Flow-cell designs

5 Chromatographic Factors Affecting Flow-Cell Design 

As well as the constraints imposed by the laws of electrochemistry, many chromatographic factors are also important in flow-cell design. Good mass transfer characteristics, electrical isolation and resistance to common HPLC solvents are important, but as with all HPLC systems extra-column band broadening must be minimal if full use is to be made of the column efficiency that can be attained. The total band broadening of an analyte peak is given by combining the individual variances (o) caused by all the components in the flow stream, plus any other factors that cannot be defined (Equation 3.2). 

The maximum permissible cell volume (Vopt) in any type of HPLC detector is approximated by  

Typically q should not exceed 10% for the narrowest peak in the chromatogram. So for a 1 mm i.d. column packed with 3 xm particles and generating 25,000 theoretical plates, the maximum detector cell volume should be of the order of 1 pL. 

Two other design aspects need to be considered. The wetted parts of the EC detector must be constmcted from materials inert to the solvents used in HPLC, such as methanol, acetonitrile and dilute aqueous buffers with pH values generally in the range 2-8. PTFE and stainless steel are the favoured materials of constraction. Although most cells will not be subject to excessive pressures, they should be able to withstand the modest back pressure created by coupling a second detector in series or the use of a flow restrictor designed to prevent bubble formation. 

There is a great variety of cell designs for electrochemical detection combined with flow analysis that can fulfill determined requirements such as high sensitivity, easy construction, operation, maintenance, minimum dead volume, speed of analysis, and small volume of sample and reagents. Designs for electrochemical cells can be quite simple - a solution 

Most often it is possible to use different detectors, from mercury drop electrodes to microelectrodes, unmodified electrodes, or electrodes with modified surfaces, with the same electrochemical cell. Moreover, cell volumes are quite variable and range from milliliters to few nanoliters, depending upon the purpose of the analytical experiment. 

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Fig. 10. Flow-through electrochemical cell designs. I, Planar geometries, thin-layer (A) and wall-jet (B) flow cell designs. II, Cylindrical geometries, open tubular (A), wire in a capillary (B), and packed-bed (C) flow cell designs...

The flow-cell design was introduced by Stieg and Nieman [166] in 1978 for analytical uses of CL. Burguera and Townshend [167] used the CL emission produced by the oxidation of alkylamines by benzoyl peroxide to determine aliphatic secondary and tertiary amines in chloroform or acetone. They tested various coiled flow cells for monitoring the CL emission produced by the cobalt-catalyzed oxidation of luminol by hydrogen peroxide and the fluorescein-sensitized oxidation of sulfide by sodium hypochlorite [168], Rule and Seitz [169] reported one of the first applications of flow injection analysis (FTA) in the CL detection of peroxide with luminol in the presence of a copper ion catalyst. They... 

Some, uPLC systems are equipped with UV absorbance detection, and other systems allow for both UV absorbance and fluorescence detection. Fluorescence detection increases the sensitivity and selectivity of certain applications and is the method of choice in many separation-based assays. The liquid (mobile phase + sample) leaving the individual flow cells designated for UV detection is transferred through capillaries to a bank of 24 flow cells designated for fluorescence detection. 

In the usual flow cell design v is always >>1 cms-1, therefore, the higher the power of v, the more efficient... 

Further sensitivity enhancements of PDA are likely to stem from advanced flow-cell design using fiber-optic technology to extend the... 

This new hyphenated analytical system integrates capillary LC with NMR detection. The capillary LC-NMR system is comprised of an NMR spectrometer equipped with a capillary flow probe and the capillary LC. The capillary flow probe has a flow-cell design with an active sample volume of only 1 or 1.5 pL. This volume is chosen to match the typical peak volumes of capillary LC separation. 

One other difference lies in the type of detection technique used, which dictates the flow-cell design. Thus, a distinction can be made in this respect between optical (absorptiometric, luminemetric) sensors, which make measurements of the bulk solution where the flow-cell is immersed, and electroanalytical (amperometric, potentiometric) sensors, where measurements are based on phenomena occurring at the electrode-solution interface. 

Fluorescence detection, because of the limited number of molecules that fluoresce under specific excitation and emission wavelengths, is a reasonable alternative if the analyte fluoresces. Likewise, amperometric detection can provide greater selectivity and very good sensitivity if the analyte is readily electrochemically oxidized or reduced. Brunt (37) recently reviewed a wide variety of electrochemical detectors for HPLC. Bulk-property detectors (i.e., conductometric and capacitance detectors) and solute-property detectors (i.e., amperometric, coulo-metric, polarographic, and potentiometric detectors) were discussed. Many flow-cell designs were diagrammed, and commercial systems were discussed. 

Figure 27.6 Recent cross-flow cell design. Working electrodes may be in parallel or series.

A modification of the design of the flow cell can also lead to a further increase in sensitivity [7,8]. The addition of a proton RF coil for selective excitation of the protons before entering the 13C flow cell enables the full build-up of the nuclear Overhauser effect in a continuous-flow system (Figure 8.1.5). The combination of this NMR flow cell design, together with the use of immobilized paramagnetic species, could lead to the registration of continuous-flow 13C NMR spectra of separated species. 

Preparative separations in the grams per injection level are different. Separations are run isocratic in 1- to 3-in columns with large pore, fully porous packings (35-60jUm). An analytical, two-pump system can just barely reach the 20-mL/min flow rates needed to run a 1-in column. Special preparative HPLC systems deliver flow rates of 50-500 mL/min to handle the larger bore columns. A stream splitter is used to send part of the flow through a refractive index detector with a flow cell designed for concentrated solutions. 

Figure 3. Partial schematic of the LED photometer electonics and flow cell designed by Betteridge et al. (24),...

Four different fluorescence flow cell designs were compared to see if they were significantly different. The following results represented relative fluorescence intensities for four replicate measurements. 

Stieg, S., and Nieman, T. A., Experimental and theoretical considerations of flow cell design in analytical chemiluminescence. Anal. Chem. 50, 401-404 (1978). 

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