Absorption detectors single wavelength

Detector Technology. For copolymer composition analysis the new diode array UV/vis detectors are extremely attractive the absorption at many wavelengths are instantaneously recorded there is only a single spectrophotometer cell so that transport time delays between detectors and axial mixing in detector cells do not confound comparison of detector response at different wavelengths and for styrene copolymers, extremely low concentrations of polymer can be detected. 

In addition to the continuum sources just discussed, line sources are also important for use in the UV/visible region. Low-pressure mercury arc lamps are very common sources that are used in liquid chromatography detectors. The dominant line emitted by these sources is the 253.7-nm Hg line. Hollow-cathode lamps are also common line sources that are specifically used for atomic absorption spectroscopy, as discussed in Chapter 28. Lasers (see Feature 25-1) have also been used in molecular and atomic spectroscopy, both for single-wavelength and for scanning applications. Tunable dye lasers can be scanned over wavelength ranges of several hundred nanometers when more than one dye is used. 

Over the past decade, there has been considerable development in imaging type detectors for the measurement of ultraviolet (UV) and visible light. These new detectors have attracted the interest of a number of analytical spectroscopists. For absorption spectroscopy, analytical chemists have traditionally used such instruments as the photometer, which uses a narrow-band light source (for example the 254 nm emission line from a low pressure Hg lamp or a continuous source with a filter), a sample cell and a photomultiplier tube (FMT) as the detector. While useful for many specific applications, the single-wavelength photometer cannot determine multiple sample components simultaneously or provide a general absorbance characterization of the system. When information at multiple wavelengths is desired,... 

In general, when the absorbance is to be measured at a single wavelength, the absorption maximum is chosen. In order that the absorbance reflects the concentration, it is required that the spectral band A A reaching the detector and preselected by the instrument parameter called bandwidth, will be much narrower (10 times) than the width of the absorption band measured at mid-height (see wavelength calibration in Section 9.8.3). 

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