Anal. Chem. 1980, 52, 605-607
Probe for the Introduction of Nonvolatile Solids into a Chemical Ionization Source by Thermal Desorption from a Platinum Wire Andries P. Bruins Stafe University, Department of Pharmacy, A. Deusinglaan 2, 97 73 A W Graningen, The Netherlands
Field Desorption (FD) is probably the most widely used method for taking mass spectra of polar and highly involatile or thermolabile organic substances. FD sources are commercially available for the majority of modern magnetic sector instruments. T h e cost of an F D source with its associated electronics may, however, be prohibitive. FD sources are not manufactured for quadrupole mass spectrometers and some, mostly older types, sector instruments. As a result, electron impact and chemical ionization mass spectrometry combined with special sample introduction techniques receive increasing attention. Baldwin and McLafferty were the first to publish CI spectra of oligopeptides by the very simple "in beam" method ( I ) . An involatile sample, applied to an extended solid probe tip is introduced into the CI plasma close to the axis of the electron beam. Thermal decomposition is greatly reduced compared with conventional solid probe introduction and the relative abundance of ions in the molecular weight region is considerably enhanced. Hansen and Munson have explored the technique systematically (21, and the application to samples of biological origin, using a Vespel tip, has been reported (3). T h e effects of sample size and heating rate on the "in beam" CI spectra have been studied extensively ( 4 ) and the use of extended tips coated with SE 30 has been published ( 5 ) . "In beam" E1 has been described by Ohashi and co-workers (6). Effective sample heating, independent of the source temperature and close to the ionization region is accomplished by the introduction of an F D emitter into a C1 ( 7 ) or E1 (8) source. It does not seem to be necessary that the emitters are activated: E1 spectra have been taken by thermal desorption of disaccharides from bare rhenium or tungsten wire (9). Combined with CI, the desorption from 0.1-mm diameter bare rhenium wire is now commercially available from Varian MAT, while Ribermag (France) offers the same option foi their quadrupole GC/MS, using 50-pm diameter bare tungsten wire. Finally it should not be overlooked that the conventional solid probe inlet system may be improved by silanizing the glass capillary ( I O ) or coating it with SE 30 (11). This paper describes a simple direct insertion probe for thermal desorption of involatile samples from a platinum wire. close to the electron beam of a CI source.
EXPERIMENTAL A Finnigan 3300 GC/MS system equipped with the standard CI source was used. Spectra were taken hy repetitive scantiing under control of the Finnigan 6110 Data System. Voltages aiid currents for negative ion operation were obtained from a hunle built supply. The ion energy was programmed typically from 6 V at m / e 100 up to -12 V at m / e 900. The electron collectur was shorted to the source block inside the vacuum feedthrough was used for the conversion dynode fur negative ion detection (12). A metal plate held at +:WKJ V wa5 placed o1)posite the ion entrance hole of the original Galileo 4'751 multipliei t u convert negative ions into positive ones. The source teniperature was kept constatit by a CKI, 405 digital temperature controller (CRL, Worthing. England) ri4ng [kit> o r iginal Finnigan thermocouple as sensor. T h e t h f " r ~ 1 N J C L ) L i pwas k repositioned and clamped inside a hole drilled in t h e source blot!, 4 mm from the solid probe inlet port. 'l'he temperature readout was calibrated against a Pt resistance thermometer inserted i n t o the source via the solid probe insertion lock. All trrxiper:iture. reported are corrected temperatures.
