Preconcentration techniques - Determination of Anions in Natural and Treated Waters - page 738

Chapter 15
Preconcentration techniques

Despite the great strides forward in analytical instrumentation that have been made in
recent years, the analyst working in the fields of potable water analysis and
environmental analysis of non-saline and saline waters finds that, frequently, the
equipment has insufficient sensitivity to be able to detect the low concentrations of
substances present in his samples with the consequence that he has to report less than the
detection limit of the method. Consequently, trends upwards or downwards in the levels
of background concentrations of these substances in the environment cannot be followed.
This is a very unsatisfactory situation which is being made worse by the extremely low
detection limits being set in new directives on levels of pollution, issued by the European
Community and other international bodies. To overcome the problem, there has been a
move in recent years, to apply preconcentration to the sample prior to analysis so that,
effectively, the detection limit of the method is considerably reduced to the point that
actual results can be reported and trends followed.
The principle of preconcentration is quite simple. Suppose that we need to determine
5ng L − 1 of a substance in a sample and that the best technique has a detection limit of
1µg L −1 (1000ng L −1 ). To reduce the detection limit to 5ng L −1 , we might, for example,
pass 1L of the sample down a small column of a substance that absorbs the substance
with 100% efficiency. We would then pass down the column 5ml of a solvent or reagent
which completely dissolves the substance from the column thereby achieving a
preconcentration of 1000/5=200. Thus, if the detection limit of the analytical method
without preconcentration were 1000ng L −1

then with preconcentration it would be

reduced to approximately 5ng L −1 .
The use of a column is but one of many possible methods of achieving
preconcentration.
A combination of preconcentration with the newest, most sensitive, and, by definition,
most expensive analytical techniques now becoming available is achieving previously
undreamt of detection limits at the very time when the requirements for such sensitive
analysis is increasing at a rapid pace. Thus the combination of preconcentration with
these techniques is enabling exceedingly low concentrations of anions in the ng L −1 or
lower range to be determined. Preconcentration prior to gas or high performance liquid
chromatography is achieving similar results in the analysis of organics.
Another aspect of preconcentration is, however, worthy of mention, particularly in the
case of smaller laboratories which cannot afford to purchase the full range of modern
analytical instrumentation. Using older, less sensitive instrumentation, preconcentration
will still achieve very useful reductions in detection limits which will be adequate in
many but not all instances. Thus, if conventional atomic absorption spectrometry
achieves detection limits of 1 and 5mg L −1 for halides in water, then a 200-fold