Interlaboratory Tests Michael Koch, M. : Interlaboratory Tests In: Wenclawiak, Koch, Hadjicostas (eds. ) © Springer-Verlag Berlin Heidelberg 2003 Quality Assurance in Analytical Chemistry – Training and Teaching
What are interlaboratory tests? § Randomly selected sub-samples from a source of material are distributed simultaneously to participating laboratories for concurrent testing Koch, M. : Interlaboratory Tests In: Wenclawiak, Koch, Hadjicostas (eds. ) © Springer-Verlag Berlin Heidelberg 2003 Quality Assurance in Analytical Chemistry – Training and Teaching
Types of interlaboratory tests method validation reference material characterization proficiency testing Koch, M. : Interlaboratory Tests In: Wenclawiak, Koch, Hadjicostas (eds. ) © Springer-Verlag Berlin Heidelberg 2003 Quality Assurance in Analytical Chemistry – Training and Teaching
Interlaboratory tests for the validation of a method § objective: best possible characterization of the method § laboratories have to use exactly the same method § assistance should be given to assure this Koch, M. : Interlaboratory Tests In: Wenclawiak, Koch, Hadjicostas (eds. ) © Springer-Verlag Berlin Heidelberg 2003 Quality Assurance in Analytical Chemistry – Training and Teaching
Interlaboratory tests for characterization of a reference material § concentration of the analyte in the material must be analysed by experienced laboratories § less experienced laboratories should not be allowed to participate § objective: best possible estimation of the “true value” of the concentration Koch, M. : Interlaboratory Tests In: Wenclawiak, Koch, Hadjicostas (eds. ) © Springer-Verlag Berlin Heidelberg 2003 Quality Assurance in Analytical Chemistry – Training and Teaching
Interlaboratory tests for proficiency testing of laboratories § objective: to get an indication of the performance of an individual laboratory or a group of laboratories as a whole § laboratories should work under routine conditions § help for the laboratory to improve its quality § can be used by customers or regulatory bodies for the selection of qualified laboratories Koch, M. : Interlaboratory Tests In: Wenclawiak, Koch, Hadjicostas (eds. ) © Springer-Verlag Berlin Heidelberg 2003 Quality Assurance in Analytical Chemistry – Training and Teaching
Objectives of proficiency tests § basic concern is accuracy § inaccuracy contains systematic and random effects § laboratory can determine, whether imprecision or bias is the reason for its inaccuracy Koch, M. : Interlaboratory Tests In: Wenclawiak, Koch, Hadjicostas (eds. ) © Springer-Verlag Berlin Heidelberg 2003 Quality Assurance in Analytical Chemistry – Training and Teaching
Motivation for the laboratories § to uncover errors that couldn’t be found with internal quality control § use as certificate for competence in this testing field for clients, authorities and accreditation bodies Koch, M. : Interlaboratory Tests In: Wenclawiak, Koch, Hadjicostas (eds. ) © Springer-Verlag Berlin Heidelberg 2003 Quality Assurance in Analytical Chemistry – Training and Teaching
Limitations § Interlaboratory tests are always retrospective § organisation, distribution of samples, analyses, evaluation take time § it is dangerous to rely on interlaboratory tests § Proficiency tests cover only a small fraction of the often wide variety of analyses § Proficiency tests do not reflect routine analyses Koch, M. : Interlaboratory Tests In: Wenclawiak, Koch, Hadjicostas (eds. ) © Springer-Verlag Berlin Heidelberg 2003 Quality Assurance in Analytical Chemistry – Training and Teaching
Standards and guidelines for proficiency testing - I § ISO Guide 43: Proficiency testing by interlaboratory comparisons § Part 1: Development and operation of proficiency testing schemes. § Part 2: Selection and use of proficiency testing schemes by laboratory accreditation bodies. § IUPAC, ISO, AOAC (1991): The International Harmonized Protocol for the Proficiency Testing of (Chemical) Analytical Laboratories. Koch, M. : Interlaboratory Tests In: Wenclawiak, Koch, Hadjicostas (eds. ) © Springer-Verlag Berlin Heidelberg 2003 Quality Assurance in Analytical Chemistry – Training and Teaching
