Anal. Chem. 1998, 70, 838-842
Electrochemical Thermospray Mass Spectrometry Instrumentation for Coupling Electrochemistry to Mass Spectrometry Gu 1 nther Hambitzer
Fraunhofer-Institut fu¨ r Chemische Technologie, Postfach 1240, D-76318 Pfinztal, Germany Joachim Heitbaum
Chemetall AG, Ernst-Reuter-Weg 14, D-60271 Frankfurt, Germany Ingo Stassen*
Fraunhofer-Institut fu¨ r Chemische Technologie, Postfach 1240, D-76318 Pfinztal, Germany
Reaction products in electrochemical processes can be identified by coupling an electrochemical thin-layer flow cell to a thermospray mass spectrometer. The performance of this analytical method, electrochemical thermospray mass spectrometry, is demonstrated. This includes the characterization of the improved electrochemical thinlayer flow cell. This cell offers the possibility to combine cyclic voltammograms with mass spectrometry. This goal was achieved, too, by the construction of a new thermospray ion source and a special vacuum recipient. It is important to get additional physical information besides pure electrochemical parameters in order to elucidate electrochemical reaction mechanisms. A mass spectrometer coupled online to an electrochemical cell can be used to identify reaction products and intermediates. During the past two decades, several processes especially for the detection of gaseous products were studied.1 The introduction of the soft ionization technique thermospray as a viable liquid chromatography/mass spectrometry interface2 allowed the identification of those species that remain in solution. It has been shown3 that coupling an electrochemical cell online with a thermospray mass spectrometer allows the measurement of mass intensities of nonvolatile products, depending on the continuously varying electrode potential. The examination of the electrooxidation of N,N-dimethylaniline at a platinum coil showed the formation of dimers and trimers in mass intensity potential curves. Volk, Yost, and Brajter-Toth4-7 have shown that the on-line coupling of electrochemistry with thermospray/tandem mass spectrometry can provide structural information about the com(1) Chang, H.; Johnson, D. C.; Honk, R. S. Trends Anal. Chem. 1989, 8, 328. (2) Blakely, C. R.; Vestal, M. L. Anal. Chem. 1983, 55, 750. (3) Hambitzer, G.; Heitbaum, J. Anal. Chem. 1986, 58, 1067. (4) Volk, K.; Yost, R.; Brajter-Toth, A. J. Electrochem. Soc. 1987, 134, 500C. (5) Volk, K.; Lee, M. S.; Yost, R.; Brajter-Toth, A. Anal. Chem. 1988, 60, 720. (6) Volk, K.; Yost, R.; Brajter-Toth, A. Anal. Chem. 1989, 61, 1709. (7) Volk, K.; Yost, R.; Brajter-Toth, A. J. Electrochem. Soc. 1990, 137, 1764.
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ponents in an electrolysis mixture without chromatographic separation. A commercial coulometric cell with a carbon electrode at different fixed potentials was used to monitor the formation of products upon the electrooxidation of biological molecules. A correlation of mass intensity potential points to cyclic voltammograms obtained in a separate cell was made. Several reaction intermediates were identified by tandem mass spectrometry, which provided insight into reaction pathways. The objective of this study is to demonstrate the performance of electrochemical thermospray mass spectrometry (ETMS). First, this includes the characterization of an electrochemical thin-layer flow cell which offers the possibility to directly correlate cyclic voltammograms to mass spectrometric intensity curves. Any smooth metal sheet can be used as the working electrode of the flow cell in different electrolyte solutions. Second, the capabilities of a new thermospray ion source and a special vacuum recipient are demonstrated. EXPERIMENTAL SECTION The experimental setup for measurements with the electrochemical thermospray mass spectrometry is shown in Figure 1. A glass flask contains the electrolyte solution with the reagents. The solution can be deaerated by helium bubbling followed by evacuation with a membrane pump. Volatile reagents are added to the deaerated solution. A double-piston HPLC pump (Latek P400, Titan) with a pulse damper (Latek Titan) and an adjustable check valve (Nupro) provides a constant flow of typically 1 mL/min at 1.5 MPa. The solution is pumped through the electrochemical thin-layer flow cell, which is arranged in a three-electrode assembly. From inlet to outlet the solution passes a small working electrode volume, schematically shown in Figure 2. A fine porous glass frit separates this volume from the counter electrode compartment. Through the counter electrode, a small volume of solution, typically 0.05 mL/min at 1.5 MPa, flows off through a ceramic frit (Schott), which serves as a flow resister. The working electrode volume is connected via a third drill hole and a Teflon tube to a glass S0003-2700(97)00753-1 CCC: $15.00
© 1998 American Chemical Society Published on Web 01/22/1998
Figure 1. Schematic drawing of the experimental setup.
