Single-beam spectrometer

In the process of performing a spectrophotometric determination of Ee, an analyst prepares a calibration curve using a single-beam spectrometer, such as a Spec-20. After preparing the calibration curve, the analyst drops the cuvette used for the method blank and the standards. The analyst acquires a new cuvette, measures the absorbance of the sample, and determines the %w/w Ee in the sample. Will the change in cuvette lead to a determinate error in the analysis Explain. 

When using a single-beam spectrometer, I0 is measured when a reagent blank is used to zero the absorbance scale. The value of I is then measured when the sample is inserted into the spectrometer. On the other hand, when using a double-beam instrument both the reagent blank, 70, and the sample, I, are measured continuously and the appropriate ratio is determined electronically. 

The laser hres at f = 0 and causes an increase in absorbance in the sample as a consequence the intensity of light reaching the detector decreases. While laser photolysis systems are normally single-beam spectrometers, in fact they behave as dual-beam instruments. The reference beam is separated from the sample beam in time, rather than space. Thus, the reference signal is acquired before laser excitation and leads to Iq. The absorbance at time t in Figure 18.3 is given by Eq. 1 ... 

The systems so far described have all been single-beam spectrometers. As in molecular spectrometry, a double-beam spectrometer can be designed. This is shown diagrammatically in Fig. 2.13. The light from the source is split into two beams, usually by means of a rotating half-silvered mirror or by a beam splitter (a 50%-transmitting mirror). The second reference beam passes behind the flame and, at a point after the flame, the two beams are recombined. Their ratio is then electronically compared. 

If we have a single-beam spectrometer, we may separately record spectra Bm(x), Um(x), and Z>M(x) and apply Eq. (45) later in the computer. With special rapid-scanning spectrometers this approach may be practical, but... 

Figure 1.6 Configurations of instruments for atomic absorption spectrometry, (a) Single-beam spectrometer (b) double-beam spectrometer.

Figure 10.11—Optical arrangement of a Fourier transform IR spectrometer, a) A 90c Michelson interferometer including the details of the beam splitter (expanded view) b) optical diagram of a single beam spectrometer (based on a Nicolet model). A weak intensity HeNe laser (632.8 nm) is used as an internal standard to measure precisely the position of the moving mirror using an interference method (a simple sinusoidal interferogram caused by the laser is produced within the device). According to the Nyquist theorem, at least two points per period are needed to calculate the wavelength within the given spectrum.

Reference and sample measurements are performed consecutively, and the resultant (sample) spectrum is obtained as the ratio of the two photon fluxes onto the detector. In a single-beam spectrometer, there are no other options in a double-beam spectrometer, the photon fluxes of the sample and reference beam path are compared. When an integrating sphere is used with two ports and a white standard in the reference position, the photon fluxes are comparable to each other, and no problems occur. Note that the ports are part of the sphere and that any material change in the reference or sample position will change the average sphere reflectance pave. The reference measurement should be conducted with exactly the same components (windows) as the sample measurement otherwise, "substitution errors" may occur. 

Infrared, like the UV and visible regions of the spectrum, provides a useful method for detecting solutes in liquid streams. The detector uses the principle of attenuated total reflectance, with a single beam spectrometer being used in conjunction with a tow volume flow cell. The cell is equipped with sodium chloride, calcium fluoride or zinc selenide windows and can be heated to temperatures greater than 100°C. The detector can... 

A single beam spectrometer recently put into operation in the author s laboratory5 is shown in Fig. 1. This spectrometer differs from others used for... 

UV/VIS absorption spectra may also be obtained with single beam instruments. In single beam spectrometers the background and sample spectra are measured one after the other. Since a chopper and reference chamber are not needed, single beam instmments are usually cheaper than double beam instruments. 

Figure 3 Principle of construction of atomic absorption spectrometers. (A) Single-beam spectrometer with electrically modulated lamp radiation (B) double-beam spectrometer with reflection and splitting of the primary radiation by a rotating, partially mirrored quartz disk (chopper). 1 - radiation source, 2 -sample cell (atomizer), 3 - monochromator, 4 - detector, 5 -electronics and readout (by permission of Wiley-VCH from Welz B and Sperling M (1999) Atomic Absorption Spectrometry, 3rd, completely revised edition. Weinheim Wiley-VCH).

Single-beam spectrometers have a single optical channel that is configured to measure either sample or reference channel but not both simultaneously. The resultant spectrum is the ratio of the transmission spectra from sample and reference measurements. In practice the response of the detector mea-... 

Spectrometers can be arranged either as single-beam or as double-beam instruments (Fig. 2.67). hi both cases, the optical properties of the sample are compared to those of a reference. In double-beam spectrometers, the light beam is divided so that one partial beam interacts with the sample, the second one with the reference. The light detector accepts both beams in parallel, and their intensity is compared. In single-beam spectrometers, the comparison of sample and reference effects is done consecutively, i.e. two consecu-... 

Figure 1.6a shows the simplest configuration of an atomic absorption spectrometer, called a single beam spectrometer . As can be seen, the lamp, the... 

Thus, the identification of the chemical form of P-bearing mineral phases is much easier than with a single-beam spectrometer. 

Gaseous absorptions These days, pollutant gases in the atmosphere, as well as carbon dioxide and water vapour, do not generally result in problems for modem spectrometers. When using older instruments, or single beam spectrometers, absorptions due to these gases may be superimposed on the observed spectrum. 

A simple single-beam spectrometer is depicted in Figure 1. The spectrometer consists of a source of radiation, a collecting mirror, an entrance slit, a dispersion element, an exit slit, and a detector. Each of these components will be examined separately. 

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