Vicki J. Barwick

Strategies for solvent selection — a literature review

Abstract Legislation restricting the use of certain chemicals will affect the choice of reagents available to analytical laboratories. Analysts may therefore be faced with the problem of finding alternative solvents for established methods, and when developing new methods will want to avoid certain types of compound (e.g., chlorinated solvents which deplete the ozone layer). A review is presented of the literature available which will help analysts to select alternative solvents, with particular emphasis on solvent extraction and liquid chromatography. Methods of classifying solvents are discussed and tables of solvent properties are given. Strategies for optimising mobile phases for high performance liquid chromatography are also described in detail.

Measurement uncertainty: Approaches to the evaluation of uncertainties associated with recovery†

A number of approaches for evaluating recovery and its contribution to uncertainty budgets for analytical methods are considered in detail. The recovery, R, for a particular sample is considered as comprising three elements, m, Rs and Rrep. These relate to the recovery for the method; the effect of sample matrix and/or analyte concentration on recovery; and how well the behaviour of spiked samples represents that of test samples. The uncertainty associated with R, u(R), will have contributions from u(m), u(Rs) and u(Rrep). The evaluation of these components depends on the method scope and the availability, or otherwise, of representative certified reference materials. Procedures for evaluating these parameters are considered and illustrated with worked examples. Techniques discussed include the use of certified reference materials and spiking studies, and the use of extraction profiling to predict recoveries. All the approaches discussed evaluate the recovery and its uncertainty for the analytical method as a whole. It is concluded that this is a useful approach as it reduces the amount of experimental work required. In addition, most of the required data are frequently available from method validation studies.

Sources of uncertainty in gas chromatography and high-performance liquid chromatography

Analysts are increasingly being required to evaluate the uncertainty associated with their methods. Indeed, estimating the uncertainty of an analytical result is an essential part of quantitative analysis. The approach of the International Organisation for Standardisation to uncertainty estimation requires the identification of the possible sources of uncertainty for a procedure, followed by the evaluation of their magnitude. A review is presented of the sources of uncertainty associated with analysis by gas chromatography and high-performance liquid chromatography. The review is intended as a source document for analysts evaluating uncertainties for chromatographic procedures. The sources of uncertainty associated with the techniques are presented, and where such data were available, quantitative estimates of their magnitude are given.

Estimating measurement uncertainty: reconciliation using a cause and effect approach

Abstract A strategy is presented for applying existing data and planning necessary additional experiments for uncertainty estimation. The strategy has two stages: identifying and structuring the input effects, followed by an explicit reconciliation stage to assess the degree to which information available meets the requirement and thus identify factors requiring further study. A graphical approach to identifying and structuring the input effects on a measurement result is presented. The methodology promotes consistent identification of important effects, and permits effective application of prior data with minimal risk of duplication or omission. The results of applying the methodology are discussed, with particular reference to the use of planned recovery and precision studies.

The evaluation of measurement uncertainty from method validation studies

A protocol has been developed illustrating the link between validation experiments and measurement uncertainty evaluation. The application of the protocol is illustrated with reference to a method for the determination of three markers (CI solvent red 24, quinizarin and CI solvent yellow 124) in fuel oil samples. The method requires the extraction of the markers from the sample matrix by solid phase extraction followed by quantification by high performance liquid chromatography (HPLC) with diode array detection. The uncertainties for the determination of the markers were evaluated using data from precision and trueness studies using representative sample matrices spiked at a range of concentrations, and from ruggedness studies of the extraction and HPLC stages.

Estimating measurement uncertainty using a cause and effect and reconciliation approachPart 2.† Measurement uncertainty estimates compared with collaborative trial expectation‡

Measurement uncertainty estimates are presented for ten analytical methods, covering over 45 combinations of analyte, matrix and concentration. The uncertainty estimates were produced using a cause and effect approach published previously. Techniques include gas and liquid chromatography, elemental analysis by graphite furnace atomic absorption spectrometry, inductively coupled plasma-mass spectrometry, inductively coupled plasma-optical emission spectrometry, and titrimetry. A range of different types of data is used, including quality control data and method validation studies. The major contributions to the uncertainty for each method and a brief description of their evaluation are given. In most cases, a combination of an experimental estimate of overall precision and the uncertainty associated with overall bias, measured as recovery, across representative sample types and analyte concentrations contributes the majority of the uncertainty, although in several cases it was essential to consider additional factors. The uncertainty estimates are also compared with collaborative trial results where available, and with reproducibility estimates obtained from the Horwitz function.

Method optimisation with the use of uncertainty budgets

Uncertainty budgets can be used for a variety of situations, e.g. reporting the total uncertainty, calculating tolerance limits or method optimisation. In this paper it is demonstrated how the use of uncertainty budgets can help in reducing the total uncertainty of an analytical method, i.e. method optimisation. In this example it has been possible to reduce the total uncertainty of a concentration determination of hydrogen peroxide (by titration) from 2.7 x 10(-2) M to 3.93 x 10(-3) M (or to about a 1/7) by changing the traceability chain and working with more pure reagents.

Evaluation of a solid phase extraction procedure for the determination of pesticide residues in foodstuffs

An evaluation of a method based on solid phase extraction (SPE) as a clean-up stage for the determination of pesticide residues in composite vegetable and cabbage samples is presented. Samples are extracted with acetonitrile and the extract is loaded onto an SPE cartridge. The pesticides are eluted with a mixture of acetonitrile/toluene (3:1 v/v) and determined by GC–MS or HPLC with fluorescence detection. Acceptable recoveries were obtained for 31 pesticides. An investigation of the effect of partial, as opposed to exact, matrix matching of standards in GC–MS analysis is also presented. While the variation using unmatched standards is visibly greater, the difference is not such as to require exact matrix matching for residue-screening work. The procedure removes the need for chlorinated solvents, reduces the volumes of other solvents used and allows increased sample throughput compared with liquid–liquid extraction procedures. © 1999 Society of Chemical Industry

Evaluation of carbon disulfide as an alternative to carbon tetrachloride for the determination of hydrocarbon oils in water by infra-red spectrophotometry

Abstract The replacement of carbon tetrachloride as an extraction solvent for the determination of hydrocarbon oils has been investigated. A detailed comparison of the performance of carbon tetrachloride and carbon disulfide has been undertaken using sets of homogeneous artificial seawater samples spiked with three different oils (gas oil, medium fuel oil and Forties field crude oil) at varying levels. An initial study of two sets of samples spiked with gas oil at different concentrations indicated problems with the homogeneity of the samples. A multiple interleaved run-off technique for sample preparation was therefore developed and this was found to significantly improve the sample homogeneity. A further three sets of samples spiked with gas, fuel and crude oil were prepared using this technique. In total, five sets of samples were analysed. In all cases, there was no significant difference between the variation in the results obtained for each solvent. In three cases the means of the results obtained u...

Perspective. Impact of key new regulations on analytical chemistry

Environmental and health and safety legislation restricting the use of certain solvents will affect the way in which analytical laboratories work. The single piece of legislation which is likely to have the greatest impact on analytical chemistry in the coming years is that enforcing the Montreal Protocol on substances which deplete the ozone layer. This will restrict the supply of a number of chlorinated solvents commonly used in laboratories including 1,1,1-trichloroethane and carbon tetrachloride. There is an urgent need for chemists to seek alternatives to these solvents, either by identifying suitable less damaging solvents, or by developing alternative technologies which require smaller solvent volumes. Analytical chemists should also be aware when developing new procedures of the general pressure (although not regulated at present) to reduce the volumes of all organic solvents used.