Proficiency schemes

Table 7.2 and Figure 7.2 show data obtained from one round of a proficiency scheme for the determination of the alcoholic strength of a spirit. The results are expressed as % alcohol by volume (%abv). When the participants results are ranked in order of magnitude, the median is the middle value, which in this case is 40.04. The assigned value is therefore 40.04 %abv. 

Fig. 1.2.1 Variation in the result reported and the reference range used in 18 laboratories reporting quantitative results for ethylmalonate in a proficiency scheme...

Table 1.2.4 The proportion of laboratories reporting of a diagnostic proficiency scheme (Northern Europe a correct diagnosis as part of the participation Diagnostic Proficiency Scheme) ... 

Post-analytically, schemes are beginning to emerge specifically to compare practice and performance between laboratories pertaining to the interpretation of test results. For instance, in the UK, NEQAS in conjunction with the National Biochemical Genetic network, MetBio.Net, are offering a scheme that provides the opportunity, when given relevant clinical details, to interpret quantitative amino acid results. This proficiency scheme can compare interpretive skills without the need to circulate scarce clinical samples. 

The development and improvement of analytical methodologies for mycotoxins has been greatly improved by the increased availability of matrix matched certified reference materials (CRMs) (Boenke, 1995) (Table 11.6). The type of matrix CRMs and concentration of the specified mycotoxin are based on the natural occurrence pattern of the toxin in specific foods and feeds. The recent availability of suitable CRMs, while being a prerequisite for the implementation of regulations and standards, will also be invaluable in many ways for the validation of new methods, solving trade disputes and for harmonising proficiency schemes. 

In general, results should be reported to such a level that rounding errors do not occur when results are used in statistical calculations. Reasonable estimation of uncertainty can be based on knowledge of the performance of the method and can make use of (11 day) method validation data (Cheeseman Wilson 1989) and/or proficiency scheme data. This assumes in both cases that an adequate number of soil matrix types are tested. It can be very instructive to compare the uncertainties produced by these two methods. Figure 1.3 attempts... 

It is essential that all proficiency scheme samples are treated as normal samples and are analysed along with the daily routine samples. One of the key purposes of a proficiency scheme is that laboratories thoroughly audit all failures and attempt to determine the cause of any failures. A laboratory should then implement suitable protocols to prevent further failures from the same cause(s). The laboratory should regularly monitor its cumulative proficiency scheme results to ensure that there are no consistent failures for any given determinands and make the results of its monitoring available to its risk assessor clients. 

The laboratory staff carrying out the analysis should participate in external quality control proficiency schemes such as AIMS (the Asbestos in Materials Scheme, which is operated by the UK Health and Safety Laboratory) and they must be able to demonstrate sufficient history of obtaining satisfactory results. 

Asbestos in Materials Proficiency Scheme Administered by the UK Health Safety Laboratory, Broad Lane, Sheffield, S3 7HQ, UK (Tel 44 (0)114 289 2000). 

The preparation of standards for the analysis of VOCs does present difficulties for many laboratories. Firstly, the common use of many solvents in laboratories means that contamination and high blanks are real problems. The analytical techniques used are highly sensitive, standard solutions are usually prepared in methanol, and therefore absorption of atmospheric contaminants readily occurs. Secondly, the volatile nature of the components may make quantitative transfer susceptible to losses. For these reasons, and because of the large number of components involved, many laboratories prefer to buy in solutions of standard mixtures, which are usually prepared gravimetrically under clean conditions. However, laboratories need to check the accuracy of such solutions, usually by comparison to solutions from other suppliers and by means of proficiency schemes, and take care to minimise evaporative losses over the time-scale of use. 

To aid control procedures, many laboratories participate in external quality assessment schemes (also termed external proficiency testing). In these assessment schemes, the pools of material may be human or animal in origin and are distributed to a number of participating laboratories for comparative purposes. Given the various analyzer technologies, it is perhaps not surprising that results obtained by various analyzers differ to some extent and these proficiency schemes do alert laboratories to poor performance. 

For the estimation of trueness, ENV-ISO 13530 recommends regular participation in external quality procedures such as interlaboratory trials and proficiency schemes for the control of trueness (bias). For internal routine action, the use of control charts, based on the mean, spiking recovery, and analysis of blanks, is recommended. In addition, the standard recommends the use of a mean and/or a range control chart and the execution of a minimum of six replicate determinations of the test sample for the calculation of the standard deviation for the control of the precision. 

Iwabuchi with coworkers designed a series of 4-silylo)yproline derivatives with a view to confer lipophilic properties to the proline motif and thereby secure a basis for the catalytic activity growth. They found that (2S,4R)- and (2R,4R)-4-silylo yprolines 30 or their BU4N salts 31 (Figure 10.3) exhibit enhanced catalytic potencies to enantioselectively convert a-sym-metric keto-aldehyde 36 to bicyclo[3.3.1]-type products 37 or ent-37 with opposite enantiopreferences and different enantiocontrolling proficiencies (Scheme 10.8). Unexpectedly, the use of carbojylate (2S,4R)-31 (5 mol%) markedly enhanced the aldolisation rate to complete the reaction in 3 h at room temperature and furnish 77% 37 with 98% diastereomeric excess and... 

If appropriate proficiency scheme results are not available this detailed examination of each step in the analytical process, the so-called bottom-up approach to uncertainty estimates, must be used. In doing so, it is necessary to use a number of equations that allow the calculation of the overall random error arising in a process containing two or more steps. This is technically referred to as the propagation of errors. The basic problem in such cases is that, if the two or more experimental steps have random error sources that are independent of each other, such errors will partly, but not wholly, cancel each other out. In the following equations the final result of the analysis is called X, and the individual analytical steps contributing to X are A, B, etc. The random errors in X, A, B, etc. (normally standard deviations) are given the symbols SX (which we wish to determine), M, 5B, etc. Then,... 

However, there is no doubt that there are different perceptions about QA even within the Western European countries. Some chemists hold strongly to the view that QA is best managed through quality systems and accreditation. Others are equally committed to having long and careful professional training combined with participation in proficiency schemes. 

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