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What is the difference between the two true values?
In most certificates of analysis, there are normally two true values listed. The first true value is the gravimetric or prepared value. The other is the certified or analytical true value.
The gravimetric true value is based on the actual weight of the analyte in a set volume of solvent. The certified true value is based on the analytical measurement of the analyte. This value is obtained by calculating the average of a set number of replicate measurements, pulled from a representative set of samples.
There is a also an uncertainty associated with the certified true value. Uncertainty of measurement comprises, in general, many components. Some of these components may be evaluated from statistical distribution of the results of series of measurements and can be characterized by experimental standard deviations. The other components, which can also be characterized by standard deviations, are evaluated from assumed probability distributions based on experience or other information.
It is understood that the result of the measurement is the best estimate of the value of the analyte, and that all components of uncertainty, including those arising from systematic effects, such as components associated with corrections and reference standards, contribute to the dispersion. The uncertainty range represents a summation of the estimates of the unpredictable variations that can reasonably be expected to occur during a measurement.
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Example of Certificate of Analysis |
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Which true value should I use?
The certified true value is the number that should be used when comparing the data.The certified value may indeed be very close to the prepared value, and therefore have a negligible error. However, the uncertainty may be very large, simply because the analyst is unsure of how close the result is to the true value. The uncertainty represents the combination of all the uncertainties of all the steps that went into the preparation of the material, and the assignment of the value. This means that the company is confident enough to certify that their preparation methods, analysis, and history with this product will provide a certified value in a range of ± x% of the certificate value.
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