(7) V. V. S.Eswara Dutt and H.A. Mottola, Anal. Cbem., 49, 776 (1977). (8) P. w. Alexander and s. H.Qureshi. J . E k t f M M I . them., 71,235(1978). (9) 0.H. Muller, J . A m . Chem. Soc., 69,2992 (1947). (IO) N. Oshino, R. Oshino, and B. Chance, Biochem. J . , 131, 555 (1973).
RECEIVED for review May 25, 1977. Accepted July 25, 1977.
This work was supported in part by the National Science Foundation (Grant CHE-76-03739). One of the authors (Ch-M. w.) gratefully acknowledges the aid of the CNRS (France) in the form of a leave of absence, and support provided by a NATO Fellowship.
Glass Lined Field Ionization Sources J.
H. McReynolds,'
N. W. Flynn, R. R. Sperry, D. Fraisse, and M. Anbar'
Stanford Research Institute, Mass Spectrometry Research Center, Menlo Park, California 94025
In using field ionization sources (for multicomponent mixture analysis and isotope dilution studies), we have observed both catalytic decomposition and strong adsorption of certain materials. Decomposition is reduced as sensitive surfaces become saturated, but this results in memory effects that are undesirable both in multicomponent analysis and in studies involving quantitation using labeled internal standards. Lower source temperatures tend to increase memory effects while reducing pyrolysis, and the opposite is true a t higher source temperatures. Some improvement is achieved by vapor phase silylation using BSTFA or other silylation reagent,s, but this treatment is not completely effective and must be repeated often with certain sample types (I,2). T o minimize these effects, glass lined versions of both multipoint array and preactivated foil field ionization sources have been developed by Spindt and Allen of SRI's Physical Electronics Group. The details of these sources have been described previously (3-5). Cross-sectional views of both sources are shown in Figure 1. The center glass tube in both sources is Corning type 7052. This tube is installed in the preoxidized Kovar mount with extra length left a t both ends. A methanol slurry of Corning 7574 solder glass frit is applied generously to both ends of the glass tube and to the recessed concave probe sealing surface. After allowing the methanol to evaporate, the assembly is fiied to approximately 750 "C using a hand torch to fuse the solder glass to the glass tubing and to the Kovar mount. After firing, the mount is annealed in an oven a t 500 "C for 15 min and cooled slowly to room temperature. Final polishing of the probe end is done with a silicon carbide charged 0.5-in. 0.d. polishing tool while turning the source mount in a lathe. The source end of the multipoint mount is polished similarly using an abrasive coated flat plate against the rotating mount. Considerable care must be exercised in polishing the tapered surface of the foil source mount, and some chipping of the glass a t the apex of the wedge normally occurs. This does not seem to affect the performance of the source since most of the sample path retains a glass surface. Figure 2 shows aliquots of the same sample, a nucleic acid hydrolysate run on a standard metal and a glass lined foil source. The almost total attenuation of guanine on the metal source is typical of this highly polar compound. This material may be observed in low yield with source temperatures greater than 250 "C, but not without considerable pyrolysis losses. The glass lined source appears to give much better results than the metal source when operated under the same conditions. To test the relative memory effects of the two source types, a sample of 40 ng of imipramine-& and 400 ng of imipramine-d6 was evaporated from the probe a t a temperature of 50-100 OC. Immediately following this analysis, a sample Present address, Department of Biophysical Sciences, School of Medicine, State University of New York at Buffalo, Buffalo, New York 14214.
r
i
Figure 1. Cross-sectional view of glass lined field ionization source. (A) Kovar mount. (B) Glass frit. (Cj Glass tube. (D) Probe sealing surface. (E) Activated foil
I I
I 100
I1 120
I.
,
I 140
160
180
m/e
Flgure 2. FIMS of nucleic acid hydrolysate. Upper: Metal source: Lower: Glass lined source; Probe temperature 60-230 OC; Source temperature: 250 O C . C = Cytosine, U = Uracil, T = Thymine, A = Adanine, G = Guanine
containing 400 ng of imipramine-& (containing
