Instrumentation atomic spectroscopy

Most chemists are familiar with atomic emission spectroscopic techniques for metal analysis of aqueous solutions and are equally aware that most of these methods cannot be readily applied to non-aqueous samples. In recent years atomic spectroscopy instrumentation has increased in sophistication allowing the analysis of a wide range of samples on a routine basis for metals content using manual or automated methods. This book aims to cover the importance of metal analysis for a range of organic samples. 

We have acquired a contract from a company called The Solution Makers located in northern New Jersey. This company prepares and sells certified standard solutions for use in accredited laboratories worldwide. They are especially known for their high-quality atomic absorption standard solutions widely used to calibrate atomic spectroscopy instruments as well as other instruments. The chemicals they use to prepare these solutions are purchased from The Inorganic Chemical Company of North America (ICCNA). 

The present book is designed to describe the basic theory of atomic spectroscopy, instrumentation, techniques, and the application of various analytical atomic spectrometric methods (AAS, plasma AES, AFS, and ICP-MS). 

The atomic spectroscopic techniques discussed in Chapters 6 and 7 and ICP-MS discussed in Chapters 9 and 10 each have advantages and disadvantages in the determination of elements in real samples. A summary of the capabilities of each technique and a comparison of the techniques is given in Appendix 7.B. Most major instrument companies have selection guides on their websites that compare their atomic spectroscopy instruments by DL, analytical working range, sample throughput, interferences, cost, and so on. 

This limitation led to the development of laser ablation as a sampling device for atomic spectroscopy instrumentation, where the sampling step was completely separated from the excitation or ionization step. The major benefit is that each step can be independently controlled and optimized. These early devices used a high-energy laser to ablate the surface of a solid sample, and the resulting aerosol was swept into some kind of atomic spectrometer for analysis. Although initially used with atomic absorption - and plasma-based emission techniques, it was not until... 

Whereas some instrumental methods produce just one signal per component, N = n, e.g., chromatographic methods, some other methods such as atomic spectroscopy generate much more signals as required, N n. 

The following block schemas show the essential instrumental features of the various atomic spectroscopy techniques. Clearly, there are many similarities between these techniques. The subsequent discussions will describe the instrumental components of these techniques. 

The selection of a technique to determine the concentration of a given element is often based on the availability of the instrumentation and the personal preferences of the analytical chemist. As a general rule, AAS is preferred when quantifications of only a few elements are required since it is easy to operate and is relatively inexpensive. A comparison of the detection limits that can be obtained by atomic spectroscopy with various atom reservoirs is contained in Table 8.1. These data show the advantages of individual techniques and also the improvements in detection limits that can be obtained with different atom reservoirs. 

If you were asked to suggest improvements to instrumentation for atomic spectroscopy what areas would you propose What is the basis for your selection ... 

Perhaps the most noticeable difference in instrumentation is the sample container used for atomic spectroscopy. This container is the source of the thermal energy needed for the conversion of ions in solution to atoms in the gas phase (and hence is called an atomizer) and in no way resembles a simple cuvette. Recall, for example, the brief discussion and photograph of the flame container in Section 7.5. 

The need for and use of thermal energy as outlined above has resulted in the invention of a number of separate and distinctly different atomizer and instrument designs, albeit based on the same theory, under the heading of atomic spectroscopy. 

An alternative approach is to analyze the samples using procedures or instrumentation that will give the maximum amount of data for each sample. For example, recent advances in atomic spectroscopy, i.e., inductively coupled argon plasma emission spectroscopy (ICP-AES), allow 20 to 30 elements to be detected simultaneously. 

Atomic absorption spectroscopy instrumentation can conveniently be considered under the following subheadings. 

Analyte is measured at parts per million ( xg/g) to parts per trillion (pg/g) levels. To analyze major constituents, the sample must be diluted to reduce concentrations to the parts per million level. As we saw in the analysis of teeth, trace constituents can be measured directly without preconcentration. The precision of atomic spectroscopy, typically 1-2%, is not as good as that of some wet chemical methods. The equipment is expensive, but widely available. Unknowns, standards, and blanks can be loaded into an autosampler, which is a turntable that automatically rotates each sample into position for analysis. The instrument runs for many hours without human intervention. 

