Densitometer slit

Densitometers use a variety of optical systems. The optics utilizes either a slit at a fixed position with a mechanism to slowly move the spectral plate across the slit, or a slit that moves across the spectral line with the photographic plate in a fixed position. Either technique permits scanning the spectral line. Slits must be sufficiently narrow so that, when centered on the image of the spectral line, no light enters the slit from alongside the line. Some densitometer slits are adjustable for length and width. Others are of fixed width, but a series of fixed slits is available for interchange. 

The factor A2 describes the influences of the profile of the analysis line and the effective measurement slit. For photoelectric measurements, the latter is the exit slit width. r, of the spectrometer. This contribution is relevant only when sjs ff < 2, which is practically the case only for photographic measurements where the line profile is scanned with a very narrow densitometer slit. For photoelectric measurements, A2=sjscff 1, as the exit slit width must be larger than the effective line width because of the thermal and mechanical stability of the system. The detection limit is a relevant figure of merit for an analytical method (Section 21.3.1). It can be written for AES ... 

For linear slit scanning densitometers it is well established that chromatographic resolution and sample -... 

Petrovic, M., M. Kastelan-Macan, K. Lazaric, and S. Babic, Vahdation of thin layer chromatography quantitation with CCD camera and slit-scanning densitometer, / AOACInt. 82 25-39 (1999). 

Quantitative pesticide analyses are performed by measuring the areas of sample and standard zones using a slit-scanning or video densitometer. Calibration curves are prepared for each analyte, and sample amounts are interpolated from the curves. Excellent accuracy and precision can be obtained because samples and standards are chromatographed and measured in parallel under the same conditions on a single TLC plate. 

According to the measuring procedure two kinds of densitometers can be distinguished, slit-beam densitometers and point scanning densitometers. With some of the instruments which are on the market both measuring procedures are possible and can be selected by means of a switch. 

Slit-beam densitometers are widely spread and have been brought onto the market by several producers such as Zeiss, Schoeffel, Camag, Shimadzu and Farrand. Results obtained from these instruments are good and the plates can be measured in a relatively short time. 

In chromatography great care must be taken in achieving uniform spots. Irregular spots cannot be measured precisely by the slit-beam densitometer. 

In order to eliminate the disadvantage, automatic devices have been developed for slit-beam densitometers which can scan a track several times 25,26). Distance and number of scan courses as well as the distance of the sample tracks are entered into the automatic instrument in advance. The highest value derived from scanning a track repeatedly is adopted as measuring value for each spot. This method of scanning is, unfortunately, time consuming. 

Computer-controlled systems were originally developed by Ebel and his team in order to achieve the most optimum positioning of any spot on a sample track in the measuring slit. Furthermore scanning during a standstill of the plate was considered an advantage, since errors which result from the time constants of densitometers and integrators could be avoided 28,29). 

A densitometer is simply a uv/vis spectrophotometer with the optics vertical rather than horizontal and narrow slits to improve resolution. The purpose of a densitometer is to measure the concentration of the separated components. Usually the separated components are stained with a dye the more concentrated the component is the more intense the color developed will be. The strip or sheet is dried possibly stripped from the backing and then placed in the densitometer, which scans across it. The model shown in Figure 27-32, p. 332, is top of the line, and has a resolution of > 40 pm and an absorbance range from 0 to 4 absorbance units and can do this better than the human eye, thus providing more useful information. 

Figure 6.20. Optical arrangement for a slit-scanning densitometer (Camag scanner 3). Identification 1 = lamp selector 2 = entrance lens slit 3 = monochromator entry slit 4 - grating 5 = mirror 6 = slit aperture disk 7 = lens system 8 = mirror 9 = beam splitter 10 = reference photomultiplier 11= TLC plate 12 = measuring photomultiplier and 13 = photodiode for transmission measurements. (From ref. [8] Marcel Dekker)...

The main improvements in quahtative and quantitative detection systems in TLC are centred around the introduction of densitometry. These involve the use of slit scanning densitometers or video or CCD camera (image processing) [130]. 

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