Interferences chemical

Interferences are physical or chemical processes that cause the signal from the analyte in the sample to be higher or lower than the signal from an equivalent standard. Interferences can therefore cause positive or negative errors in quantitative analysis. There are two major classes of interferences in AAS, spectral interferences and nonspectral interferences. Nonspectral interferences are those that affect the formation of analyte free atoms. Nonspectral interferences include chemical interference, ionization interference, and solvent effects (or matrix interference). Spectral interferences cause the amount of light absorbed to be erroneously high due to absorption by a species other than the analyte atom. While all techniques suffer from interferences to some extent, AAS is much less prone to spectral interferences and nonspectral interferences than atomic anission spectrometry and X-ray fluorescence (XRF), the other major optical atomic spectroscopic techniques. 

A serious source of interference is chemical interference. Chemical interference occurs when some chemical component in the sample affects the atomization efficiency of the sample compared with the standard solution. The result is either an enhancement or a depression in the analyte signal from the sample compared with that from the standard. This effect is associated most commonly with the predominant anions present in the sample. The anion affects the stability of the metal compound in which the analyte is bound, and this, in turn, affects the efficiency with which the 

There are three ways of compensating for chemical interference. The first approach is to match the matrix of the standards and samples that is, to have the same anion(s) present in the same concentrations in the working standards as in the samples being analyzed. This supposes that the samples have been thoroughly characterized and that their composition is known and constant. This may be the case in industrial production of a material or chemical, but often, the sample matrix is not well characterized or constant. 

The third approach is to eliminate the chemical interference by switching to a higher-temperature flame, if possible. For example, when a nitrons oxide-acetylene flame is used, there is no chemical interference on Ca from phosphate, because the flame has sufficient energy to decompose the calcium phosphate molecules. Therefore, no lanthanum addition is required. 

A fourth possible approach is the use of the method of standard additions (MSA), discussed in Chapter 2. This approach can correct for some chanical interferences but not aU. For example, in the graphite furnace, if the analyte is present as a more volatile compound in the sample than the added analyte compound, MSA wiU not work. The analyte form in the sample is lost prior to atomization as a result of volatilization, while the added analyte compound ranains in the furnace until atomization therefore, the standard additions method will not give accurate analytical results. 

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For a method to be useful it must provide reliable results. Unfortunately, methods are subject to a variety of chemical and physical interferences that contribute uncertainty to the analysis. When a method is relatively free from chemical interferences, it can be applied to the determination of analytes in a wide variety of sample matrices. Such methods are considered robust. 

Minimizing Chemical Interferences The quantitative analysis of some elements is complicated by chemical interferences occurring during atomization. The two most common chemical interferences are the formation of nonvolatile compounds containing the analyte and ionization of the analyte. One example of a chemical interference due to the formation of nonvolatile compounds is observed when P04 or AP+ is added to solutions of Ca +. In one study, for example, adding 100 ppm AP+ to a solution of 5 ppm Ca + decreased the calcium ion s absorbance from 0.50 to 0.14, whereas adding 500 ppm POp to a similar solution of Ca + decreased the absorbance from 0.50 to 0.38. These interferences were attributed to the formation of refractory particles of Ca3(P04)2 and an Al-Ca-O oxide. 

Accuracy When spectral and chemical interferences are minimized, accuracies of 0.5-5% are routinely possible. With nonlinear calibration curves, higher accuracy is obtained by using a pair of standards whose absorbances closely bracket the sample s absorbance and assuming that the change in absorbance is linear over the limited concentration range. Determinate errors for electrothermal atomization are frequently greater than that obtained with flame atomization due to more serious matrix interferences. 

Sensitivity is also influenced by the sample s matrix. We have already noted, for example, that sensitivity can be decreased by chemical interferences. An increase in sensitivity can often be realized by adding a low-molecular-weight alcohol, ester, or ketone to the solution or by using an organic solvent. 

Accuracy The accuracy of a fluorescence method is generally 1-5% when spectral and chemical interferences are insignificant. Accuracy is limited by the same types of problems affecting other spectroscopic methods. In addition, accuracy is affected by interferences influencing the fluorescent quantum yield. The accuracy of phosphorescence is somewhat greater than that for fluorescence. 

Atomic emission is used for the analysis of the same types of samples that may be analyzed by atomic absorption. The development of a quantitative atomic emission method requires several considerations, including choosing a source for atomization and excitation, selecting a wavelength and slit width, preparing the sample for analysis, minimizing spectral and chemical interferences, and selecting a method of standardization. 

Choice of Atomization and Excitation Source Except for the alkali metals, detection limits when using an ICP are significantly better than those obtained with flame emission (Table 10.14). Plasmas also are subject to fewer spectral and chemical interferences. For these reasons a plasma emission source is usually the better choice. 