Figure 3. Schematic drawing of the thin-layer flow cell: top view.
Figure 2. Schematic drawing of the thin-layer flow cell: side view.
flask containing the calomel reference electrode (SCE). The connection serves as a Luggin capillary tube. The flow through the Luggin capillary is adjusted to 0.01 mL/min at 1.5 MPa. As shown in Figure 3, the cell consists of two structural members made of Kel-F. Fixing a glass frit in the top, the oval cavity (14 × 10 mm) with a depth of 0.2 mm is made by a mill cutter. The commercial Minstac system (Lee Instac) is used for tubing and fitting. The counter electrode is a spiral wound gold wire (0.7-mm diameter) and is inserted in an Instac three-boss manifold. The working electrode is an oval metal sheet with dimensions of 15 × 11 × 0.1 mm and lies in an appropriate oval cavity in the base of the cell. Electrical contact is ensured by a truncated gold wire inserted from the bottom.
The members of the cell are squeezed together by screws. Since the area of the working electrode volume in the top is smaller than the metal sheet in the base, the border of the oval cavity is pressed on the metal. The hydrophobic Kel-F tightly closes off this volume with respect to the solution. A Viton gasket provides the gas-tightness. The electrochemical thin-layer flow cell is built in the threeelectrode arrangement and allows the use of any metal sheet as the working electrode. The materials of the cell and the fittings (Kel-F, Teflon, glass) are chemically resistant against most of the reagents. Since there are pressures up to 2 MPa, caused by vaporization in the capillary tube, the system was tested up to 3.5 MPa. A porous glass frit separates counter and working electrode compartments of the flow cell. Glass frits are employed in the usual three-electrode assembly to avoid mixing of the electrolyte solution in the working and the counter electrode compartments. The installation of a glass frit in the thin-layer flow cell is for two important reasons: (1) The use of a glass frit with a sufficient small pore size ensures that the volume of the electrolyte solution in front of the working electrode is an almost entirely separated area with respect to the liquid flow. This means that the exchange of the solution in the working electrode area with the counter electrode compartment is small compared to the total liquid flow. Therefore, the flow resistance in the frit has to be considerably higher than the flow resistance in the thin layer above the working electrode. The flow through the frit was experimentally determined to be not more than 5% of the total liquid flow through the electrochemical cell. (2) The frit minimizes the iR drop in the electrolyte solution along the working electrode. This ensures that the thin-layer flow cell allows the measurement of “regular” cyclic voltammograms. Analytical Chemistry, Vol. 70, No. 5, March 1, 1998
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Figure 4. Schematic drawing of the thermospray ion source with (A) capillary tube, (B) jet chamber, (C) exit cone, (D) terminals/ thermocouple for exit cone, and (E) connection to rotary vane pump.
A small part of the electrolyte solution should flow through the frit into the counter electrode compartment for the following reasons: (1) Substances that are generated at the working electrode enter the pores of the frit and may cause memory effects. This is prevented by a small flow through the frit. (2) Products that are formed at the counter electrode do not enter the working electrode compartment. (3) The counter electrode compartment is easily filled with electrolyte solution. Additionally, the counter electrode stays in contact with the solution, even in the case of strong gas evolution. As schematically shown in Figure 4, the outlet Teflon tube (i.d. 0.3 mm) of the electrochemical flow cell is directly connected via a low dead volume union to the stainless steel capillary tube (A) of the thermospray ion source. The main part of the ion source is outside the vacuum recipient. The connection to the recipient is simply made by a small flange DN 50 KF. The capillary tube (i.d. 0.1 mm, o.d. 0.4 mm, Hamilton) is directly heated over a length of ∼200 mm. A constant temperature is adjusted by a home-built vaporizer controller. The temperature is measured about ∼70 mm from the beginning of the capillary tube by a thermocouple. The end of the capillary tube is welded into a copper disk (6mm diameter). A sharpened bored stainless steel screw cuts a gastight border and presses the copper disk against the tempered copper block providing a good thermal contact. The jet chamber (length 200 mm) is a tempered copper tube which is brazed at 840
