Post-column solvent extraction

Since droplet formation is a particular problem with aqueous mobile phases, continuous post-column solvent extraction, in which the solutes are extracted into an immiscible organic mobile phase, has been proposed [4]. The mobile phase reaching the belt thus becomes totally organic in nature and much more easily removed. The major disadvantage of this approach is the possible loss of analyte during the extraction procedure. 

Fla- 4-27 Incorporation of a continuous liquid-liquid extraction module (a) prior to and (b) after the chromatographic column in HPLC. (A) Determination of fat-soluble vitamins in pharmaceutical preparations (B) post-column solvent change-over. (Reproduced from [28] with permission of Elsevier). 

Involatile inorganic buffers, when used as mobile-phase additives, are the prime cause of blocking of the pinhole. The situation can be alleviated either by replacing them by a more volatile alternative, such as ammonium acetate, or by using post-column extraction to separate the analytes from the buffer, with the analytes, dissolved in an appropriate organic solvent, being introduced into the mass spectrometer. 

Fig. 3.7 Normal-phase-HPLC chromatograms of PA fractions generated by differential solvent extraction of crude GSE (protocol 1). (a) Fraction 1, (b) Fraction 2, (c) Fraction 3, (d) Fraction 4, (e) Fraction 5, and (f) Fraction 6. Compounds were detected with post-column deiivatization using DMACA...

Carbamate type and its Fruit and Solvent extraction, clean-up on HPLC with post-column [87]... 

Carbamate Grain, fruit and vegetables Solvent extraction clean-up on amino-bonded silica HPLC-post-column reaction with o-phthaldehyde - [88]... 

In normal high pressure liquid chromatography, typical sample volumes are 20-200 p.L this can become as little as 1 nL in capillary HPLC. Pretreatment of the sample may be necessary in order to protect the stationary phase in the column from deactivation. By employing supercritical fluids such as carbon dioxide, pretreatment can be bypassed in many instances so that whole samples from industrial and environmental matrices can be introduced directly into the column. This is due to the fact that the fluid acts as both extraction solvent and mobile phase. Post-column electrochemistry has been demonstrated. For example, fast-scan cyclic voltammo-grams have been recorded as a function of time after injection of microgram samples of ferrocene and other compounds in dichloromethane solvent and which are eluted with carbon dioxide at pressures of the order of 100 atm and temperatures of 50°C the chromatogram is constructed as a plot of peak current vs. time [18]. 

The post-column connection of an HPLC to FI liquid-liquid extraction appears to present no major difficulties, and effluent from the chromatograph column may be connected to the solvent segmentor of the extraction system or between this point and the extraction coil. 

This extended series of developments culminated in the thermospray design (Blakley 1983) that was subsequently commercialized as an LC/MS interface. A solution of the analyte (e.g., HPLC eluant) and a volatile buffer (typically O.IM ammonium acetate added post-column) was evaporated from a heated capillary at a flow rate of up to LSml/min into a heated chamber (whence the name thermospray ), forming a mist of droplets containing relatively involatile analytes and solvent vapor as the solvent evaporated the analyte formed adducts with ions from the added salt. It is believed that the formation of free gaseous ions from the microdroplets then proceeds in a manner similar to that discussed for ESI in Section 5.3.6. Most of the neutrals are removed by a vacuum pump and the ions are extracted orthogonally by some electrostatic lenses and a repeUer through a pinhole (restricted to 25 (j,m to protect the vacuum in the mJz analyzer, typically a quadrupole because of its better tolerance to poor vacuum). Such an arrangement is found to be efficient... 

HPLC-UV-NMR is a powerful technique for the identification and characterization of flavonoids. However, there are drawbacks, as NMR remains rather insensitive because of the need for solvent suppression, which has restricted the observable NMR range. Recently, two major research developments in HPLC-UV-NMR are post-column solid-phase extraction (HPLC-UV-SPE-NMR) and combination of HPLC-UV-SPE with capillary separations and NMR detection [89]. A post-column treatment of analyte focusing and multiple trapping through a SPE has solved the problem of sensitivity and solvent suppression. The separation and elucidation of three C-methylated flavanones and five dihydrochalcones from Myrica gale seeds have been achieved by HPLC-DAD-SPE-NMR and... 

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