System peaks Speciation

Takatera and Watanabe [41] used this technique for the speciation of iodide ion, I-, and five iodo amino acids (monoiodotyrosine (MIT), diiodotyrosine (DIT), 3,3,5-triiodothyromine (T3), 3,3,5 -triiodothyromine (rT3), and thyroxine (T4)) which are all found in thyroid hormones. The speciation of these compounds in clinical samples such as blood plasma and urine may assist in the identification of thyroid diseases. The RPLC-ICP-MS system was able to detect all of the I-containing compounds with no interferences. Detection limits were in the range 35-130 pg for the six compounds using a 50% methanol eluent. Detection limits were better for species eluted at a shorter retention time since the peak shapes were sharper. The detection limits calculated were an order of magnitude lower than for methods where UV absorbance detection was used. 

Liquid chromatography (LC) is the most commonly used technique for trace element speciation with ICP-MS detection. The mobile phase flow rates used with most LC techniques (0.5-2.0 mL min-1) are compatible for ICP-MS introduction using conventional sample introduction systems (pneumatic nebulization with cross flow and concentric nebulizers and double-pass spray chambers). An interface, known as a transfer line, must be constructed to allow connection between the outlet of the LC column and the nebulizer of the ICP-MS. Inert plastic tubing is commonly used for this purpose with the inner diameter and length kept to 20-50 cm in order to minimize peak broadening. 

The first CE-ICP-MS coupled system was described by the Olesik group [100] for potential speciation studies. The need for an interface, with low dead volume to minimize peak broadening, was identified. In this interface, the capillary was grounded by coating 5 cm of the end with a controlled thickness of silver paint. The EOF was approximately 0.05 pL/min"1 however, the liquid flow rate increased, as a result of a vacuum effect from the nebulizer gas flow, which, in turn, resulted in a parabola-shaped velocity profile. This caused beak broadening to occur. 

A strong preference in speciation analysis is to use a separation step that can be combined with a detection step in an on-line system [45]. In such coupling, analytical selectivity relies on the application of different chromatographic or electrophoretic methods, while the use of atomic spectrometric techniques assures high sensitivity and f>t-for-purpose limits of detection (LoDs). However, hyphenated techniques with element-specif>c detection do not provide structural information on the species. If appropriate standards are available, the assignation of chromatographic peaks can be accomplished by spiking experiments. On the... 

Another approach to the assessment of ion speciation in the resin and solution phases with equal ility for the determination of formation constants of complexes in these two environments is provided by NMR spectroscopy. The method can provide sharp, well-separated signals for each successive complex species formed, the peak area being proportional to the atomic concentration of the element being measured, irrespective of its complex form. For a completely labile system, a well-defined chemical shift change is often observed with successive complex formation. These features make this analytical procedure well suited for study of metal ion complexation by the ion-exchanger phase. 

This example illustrates the potential of TPR. Quantitative information is obtained on the speciation of a complex catalyst system. On the other hand, additional techniques are needed to get a molecular picture. Furthermore, from the results major elements are obtained for a recipe for calcination and reduction treatments. For instance, calcination should take place at conditions such that nitrates are completely removed the temperature should exceed 650 K. Furthermore, in general, Co ions should not diffuse into the support. As a consequence, a temperature of 900 K should not be exceeded (see Fig. 12.3 above a calcination temperature of 925 K the high temperature peak increases strongly due to the... 

In speciation, glow discharges are excellent detectors for GC work as shown earlier. In addition to the low power and pressure ICPs they can be used successfully for element-specific detection for gas chromatography. An rf-GD-MS system has been used as a detector for GC by Olson et al. [661], The set-up should consist of a temperature-controlled transfer line of stainless steel from the exit of the GC to the inlet of the GD source. The system has been tested with tetraethyl-Pb, tetraethyl-Sn and tetrabutyl-Sn and provided useful structural information for the identification of these compounds through the observation of fragment peaks the detection limits were down to 1 pg. 

Solvent extracts are analysed by an HPLC system with a UV and/or fluorescence detector. The chromatogram can produce either a total PAH result, or can be fine-tuned to give speciated compounds. The method is specific and sensitive, but may suffer from co-elution, and the limit of detection for each PAH is 1 mg/kg. For soil analysis, this is not the preferred technique as build up of matrix components on the HPLC column can result in small shifts in retention times leading to mis-identification of peaks. 

The speciation of vanadate in aqueous systems is found to be both concentration and pH dependent. The NMR spectrum of the vanadate solution contains four main peaks at —542, —564, —574 and —582 ppm, which are attributed to vanadate monomer, dimer, tetramer and pentamer, respectively [166,167]. A dynamic analysis combined with exchange rates determined by using the 2D EXSY NMR spectrum, allowed quantification of exchange pathways, and has yielded information about the kinetic stability of the vanadate oligomers (Figure 5.9) [167]. The major pathway for monomer formation is unimolecular decomposition of the dimer conversely, the major pathway for... 

The main application of the LC/AAS system is to help determine metal speciation in samples, not merely to identify the presence of a particular element. It is not enough to detect the presence of lead, mercury or chromium, but to be able to identify the form in which they are present. Depending upon the chemical form of a mercury compound it may or may not be toxic. Similarly if chromium is present in the tertiary form it is not particularly dangerous conversely, in its sixth valency state, it is strongly carcinogenic. It follows that the liquid chromatograph can be employed to separate the different species and the atomic spectrometer can identify and confirm the presence of a specific element in the appropriate peaks. 

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