Liquid Chromatography Detectors - The UV Detectors > The Fixed Wavelength UV Detector > Page 40
Thus. two detectors, having the same sensitivity defined as the minimum detectable change in absorbence, will not necessarily have the same sensitivity with respect to solute concentration. Only if the path lengths of the two sensors are identical will they also exhibit the same concentration sensitivity. This can cause some confusion as it would be expected that two instruments having the same spectroscopic sensitivity would also have the same chromatographic sensitivity. To compare the sensitivity of two detectors given in units of absorbence the path lengths of the cells in each instrument must be taken into account.
UV detectors can be used with gradient elution providing the solvents do not absorb significantly over the wavelength range that is being used for detection. In reversed phase chromatography, the solvents usually employed are water, methanol, acetonitrile and tetrahydrofuran (THF), all of which are transparent to UV light over the total wavelength range normally used by UV detectors. In normal phase operation more care is necessary in solvent selection as many solvents that might be appropriate as the chromatographic phase system are likely to absorb UV light very strongly. The n-paraffins, methylene dichloride, aliphatic alcohols and THF are useful solvents that are transparent in the UV and can be used with normal distribution systems (e.g. a polar stationary phase such as silica gel).
The Fixed Wavelength UV Detector
The fixed wavelength UV detector uses light of a single wavelength (or nearly so) which is produced by a specific type of discharge lamp. The most popular lamp is the low pressure mercury vapor lamp, which generates most of its light at a wavelength of 254 nm. Other lamps that could be used are the low-pressure cadmium lamp which generates the majority of its light at 225 nm and the low pressure zinc lamp that emits largely at 214 nm. None of the lamps are strictly monochromatic and light of other wavelengths is always present but usually at a significantly lower intensity. The emission spectra of the mercury, cadmium and zinc lamps are shown in figure 24. It is seen that to obtain monochromatic light an appropriate filter would be needed. The low pressure mercury light source (wavelength 253.7 nm) provides the closest to true monochromatic light of all three lamps. However, there is light present of significant intensity below 200 nm, but light of such wavelengths is generally absorbed by the mobile phase.