Nonspectral Interferences

A serious source of interference is chemical interference. Chemical interference occurs when some species in the sample affects the atomization efficiency of the sample compared 

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 nitrous 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 MSA, discussed in Chapter 2. This approach can correct for some chemical interferences but not all. For example, in the graphite furnace, if the analyte is present as a more volatile compound than the added analyte compound, MSA may not work. If the analyte form in the sample is lost prior to atomization as a result of volatilization, while the added analyte compound remains in the furnace until atomization, the standards additions method will not give accurate analytical results. 

Other potential sources of interference are the sample matrix and the solvent used for making the sample solution. The sample matrix is anything in the sample other than the analyte. In some samples, the matrix is quite complex. Milk, for example, has a matrix 

---

Althoi h nonspectral interference effects are generally less severe in ICP-OES than in GFAA, FAA, or ICPMS, they can occur. In most cases the effects produce less than a 20% error when the sample is introduced as a liquid aerosol. High concentrations (500 ppm or greater) of elements that are highly ionized in the... 

Errors due to nonspectral interferences can be reduced via matrix matching, the method of standard additions (and its multivariant extensions), and the use of internal standards. ... 

Different analytical procedures have been developed for direct atomic spectrometry of solids applicable to inorganic and organic materials in the form of powders, granulate, fibres, foils or sheets. For sample introduction without prior dissolution, a sample can also be suspended in a suitable solvent. Slurry techniques have not been used in relation to polymer/additive analysis. The required amount of sample taken for analysis typically ranges from 0.1 to 10 mg for analyte concentrations in the ppm and ppb range. In direct solid sampling method development, the mass of sample to be used is determined by the sensitivity of the available analytical lines. Physical methods are direct and relative instrumental methods, subjected to matrix-dependent physical and nonspectral interferences. Standard reference samples may be used to compensate for systematic errors. The minimum difficulties cause INAA, SNMS, XRF (for thin samples), TXRF and PIXE. 

The concept of selectivity and specificity has been applied to characterize interferences appearing in two different ICP-MS techniques (Horn [2000]). Classical ICP-MS with pneumatic nebulization and ETV-ICP-MS are compared for the determination of traces of zinc in sea-water. Whereas spectral interferences decrease using the ETV device, nonspectral interferences increase significantly (Bjorn et al. [1998]). A quantitative comparison of the both analytical procedures, here called PN (pneumatic nebulization) and ETV (electrothermal vaporization, Sturgeon and Lam [1999]) is possible by means the specificity as a function of the Zn concentration (Horn [2000]). The spectral interferences on the four zinc isotopes are listed in Table 7.4. 

With this technique, problems may arise with interference, such as background absorption—the nonspecific attenuation of radiation at the analyte wavelength caused by matrix components. To compensate for background absorption, correction techniques such as a continuous light source (D2-lamp) or the Zeeman or Smith-Hieftje method should be used. Enhanced matrix removal due to matrix modification may reduce background absorption. Nonspectral interference occurs when components of the sample matrix alter the vaporization behavior of the particles that contain the analyte. To compensate for this kind of interference, the method of standard addition can be used. Enhanced matrix removal by matrix modification or the use of a L vov platform can also reduce nonspectral interferences. Hollow cathode lamps are used for As, Cu, Cr, Ni, Pb, and Zn single-element lamps are preferred, but multielement lamps may be used if no spectral interference occurs. 

The ET-AAS technique is especially prone to nonspectral interferences, that is, effects on the formation of atoms. Such interferences cannot be eliminated by BC. Some of these effects can, however, be corrected by the use of the standard addition procedure. To investigate whether standard addition is necessary, the... 

However, it must be kept clear in mind that direct instrumental detection methods for trace substances are physically relative methods which require calibration, during which systematic errors, caused for instance by spectral and nonspectral interferences, may occur. Relative methods are in fact matrix-dependent and would require the analysis of Certified Reference Materials (CRMs) in order to guarantee the good quality of the analytical data. Unfortunately, CRMs are not available for polar snow and ice and hence the only way to assure the quality of the data is, whenever possible, to make careful intercomparisons of the techniques able to measure the same analytes with different approaches. 

Interferences in AA spectroscopy are divided into spectral and nonspectral interferences. 

Nonspectral interferences are either nonspecific or specific. Nonspecific interferences affect the nebulization by altering the viscosity, surface tension, or density of the analyte solution, and consequently the sample flow rate. Certain contaminants also decrease the desolvation and atomization efficiency by lowering the atomizer temperature. Specific interferences are also called chemical interferences because they are more analyte dependent. Solute volatilization inter-... 

