferrocene (Ell2 efficient manner, Z’ES must be s d c i e n t to oxidize/reduce the molar = 0.31 V versus SCE)20or anthracene (Ell2 = 1.19 V versus equivalent of the analyte present plus the molar equivalent of all SCE),I3 were added to the sample solution. The dotted line other species present in the solution that are more easily oxidized/ spectrum in Figure 7a was obtained from the injection of the reduced than the analyte. Moreover, whether or not a particular porphyrin sample containing no other added electroactive species. redox reaction takes place, and the extent to which that reaction As expected on the basis of the previous experiments, the takes place, is governed by the magnitude of i ~ sby , the respective magnitude of Z’ES (-6 x A) is sufficient to convert only a 3-5 concentrations and redox potentials of all the species in solution, pM portion of the 11pM porphyrin sample to the radical cation. and by the flow rate of solution through the capillary. Thus, for When 3.0 pM ferrocene was added to the sample, the amount of a given ~ E S ,reducing the flow rate provides one simple means to oxidized Ni1IOEP was observed to decrease (data not shown) increase the extent of analyte electrolysis, because of the because ferrocene is easier to oxidize than NiI’OEP and is decreased flow rate of the analyte through the capillary (eq 1) therefore oxidized first. Following oxidation of the 3.0 pM of and because of the increased electrolysis time (eq 3). Another ferrocene, oxidation of only 1-2 pM porphyrin was required to means to accomplish this task would be to increase the magnitude maintain the remainder of the faradaic current. The dotted line of Z’ES. Given an upper limit to the voltage that can be applied to spectrum in Figure 7b was obtained from an injection of the the ES capillary of 4-5 kV for optimized ESMS conditions, a porphyrin sample containing 30 pM ferrocene. In this case, a simple means to substantially increase Z’ES with a fixed solvent sufficient amount of ferrocene is present to supply all of the composition is to increase the conductivity of the solution by faradaic current and porphyrin oxidation does not take place. Thus, adding electrolyte to the solvent system. However, a judicious one can presume that EEISnever reaches a value sufficient to choice of the electrolyte must be made so that the gas-phase ion oxidize the porphyrin. signal from the newly formed analyte ion is not suppressed.3O The dotted line spectrum in Figure 7c demonstrates the effect Additionally, one might reduce the magnitude of Z‘ES necessary to on the extent of porphyrin oxidation brought about by adding an oxidize/reduce a given amount of analyte by eliminating from the electroactive species to a solution more difficult to oxidize than solvent system all species whose redox potentials are lower than the porphyrin, viz., anthracene. Sufficient anthracene is present that of the analyte. This might include using a diflicult to oxidize to supply all of the faradaic current (i.e., 28 pM), but the presence of the anthracene does not affect the extent of porphyrin oxidation. electrode material such as platinum to lessen the possibility of This can be observed by comparing the dotted l i e spectrum in the oxidation of the electrode as a contributor to the faradaic Figure 7c with the dotted line spectrum in Figure 7a. The fine current.30 structure noted on the short-wavelength side of the radical cation Certainly other implications of the CCE nature of the ES ion absorption peak in Figure 7c is due to the absorption peaks of source in regard to ESMS will be manifest with continued study. the neutral anthracene in the solution. The extent of porphyrin However, this CCE device may also have utility when used in oxidation is not affected under these conditions, because a conjunction with other types of detectors. For example, a setup sufficient amount of the more easily oxidized porphyrin is present similar to the ES/diode array system shown in Figure 1 might to maintain the required faradaic current. Therefore, the value be a useful tool in certain types of spectroelectrochemical of EEISremains below that necessary to oxidize anthracene. experiments that may be carried out using a CCE cell rather than Analytical Implications. The results detailing the solutiona controlled-potential electrolytic (CPE) cell.31 phase detection of the products of Nil*OEP oxidation under a variety of ES ion source conditions provide proof of the electrolytic nature of ES and definitive proof that solution species can be ACKNOWLEDGMENT involved in the redox reactions. Furthermore, these results The authors thank Dr. Scott A. McLuckey (ORNL) for critical demonstrate that the ES ion source is a controlled-current source review of the manuscript and Prof. Chris G. Enke (University of and the electrolytic nature of the ES device is analogous to New Mexico) for many helpful discussions and suggestions. Prof. controlled-current electrolysis carried out in a flow cell. Under Kelsey D. Cook (University of Tennessee) is thanked for access the typical conditions of operation and use of an ES ion source in to the detector used to measure solution conductivities. F.Z. combination with mass spectrometry, the knowledge that the acknowledges an appointment to the United States Department electrolytic nature of ES is analogous to a CCE cell might appear of Energy, Laboratory Cooperative Postgraduate Research Trainto be of little analytical consequence. In comparison to convening Program, administered jointly by the Oak Ridge Institute for tional electrolysis studies, the analytes that are normally the Science and Education (ORISE) and OWL. This research was subject of analysis in ESMS are “spectator” electrolytes in the sponsored by the United States Department of Energy, Office of solvent system. The species that participate in the redox reacBasic Energy Sciences under Contract DE-AC0584QR21400with tions, i.e., the main subject of analysis in conventional electrolysis Martin Marietta Energy Systems, Inc. studies, have been of little interest in ESMS.13J4 However, recognition that the ES ion source operates in a fashion analogous to a CCE cell has at least one major analytical hnplication in regard to ESMS. That is, this characterization provides the information Received for review February 6, 1995. Accepted June 7, necessary to maximize the efficiency of the faradaic process in 1995.B the ES capillary for ionization of neutral analytes in solution for AC950128C subsequent gas-phase detection by the mass s p e c t r ~ m e t e r . ~ ~ J ~ . ~ ~ (31) Heineman, W. R. J. Chem. Educ. 1983,60,305-308.
@Abstractpublished in Advance ACS Abstracts, July 15, 1995.
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