Double-beam UV/VIS

Infrared instruments using a monochromator for wavelength selection are constructed using double-beam optics similar to that shown in Figure 10.26. Doublebeam optics are preferred over single-beam optics because the sources and detectors for infrared radiation are less stable than that for UV/Vis radiation. In addition, it is easier to correct for the absorption of infrared radiation by atmospheric CO2 and 1420 vapor when using double-beam optics. Resolutions of 1-3 cm are typical for most instruments. 

The UV absorption spectra of niclosamide in methanol, methanolic 0.1 N HC1 and methanolic 0.1 N NaOH are shown in Fig. 3. The figures were recorded using a Perkin-Elmer double beam model 550s UV-Vis spectrophotometer. The values of the log emax (log molar extinction coefficient) at their corresponding wavelength maximum in nanometers (2max) are shown in the following table. 

Let us begin with the instrumentation. Dispersive IR instruments, similar to the double-beam instruments described for UV-VIS spectrophotometry, have been used in the past but have become all but obsolete. While some laboratories may still use these instruments, we will not discuss them here. 

The above two modifications have been duly incorporated in a double-beam spectrophotometer. Figure 21.6, depicts the schematic diagram of the optical system involved in a Lambda-2 microcomputer-controlled UV-VIS Spectrophotometer (Perkin-Elmer). 

Conventional scanning UV-vis spectrophotometers come in two confignrations single beam and double beam. In general, scanning UV-vis spectrophotometers offer snperior stray light performance to an... 

Spectrometer type and manufacturer Super Scan 3 (Varian) UV-VIS-spectrophotometer, double beam ... 

Figure 10.9—Schematic diagram of various infrared spectrometers, a) Single beam model its principle is still used for measurements at a single wavelength b) double beam model c) single beam Fourier transform instrument. Contrary to UV/VIS spectrometers, the sample is placed immediately after the light source. Since photon energy in this range is insufficient to break chemical bonds and degrade the sample, it can be permanently exposed to the full radiation of the source.

N, adsorption for determination of pore size distribution(BJH method) using a Micrometries ASAP 2000 automatic analyzer. FTIR spectra were recorded on a PerkinElmer 221 spectrometer and UV-vis spectroscopic measurments were carried out using Varian CARY 3E double beam spectrometer in the range of 190 - 820 nm. 

All UV/vis spectra were recorded on a Varian Cary 50 Scan spectrophotometer and a PERKIN ELMER UV/vis Spectrometer Lambda 2 (double beam) in solution. Absorption maxima Xmax are given in nm. 0.4 cm quartz cuvettes were used for all measurements. 

A more sophisticated instrument is the Farrand VIS-UV Chromatogram Analyser CFigure 3) that can also be used to measure the fluorescence spectra as well as to obtain reliable quantitative data. This particular instrument is equipped with a Xenon lamp that emits a continuous spectrum of wavelengths. It has two monochromators and two filters, for both the excitation and emission modes and can be operated either in double-beam ratio or single beam. 

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