Laser assisted field desorption mass spectrometry II: Metals and alloys

Abstract

The first application of Field Desorption Mass Spectrometry (FD-MS) to the analysis of metals and some widely-used, industrial alloys is reported. Fine metal powders of tungsten, tantalum, antimony, iron, hafnium, palladium, zinc, copper, uranium, manganese, tin, titanium, nickel, zirconium, gold, silver, scandium and aluminium were applied to the conventionally used high-temperature carbon emitter and desorbed using direct or indirect heating. The intensities of the obtained ion currents for the cations of these elements increased in the sequence of this listing. During ion formation the sample and emitter temperature in the target area of an argon ion laser on the emitter surface was measured by means of a micropyrometer. These investigations gave clear evidence that the onset for the production of metal cations from the non-ionic samples occurs close to the melting point of the corresponding element. Intense ion currents are recorded in many cases slightly above this temperature (approx. 50° C). For alloys a fractionated desorption of the metallic components occurred in a wider temperature range and the corresponding ion intensity/temperature profiles (thermograms) for the alloys Cr80/Al20, Cu70/Zn30 and Fe71/Cr18/Ni8/Mo3 are given and interpreted. These more qualitative pilot studies demonstrate convincingly the utility and versatility of laser assisted FD-MS for the detection and identification of metals, accompanying trace metals and organic/inorganic trace impurities. The sample amount required for analysis is some micrograms, but only between 10−12 and 10−9 g are actually desorbed, depending mainly upon the laser adjustment and the kind of metal which is under investigation. While direct heating of the FD emitter leads to destruction between 1,300 and 1,400° C, the use of the laser for indirect heating extends the applicability well above 3,000° C and opens up the field of high-temperature chemistry.