The Fixed Wavelength UV Detector

There are two common types of UV detector, the fixed wavelength detector and the variable wavelength detector, both of which are commercially available. The two types differ greatly in design and function and therefore will be described separately. 

The difference signal is then fed to an appropriate amplifier to render the output linearly related to solute concentration and then to a recorder. 

One of the first effective small volume UV detectors suitable for use with high efficiency columns was that described by Horvath and Lipsky in 1966 (1). In 1968 Kirkland (2) described in detail another small volume detector which subsequently, in a slightly modified form, provided the basic design for the first DuPont UV detector. The cell was cylindrical in shape, 1 mm in diameter and had a path length of 1 cm giving a total cell volume 7.5 iil. Kirkland also used the 254 nm UV light emitted from a low-pressure mercury vapor lamp as the light source and claimed a noise level of + 0.0002 absorbance at a full scale sensitivity of 0.01 

Column Mobile Phase Flow Rate Detection  

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

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... 

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. 

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. 

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 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... 

Classical LC detectors (refractive index, fixed wavelength UV absorbance at 254 or 280 nm) have lacked the sensitivity to allow direct analysis of cannabi-noids in biological fluids. However, recent development of variable wavelength absorbance detectors extending into the 195-220nm UV region and of fluorescence detectors for HPLC led the authors to initiate... 

Clove oil may be obtained from cloves by steam distillation. A number of other spices may be used for extracting other oils (9,10) whose structures are shown in Table 16-1. The essential oils are rich in one main compound and since all components are UV absorbing, the fixed wavelength detector at 254 nm is adequate for detection. Liquid chromatography is a rapid analytical tool in analyzing the essential oils. 

Fixed and variable wavelength UV detectors. This is probably the HPLC detector found in most labs. Configurations vary from fixed wavelength (commonly 254 nm) to multi-wavelength and stop run scan-... 

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