Double-Beam Spectrophotometry

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. 

Part—IV has been entirely devoted to various Optical Methods that find their legitimate recognition in the arsenal of pharmaceutical analytical techniques and have been spread over nine chapters. Refractometry (Chapter 18) deals with refractive index, refractivity, critical micelle concentration (CMC) of various important substances. Polarimetry (Chapter 19) describes optical rotation and specific optical rotation of important pharmaceutical substances. Nephelometry and turbidimetry (Chapter 20) have been treated with sufficient detail with typical examples of chloroetracyclin, sulphate and phosphate ions. Ultraviolet and absorption spectrophotometry (Chapter 21) have been discussed with adequate depth and with regard to various vital theoretical considerations, single-beam and double-beam spectrophotometers besides typical examples amoxycillin trihydrate, folic acid, glyceryl trinitrate tablets and stilbosterol. Infrared spectrophotometry (IR) (Chapter 22) essentially deals with a brief introduction of group-frequency... 

The drug was determined by second derivative spectrophotometry using a Uvidec 610 double beam spectrophotometer, and the absorbance was measured at 259.2 nm. Beer s law was obeyed in range of 10-30 pg/mL. 

Noise in UV-vis spectrophotometry refers to uncertainties caused in the measurement of the absorbance signal. Essentially, there are two sources of noise. One isdependent on the source intensity (Schott noise) and the other independent of it. The effect of noise in spectrophotometric measurement can be significantly reduced if the concentration of the analyte is adjusted such that measured absorbance is between 0.3 and 1.2 absorbance units. A significant source of noise in double-beam instruments arises when sample and reference cells are not positioned properly. Minor imperfections in the cells cause reflections and scattering... 

Spectrometers that use phototubes or photomultiplier tubes (or diode arrays) as detectors are generally called spectrophotometers, and the corresponding measurement is called spectrophotometry. More strictly speaking, the journal Analytical Chemistry defines a spectrophotometer as a spectrometer that measures the ratio of the radiant power of two beams, that is, PIPq, and so it can record absorbance. The two beams may be measured simultaneously or separately, as in a double-beam or a single-beam instrument—see below. Phototube and photomultiplier instruments in practice are almost always used in this maimer. An exception is when the radiation source is replaced by a radiating sample whose spectrum and intensity are to be measured, as in fluorescence spectrometry—see below. If the prism or grating monochromator in a spectrophotometer is replaced by an optical filter that passes a narrow band of wavelengths, the instrument may be called a photometer. 

The spectrometer system for AAS can be configured as a single-beam system, as shown in Fig. 6.8, as a double-beam system, shown in Fig. 6.14, or as a pseudo-double-beam system, which will not be discussed. (See the reference by Beaty and Kerber for a description of this system.) Note that in AAS the sample cell is placed in front of the monochromator, unlike UV/VIS spectrometers for molecular absorption or spectrophotometry, where the sample is placed after the monochromator. 

From the above it Is clear that dual wavelength spectrophotometry provides information from two wavelengths per unit time. All other factors being equal, the resultant data should be more useful than data from a double beam spectrophotometer. 

Instruments with single and double beam optics are available. The double beam optical arrangement Is more or less similar to that of double beam apparatus in absorption spectrophotometry. 

Table 1.4 opposes double-beam UVA IS spectrophotometry in solution to the use of a reflectance sphere (standard for industry). Recently also handheld reflection spectrometers have been introduced. 

Table 1.4. Reflectance and conventional double-beam UVA IS spectrophotometry ...

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