Figure 21-6 An electrically heated graphite furnace for atomic spectroscopy (—38 mm long, in this case). [Courtesy Instrumentation Laboratory, Wiirrington, MA.]...

The besl isolation of radiant energy can he achieved with flame spectrometers that incorporate either a prism sir grating monochromator, those with prisms having variable gauged entrance and exii slits. Both these spectrometers provide a continuous selection of wavelengths with resolving power sufficient lo separate completely most of the easily excited emission lines, and afford freedom from scattered radiation sufficient lo minimize interferences. Fused silica or quartz optical components are necessary to permit measurements in Ihe ultraviolet portion of the spectrum below 350 nanometers Sec also Analysis (Chemical) Atomic Spectroscopy Photometers and Spectra Instruments. 

Once the sample preparation is complete, the analysis is carried out by an instrument of choice. A variety of instruments are used for different types of analysis, depending on the information to be acquired for example, chromatography for organic analysis, atomic spectroscopy for metal analysis, capillary electrophoresis for DNA sequencing, and electron microscopy for small structures. Common analytical instrumentation and the sample preparation associated with them are listed in Table 1.1. The sample preparation depends on the analytical techniques to be employed and their capabilities. For instance, only a few microliters can be injected into a gas chromatograph. So in the example of the analysis of pesticides in fish liver, the ultimate product is a solution of a few microliters that can be injected into a gas chromatograph. Sampling, sample preservation, and sample preparation are... 

However, as real-world analytical problems are most often neither ideal nor routine, analytical instrumentation supplemental to atomic spectroscopy would be a great advantage in developing methods for non routine applications. Thus, it is becoming apparent that the well-equipped analytical laboratory of the future will incorporate both atomic spectroscopy and ion chromatography. 

Atomic absorption remained the technique of choice until relatively recently. However, with the introduction of plasma sources, atomic emission, in the form of inductively coupled plasma spectroscopy, has made a comeback. This development is now receiving historical attention, and was the subject of a symposium held in 1999. Papers discussed atomic emission analysis prior to 1950,206 the fact that emission techniques developed continuously, even in the period when absorption methods were dominant,207 and the development of the plasma sources on which the new techniques depend.208 Also discussed was the powerful hyphenated technique of ICP-MS,209 and the history of one of the leading manufacturers of atomic emission instruments.210... 

TABLE 3.1. Instrumental LoDs in pg 1 From Guide to Atomic Spectroscopy Techniques and Applications, Perkin-Elmer, 2000 ... 

The precision of an instrument must be considered. Many typical measurements, for example, in atomic spectroscopy, are recorded to only two significant figures. Consider a dataset in which about 95 % of the readings were recorded between 0.10 and 0.30 absorbance units, yet a statistically designed experiment tries to estimate 64 effects. The /-test provides information on the significance of each effect. However, statistical tests assume that the data are recorded to indefinite accuracy, and will not take this lack of numerical precision into account. For the obvious effects, chemo-metrics will not be necessary, but for less obvious effects, the statistical conclusions will be invalidated because of the low numerical accuracy in the raw data. 

The use of analytical atomic spectroscopy in clinical chemistry has developed rapidly over the last 20 years and there is now adequate knowledge and instrumentation available for the measurement of a wide range of elements (C12, H25, M4, W25) in concentrations as low as 1 ng/ml or amounts as small as 10" g. The cost of the instruments ranges from 100 ( 240) for the simplest flame photometer to 50,000 ( 120,000) for an advanced direct reading spectrometer with data handling facilities. 

X-Ray fluorescence is nondestructive and has significant advantages in simultaneous multielement analysis and ultramicroanalysis using electron beam excitation. It has found widespread industrial applications but as instrumentation is costly and complex in comparison with analytical atomic spectroscopy, the technique is not suitable for routine use in clinical chemistry. It seems unlikely that it can ever be more than a research tool. 

The physico-chemical properties of the analytes and the way they reach the detector have made atomic spectroscopy the detection technique of choice in most instances. A heated quartz cell or a similar device is connected directly to the gas outlet of the separation cell [26]. The use of an atomic fluorescence detector has provided methods for selenium [25,27] and mercury [28,29] that possess excellent analytical features and use inexpensive instruments. On a less affordable level are ICP emission [30] and atomic emission cavity spectrometers [31]. 

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