In my previous blog, I discussed how your laboratory measurements could be precise (the results are reproducible), yet inaccurate (the value is wrong).
It is comparable to my target results from archery practice where I had consistency shooting at a particular area of the target (-performing good lab technique and producing similar values each trial), however, I was not hitting the bullseye (i.e, achieving the correct concentration).
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Here are some reasons for the laboratory results to be off:
- Mistakes in the calibration standards.
Many labs make their own standards (from stock solutions or even crystalline solids) and if there is an error in the dilution process, this will simply result in your measurements being be off. - Interferences.
A consistent interference is present in the extraction and preparation process that adds a set amount of your analyte - maybe a contaminated solvent or sorbent that the lab is using. It is a constant amount adding to the results causing all them to be higher than what is actually in the sample. - Calculation errors.
This is pretty common. The math is wrong in how the amount of the substance is being calculated. This is an easy fix if the calculation error can be found.
There are probably many more reasons why precision may be good but accuracy is off for your analytical measurements.
Build in checks and balances to ensure that your next project can identify these errors before you use and rely on the data for your litigation cases.
One of the best ways to do this is provide a sample where the concentration of the target analyses is known. Standard reference materials are available for most matrices from many different sources. NIST is a good place to start. If there isn’t an SRM available, you may need to make your own… but that is a story for another blog.
What do you look for in expert witnesses and consultants for your litigation cases?
