Ion Chromatography - The Thermospray Interface 2




Figure 40. The Thermospray LC/MS Interface


Using a thermal spray system with a 0.1M ammonia acetate buffer, and a mobile phase that contained a high proportion of water particularly high sensitivities were achieved (an example of a mobile phase that might well be used in an ion chromatograph). The optimum interface temperature depended on the mobile phase composition, which could be determined by employing the maximum solvent buffer ion intensities. The spectra obtained from thermospray ionization are similar to those produced by chemical ionization when ammonia is employed as the chemical ionization reagent. The ions seem to be formed by protenation, resulting in ammonium addition and proton-bound solvent molecular clusters. The thermospray ionizing process seems to be very soft and produces very few fragment ions.


Blakely and Vestal (2) used a thermospray interface in tandem with a quadrapole mass spectrometer, and demonstrated that at flow rates up to 2ml/min. it could provide stable vaporization and ionization with aqueous mobile phases. This is important for the system to be used with ion chromatography. If the mobile phase contained a significant amount of ions (ca 10-4 to 1.0M), no extra thermal ionization source is necessary to allow detection of many non-volatile solutes at the sub-microgram level. It was also shown that when using weakly ionized mobile phases, an additional conventional electron beam is necessary to provide gas-phase reagent ions for chemical ionization to be successful.


A more recent form of thermospray interface, the Vestec Model 201 used by Via and Taylor (3), is shown in figure 41. This interface could handle flow rates of up to 1.5ml/min but two oil diffusion pumps backed by a single mechanical pump was required to hold the necessary vacuum. The two pumps were used to differentially exhaust the vacuum manifold, the source, and the analyzer regions of a quadrapole mass spectrometer. A further two diffusion pumps (backed by a another single mechanical pump) were also coupled directly to the source, opposite the sample inlet.