surements of the sensitivity coefficients of a whole number of pure hydrocarbons of various groups. The suggested procedure for analyses of saturated petroleum fractions is far less time- and labor-consuming than the iterative procedure, particularly for larger series of analyses and in all those cases where an accuracy of &IO% re1 proves to be satisfactory. For a reliable determination of paraffins, it is desirable to know, a t least approximately, the normal to isoparaffins ratio, the mean sensitivity coefficients of which differ rather considerably (by 0.960 and 0.704, respectively). If this ratio is not known, it is better to employ the mean value of the sensitivity coefficients of both these groups. Preliminary results have shown that the above procedure may be advantageously applied to mixtures of saturated hydrocarbons up to C15-CI6, where the presence of aromat-
ics sufficiently suppresses the adsorption effects of higher molecular weight saturated hydrocarbons.
LITERATURE CITED (1) W. L. Mead, Anal. Chem., 40, 743 (1968). (2) K. G. Hippe and H. D.Beckey, ErdoelKohle, Erdgas, Petrochem., 24, 620 (1971). (3) M. Ryska, M. Kuras, and J. Mostecky. lnt. J. Mass Spectrom. /on Phys., 16, 257 (1975). (4) ASTM D 2789-71, American Society for Testina and Materials. Philadelphia, Pa., 1971. (5) L. R. Snyder, H. E. Howard, and W. C. Ferguson. Anal. Chem., 35, 1676 (19631. (6) W.L. ‘Mead, British Petroleum Research Center, Sunbury on Thames, England, 1974, private communication.
RECEIVEDfor review March 27, 1975. Accepted September 19, 1975.
Field Ionization-Field Desorption Source for Nonfragmenting Mass Spectrometry Michael Anbar” and Gilbert A. St. John Mass Spectrometry Research Center, Stanford Research Institute, Menlo Park, Calif. 94025
A field ionization and desorption source has been deveioped, comprising a rough metal surface at the end of a short, thin metal rod, and a heatable structure which allows its reproducible placement within 50 pm from a counter electrode. The field desorbing rod is extremely simple to prepare and a new one may be used for each sample. This source has been successfully operated at ambient temperature to 350 ‘C. A number of examples of field ionization and field desorption spectra of inorganic and organic compounds obtained by this source are presented. Different ionization processes which take place under field desorption conditions are discussed. Most important among these is the ionizatlon of materials dissolved in a polar organic poiymeric matrix.
Field ionization has drawn considerable interest in recent years because of its ability to produce primarily unfragmented mass spectra (1-3). The analytical applications of this technique have been described before (1-3). The gist of this technique is an efficient ionization source. A number of types of field ionizers have been developedthe Beckey carbonaceous dendrite tungsten wire source ( I ) , the nickel dendrite source ( 4 , 5 ) , the Robertson razor blade edge source ( 6 , 7 ) ,and the SRI porous multipoint source (2).The latter has a fairly high overall ionization efficiency because of the effective sample feed through the porous backing. A large fraction of the energetic ions produced are, however, lost because of their divergence, and only a small fraction can be effectively mass analyzed. Still, reasonable overall ionization X transmittance efficiencies are attained with the porous sources; ranging from 3 X 101j ions/mol in a 90’ sector magnet with a resolution of 500, to 1x ions/mol (at mass 126) using a Yg-inch quadrupole analyzer with the same resolution. These efficiencies are sufficient to allow a large variety of analytical applications (2, 8). 198
ANALYTICAL CHEMISTRY, VOL. 48, NO. 1, JANUARY 1976
The usefulness of the nonfragmenting nature of field ionization can be extended to nonvolatile substrates if these are placed directly on the highly curved surfaces which are necessary for field ionization. This mode of ionization has been developed by Beckey et al. and named “field desorption” (9). By placing the material directly on the active surface, one also overcomes the shortcoming of low sample-feed efficiencies of nonporous field ionization sources, and overall ionization X transmittance efficiencies, allowing the detection of lo-” g of material, have been achieved for magnetic sector mass spectrometers with the carbonaceous dendrite sources (9). The SRI multipoint structures are much too expensive to be used as field desorption sources. Whereas they have been used for hundreds of hours in the field ionizing mode, their use is limited to very few analyses in the desorption mode. Also, the carbonaceous dendrite sources are difficult to produce, though their fabrication is not as difficult as that of the SRI multipoint structures. In any case, we looked for as simple a structure as possible which would facilitate field desorption, so that it can be discarded after each sample. This would alleviate cleanup and memory problems. We have found that rough metal surfaces formed by breakage of hard brittle metals, like tungsten or titanium, are effective field ionizers (or field desorbers). Also, iron surfaces may be roughened by acid etching to allow field ionization, We then modified the SRI field ionization source structure, substituting the field ionizing multipoint pedestal (Figure 1) by a rough-ended metal rod (Figure 2). This arrangement allows the rapid interchange of the two types of ionizers and necessitates minimal readjustments of the ion optics system. In this exploratory study, we have used a low resolution 45’ sector magnet which was manually scanned. Obviously, higher quality mass spectra might be obtained using a more advanced mass analyzer. We shall now describe in greater detail the new ionization source and illustrate its performance with a number of examples.
COUNTER ELtCTROOE
/
KOVAR MOUNT ASSEMsLY
a
PO~NTS
VAPORIZED SAMPLE
n
n
b Figure 1. Schematic cross section of multipoint ionizer showing solid sample probe COUNTER LLECTRODE
/
DESORPTION TIP BROKEN TUNGSTEN R O O ,
C
Flgure 2. Field desorption ionizer with sapphire insulators
EXPERIMENTAL Description of Ionization Source. Desorption ionization takes place from the end of B 5s-inch X 1-inch thin rod (or wire) fixed in close proximity to a 250-mesh nickel counter electrode or a 250-wm slit counter electrode made from 125 wm molybdenum shim stock. Tungsten rod of 5s-inch diameter was notched at 1-inch intervals on an abrasive wheel and freshly broken for each sample. The hroken surface is microcrystalline enough to allow field ionization (Figure 3a). When using iron wire for a desorber, 1-inch lengths were cut from a long wire and the end was sharpened to a truncated cone on an abrasive wheel to a final diameter of 0.5-1 mm. The surface of the end was etched in HCI to increase the roughness and surface area (Figure 36). A dip in gold chloride (chloroauric acid) deposits colloidal gold on the tip upon drying (Figure 3c). No significant differences in the performance of the three types of ionizing surfaces were found, thus the broken tungsten rods are being routinely used hecause of the simplicity of their preparation. The source structure consists of a modified version of SRI's standard field ionizing source. In place of the pedestal upon which the field ionizing paints are deposited, a hollow pedestal with a hole to clear X s inch is substituted (Figure 2). The ionizing surface with the sample deposited on it is introduced directly into the ionizing region from the back through a vacuum lock. The insertion rod is %-inch by 9-inch insulating rod with a stainless tip '/-inch X 1-inch threaded into its end. The steel tip is drilled to receive and clamp the 'hs-ineh desorption wire. The insulating rod used is made of Teflon for law temperature operation (
