Column Washing and Equilibration

In on-line elution, the kinetic features of the process are much more important than for off-line batch procedures. Weak eluents requiring long equilibrium periods may be used successfully for off-line procedures, but caimot be used for on-line applications, since slow elutions may significantly degrade the enrichment factors and/or the concentration efficiencies. 

The eluent should not attack the packing material noticeably for at least hundreds of elutions. This may not be a vital factor for off-line procedures, since sometimes the sorbent may even be digested to release the analytes but an on-line system is expected to run for days and weeks without changing the columns. Highly concentrated acids and bases may be effective eluents and may not be harmful to the sorbent however, they may create problems in some detectors such as the AAS nebulizing systems through corrosion or blockages. On-line modification or dilution of the eluates are feasible, but only at the expense of a sacrifice in sensitivity. 

Schlieren effects occurring at the interface between sample/carrier and eluent in spectrophotometric determinations can be a serious source of interference (cf. Sec. 4.6.6). Therefore, the reftractive index of the eluent should be as close as possible to that of the interfacing sample, carrier, or wash solution in such applications. 

In the atomic absorption spectrometer, organic solvent eluents may create additional enhancement effects. The effect of organic solvents in flame AAS is 

The elution flow-rate is an important parameter in column preconcentration which is usually optimized for maximum sensitivity. However, the speed of elution is also a crucial factor for the efficiency of on-line preconcentration systems, because in most cases the eluent flow is connected directly (some after merging with reagent streams) with the detector. This is particularly important for detection systems which require a certain sample delivery rate for optimum response, e.g., the flame AA or ICP spectrometer, and will be discussed in more detail in section 4.6.3. 

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