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ANALYTICAL CHEMISTRY, VOL. 52, NO. 3, MARCH 1980
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Flgure 1. Construction of the Pt wire probe. (1) CI source block with original ceramic spacer and input adapter; (2) 0.2-mm diameter Pt wire spotwelded to 3; (3) 0.6-mm Pt wire molten into 4; (4) tube, AR soda glass, 4.2 mm 0.d. X 2 mm i.d.; (5)0.5-mm diameter copper wire, spotwelded to 3, insulated with Teflon sleeve; (6) '/, inch 0.d. X 4.8 mm i.d. stainless steel tube; (7) standard '/,-inch Swagelok nuts; (8) '/,-inch Vespel ferrule; (9) '/,-inch Swagelok union body; (10) home made 4.5-mm i.d. Teflon ferrule; (11) '/,-inch Swagelok union with two 4.5-mm i.d. Teflon
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example, desorbed from the Pt wire shows the [M + HI- ion as the base peak. A successive loss of water molecules from this ion is not observed, and the [M - H - H 2 0 ] - ion has a relative abundance of only 1% . Figure 3 gives the spectrum of a complex glycoside of high molecular weight. The sequence of the sugar units can be derived from the fragment ions. It is the experience in field desorption mass spectrometry that fragmentation can be controlled by a proper selection of the wire current. T h e same effect has been observed in
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Extracted ion current profiles of three peaks from the spectrum of digltoxigenlne moncdigitoxoside. Current program rate 0.8 A/min Figure 4.
FD-CI (7) and is demonstrated in Figure 4 for the desorption of a sample from the Pt wire. The relative abundance of the ion a t mle 373 reaches its maximum a t a lower wire current than the [M - HI- ion a t mle 503. This indicates t h a t m l e 373 is not solely a fragment of the complete glycoside, but also the [M - HI- ion of the aglycone digitoxigenine which is present as an impurity. The formation of mle 129, observed a t a relatively high wire current, is enhanced by thermal excitation by a high wire temperature. Figure 5 gives an indication of the sample size required. One microliter of a solution of digitoxine, containing 113 n g l p L was applied to the wire, and a selected ion current profile was obtained by single ion monitoring of mle 763, the [M - HIion. Close inspection of the base line on the display unit of the data system revealed an S I N ratio of approximately 250. Figure 5 was obtained under average operating conditions (source used for a few weeks, emission current 100 PA, electron multiplier a t 1300 V), without attempts to reach the optimum
Anal Chem 1980, 52, 607-608
U!GITOXINE
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607
is easy to operate, and is protected against mechanical damage during insertion through the vacuum lock.
ACKNOWLEDGMENT The author is grateful to J. Roede for building the negative ion source supply, to F. P. Buss and A. Oosterhoff for constructing the Pt wire probe, and to J. v.d. Greef, University of Amsterdam, for practical information on field desorption.
LITERATURE CITED
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Figure 5. Selected ion current profile of the [M - HI- ion of digitoxine by OH- negative ion CI. Current program r a t e 2 A/min. Source temperature 1 7 0
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performance of the instrument. It thus seems to be possible to record a full spectrum of 100 ng under favorable conditions, but 1 pg, collected for example by preparative chromatography, would be a more realistic sample size. When 10 ng of digitoxine was introduced, however, no signal was observable by single ion monitoring. Essentially the same results have been reported recently: 100 ng of a disaccharide gave both molecular weight information and fragment ions using the Ribermag desorption CI probe with NH, as the reactant gas, but 10 ng only produced fragment ions (15). Apparently, a certain amount of sample is consumed by decomposition on active sites on the metal wires. In conclusion, the Pt wire probe is constructed from inexpensive components, can handle complex sample molecules,
