Single-element techniques

Type standardization Provided that the total specimen absorption does not vary significantly over a range of analyte concentrations, and provided that enhancement effects are absent and that the specimen is homogeneous, a hnear relationship will be obtained between analyte concentration and measured characteristic line intensity. Where these provisos are met, type standardization techniques can be employed. 

In this way, linear calibration curves can be used to determine trace and minor element concentrations in alloys, mineral pellets and liquids provided that the major element concentrations of standards and unknowns are very similar. In this case, the matrix effect remains the same. 

To determine major and minor elements in complex samples, more elaborate matrix correction algorithms need to be appHed. They can be roughly divided into two categories the influence coefficient methods and the fundamental parameter method. 

Influence coefficient methods AU these models have essentially the same form  

Fundamental parameter method The fundamental parameter method is based on the physical theory of X-ray production rather than on empirical rdations between observed X-ray count rates and concentrations of standard samples. In general, the observed XRF count rate Rj of (the K line of) an element i, obtained by polychromatic exdtation of a sample with thickness d and density p, can be written as  

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Atomic absorption spectroscopy is more suited to samples where the number of metals is small, because it is essentially a single-element technique. The conventional air—acetylene flame is used for most metals however, elements that form refractory compounds, eg, Al, Si, V, etc, require the hotter nitrous oxide—acetylene flame. The use of a graphite furnace provides detection limits much lower than either of the flames. A cold-vapor-generation technique combined with atomic absorption is considered the most suitable method for mercury analysis (34). 

As mentioned, thermal ionization mass spectrometry is the area in which isotope dilution developed and in which it has received the widest range of applications. One of thermal ionization s major limitations is that it is essentially a single-element technique in no way can it be considered multielement in the sense that numerous elements can be assayed in a single analysis. It is thus highly desirable to mate isotope dilution with multielement analysis capability. Spark source mass spectrometry for years dominated elemental analysis, but the nature of the samples (solids) made use of isotope dilution difficult. Use of a multielement spike was reported as long ago as 1970 by Paulsen et al. [17], however, and more recently by Carter et al. [18] and by Jochum et al. [19,20]. 

Multi-elemental techniques versus single element techniques... 

The radiation source used in AAS is an HCL or an EDL, and a different lamp is needed for each element to be determined. Because it is essentially a single-element technique, AAS... 

Calibration of AAS methods can be performed by the use of an external calibration curve or by MSA both calibration methods were presented in Chapter 2. Internal standards are not used in AAS, because it is usually a single-element technique we cannot measure an internal standard element at the same time we measure the analyte. 

Like FAAS, ETA-AAS is a single element technique and is specific for the element being determined. Two major advantages of ETA-Aas over FAAS are small sample volume and high sensitivity. Usually, 5 to lOpl of sample is needed for... 

Since AAS is classically a single-element technique, there is an increasing trend to overcome this Hmitation by the development of simultaneous multi-element spectrometers with multichannel detection. Such instmments are presently available on the market, and allow the simultaneous determination of up to six elements. As these are GF instruments, they must however be used under compromise conditions for the ashing and atomisation steps. 

The radiation source used in AAS is an HCL or an EDL, and a different lamp is needed for each element to be determined (except for the new continuum source system discussed earlier). Because it is essentially a single-element technique, AAS is not well suited for qualitative analysis of unknowns. To look for more than one element requires a significant amount of sample and is a time-consuming process. For a sample of unknown composition, multielement techniques such as XRF, ICP-mass spectrometry (ICP-MS), ICP-OES, and other atomic emission techniques are much more useful and efficient. 

The disadvantage for SS-AAS is that the method is a single element technique, and for both SS-AAS and SS-ICP that special apparatus and a homogeneous solid are required. On the other hand, it can be stated that this technique is useful to determine the homogeneity of a solid. For SS-AAS an excellent background correction technique, e.g., use of the Zeeman effect, is needed to correct for the high nonanalyte peak due to the high amount of mass which may be analyzed. 

Another area of concern with regard to contamination is in the selection of calibration standards. Because ICP-MS is a technique capable of quantifying up to 75 different elements, it will be detrimental to the analysis to use calibration standards that are developed for a single-element technique such as atomic absorption. These single-element standards are usually certified only for the analyte element and not for any others, although they are often quoted on the certificate. It is therefore absolutely critical to use calibration standards that have been specifically made for a multielement technique such as ICP-MS. It does not matter whether they are single or multielement standards, as long as the certificate contains information on the suite of analyte elements you are interested in as well as any other potential interferents. 

This is also mainly a single-element technique, although multielement instrumentation is now available. It works on the same principle as flame AA, except that the flame is replaced by a small heated tungsten filament or graphite tube. The other major difference is that in ETA, a very small sample (typically, 50 pL) is injected onto the filament or into the tube, and not aspirated via a nebulizer and a spray chamber. Because the ground-state atoms are concentrated in a smaller area than a flame, more absorption takes place. The result is that ETA offers about 100 times lower detection limits than EAA. 

As with flame AA, ETA is basically a single-element technique, although multielement instrumentation is available from some vendors. Because of the need to thermally and sometimes chemically pretreat the sample to remove solvent and matrix components prior to atomization, ETA has a relatively low sample throughput. A typical graphite furnace determination normally requires 2-3 min per element per replicate, although multielement systems are capable of achieving up to six elements in the same amount of time. 

Analytical performance can mean different things to different people. The major reason that the trace element community was attracted to ICP-MS almost 20 years ago was its extremely low multielement DLs. Other multielement techniques, such as ICP-OES, offered very high throughput but just could not get down to ultratrace levels. Even though ETA offered much better detection capability than ICP-OES, it did not offer the sample thronghpnt capability that many applications demanded. In addition, ETA was predominantly a single-element technique and so was impractical for carrying out rapid multielement analysis. These limitations quickly led to the commercialization and acceptance of ICP-MS as a tool for rapid ultratrace element analysis. However, there are certain areas where ICP-MS is known to have weaknesses. For example, dissolved solids for most sample matrices must be kept below 0.2%, otherwise it can lead to serious drift problems and poor precision. 

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