Page 273 - Tandem Techniques

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conductivity detector. Using the diode array detector, and choosing the optimum wavelength to
reconstruct the chromatogram, the high chromatographic selectivity of the reversed phase can be
exploited. It should be said that, in the past, the multi-wavelength dispersive detector has also proved
extremely useful for this purpose. It has provided adequate sensitivity, good versatility, a linear
response and a resolution that is not limited by diode content. It was found, however, to be somewhat
bulky (due to the need for a relatively large internal 'optical bench'). Furthermore, it has a mechanically
operated wavelength selector and requires a stop/flow procedure to obtain spectra 'on-the-fly'. More
important, the dispersive instrument can only provide absorption data at one specific wavelength in the
continuous monitoring mode. Consequently, if the separation is to be monitored at another wavelength
the separation must be repeated. The diode array detector has the same advantages as the dispersive
instrument, but none of the disadvantages and provides multi-wavelength detection in a single
chromatographic separation.

The manner in which the resolution of the diode array detector is restricted can be examined as follows.
The resolution of the diode array detector (Dl), will depend on the number of diodes (n) in the array,
and also on the range of wavelengths covered (l2 - l1).

Thus

It is seen that the ultimate resolving power of the diode array detector will depend on the semiconductor
manufacturer, and on how narrow the individual photo cells can be commercially fabricated. Most
instruments have a data processing package that will permits the separation to be monitored in real time
by at least one diode, so that the chromatogram can be followed as the separation develops. By noting
the elution time of a particular peak, a spectrum of the solute can be obtained by recalling the output of
all the diodes from memory, at that particular time. This gives directly the spectrum of the solute, i.e. a
curve relating adsorption and wavelength. The liquid chromatograph/UV spectrometer tandem system
(the diode array detector or the dispersive UV spectrometer) can be used