When possible, quantitative analyses are best conducted using external standards. Emission intensity, however, is affected significantly by many parameters, including the temperature of the excitation source and the efficiency of atomization. An increase in temperature of 10 K, for example, results in a 4% change in the fraction of Na atoms present in the 3p excited state. The method of internal standards can be used when variations in source parameters are difficult to control. In this case an internal standard is selected that has an emission line close to that of the analyte to compensate for changes in the temperature of the excitation source. In addition, the internal standard should be subject to the same chemical interferences to compensate for changes in atomization efficiency. To accurately compensate for these errors, the analyte and internal standard emission lines must be monitored simultaneously. The method of standard additions also can be used. 

What type of chemical interference in the sample necessitates the use of the method of standard additions ... 

Accuracy When spectral and chemical interferences are insignificant, atomic emission is capable of producing quantitative results with accuracies of 1-5%. Accuracy in flame emission frequently is limited by chemical interferences. Because the higher temperature of a plasma source gives rise to more emission lines, accuracy when using plasma emission often is limited by stray radiation from overlapping emission lines. 

Sensitivity Sensitivity in flame atomic emission is strongly influenced by the temperature of the excitation source and the composition of the sample matrix. Normally, sensitivity is optimized by aspirating a standard solution and adjusting the flame s composition and the height from which emission is monitored until the emission intensity is maximized. Chemical interferences, when present, decrease the sensitivity of the analysis. With plasma emission, sensitivity is less influenced by the sample matrix. In some cases, for example, a plasma calibration curve prepared using standards in a matrix of distilled water can be used for samples with more complex matrices. 

More subtle modes of action are also possible since the response to hormone receptor binding is complex and could be affected by chemical interference with receptor-related proteins, DNA methylation or histone acetylation. Dioxin (TCDD), for example, reduces the ability of the oestrogen-receptor complex to bind to the oestrogen response element of DNA, reducing gene transcription. ... 

They chemically interfere with the flame propagation mechanism. 

These factors may be broadly classified as (a) spectral interferences and (b) chemical interferences. 

The production of ground-state gaseous atoms which is the basis of flame spectroscopy may be inhibited by two main forms of chemical interference (a) by stable compound formation, or (b) by ionisation. 

The determination of magnesium in potable water is very straightforward very few interferences are encountered when using an acetylene-air flame. The determination of calcium is however more complicated many chemical interferences are encountered in the acetylene-air flame and the use of releasing agents such as strontium chloride, lanthanum chloride, or EDTA is necessary. Using the hotter acetylene-nitrous oxide flame the only significant interference arises from the ionisation of calcium, and under these conditions an ionisation buffer such as potassium chloride is added to the test solutions. 

At this point, the solution containing the component to be measured (Ax) also contains any other compounds from the original matrix that are soluble in the solvent used in the analysis. For the analysis to be accurate, other components in the matrix cannot interfere by eluting at the same retention time as the components to be measured. For accurate MS analyses, the matrix component must not interfere with production of the ions being measured for either the internal standard or the component to be measured. In some cases, to eliminate interferences, it may be necessary to increase the resolution of the mass spectrometer by narrowing the mass window being monitored. Alternatively, MS/MS can be used to avoid chemical interference (see Chapter 1). 

Selectivity and Interference Selectivity means that only that species is measured which the analyst is looking for. A corollary is the absence of chemical interferents. A lack of selectivity is often the cause of nonlinearity of the calibration curve. Near infra-red spectroscopy is a technology that exemplifies how seemingly trivial details of the experimental set-up can frustrate an investigator s best intentions Ref. 124 discusses some factors that influence the result. 

The advantage of the inverse calibration approach is that we do not have to know all the information on possible constituents, analytes of interest and inter-ferents alike. Nor do we need pure spectra, or enough calibration standards to determine those. The columns of C (and P) only refer to the analytes of interest. Thus, the method can work in principle when unknown chemical interferents are present. It is of utmost importance then that such interferents are present in the Ccdibration samples. A good prediction model can only be derived from calibration data that are representative for the samples to be measured in the future. 

Today, ICP-AES is an indispensable inorganic analytical tool. However, because of the high plasma temperature, ICP-AES suffers from some severe spectral interferences caused by line-rich spectra of concomitant matrix elements such as Fe, Al, Ca, Ni, V, Mo and the rare-earth elements. This is at variance with AAS. The spectral interference can of course be minimised by using a (costly) high-resolution spectrometer. On the other hand, the high temperature of the ICP has the advantage of reducing chemical interferences, which can be a problem in AAS. 

Atomic absorption takes advantage of the fact that most of the atoms remain in the ground state, and are capable of absorbing radiation of the appropriate wavelength corresponding to Ah. Whereas a hot flame is preferred for flame photometry, a cooler flame is preferred for atomic absorption, except in cases where chemical interference may occur. 

Table 1 lists the temperatures of some commonly used flames for atomic absorption. A cool flame such as argon-hydrogen-entrained air or air-coal gas is usually not preferred because of increased danger of chemical interferences (see below). The most commonly used flame is the air-acetylene flame. 

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