1998
(9) (10) (11)
(12)
ANALYTICAL CHEMISTRY, VOL. 51, NO. 12, OCTOBER 1979 and Chemical Ionization from a Combined Gas Chromatograph-Mass Spectrometer System", 172nd National Meeting, American Chemical Society,Division of Fuel Chemistry, Vd. 21, No. 6 (Aug. 29-Sept. 3, 1976). J. T. Kwan, J. I.S.Tang, W. H. Wong, and T. F. Yen, "Application of Liquid Chromatography to Monltw Biological Treatment of Oil Shale Retort Water", Prepr., Div. Pet. Chem., Am. Chem. Soc., 22, 823 (1977). W. D. Felix, D.S.Farrier, and R. E. Poulson, "High Performance Liquid Chromatographic Characterization of Oil Shale Retort Waters", USERDA, Omega-9 CIP Document No. 3 (1977). H. A. Stuber and J. A. Leenheer, "Fractionation of Organic Solutes in 011 Shale Retort Waters for Sorption Studies on Processed Shale", presented at the 175th National Meeting of the American Chemical Society, Anaheim, Calif., March 12-17, 1978. R. M. Silverstein, G. C. Bassler, and T. C. Morrill, "Spectrometric Identification of Organic Compounds", John Wiley and Sons, New York, 1974.
(13) H. A. Szymanski and R. E. Erickson, "Infrared Band Handbook", Vol. 1. Plenum Press, New York, 1970. (14) C. D. Becker, W. G. WoodfieM, and J. A. Strand, "Solvent Refined Coal Studies: Effects and Characterization of Treated Solvent Refined Coal Effluent", PNL-2606, Pacific Northwest Laboratory, Richland, Wash., 1978. (15) J. A. Leenheer and E. W. D. Huffman, Jr., U . S . J. Res. U.S. Geol. Survey, 4, 737 (1976).
RECEIVED for review February 5 , 1979. Accepted March 29, 1979. This work was supported by the US.Department of Energy under Contract EY-76-C-06-1830. Brand names are used for reader convenience but use does not constitute endorsement by Battelle Memorial Institute.
Noble Metal Field Ionization/Field Desorption Emitters Generated by Electrochemical Deposition Guenter Semrau and Joachim Heitbaum" Institute of Physical Chemistry, University
of Bonn, Wegelersrr. 12, 0-5300Bonn, West Germany
An ion emitter for field ionizatlon and desorption mass spectrometry is presented consistlng of a 10-Cc.m Pt wire covered with Pt microneedies formed by cathodic metal deposition from an aqueous 1.5 M sodium hexachioropiatinate solution at 85 O C using a galvanic pulse technlque. The emitter has good mechanical strength and can be used several times when standard cleaning procedures are applied. Its emission properties are comparable to that of the well known carbon dendrite emitter produced at high temperatures.
properties, although the peak intensities are one to two orders of magnitude smaller than those obtained with the carbon dendrite emitter used as a reference standard. The formation of MeOH layers in the case of the Ni and Co emitters can be well understood having in mind that hydrogen evolution occurs parallel to the cathodic metal deposition. Thus, the pH of the solution is locally increased near the electrode because of the H30+ consumption or OHformation, respectively, according to
H30++ eSince the invention of field ionization (FI) and field desorption (FD) mass spectrometry, special attention was given to the development of ion emitters. Up to now, there has been an almost exclusive use of wire emitters covered with carbon dendrites generated by pyrolysis of benzonitrile at low pressure and high electric field (1-3). Their advantages of high mechanical strength and chemical resistivity compete with the long time needed for preparation (almost 8 h). For this reason, alternative methods have been proposed recently such as the "high-rate growth of dendrites" ( 4 ) or the "high temperature growth of silicon whiskers on gold coated tungsten wires" ( 5 ) . Although the times of preparation were reduced to several minutes with the latter methods, all three types of emitters mentioned so far have to be formed under vacuum conditions and rather complicated temperature programs have to be observed under careful gas pressure control. These experimental difficulties can be avoided and even shorter preparation times (less than 1min) can be achieved when dendritic deposits are produced electrochemically as was pointed out by Goldenfeld (6). Using a programmable pulse generator, Bursey et al. (7-9) prepared F I / F D emitters by cathodic metal deposition on tungsten wires from aqueous solutions of simple Ni or Co salts. These emitters, however, exhibit almost insulating or semiconducting properties a t room temperature because they contain intrinsic oxide layers of the respective metal (10). Electrically conductive microneedles are obtained only when the Ni or Co emitters are heat treated in vacuum. Therefore, only heat activated emitters show satisfactory ionization 0003-2700/79/035 1-1998$01.OO/O
or
H 2 0 + e-
-
;H2 1
+ H2O
1 2
+ OH
-H2
This causes hydrolysis of the metal ions resulting in the formation of, for example, colloidal NiOH which occasionally may be incorporated into the deposit. The formation of metal hydroxides will not take place when noble metals are deposited such as Pt group metals or Au. Therefore the generation of Pt emitters for FI/FD applications was studied and is described below. These emitters are free of intrinsic oxide, no heat treatment being necessary. They are easy to prepare and the dendrites show good mechanical strength and adhesion to the underlying wire. Moreover, the Pt emitters can be used several times and standard cleaning procedures may be used such as washing in organic solvents or even in chromic acid. Furthermore, their morphology can easily be adjusted to the analytical problem in question by varying the number of cathodic pulses. And finally, they may be of special interest for the investigation of surface reactions.