Standards and guidelines for proficiency testing - II § International laboratory accreditation cooperation (ILAC): Guidelines for the requirements for the competence of providers of proficiency testing schemes. § Draft ISO 13528: Statistical Methods for the Use in Proficiency Testing by Interlaboratory comparisons. Koch, M. : Interlaboratory Tests In: Wenclawiak, Koch, Hadjicostas (eds. ) © Springer-Verlag Berlin Heidelberg 2003 Quality Assurance in Analytical Chemistry – Training and Teaching
Demands on the provider personnel § § special organizational capabilities technical experts for the analysis statisticians all staff have to be competent for the work it is responsible for Koch, M. : Interlaboratory Tests In: Wenclawiak, Koch, Hadjicostas (eds. ) © Springer-Verlag Berlin Heidelberg 2003 Quality Assurance in Analytical Chemistry – Training and Teaching
Demands on the provider – Planning - I § The interlaboratory test should be carefully prepared. § The planning must be documented before the start of the test Koch, M. : Interlaboratory Tests In: Wenclawiak, Koch, Hadjicostas (eds. ) © Springer-Verlag Berlin Heidelberg 2003 Quality Assurance in Analytical Chemistry – Training and Teaching
Demands on the provider – Planning - II § The plan should typically include: § name and address of the PT provider § name and address of the coordinator and other personnel § nature and purpose of the PT scheme § procedure for the manner in which the participants are selected or criteria which have to met before participation is allowed § name and address of the laboratory performing the scheme (e. g. sampling, sampling processing, homogeneity testing and assigning values) and the number of expected participants. Koch, M. : Interlaboratory Tests In: Wenclawiak, Koch, Hadjicostas (eds. ) © Springer-Verlag Berlin Heidelberg 2003 Quality Assurance in Analytical Chemistry – Training and Teaching
Demands on the provider – Planning - III § Planning content (contd. ): § nature of the test items and of the tests selected § description of the manner in which the test items are obtained, processed, checked and transported. § description of the information that is supplied to participants and the time schedule for the various phases. § information on methods or procedures which participants may need to use to perform the tests or measurements. Koch, M. : Interlaboratory Tests In: Wenclawiak, Koch, Hadjicostas (eds. ) © Springer-Verlag Berlin Heidelberg 2003 Quality Assurance in Analytical Chemistry – Training and Teaching
Demands on the provider – Planning - IV § Planning content (contd. ): § outline of the statistical analysis to be used. § description of how the assigned value is determined. § description of the data or information to be returned to participants. § basis techniques and methods used for evaluation § description of the extent to which the test results are to made public. Koch, M. : Interlaboratory Tests In: Wenclawiak, Koch, Hadjicostas (eds. ) © Springer-Verlag Berlin Heidelberg 2003 Quality Assurance in Analytical Chemistry – Training and Teaching
Demands on the provider – Data-processing equipment § Equipment should be adequate for § data processing § statistical analysis § to provide timely and valid results § Software must be § verified and § backed up Koch, M. : Interlaboratory Tests In: Wenclawiak, Koch, Hadjicostas (eds. ) © Springer-Verlag Berlin Heidelberg 2003 Quality Assurance in Analytical Chemistry – Training and Teaching
Demands on the provider – Test item preparation and management - I § For the selection of the test item all characteristics that could affect the integrity of the interlaboratory comparison should be considered § § homogeneity stability possible changes during transport effects of ambient conditions (e. g. temperature) Koch, M. : Interlaboratory Tests In: Wenclawiak, Koch, Hadjicostas (eds. ) © Springer-Verlag Berlin Heidelberg 2003 Quality Assurance in Analytical Chemistry – Training and Teaching
Demands on the provider – Test item preparation and management - II § samples used in the proficiency test should be similar to the samples that are routinely analysed in the laboratories § sample amount § surplus of sample can be used as reference material § surplus can be used to make excessive effort on the analyses Koch, M. : Interlaboratory Tests In: Wenclawiak, Koch, Hadjicostas (eds. ) © Springer-Verlag Berlin Heidelberg 2003 Quality Assurance in Analytical Chemistry – Training and Teaching
