Anal. Chem. 1989, 61,2124-2126
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to form hvdroeen bonds as evident from the high ” value of the i
”
1-butanol probe. The chiral rJolvsiloxane derived from (RRbtartramide - should find extensive application in chiral recognition.
LITERATURE CITED (1) Frank, H.; Nicholson, G. J.; Bayer, E. J. Cbromatogr. Sci. 1977, 5 2 , 174-176. (2) Nicholson, G. J.; Frank, H.; Bayer, E. HRC CC, J. Higb Resolut. Chromatogr. Chromatogr. Commun. 1979, 2, 411-415. (3) Koppenhoefer, 6.; Allmendinger, H.; Nicholson, G. J : Bayer, E. J. Chromatogr, 1983, 260, 63-73. (4) Frank, H.; Nicholson, G. J.; Bayer, E. Angew. Chem., I n t . Ed. Engl. 1978, 17,363-365. (5) Koppenhoefer, 6.; Walser, M.; Bayer, E. J . Chromatogr. 1986, 358,
(7) Koch, E.; Nicholson, J.; Bayer, E. HRC CC, J. High Resolut. Chromatogr. Chromatcgr. Common. 1984, 7, 398-403. (8) Saeed, T.; Sandra, P.; Verzele. M. J. Cbromatogr. 1979, 786, 6 1 1-6 18. (9) Konig, W. A.; Benecke, I.; Sievers, S. J . Cbromatogr. 1981, 217, 71-79. (10) Dobashi. Y.; Hara, S. J. Am. Chem. Soc. 1985. 707,3406-3411. (11) Dobashi. Y.; Hara, S. Tetrahedron Lett. 1985, 2 6 . 4217-4220. (12) Dobashi. Y.; Hara, S. J. Org. Chem. 1987, 5 2 , 2490-2496. (13) Konig, W. A,; Benecke, I. J. Chromatogr. 1981, 209,91-95. (14) Konig, W. A.; Benecke, I.; Sievers, S. J. Chromatogr. 1982, 238, 427-432. (15) Frank, H.; Nicholson, G. J.; Bayer, E. J. Chromatogr. 1978, 146, 197-206. (16) Bradshaw, J.; Aggarwal, S. K.; Rouse, C. A.; Tarbet, B. J.; Markieds, K. E.; Lee, M. L. J. Cbromatogr. 1987, 405, 169-177.
159-168. (6) Koppenhoefer, 6.: Allmendinger, H. Chromatograpbia 1986, 2 7 , 503-508
RECEIVED for review May 1, 1989. Accepted June 1, 1989.
Fabrication and Characterization of Glassy Carbon Linear Array Electrodes L. J o s e p h Magee, Jr., a n d J a n e t Osteryoung* Department of Chemistry, S t a t e University of N e w York at Buffalo, Buffalo, N e w York 14214 Arrays of electrodes for use as electrochemical detectors in flowcells are the subject of many ongoing investigations. Originally developed to take advantage of the properties displayed by microelectrodes (i.e. enhanced current densities from nonplanar diffusional contributions to the net current, low iR drop characteristics, and a decreased dependence on convection) and to generate larger, more easily measured currents, arrays of electrodes have been fabricated from various materials and in different geometric configurations (1-9). However, because of the low dead volumes required of flowcell detectors, under conditions typical of liquid chromatography or flow injection analysis, the linear velocities of the fluids flowing through them are too high for nonplanar diffusion to be a factor. Only in extreme cases (very low flow rates and extremely small electrode size) will nonplanar diffusion affect the measured currents in these flowcell detectors. Nevertheless, measured current densities of electrode arrays are almost always greater than those obtained for single electrodes of similar active electrode surface area under identical hydrodynamic conditions. This increase in current density is attributed to the reestablishment of the bulk concentration of the electroactive species as it travels across the insulating regions between electrode elements of the array (10-12). Consequently, each electrode element of the array “sees” bulk or near-bulk concentrations. This situation contrasts greatly with the depletion across the surface of a large electrode. Electrode arrays have been fabricated in a number of geometries. The simplest geometry from the standpoint of fabrication is random. Random arrays can be made by combining powders or chips of electrode material with an insulator (e.g., plastic) ( I , 2). Another type of random array is made by using a reticulated electrode material (e.g. reticulated vitreous carbon) with an insulator to fill the pores. Although difficult to characterize geometrically, random arrays are fairly simple to construct from readily available materials (3). Arrays of electrodes based on disks have also been fabricated ( 4 ) . These arrays were constructed by sandwiching carbon fibers between glass microscope slides and sealing them in epoxy. The most popular type of array geometry is the linear array. This electrode geometry is the most efficient in a flowcell when the lines are oriented opposed to the direction of flow, allowing one to obtain the highest current densities and net currents in the space allowed by flowcell dimensions. The most com-
