Liquid chromatography multidimensional

Liquid chromatography (LC) is a good alternative to GC for polar or thermolabile eompounds. While polar eompounds need to be derivatized for GC analysis, this is therefore not neeessary for LC analysis. 

When environmental samples are analysed by reverse-phase liquid ehromatogra-phy, the most widely used teehnique, polar interferenees usually appear (ions, plus humie and fulvie aeids). This makes it diffieult to determine more polar eompounds that elute in the first part of the ehromatogram. This is speeially important when deteetion is not seleetive, e.g. UV deteetion, whieh is one of the most eommon teeh-niques in routine analysis. In sueh eases, multidimensional ehromatography plays an important role. 

The applieation range of eoupled-eolumn teehnology is determined by the separation power of the first eolumn. In general, it ean be said that low resolution favours multiresidue methods (MRMs), while high resolution leads to methods for a single analyte or for a group of analytes with similar properties. 

Multidimensional LC-LC, using two high-resolution eolumns with orthogonal separation meehanisms, has only a few applieations in environmental analysis. The limitations that sueh a multidimensional system has with regard to seleetivity must 

In spite of Such limitations, some examples can be found in literature. For example, a reversed-phase Cjg column has also been coupled to a weak ion-exchange column to determine gluphosinate, glyphosate and aminomethylphosphonic acid (AMPA) in environmental water (28). This method will be described further below. 

Principles and Characteristics The use of a liquid chromatographic separation as a means of preparing samples for subsequent analysis by another chromatographic separation is well established. The goal of such cleanup separations is to reduce the complexity of the 

The use of LC as a sample pretreatment of GC has received considerable attention. The separation mechanisms in LC and GC are complementary, providing a powerful combined tool for the separation of complex 

The main reason for pursuing on-line LC-GC is that FIPLC provides far better resolution than conventional techniques of sample preparation (e.g. involving 

subcritical (hot/liquid) water can be used as a mobile phase for packed-column RPLC with solute detection by means of FID [942]. In the multidimensional on-line PHWE-LC-GC-FTD/MS scheme, the solid sample is extracted with hot pressurised water (without the need for sample pretreatment), and the analytes are trapped in a solid-phase trap [943]. The trap is eluted with a nitrogen flow, and the analytes are carried on to a LC column for cleanup, and separated on a GC column using the on-column interface. The closed PHWE-LC-GC system is suitable for many kinds of sample matrices and analytes. The main benefit of the system is that the concentration step is highly efficient, so that the sensitivity is about 800 times better than that obtained with traditional methods [944]. Because small sample amounts are required (10 mg), special attention has to be paid to the homogeneity of the sample. The system is 

In on-line multidimensional HPLC (MDHPLC) two relatively high-efficiency columns are coupled in an instrument, via the use of valves, traps and other means. In LC-LC the precolumn is used for sample cleanup and prefractionation, before introduction of the fraction of interest to the analytical column. Much of the instrumentation for MDHPLC is the same as that in conventional one-dimensional experiments. However, the additional complexity of MDHPLC experiments leads to greater difficulties than those found in conventional HPLC  

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EXAMPLES OE MULTIDIMENSIONAL LIQUID CHROMATOGRAPHY IN ENVIRONMENTAL ANALYSIS... 

J. V. Posluszny and R. Weinherger, Detemiination of drug substances in biological fluids by diiect injection multidimensional liquid chromatography with a micellar cleanup and reversed-phase clnomatography , Chem. 60 1953-1958(1988). 

Multidimensional liquid chromatography encompasses a variety of techniques used for seunple separation, cleanup and trace enrichment [12,279-289]. A characteristic feature of these methods is the use of two or more columns for the separation with either manual or automatic switching by a valve interface of fractions between columns. These techniques require only minor modification to existing equipment, and of equal importance, enable the sample preparation and separation procedures to be completely automated. 

SEC in combination with multidimensional liquid chromatography (LC-LC) may be used to carry out polymer/additive analysis. In this approach, the sample is dissolved before injection into the SEC system for prefractionation of the polymer fractions. High-MW components are separated from the additives. The additive fraction is collected, concentrated by evaporation, and injected to a multidimensional RPLC system consisting of two columns of different selectivity. The first column is used for sample prefractionation and cleanup, after which the additive fraction is transferred to the analytical column for the final separation. The total method (SEC, LC-LC) has been used for the analysis of the main phenolic compounds in complex pyrolysis oils with minimal sample preparation [974]. The identification is reliable because three analytical steps (SEC, RPLC and RPLC) with different selectivities are employed. The complexity of pyrolysis oils makes their analysis a demanding task, and careful sample preparation is typically required. 

Multidimensional liquid chromatography and tandem mass spectrometry... 

One attempt to overcome these disadvantages has been to use multidimensional liquid chromatography (LC) followed directly by tandem mass spectrometry to separate, fragment and identify proteins (Link et al., 1999). In this process, a denatured and reduced protein mixture is digested with a protease to create a collection of peptides (Fig. 2.6). The peptide mixture is applied to a cation exchange column and a fraction of these peptides are eluted based on charge onto a reverse-phase column. The... 

Multidimensional Liquid Chromatography Theory and Applications in Industrial Chemistry and the Life Sciences, Edited by Steven A. Cohen and Mark R. Schure. 

Two books of interest have covered some of the ground for multidimensional liquid chromatography in various forms. These include the book Multidimensional Chromatography, edited by Cortes (1990) and Multidimensional Chromatography by Mondello et al. (2002). 

METHOD DEVELOPMENT IN COMPREHENSIVE MULTIDIMENSIONAL LIQUID CHROMATOGRAPHY... 

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