Detectors absorbance ratios

Confirmatory analysis of suspected liquid chromatographic peaks is usually accomplished by a photodiode array detector that continuously collects spectral data during the chromatographic separation and further compares the spectrum (200-550 nm) of the eluted suspected compound with that of a standard (37, 38, 66, 161, 163, 166-168, 178, 180, 181). Online absorbance ratio techniques combined witlr off-line thin-layer chromatography have been also reported (171). Although tliese confirmation techniques are relatively simple, their sensitivity is not generally adequate to identify trace levels of residual nitrofurans in edible animal products. 

This technique depends on the fact that the three uranium L x-rays, which accompany 25% of all 23sp decays, are of slightly differing energies and therefore have different absorption coefficients in the limestone. Figure 2 shows the spectrum of these L x-rays observed through several samples of wet limestone. These were thin measured slices of the limestone which were interposed as absorbers between a source of 238p radiation and the detector. The ratio of the intensities of the L1/L2 x-rays decreases with the thickness of the stone similarly the L3/L2 ratio increases with thickness. It is a simple matter to calibrate... 

In the double-beam system, the source radiation is split into two beams of equal intensity. The two beams traverse two light paths identical in length a reference cell is put in one path and the sample cell in the other. The intensities of the two beams after passing through the cells are then compared. Variation in radiation intensity due to power fluctuations, radiation lost to the optical system (e.g., cell surfaces, mirrors, etc.), radiation absorbed by the solvent, and so on should be equal for both beams, correcting for these sources of error. A dispersive spectrometer used for absorption spectroscopy that has one or more exit slits and photoelectric detectors that ratio the intensity of two light beams as a function of wavelength is called a spectrophotometer. 

In addition, the absorbance ratio has been shown to be a satisfactory substitute for the diode array detector because it monitors the intrinsic variability of methods, allowing for the development of a new protocol for distinct matrices. It should be borne in mind that minor variations required in the A obviously depend on grooves originating from a prism or comb, which are the principal determining factors... 

Due to the conversion process an absorbed photon give rise to less than one electron generated in the CCD. This phenomenon, also called a "quantum sink" shows that the detector is degrading the S/N ratio of the image. The quality of an image being mainly limited by the quantum noise of the absorbed gamma this effect is very important. 

If the wavelength of maximum absorption of the analyte (Xmax) is known, it can be monitored and the detector may be considered to be selective for that analyte(s). Since UV absorptions are, however, generally broad, this form of detection is rarely sufficiently selective. If a diode-array instrument is available, more than one wavelength may be monitored and the ratio of absorbances measured. Agreement of the ratio measured from the unknown with that measured in a reference sample provides greater confidence that the analyte of interest is being measured, although it still does not provide absolute certainty. 

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