EXPERIMENTAL Electrolytic Cell and Solution. The experimental setup for the electrochemical metal deposition is schematically shown in Figure 1. Emitter bases were constructed from two 1.0-mm diameter Vacon ports rigidly held approximately 1.5 mm apart by a glass spacer with glass to metal seals. A 10-bm length of platinum wire was spotwelded to this base and mounted in a holder which allowed accurate and reproducible adjustment of distance between the emitter wire and the counter electrode 0 1979 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 51, NO. 12, OCTOBER 1979
1999
a
Figure 1. Electrolytic cell with emitter holder (a),emitter (b), counter electrode (c),and pulse generator (d) (platinum wire, 1.0-mm diameter) placed beneath it and running parallel to it a t a distance of about 1.5-2.0 mm diameter. In order to prevent metal deposition on the ports, those had been isolated by a quickly curing resin before metal deposition was started. The electrolyte consisted of an aqueous 1.5 M sodium hexachloroplatinate solution which was obtained by neutralizing hexachloroplatinic acid with Na2C03. The acid itself was not favorable for the platinum deposition because at low pH values the hydrogen evolution exceeds the metal deposition. Moreover, in the latter case the deposit is a spongy thin layer which does not adhere to the platinum wire. In the neutral electrolyte, however, the hydrogen evolution is largely suppressed and needle-like platinum deposits can be obtained by cathodic reduction. Conditions for the Deposition. The metal deposition was performed at an elevated temperature of about 90 "C by rectangular current pulses using a programmable pulse generator as described by Rechsteiner et al. (9). This electronic device delivers any number between 1 and 255 of pulses of variable width (0.8-80 ms) separated by a pulse pause of 100 ms. The pulse height can be changed from -41 V to 0 V. Optimal conditions to produce Pt emitters are a pulse height of -34 V and a pulse width of 4 ms. The number of pulses choosen determines the surface morphology varying over a wide range. Mass Spectrometer. FI mass spectra were obtained using a single focusing 60" magnetic mass spectrometer with standard equipment. FD experiments were performed on a quadrupole mass filter, the spectra being recorded on a multichannel analyzer.
RESULTS AND DISCUSSION Morphology of Pt Emitters. T h e other experimental parameters being fixed, the morphology of the Pt emitters depends only on the number of cathodic pulses applied as can be seen from the examples given below. In Figure 2 the micrographs of a n emitter obtained with 32 pulses are given showing a very rough surface, the growth of longer dendrites being largely restricted. This type of emitter was found most favorable for F D applications. Because of its high surface area, large amounts of substance can be loaded thereby producing intensive spectra. On the other hand, interference of field ionization is low and shielding effects due to long tips can be neglected. I t should be mentioned that this type of emitter readily accepts droplets of aqueous solutions, a problem which sometimes arises when organic substances are t o be analyzed from electrolytes by alkali ion attachment (11). Figure 3 shows a typical FI emitter obtained with 64 pulses. It is covered with quite a number of metallic needles and protrusions, the radii of curvature a t the top lying between 500 and 1000 A as was estimated from scanning electron microphotographs.