Demands on the provider Homogeneity - I § The PT provider has to ensure that every laboratory will receive samples that do not differ significantly in the parameters to be measured § documented procedure for establishing this homogeneity § degree of homogeneity evaluation of the laboratories results must not be significantly affected § any variation between the portions must be negligible in relation to the expected variations between the participants Koch, M. : Interlaboratory Tests In: Wenclawiak, Koch, Hadjicostas (eds. ) © Springer-Verlag Berlin Heidelberg 2003 Quality Assurance in Analytical Chemistry – Training and Teaching
Demands on the provider Homogeneity - II § true solutions are homogeneous at a molecular level § for solid samples special care on the homogenisation § a formal homogeneity check is described in the „International harmonized protocol. . . “ Koch, M. : Interlaboratory Tests In: Wenclawiak, Koch, Hadjicostas (eds. ) © Springer-Verlag Berlin Heidelberg 2003 Quality Assurance in Analytical Chemistry – Training and Teaching
Demands on the provider Stability - I § test material must be sufficiently stable § under the conditions of storage and distribution to the participants § for the time period from producing the samples until the analyses in the participant’s laboratory § this stability has to be tested by the PT provider Koch, M. : Interlaboratory Tests In: Wenclawiak, Koch, Hadjicostas (eds. ) © Springer-Verlag Berlin Heidelberg 2003 Quality Assurance in Analytical Chemistry – Training and Teaching
Demands on the provider Stability - II § Analysing a part of the samples after the estimated time necessary for the distribution § differences in the results may be due to instability or to between-batch variability in the organiser’s laboratory § information may be derived from the organiser’s prior experience or obtained from technical literature § accelerated stability testing by worsening the ambient conditions for the samples Koch, M. : Interlaboratory Tests In: Wenclawiak, Koch, Hadjicostas (eds. ) © Springer-Verlag Berlin Heidelberg 2003 Quality Assurance in Analytical Chemistry – Training and Teaching
Demands on the provider Stability - III § the organiser has to ensure that the changes due to instability do not significantly affect the evaluation of the laboratories’ performance Koch, M. : Interlaboratory Tests In: Wenclawiak, Koch, Hadjicostas (eds. ) © Springer-Verlag Berlin Heidelberg 2003 Quality Assurance in Analytical Chemistry – Training and Teaching
Choice of analytical method § Normally the laboratory should use its routine method § the choice might be limited by e. g. legal regulations § organiser should ask for details § to conduct a method specific evaluation § to give comments on the methods used Koch, M. : Interlaboratory Tests In: Wenclawiak, Koch, Hadjicostas (eds. ) © Springer-Verlag Berlin Heidelberg 2003 Quality Assurance in Analytical Chemistry – Training and Teaching
Method specific evaluation Koch, M. : Interlaboratory Tests In: Wenclawiak, Koch, Hadjicostas (eds. ) © Springer-Verlag Berlin Heidelberg 2003 Quality Assurance in Analytical Chemistry – Training and Teaching
Determination of the assigned value § one of the most critical features of a proficiency test § inappropriate value will drastically reduce the value of the scheme § the same problem as in the certification of a reference material § but the organiser of a proficiency test cannot expend the same amount of effort Koch, M. : Interlaboratory Tests In: Wenclawiak, Koch, Hadjicostas (eds. ) © Springer-Verlag Berlin Heidelberg 2003 Quality Assurance in Analytical Chemistry – Training and Teaching
Assigned value – certified reference material § ideal test material for a proficiency test § disadvantages § high costs § limited availability § in the necessary quantity § and concentration range § CRM‘s have to be stable for months and PT often deals with more or less instable samples (foodstuffs, biomedical, environmental samples) Koch, M. : Interlaboratory Tests In: Wenclawiak, Koch, Hadjicostas (eds. ) © Springer-Verlag Berlin Heidelberg 2003 Quality Assurance in Analytical Chemistry – Training and Teaching
