UV detector fixed wavelength

As a result of its highly polar character, silica gel is particularly useful in the separation of polarizable materials such as the aromatic hydrocarbons and polynuclear aromatics. It is also useful in the separation of weakly polar solute mixtures such as ethers, esters and in some cases, ketones. The mobile phases that are commonly employed with silica gel are the n-paraffins and mixtures of the n-paraffins with methylene dichloride or chloroform. It should be borne in mind that chloroform is opaque to UV light at 254 nm and thus, if a fixed wavelength UV detector is being used, methylene dichloride might be a better choice. Furthermore, chloroform is considered toxic and requires special methods of waste disposal. Silica gel is strongly deactivated with water and thus, to ensure stable retentive characteristics, the solvent used for the mobile phase should either be completely dry or have a controlled amount of water present. The level of water in the solvent that will have significant effect on solute retention is extremely small. The solubility of water in n-heptane is... 

There are two types of UV detector the fixed wavelength detector and the multi-wavelength detector. A diagram of a fixed wavelength UV detector is shown in figure 5. 

Distinguish between the fixed-wavelength UV detector and the variable-wavelength UV detector in terms of design and use. 

The use of a fixed wavelength UV detector for liquid chromatographic separations was first described by Horvath and Lipsky in 1966 [1], and is possibly the most popular HPLC detector in general use today. Although other detection techniques are more sensitive, the UV detector provides a simple and universal answer to the majority of HPLC applications [2]. Developed in 1982, the diode array UV detector measures the full absorption spectrum of each analyte peak, and was a... 

The sensitivity of the UV absorption function is about 1.7 x 10" g/ml for toluene with a linear dynamic range of about 1.5 x lo. These specifications compare well with those expected for a fixed wavelength UV detector. The fluorescence function provides a sensitivity of about... 

Most commercial fixed-wavelength UV detectors take advantage of the intense line source of 254-nm radiation in the low-pressure mercury arc lamp. The high intensity of the radiation provides excellent detectability for the small-aperture microvolume flow cells required in HPLC. Concentration of most of the radiation in a narrow-wavelength band places less demand on optical filters and enhances the linear range of the detector. 

In commercial fixed wavelength UV detectors, Xmax is often set at 254 nm. This specific wavelength will provide a wide range of applications, giving strong absorption bands for biologically important compounds such as amino acids, proteins, enzymes and nucleic acid constituents. 

A diagram of a fixed wavelength UV detector is shown in figure 7.7. 

Figure 2.7 Schematic of fixed wavelength UV detector. By permission of Millipore/Waters.

Besides fixed-wavelength UV detectors, which usually operate at 254 nm. modem UV-VIS de-... 

Diode array detector (DAD) DAD is used to measure molecular absorption of at a certain ultraviolet-visible (UV/Vis) wavelength. The amount of light absorbed will depend on the amount of a particular compound that is passing through the beam at the time and the absorbance of that compound at a particular UV wavelength. The difference between a DAD and a fixed wavelength UV detector is that a DAD can scan across a predetermined wavelength. 

The two other detectors are based on absorbance. The fixed-wavelength UV detector essentially is a marriage of a flow cell and a UV/VIS spectrophotometer. Absorbance at one wavelength at a time can be detected, but most such detectors can operate at more than one wavelength. A photodiode... 

The sensitivity of the UV absorption function was 1.7 x 10 g/ml of toluene with a linear dynamic range of about 1.5 x 10. These specifications compare well with those of the standard fixed wavelength UV detector. The fluorescence function provided a sensitivity of 2.5 x 10 g/ml for dansyl iso-leucine and a linear dynamic range of 1.2 x 10. Finally, the sensitivity of the conductivity function to sodium chloride was shown to be 5 x 10 g/ml with a linear dynamic range of 3 x 10. The response indices were 0.975, 0.95, 1.042 for the UV function, the fluorescence function and the conductivity function respectively. The dispersion of the cell was small, equivalent to a standard deviation of about 2.8 iiU... 

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