Anal. Chem. 1983, 55, 1170-1171
1170
Liquid Metal Substrate for Dynamic Secondary Ion Mass Spectrometry Sir: Molecular secondary ion mass spectrometry (SIMS) is widely used today to determine the molecular structure of a variety of organic and inorganic compounds (1,2). Although SIMS was originally developed as a surface analytical tool for in-depth and surface analysis studies, it has become an important new "soft" ionization method for the analysis of nonvolatile and thermally labile compounds (3). One recent variation of the SIMS methodology that has been extremely successful in the analysis of organics and biomolecules is called FAB, fast atom bombardment (4), or liquid SIMS. In liquid SIMS, the compound to be analyzed is dissolved in a liquid matrix, such as glycerol, at some optimum concentration in order to provide a surface which, during particle bombardment, is constantly replenished with sample molecules that diffuse from the bulk to the surface. With the liquid matrix, therefore, one can use a primary atom beam, with densities of 1O1Oto 10l1(atoms/cm2)/s, to produce intense and long-lived molecular secondary ion emission (5). One objective of this study is to investigate the use of liquid metal surfaces, such as gallium, as sample substrates for dynamic SIMS analyses. Previously, Fine and co-workers observed that bombarding the surface of gallium with an electron beam caused the mass transport of impurities to the beam where they were desorbed, and thereby produced a clean metal surface (6). We discovered that ion bombardment of the liquid metal surface, upon which a sample was deposited, resulted in movement of the sample species toward the primary ion beam where they were desorbed and finally detected by the mass analyyer. The liquid meta provides an inert, smooth, conducting surface that could of er several advantages over other substrates for certain SIMS applications. We report the use of a liquid gallium alloy as a substrate in SIMS for the study of some polycyclic aromatic compounds (PAC) with an ion microscope.
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EXPERIMENTAL SECTION The compounds analyzed were two sets of PAC isomers: (1) phenanthrene and anthracene, three-ring C14H10, molecular weight = 178; and (2) fluoranthene and pyrene, four-ring ClaHlo,molecular weight = 202. Samples of these compounds on silver or indium foils were prepared either by depositing 1 WLof a lo-' M solution of the PAC in chloroform onto a piece of the metal or by burnishing the pure PAC, or a mixture of the PAC with NH4C1, onto the metal foil. Samples of PACs on liquid metal were prepared by depositing 1 or 2 p L of a chloroform solution of the PAC onto the gallium. Because gallium is a solid at room temperature, a small amount of indium was added to the gallium to decrease its melting point. The result was an alloy which is liquid at room temperature. A small bead, approximately 0.5 cm in diameter, of this alloy was placed onto a piece of glass and set in a disk-shaped sample holder. The PAC solution was deposited over the surface of the liquid metal. In order to prevent loss of the liquid substrate in the vacuum interlock system, the sample was cooled before placement in the instrument. During the analysis, the substrate was liquid. Secondary ion mass spectra were obtained with a CAMECA IMS-300 ion microscope using a 5.5-keV 02+ion beam with a current density in the range of (1 to 5) X A/cm2. This instrument and its analytical applications have been described (7). The mass range scanned is 1to 250 amu and the secondary ions are detected with the converter cathode/photomultiplier. One advantage of this spectrometer over conventional FAB instruments is that the sample is immersed in the full extraction field of the spectrometer (i.e., -1000 V/mm) resulting in much higher ion transmission.
Table I. The Intensity of the Molecular Ion Emission from Various Sample Substrates compound phenanthrene anthracene fluoranthene pyrene
molecular ion emission, counts/s Ag or In Ga/In alloy glycerolc N.D.' N.D. N.D. N.D.
1900-2500 1900-2500 3100--3800
3100-3800
N.D. N.D. N.D. N.D.
a Not detected (