Assigned value – Consensus of „Expert Laboratories” - I § mean of analysis by expert laboratories § with high precision reference methods and traceable materials for calibration § if different physico-chemical methods are used and the same results are obtained, it is more probable that the value is near to the „true“ value Koch, M. : Interlaboratory Tests In: Wenclawiak, Koch, Hadjicostas (eds. ) © Springer-Verlag Berlin Heidelberg 2003 Quality Assurance in Analytical Chemistry – Training and Teaching
Assigned value – Consensus of „Expert Laboratories” - II § disadvantages § very much effort to ensure the accuracy of the reference measurements § „nobody is perfect“ § there might be doubts among the participants if the result of the expert laboratories deviates from the mean of the participants Koch, M. : Interlaboratory Tests In: Wenclawiak, Koch, Hadjicostas (eds. ) © Springer-Verlag Berlin Heidelberg 2003 Quality Assurance in Analytical Chemistry – Training and Teaching
Assigned value – Formulated or “Synthetic” test materials - I § materials, spiked with the analyte to a known extent § can be made with extremely accurate amounts by gravimetric or volumetric methods § If material does not contain significant amounts of the analyte § assigned value directly from added amount § If material contains analyte, this amount has to be characterized very well. § method to calculate this content from proficiency test was recently developed by the author Koch, M. : Interlaboratory Tests In: Wenclawiak, Koch, Hadjicostas (eds. ) © Springer-Verlag Berlin Heidelberg 2003 Quality Assurance in Analytical Chemistry – Training and Teaching
Assigned value – Formulated or “Synthetic” test materials - II § disadvantages § difficult to achieve sufficient homogeneity, especially with solid materials § analyte might be bound in a different chemical form § Especially in solid materials the originally contained analyte might be bound more strongly to the matrix Koch, M. : Interlaboratory Tests In: Wenclawiak, Koch, Hadjicostas (eds. ) © Springer-Verlag Berlin Heidelberg 2003 Quality Assurance in Analytical Chemistry – Training and Teaching
Assigned value – Consensus of participants - I § easiest and cheapest way used very often § If method for analysis is easy and straightforward good estimate of „true“ value § If a „convention method“ (an empirically defined method) is used, the consensus value is the only possibility Koch, M. : Interlaboratory Tests In: Wenclawiak, Koch, Hadjicostas (eds. ) © Springer-Verlag Berlin Heidelberg 2003 Quality Assurance in Analytical Chemistry – Training and Teaching
Assigned value – Consensus of participants - II § disadvantages § Consensus value might be seriously biased (e. g. analyses of highly volatile substances) § there might be no consensus at all § e. g. if two analytical methods are used, where one is biased § these circumstances are not uncommon in trace analysis § care should be taken to decide whether a consensus value really is good choice Koch, M. : Interlaboratory Tests In: Wenclawiak, Koch, Hadjicostas (eds. ) © Springer-Verlag Berlin Heidelberg 2003 Quality Assurance in Analytical Chemistry – Training and Teaching
Methods to calculate consensus value – Arithmetic mean § requires an outlier test § but these tests are often not very satisfactory, especially if many outliers are present § outlier tests assume normal distribution which is normally not true in trace analysis Koch, M. : Interlaboratory Tests In: Wenclawiak, Koch, Hadjicostas (eds. ) © Springer-Verlag Berlin Heidelberg 2003 Quality Assurance in Analytical Chemistry – Training and Teaching
Methods to calculate consensus value – Median § not affected by outlying data § makes not full use of the information content of the data § if the distribution is skewed, the median is biased Koch, M. : Interlaboratory Tests In: Wenclawiak, Koch, Hadjicostas (eds. ) © Springer-Verlag Berlin Heidelberg 2003 Quality Assurance in Analytical Chemistry – Training and Teaching
Methods to calculate consensus value – Robust mean § „trimmed“ data; a certain part of the data on both tails of the data set is excluded prior to the calculation of the mean § e. g. mean of interquartile range § mean of data between the first and the third quartile of the data set § or Huber statistics Koch, M. : Interlaboratory Tests In: Wenclawiak, Koch, Hadjicostas (eds. ) © Springer-Verlag Berlin Heidelberg 2003 Quality Assurance in Analytical Chemistry – Training and Teaching
