Language of Analytical Chemistry: Protocols

Protocols

Earlier we noted that a protocol is a set of stringent written guidelines, specifying an exact procedure that must be followed if results are to be accepted by the agency specifying the protocol. Besides all the considerations taken into account when de- signing the procedure, a protocol also contains very explicit instructions regarding internal and external quality assurance and quality control (QA/QC) procedures.10 Internal QA/QC includes steps taken to ensure that the analytical work in a given laboratory is both accurate and precise. External QA/QC usually involves a process in which the laboratory is certified by an external agency.

As an example, we will briefly outline some of the requirements in the Envi- ronmental Protection Agency’s Contract Laboratory Program (CLP) protocol for the analysis of trace metals in aqueous samples by graphite furnace atomic ab- sorption spectrophotometry. The CLP protocol (Figure 3.7) calls for daily stan- dardization with a reagent blank and three standards, one of which is at the labo- ratory’s contract required detection limit. The resulting calibration curve is then verified by analyzing initial calibration verification (ICV) and initial calibration blank (ICB) samples. The reported concentration of the ICV sample must fall within ±10% of the expected concentration. If the concentration falls outside this limit, the analysis must be stopped and the problem identified and corrected be- fore continuing.

After a successful analysis of the ICV and ICB samples, standardization is rever- ified by analyzing a continuing calibration verification (CCV) sample and a contin- uing calibration blank (CCB). Results for the CCV also must be within ±10% of the expected concentration. Again, if the concentration of the CCV falls outside the es- tablished limits, the analysis must be stopped, the problem identified and corrected, and the system standardized as described earlier. The CCV and the CCB are ana- lyzed before the first and after the last sample, and after every set of ten samples. Whenever the CCV or the CCB is unacceptable, the results for the most recent set of ten samples are discarded, the system is standardized, and the samples are reana- lyzed. By following this protocol, every result is bound by successful checks on the standardization. Although not shown in Figure 3.7, the CLP also contains detailed instructions regarding the analysis of duplicate or split samples and the use of spike testing for accuracy.