Page 342 - Tandem Techniques

Page 326

operating procedures that were entailed. However, the problem of sensitivity appears to have been
solved by employing the FTIR instrument with the right type of interface. As already stated, as a result
of the extensive research that been carried out on the development of interfaces for LC/FTIR tandem
instruments, the use of the technique is likely to increase significantly in the future.

Synopsis

Initially LC/IR was carried out off-line, fractions of the eluent being collected, and the IR spectra being
taken in a bromide disk, or Nujol mull. The advent of the FTIR instrument improved the IR sensitivity,
and as the spectra could be stored, background subtraction was now possible. However, this procedure
gave a very low signal to noise, and thus poor overall sensitivity. Transport interfaces were the first type
to be developed which were, in effect, fraction collecting devices, that subsequently took the collected
sample to the IR measuring system to obtain a spectrum. The transport concept evolved through a
potassium bromide plate, a disk with a metallic reflective surface, to a IR transparent disk made of
germanium. A number of different nebulizing jets were also developed, to deposit the solute on the disk
more efficiently, including a supersonic nebulizer. Transmission and/or reflection spectra were taken
by an FTIR spectrometer, and sensitivities of 50-100 ng per spectra were obtained using these methods.
Flow-through cells were developed by drilling holes in alkali metal halide crystals, but were found to
have very limited sensitivity. Membrane extraction techniques were also investigated, which although
cleverly devised and carefully constructed, could not match the sensitivity obtained from the disk
transport system. A recent interesting innovation involved the use of a Raman spectrometer to monitor
LC column eluents. The eluent from the column was mixed with a silver dispersion which strongly
enhances the Raman signal, and the mixture then passed through a cell illuminated by a laser beam. The
scattered light was then focused onto a monochromator and a spectrum obtained in the usual manner.
This device is relatively new, and the actual sensitivities that were obtained are difficult to determine
from the literature, but without doubt this concept holds exciting prospects for the future.