The main concern of a lab that makes chemical analyses or other types of tests is to ensure that the results obtained in their measurements are correct, of course. There are ways to evaluate the tendency of a test method or chemical analysis, often involving the use of certified reference materials, or patterns, or the participation in proficiency testing schemes of proficiency, or other types inter comparisons.
Trend assessments methods of chemical analysis, using certified reference materials, may seem complicated, expensive and time-consuming experiments. The literature sometimes confuse or scare the user with numerous metrological terms unknown and seemingly complex statistical models. The fact is that many times the lab acquires certified reference materials and do not use them effectively. We know of cases where if wasted material with exaggerated experiments needlessly. There are examples where the materials purchased are stored safely and little are used, for lack of a simple guideline. Sometimes all we have is qualitative statements, "method is OK," method passes, or is in the range ".
In this article I present a simple script to use reference materials to evaluate quantitatively a method of measurement is accurate or if you have trend error (bias). This script is not complete, nor the best way of assessing the trend, but allows a reasonable assessment for most cases. To follow this roadmap, the laboratory should be your estimation of uncertainty of measurement with the method used.
A script
1) Do no determinations of the analyte of MRC using your method of measurement,
2) Calculate the average Clab of concentrations obtained,
3) raise your estimation of measurement uncertainty in the lab, ulab,
3) see what is the certified concentration (C)mrc, and the standard uncertainty of the certified concentration umrc
3) If the equation below is true, the method can be considered accurate:
Note that the uncertainties above, written with the letter "u", are standard uncertainties, are not expanded uncertainties. In the certificates of reference materials, are generally presented the expanded uncertainties UCRM, then convert them into standard uncertainties uCRM, dividing by the coverage factor k the estimate of uncertainty:
Usually k = 2:00 pm chemical analysis, then:
So If the equation below is true, the method can be considered accurate::
An example
A laboratory used the IPT reference material to check your 41B measurement of sulfur. He made six determinations and obtained an average of 0.298%. The uncertainty of sulphur content by the method used is 0.008%. The sulphur content certificate on IPT 41B are (0.322 ± 0.003)%. The verification is the following:
Using this equation, we can conclude that the range of variation of the mean in the lab should be (0.322 ± 0.016)% of sulphur, when using the IPT 41B. So using this method, the average grade must be between% and 0.306 0.338% so that the method is considered accurate. This did not occur, because the average value was 0.298%, and concluded that the method has a tendency.
Going on trend, the responsible should do a study to identify the source of trend, for example, at the point of the calibration curve, in the process of weighing sample, etc.