Trace enrichment

This is normally carried out off-line using commercially available bonded-phase extraction cartridges such as Bond-Elut, Sep-Pac and Baker-Bond. 

Analytical columns can also be used for trace enrichment by the con- 

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Trace enrichment and sample clean-up are probably the most important applications of LC-LC separation methods. The interest in these LC-LC techniques has increased rapidly in recent years, particularly in environmental analysis and clean-up and/or trace analysis in biological matrices which demands accurate determinations of compounds at very low concentration levels present in complex matrices (12-24). Both sample clean-up and trace enrichment are frequently employed in the same LC-LC scheme of course, if the concentration of the analytes of interest are Sufficient for detection then only the removal of interfering substances by sample clean-up is necessary for analysis. 

Trace enrichment or preconcentration by LC-LC methods are based on the possibility that the analytes will be retained as a narrow zone on the top of the first column when a large volume of sample is pumped trough the column. Good reproducibility can be achieved when the column capacity is not exceeded and the column is not overloaded. Trace enrichment is usually performed when relatively non-polar... 

Figure 5.3 Analysis of 100 ml of (a) surface water and (b) drinking water sample spiked with 0.1 pig/ml of microcystins, using column-switching HPLC 1, microcystin-RR 2, microcystin-YR 3, microcystin-LR. Reprinted from Journal of Chromatography A, 848, H. S. Lee et al, On-line trace enrichment for the simultaneous determination of microcystins in aqueous samples using high performance liquid chromatography with diode-array detection , pp 179-184, copyright 1999, with permission from Elsevier Science.

J. Slobodnik, H. Lingeman and U. A. Th Brinkman, Large-volume liquid cliromato-grapliic trace-enrichment system for environmental analysis , Chromatogmphia 50 141-149 (1999). 

Multidimensional HPLC offers very high separation power when compared to monodimensional LC analysis. Thus, it can be applied to the analysis of very complex mixtures. Applications of on-line MD-HPLC have been developed, using various techniques such as heart-cut, on-column concentration or trace enrichment applications in which liquid phases on both columns are miscible and compatible are frequently reported, but the on-line coupling of columns with incompatible mobile phases have also been studied. 

H. Iith, R. Tocklu, K. Welten, G. J. de Jong, R. W. Frei and U. A. Th Brinkman, Trace enrichment on a metal-loaded thiol stationary phase in liquid cliromatography effect of analyte sti ucture and pH value on the (de)soiption behaviour , J. Pharm. Biomed. Anal. 7 1679-1690(1989). 

These small columns,(usually 10 mm X 1-4.6 mm i.d.) are normally packed with 10-40 p.m sorbents such as Cig-bonded silica, Cg-bonded silica or styrene-divinylbenzene copolymer. These sorbents are not very selective and more selective sorbents, such as the immunosorbent (94), have also been used with good results. Coupling of SPE-gas chromatography is in fact the one most often used in environmental analysis because it reaches a high level of trace enrichment, eliminates water and elutes retained compounds easily with an organic solvent that can be injected into the gas chromatograph. 

J. Slobodnik, A. J. H. Router, J. J. Vreuls, I. Eiska and U. A. Th Brinkman, Monitoring of organic micropollutants in surface water by automated on-line trace-enrichment liquid and gas cliromatographic systems with ultraviolet diode-array and mass spectro-metric detection , J. Chromatogr. 768 239-258 (1997). 

C. Aguilar, R Bomtll and R. M. Marce, Determination of pesticides by on-line trace enrichment-reversed-phase liquid cliromatogr aphy-diode-array detection and confirmation by particle-beam mass spectrometry , Chromatographia 43 592-598 (1996). 

E. Pocurull, R. M. Marce and E. Bonttll, Determination of phenolic compounds in natural waters by liquid cliromatography with ulrtaviolet and elecrtochemical detection after on-line trace enrichment , / Chromatogr. 738 1-9(1996). 

Noroozian E, Maris FA, Nielen MWF, et al. 1987. Liquid chromatographic trace enrichment with on-line capillary gas chromatography for the determination of organic pollutants in aqueous samples. Journal of High Resolution Chromatography and Chromatography Communications 10 17-24. 

Activated carbon disks have also been used for the extraction of polar pesticides from environmental water samples. Slobodnik et alP used an Empore activated carbon disk (EACD) and Envi-Carb graphitized carbon black (GCB) and CPP-50 graphitized carbon for the trace enrichment of polar pesticides from water. 

Polymeric precolumns of styrene-divinylbenzene were used by Aguilar et al. to monitor pesticides in river water. Water samples (50 mL) were trace enriched on-line followed by analysis using LC combined with diode-array detection. LC atmospheric pressure chemical ionization (APCI) MS was used for confirmatory purposes. It was found that after the pesticides had been extracted from the water sample, they could be stored on the precartridges for up to 3 months without any detectable degradation. This work illustrates an advantage of SPE for water samples. Many pesticides which may not be stable when stored in water, even at low temperature, may be extracted and/or enriched on SPE media and stored under freezer conditions with no detectable degradation. This provides an excellent way to store samples for later analysis. 

In summary, the development of materials for the extraction of pesticides from water samples has progressed from simple liquid-liquid extraction for the principal active compound to sophisticated SPE media capable of exclusively trapping the target pesticide and metabolites selectively. The development of alkyl bonded phase silica cartridges and extraction disks combined with on-line extraction techniques is currently the principal means used for the extraction and trace enrichment of pesticides and metabolites from water. 

Starting aaterial for their preparation and subsequent activation. Graphitized carbon blacks have low surface areas, usually less than 100 K /g and typically 5-30 u /g. They have a low capacity for low molecular weight analytes but have been used successfully for the isolation of heavier nonpolar analytes that might be difficult to recover from more active forms of carbon [169,170]. The most common form of carbon used for trace enrichment is granular activated carbon, generally prepared by tbe low temperature... 

MDLTIDIIIENSIONAL AND MOLTIMODAI. SAMPLE FSACTIONATIOH AND TRACE ENRICHMENT TECHNIQUES... 

The simplest type of two-dimensional gas diromatography for heart cutting or trace enrichment using a packed precolumn and a capillary column is shown in Figure 8.16 [205]. Almost any modem gas chromatograph could be converted into a similar unit with the addition of a few auxiliary components. Preliminary separation t2dces place on the packed column, the effluent from which is directed either to a vent or to the capillary inlet by the Deans switch, nie effluent reaching the capillary inlet is split three ways. One portion passes to a detector used to monitor the preseparation, a second portion enters the cap Bry column and is... 

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. 

Small molecule size-c Ecluslon chrmutography 441 Soap-film meter (GC) 4, 235 Solid-phase extraction 777 cartridges 777 membranes 780 optimization 777, 783 sorbents 778, 785 trace enrichment 777, 783 Solubility parameters 460 Solvatochromic parameters GC 191... 

Sorption Techniques for Trace Enrichment of Organics in Water.783... 

Multidimensional and Multimodal Sample Fractionation and Trace Enrichment Techniques.790... 

SORPTION TECHNIQUES FOR TRACE ENRICHMENT OF ORANICS IN HATER... 

Dutton, J., Hodgkinson, A. J., Hutchinson, G., and Roberts, N. B., Evaluation of a new method for the analysis of free catecholamines in plasma using automated sample trace enrichment with dialysis and HPLC, Clin. Chem., 45, 394, 1999. 

Trace enrichment, cleanup Solid-phase extraction SPE-GC... 

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