1686
Anal. Chem. 1986, 58, 1686-1690
Particle Induced Desorption Mass Spectrometry in a Microscopic Mode Paul E. Filpus-Luyckx' and E. A. Schweikert* Center for Chemical Characterization & Analysis, Department of Chemistry, Texas A&M University, College Station, Texas 77843-3144
ParHck Induced deeorpHon mas spectrometry has been applied as d mlcrochemkal technlque wlng microbeams of 84-MeV MKr7+. The timesf-fllght mass spectrometer allows slrnuiianeous muttbnass determinations of atomlc, Isotoplc, and molecular positive and negative lons. Mass resolution of up to 296 was measwed for masses ranging to 653 amu. In a scanning mode, the technique can resolve features 60 pm In size. Qualttatlve scanning shows the mass spectra changlng wlth differences In chemlcai composttion.
detected by a microchannel plate (MCP) detector. This detector consisted of two microchannel plates mounted in the chevron configuration (18 mm diameter, nonimaging quality from Galileo Corp.). Each plate had an electron gain of approximately IO4for a 1OOO-V potential drop (5). The plate output was collected in a conical anode machined for a 5 0 4 output impedance to minimize pulse ringing (6). The MCP signal was preamplified by a custom-built fast preamplifier then discriminated by a constant-fraction discriminator. The further electronics setup consisted of a standard TOF assembly. A detailed description is provided in ref 3. Finally,the time-tuamplitude converter outputs were collected and analyzed with the Cyclotron Institute VAX 11/780 computer and the data acquisition program Vulcan (7).
Particle induced desorption mass spectrometry (PDMS) is recognized as a unique analytical technique for the characterization of large nonvolatile biomolecules (1). The desorption phenomenon itself is remarkable for its efficiency (ions detected/100 incident heavy ions), reaching, for example, several tens of percent for Cs+ from CsI (2). Thus, mass spectra of desorbed species can be obtained with a small number of incident ions (say a few thousand). With this in mind, one can envision the application of PDMS for the analysis of small areas. This study deals with desorption induced by microbeams of high-energy heavy ions and the analysis of the desorbed species via time-of-flight mass spectrometry to deduce chemical composition information from areas on the order of micrometers in size.
RESULTS AND DISCUSSION Mass Spectrometry. As already noted, the desorbed species were characterized via time-of-flight mass spectrometry. The mass spectrometric capabilities of the TOF-MS system were evaluated by analyzing alkali halide targets. The data were obtained with microbeams collimated to 1, 10, or 50 Km in diameter. Representative qualitative results are presented in Figure 2 for the positive ions desorbed from CsI, RbCl, and KCl targets. Each spectrum shows H+, Hz+, the principal alkali cation, and additional masses. The CsI spectrum contains peaks that are attributed to sodium and potassium contaminants. In the RbCl spectrum, organosilicon and hydrocarbon contaminant peaks from the vacuum pump oil and the target backing are evident. In both the KCl and RbCl spectra, the isotopic peaks of the alkali ions are shown, 39K:41K and s5Rb:s7Rb,respectively. The KC1 spectrum also contains a K2C1+peak that does, however, not show resolved isotopic multiplet peaks. Negative ion spectra for NaC1, mixed CsI/RbCl, and CsI targets are presented in Figure 3. The peaks due to the organosilicon and hydrocarbon contaminants are common to all three spectra. The two targets with an iodide layer show I- peaks,and the two targets with a chlorine layer show a broad 35-37-amu peak, in agreement with the composition of the thin f i b . These examples demonstrate that PDMS is capable of simultaneous multimass identification of organic and inorganic ions of atomic and molecular species, either positively or negatively charged. The mass resolution of the spectrometer, Le., the ratio of the mass to the resolvable mass difference a t that mass, was measured for masses up to 653 amu (Cs31z+)and ranged from 12 to 296. The mass resolution varies with mass in a timeof-flight spectrometer, as evidenced by differentiating the kinetic energy equation for a fixed time resolution (3). This is in contrast to magnetic and electrostatic sector instruments. Quantitative measurements were made with a series of alkali halide targets analyzed several times each. The reproducibility of the yield measurements was quite variant. For example, the desorption yields of Cs+ varied from 12 to 69% for the same target measured on different dates. The experimental yields were found in these preliminary experiments to be sensitive to changes in vacuum levels, response of the microchannel plates, and especially changes in the condition of
EXPERIMENTAL SECTION The ion beams used in this work were produced at the Texas A&M University 88-in. variable-energycyclotron. Beams of 84MeV s4Kr7+were used. The experimental work was carried out in a vacuum chamber constructed for time-of-flight spectrometry and maintained at