b
Figure 2. Pt emitter deposited from 1.5 M NaPtCI, solution, T = 90 'C, t , = 4 ms, U p = -34 V, n = 32; magnifications: (a) 2300X, (b) 5800X
a
b
Figure 3. Pt emitter deposited from 1.5 M NaPtCi, solution, T = 90 'C, t , = 4 ms, U p = -34 V, n = 64; magnifications: (a) 580X, (b) 2300X
A further increase of the number of pulses leads to a spongy intergrowth of the metal deposits as is demonstrated in Figure 4. These structures are no longer favorable for mass spectrometric investigations. All emitters shown were pure metallic as was proved by TEM.
2000
ANALYTICAL CHEMISTRY, VOL. 51, NO. 12, OCTOBER 1 9 i79
-1
I nt
I
IM+Nal+ 365
I00
200
Figure 6. FD mass spectrum of sucrose mA
a
300
d e +
+ NaI, heating current 26
As was stated above, no heat treatment of the Pt emitters proved to be necessary as in the case of the Ni and Co emitters mentioned. FD Mass Spectrometry. The Pt emitter prepared with 32 pulses (cf. Figure 2) was used for FD measurements. Typical FD spectra of adenosine and succrose obtained with this emitter are shown in Figures 5 and 6, respectively. The emitters can be used repeatedly. Their activity diminishes, however, when they are charged with strong heating currents.
b Flgure 4. Pt emitter deposited from 1.5 M NaPtCI, solution, T = 90 O C , f, = 4 ms, U p = -34 V, n = 320; magnifications: (a) 590X, (b) 2400X
CONCLUSIONS The Pt emitters presented in this paper are easily and quickly prepared and well suited for FI/FD mass spectrometry. They are pure metallic and can therefore be cleaned by standard methods without being damaged. Furthermore, they might be of interest for the investigation of surface reactions. ACKNOWLEDGMENT The authors are indebted to F. W. Rollgen for helpful discussions and to A. Maas for taking the scanning electron microphotographs. LITERATURE CITED
a
b
Figure 5. FD mass spectrum of adenosine, heating currents 20 mA (a) and 18 mA (b)
FI Mass Spectrometry. In order to show the feasibility of the Pt emitters for FI mass spectrometry, n-heptane spectra were recorded using the emitter formed with 64 pulses (cf. Figure 3). The line intensities equal to those obtained with a pyrocarbon emitter under the same conditions. The medium field strength was estimated from the ratio of the CzH5+and C7HI6+peaks in the mass spectrum. This ratio being about 2 x lo-*, a medium field strength of some lo' V/cm is obtained.
(1) H. D. Beckey, E. Hilt, and H. R. Schuken, J . Physics E, 6, 1043 (1973). (2) H. D. Beckey, "Field Ionization Mass Spectrometry", Pergamon Press, Elmsford, N.Y., 1971. (3) H. D. Beckey and H. R. Schulten, Angew. Chem., 87, 425 (1975). (4) H. 8. Linden, E. Hilt, and H. D. Beckey, J . Phys. E, 11, 1033 (1978). (5) T. Matsuo, H. Matsuda, and I. Katakuse, Anal. Chem., 51, 69 (1979). (6) I.v. Goklenfeld, R. N. Bondarenko, and V. G. Golovaty, Prib. Tekh. Eksp., 3, 166 (1973). (7) R. M. Wightman, D. M. Hinton, M. C. Sammons, and M. M. Bursey, Inf. J . Mass Spectrom. Ion Phys., 17, 208 (1975). (8) M. M. Bursey, C. E. Rechsteiner, M. C. Sammons, D. M. Hinton, T. S. Colpitts, and K. M. Tvavonas, J . Phys. E, 9, 145 (1976). (9) C. E. Rechsteiner, D. E. Mathis, M. M. Bursey, and R. P. Buck, Biomed. Mass. Spectrom., 4, 52 (1977). (10) F. W. Rollgen, H. J. Heinen, U. Giessmann, and S. J. Reddy, Int. J. Mass. Spectrom. Ion Phys., 24, 235 (1977). (11) H. J. Heinen, U. Giessmann, and F. W. Rollgen, Org. M a s Spectrom., 12, 715 (1977).
RECEIVED for review March 3, 1979. Accepted July 9, 1979.
