Instruments single beam

Double-beam AA spectrophotometers are still marketed by instrument manufacturers. A double-beam system compensates for changes in lamp intensity and may require less frequent re-zeroing than a single-beam instrument. These considerations had more merit some years ago when hollow cathode lamps suffered from some instability. It should be noted, however, that the optical... 

In practical situations the absorbance of a sample is determined by making two measurements, the first to determine 70 and the second to determine I. The determination of I0 is used to cancel a large number of experimental factors that could affect the result. When measuring I0 the sample container must closely match the unknown container in all ways except for the analyte content. The cuvettes should be a matched pair if a double beam instrument is used and the same cuvette can be used for both the blank and sample with a single beam instrument. The blank solution filling the cuvette should be identical to the solvent that the sample is dissolved in, except for the sample itself. If done correctly, the least-squares line for the calibration graph will come very close to the 0,0 point on the graph. 

Simple spectrometers that cover the region from 350 to 1000 nm are available for modest cost and are useful for routine analysis. These spectrometers are usually single beam instruments that are set up according to the block diagram in Fig. 5.15, and Fig. 5.16 illustrates the actual configuration of a commercial instrument. 

Fig. 5.16. A schematic diagram of a single beam instrument, the Genesis 2000 produced by Thermo Spectra. Layout of parts for a UV-Vis spectrometer (with permission of Thermo inc.).

Figure 3.1 Schematic diagram of an AAS spectrometer. A is the light source (hollow cathode lamp), B is the beam chopper (see Fig. 3.2), C is the burner, D the monochromator, E the photomultiplier detector, and F the computer for data analysis. In the single beam instrument, the beam from the lamp is modulated by the beam chopper (to reduce noise) and passes directly through the flame (solid light path). In a double beam instrument the beam chopper is angled and the rear surface reflective, so that part of the beam is passed along the reference beam path (dashed line), and is then recombined with the sample beam by a half-silvered mirror.

Figure 3.2 Beam chopper in AAS. In a single beam instrument it is mounted vertically off-centre, so that it chops the beam. In a dual-beam instrument it is angled and mirrored so that it alternately allows the sample beam through and reflects the reference beam along the secondary path.

Like much instrumentation working in the IR/visible/UV region of the spectrum, most modern UV/visible spectrometers are of the dual-beam type, since this eliminates fluctuations in the radiation source. The principle of this has been described in detail in Section 3.2. Radiation from the source is split into two by a beam splitter, and one beam is passed through the sample cell (as in a single beam instrument). The other beam passes through a reference cell, which is identical to the sample cell, but contains none of the analyte... 

The single beam instrument measures directly the amount of energy transmitted by the sample. They give the most accurate transmittance measurements and is particularly helpful for quantitative analysis. It has simpler and more reliable systems than double beam. 

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. 

What problem exists if you want to use a single-beam instrument for precise work, even if it has a rapid scanning capability ... 

What are the advantages of a double-beam instrument over a single-beam instrument ... 

Rapid-scanning single-beam instruments exist. The absorption spectrum of the blank is first obtained, followed by that of the sample. The sample scan is then adjusted to its proper measurement by using the spectrum of the blank. 

A basic photometer (Figure 2.17) is a single-beam instrument in which there is only one light path and the instrument is standardized using a blank solution, which is replaced with the sample to obtain a reading. The major problem with a single-beam design lies in the fact that the absorbance value recorded for the... 

ISO 1600 1990 Plastics - Cellulose acetate - Determination of light absorption on moulded specimens produced using different periods of heating ISO 13468-1 1996 Plastics - Determination of the total luminous transmittance of transparent materials - Part 1 Single-beam instrument ISO 13468-2 1999 Plastics - Determination of the total luminous transmittance of transparent materials - Part 2 Double-beam instrument ISO 14782 1999 Plastics - Determination of haze for transparent materials... 

During in situ UV-vis spectroelectrochemical work, it is easier to obtain spectra by using a single-beam instrument. At the start of the experiment, the analyst sets the absorbance to zero with the in situ cell placed in the path of the beam, so the cell then acts as a spectroscopic blank or ( reference ). Any changes in absorption will relate to the changes in the amounts of each of the redox states within the cell, rather than from the cell itself. 

Fourier transform infrared spectrometers first appeared in the 1970s. These single beam instruments, which differ from scanning spectrometers, have an interferometer of the Michelson type placed between the source and the sample, replacing the monochromator (Figs 10.9c and 10.11). 

Double beam spectrophotometers allow differential measurements to be made between the sample and the analytical blank. They are preferable to single beam instruments for measurements in problematic solutions. For high performance instruments, the bandwidth can be as low as 0.01 nm. 

Figure 12.9—OpticaI scheme of a spectrofluorimeter having two detectors, one of which is used to control the intensity of the light source. A fraction of the incident beam is reflected by the beam splitter and monitored by a photodiode to control the intensity of the incident beam. Comparison of the signals obtained from both detectors allows the elimination of any drift in the light source. This procedure, for single beam instruments, gives approximately the same stability as with double beam instruments. (Model F4500 reproduced by permission of Shimadzu.)...

The optical scheme of an atomic absorption spectrophotometer is illustrated in Fig. 14.4, which shows a basic single beam instrument. 

A single-beam spectrophotometer is inconvenient because the sample and reference must be placed alternately in the beam. For measurements at multiple wavelengths, the reference must be run at each wavelength. A single-beam instrument is poorly suited to measuring... 

Sample chambers for spectrometers come in two varieties—those holding only one cuvette at a time (single-beam) and those holding two cuvettes, one for a reference, usually solvent, and one for sample (double-beam). In a double-beam instrument, the sample spectrum is continuously corrected by subtraction of the reference spectrum. In the past, single-beam instruments were usually less expensive but more cumbersome to use because reference and sample cuvettes required constant exchange. However, modern singlebeam instruments with computer control and analysis can be programmed to correct automatically for the reference spectrum, which may be stored in a memory file. The use of both types of instruments is outlined in the applications section. 

For fixed-wavelength measurements with a single-beam instrument, a cuvette containing solvent only is placed in the sample beam and the instrument is adjusted to read zero absorbance. A matched cuvette containing sample plus solvent is then placed in the sample chamber and the absorbance is read directly from the display. The adjustment to zero absorbance with only solvent in the sample chamber allows the operator to obtain a direct reading of absorbance for the sample. 

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