mon method for fabricating linear arrays is by thin-film technology and lithography. Usually gold is vapor-deposited onto a substrate (5-8). The major drawbacks of these lithographically fabricated arrays arise from their three-dimensional nature. The gold lines are on top of the substrate and are on the order of 300-600 nm thick. Apparently no turbulence is caused by these lines in flowcells. However the layers tend to separate under voltammetric conditions in solution and are mechanically fragile. Thus their utility as something other than laboratory curiosities is limited, because they cannot be subjected to mechanical polishing, a routine procedure for the maintenance of electrodes. Another method involves etching a pattern in a Macor substrate and filling in the resulting grooves with a gold filler (9). The resulting inlaid electrode can be polished mechanically, but the etching procedure seems difficult to control. Linear arrays based on carbon have not been described. Carbon electrodes, and especially glassy carbon electrodes, are probably the most common type in use today. They are used routinely as is or as the substrate for surface-modified and mercury-coated electrodes. Probably glassy carbon has not been used to make electrode arrays because of the nature of the material itself. Glassy carbon is a very hard, brittle material that shatters quite easily. Though the material is available in a number of shapes and sizes, including rods, plates, disks, tubes, cones, and crucibles, the only geometry that has had any extensive use as an electrode material has been the rod. Rods of glassy carbon, typically 3 mm in diameter, are readily sealed in Teflon, Kel-F, or some other insulating material to yield a disk electrode of 3-mm diameter. Machining techniques are available, however, that might allow one to fabricate electrode geometries other than a simple disk. This paper describes a method for fabricating linear arrays of electrodes for use in a flowcell detector.
EXPERIMENTAL SECTION Electrode Fabrication. Electrodes were fabricated from glassy carbon plate treated t o 2500 “C available from Atomergic Chemetals, Plainview, NY. The plate came in a 4 in. X 4 in. piece that was cut into 1 in. X 1 in. squares with a diamond saw. A series of grooves were cut in a 1 in. X 1 in. piece with a dicing saw (Tempress Model 602, Sola Basic Industries, Los Gatos, CA). Dicing saws are used routinely in the fabrication of microelectronic circuits to cut up silicon wafers. The cutting is done with a diamond impregnated wheel. This method worked quite well for
0003-2700/89/0361-2124$01.50/00 1989 American Chemical Society
ANALYTICAL CHEMISTRY. VOL. 61. NO. 18, SEPTEMBER 15. 1989
* 2125
Table 1. Physical Characteristics of Two Linear Array Electrodes and a Single Disk Electrode
line I line I1 disk
line width,
gap width,
length,
Irm
Irm
mm
no. of elements
60
65
85
90
4.5 4.5
24 21
I
= 1.5 mm
total active surface area, mm2 6.48 8.03 7.10
1
Table 11. Steadystate Response of a Series of Electrodes of Various Geometries' flow rate, mL/min 0.2 0.4
0.6 0.8
1.0 1.2 1.4 1.6 1.8 2.0 '4 p M
current density, nA/mmz disk line I line 11 9.18 ~
~~
11.55 13.30 14.82 15.94 16.96 17.80 18.59 19.32 20.11
lfi.67 ~~
~
21.14 24.38 27.16 29.48 31.56 33.41 35.26 36.81 38.35
14.70. 18.80 21.79 24.16 26.46 28.14 29.58 31.32 32.44 33.94 ~~
solution of K,Fe(CN)&3H20in 0.05 M KCI.