Methods to calculate consensus value – Robust mean – Huber statistics § Iterative process § define initial value for m as median of all data § all data outside m± 1. 5*STD are set to m+1. 5*STD or m-1. 5*STD § new value for m is calculated as arithmetic mean of the new data § repeat until there are no changes Koch, M. : Interlaboratory Tests In: Wenclawiak, Koch, Hadjicostas (eds. ) © Springer-Verlag Berlin Heidelberg 2003 Quality Assurance in Analytical Chemistry – Training and Teaching
Performance scoring § assigned value is the target § for the assessment of laboratories a accepted range is necessary § prescribed range originating from the demands put on the analysis (fitness for purpose) § calculated from the standard deviation of the data set Koch, M. : Interlaboratory Tests In: Wenclawiak, Koch, Hadjicostas (eds. ) © Springer-Verlag Berlin Heidelberg 2003 Quality Assurance in Analytical Chemistry – Training and Teaching
Performance scoring – Tolerance range from STD § normally distributed set of data § 95, 5% of the values inside a range of ± 2σ § 99, 7% of the values inside a range of ± 3σ § on a confidence level of 95, 5 % all accurate data are inside µ± 2σ Koch, M. : Interlaboratory Tests In: Wenclawiak, Koch, Hadjicostas (eds. ) © Springer-Verlag Berlin Heidelberg 2003 Quality Assurance in Analytical Chemistry – Training and Teaching
Performance scoring – Z-score § the deviation from the assigned value in standard deviation units n the standard deviation is calculated after exclusion of outlier or with robust statistics Koch, M. : Interlaboratory Tests In: Wenclawiak, Koch, Hadjicostas (eds. ) © Springer-Verlag Berlin Heidelberg 2003 Quality Assurance in Analytical Chemistry – Training and Teaching
Performance scoring – Classification of the Internat. Harmonized Protocol § |Z-score| 2 § 2<|Z-score| 3 § |Z-score|>3 - satisfactory questionable unsatisfactory § Z-scores are common practise in the assessment of laboratory results Koch, M. : Interlaboratory Tests In: Wenclawiak, Koch, Hadjicostas (eds. ) © Springer-Verlag Berlin Heidelberg 2003 Quality Assurance in Analytical Chemistry – Training and Teaching
Z-score - diagramm Koch, M. : Interlaboratory Tests In: Wenclawiak, Koch, Hadjicostas (eds. ) © Springer-Verlag Berlin Heidelberg 2003 Quality Assurance in Analytical Chemistry – Training and Teaching
Statistical distribution § Data near to the limit of determination are not normal distributed § otherwise there should be negative values with a finite probability § tolerance limits should be asymmetrical (more narrow below the assigned value, more wide above it) Koch, M. : Interlaboratory Tests In: Wenclawiak, Koch, Hadjicostas (eds. ) © Springer-Verlag Berlin Heidelberg 2003 Quality Assurance in Analytical Chemistry – Training and Teaching
Solution approaches for assymetrical tolerance limits § logarithmic normal distribution § take the logarithm of the values prior to statistical calculations § Modification of Z-scores with g = quality limit for Z and k 1, k 2 =correction factors = rel. standard deviation = distribution function of standard normal distribution 1 - = confidence level (here: 0, 955) Koch, M. : Interlaboratory Tests In: Wenclawiak, Koch, Hadjicostas (eds. ) © Springer-Verlag Berlin Heidelberg 2003 Quality Assurance in Analytical Chemistry – Training and Teaching
Laboratory assessment § by combination of single value assessment § involves danger of misinterpretation § a laboratory can measure one parameter permanently wrong, but nevertheless is positively assessed Koch, M. : Interlaboratory Tests In: Wenclawiak, Koch, Hadjicostas (eds. ) © Springer-Verlag Berlin Heidelberg 2003 Quality Assurance in Analytical Chemistry – Training and Teaching
Combined assessment according to Intern. Harmon. Protocol. . . - RSZ § RSZ (rescaled sum of z-scores) § RSZ = z/√m with m = number of scores § same scale as z-score § negative assessment, if all values are within the tolerance but a little biased in the same direction § errors with opposite sign cancel each other out Koch, M. : Interlaboratory Tests In: Wenclawiak, Koch, Hadjicostas (eds. ) © Springer-Verlag Berlin Heidelberg 2003 Quality Assurance in Analytical Chemistry – Training and Teaching
