Types of experimental error

Random or indeterminate errorArise 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 errorCan be determined and corrected for.

Example: improperly calibrated instrument

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

CHM 235---Dr. Skrabal

Accurate and precise

Not accurate, not precise Accurate, not 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 methodsBlank analysesUse 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 0.002 0

10.0 0.120 0.118

20.0 0.242 0.240

50.0 0.605 0.603unknown 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. Available from National Research Council of Canada.

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

From D.C. Harris (2003) Quantitative Chemical Analysis, 6th Ed.

Lead in polyethylene intercomparison

Multiple method comparison

• Use different methods to analyze for the same analyte in the same sample

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.