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one end into the electrically heated copper block. The temperature is measured by a thermocouple placed in the copper block and regulated by a current controller. The jet chamber is fixed in the main chamber (B) of the thermospray ion source by a Swagelok fitting with a Vespel ferrule. This provides gastight thermal and electrical isolation from the main chamber up to 300 °C. The exit cone (C) with an orifice of 0.8-mm diameter works as a skimmer. It is mounted on a bored Vespel cylinder at the end of the main chamber. The cone is separately heated to a desired temperature measured by a thermocouple (D). The source potential for the positive ion mode is usually 15 V against the mass potential of the quadrupole mass filter. By the rapid vaporization of the electrolyte solution, a small amount of ions are transferred from the solution into the vapor phase. A part of the ion-containing vapor is forced through the orifice of the exit cone to the second chamber pumped by a turbomolecular pump (Balzers TPH 330 halogen-resistant) to 10-3 mbar. Those cations entering the second chamber are accelerated by a potential of 30 V to the inlet cone of the third chamber. The ions pass a stainless steel net with an optical transparency of 60%, allowing the permanent measurement of proportional total ion current by an electrometer (Balzers). The third chamber is pumped below 10-5 mbar by a turbomolecular pump (Balzers TPU 110). Here, the mass analysis is performed by a Balzers QMG 511 quadrupole mass filter with a mass range of 1024 amu. The major part of the vapor flows around the skimmer back through the main chamber of the thermospray ion source (E). A membrane pressure controller (Leybold) holds the vapor pressure in the main chamber typically at 10 mbar. The vapor is pumped by a liquid nitrogen baffle and a halogen-resistant rotary vane vacuum pump (Balzers DUO 008). A special thermospray ion source was designed for the coupling of the thin-layer flow cell to the thermospray mass spectrometer . As shown in Figure 4, most parts of the ion source such as the directly heated capillary tube were built outside the recipient. There are practical reasons for this setup: (1) When stronger acids are examined, e.g., 0.1 M sulfuric acid, as electrolyte solution, the heated capillary tube could burst open by local corrosion reactions. By placing the tube outside the recipient neither detection instruments nor pumps could be destroyed. (2) A short and easy tube connection between cell and capillary tube is possible. (3) During spraying, the position of the thermocouple could be varied easily to optimize to high ion currents with low fluctuations. (4) The capillary tube is simple to make and quick to install. The electrical equipment consists of a home-built potentiostat, a programmable sweep generator (Hopf), and a X/Y(t) recorder (Kipp & Zonen). The acquisition of the electrochemical and mass spectrometric data and the control of the mass spectrometer were done by personal computer (IBM PC AT) with an analogue and digital I/0 board (Compmess C 1301) using a self-made software. RESULTS AND DISCUSSION Thermospray Mass Spectra. The thermospray ion source constructed is different in some details (e.g., the separated heated
Figure 5. Thermospray mass spectrum of 0.1 M NH4OAc: flow rate, 1 mL/min; capillary temperature, 158 °C; jet chamber temperature, 175 °C; exit cone temperature, 182 °C.
Figure 6. Thermospray mass spectrum of 10-3 M EMA in 0.1 M NH4OAc: flow rate, 1 mL/min; capillary temperature, 160 °C; jet chamber temperature, 200 °C; exit cone temperature, 200 °C.
jet chamber and the exit cone within the axis of the capillary tube) from the usual sources. But, as shown in Figure 5,the ion source delivers the usual thermospray mass spectra. This spectrum is an example of a primary thermospray ionization process, which means the direct transfer of a small amount of ions out of the electrolyte solution into the vapor phase. Spraying of 0.1 M NH4OAc solution results in clusters of NH4‚nH2O, NH4‚NH3‚(n 1)H2O, CH3CONH3‚nH2O, and CH3CONH3‚HOAc‚nH2O. Reagents with high gas-phase basicity are attached by ions in the heated jet chamber as a secondary ionization process.8 For this process, a mass spectrum of 10-3 M N,N-ethylmethylaniline (EMA) in 0.1 NH4OAc is shown in Figure 6. This dialkylaniline, which is not protonated in neutral solution, and its cluster with H2O dominate the mass spectrum. Here the ammonium ions (8) Vestal, M. L. Ion Formation from Organic Solids; Benninghoven, A. Ed.; Springer: Heidelberg, 1983.
