Sample-introduction methods atomic spectroscopy

TABLE 8-2 Methods of Sample Introduction in Atomic Spectroscopy... 

Amongst the wide range of sample introduction methods available for atomic emission spectroscopy, chromatographic methods are most popular as they transform a complex mixture into a time-resolved separated analyte stream [49]. 

For the first five atomization sources listed in Tabic 8-1, samples are usually introduced in the form of aqueous solutions (occasionally, nonaqueous solutions are used) or less often as slurries (a slurry is a suspension of a finely divided powder in a liquid). For samples that are difficult to dissolve, however, several mcth(xls have been used to introduce samples into the atomizer in the form of solids or finely dispersed powders. Generally, -solid sample-introduction techniques are less reproducible and more subject to various errors and as a result are not nearly as widely applied as aqueous solution techniques. Table 8-2 lists the common sample-introduction methods for atomic spectroscopy and the type of samples to which each method is applicable. 

To examine a sample by inductively coupled plasma mass spectrometry (ICP/MS) or inductively coupled plasma atomic-emission spectroscopy (ICP/AES) the sample must be transported into the flame of a plasma torch. Once in the flame, sample molecules are literally ripped apart to form ions of their constituent elements. These fragmentation and ionization processes are described in Chapters 6 and 14. To introduce samples into the center of the (plasma) flame, they must be transported there as gases, as finely dispersed droplets of a solution, or as fine particulate matter. The various methods of sample introduction are described here in three parts — A, B, and C Chapters 15, 16, and 17 — to cover gases, solutions (liquids), and solids. Some types of sample inlets are multipurpose and can be used with gases and liquids or with liquids and solids, but others have been designed specifically for only one kind of analysis. However, the principles governing the operation of inlet systems fall into a small number of categories. This chapter discusses specifically substances that are normally liquids at ambient temperatures. This sort of inlet is the commonest in analytical work. 

The development of solvent-impregnated resins and extraction-chromatographic procedures has enabled the automation of radiochemical separations for analytical radionuclide determinations. These separations provide preconcentration from simple matrices like groundwater and separation from complex matrixes such as dissolved sediments, dissolved spent fuel, or nuclear-waste materials. Most of the published work has been carried out using fluidic systems to couple column-based separations to on-line detection, but robotic methods also appear to be very promising. Many approaches to fluidic automation have been used, from individual FI and SI systems to commercial FI sample-introduction systems for atomic spectroscopies. 

Tt may be safe to say that the interest of environmental scientists in airborne metals closely parallels our ability to measure these components. Before the advent of atomic absorption spectroscopy, the metal content of environmental samples was analyzed predominantly by wet or classical chemical methods and by optical emission spectroscopy in the larger analytical laboratories. Since the introduction of atomic absorption techniques in the late 1950s and the increased application of x-ray fluorescence analysis, airborne metals have been more easily and more accurately characterized at trace levels than previously possible by the older techniques. These analytical methods along with other modem techniques such as spark source mass spectrometry and activation analysis... 

In the last 20 years atomic spectroscopy has made great strides, particularly with the introduction of new improved optic designs and detection methods. These improvements have led to superior resolution of the wavelengths of the excited atoms and detection techniques measuring lower levels of metals with ease. After a slow and problematic start, inductively coupled plasma optical emission spectrometry (ICP-OES) has become an established technique in most laboratories analysing a wide range of sample matrices reporting accurate and precise results. 

Magnetic sector instruments are further distinguished by their method of sample introduction - how the sample is put into the electromagnetic field. Many instruments today use a plasma, identical to that in optical ICP spectroscopy (ICP-MS). In the mass spec, the plasma is used to ionize the atoms. Although the plasma emits the characteristic spectra used in ICP-OES, this visible light output is not used by the mass spectrometer. Figure 4.25 shows the ICP-MS in the Laboratory for Archaeological Chemistry at the University of Wisconsin-Madison. 

The detection systems used with HPLC can be broadly divided into three approaches photometry, plasma techniques (ICPAES, ICPMS), and cold vapour atomic absorption and fluorescence spectroscopy (CV-AAS, CV-AFS). The method with the lowest limits of detection (LOD) with sample introduction via a direct injection nebulizer used ICP-MS. An HPLC system coupled to atmospheric pressirre chemical ionization MS was used to identify methyl mercury spiked into a fish tissue CRM (DORM-1, NRCC). This type of system has a significant advantage over elemental detection methods because identification of the species present is based on their structure, rather than matching the analyte s retention time to that of a standard. 

Atomic fluorescence flame spectrometry is receiving increased attention as a potential tool for the trace analysis of inorganic ions. Studies to date have indicated that limits of detection comparable or superior to those currently obtainable with atomic absorption or flame emission methods are frequently possible for elements whose emission lines are in the ultraviolet. The use of a continuum source, such as the high-pressure xenon arc, has been successful, although the limits of detection obtainable are not usually as low as those obtained with intense line sources. However, the xenon source can be used for the analysis of several elements either individually or by scanning a portion of the spectruin. Only chemical interferences are of concern they appear to be qualitatively similar for both atomic absorption and atomic fluorescence. With the current development of better sources and investigations into devices other than flames for sample introduction, further improvements in atomic fluorescence spectroscopy are to be expected. 

Flow injection analysis and atomic emission spectroscopy (FIA-AES) Flow injection analysis (FIA) is a method in which small sample volumes, typicaUy about 10— 200 pL, are injected into a continuously flowing carrier stream. Corresponding transient signals can then be monitored each time a sample is introduced into the atomiser. The primary advantage of FIA as a means of sample introduction... 

Another method included in this chapter is ICP-MS, which, although not based on atomic spectroscopy in a true sense, utilizes the same instrumental approach as ICP-AES for sample introduction. The difference is that analyte quantification takes place using a mass spectroscopy detector. 

Most of these techniques have either limited applicability or suffer from inconsistent precision and accuracy and therefore have not been adopted as routine approaches. Laser ablation is probably one of the most promising methods in the above list, with high potential to provide an alternative sample introduction route for the different atomic spectroscopy techniques. 

Atomic emission spectroscopy is one of the most useful and commonly used techniques for analyses of metals and nonmetals providing rapid, sensitive results for analytes in a wide variety of sample matrices. Elements in a sample are excited during their residence in an analytical plasma, and the light emitted from these excited atoms and ions is then collected, separated and detected to produce an emission spectrum. The instrumental components which comprise an atomic emission system include (1) an excitation source, (2) a spectrometer, (3) a detector, and (4) some form of signal and data processing. The methods discussed will include (1) sample introduction, (2) line selection, and (3) spectral interferences and correction techniques. 

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