Combined assessment according to Intern. Harmon. Protocol. . . - SSZ § SSZ (sum of squared z-scores) § different scale, because 2 -distributed § doesn‘t consider the sign of z-scores Koch, M. : Interlaboratory Tests In: Wenclawiak, Koch, Hadjicostas (eds. ) © Springer-Verlag Berlin Heidelberg 2003 Quality Assurance in Analytical Chemistry – Training and Teaching
Combination of single values assessments § just counting positive and negative assessments of all values § the absolute value of the z-score is not considered § e. g. assessment in the proficiency tests of german water authorities § 80 % of the values – |Zu-score| 2 § 80 % of the parameters successful Koch, M. : Interlaboratory Tests In: Wenclawiak, Koch, Hadjicostas (eds. ) © Springer-Verlag Berlin Heidelberg 2003 Quality Assurance in Analytical Chemistry – Training and Teaching
Test scheme reports § should be distributed to the laboratories as soon as possible § normally not later than 1 month after deadline for the analytical results § laboratories need quick feedback for corrective actions § laboratories should be identified in the report by test specific codes to maintain confidentiality Koch, M. : Interlaboratory Tests In: Wenclawiak, Koch, Hadjicostas (eds. ) © Springer-Verlag Berlin Heidelberg 2003 Quality Assurance in Analytical Chemistry – Training and Teaching
Test scheme reports – ISO Guide 43 – contents - I § Name and address of the organisation conducting or coordinating the test scheme § Names and affiliations of persons involved in the design and conduct of the test scheme § Date of issue of the report § Report number and clear identification of the test scheme § Clear description of item or materials used, including details of sample preparation and homogeneity testing § Laboratory participation codes and test results Koch, M. : Interlaboratory Tests In: Wenclawiak, Koch, Hadjicostas (eds. ) © Springer-Verlag Berlin Heidelberg 2003 Quality Assurance in Analytical Chemistry – Training and Teaching
Test scheme reports – ISO Guide 43 – contents - II § Statistical data and summaries, including assigned values and range of acceptable results § Procedures used to establish any assigned value § Details of the traceability and uncertainty of any assigned value § Assigned values and summary statistics for test methods/procedures used by other participating laboratories (if different methods are used by different laboratories) § Comments on laboratory performance by the coordinator and technical advisers Koch, M. : Interlaboratory Tests In: Wenclawiak, Koch, Hadjicostas (eds. ) © Springer-Verlag Berlin Heidelberg 2003 Quality Assurance in Analytical Chemistry – Training and Teaching
Test scheme reports – ISO Guide 43 – contents - III § Procedures used to design and implement the test scheme (which may include reference to test scheme protocol) § Procedures used to statistically analyse the data § Advice, where appropriate, on the interpretation of the statistical analysis Koch, M. : Interlaboratory Tests In: Wenclawiak, Koch, Hadjicostas (eds. ) © Springer-Verlag Berlin Heidelberg 2003 Quality Assurance in Analytical Chemistry – Training and Teaching
Certificate § If the proficiency test scheme has regulations for the assessment of the laboratories on the basis of the data (successful / not successful) a certificate should be sent to the laboratory in case of successful participation. § In many cases these certificates are used by the laboratories for demonstrating competence to their customers, i. e. for advertising. Koch, M. : Interlaboratory Tests In: Wenclawiak, Koch, Hadjicostas (eds. ) © Springer-Verlag Berlin Heidelberg 2003 Quality Assurance in Analytical Chemistry – Training and Teaching
Certificate – example Koch, M. : Interlaboratory Tests In: Wenclawiak, Koch, Hadjicostas (eds. ) © Springer-Verlag Berlin Heidelberg 2003 Quality Assurance in Analytical Chemistry – Training and Teaching
Confidentiality § normally in all PT schemes the identity of all laboratories are kept confidential § public reports about poor performance of a laboratory in a proficiency test could be the economic ruin of this laboratory § identity should be known only to a small number of persons § These persons must be regularly instructed about there duty to keep this information confidential § the coordinating body may be required to report poor performance to a particular authority § participants should be notified of this possibility Koch, M. : Interlaboratory Tests In: Wenclawiak, Koch, Hadjicostas (eds. ) © Springer-Verlag Berlin Heidelberg 2003 Quality Assurance in Analytical Chemistry – Training and Teaching
