Liquid chromatography coupled with supercritical fluid extraction

M. Ashraf-Khorassani, M. Barzegar and Y. Yamini, On-line coupling of supercritical fluid extraction with high performance liquid chromatography , J. High Resolut. Chromatogr. 18 472-476 (1995). 

One example of normal-phase liquid chromatography coupled to gas chromatography is the determination of alkylated, oxygenated and nitrated polycyclic aromatic compounds (PACs) in urban air particulate extracts (97). Since such extracts are very complex, LC-GC is the best possible separation technique. A quartz microfibre filter retains the particulate material and supercritical fluid extraction (SPE) with CO2 and a toluene modifier extracts the organic components from the dust particles. The final extract is then dissolved in -hexane and analysed by NPLC. The transfer at 100 p.1 min of different fractions to the GC system by an on-column interface enabled many PACs to be detected by an ion-trap detector. A flame ionization detector (PID) and a 350 p.1 loop interface was used to quantify the identified compounds. The experimental conditions employed are shown in Table 13.2. 

The method based on immunosorbents coupled on-line with liquid chromatography-atmospheric pressure chemical ionization mass spectrometry [109], discussed in section 9.4.2.1, has been applied to the determination of substituted urea type herbicides. Supercritical fluid extraction with methanol modified carbon dioxide has been applied to the determinants of sulfonyl urea herbicides in soil [261],... 

McNally and Wheeler [364] used supercritical fluid extraction coupled to supercritical fluid chromatography to determine sulfonylurea herbicides in soil. Klatterback et al. [365,366] used supercritical fluid extraction with methanol-modified carbon dioxide followed by high-performance liquid chromatography with UV detection to determine sulfonylurea herbicides obtained on a Cis solid-phase extraction disc. Alternatively the determination was carried out by gas chromatography of the dimethyl derivatives of the sulfonylurea herbicides, employing an electron capture or a NP detector on the gas chromatograph. 

Figure 6.1 Bar-graph of MeHg in CRM 580. The results correspond to six replicate determinations as performed by different laboratories using various methods. MEANS indicates the mean of laboratory means with 95% confidence interval. Abbreviations-. CVAAS, cold vapour atomic absorption spectrometry CVAFS, cold vapour atomic fluorescence spectrometry ECD, electron capture detection GC, gas chromatography HPLC, high-performance liquid chromatography ICPMS, inductively coupled plasma mass spectrometry MIP, microwave induced plasma atomic emission spectrometry QFAAS, quartz furnace atomic absorption spectrometry SFE, supercritical fluid extraction.

This overview is focused on the on-line coupling of pressurized liquid extraction (PLE), microwave-assisted extraction (MAE), supercritical fluid extraction (SEE) and sonication-assisted extraction (SAE) with liquid and gas chromatography for the analysis of solid agricultural and food samples. In addition, head-space techniques and direct thermal extraction are discussed. 

The most common extraction techniques for semivolatile and nonvolatile compounds from solid samples that can be coupled on-line with chromatography are liquid-solid extractions enhanced by microwaves, ultrasound sonication or with elevated temperature and pressures, and extraction with supercritical fluid. Elevated temperatures and the associated high mass-transfer rates are often essential when the goal is quantitative and reproducible extraction. In the case of volatile compounds, the sample pretreatment is typically easier, and solvent-free extraction methods, such as head-space extraction and thermal desorption/extraction cmi be applied. In on-line systems, the extraction can be performed in either static or dynamic mode, as long as the extraction system allows the on-line transfer of the extract to the chromatographic system. Most applications utilize dynamic extraction. However, dynamic extraction is advantageous in many respects, since the analytes are removed as soon as they are transferred from the sample to the extractant (solvent, fluid or gas) and the sample is continuously exposed to fresh solvent favouring further transfer of analytes from the sample matrix to the solvent. 

The use of separation techniques, such as gel permeation and high pressure liquid chromatography interfaced with sensitive, silicon-specific aas or ICP detectors, has been particularly advantageous for the analysis of silicones in environmental extracts (469,483—486). Supercritical fluid chromatography coupled with various detection devices is effective for the separation of silicone oligomers that have molecular weights less than 3000 Da. Time-of-flight secondary ion mass spectrometry (TOF-sims) is applicable up to 10,000 Da (487). 

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],... 

Wippo and Stan, 1997). Other on-line couplings have included TLC with gas chromatography, supercritical fluid extraction, and the thermal separation technique (Kovar and Morlock, 1996). Somsen et al. (1995) have described couplings of colunm liquid chromatography with TLC and spectrometric methods (FTIR, SERS, and fluorescence spectrometry). 

There are a number of online methods coupling SFE to other analytical techniques such as gas chromatography (GC), capillary or packed column supercritical fluid chromatography (SEC), and liquid chromatography (EC). The online techniques are rather sensitive because the entire portion of extracted components is transferred to the column. A disadvantage with online approaches is that coextracted fat, water, and sometimes modifier may interfere in the final analysis. Additionally overloading of the analytical column must be avoided. Therefore, only small samples are usually extracted with online methods in SFE. A comprehensive discussion of online methods can be found in Further Reading. 

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