Multiplewavelength Detector

The multiple-wavelength detector requires a source that emits light over a wide range of wavelengths and, with the aid of a monochromator, light of a specific wavelength can be selected for detection purposes. Detector sensitivity can be improved by selecting light of a wavelength at which the solute has its maximum absorption. Alternatively, the emerging solution peak can be scanned over a range of wavelengths, and the absorption spectra of eluted substances can be used for identification purposes.

There are two basic types of multiple-wavelength detector: the dispersion detector and the diode array detector, the latter being the more popular. In fact, there are currently very few dispersion instruments commercially available but as there are many still in use, their characteristics will be discussed.

The two types of multiple-wavelength detectors have important differences. In the dispersive instrument, the light is dispersed before it enters the sensor cell, and thus virtually monochromatic light passes through the cell. However, if the incident light can excite the solute and cause fluorescence at another wavelength, then the light falling on the photocell will contain that incident light that has been transmitted through the cell, together with any fluorescent light that may have been generated in the cell. Consequently, the light monitored by the photocell will not be solely monochromatic and light of another wavelength, if present, could impair the linear nature of the response. In most cases this effect is negligible, but with some substances it may be quite significant. The diode array detector operates in quite a different manner. Light of all wavelengths generated by the deuterium lamp is transmitted through the cell, and the transmitted light is then dispersed over an array of diodes. In this way, absorption at discrete groups of wavelengths is continuously monitored at each diode. However, the light falling on a discrete diode may not be solely that transmitted through the cell from the source, but may contain fluorescence light excited by light of a shorter wavelength. In this case, the situation is exacerbated by the fact that the cell contents are exposed to all the light emitted from the lamp, and so fluorescence is more likely. In general, this means that, under some circumstances, the transmitted light measured may also contain fluorescent light. As a consequence, the absorption spectrum obtained for a given substance may be degraded from the true absorption curve.

The ideal multiple-wavelength detector would be a combination of both the dispersion instrument and the diode array detector. This system would allow a monochromatic light beam to pass through the detector and then the transmitted beam would itself be dispersed again on to a diode array. Only that diode corresponding to the wavelength of the incident light would be used for monitoring the transmission. In this way any fluorescent light would strike other diodes, the true absorption would be measured and, if the incident light is scanned, accurate absorption spectra could be obtained.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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