Non-specific absorption

Again, care has to be taken for the non-ideal (or real) behavior of the measurement system. Applications are limited by non-specific absorption of molecules on the surface, mass transfer effects (under conditions of laminar flow a 1-5-pm layer between sensor surface and volume flow is not whirled and has to be passed by passive diffusion) or limited access for the immobilized molecules [158-160]. 

Solvents such as chloroform, carbon tetrachloride, hexane and benzene gave absorption signals because of their non-specific absorption at the lead resonance line. Although these solvent peaks generally emerged well before the lead compounds, the use of the background corrector is recommended to eliminate these potential interferences. 

The disappearance of all but the non-specific absorption from the spectrum with increasing acidity reflects a loss of conjugation as the doubly charged cation is formed (equation ii) this transformation is represented by the formation of (LXVII) (BH ) which permits the widest charge separation. 

Figure 2.15 shows an instrument capable of doing this simultaneously and automatically. Lead is particularly prone to this problem, and Fig. 2.16 shows how background correction can be used to remove the interference of non-specific absorption when determining lead in chromium. Notice that the precision is also improved, mainly because the effects which give rise to the background are not very reproducible. 

Spectral interferences from the overlap of molecular bands and lines (e g. the calcium hydroxide absorption band on barium at 553.55 nm) cannot be so easily dismissed. Lead seems to be particularly prone to such non-specific absorption problems at the 217.0 nm line (e g. sodium chloride appears to give strong molecular absorption at this wavelength). This type of problem is encountered in practical situations, but can sometimes be removed by the technique of background correction (see Section 2.2.5.2). 

A spectrometer with rapid response electronics should be used for electrothermal atomization, as it must follow the transient absorption event in the tube. Automatic simultaneous background correction (see Section 2.2.5.2) is virtually essential, as non-specific absorption problems are very severe. It is important that the continuum light follows exactly the same path through the furnace as the radiation from the line source (assuming a deuterium lamp is being used rather than Smith-Hieftje or Zeeman effect). The time interval between the two source pulses should be as short as possible (a chopping frequency of at least 50 Hz) because of the transient nature of the signal. 

Antibody responses in the H. diminuta mouse system have been reported from a number of workers and isotypes of IgA, IgG and IgM have been found on this cestode. Moreover, their titres increased coincidently with worm rejection and darkened areas suggested that these surface binding antibodies have a functional role in inducing morphological alterations. It is not known, however, whether the presence of these antibodies on the surface is due to specific or non-specific absorption (555). In this system, passive protection of mice by transfer of immune sera has not been demonstrated. 

The relatively simple procedure of analysing waters for trace metals using AAS is complicated by the fact that a number of metal determinations suffer from interferences. These interferences are well documented in the literature and there are now standard methods for overcoming them. In the main, interferences encountered in the atomic absorption spectrophoto-metric analysis of water can be divided into four categories chemical interferences, matrix interferences, non-specific absorption and ionisation. 

Fig. 2. (a) Method of standard additions, (b) Error introduced due to non-specific absorption. 

Background interference. Correction for non-specific absorption is sometimes essential in geochemical work (section I.D.) and is most conveniently carried out by means of a continuum source. However, not all systems available are completely effective, and this feature should be checked before purchase of an instrument. It is essential that the beams from the hollow-cathode lamp and the continuum lamp traverse exactly equivalent paths through the flame, and can be accurately balanced for energy. 

Check that there are no background (scatter or non-specific absorption) effects by using a hydrogen or deuterium lamp or a nearby non-absorbing line. 

The classic example of a pharmacopoeial assay based on the Morton-Stubbs correction is that for vitamin A alcohol and the ester. Other techniques proposed for the correction of non-specific absorption include difference spectrophotometry, second derivative spectrophotometry, the use of orthogonal polynomials, and chemical or physical transformation of the drug to give absorption at a longer wavelength. 

To ensure that the absorption is proportional to the concentration level of the atoms, only the atoms must absorb. In reality, the flame itself, certain molecular species and solid particles contained in the flame can also absorb. These non specific absorptions, also called background, can be corrected using one of the following two methods, which are the most commonly used on commercial instruments ... 

We will concentrate here on correction using a continuous emission lamp. The method consists of measuring, alternatively, the atomic absorption from the line of the element and the non specific absorption from a continuous spectrum lamp, over an range centred on the line and defined by the monochromator bandwidth. As this is much greater than the width of the line being analysed, we can consider that the second measurement corresponds solely to continuous (non specific) absorption. Continuous spectrum lamps used to correct the background arc ... 

The measure of absorption specific to an element may be subject to interference from non specific absorptions and various interactions that may be corrected or compensated for by a variety of methods. 

Molecular compounds from the matrix which are not dissociated may, however, lead to broadband absorption spectra whereas small solid particles in the flame may diffuse the light over an extensive range of wavelengths. When these non specific absorptions are superimposed on the atomic absorption at the wavelength of the element to be determined, it is necessary to measure the non specific absorptions and correct the total absorption. 

The principal methods recommended for measuring non specific absorption or background in atomic flame absorption spectrometry are as follows ... 

The instrument is calibrated for a given element for each series of samples. Direct calibration requires detailed knowledge of the milieu to be analysed. Precise results depend on the composition of the calibration solutions being as close as possible to that of the solutions to be analysed. The standard addition method can be u.sed in cases where it is not possible to produce external reference solutions similar to the solutions to be analysed. This method should be used with considerable caution because it assumes that the absorption is due solely to the element under analysis and, in particular, that the non specific absorption is fully corrected for. If this is not the case, any interfering absorption leads to an overestimate of the values observed. On the other hand, this method does have the advantage of eliminating the matrix effect. 

The interaction of nanoparticles with the proteins is governed from the same type of interactions described for carbon nanotubes. Since NPs carry charges, they can electrostatically adsorb biomolecules with different charges, which depend on the pH that the immobilization takes place and the pi of the protein [3,191]. Moreover, hydrophobic interactions, hydrogen bonds and non-specific absorption can play a role for enzyme non-covalent adsorption onto the surface of nanoparticles. 

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