Spectrophotometers designs

There was more. A strain gauge on each mask would indicate the airflow, and a small tube would divert a sample of the aerosol to the spectrophotometers, designed to calculate cumulative dosage in real time. Airtight Plexiglas windows would provide visual access to the volunteers. Auditory input would feed into each soldier s earpiece from a microphone in the booth. 

Any advanced absorbance/fluorescence spectrophotometer designed for routine acquistion of absorption or emission on the subsecond time scale. The basic goal is to obtain a series of complete UV/visible or fluorescence spectra as a function of time, usually after samples are mixed in a stopped-flow device. Such data help the investigator to infer the most likely structures of transient intermediates whose electronic spectra or fluorescence spectra can be determined by deconvoluting the spectra with appropriate reaction kinetic simulation software or by some other global analysis method (Fig. 1). 

Insufficient resolution leads to a decrease in the extinction coefficient across the wavelength axis, and therefore inaccurate quantitation results. The sensitivity of the measurement is also compromised. From a qualitative point of view, the fine features in the spectrum may be lost. The resolution of a UV-Vis spectrophotometer is related to its spectral bandwidth (SBW). The smaller the spectral bandwidth, the finer the resolution. The SBW depends on the slit width and the dispersive power of the monochrometer. Typically, only spectrophotometers designed for high-resolution work have a variable slit width. Spectrophotometers for routine analysis usually have a fixed slit width. For diode array instruments, the resolution also depends on the number of diodes in the array. 

The rate at which liquid crystalline bile salt monoolein aggregates were dispersed by bile salt solutions was determined by Quentin Gibson (Cornell University, Ithaca, N. Y.) using a rapid mixing stop-flow spectrophotometer designed in his laboratory (4). 

Once the sample has been introduced into the emission source, atomized, and excited, the emitted photons are diffracted by an optical system consisting of slits, mirrors, and gratings, which focus the spectral lines onto a detector. This section discusses the types of gratings and spectrophotometer designs that can be used in AES. 

In addition to geometric requirements, AS/NZ 4399 describes the effect of fluorescent samples on diffuse transmittance measurements for two types of spectrophotometer designs. The discussion is specific to whether the instrument monochromator is placed before or after the sample in other words, whether the sample illumination is monochromatic or polychromatic. 

The second type of spectrophotometer design uses polychromatic illumination of the sample and monochromatic detection. Placing the monochromator after the sample separates both A, and Af before they are detected. The division of wavelengths can be distributed spatially as depicted in Fig. 13. 

This paper is not hardware-oriented and will treat this topic superficially. Modern spectrophotometer designs tend to change rapidly and the only way to keep abreast of developments is to maintain contact with the various manufacturers and attend instrument expositions occasionally. Table II is a representative list of present-day recording, doublebeam, ultraviolet, visible, and near-infrared spectrophotometers. It is not inclusive. The specifications are from the manufacturer s sales literature. Their claims as to photometric and wavelength accuracy and reproducibility are not included since we feel that they may not all be on equivalent bases. 

N. L. Alpert, Infrared Filter Grating Spectrophotometers—Design and Properties, Appl. Opt. 1 437 (1962). 

Atomic absorption spectrophotometers (Figure 10.37) are designed using either the single-beam or double-beam optics described earlier for molecular absorption spectrophotometers (see Figures 10.25 and 10.26). There are, however, several important differences that are considered in this section. 

Measurement of Whiteness. The Ciba-Geigy Plastic White Scale is effective in the visual assessment of white effects (79), but the availabihty of this scale is limited. Most evaluations are carried out (ca 1993) by instmmental measurements, utilising the GIF chromaticity coordinates or the Hunter Uniform Color System (see Color). Spectrophotometers and colorimeters designed to measure fluorescent samples must have reversed optics, ie, the sample is illuminated by a polychromatic source and the reflected light passes through the analy2er to the detector. 

Nuclear Magnetic Resonance Spectroscopy. Bmker s database, designed for use with its spectrophotometers, contains 20,000 C-nmr and H-nmr, as weU as a combined nmr-ms database (66). Sadder Laboratories markets a PC-based system that can search its coUection of 30,000 C-nmr spectra by substmcture as weU as by peak assignments and by fiiU spectmm (64). Other databases include one by Varian and a CD-ROM system containing polymer spectra produced by Tsukuba University, Japan. CSEARCH, a system developed at the University of Vieima by Robien, searches a database of almost 16,000 C-nmr. Molecular Design Limited (MDL) has adapted the Robien database to be searched in the MACCS and ISIS graphical display and search environment (63). Projects are under way to link the MDL system with the Sadder Hbrary and its unique search capabiHties. 

