types of experimental error random or indeterminate error arise from inherent limitations in...
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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.