Absorbance signal

In the previous section it has been shown that the measured sample absorbance may be higher than the true absorbance signal of the analyte to be determined. This elevated absorbance value can occur by molecular absorption or by light scattering. There are three techniques that can be used for background correction the deuterium arc the Zeeman effect and the Smith-Hieftje system. 

The signal part of the S/N ratio that concerns us is the way these expressions vary with the concentration of the analyte. Therefore, from equation 57-29 we obtain, for the absorbance signal ... 

But things are not so simple. In this examination we have so far looked only at a derivative calculated from adjacent data points. What happens when we calculate a two-point derivative based on non-adjacent data points In fact we have already considered this question qualitatively in our previous chapter [3], when we noted that using the optimum spacing will result in an improved S/N ratio for the derivative. Of course, improved in this case is in comparison to the derivative computed using adjacent data points, it must be determined on a case-by-case basis whether the improvement is sufficient to exceed that of the actual direct absorbance signal. 

Figure 5.18. Absorbance signals of test elements compared to background absorbance generated by seawater during atomisation for a pyrolysis performed at 380 °C (A), 630 °C (B), 850 °C (C), and 1400 °C (D), in the presence of NH4NO3 (4%). Source [681]...

An alternative to derivatizing carbohydrates is the use of indirect photometric detection. In this method, a detectable co-ion in the electrolyte is added to the buffer system generating a steady state absorbance signal in the detector. As the analyte ions migrate in front of the detector window, they displace the detectable co-ion and cause a decrease or negative response in the detector signal. This method provides universal detection of all anions or cations. Since most carbohydrates are not ionized... 

In the method described by Willie et al. [167] atomic absorption measurements were made with a Perkin-Elmer 5000 spectrometer fitted with a Model HGA 500 graphite furnace and Zeeman effect background correction system. Peak absorbance signals were recorded with a Perkin-Elmer PRS-10 printer-sequencer. A selenium electrodeless lamp (Perkin-Elmer Corp.) operated at 6W was used as the source. Absorption was measured at the 196.0nm line. The spectral band-pass was 0.7nm. Standard Perkin-Elmer pyrolytic graphite-coated tubes were used in all studies. 

FIGURE 9.17 An illustration of a temperature program for a graphite furnace experiment (left), and the absorbance signal that results (right). The absorbance signal corresponds to the third temperature plateau. See text for a more detailed explanation. 

Why is the absorbance signal developed in the case of the graphite furnace AA technique said to... 

The absorbance signal originates from a very small volume of solution placed in the furnace, and since the furnace is continuously flushed with an inert gas, the vapors from this volume are swept out of the furnace after a short time. 

Detection Limit It may be defined as the concentration (meg ml-1) of an element that gives rise in the shifting of absorbance signal to an amount which equals to the peak-to-peak noise of the base-line. 

The effect of multiple-order diffraction, if not corrected, is to introdnce a stray light signal at the detector, which badly affects the measnred linearity of the required absorbance signal. In the example above, if the required measurement is at 2000nm, and only 0.1% of stray light at lOOOnm reaches the detector, then appreciable absorbance nonlinearity is seen at absorbances above 2.0 AU. 

Absorbance signals seen in NIR consist of combination and overtone bands of hydrogen bonds such as C-H, N-H, 0-H, and S-H, which are aroused by large force constants and small mass. NIR spectra thus cover precious information on chemical as well as physical properties of analyzed samples due to characteristic reflectance and absorbance patterns [121-123], which makes this analysis method applicable to the characterization of monolithic stationary phases. 

The stabilized temperature platform furnace (STPF) concept was first devised by Slavin et al. It is a collection of recommendations to be followed to enable determinations to be as free from interferences as possible. These recommendations include (i) isothermal operation (ii) the use of a matrix modifier (iii) an integrated absorbance signal rather than peak height measurements (iv) a rapid heating rate during atomization (v) fast electronic circuits to follow the transient signal and (vi) the use of a powerful background correction system such as the Zeeman effect. Most or all of these recommendations are incorporated into virtually all analytical protocols nowadays and this, in conjunction with the transversely heated tubes, has decreased the interference effects observed considerably. 

Fig. 16. Change of the absorbance signal as a function of time for (a) cyclohexene and (b) TBHP on an uncoated ZnSe IRE (thin line) and on a ZnSe IRE coated with a methyl-modified Ti-Si aerogel catalyst (solid line). At time t = 0, the concentration at the inlet of the ATR flow-through cell was switched from 0 to 3 mmol/L, and at t = 122 s, it was switched back again (50).

AFS involves the emission of photons from an atomic vapour that has been excited by photon absorption. For low absorbance signals (and thus for low... 

During the extraction method described by Baucells et al. [53] for the determination of molybdenum, the dry residue was solubilised with nitric acid. To observe the influence of nitric acid concentration on the absorbance signal of molybdenum, different acid concentrations were used. There was a decrease of 22.86% in the peak height when 10% nitric acid was present compared with no concentrated nitric acid. 

On-column UV absorbance detection is by far the most common method of detection in CE today. Many compounds of interest absorb light to some extent in the UV region without any chemical modification. Detector components are fairly robust and inexpensive, and little operator skill is required. For these reasons, most commercial CE instruments are equipped with a standard UV absorbance detector. However, as absorbance signals are directly proportional to the optical pathlength (Beer s Law), the 10-100 xm internal diameter of capillaries used in CE yield rather disappointing detection limits in the range of 10-5-10-7M (7). 

These simple one-wavelength calibration models with no intercept term are severely limited. Spectral data is used from only one wavelength, which means a lot of useful data points recorded by the instrument are thrown away. Nonzero baseline offsets cannot be accommodated. Worst of all, because spectral data from only one wavelength is used, absorbance signals from other constituents in the mixtures can interfere with analysis. Some of the problems revealed for models without an intercept term can be reduced when an intercept term is incorporated. 

In flame spectrometry, physical interferences are related to transport of determinant from sample solution to the flame. The pneumatic nebulizer functions not only as a spray generator, but also as a pump.1,2 Anything which influences the pumping rate will influence the size of the absorbance signal obtained. The pumping, or aspiration, rate is most sensitive to changes in viscosity of the sample solutions. 

In the Smith-Hieftje system, the lamp power is subjected to short pulses of high current.17 This causes momentary bursts of high atom concentration in the hollow cathode. The emission line profile is broadened as an atom cloud forms just outside the cathode, which causes absorption at the centre of the emitted line profile, as shown in Figure 7. Effectively the single narrow emission line is split into a pair of lines immediately adjacent to the original line centre. Thus, in the normal mode, atomic and molecular absorption are measured, but in the pulsed mode, only molecular absorbance is monitored. The difference between the two signals provides a corrected atomic absorbance signal. 

For some elements, especially those which tend to form thermally stable oxides, fuel-to-oxidant ratio may have a dramatic effect upon atomic absorbance signal. Figure 4, for example, illustrates the effect of increasing fuel flow upon aluminium determination. 

Conventional pneumatic nebulizers typically consume sample solution at the rate of ca. 5-8 ml min-1. Thus generally, when flame spectrometry is used on a routine basis, 2-5 ml of sample solution is used per determination. However it is possible to employ much smaller volumes of sample solution.16 Figure 3, for example shows typical atomic absorption signals for the nebulization of 0.01, 0.02, and 0.05 ml of a 1 mg l-1 standard solution, as recorded on a storage oscilloscope, compared with the signal from continuous nebulization. It is clear that only about 0.04 ml of solution is required to obtain the maximum absorbance signal. 

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