Baldwin. M. A,; McLafferty, F. W. Org. Mass Spectrom. 1973, 7, 1353. Hansen. G.: Munson. B. Anal. Chem. 1978. 50. 1130. Cotter, R. J. Anal. Chern. 1979, 51, 317. Cotter, R. J.; Fenselau, C. Biomed. Mass Spectrom. 1979, 6 , 287. Carroll, D. 1.; Dzidic, I.; Horning, M. G.; Montgomery, F. E.; Nowlin, J. G.; Stillwell, R . ; Thenot, J. P.; Horning, E. C. Anal. Chem. 1979, 51, 1858. Ohashi. M.; Yamada. S.; Kudo, H.; Nakayama, N. Biorned. Mass Spectrom. 1978, 5 1 , 578. Hunt, D. F.; Shabanowitz, J.; Botz, F. K.; Brent, D. A. Anal. Chem. 1977, 49, 1160. Soltmann, B.; Sweeley, C. C . ; Holland, J. F. Anal. Chsm. 1977, 49, 1164. Anderson, W. R.; Frick, W.; Daves, G. D. J . Am. Chem. SOC.1978, 100, 1974. Raaymakers, J. G. A. M.; Engel, D. J. C. Anal. Chem. 1974, 46, 1357. Thenot, J. P.: Nowlin, J.; Carroll, D. 1.; Montgomery, F. E.; Horning, E. C. Anal. Chem. 1979, 51, 1101. Stafford, G.; Reeher, J.; Smith, R . ; Story, M. I n "Dynamic Mass SDectrometry", Vol. 5, Price, D., Todd, J. F. J., Eds.; Heyden and Son: London, 1977, p 55. Smit, A. L. C.; Field, F. H. J . Am. Chem SOC.1977, 99, 6471. Bruins, A. P., paper presented at the 8th International Mass Spectrometrv Conference. Auoust 12-18. 1979 Oslo. Norway; Adv. Mass siectrom. VOI. 8 , in bress. Prome, J. C.; Beaugrand, C., papers presented at the Ribermag GC/ MS/DS Symposium, October 17-18, 1979, Antwerpen, Belgium.
RECEIVED for review August 14, 1979. Accepted December 10,1979.
Simple Procedure for Producing a Hanging Mercury Drop Electrode of Constant Surface Area Anthony J. Bellamy Chemistry Department, University of Edinburgh, West Mains Road, Edinburgh EH9 3JJ, Scotland
T h e use of the hanging mercury drop electrode (HMDE) for many quantitative electroanalytical measurements, e.g., voltammetry, anodic stripping analysis, requires that the electrode area is constant, and in some cases known. This is usually achieved in two ways: (i) one or more drops of mercury from a dropping mercury electrode, of known characteristics, are collected in a scoop and transferred to a small amalgamated platinum disk, (ii) a glass capillary tube connected to a closed mercury reservoir is fed by advancing a calibrated, threaded plunger into the reservoir ( I , p 85). We have found that the following simple procedure produces a HMDE with a reproducible surface area (standard deviation *1 YO),and is more convenient than either of the above methods. A small platinum disk electrode was prepared by sealing a short length of 0.635-mm diameter platinum wire into the end of a soft glass tube (7-mm 0.d.); the sealed end of the glass tube and wire were ground flat and then polished with 0.3-pm alpha alumina. The platinum disk was made the cathode (Pt wire anode) in the electrolysis of 1 M perchloric acid (3-V battery); hydrogen was discharged for 10 min; and then the disk was amalgamated by plunging it below the surface of a mercury pool while still connected to the battery (2). The 0003-2700/80/0352-0607$01 O O / O
electrical connection was removed, the electrode was withdrawn, washed with distilled water and acetone, and, after drying in air, was stored in a test tube (ca. 10-mm i.d.) containing mercury to a depth of ca. 20 mm, with the electrode tip ca. 5 mm below the surface. The surface area of the mercury drop adhering to the platinum disk when the electrode is removed from the storage tube was found to be critically dependent upon the method of withdrawal. If the storage tube is held vertically and the electrode is withdrawn vertically (Figure lA, method A), the area is irreproducible (see Table I, experiments 6-10). If, however, the storage tube is held almost horizontally, so that the mercury is about to run along the tube, and the end of the electrode is moved from below to above the mercury surface by downward movement of the outer end of the electrode (Figure 1B,method B), the area is very reproducible (see Table I, experiments 1-5). The mercury drops produced by method A are visibly larger and more rounded than those produced by method B. T h e constancy of the area of the mercury drops produced by method B was demonstrated by using the HMDE for the linear sweep voltammetry of the first electron transfer of C 1980 American Chemical Society
