Supercritical fluid chromatography multidimensional

SFC-SFC is more suitable than LC-LC for quantitation purposes, in view of the lack of a suitable mass-sensitive, universal detector in LC. Group quantitation can be achieved by FID. The ideal SFC-SFC system would consist of a short (10-30 cm) packed-capillary primary column, interfaced to a long (5-10m) open-tubular column, but such a combination is difficult to realise, due to the different flow-rates required for each column type. Coupled SFC-SFC is often configured with a solute concentration device prior to valve switching on to the SFC. The main approaches to this concentration stage are the use of absorbent material or cryofocusing. Davies el at. [924] first introduced two-dimensional cSFC (cSFC-cSFC), and its use has been reported [925,926]. 

Alternatively, the LC dimension of LC-GC may be replaced by packed-column SFC, in order to improve the compatibility between two mobile phases and to allow the FID to be used for both separations. Because of the relatively nonpolar nature of scCCL. SFC-GC is particularly recommended as a substitute for many normal-phase LC-GC analyses. The techniques developed for solvent evaporation at the LC-GC interface are often not required in SFC-GC, because the solutes are deposited at the front of the GC column when CCL decompresses into a gas at the end of the SFC column. 

SFC-TLC is largely unexplored. Stahl [927] developed a device for supercritical fluid extraction with deposition of the fluid extracts on a moving TLC plate. Wunsche et al. [928] have described an automated apparatus for direct pSFC-TLC coupling. Compared to collecting the effluent from the SFC in decompression vessels, the direct deposition of the effluent on the TLC plate leads to significant losses of analytes. Multidimensional SFC has been reviewed [929]. 

Applications On-line pSFC-GC has been applied to the analysis of fossil fuels, such as group-type separations of high-olefin gasoline (saturates, olefins and aromatics) [930]. No significant applications concerning polymer/additive analysis can be mentioned. 

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Figure 12.20 SFC-GC analysis of a sample of aviation fuel (a) SFC separation into two peaks (b and c) coixesponding GC ttaces of the respective peaks (flame-ionization detection used throughout). Reprinted from Journal of High Resolution Chromatography, 10, J. M. Levy et ah, On-line multidimensional supercritical fluid chromatography/capillary gas chromatography , pp. 337-341, 1987, with permission from Wiley-VCH.

J. M. Levy, J. P. Guzowski and W. E. Huhak, On-line multidimensional supercritical fluid chromatography/capillary gas chromatography. J. High Resolut. Chromatogr. 10 337-341 (1987). 

Yarita, T., A. Nomura, and Y. Horimoto, 1994. Type analysis of citrus essential oils by multidimensional supercritical fluid chromatography/gas chromatography. Ana/. ScL, 10 25-29. 

Figure 12.23 SFC-SFC analysis, involving a rotaiy valve interface, of a standard coal tar sample (SRM 1597). Two fractions were collected from the first SFC separation (a) and then analyzed simultaneously in the second SFC system (h) cuts a and h are taken between 20.2 and 21.2 min, and 38.7 and 40.2 min, respectively. Peak identification is as follows 1, tii-phenylene 2, chrysene 3, henzo[g/ i]perylene 4, antliracene. Reprinted from Analytical Chemistry, 62, Z. Juvancz et al, Multidimensional packed capillary coupled to open tubular column supercritical fluid chromatography using a valve-switcliing interface , pp. 1384-1388, copyright 1990, with permission from the American Chemical Society.

Figure 12.24 Schematic diagram of the multidimensional packed capillary to open tubular column SFC-SFC system. Reprinted from Analytical Chemistry, 62, Z. Juvancz et al., Multidimensional packed capillary coupled to open tubular column supercritical fluid chromatography using a valve-switching interface , pp. 1384-1388, copyright 1990, with permission from the American Chemical Society.

Other multidimensional systems, such as supercritical fluid chromatography (SFC-GC or LC-SFC), will not be described here because, although some applications to environmental analysis have been described (4, 7-9), they have not been very widely used in this field. 

