Valid Analytical Measurement (Week 3) Flashcards
Valid analytical measurements
Results of analysis need to be fit for purpose.
Results obtained in one lab need to be consistent with results obtained in another.
Results obtained at different times need to be consistent.
Results need to fulfil customers’ requirements.
Unreliable results can cost a laboratory financially, in reputation, and possibly with lost accreditation.
VAM initiative
Launched by DTI in 1988
Aims to improve quality of analytical measurement
Provides a framework to enable organisations to deliver reliable results consistently.
International harmonisation of measurement - results in different countries
Relate measurements to international standards
VAM principle 1
Analytical measurements should be made to satisfy an agreed requirement.
The analysis is performed to solve a defined problem.
The analyst needs to understand the problem to be able to select the most appropriate technique.
VAM principle 1 steps
Identify a responsible analyst.
Define how the samples are to be obtained – the sampling procedure. A poor sampling procedure will cause all results to be unreliable.
Discuss with the client how the results are going to be used – this will determine the required confidence levels.
Discuss with the client the critical and non-critical issues, e.g. speed or accuracy. These will affect the choice of analysis.
VAM principle 2
Analytical measurements should be made using methods and equipment which have been tested to ensure they are fit for purpose.
Measurements and methods must be applied within their stated scope, e.g. levels of analyte and types of sample matrix.
VAM principle 2 steps
Careful choice of method is important.
Ensure that the method validation covers the intended use, e.g. with regards to accuracy and measurement uncertainty.
Ensure that your method can provide acceptable results which achieve the required specification from the client.
Ensure that instruments have been calibrated.
Use appropriate reference materials.
VAM principle 3
Staff making analytical measurements should be both qualified and competent to undertake the task.
Analysts who understand the method they are using will achieve more reliable results.
Staff should be required to show that they can produce reliable results before they are allowed to work without close supervision.
VAM principle 3 steps
Ensure that staff have been appropriately trained.
Analysts need to be sufficiently competent and knowledgeable to be able to know when results are unusual or anomalous.
VAM principle 4
There should be a regular independent assessment of the technical performance of a laboratory (preferably by participating in national and international proficiency testing schemes).
External benchmarking provides a lab feedback on its performance compared to other labs.
The entire process from sample receipt to reporting of results should be assessed.
VAM principle 4 steps
Participate in proficiency testing schemes.
Where no suitable schemes are available, analyse check samples from the client, where possible.
VAM principle 5
Analytical measurements made in one location should be consistent with those elsewhere.
Disagreements over results between labs can cost money and cause delays.
Regulations cannot be enforced if results of analysis are disputed.
VAM principle 5 steps
Use reference materials as QC samples in conjunction with control charts to ensure that analysis continues to give acceptable results.
Ensure reference materials are traceable back to calibration.
The identity of the reference material and/or the concentration of the reference material has been confirmed with national or international standards or methods.
(This will be stated in the documentation that comes with the reference material.)
VAM principle 6
Organisations making analytical measurements should have well defined quality control and quality assurance policies.
Labs need to have procedures that will ensure that they carry out their analyses in a valid manner.
Laboratory records need to be sufficient to enable results to be checked or the analysis re-constructed.
VAM principle 6 steps
The laboratory needs to have an appropriate quality management system (QMS).
A QMS encompasses quality management, quality assurance and quality control and policies.
For analytical laboratories, accreditation to ISO 17025 is the relevant quality standard.
A QMS requires that QA procedures must be audited and reviewed.
Certified reference materials
Measurements need to traceable back to calibration.
This ensures that results from different laboratories are comparable.
This also ensures that methods can be validated.
Traceability for measuring physical characteristics is straightforward.
Example - mass is traceable via calibration of balances using certified weights, through the UK standard kilogram in the National Physical Laboratory (NPL), known as kilogram-18, which itself is checked against the internationally agreed kilogram. (The kilogram is now defined by the exact value of a constant, Planck’s constant, which will not change over time.)
CRM 2
Traceability is more difficult for chemicals.
–Is the material authentic? – is it the material that is named on the label?
–What is the concentration of the material?
Ensuring CRM
Traceability is ensured by using Certified Reference Materials (CRMs)
CRMs are materials produced and characterised in a technically valid manner.
CRMs are accompanied by documentation from the certifying body detailing the composition and the characterisation process.
CRMs come with a stated uncertainty – the limits within which the true value of the CRM will lie.
Types of CRM
Single substance CRM:
A pure material which is certified for chemical purity, or for a physical property such as conductivity.
Matrix material CRM:
The analyte is supplied in a particular matrix. The same analyte may be in several different matrices, e.g. hexestrol, diethylstilbestrol and dienestrol (used as growth promoters in cattle) in bovine urine or bovine muscle.
CRM uses
CRMs are used as primary reference materials.
CRMs are used to calibrate secondary reference materials.
CRMs are very expensive, therefore they themselves would not be used on a day-to-day basis.)
CRM uses
Calibration and verification under routine conditions, using secondary reference materials derived from the CRM.
Internal QC and QA schemes.
Verification of correct application of standardised methods.
Development and validation of new methods.
Calibration of other materials.
CRM - Determining true value?
Definitive method
Independent measurement method
Inter-laboratory consensus method
CRM definite method
Single lab (usually) The method has high precision and zero systematic error E.g. isotope dilution mass spectrometry
CRM - indefinite method
Single lab (usually) Two or more reliable methods All results fall within the required end-use uncertainty.
CRM Inter-laboratory consensus method
Several labs (~5-20) analyse replicate samples. Each lab uses their own method or a specified method.
CRM measurement uncertainty
A value determined in any laboratory is unlikely to be exactly the same as that of the certified value of CRM.
The key question is, is the value determined in the lab within the specified uncertainty in the CRM value?
Uncertainty in the CRM value is part of the calculation for the overall measurement uncertainty. (See later lectures.)
Results will be slightly different every time - theres a range, does value you get lie within this area or out?