Spectrometric methods require a prior sampling preparation containing a separation step. The separation step is necessary especially to eliminate interference. Nonspectral interferences in flame atomic absorption spectrometry can be overcome by using a calibration model.221 The model uses two independent variables for analyte quantification (the amount of the sample and the amount of analyte added) the measured absorbance is the dependent variable. To control the matrix interferences without prior knowledge of the matrix composition, it is necessary to carry out nine calibration points to obtain accurate analytical information. This confers high reliability of the analytical information for determination of trace elements in complex matrices. 

Graphite furnace atomizers experience significant nonspectral interference problems, some of which are unique to the furnace. Compensation or elimination of these interferences is different than what is done in flame atomizers. 

Can a given nonanalyte element present in a sample cause both spectral and nonspectral interferences in atomic emission spectrometry Explain, with diagrams as necessary. 

By definition, an interference effect occurs when the analytical signal is changed by the sample matrix compared with the reference or calibration standard, typically an acidified aqueous solution. This article is only concerned with nonspectral interferences in ET-AAS spectral interferences are considered elsewhere. It has been demonstrated in ET-AAS that the atomization efficiency (conversion of analyte to free atoms) is 100% for the majority of elements in simple solutions, which means that, in most cases, only negative nonspectral interferences can occur, i.e., the signal can only be reduced by the presence of... 

The practical applicability of absorption and emission spectrometric techniques is frequently restricted by spectral and nonspectral interferences due to their inherently low tolerance limit to sample constituents present in complex matrices. The most adequate way of alleviating the interfering effects consists of performing appropriate online sample pretreatment prior to analysis using flowing stream schemes. In many... 

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. 

Chemical modification, also commonly called matrix modification, is the addition of one or more reagents to the graphite tube along with the sample. The use of these chemical modifiers is to control nonspectral interferences by altering the chemistry occurring inside the furnace. The reagents are chosen to enhance the volatility of the matrix or to decrease the volatility of the analyte or to modify the surface of the atomizer. The use of a large amount of chemical modifier may, for... 

Excitation and ionization interferences are nonspectral interferences. When a sample is aspirated into a flame, the elements in the sample may form neutral atoms, excited atoms, and ions. These species exist in a state of dynamic equilibrium that gives rise to a steady emission signal. If the samples contain different amounts of elements, the position of equilibrium may be shifted for each sample. This may affect the intensity of atomic emission. For example, if sodium is being determined in a sample that contains a large amount of potassium, the potassium atoms may collide with unexcited sodium atoms in the flame, transferring energy in the collision and exciting... 

Nonspectral interferences affect the analyte signal, i.e., the number of analyte atoms in the absorption volume (absolute or per unit time) directly. They are best classified according to the place or stage at which the particular interference occurs, i.e., transport, volatilization, vapor phase, or spatial distribution interferences. Because the mechanisms of nonspectral interferences depend very much on the particular atomizer used, they will be discussed in detail in the following Sects. 2.2, 3.2, and 4.2. Nonspecific interferences, such as transport interferences caused in the... 

Among the nonspectral interferences transport interferences in the nebulizer are relatively common in the analysis of body fluids. This is certainly no problem when 10- or 20-fold diluted serum is used for the determination of the electrolytes. If, however, undiluted or only slightly diluted body fluids are aspirated directly, the viscosity of these liquids can impair aspiration rate and nebulization efficiency relative to the reference solutions used. If the sample solution cannot be diluted sufficiently to avoid this interference, a frequently used alternative is matrix-matched standards, i.e., reference solutions with a viscosity close to that of the samples. Another alternative is to use the method of additions, which can perfectly correct for this interference. This calibration technique, however, is labor-intensive and time consuming, and is restricted to the linear part of the calibration curve. Viscosity of the sample solutions is much less of a problem when FI techniques are used for sample introduction. This is because samples are not aspirated but pumped to the nebulizer, because much smaller sample volumes are used, and because the sample is always in a carrier solution which supports nebulization and removes all potential residues in the nebulizer-bumer system. 

Gas phase interferences due to compound formation of the analyte element with a concomitant should not be very significant in ETAAS because a much longer time is available for dissociation compared to FAAS. It was shown by high-temperature equilibrium calculations that gas phase interferences at the temperatures used in ETAAS should actually be rather insignificant [18], The reason why the literature is nevertheless full of reports on such interferences is largely due to an improper use of this technique. Slavin et al. [19], based on the systematic work of L vov [20], introduced a concept which they called stabilized temperature platform furnace (STPF). It is in essence a package of measures which eliminates most nonspectral interferences in ETAAS by atomization under local thermal equilibrium conditions. 

---