If simple sample pretreatment procedures are insufficient to simplify the complex matrix often observed in process mixtures, multidimensional chromatography may be required. Manual fraction collection from one separation mode and re-injection into a second mode are impractical, so automatic collection and reinjection techniques are preferred. For example, a programmed temperature vaporizer has been used to transfer fractions of sterols such as cholesterol and stigmasterol from a reversed phase HPLC system to a gas chromatographic system.11 Interfacing gel permeation HPLC and supercritical fluid chromatography is useful for nonvolatile or thermally unstable analytes and was demonstrated to be extremely useful for separation of compounds such as pentaerythritol tetrastearate and a C36 hydrocarbon standard.12... 

Juvancz, Z. Payne, K.M. Markides, K.E. Lee, M.L. Multidimensional packed capillary coupled to open tuhular column supercritical fluid chromatography using a valveswitching interface. Anal. Chem. 1990, 62, 1384—1388. 

The term multimodal has been used in two ways in TLC, to designate layers such as bonded cyano sorbents that can operate with two or more mechanisms (see Section IV.C) or, in the context of this section, to specify multidimensional separations that are performed by coupling TLC, HPTLC, or OPLC (223) with another technique, such as gas chromatography, supercritical fluid chromatography (224), countercurrent chromatography (225), and, most commonly, HPLC (145, 226-228), in order to improve the separation capacity available from either of the individual methods. For example, the combination of adsorption AMD-HPTLC and partition HPLC for water analysis produced as many as 700 individual densitometric peaks (49). Multimodal TLC separations have been reviewed (229-231). 

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). 

Although SFE and SFC share several common features, including the use of a supercritical 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 (chromatography) 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 decrease 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... 

IRMS LC MDGC MS MSA NIF NMR OAV OSV PCA RAS RP SDE SFE SIM SNIF SPME TIC TLC Stable Isotope Ratio Mass Spectrometry Liquid Chromatography MultiDimensional Gas Chromatography Mass Spectrometry Multivariate Sensory Analysis Nasal Impact Frequency Nuclear Magnetic Resonance spectroscopy Odor Activity value Odor Spectrum Value Principal Component Analysis Retronasal Stimulation Reversed Phase Simultaneous steam Distillation Extraction Supercritical Fluid Extraction Selected Ion Monitoring Surface of Nasal Impact Frequency Solid Phase Micro Extraction Total Ion Current Thin Layer Chromatography... 

See also Air Analysis Sampling. Extraction Solvent Extraction Multistage Countercurrent Distribution Supercritical Fluid Extraction. Gas Chromatography Multidimensional Techniques. Mass Spectrometry Time-of-Flight. Quality Assurance Quality Control Reference Materials. Water Analysis Overview. 

The use of essential oils is increasing because of the increase in the number of their apphcations and in the framework of natural and environmentally friendly materials. Many times the analysis of their components is quite complex due to the high number and the diversity of compounds in their composition. In this entry a general overview of the extraction methods is given by comparing conventional hquid-liquid and sohd-hquid methods with new alternative ones, such as supercritical fluid extraction and microwave-assisted extraction. Gas chromatography methods and examples are treated and important issues such as detection systems, modem hbraries for compounds identification, as well as multidimensional or hyphenated techniques are discussed. The use of these modem techniques and methods has improved resolution and sensitivity in essential oils determination and could open the possibihty of future work in this area of chromatography. 

Coupled systems include multidimensional and multimodal systems. Multidimensional chromatography involves two columns in series preferably two capillary columns, with different selectivity or sample capacity, to optimize the selectivity of some compounds of interest in complex profiles or to provide an enrichment of relevant fractions. In multimodal systems, two chromatographic methods or eventually a sample preparation unit and a chromatographic method are coupled in series. Coupled systems that received much interest in recent years are multidimensional CGC (MDCGC), the combination of high-performance liquid chromatography with CGC (HPLC-CGC) and the on- or off-line combination of supercritical fluid extraction with CGC (SFE-CGC). Multidimensional and multimodal techniques in chromatography arc described in detail in [65],... 

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