Indirect Methods Involving On-line Continuous Precipitation

1 Indirect Methods Involving On-line Continuous Precipitation 

Hitherto all applications using FI continuous precipitation systems to determine anions had as detector the flame AA spectrometer. The prerequisite is that a suitable cation (tag element) is required which can selectively precipitate the anion analyte. The high sensitivity of the AA detector for the determination of most metals can then be readily exploited to perform a sensitive indirect measurement of the precipitated anion via the tag element, following an on-line separation. Other methods, e.g. VIS-UV spectrophotometry, are less attractive in this respect, as an indirect method involving separation usually will not shou much superiority over direct methods. 

The indirect methods for the determination of anions by on-line precipitation using A AS are shown in Table 7.1. It can be seen that methods with a dissolution step generally have much lower sampling frequencies than those without dissolution. One has to remember, however, that the filters have to be cleaned periodically if the precipitates are not dissolved within each cycle. Therefore, the differences in real sample throughputs will not be that significant if the time for the off-line cleaning of filters is taken into account. 

When organic constituents in a sample selectively form insoluble products with certain metals, such properties can be utilized to develop sensitive indirect methods for the oiganics by using AAS to determine the tag metals following a separation of the precipitate, as for anion determinations. Unfortunately, reactions selective enough to determine the organic analytes in even moderately complex matrices are few. Such methods are nonetheless quite ideal for pharmaceutical preparations where the matrices are often simple and controlled. The highest selectivity is therefore not necessary. FI on-line precipitation methods provide a much more efficient alternative compared to the manual batch methods. 

The various applications in this field are also shown in Table 1. Manifolds used for the determinations were quite similar to those used for the indirect determination of anions. 

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