Types of experimental error Random or indeterminate error Arise from inherent limitations in ability to make measurements. Assumed to “cancel out” over time. Can minimize with experimental setup, but can never eliminate completely. Examples: electrical noise; differences in visual determination. Systematic or determinate error Can be determined and corrected for. Example: improperly calibrated instrument

CHM 235---Dr. Skrabal Precision vs. accuracy Two important concepts regarding data are accuracy and precision Accuracy: Closeness to a “true” value. Must eliminate systematic error to assure accuracy. Precision: Reproducibility

Not accurate, not precise Accurate and precise Precise, not accurate

Assessing systematic error (and accuracy) Can never really know a “true” value, but there are practical methods for assessing accuracy and for detecting systematic error Four important methods Blank analyses Use of certified reference materials or standard reference materials (SRM and CRM) Interlaboratory comparison (round-robin experiment) Multiple method comparison

Blank analyses Analysis performed using some analytical procedure with everything but the analyte Purpose is to detect whether or not there is analyte present in reagents, water, laboratory implements, etc. used for an analytical procedure For example, for analysis of an analyte in an aqueous solution, use high-purity water in place of sample and measure as normal. Signal from blank is subtracted from each and every sample signal

Example: Blank analyses For a spectrophotometric analysis, a series of standards is prepared and tested. Then an unknown is analyzed. Note the blank value is subtracted from each standard and sample measurement. Conc. (mg/L) Absorbance Blank- corrected absorbance 0.002 10.0 0.120 0.118 20.0 0.242 0.240 50.0 0.605 0.603 unknown 0.473 0.471

Certified (or standard) reference materials CRMs (or SRMs) are available for many different types of analytes in many different media.

CRM for trace metals in estuarine water CRM for trace metals in estuarine water. Available from National Research Council of Canada.

Fish tissue CRM for various organic analytes Fish tissue CRM for various organic analytes. Available from National Research Council of Canada.

Interlaboratory comparison (round-robin experiment) Sample containing analyte of interest is distributed to different laboratories for analysis (single blind or double blind) Results analyzed for similarity; compared to certified value if available

Lead in polyethylene intercomparison From D.C. Harris (2003) Quantitative Chemical Analysis, 6th Ed.

Multiple method comparison Use different methods to analyze for the same analyte in the same sample

CRM for trace metals in estuarine water CRM for trace metals in estuarine water. Available from National Research Council of Canada. Note that most metals were determined using completely different analytical methods.