Let us examine some batch results. In trials in which 5 mL of a dye solution was added by pipet (with pressure) to 10 mL of water in a 25-mL flask, which was shaken to mix (as determined visually), and the mixed solution was delivered into a 3-mL rectangular cuvette, it was found that = 3-5 s, 2-4 s, and /obs 3-5 s. This is characteristic of conventional batch operation. Simple modifications can reduce this dead time. Reaction vessels designed for photometric titrations - may be useful kinetic tools. For reactions that are followed spectrophotometrically this technique is valuable Make a flat button on the end of a 4-in. length of glass rod. Deliver 3 mL of reaction medium into the rectangular cuvette in the spectrophotometer cell compartment. Transfer 10-100 p.L of a reactant stock solution to the button on the rod. Lower this into the cuvette, mix the solution with a few rapid vertical movements of the rod, and begin recording the dead time will be 3-8 s. A commercial version of the stirrer is available. 

Photoelectric-Colorimetric Method. Although the recording spectrophotometer is, for food work at least, a research tool, another instrument, the Hunter multipurpose reflectometer (4), is available and may prove to be applicable to industrial quality control. (The newer Hunter color and color difference meter which eliminates considerable calculation will probably be even more directly applicable. Another make of reflection meter has recently been made available commercially that uses filters similar to those developed by Hunter and can be used to obtain a similar type of data.) This instrument is not a spectrophotometer, for it does not primarily measure the variation of any property of samples with respect to wave length, but certain colorimetric indexes are calculated from separate readings with amber, blue, and green filters, designated A, B, and G, respectively. The most useful indexes in food color work obtainable with this type of instrument have been G, which gives a... 

The instrumentation for sensors based on absorption measurements can be designed on the traditional spectrophotometers by using a flowthrough cell for automatic sampling with the sensors mounted inside the flow-through cell shown in Fig. 20a.3. For remote optical sensing using optical fibers, the chromophores can be immobilized in reflective... 

Aquatec Modular, semi-or fully automatic operation. Microprocessor controlled. A dedicated instrument designed for water analysis, i.e. dedicated method cassettes for phosphate and chloride, 600-100 samples h Flow through spectrophotometer (400-700nm)... 

FIA star 5010 Modular, semi- or fully automatic operation. May be operated with process controller microprocessor. Can be set up in various combinations with 5017 sampler and superflow software which is designed to run on IBM PC/XT computer 60-180 samples h Dialysis for in-line sample preparation and in-line solvent extraction.Thermostat to speed up reactions. Spectrophotometer (400-700nm) or photometer can be connected to any flow through detector, e.g. UV/visible, inductively coupled plasma, atomic absorption spectrometer and ion-selective electrodes... 

Let us dwell on Figure 6.4 for a moment. The standards and sample solutions are introduced to the instrument in a variety of ways. In the case of a pH meter and other electroanalytical instruments, the tips of one or two probes are immersed in the solution. In the case of an automatic digital Abbe refractometer (Chapter 15), a small quantity of the solution is placed on a prism at the bottom of a sample well inside the instrument. In an ordinary spectrophotometer (Chapters 7 and 8), the solution is held in a round (like a test tube) or square container called a cuvette, which fits in a holder inside the instrument. In an atomic absorption spectrophotometer (Chapter 9), or in instruments utilizing an autosampler, the solution is sucked or aspirated into the instrument from an external container. In a chromatograph (Chapters 12 and 13), the solution is injected into the instrument with the use of a small-volume syringe. Once inside, or otherwise in contact with the instrument, the instrument is designed to act on the solution. We now address the processes that occur inside the instrument in order to produce the electrical signal that is seen at the readout. 

Some spectrophotometers are single-beam instruments, and some are double-beam instruments. In a double-beam instrument the light beam emerging from the monochromator is split into two beams at some point between the monochromator and the detector. The double-beam design provides certain advantages that we will discuss shortly. 

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