Collusion and falsification of results § PT schemes often are not only a help for the laboratories to improve their quality but also a control tool for accreditation bodies, customers and authorities § their may be a tendency among some participants to give a falsely optimistic impression of their capabilities Koch, M. : Interlaboratory Tests In: Wenclawiak, Koch, Hadjicostas (eds. ) © Springer-Verlag Berlin Heidelberg 2003 Quality Assurance in Analytical Chemistry – Training and Teaching
Collusion § must not be possible n as many concentration levels as possible Koch, M. : Interlaboratory Tests In: Wenclawiak, Koch, Hadjicostas (eds. ) © Springer-Verlag Berlin Heidelberg 2003 Quality Assurance in Analytical Chemistry – Training and Teaching
Number of multiple measurements § example from reality: § routine: single measurement § asked in proficiency test: double measurement § executed in proficiency test: (!) measurements independent 40 § therefore: limitation of sample amount, where possible Koch, M. : Interlaboratory Tests In: Wenclawiak, Koch, Hadjicostas (eds. ) © Springer-Verlag Berlin Heidelberg 2003 Quality Assurance in Analytical Chemistry – Training and Teaching
Level-by-level evaluation for different concentrations § can lead to injustice tolerance limits / standard deviation in % aluminum 120 100 80 60 40 20 0 -20 -40 -60 -80 0, 050 0, 075 0, 100 0, 125 0, 150 0, 175 0, 200 concentration in mg/l n a procedure for a common evaluation can be found in a German standard (DIN 3840245: 2003) Koch, M. : Interlaboratory Tests In: Wenclawiak, Koch, Hadjicostas (eds. ) © Springer-Verlag Berlin Heidelberg 2003 Quality Assurance in Analytical Chemistry – Training and Teaching
Effort for the laboratory § analysis of the samples § should not exceed the effort for routine samples § in reality not insignificant § participation fee Koch, M. : Interlaboratory Tests In: Wenclawiak, Koch, Hadjicostas (eds. ) © Springer-Verlag Berlin Heidelberg 2003 Quality Assurance in Analytical Chemistry – Training and Teaching
Participation fee § normally between 300 US-$ and 1000 US-$ (depending on matrix and parameters) Koch, M. : Interlaboratory Tests In: Wenclawiak, Koch, Hadjicostas (eds. ) © Springer-Verlag Berlin Heidelberg 2003 Quality Assurance in Analytical Chemistry – Training and Teaching
Benefits - I § regular, external and independent check on data quality § Assistance in demonstrating quality and commitment to quality issues § Motivation to improve performance § Support for accreditation/certification to quality standards § Comparison of performance with that of peers Koch, M. : Interlaboratory Tests In: Wenclawiak, Koch, Hadjicostas (eds. ) © Springer-Verlag Berlin Heidelberg 2003 Quality Assurance in Analytical Chemistry – Training and Teaching
Benefits - II § Assistance in the identification of measurement problems § Feedback and technical advice from organisers (reports, newsletters, open meetings) § Assistance in the evaluation of methods and instrumentation § A particularly valuable method of quality control where suitable reference materials are not available Koch, M. : Interlaboratory Tests In: Wenclawiak, Koch, Hadjicostas (eds. ) © Springer-Verlag Berlin Heidelberg 2003 Quality Assurance in Analytical Chemistry – Training and Teaching
Benefits - III § Assistance in training staff § Assistance in the marketing of analytical services § Savings in time/costs by reducing the need for repeat measurements § A guard against loss of reputation due to poor performance § Increased competitiveness Koch, M. : Interlaboratory Tests In: Wenclawiak, Koch, Hadjicostas (eds. ) © Springer-Verlag Berlin Heidelberg 2003 Quality Assurance in Analytical Chemistry – Training and Teaching
Benefits - costs § the costs are noticed immediately § benefits are difficult to quantify in monetary terms § the succesful participation often is a important proof of competence § and therefore compensate for the costs of participation Koch, M. : Interlaboratory Tests In: Wenclawiak, Koch, Hadjicostas (eds. ) © Springer-Verlag Berlin Heidelberg 2003 Quality Assurance in Analytical Chemistry – Training and Teaching
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