Multidimensional sample preparation

In order to reduce or eliminate off-line sample preparation, multidimensional chromatographic techniques have been employed in these difficult analyses. LC-GC has been employed in numerous applications that involve the analysis of poisonous compounds or metabolites from biological matrices such as fats and tissues, while GC-GC has been employed for complex samples, such as arson propellants and for samples in which special selectivity, such as chiral recognition, is required. Other techniques include on-line sample preparation methods, such as supercritical fluid extraction (SFE)-GC and LC-GC-GC. In many of these applications, the chromatographic method is coupled to mass spectrometry or another spectrometiic detector for final confirmation of the analyte identity, as required by many courts of law. 

Although SFE and SFC share several common features, including the use of a superaitical fluid as the solvent and similar instrumentation, their goals are quite distinct. While SFE is used mainly for the sample preparation step (extraction), SFC is employed to isolate (chr-omatography) individual compounds present in complex samples (11 -15). Both techniques can be used in two different approaches off-line, in which the analytes and the solvent are either vented after analysis (SFC) or collected (SFE), or on-line coupled with a second technique, thus providing a multidimensional approach. Off-line methods are slow and susceptible to solute losses and contamination the on-line coupled system makes possible a deaease in the detection limits, with an improvement in quantification, while the use of valves for automation results in faster and more reproducible analyses (16). The off-line... 

Although on-line sample preparation cannot be regarded as being traditional multidimensional chromatography, the principles of the latter have been employed in the development of many on-line sample preparation techniques, including supercritical fluid extraction (SFE)-GC, SPME, thermal desorption and other on-line extraction methods. As with multidimensional chromatography, the principle is to obtain a portion of the required selectivity by using an additional separation device prior to the main analytical column. 

The coupling of supercritical fluid extraction (SEE) with gas chromatography (SEE-GC) provides an excellent example of the application of multidimensional chromatography principles to a sample preparation method. In SEE, the analytical matrix is packed into an extraction vessel and a supercritical fluid, usually carbon dioxide, is passed through it. The analyte matrix may be viewed as the stationary phase, while the supercritical fluid can be viewed as the mobile phase. In order to obtain an effective extraction, the solubility of the analyte in the supercritical fluid mobile phase must be considered, along with its affinity to the matrix stationary phase. The effluent from the extraction is then collected and transferred to a gas chromatograph. In his comprehensive text, Taylor provides an excellent description of the principles and applications of SEE (44), while Pawliszyn presents a description of the supercritical fluid as the mobile phase in his development of a kinetic model for the extraction process (45). 

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. 

Wagner, K., Miliotis, T., Marko-Varga, G., Bischoff, R., Unger, K.K. (2002). An automated online multidimensional HPLC system for protein and peptide mapping with integrated sample preparation. Anal. Chem. 74, 809-820. 

Achiral-chiral multidimensional chromatography remains one of the best ways to separate chiral analytes from interfering matrix components or other compounds. The flexibility offered by different operation modes, stationary and mobile phases, and configurations allows analysis methods to be tailored to the analytical problem. By offering possible configurations for both online sample cleanup and concentration, achiral/chiral LC/LC reduces manual sample preparation. The ability to be coupled to... 

A major improvement came with digital electronics, which allows high-resolulion liming (0.1 second) and exact repeatability. Precise timing of sequential events is essential to multidimensional sample preparation utilizing column separation operators and switching valves. 

Authenticity evaluation has recently received increased attention in a number of industries. The complex mixtures involved often require very high resolution analyses and, in the case of determining the authenticity of natural products, very accurate determination of enantiomeric purity. Juchelka et al. have described a method for the authenticity determination of natural products which uses a combination of enantioselective multidimensional gas chromatography with isotope ratio mass spectrometry (28). In isotope ratio mass spectrometry, combustion analysis is combined with mass spectrometry, and the 13C/12C ratio of the analyte is measured versus a C02 reference standard. A special interface, employing the necessary oxidation and reduction reaction chambers and a water separator, was used employed. For standards of 5-nonanone, menthol and (R)-y-decalactone, they were able to determine the correct 12C/13C ratios, with relatively little sample preparation. The technical details of multidimensional GC-isotope ratio MS have been described fully by Nitz et al. (29). A MDGC-IRMS separation of a natural ds-3-hexen-l-ol fraction is... 

It is interesting that, in analytical chemistry, besides the efforts to increase the sample throughput and to decrease the detection limits, another trend can be observed which is directed to the analysis of more and more complex mixtures without laborious sample preparation and separation steps. This development was triggered by the requirements of bio- and environmental analysis and is closely connected to the development of multidimensional analytical methods, as well as hyphenated techniques which provide much more selectivity than one-dimensional analytical methods. Among the range of hyphenated techniques, those which combine a high separation efficiency with a maximum of structural information are of particular importance. These are hyphenated techniques such as GC-MS, LC-MS, LC-NMR and LC-NMR-MS. 

The fragmentation rate can be enhanced by either increasing the scanning speed of the instalments or by reducing the complexity of peptide mixtures prior to MS detection. Scan rate has significantly evolved during recent years, and the current generation of MSs reaches scan rates around 100 Hz, which represents a qualitative improvement compared to older models. Similarly, sample preparation has also been refined, and multidimensional separation at the level of proteins, proteolytic peptides, or both have made it possible to achieve a comprehensive analysis of nearly the entire human proteome... 

Coupled-column separations or multidimensional chromatography can be considered as a sample preparation form, as one column is used to derive fractions for the second column. It provides a two dimensional separation in which sample substances are distributed over a retention plane formed by the operation of two independent columns. This type of two dimensional based separation method is more powerful than a single dimensional based one. A retention plane has more peak capacity than a retention line and so can accommodate much more complex mixtures. Component identification is more reliable because each substance has two identifying retention measures rather than one. These type of combinations offer high selectivity and high sensitivity, and could be used with less expensive and more robust detectors (e.g., flame ionization). ... 

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