:I
a
I I
i
1
i
Figure 1. Grooves and ridges machined into a glassy carbon plate. (ai Drawing. lb) Photomicrograph of the end of a glassy carbon plate. Grooves are 90 ilm wide and lines are 85 pm wide. IC) Photomicrcgraph of the top of a glassy carbon plate. Grooves are 65 mn wide (black) and lines are 60 ilm wide (gray). putting grooves in the glassy carbon plate, The glassy carbon plate was mounted on the chuck, and the depth of cut was set to 0.010 in. A feed rate of about 3 in./min was used. Making a series of equally spaced grooves was greatly simplified by the fact that the saw has an automatic feed and jog setting. The saw will make a cut, return to the original position, move the workpiece a predetermined amount. and make another cut. This procedure runs continuously until the saw is turned off. After a series of grooves was made in the center of the 1 in, X 1 in. plate. the remaining surface was cut back, again by using the dicing saw. leaving a series of ridges in the middle of the plate (Figure 1). The dimensions in Figure I b were obtained by using a calibrated grid on a microscope, This plate was then cut into three equally sized pieces, each of which could he used to make a flowcell detector. The machined piece was cleaned in an ultrasonic bath to prepare the glassy w b o n for sealing in an insulator and to remove any chips and particles remaining in the grooves from machining. A piece of this machined glassy carbon was potted in Maraglas epoxy (Acme Chemical and Insulation Co., New Haven, CT, No. 658 resin and No. 558 hardener) to fill the grooves and form the
body of the flowcell. Electrical contact was made fmm the bottom with a brass screw. The electrode assembly was prepared far use by first grinding off the excess epoxy coating the glassy carbon with 600-grit sandpaper. The surface was then polished with aluminafwater slurries on a polishing cloth starting with 1-am alumina and working down to 0.05 Irm alumina for the final polishing. Some difficulties with bubbles in the epoxy yielded imperfect seals with the carbon, resulting in a background current density roughly 10 times (7 nA/mm2) that for the commercial detector. Adequate seal between insulator and conductor is important for any embedded electrode. High background currents arising from imperfect seals might be expected to vary linearly with the perimeter of the seal. In this case the perimeter of the arrays is about 20 times the perimeter of the circular commercial electrode. Equipment and Reagents. The flow system used to characterize these electrodes consisted of an LKB Model 2150 highperformance liquid chromatography pump and a BAS Model LC 1TA electrochemical flowcell detector with a BAS Model LC 4A amperometric detector. The mobile phase was 0.1 M KCI (Mallinckrodt reagent grade) in deionized water. The anal* was K,Fe(CN)6.3H,0 (Baker reagent grade).
RESULTS AND DISCUSSION Two different glassy carbon line array electrodes were fabricated and characterized. T h e physical characteristics of these two electrodes, as well as the characteristics of the disk electrode with which they are compared, are given in Table 1. An experiment was run to determine the current densities of these two electrodes under steady-state conditions and to compare the results with those obtained for the single disk electrode under similar conditions. This was done by continuously pumping a solution of analyte (4 r M Fe(CN)$-) in the mobile phase (0.1 M KCI) through the detector a t a series of flow rates. T h e results of these experiments are given in Table 11. As expected, the linear arrays of glassy carbon yielded higher current densities than the single disk. Plots of log current vs log flow rate using the data of Table I1 give slopes of 0.36 for the two arrays and 0.34 for the disk electrode. These slopes agree closely with the one-third power dependence on flow rate established for the channel flowcell (13, 14).
I t is apparent from the photomicrographs that the dicing saw is capable of making very small, chip-free lines in the
Anal. Chem. 1989, 6 1 , 2126-2128
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glassy carbon. Line widths as small as 60 wm were obtained at a fairly high feed rate. It should be possible to make even narrower lines by using lower feed rates. Also worth pointing out is the versatility of the dicing saw for making cuts of different sizes. Changing the spacing between blade cuts merely requires the setting of a switch. In addition, gaps of different widths can be made by making multiple cuts with the blade or by using blades of different widths. The success achieved with the dicing saw in making smooth lines of glassy carbon in micrometer dimensions in such an easy fashion makes the technique noteworthy. The range of application can probably be extended to other electrode materials such as gold or platinum and to other sealing materials such as other epoxies or Pyrex (15, 16).
ACKNOWLEDGMENT We thank Mike Jackson and the Rochester Institute of Technology for the use of the dicing saw. Registry No. Fe(CN),4-, 13408-63-4;carbon, 7440-44-0.
LITERATURE CITED (1) Weisshaar. 5 3 , 1809
D.E.: Tallman, D.E.;Anderson, J. L. Anal. Chem. 1981,
(2) Falat, L: Cheng, H. Y. Anal. Chem. 1982, 5 4 , 2109. (3) Wang, J.: Frelha, B. A. J. Chromatogr. 1984, 298, 79. (4) Caudill, W. L.; Howell, J. 0.; Wightman, R. M. Anal. Chem. 1982, 5 4 , 2531. (5) Bond, A. M.; Henderson, T. L. E.;Thorman, W. J. Phys. Chem. 1986, 9 0 , 2911. (6) Thorman, W.; van den Bosch, P.; Bond, A. M. Anal. Chem. 1985, 5 7 , 2764. (7) Fosdick, L. E.;Anderson, J. L. Anal. Chem. 1986, 5 8 , 2481. (8)Fosdick, L. E.; Anderson, J. L.: Baginski, T. A,; Jaeger, R. C. Anal. Chem. 1986, 5 8 , 2750. (9) DeAbreu, M.; Purdy, W. C. Anal. Chem. 1987, 5 9 , 204. (IO) Moldoveanu. S.: Anderson, J. L. J. Electroanal. Chem. Interfacial Electrochem. 1985, 185, 239. (11) Anderson, J. L.: Ou. T. S.:Moldoveanu, S. J. Electroanal. Chem. Interfacial Electrochem. 1985, 196, 213. (12) Cope, D. S.;Tallman, D. E. J. Nectroanal. Chem. Interfaclal Electrochem. 1986, 205, 101.
