Ion Chromatography - Ion Chromatography - Spectroscopy Systems



Ion Chromatography - Spectroscopy Systems.


An instrument that combines a separation technique with a spectrometer is probably the most powerful tool that the modern analyst has available. The first tandem system to be developed was the gas chromatograph with the mass spectrometer and later the gas chromatograph with the IR spectrometer. These were a natural first development, as interfacing the eluent from a gas chromatograph with a spectrometer was a relatively simple problem. The GC eluent carrying the solute vapour could be passed directly into the spectrometer using a Bieman or Ryhage concentrator when necessary. The combination of the liquid chromatograph with a spectrometer, however, was a far more difficult problem. The mobile phase had to be removed or at least reduced to a level where it did not interfere with the function of the mass spectrometer. The first interface was developed by McLafferty, which was a direct inlet system that had very limited application in its original form but, nevertheless, was the first successful interface to be practically useful. There are now a number of interfaces available, some based on the direct inlet system others on the transport system and others employing more ingenious devices. In the LC/MS system developed by McLafferty a small fraction of the column eluent was split from the main stream, and passed directly into the source of a high-resolution mass spectrometer by means of capillary tube. McLafferty avoided the problem of solvent interference in the mass spectrometer by choosing a solvent that would act as a chemical ionization reagent. Unfortunately this could not be used generally as it restricted the choice of mobile phase and made the system almost impossible to use with an ion chromatograph.


There are, nevertheless, a number of different LC/spectrometer interfaces that have been used very successfully with the ion chromatograph and some of these will now be discussed..


Direct Inlet LC/MS Interface


In a similar manner to McLafferty's technique, with his direct inlet interface, a portion of the eluent can be injected directly into the ion source of a mass spectrometer via a suitable restriction and with no prior concentration. A diagram of the basic direct inlet interface for a mass spectrometer is shown in figure 39.


The restriction that limits the flow of mobile phase into the mass spectrometer ion source consists of an orifice 2-5mm in diameter through which the column eluent is forced. The resulting liquid jet rapidly breaks up into fine spray. The droplets are then vaporized in a heated chamber, and the sample vapour and solvent vapour then enter the ionization chamber. The tip of the jet can be cooled to prevent premature evaporation of the drops of eluent. The spectrometer pumping system normally cannot cope with the vapour evolved from much more that a few microliters of liquid, consequently, the column eluent must be split and, thus, the relative flow rates are adjusted by a down-stream needle valve.


Figure 39. The Direct Inlet LC/MS Interface


In general, (as found by McLaffery) the interface needs to be employed with a chemical ionization interface using the solvent vapour as the reagent gas. The disadvantage of this type of interface is that the small orifice can easily become blocked. This direct inlet interface is best used for solutes that have a reasonable vapour pressure at the source temperature. The interface as described would not function well for an ion chromatograph spectrometer tandem system.