Fixedwavelength Detector

The fixed-wavelength detector functions with light of a single wavelength (or nearly so) generated by a specific type of discharge lamp. The most popular lamp for this purpose is the low pressure mercury vapour lamp, which generates most of its light at 254 nm. There are other lamps that are used with fixed-wavelength UV detectors: the low pressure cadmium lamp which generates light predominantly at 225 nm and the low pressure zinc lamp that emits light at 214 nm. None of the lamps are monochromatic and light of other wavelengths is always present but usually at a significantly lower intensity. The low pressure mercury light source (actual maximum emission wavelength 253.7nm) is the lamp which is most frequently used in the fixed-wavelength detector.

The typical optical system of a fixed-wavelength UV detector is shown in Figure 2. Light from the UV source is collimated by a suitable lens through the sample cell and the reference cell, and then on to two photocells. The cells are cylindrical, with quartz windows at either end. The reference cell compensates for any absorption that the mobile phase might have at the sensing wavelength. The outputs from the two photocells are passed to a signal-modifying amplifier so that the output is linearly related to the concentration of solute being detected.

For reasons already discussed, modern sensor cells have angular conduits that form a Z shape to reduce dispersion. The UV detector can be fairly sensitive le

Figure 2 A fixed-wavelength UV detector.

to both flow rate and pressure changes but this instability can be greatly reduced if the sensor is well thermostatted. The fixed wavelength UV detector was once one of the most commonly used LC detectors; it is sensitive, linear and relatively inexpensive. However, today the diode array detector is the most popular, despite its much higher price. The sensitivity (minimum detectable concentration) of the fixed-wavelength detector to compounds with favourable detection properties is about 5 x 10"8 gmL"1, with a linear dynamic range of about three orders of magnitude.

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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