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solute that would provide a signal to noise ratio of 2, was about 16 ng. However, it would appear from
the spectra on the right that to obtain a spectrum that has sufficient information for sample
identification, the mass of solute injected must be greater than 31 ng.

Solvent elimination from transport systems, such as those previously described, has proved to be
relatively easy with non-aqueous mobile phases. However, the majority of LC separations are carried
out employing reverse phase columns. Such columns require the use of mobile phases with high water
contents, and such mixtures do not evaporate easily. Poor volatility causes the solute deposits to be
smeared into one another, which can seriously impair the separation. In addition, the presence of water
in the mobile phase will also restrict the choice of the transport medium, as it must not be soluble, or
affected by water. Another problem associated with the deposition of the solute on the transport
medium is the need to keep the area of deposition as small as possible, which is essential to prevent one
spot merging into its neighbor.
Lange and Griffiths [10] extended the development of the LC/FTIR interface using concentric flow
nebulization to improve the deposition of the solute, and to focus the solute onto a narrow spot. The
different types of nebulizers they examined are depicted in Figure 8.6. The first design was merely an
emulation of the thermospray employed as an interface in LC/MS. Although satisfactory for interfacing
with a mass spectrometer, the thermal spray jet gave a diffuse distribution of eluent, and spots of
sufficiently small diameter could not be obtained. The second method, termed by the authors
hydrodynamic focusing, employed a concentric gas flow which certainly helped reduce the size of the
spot due to the Bernoulli effect. However, when the diameter of the jet becomes too small, the jet
breaks up again and a diffuse deposition occurs. To aid in the evaporation, and to reduce the jet
diameter still further, the jet was heated as in the thermospray method. For optimum performance, the
inner jet should protrude about I mm beyond the outer tube carrying the nebulizing gas. This provides
the required narrow jet and small spot diameter; this form of deposition was termed concentric flow
nebulization.