Impurity analysis instrumental techniques

Chemical Properties. Elemental analysis, impurity content, and stoichiometry are determined by chemical or instrumental analysis. The use of instrumental analytical methods (qv) is increasing because these are usually faster, can be automated, and can be used to determine very small concentrations of elements (see Trace and RESIDUE ANALYSIS). Atomic absorption spectroscopy and x-ray fluorescence methods are the most useful instrumental techniques in determining chemical compositions of inorganic pigments. Chemical analysis of principal components is carried out to determine pigment stoichiometry. Analysis of trace elements is important. The presence of undesirable elements, such as heavy metals, even in small amounts, can make the pigment unusable for environmental reasons. 

There is a need to evaluate, by newer thermal analysis instrumentation and techniques, polymers previously only briefly characterized, emphasizing those products which show potential industrial application or other meritorious property. Such products should be well characterized with regard to such factors as chain length, molecular weight distribution, endgroup, purity, nature and amount of impurities, and actual morphological structure of the polymer. 

The use of modern analytical instruments has greatly expanded the analyst s ability to determine impurities in silicates. Wet chemical methods usually are far too tedious, suffer from substantial interference or are not sensitive enough for impurity analysis. Even some instrumental techniques are subject to interferences, requiring separation to be used in the analytical procedure. The analyst must also decide on the sensitivity required since lowering detection limits usually increases the cost of analysis and the sophistication of the analytical procedure. 

INSTRUMENTAL TECHNIQUES FOR SILICATE IMPURITY ANALYSIS Detection of Cations... 

It is very important to note that the metallic impurities in CNTs are the important factor in inducing significant toxic responses. Therefore, a quantitative measurement of the concentration of metal impurities in CNTs is key, although this is very difficult. Recently, neutron activation analysis (NAA) technique as a non-destructive standard method has been used to quantitatively analyse the metal impurities in CNTs, and ICP-MS is regarded as a practical analytical method. In the absence of a true reference material for CNTs, the NAA method can provide the best estimate of the true value of metallic impurities in CNTs, while ICP-MS is a desktop instrumental... 

Analysis of such small amounts of materials requires particularly sensitive instrumental techniques, which should ideally be able to provide separation from any contaminants and an unambiguous identification. GC-MS is the ideal tool for this as this not only provides a separation from any impurities, but also gives an absolute identification in the mass spectrum. 

Atomic absorption spectroscopy of VPD solutions (VPD-AAS) and instrumental neutron activation analysis (INAA) offer similar detection limits for metallic impurities with silicon substrates. The main advantage of TXRF, compared to VPD-AAS, is its multielement capability AAS is a sequential technique that requires a specific lamp to detect each element. Furthermore, the problem of blank values is of little importance with TXRF because no handling of the analytical solution is involved. On the other hand, adequately sensitive detection of sodium is possible only by using VPD-AAS. INAA is basically a bulk analysis technique, while TXRF is sensitive only to the surface. In addition, TXRF is fast, with an typical analysis time of 1000 s turn-around times for INAA are on the order of weeks. Gallium arsenide surfaces can be analyzed neither by AAS nor by INAA. 

As liquid chromatography plays a dominant role in chemical separations, advancements in the field of LC-NMR and the availability of commercial LC-NMR instrumentation in several formats has contributed to the widespread acceptance of hyphenated NMR techniques. The different methods for sampling and data acquisition, as well as selected applications will be discussed in this section. LC-NMR has found a wide range of applications including structure elucidation of natural products, studies of drug metabolism, transformation of environmental contaminants, structure determination of pharmaceutical impurities, and analysis of biofiuids such as urine and blood plasma. Readers interested in an in-depth treatment of this topic are referred to the recent book on this subject [25]. 

A convenient method is the spectrometric determination of Li in aqueous solution by atomic absorption spectrometry (AAS), using an acetylene flame—the most common technique for this analyte. The instrument has an emission lamp containing Li, and one of the spectral lines of the emission spectrum is chosen, according to the concentration of the sample, as shown in Table 2. The solution is fed by a nebuhzer into the flame and the absorption caused by the Li atoms in the sample is recorded and converted to a concentration aided by a calibration standard. Possible interference can be expected from alkali metal atoms, for example, airborne trace impurities, that ionize in the flame. These effects are canceled by adding 2000 mg of K per hter of sample matrix. The method covers a wide range of concentrations, from trace analysis at about 20 xg L to brines at about 32 g L as summarized in Table 2. Organic samples have to be mineralized and the inorganic residue dissolved in water. The AAS method for determination of Li in biomedical applications has been reviewed . 

Depth scale calibration of an SIMS depth profile requires the determination of the sputter rate used for the analysis from the crater depth measurement. An analytical technique for depth scale calibration of SIMS depth profiles via an online crater depth measurement was developed by De Chambost and co-workers.103 The authors proposed an in situ crater depth measurement system based on a heterodyne laser interferometer mounted onto the CAMECA IMS Wf instrument. It was demonstrated that crater depths can be measured from the nm to p,m range with accuracy better than 5 % in different matrices whereas the reproducibility was determined as 1 %.103 SIMS depth profiling of CdTe based solar cells (with the CdTe/CdS/TCO structure) is utilized for growing studies of several matrix elements and impurities (Br, F, Na, Si, Sn, In, O, Cl, S and ) on sapphire substrates.104 The Sn02 layer was found to play an important role in preventing the diffusion of indium from the indium containing TCO layer. 

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