Solid-phase extraction, with offline

More success was obtained using on-line solid phase extraction before offline analysis by capillary electrophoresis with matrix-assisted laser-desorption ionization (MALDI) MS [18]. Because microdialysis is a method used for analysis of small molecules and peptides, MALDI is not used frequently with microdia lysis sampling. Another disadvantage is that MALDI cannot be used on-line with the separation because it is a vacuum ionization technique. However, if the pep-tide is large enough (0 1000 Da) MALDI can be useful. For the analysis of peptides in dialysate an appropriate separation is important before mass spectrometric detection. In a comparison with direct sampling of dialysate in MALDI, capillary electrophoresis provides the high efficiency separations necessary to resolve all... 

Aguilar, C., Bomil, F., and Marce, R. M., Online and offline solid-phase extraction with styrene-divinylbenzene membrane extrachon disks for determining pesticides in water by reversed-phase liquid chromatography-diode array detection, J. Chromatogr. A, 754, 77-84, 1996. 

Offline extraction is the easiest and most common way for combining affinity columns with other analytical techniques. This typically involves the use of antibodies that are immobifized and packed into a disposable syringe or solid-phase extraction cartridge. After conditioning the affinity column with the necessary application buffer or conditioning solvents. 

Automation of solid-phase extraction for riboflavin separation has been carried out in both offline and online modes, in the last case with direct coupling to the injection valve of HPLC equipment. 

Some techniques that combine the properties of extraction and cleanup are supercritical fluid extraction (SEE) and matrix solid-phase dispersion (MSPD). Supercritical fluids, i.e., at a temperature and pressure in excess of their critical point, have unique properties for selective extraction of analytes from a sample. Solid samples are mixed with an inert dispersant, such as hydromatrix, and the mixture packed into the cell of the SEE apparatus. The sample is extracted with supercritical CO2, with or without addition of organic modifier, and the extracted analytes may be collected inline or offline on suitable adsorbents (Figure 3). Further cleanup of the sample extract may be performed using SPE. MSPD is based on intimate mixing of animal tissue sample with a bonded silica, such as Cig, and packing of the blended material into a column from which interferences can be eluted by washing with solvents and the analytes eluted using a selective solvent. 

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