Online dialysis

To reduce coextractives in the primary sample extract and concentrate the analyte(s), various types of sample cleanup procedures can be applied. They include conventional liquid-liquid partitioning, solid-phase extraction, matrix solid-phase dispersion, and online dialysis and subsequent trace enrichment (Table 29.5). In many applications, more than one of these procedures is applied in combination to decrease the background noise of the detector, thus making it possible to quantify trace level residue concentrations. 

Online dialysis followed by trace enrichment of the analytes can also be employed for the determination of nitro furans in foods of animal origin. The efficiency of this technique has been demonstrated in a method for the determina-... 

H2O extn, purification by online dialysis and subsequent trace enrichment on reversed phase, 40 m, preconcn column and switching to analytical column... 

Online dialysis and subsequent trace enrichment has been further described for isolation/purification of flumequine residues from fish muscle (203), or oxolinic acid and flumequine from chicken liver (193) and salmon muscle (204). This involves online purification by diphasic dialysis membrane and trapping of the analytes onto a liquid chromatographic preconcentration column (reversed-phase Ci8 or polymeric), rinsing of the coextracted materials to waste, and finally flushing of die concentrated analytes onto the analytical column. 

Eng, G.Y. Maxwell, R.J. Cohen, E. Piotrowski, E.G. Eiddler, W. Determination of flumequine and oxolinic acid in fortified chicken tissue using online dialysis and high-performance liquid-chromatography with fluorescence detection. J. Chromatogr. 1998, 799, 349-354. 

Fang, Q., Sun, Y., and Fang, Z. Automated continuous monitoring of drug dissolution process using a flow injection online dialysis sampling, solvent extraction separation spectrophotometric system. Presenilis J. Anal. Chem. 364(4) 347—352,1999. 

Examples of basic studies of offline extraction and preconcentration of pesticide residues using other techniques, such as online dialysis, steam distillation, supercritical fluid extraction, pressurized liquid extraction, cloud point extraction, or liquid-liquid membranes, have been reported. The large amounts of matrix coextractives and the need for clean extracts in CE/ultraviolet (UV) analysis are the main reasons for their scarce application. 

Figure 6 shows typical online dialysis configurations. A continuously working dialysis cell can be coupled directly to the valve IV (Figure 6A) injecting the prepurified and conditioned sample solution into a nonsegmented carrier flow stream, e.g., of liquid column chromatography. The analytes are separated... 

When developing a method should an assay be automated immediately or be developed manually first and automated later While no definite rules can be stated, it may well be easier for methods to be developed manually, first and then automated later. The main exceptions to this statement are column-switching in liquid chromatography (LC) and online dialysis, which can only be developed as automated methods. Another exception is when an existing automated method can be modified easily for use with new analytes, typically those that are structurally similar. 

Kritsunankul et al. [76] proposed flow injection online dialysis for sample pretreatment prior to the simultaneous determination of some food additives by HPLC and UV detection (FID-HPLC). For this, a liquid sample or mixed standard solution (900 pL) was injected into a donor stream (5%, w/v, sucrose) of a FID system and was pushed further through a dialysis cell, while an acceptor solution (0.025 mol/L phosphate buffer, pH 3.75) was held on the opposite side of the dialysis membrane. The dialysate was then flowed to an injection loop of the HPLC valve, where it was further injected into the HPLC system and analyzed under isocratic reversed-phase HPLC conditions and UV detection (230 nm) (Figure 24.6). The order of elution of five food additives was acesulfame-K, saccharin, caffeine, benzoic acid, and sorbic acid, with an analysis time of 14 min. This system has advantages of high degrees of automation for sample pretreatment, that is, online sample separation and dilution and low consumption of chemicals and materials. 

FIGURE 24.6 FIA manifold for online dialysis coupled with HPLC. AS acceptor stream C carrier DC dialysis cell DM dialysis membrane DS donor stream HPLC high-performance liquid chromatography IV injection valve MCI and MC2 mixing coils PI and P2 pumps S sample W waste. 

Saccharin is also determined with FIA methodologies, which allow multianalyte determination. These methodologies include methods cited previously for acesulfame-K and/or cyclamate a method based on electrochemical detection of these three artificial sweeteners using stabilized systems of filter-supported BLMs [72] online dialysis for sample pretreatment prior to the simultaneous determination of saccharin and caffeine, benzoic acid, and sorbic acid by HPLC-FID [76] and molecular spectroscopic methods in the UV region based on the transient retention of analytes on solid phases, silica Cjg [74], and quaternary amine ion exchanger [75]. 

---