Diode array/rapid scan spectrophotometers

These methods of obtaining complete ultraviolet-visible spectra can be very time-consuming. However, rapid scanning spectrophotometers based on optical diode array detectors are now eormnereially available and are capable of recording complete spectra in very short time intervals (1 second or less). They can therefore be used to obtain spectra of separated components without stopping eluent flow or to detect at several wavelengths simultaneously. 

Spectral resolution is a measure of the separation between two adjacent wavelengths. The two adjacent peaks are said to be resolved when the minimum absorbance between the two peaks is 80%i or lower of the maximum absorbance. Most double-beam spectrophotometers have resolution below 0.5, typically 0.1. However, many diode array and rapid scan spectrophoto-metric detectors have resolution between 1 and 2 nm. 

Dispersive spectrophotometers described so far scan through a spectrum one wavelength at a time. A diode array spectrophotometer records the entire spectrum at once. The entire spectrum of a compound emerging from a chromatography column can be recorded in a fraction of a second by a photodiode array spectrophotometer. At the heart of rapid spectroscopy is a photodiode array such as the one in Figure 19-11, which contains 1 024 individual semiconductor detector elements (diodes) in a row. 

Especially in UV-VIS spectroscopy, improved diode-array spectrophotometers or other fast spectral devices will be favorable for the fine resolution as well as the enhacement of sensitivity of spectra and other electric signals. By rapid accumulation of many scans, the best method for noise elimination can be employed from a rational point of view, and in the shortest amount of time, without altering the shape of the signals. 

Diode array detection (dispersive Fig. 3) offers the advantage of the absence of moving parts, extending the longevity and reliability of such systems compared to more traditional spectrophotometers. The main advantage of these instruments is the rapidity with which data can be collected (e.g., on the millisecond scale versus the traditional scanning instrument that makes spectral measurements on the seconds to minutes scale). 

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