ABSTRACT
The coupling of liquid chromatography (LC) to mass spectrometry (MS) makes it possible to analyze nonvolatile and thermally labile compounds not amenable to analysis using gas chromatography (GC)-MS. However, the typical LC flow rate of I mVmin corresponds to ca. 1 Vmin of gas when volatilized at STP. In order to be compatible with the ca. 10·5 torr source pressure required for efficient ionization by electron impact (El), LC/MS requires efficient removal of the mobile phase flowing into the interface. In pursuit of this goal, mass spectroscopists heralded the development of the particle beam (PB) interface that selectively enriched analytes and removed the bulk of mobile phase molecules (1). This interface consists of an aerosol generator (nebulizer), a desolvation chamber, an expansion nozzle and a momentum separator (see Figure 1). Liquid droplets containing analyte and LC mobile phase are formed by gas nebulization. In the ideal situation, the droplets are desolvated into particles containing only analyte. The particles thus formed expand supersonically through the nozzle into the two-stage momentum separator where the majority of the lighter He nebulizing gas and mobile phase molecules are removed via the pumping lines. Finally, the high momentum analyte particles enter the mass spectrometer ion source where volatilization and ionization occurs.
