Systems for On-line Separation of Interferents

The manifold designs are usually simple and straightforward when on-line columns are used to enhance sensitivity by retaining the interferents this procedure is often termed on-line clean-up of the sample. Columns which selectively retain the interferent. but allow the analyte to pass freely, are connected in the sample introduction line either before of behind the injection valve (Fig. 4.3 a. b). Szpunar-Lobinska et al.[39] compared the two approaches in the determination of biuret in urea fertilizers by spectrophotometry, and strongly recommended the second one (Fig. 4.3 b) because  

The capacity of the columns used in such applications may be larger than those used for preconcentration purposes since dispersion of sample is less important at this stage. This makes it possible to p eriodically regenerate or replace the columns after a large number of determinations. In the foregoing example, regeneration of the column was required only every other day [39]. 

The clean-up separation manifolds are quite similar for different detection systems. They can also be integrated with separation manifolds based on other principles such as gas-liquid, or liquid-liquid separation. A good example is that described by Marshall and van Staden [33]. These workers used an on-line ion-exchange column to remove interfering base metals in the determination of selenium and arsenic with a FI hydride generation AAS system, which also incorporated a gas-liquid separator. The system is shown schematically in Fig. 4.4. More recently, Tyson et al.[40] reported on a column separation system which effectively removed the interference of large concentrations of copper in the determination of hydride forming elements. 

When the analyte is retained on the column instead of the interferents, the FI manifold resembles those used for on-line preconcentration purposes, because finally the analyte will have to be eluted in each cycle in order to be determined in the detector. The only difference would be in the amount of sample loaded. The reader may refer to the manifolds used for column preconcentration described in the sections which follow. 

sample C ion-exchanger column CR, buffer carrier A, acid R, re-ductant SR gas-liquid separator W, waste An argon flow AAS. heated quartz atomizer [33]. 

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