Coupled chromatographic techniques

P. Van Zoonen, E. A. Hogendoom, G. R. Van der Hoof and R. A. Baumann, Selectivity and sensitivity in coupled chromatographic techniques as applied in pesticide residue analysis . Trends. Anal. Chem. 11 11-17 (1992). 

Ultimately major efforts to develop coupled chromatographic techniques have been performed to alleviate the problem of manual sample pretreatment and to enhance sensitivity and selectivity in the analysis of PAHs in foodstuffs (192) and environmental samples (193-195). Liquid chromatography/MS (196,197), GC/MS (175), HPLC with UV absorbance, fluorescence (177) (see Fig. 4), or electrochemical (ED) detection (179), and ELISA immunoassay (198) have been successfully used for the determination of HAAs. 

The results demonstrate that coupled chromatographic techniques with multiple detectors permit the determination of average composition data, heterogeneity parameters, and separations of homopolymers and copolymers. The methodology reviewed here enables a distinction to be made between copolymers and polymer blends. 

Coupled chromatographic techniques Several chromatography techniques (C) have been used for sulfur determination. These techniques allow generally very sensitive detection of sulfur-containing compounds (see Table 1). Usually, sulfur is first oxidized followed by ion chromatography. Typically, such analyses require sophisticated chemical procedures but this is not always the case for example, the technique using pyrolysis-gas chromatography (Py-GC) equipped with a flame photometric detector (FPD) does not require sample pretreatment. 

The design of these interfaces is dictated, in large part, by the method used to ionize the molecules, which in turn is dictated by the nature of the molecules themselves, i.e. volatile, nonvolatile, polar, nonpolar, low mass, high mass etc., as well as the need to couple chromatographic techniques that operate at or near atmospheric pressure to the vacuum requirements of the MS. 

Trathnigg, B., Hreczuch, W. 2000. Characterization of fatty ester ethoxylates by coupled chromatographic techniques. Proceedings of the 5th World Surfactant Congress. Firenze, 1 472-480. 

The unmatched capacity of MS to reveal information useful to identify the organic components of artistic, historic, and archaeological objects with further possibilities for their detection and quantification at trace levels in complex matrices derives from the ability to couple chromatographic techniques with mass spectrometers, thereby allowing separation of the individual molecular species with on-line mass spectrometric analysis of the eluting components. 

Oxygen and nitrogen also are deterrnined by conductivity or chromatographic techniques following a hot vacuum extraction or inert-gas fusion of hafnium with a noble metal (25,26). Nitrogen also may be deterrnined by the Kjeldahl technique (19). Phosphoms is determined by phosphine evolution and flame-emission detection. Chloride is determined indirecdy by atomic absorption or x-ray spectroscopy, or at higher levels by a selective-ion electrode. Fluoride can be determined similarly (27,28). Uranium and U-235 have been determined by inductively coupled plasma mass spectroscopy (29). 

Supercritical Fluid Techniques Coupled with Chromatographic Techniques... 

Chlorophenoxy acids are relatively polar pesticides which are usually determined by LC because volatile derivatives have to be prepared for GC analysis. This group of herbicides can be detected by multiresidue methods combined with automated procedures for sample clean-up, although selectivity and sensitivity can be enhanced by coupled-column chromatographic techniques (52). The experimental conditions for Such analyses are shown in Table 13.1. 

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. 

The PSP toxins represent a real challenge to the analytical chemist interested in developing a method for their detection. There are a great variety of closely related toxin structures (Figure 1) and the need exists to determine the level of each individually. They are totally non-volatile and lack any useful UV absorption. These characteristics coupled with the very low levels found in most samples (sub-ppm) eliminates most traditional chromatographic techniques such as GC and HPLC with UVA S detection. However, by the conversion of the toxins to fluorescent derivatives (J), the problem of detection of the toxins is solved. It has been found that the fluorescent technique is highly sensitive and specific for PSP toxins and many of the current analytical methods for the toxins utilize fluorescent detection. With the toxin detection problem solved, the development of a useful HPLC method was possible and somewhat straightforward. 

HPLC (in both NP and RP modes) is quite suitable for speciation by coupling to FAAS, ETAAS, ICP-MS and MIP-MS [571,572]. Coupling of plasma source mass spectrometry with chromatographic techniques offers selective detection with excellent sensitivity. For HPLC-ICP-MS detection limits are in the sub-ng to pg range [36]. Metal ion determination and speciation by LC have been reviewed [573,574] with particular regard to ion chromatography [575]. 

ESI-MS is the most successful method of coupling a condensed phase separation technique to a mass spectrometer. Because the input to ESI is a liquid, electrospray serves as an interface between the mass spectrometer and liquid chromatographic techniques, including SEC and CE (capillary electrophoresis). In LC-MS the flow-rate should lie in the range recommended for the HPLC pump and the mass spectrometer (typically 0.001 -l.OmLmin-1). Recent advances in (nano)electrospray technology include the development of the use of very low solvent flow-rates (30 to 1000nLmin-1) [130,131],... 

Table 7.6 Main features of coupled SFE-chromatographic techniques...

Applications SFE-SFC solves problems in such diverse areas as polymers/monomers, oils/lubricants, foods, pharmaceuticals, natural products, specialty chemicals, coatings, surfactants and others. Off-line SFE-SFC survives alongside on-line determinations of additives, because of the need for representative sample sizes. Off-line SFE-SFC was used for extraction of AOs from PP [102]. In cases where the analyst wishes to perform further analysis on the extracted species, it is useful to be able to isolate the extract from the solvent. The ability to remove the solvent easily is particularly important when SFE is coupled on-line to chromatographic techniques, but is equally important for trace analysis when it is useful to concentrate... 

Plasmas compare favourably with both the chemical combustion flame and the electrothermal atomiser with respect to the efficiency of the excitation of elements. The higher temperatures obtained in the plasma result in increased sensitivity, and a large number of elements can be efficiently determined. Common plasma sources are essentially He MIP, Ar MIP and Ar ICP. Helium has a much higher ionisation potential than argon (24.5 eV vs. 15.8 eV), and thus is a more efficient ionisation source for many nonmetals, thereby resulting in improved sensitivity. Both ICPs and He MIPs are utilised as emission detectors for GC. Plasma-source mass spectrometry offers selective detection with excellent sensitivity. When coupled to chromatographic techniques such as GC, SFC or HPLC, it provides a method for elemental speciation. Plasma-source detection in GC is dominated by GC-MIP-AES... 

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