Emission spectroscopy, accuracy precision

Chapters 7 and 8 describe two major techniques for the monitoring of trace elements in environmental samples atomic absorption (AA) and inductively coupled plasma-atomic emission spectroscopy (ICP). AA is most ideally suited for analyses where a limited number of trace metal concentrations are needed with high accuracy and precision. ICP has the advantage of multielement analysis with high speed. 

Research by Burton and Price (38) demonstrated that Ba/Sr ratios generated by ICP emission spectroscopy (ICP-ES) can be used to infer the diet (marine verses terrestrial) of prehistoric populations. In this experiment we duplicate results obtained by Burton and Price for the Paloma samples. Our results show that the Ba/Sr ratios obtained by LA-ICP-MS are comparable in precision and accuracy to ICP-ES data (Figure 12). Although it is not unexpected that a coastal population would rely heavily upon marine resources, there are applications where this type of research would have value. What we have done here is demonstrate the efficacy of LA-ICP-MS to this line of research by demonstrating that it is possible to generate results similar to those obtained by other analytical techniques. 

Since the mid-1960s, a variety of analytical chemistry techniques have been used to characterize obsidian sources and artifacts for provenance research (4, 32-36). The most common of these methods include optical emission spectroscopy (OES), atomic absorption spectroscopy (AAS), particle-induced X-ray emission spectroscopy (PIXE), inductively coupled plasma-mass spectrometry (ICP-MS), laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS), X-ray fluorescence spectroscopy (XRF), and neutron activation analysis (NAA). When selecting a method of analysis for obsidian, one must consider accuracy, precision, cost, promptness of results, existence of comparative data, and availability. Most of the above-mentioned techniques are capable of determining a number of elements, but some of the methods are more labor-intensive, more destructive, and less precise than others. The two methods with the longest and most successful histoty of success for obsidian provenance research are XRF and NAA. 

Argon plasma offers a number of advantages as a source for emission spectroscopy. Argon is an inert gas and will not react with the sample so chemical interference is greatly reduced. At plasma temperatures, atomization is complete and elemental spectra do not reflect molecular components. Detection limits are high for most elements. Accuracy and precision are excellent. In addition, ICP/OES requires less sample preparation and less sample amount than other techniques. 

The relative accuracy and precision obtained by arc and spark emission spectroscopy is commonly about 5%, but may be as poor as 20-30%. Arc emission is much more prone to matrix effects than spark emission due to the lower temperature of the discharge. Both arc and spark excitation may require matrix matching of sample and standards for accurate analyses, and usually require the use of an internal standard. 

For analytical spectroscopy it is necessary to have the arc or spark excitation stand rigidly mounted on an optical bench. Care in optical alignment and care in spacing the electrodes are necessary and each adjustment must be reproducible. Early work in analytical emission spectroscopy made use of very simple electrode holders, open to the surroundings. With the development of improved, more complicated power sources and the need to further develop accuracy and precision, many changes in electrode holders and accessories occurred. The result has been the development of versatile enclosed units for excitation of analytical samples. 

The sensitivity, accuracy, and precision of solid sample analysis were greatly improved by coupling of LA with ICP-OES/MS. The ablated species are transported with a carrier gas (usually argon) into the plasma torch. Additional atomization. excitation and ionization of the ablated species in a stationary hot plasma provide a dramatic increase in the sensitivity of emission detection (LA-ICP-OES) or detection of ions (LA-ICP-MS). The efficiency of the transport of ablated species into an ICP strongly depends on the size of the particles. The optimal conditions for ablation in the ca.se of LA-ICP differ significantly from the optimal conditions for LIBS because the efficient transport of the ablated matter to an ICP requires a fine aerosol (with solid particle diameters less than a few micrometers), whereas direct optical emission spectroscopy of the laser plume needs excited atoms and ions. 

Spectroscopic methods for the deterrnination of impurities in niobium include the older arc and spark emission procedures (53) along with newer inductively coupled plasma source optical emission methods (54). Some work has been done using inductively coupled mass spectroscopy to determine impurities in niobium (55,56). X-ray fluorescence analysis, a widely used method for niobium analysis, is used for routine work by niobium concentrates producers (57,58). Paying careful attention to matrix effects, precision and accuracy of x-ray fluorescence analyses are at least equal to those of the gravimetric and ion-exchange methods. 

The method of standard additions is widely used in atomic spectroscopy (e.g. determination of Ca2+ ions in serum by atomic emission spectrophotometry) and, since several aliquots of sample are analysed to produce the calibration graph, should increase the accuracy and precision of the assay... 

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