Mass Spectrometry Of Nonvolatile Compounds By Desorption From Extended Probes

The technique has been called “direct Cl,” “in-beam,” “direct exposure,” “desorption Cl,” and “surface ionization,” and mass spectroscopists have been using variations of the technique ever since McLafferty and Baldwin (1) introduced it in 1973. The idea is very simple: Nonvolatile samples are coated on a surface and inserted directly inside an ion source by means of some extended direct insertion probe-an idea that can actually be traced back to R. I. Reed (2) and others who reported improvements in their spectra when the sample was placed inside of the source. This replaces the usual practice of vaporizing the sample outside of the source, which generally results in pyrolysis, and it gives a much better chance of observing molecular ions in the mass spectra. (See Figure 1.) Since its introduction, investigators have examined many compounds on a number of different surfaces, examined heating rates (fast and slow), coated samples onto filaments or activated emitters (borrowed from field desorption sources), and argued about whether field desorption itself really needs the field. It has contributed to our understanding of other desorption methods, such as field desorption (FD), plasma desorption, and laser desorption, and demanded some revision of our idea of volatility, under conditions in which sample-surface interactions now become more important than the sample-sample interactions that normally govern the behavior of the vaporization process.