Sample analysis techniques involved

Isotopic dilution analysis is widely used to determine the amounts of trace elements in a wide range of samples. The technique involves the addition to any sample of a known quantity (a spike) of an isotope of the element to be analyzed. By measuring isotope ratios in the sample before and after addition of the spike, the amount of the trace element can be determined with high accuracy. The method is described more fully in Figure 48.13. 

Microscopy (qv) plays a key role in examining trace evidence owing to the small size of the evidence and a desire to use nondestmctive testing (qv) techniques whenever possible. Polarizing light microscopy (43,44) is a method of choice for crystalline materials. Microscopy and microchemical analysis techniques (45,46) work well on small samples, are relatively nondestmctive, and are fast. Evidence such as sod, minerals, synthetic fibers, explosive debris, foodstuff, cosmetics (qv), and the like, lend themselves to this technique as do comparison microscopy, refractive index, and density comparisons with known specimens. Other microscopic procedures involving infrared, visible, and ultraviolet spectroscopy (qv) also are used to examine many types of trace evidence. 

ICPMS is uniquely able to borrow a quantitation technique from molecular mass spectrometry. Use of the isotope dilution technique involves the addition of a spike having a different isotope ratio to the sample, which has a known isotope ratio. This is usefiil for determining the concentration of an element in a sample that must undergo some preparation before analysis, or for measuring an element with high precision and accuracy. ... 

The second technique involves direct sampling of the reactor effluent (Figure 5-2). In this technique, a sample of reactor effluent is collected in an aluminized polyester bag for separation and analysis. 

It is worth briefly pointing out the difference between in-line and online analysis. In-line analysis does not involve removal of the sample from the reaction vessel, for example in determining water or oxygen content. On-line analysis does involve removing a sample, usually as a side stream, which adds to the complexity of the plant since the sample off-take equipment will often need to be built to the same integrity as the plant. There are four common techniques employed ... 

Classical LLEs have also been replaced by membrane extractions such as SLM (supported liquid membrane extraction), MMLLE (microporous membrane liquid-liquid extraction) and MESI (membrane extraction with a sorbent interface). All of these techniques use a nonporous membrane, involving partitioning of the analytes [499]. SLM is a sample handling technique which can be used for selective extraction of a particular class of compounds from complex (aqueous) matrices [500]. Membrane extraction with a sorbent interface (MESI) is suitable for VOC analysis (e.g. in a MESI- xGC-TCD configuration) [501,502]. 

After matrix removal, samples can be measured using various techniques, such as AAS, AES, ICP, etc. Traditional chemical analysis methods, involving separation and gravimetric, titrimetric or polarographic determination of the elements, are being replaced by a wide selection of instrumental methods. 

Principles and Characteristics The fastest growing area in elemental analysis is in the use of hyphenated techniques for speciation measurement. Elemental spe-ciation analysis, defined as the qualitative identification and quantitative determination of the individual chemical forms that comprise the total concentration of an element in a sample, has become an important field of research in analytical chemistry. Speciation or the process yielding evidence of the molecular form of an analyte, has relevance in the fields of food, the environment, and occupational health analysis, and involves analytical chemists as well as legislators. The environmental and toxicological effects of a metal often depend on its forms. The determination of the total metal content... 

When heated, many solids evolve a gas. For example, most carbonates lose carbon dioxide when heated. Because there is a mass loss, it is possible to determine the extent of the reaction by following the mass of the sample. The technique of thermogravimetric analysis involves heating the sample in a pan surrounded by a furnace. The sample pan is suspended from a microbalance so its mass can be monitored continuously as the temperature is raised (usually as a linear function of time). A recorder provides a graph showing the mass as a function of temperature. From the mass loss, it is often possible to establish the stoichiometry of the reaction. Because the extent of the reaction can be followed, kinetic analysis of the data can be performed. Because mass is the property measured, TGA is useful for... 

Falkner and Edmond [334] determined gold at femtomolar quantities in seawater by flow injection inductively coupled plasma quadrupole mass spectrometry. The technique involves preconcentration by anion exchange of gold as a cyanide complex, [AulCNjj], using 195Au radiotracer (ti/2 = 183 days) to monitor recoveries. Samples are then introduced by flow injection into an inductively coupled plasma quadrupole mass spectrometer for analysis. The method has a detection limit of 10 fM for 4 litres of seawater preconcentrated to 1 ml, and a relative precision of 15% at the 100 fM level. 

Instrumental analysis can also involve chemical reactions, but it always involves modern sophisticated electronic instrumentation. Instrumental analysis techniques are high-tech techniques, often utilizing the ultimate in complex hardware and software. While sometimes not as precise as a carefully executed wet chemical method, instrumental analysis methods are fast and can offer a much greater scope and practicality to the analysis. In addition, instrumental methods are generally used to determine the minor constituents or constituents that are present in low levels, rather than the major constituents of a sample. We discuss wet chemical methods in Chapters 3 and 5. Chapter 15 is concerned with physical properties Chapters 7 to 14 involve specific instrumental methods. 

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