Emission spectroscopy, accuracy

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. 

The increase in accuracy afforded by a radiochemical separation is absolutely necessary in the determination by NAA of trace elements in the coals selected as standards. The fact that interferences from the coal matrix are removed by a radiochemical separation is the advantage of this method of analysis over such instrumental methods as x-ray fluorescence and emission spectroscopy. 

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. 

Inductively coupled plasma, ICP, atomic emission spectroscopy has made the determination of wear metals very easy and used oils can be scanned for the presence of at least 20 elements in less than 1 min. Wear trends can be obtained by comparing wear metals from a series of samples. However, it should be noted that the accuracy of the determination can be limited by the particle sizes present. 

High-sensitivity white-light absorption spectroscopy has several advantages in the spectroscopic determination of gas kinetic temperatures. The method is noninvasive, in situ, and relatively simple. Multichannel detection allows high signal-to-noise ratios even in absorption, as well as accurate determination of relative line intensities without difficulties due to lamp drift. Additionally, the determination of relative rotational population distributions in ground or metastable electronic levels of diatomic molecules alleviates the concerns associated with the accuracy of rotational temperature analysis using optical-emission spectroscopy. 

The disadvantage of film detection is that the plate must be developed and the lines identified to obtain the mass spectrum. Also, intensity data suffer in accuracy (at best, ion abundances can be measured to 10% relative error). Identification of line position and intensity is done with a microdensitometer (as in x-ray crystallography or emission spectroscopy) high-resolution measurements can be made in this manner. The densitometer is usually interfaced to a computer. 

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. 

These relatively high concentrations measured by FDMS can also be detected by other methods with adequate accuracy, although the FD method has the advantage that tissues need no pretreatment. Parallel measurements of the same preparations by atomic absorption and emission spectroscopy have revealed no significant deviations from the authors results. 

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. 

In metallurgy, alloy composition can be rapidly determined and unknown samples identified rapidly. XRF has an advantage over wet chemistry in that all the components can be measured due to the wide dynamic range of XRF. For example, in the analysis of nickel alloys, a wet chemical approach would measure all the other elements and calculate the Ni content as the balance. With XRF, the major element, nickel, as well as the minor and trace components can be measured accurately. For high-grade steel and alloys with multiple major components, WDXRF achieves better accuracy and repeatability than optical emission spectroscopy (OES). 

The uncertainties Ay/yhave been calculated taking into account the errors on Io/Iat and L (thickness of the recombination boundary layer) but also on the flow parameters the diffusion coefficient Do,air determined using the Chapman-Enskog theory, the mean square atomic velocity V determined using the gas kinetic theory (rarefied gas). The accuracy on these two last values is due essentially to that of the gas temperature, measured by emission spectroscopy (N2 rotational temperature), this leads to a total accuracy of 35%. 

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. 

CIR-FTIR spectroscopy provides a direct technique for studying in situ hydrous metal oxide surfaces and molecules adsorbed on these surfaces (37). By itself, FTIR spectrometry is a well established technique which offers numerous advantages over dispersive (grating) IR spectrometry (1) improved accuracy in frequency measurements through the use of a HeNe laser (2) simultaneous frequency viewing (3) rapid, repetitive scanning which allows many spectra to be collected in a small time interval (4) miriimal thermal effects from IR beam and (5) no detection of sample IR emissions (38). 

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