Computer-based spectrophotometers

Figure 16-1 shows a typical output from a commercial IR spectrophotometer. Although the y-axis is shown as linear in transmittance, modern computer-based spectrophotometers can also produce spectra that are linear in absorbance. The abscissa in this spectrum is linear in wavenumbers with units of reciprocal centimeters. A wavelength scale is also shown at the top of the plot. Computer-based spectrophotometers can also produce a variety of other spectral formats such as linear in wavelength, baseline corrected, and derivative and smoothed spectra,... 

For reference, a spectrophotometer/colorimeter with direct concentration readout costs on the order of 2000 a digital Na/K flame photometer with an automatic sample-diluter, about 4000 a flexible, computer-controlled, single-channel analyzer under 25,000 a computer-based parallel-fast analyzer, about 50,000 and a multi-channel analyzer from about 80,000 to well over 200,000. 

These classical-type cuvette-based studies are still in widespread use today. Spectrophotometers are both common in the biochemical laboratories and easy to use. Spectral resolution is excellent (typically <1 nm) and the data easily collected and analyzed with modern computer-based systems. One major drawback of these instruments is that the observed spectrum is a weighted average of all... 

The reaction of Ru(III) chelate with diimine is about 99 times more efficient than that of Ru(III) with hydrazine. Computer-simulated chemiluminescence time curves based on the kinetic data of the above reaction scheme exactly matched light-intensity time curves recorded in a stopped-flow spectrophotometer 166h At high hydrazine concentra-... 

Although standard IR spectrometers are used for studying the amide bands, FTIR spectrometers are more accurate and reliable. FT-IR spectrophotometers are based upon the Michelson interferometer. A typical instrument (Fig. 7.1) comprises an optical bench housing the interferometer, sample, infrared source and detector, coupled to a computer, which controls the spectral scanning, analysis and data processing (for review see Griffiths, 1980). 

Although a computer cannot yet be considered as a normal part of laboratory equipment, most of the toxicological laboratories in Europe and North America are fully equipped for instrumental work. Many hospital laboratories on the other hand do not possess anything more sophisticated than a UV spectrophotometer, while in Asia and Africa, where the majority of cases of poisoning by alkaloids occur, only the simplest equipment is available. In order to be of the widest use, the scheme given below is based on paper and thin-layer chromatography with the help of UV spectrophotometry if it is available. 

The AD-interface connecting the analyser and the data unit is an 8-channel serial AD-interface with 12-bit resolution. The ADC is based on a Linear Technology chip (LTC1290) and requires only a few additional components (a print board, complete with all components can be purchased for about 50 US from Conrad Electronic, D-92240 Hirschau). This interface is connected to a maximum of 8 spectrophotometer analogue outputs (or other instruments, e.g., an attached conductivity sensor) and to a serial port of the PC. One of the analogue inputs reads the sampler status. A serial ADC allows longer distance between wet analysis and dry data treatment (about 15 m). If parallel ADC interface cards mounted in the computer are used, the wet-analyser and computer have to be close together (less than 5 m). No additional hardware is required. 

Many mixlern photometers and spectrophotometers are based on a double-beam design. Figure 13-13b illustrates a double-beam-in-space instrument in which two beams are formed in space by a V-shape mirror called a beamspHuer. One beam passes through the reference solution to a pholodetector, and the second simultaneously traverses the sample to a second, matched detector. The two outputs are amplified, and their ratio (or the logarithm of their ratio) is determined electronically or by a computer and displayed by the readout device. With manual instruments, the measurement is a two-step operation involving first the zero adjustment with a shutter in place between selector and... 

In recent years, a new generation of colorimeters based on spectrophotometers have been developed. The spectrophotometric colorimeter does not mimic the human eye. Instead, it makes a spectrophotometric measurement at sixteen 20-nm intervals over the entire range of the visible spectrum. The percentage reflectance value obtainedby the spectrophotometer is converted into tristimulus values through the use of a microprocessor. One other useful feature added to these spectrophotometric colorimeters is the ability to select various types of CIE illuminants. Even though the actual light source remains the same, the microprocessor computes the colors that would be seen if the samples were viewed under various illuminants. 

There have been many new developments in color measurement systems technology in recent years. A major breakthrough is in the area of portable color measurement techniques. Newly developed portable spectrophotometers (Figure 6-13h) now make it possible to measure and analyze data on the production floor. Some portable spectrophotometers are also capable of displaying an actual spectral reflectance curve. Bench-top spectrophotometers have been updated to allow more computer control of the lens, UV filter, and specular port. The ability to calibrate the UV component of the color spectrum is an important feature for measuring fluorescent colors accurately. Advances in windows-based software has improved the capabilities for statistical analysis of color measurements, color corrections, and color formulation. 

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