Single spectral interferences

In AAS, the excitation source inert gas emission offers a potential background spectral interference. The most common inert gases used in hollow cathode lamps are Ne and Ar. The data taken for this table and the other tables in this book on lamp spectra are from HCLs however, electrodeless discharge lamps emit very similar spectra. The emission spectra for Ne and Ar HCLs and close lines that must be resolved for accurate analytical results are provided in the following four tables. This information was obtained for HCLs and flame atom cells and should not be considered with respect to plasma sources. In the Type column, I indicates that the transition originates from an atomic species and II indicates a singly ionized species. 

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 photomultiplier detects both the thermal emission from the determinant and also any other atomic or molecular emission from either concomitant elements present in the sample or from the flame itself. Figure 8, for example, shows a typical section of a flame emission spectrum. While it is possible for some determinations by FES to work at a single fixed wavelength, as in flame AAS, it is advisable, at least initially, to scan the emission spectrum in the vicinity of the wavelength of interest to confirm the absence of spectral interferences. In any event, regular re-zeroing and aspiration of an appropriate standard to check for signal drift is essential. 

In environmental analysis, flame photometry is most widely used for the determination of potassium, which emits at 766.5 nm. It is also often used for the determination of sodium at 589.0 nm, although spectral interference problems (see Chapter 3) then may be encountered in the presence of excess calcium because of emission from calcium-containing polyatomic species. Molecular species are more likely to be found in cooler flames than in hotter flames. Some instruments use single, interchangeable filters, while others have three or more filters, for example for the determinations of potassium, sodium and lithium,... 

These can be performed successfully with AES. Indeed, the unambiguous detection and identification of a single non-interfered atomic spectral line of an element is sufficient to testify to its presence in the radiation source and in the sample. The most intensive line under a set of given working conditions is known as the most sensitive line. These elemental lines are situated for the various elements in widely different spectral ranges and may differ from one radiation source to another, as a result of the excitation and ionization processes. Here the temperatures of the radiation sources are relevant, as the atom and ion lines of which the norm temperatures (see Section 1.4) are closest to the plasma temperatures will be the predominant ones. However, not only will the plasma temperatures but also the analyte dilutions will be important, so as to identify the most intensive spectral lines for a radiation source. Also the freedom from spectral interferences is important. 

Spectral interferences also result from the presence of combustion products that exhibit broadband absorption or particulate products that scatter radiation. Both reduce the power of the iraiismillcd beam and lead to positive analytical errors. When the source of these products is Ihe fuel and oxidant mixture alone, the analytical data c in be corrected by making absorption measurements while a blank is aspirated into the flame. Note that this correction must be used with both double-beam and single-beam inslrilmcnls because Ihe reference beam of a double-beam instrument does not pass through Ihe Hamc (see Figure 9-13b). 

Generally, for confirmation of the presence of a trace amount of a given element, two or three raies ultimes should be detected. Confirmation based on a single RU line is unreliable because of the possibibty of spectral interference from other elements. 

Because many elements have several strong emission Hnes, AES can be regarded as a multivariate technique per se. Traditionally, for quantitative analysis in atomic emission spectroscopy, a single strong spectral line is chosen, based upon the criteria of Hne sensitivity and freedom of spectral interferences. Many univariate attempts have been made to compensate spectral interferences by standard addition, matrix matching, or interelement correction factors. However, all univariate methods suffer from serious limitations in a complex and Hne-rich matrix. 

An objective evaluation of the methods on the basis of literature data is hardly possible, because of differences in experimental conditions, sample, amount of sample available, and measurement time. Therefore, the detection limit values stated in the literature given in Tables 1-4 should be considered, at best, approximate. But it is clear that there is no single instrumental technique that meets all the analytical requirements. For example, some methods may be applicable over only a limited concentration range, may be subject to matrix effects or spectral interferences, or have a limited availability. Also, sometimes nondestructive fast methods are needed. The choice of an instrumental method depends on the material to be analyzed and the type of analysis required. 

A simple chemical system can consist of a pure single component, or of a single component or more than one component in a mixture with no spectral interference it is assumed that the radiation absorption by one component is not affected by the presence of other components. In simple systems, absorbance peak height measurements, directly or by using a selected baseline [35], are often employed for calibration and analysis. Because of intrinsic instrumental errors the practical limit for usable absorbance values is about three. Peak height measurements are also sensitive to changes in instrumental resolution and can vary considerably from instrument to instrument. To circumvent these problems, an alternative method is the use of integrated absorbance or peak area [10],... 

Power of Detection. For optimum power of detection, the analyte density in the plasma, the ionization, and the ion transmission must be maximized. The necessary power is 0.6-2 kW with the sample ca. 10-15 mm above the tip of the injector. The detection limits, obtained at single element optimum conditions, differ considerably from those at compromise conditions, but are still considerably lower than in ICP-AES (Table 6). For most elements they are in the same range, but for some they are limited by spectral interference. This applies to As ( As" with Ar CF), Se ( Se with Ar Ar ), and Fe ( Fe with Ar O ). The acids present in the measurement solution and the material of which the sampler is made (Ni, Cu, etc.) may have considerable influence on these sources of interference and the detection limits for a number of elements. The detection limits for elements with high ionization potential may be even lower when they are detected as negative ions (for CF Cl = 5 ng/mL and for Cr Cl = I ng/mL). 

Single-collector instruments also prove very usefid for mass content determinations via isotope dilution, as carefiil estimation of all quantities that influence the uncertainty budget demonstrates that the precision on the isotope amount ratio is typically not the dominant factor for high-precision measurements. Often, the accuracy of a mass content measurement will hardly improve through the use of MC-ICP-MS instruments as other influence quantities, such as uncontrolled spectral interferences and sample inhomogeneities, typically deserve more attention. 

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