也相当于reference sample or labortary control sample Quality Control Samples by Tim Loftus James Dux, Ph.D. describes quality control in his book, Handbook of Quality Assurance for the Analytical Chemical Laboratory, as “those operations undertaken in the laboratory to ensure that the data are generated within known probability limits of accuracy and precision.” In other words, it is the stuff that you do that shows whether your lab results are most likely right. Unfortunately, many laboratories do not have the quality control they should have to produce unquestionable results. Reasons can vary, but most people find it hard to add 10% to 20% more analyses from QC samples while their normal sample output remains the same. It seems like a waste of time for a technician who knows that he is performing a test method properly.
There may be more to it than that, though. The technician may be performing the test properly, but is the test equipment working properly? Are the reagents OK? Is there interference in the sample? QC samples, run along with “real” samples, are to help determine just these types of errors. Any problem with equipment, reagents, and yes, technician error, will produce bad results. Bad results may show an NPDES permit violation when, in fact, there is no violation. QC samples, when they fall within acceptable levels, will help to validate the results of the “real” samples.
QC samples should be run with every batch of samples analyzed. Generally 10% to 20% of your analysis run should be QC samples. Common QC samples include:
Method blank – This is reagent water that has gone through the same procedure and has all the reagents as the other samples. Method blanks are useful in determining contamination. Ideally, the method blank should not be greater than the method detection limit.
Known – This is a sample that has a known amount of analyte in it. The sample can be made in-house or it can be from an outside source. If it is made in-house, it should be made from a different source of chemicals than what the test standards are made from. The value of the known sample should fall within the result range of the other samples. Knowns help determine the accuracy of an analysis.
Spikes – A spike sample is a “real” sample that has a known amount of analyte added to it. The difference between the sample and the spiked sample is determined and the percent recovery is calculated. Spikes help to determine if interferences are present. Generally, 85% to 115% recovery is acceptable.
Duplicate – Two samples are taken side by side using the same procedures and tested separately. Duplicate samples used to evaluate analytical or measurement precision.
Replicate – A single sample that is tested twice within the batch. The results indicate precision.
Quality control also includes control charts, standardization of equipment, cleaning glassware, and a number of other issues, some of which will be the subject of future articles. However, keep in mind that not all of these QC suggestions can be used with all test methods. What may be used with phosphorus analysis may not necessarily be appropriate with pH analysis or coliform analysis.
While the above suggestions are only meant as a rough guideline, there may be other requirements for QC samples in your NPDES permit or state certification program. For those of you who do not have a formal QC plan, consider incorporating some of these samples in your analysis runs. It will take more of your time, but with good quality control, you will not have to defend your results. Your results will defend themselves.
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