formed in the primary ionization process transfer protons to the aniline molecules in the vapor phase. Low peak heights of ammonium clusters are caused by a nearly complete secondary ionization process. This is caused by the long reaction time in the long jet chamber compared to shorter times in the usual ion sources. With increasing time, the gas-phase acid/base reaction between ammonium ion and the aniline derivative should lead to a thermodynamic equilibrium.9 However, the low current of the gas-phase dimer (EMA-EMAH+) indicates that the proton transfer between the anilines is very small. This is a typical example for the kinetic control of a secondary ionization process, which is characterized by an ion formation, at least for volatile reagents, proportional to the concentration in the liquid phase.9 Total Ion Current. In parallel to the recording of mass spectra, the total ion current (TIC) was measured at the net by an electrometer amplifier. The total ion current of 0.1 M NH4OAc at a flow rate of 1 mL/min was ∼2 × 10-9 A. This is 10 times lower compared to the TIC of a usually designed ion source. This decrease could be caused by the greater length of the jet chamber leading to a higher rate of ion recombination in the gas phase and/or of wall impact. The on-line measurement of the TIC allows one to control the performance of the thermospray ionization system. By using a carefully deaerated solution and optimizing the thermospray parameters, the short-time TIC fluctuations measured by applying a low-pass filter with 90% rising time at 100 µs were within (5%. The long-time stability within 30 min was achieved by applying a low-pass filter at 100 ms. When the electrochemical cell is coupled to the thermospray mass spectrometer, the on-line record of TIC helps to distinguish between fluctuations by thermospray ionization and the change of ion currents caused by an electrochemical process. Thin-Layer Flow Cell Coupled to a Thermospray Mass Spectrometer. The thin-layer flow cell was connected to the capillary tube of the thermospray mass spectrometer in order to verify the feasibility of ETMS for electrochemical measurements. A solution of 10-3 M EMA in 0.1 M NH4OAc was pumped with a flow rate of 1 mL/min through the cell into the capillary tube. The time response of the electrochemical current and the mass intensity of EMAH+ at m/z ) 136 were recorded (Figure 7). The potential was switched from open circuit to +1600 mV/SCE. The electrochemical current ICV declines, and the consumption of EMA is obvious by the decrease of the ion current IMS. The decline starts after a dead time of ∼0.3 s, which depends on the flow rate and the tube volume between cell and capillary tube. The time when 90% of the final mass intensity of EMAH+ is reached is ∼1.2 s. The time response of the mass spectrometric current is fast enough to measure slow mass spectrometric cyclic voltammograms. To give a first impression of the capability of the method ETMS, a mass spectrometric cyclic voltammogram is displayed in Figure 8. The positive sweep of a cyclic voltammogram for the oxidation of 10-3 M EMA in 0.1 M NH4OAc at a gold electrode with a sweep rate of 6 mV/s is shown. The ion current of EMAH+‚2H2O with m/z ) 172 starts to decrease with rising anodic current. The ion currents of two products with m/z ) 255 and 241 increase with rising anodic current up to the diffusion (9) Alexander, A. J. A.; Kebarle, P. Anal. Chem. 1986, 58, 471.
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Figure 8. Mass spectrometric cyclic voltammogram of 10-3 EMA in 0.1 M NH4OAc: sweep rate, 6 mV/s; working electrode, gold; IMS in arbitrary units. (A) ion current m/z ) 172; (B) total ion current; (C) electrochemical current; (D) ion current m/z ) 255; (E) ion current m/z ) 241.
Figure 7. Time response of electrochemical current ICV and ion current IMS to a potential step from open circuit to the oxidation potential of EMA: electrolyte solution, 10-3 M EMA in 0.1 M NH4OAc; flow rate, 1 mL/min; capillary temperature, 165 °C; jet chamber temperature, 200 °C; exit cone temperature, 200 °C.
limitation. This figure clearly demonstrates the potential-dependent formation of different oxidation products, as well as the consumption of the educt with rising anodic current. Additionally, the total ion current is shown to illustrate the stability of the ion signals in the mass spectrometer. The products of the anodic oxidation of EMA can be identified by conducting the experiments with deuterated compounds. The formation of dimers and trimers, as well as dealkylated products, was demonstrated. Besides EMA, other alkylated anilines have been examined with the experimental setup described above. The complex reaction mechanism of the anodic oxidation of different N,Ndialkylanilines was clarified by the use of ETMS. These results will be presented in a following paper.10
etry. This aim was reached by the development of an appropriate thin-layer flow cell. This cell is suitable for electrochemical measurements in the usual three-electrode assembly. The flow cell can be easily connected to the capillary tube of a thermospray ion source. The new thermospray ion source and a special vacuum recipient deliver high and stable ion currents which are necessary to detect reaction products during electrochemical processes. The electrochemical behavior of various substances in different electrolyte solutions can be examined. The response time of the experimental setup is short enough to record slow mass spectrometric cyclic voltammograms. This leads to the elucidation of complex reaction mechanisms which will be shown elsewhere. Further work is focused on the examination of strongly acidic and water-free organic electrolyte solutions. This is achieved by the construction of a new thermospray ion sources with enhanced corrosion stability and a dual-beam ion source, respectively. ACKNOWLEDGMENT The authors thank F. W. Ro¨llgen for helpful discussions.
CONCLUSIONS Electrochemistry and mass spectrometry have successfully been combined in electrochemical thermospray mass spectrom-
Received for review July 14, 1997. Accepted December 8, 1997.
(10) Hambitzer, G.; Heitbaum, J.; Stassen, I., submitted to J. Electroanal. Chem.
AC970753C
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