(13) Sparrow, E. M. National Advisory Committee for Aeronautics TN 3331, 1955. (14) Weber, S.G.; Purdy, W. C. Anal. Chim. Acta 1978, 100, 531. (15) Sambell. R. A. J.; Bowen, D. H.; Phillips, D. C. J. Mater. Sci. 1972, 7 , 663. (16) Sambell, R. A. J.: Briggs, A.; Phillips, D. C.; Bowen. D. H. J. Mater. S o . 1972. 7, 676.
RECEIVED for review March 8,1989. Accepted May 12,1989. This work was supported in part by the National Science Foundation under Grant CHE 8521200.
Tuning and Calibration in Thermospray Liquid Chromatography/Mass Spectrometry Using Trifluoroacetlc Acid Cluster Ions Steven J. Stout* and Adrian R. daCunha American Cyanamid Company, Agricultural Research Division, P.O. Box 400, Princeton, New Jersey 08540
INTRODUCTION Liquid chromatography/mass spectrometry (LC/MS) is a rapidly developing technique for the analysis of complex mixtures not amenable to gas chromatography/mass spectrometry (GC/MS) techniques ( I ) . Of the several approaches for interfacing liquid chromatography with mass spectrometry, thermospray (TSP) LC/MS appears to be the one best suited for the analysis of polar and labile organic compounds (2-6). One of the major drawbacks of T S P LC/MS is the need to install a separate ion volume which then must be tuned and calibrated. As discussed by Heeremans et al. (7), current methods of tuning TSP LC/MS suffer from serious shortcomings. Tuning on a solution of a particular analyte may not be applicable for the analysis of unknown compounds or when limited quantities of sample are available. Tuning solutions of poly(propy1ene glycol) (PPG), poly(ethy1ene glycol) (PEG) (8, 9),and sodium acetate ( I O ) offer a more universal method of tuning but result in rapid contamination of the ion source and memory effects. T o overcome these shortcomings, Heeremans et al. (7) reported adding volatile acetic acid to an ammonium acetate eluent. Tuning on acetic acid-ammonia cluster ions from m/z 100 to lo00 was achieved with no ion source contamination. In this paper, we report on the use of trifluoroacetic acid to generate cluster ions for TSP LC/MS tuning and Calibration to m / z 4000 (the upper mass limit of our instrumentation) without ion source contamination. Additionally, the same tuning solution can be used for tuning in the negative ion mode of operation.
EXPERIMENTAL SECTION The experiments were performed with a TSQ-70 triple-stage quadrupole system equipped with a thermospray interface
* Author to whom correspondence should be addressed. 0003-2700/89/0361-2126$01.50/0
(Finnigan-MAT Corp., San Jose, CA). Operational parameters specific to the thermospray interface included the following: vaporizer temperature, 90 "C; aerosol temperature, 230 "C; repeller voltage, 70 V; mass spectrometer high vacuum, 2.7 X Torr. Solvent delivery was performed with an AB1 Kratos Spectroflow Model 400 LC pump. The mobile phase was CH,OH/H,O/trifluoroacetic acid (15/84.5/0.5,0.1 M ammonium acetate), flowed at 1.5 mL/min, and gave a back pressure of approximately 30 bar. ["NIAmmonium acetate (99% 15Nenriched) was obtained from ICON Services (Summit, NJ). Analytical reagent grade [14N]ammonium acetate was obtained from Mallinckrodt, Inc. (Paris, KY).
RESULTS AND DISCUSSION Since Heeremans (7)demonstrated tuning and calibration of a TSP LC/MS ion source to m / z 1000 using acetic acid (H0Ac)-ammonia cluster ions, we anticipated the greater mass of trifluoroacetic acid (TFA) would approximately double this mass range if TFA (molecular weight 114) exhibited the same level of cluster ion formation as acetic acid (molecular weight 60). As shown in Figure 1, this objective was achieved with cluster ions covering the mass range of m / z 100-2000. The predominant series of TFA-ammonia cluster ions corresponds to (TFA),(NH3),(NH4)+. This series starts with the (TFA)(NH,)(NH,)+ ion at m / z 149 and repeats in increments of 131 u (equivalent to TFA + NH,). This pattern is especially evident above m / z 400. Several ions below m / z 400 correspond to additions of NH3 to this series. Ions at m / z 192 (base peak) and m / z 175 correspond to (TFA)(HOAc)(NH4)+and m / z 192 - NHJ, respectively. The origins of the m / z 119 and 110 ions are uncertain at the present time. With switching to the high mass range of the instrument, the (TFA),(NH3),(NH4)+series is found to extend to m / z 4000 (Figure 2), the upper mass limit of the instrument. Unfortunately, extending the mass range is also accompanied by a degradation in mass resolution. 1989 American Chemical Society
