1. 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

2. CHM 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

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

4. 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

5. 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

6. 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

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

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

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

10. 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

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

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

13. 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.