Instant Notes: Analytical Chemistry

versatile as they allow monitoring at any wavelength within the working

range of the detector to give the optimum response for each solute. They

employ deuterium and tungsten lamp sources for the UV and visible

regions, respectively, a diffraction grating monochromator for wave-

length selection and a photomultiplier detector. Many are computer-

controlled for programmable wavelength switching during a separation

to optimize sensitivity and selectivity.

(iii)Photodiode-array detectorsare spectrometers with fixed optics and a

detection system consisting of one or two arraysof photodiodeson a

silicon chip positioned to receive radiation dispersed by a diffraction

grating (Fig. 6). Electronic scanning, digitizing and processing of the

signals by a microcomputer enables ‘snapshots’ of the complete spec-

trum of the flowing eluent to be collected and stored every 0.1 s. The

spectra and the developing chromatogram at any wavelength can be

displayed on a VDU screen in real time and subsequently shown as a

3-D color plot of absorbance, wavelength and time (Fig. 7). The data can

be manipulated and re-plotted on the screen, and comparisons made

with library spectra for identification purposes.

D6 – HPLC: principles and instrumentation 163

Mirror

Reference

photo diode

Flow cell

Sample

photo diode

Mirror

Entrance slit

Beam splitter

Mirror

Grating

Prealigned lamp

Fig. 5. UV-visible variable-wavelength spectrophotometric detector.

Concave

holographic grating

Slit

Flow cell Aperture D2 lamp

Lens

2 & 3

Shutter Lens 1

Diode array

Spectrum

190–800 nm

Fig. 6. Diode-array detector (DAD).