On-line diluter for atomic absorption spectrophotometry

The short calibration range of atomic-absorption spectrophotometers and the widely varying sensitivities available for various elements tend to complicate the analysis of environmental materials. The complication stems from (a) the wide range of analyte concentrations found, which requires extra dilution for sporadic samples, and (b) the systematic dilution of every sample solution needed when combinations of certain elements are determined. For example, in the analysis of soils for cadmium and zinc, different dilutions are required for the two elements because of their very different abundances, and the absence of medium-sensitivity lines for zinc. These complications do not constitute a problem for small analytical loads. However, in long runs, and especially when automatic systems are employed, the retrospective replacement of out-of-range measurements and the use of two separate dilutions for difierent elements add substantially to the workload. There are a number of discrete dilution systems available commercially,l although we are aware of no previous examples of an on-line continuous dilution system for atomic absorption. Here we describe an on-line diluter that can obviate manual intervention. If a test solution is found to have a concentration above the useful range the flow can be immediately diverted through the diluter to supply a fixed-ratio dilution to the nebuliser. The device described here is hand operated, but could readily be adapted to automatic control. The mixing chamber produces turbulent mixing with minimum flow resistance, which are the conditions required for this application.