On-line matrix removal

Miro M, Jonczyk S, Wang J and Hansen EH (2003) Exploiting the bead-injection approach in the integrated sequential injection lab-on-valve format using hydrophobic packing materials for on-line matrix removal and preconcentration of trace levels of cadmium in environmental and biological samples via formation of non-charged chelates prior to ETAAS detection. J. Anal Atom Spectrom 18 BOOB. 

Amperometric detectors, or specifically their electrodes, need much more care and maintenance and are therefore used only in cases where conductivity measurements are not possible or sensitive enough. Analytes range from sugars, metal-oxo-anions, to toxic anions, such as cyanide and sulphide. The determination of cyanide and sulphide in a drug substance is shown in Fig.5. The sample is analyzed after on-line matrix removal and accumulation of the analyte (a system similar to that described by Reust et al. in ) amperometrically on a silver electrode. 

Off-line dicarbamate solvent extraction and ICP-MS analysis [317] provided part-per-trillion detection limits Cd (0.2 ppt), Co (0.3 ppt), Cu (3 ppt), Fe (21 ppt), Ni (2 ppt), Pb (0.5 ppt), and Zn (2 ppt). Off-line matrix removal and preconcentration using cellulose-immobilized ethylenediaminetetraacetic acid (EDTA) have also been reported [318]. Transition metals and rare earth elements were preconcentrated and separated from the matrix using on-line ion chromatography with a NTA chelating resin [319]. Isotope-dilution-based concentration measurement has also been used after matrix separation with a Chelex ion-exchange resin [320]. The pH, flow rate, resin volume, elution volume, and time required for isotope equilibration were optimized. A controlled-pore glass immobilized iminodiacetate based automated on-line matrix separation system has also been described [321]. Recoveries for most metals were between 62% and 113%. 

Flow-injection (FI) on-line analyte preconcentration and matrix removal techniques greatly enhance the performance of atomic spectrometry [348], By using USN with membrane desolvation (MDS) as the interface, FI sorbent extraction can be directly coupled with ICP-MS for the analysis of organic solutions [349]. 

A typical extraction manifold is shown in Figure 13.2. The sample is introduced by aspiration or injection into an aqueous carrier that is segmented with an organic solvent and is then transported into a mixing coil where extraction takes place. Phase separation occurs in a membrane phase separator where the organic phase permeates through the Teflon membrane. A portion of one of the phases is led through a flow cell and an on-line detector is used to monitor the analyte content. The back-extraction mode in which the analyte is returned to a suitable aqueous phase is also sometimes used. The fundamentals of liquid liquid extraction for FIA [169,172] and applications of the technique [174 179] have been discussed. Preconcentration factors achieved in FIA (usually 2-5) are considerably smaller than in batch extraction, so FI extraction is used more commonly for the removal of matrix interferences. 

Hollenbach et al. captured "Tc from standards or soil sample digestates on a TEVA-Resin column for on-line purification and preconcentration prior to ICP-MS determination.49 A wash solution of 0.5 M HN03 was used to remove interferences prior to elution with 8 M HN03 solution. A Re isotope, which behaves similarly to pertechnetate on TEVA-Resin, was used as an internal standard. Recoveries from the column varied from 97 to 99.5%, and columns could be reused over a hundred times. The use of the on-line column separation reduced detection limits by 10-fold and alleviated matrix and isobaric interferences compared to direct sample injection. This pioneering study adapted FIAS-200 and FIAS-400 FI systems to perform sample injection and extraction-chromatographic separations upstream from the ICP-MS. 

Truscott, J. B., Bromley, L., Jones, P., Evans, E. H., Turner, J., and Fairman, B., Determination of natural uranium and thorium in environmental samples by ETV-ICP-MS after matrix removal by on-line solid phase extraction, J. Anal. At. Spectrom., 14, 627-631, 1999. 

In the analysis of high-purity substances, general matrix removal is often very important so as to pre-concentrate the elements to be determined. To this aim all separation techniques such as ion exchange, liquid-liquid extraction of a metal complex with organic solvents, fractionated crystallization, precipitation and coprecipitation as well as electrochemical methods may be used (for a systematic treatment, see Ref. [300]). These principles can also be applied in on-line systems, as is now possible with solid phase extraction. Here matrix elements or the analytes can be adsorbed as complexes onto the column and eluted for direct determination by AAS. 

A reduction of the interferences resulting both from spectral overlap and ionization can be realized in a number of cases by removal of acids and matrix residuals, as shown by on-line removal of these species by separations on solid phases, which has now been tested in the case of exchanges made at modified nebulizer surfaces [517]. 

Although many individual choices for sample preparation exist, sometimes the combination of two techniques yields a more desirable result. Within a drug discovery environment, however, throughput and cost are important factors that influence the choice of sample-preparation methods. The protein-precipitation procedure can quickly yield a sample that is ready for analysis, but its potential for carryover of matrix interferences is problematic. The isolated supernatant can be filtered or centrifuged before injection, but these procedures remove only particulates or proteinaceous material, not the materials that cause matrix interferences. Thus, protein precipitation is sometimes followed by LLE, SPE, or an on-line technique for a more selective cleanup before analysis. A particularly challenging sample-preparation requirement is the analysis of animal tissues, as is commonly performed in drug-discovery support laboratories that perform drug uptake studies. Here, a series of sequential sample-preparation steps are common [121]. 

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