Separation Versus Preconcentration
Two frequently encountered analytical problems are: (1) the presence of matrix components interfering with the analysis of the analyte; and (2) the presence of an- alytes at concentrations too small to analyze accurately. We have seen how a separa- tion can be used to solve the former problem. Interestingly, separation techniques can often be used to solve the second problem as well. For separations in which a complete recovery of the analyte is desired, it may be possible to transfer the analyte in a manner that increases its concentration. This step in an analytical procedure is known as a preconcentration.
Two examples from the analysis of water samples illustrate how a separation and preconcentration can be accomplished simultaneously. In the gas chromato- graphic analysis for organophosphorous pesticides in environmental waters, the an- alytes in a 1000-mL sample may be separated from their aqueous matrix by a solid- phase extraction using 15 mL of ethyl acetate. After the extraction, the analytes are present in the ethyl acetate at a concentration that is 67 times greater than that in the original sample (if the extraction is 100% efficient). The preconcentration of metal ions is accomplished by a liquid–liquid extraction with a metal chelator. For example, before their analysis by atomic absorption spectrophotometry, metal ions in aqueous samples can be concentrated by extraction into methyl isobutyl ketone (MIBK) using ammonium pyrrolidine dithiocarbamate (APDC) as a chelating agent. Typically, a 100-mL sample is treated with 1 mL of APDC, and extracted with ten mL of MIBK. The result is a ten-fold increase in the concentration of the metal ions. This procedure can be adjusted to increase the concentrations of the metal ions by as much as a factor of 40.