Page 345 - Tandem Techniques

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Chapter 9
Liquid Chromatography/Mass Spectroscopy (LC/MS) Tandem Systems

Tandem instruments involving the combination of the liquid chromatograph and the mass spectrometer
are generally far more complicated than their GC/MS counterparts. There are two reasons for this; first
the substances separated by liquid chromatography are, in the main, relatively involatile; second, as a
result of their poor volatility, the simple electron impact and chemical ionization sources are no longer
adequate. This situation has evoked the development of a number of unique ionization procedures and
interfaces, exclusively for use with LC/MS tandem instruments.

The problem of ionizing involatile materials, however, is not restricted to the successful use of the mass
spectrometer with the liquid chromatograph, the difficulty also arises with the normal use of the mass
spectrometer. As the temperature of a sample is raised its volatility increases, but so does the rate of
thermal decomposition. Small molecular weight substances, and those of low polarity, can usually be
rendered sufficiently volatile by merely raising the temperature. Strongly polar and high molecular
weight materials, on the other hand, start to thermally decompose, before the parent material has
sufficient vapor pressure to provide ions by electron impact or chemical ionization. It follows that
alternative methods of ionization are necessary, in order to obtain mass spectra of such compounds. A
number of these alternative ionization methods can be easily incorporated into an interface that will
operate with a liquid