November 2006, Vol. 18, No.11
Test Your Knowledge of Laboratory Quality Control
Multiple Choice Questions:
In quality assurance–quality control terms, the initials RPD stand for relative percent difference. Why is this term used in expressing duplicate results?
A. It minimizes the effect of round-off errors.
B. It explains the reason for the gap between a true value and a measured one.
C. It minimizes the effect of wide concentration ranges in measurements.
D. It corrects for missing data points.
Which of the following quality control sample types would be used to measure precision?
A. Matrix spike.
B. Matrix duplicate.
C. Reagent blank.
D. Check standard.
In a matrix spike quality control sample, the following data are known.
Spike concentration = 15.5 mg/L
Sample concentration = 4.5 mg/L
Measured concentration in matrix spike = 21.8 mg/L
Based on calculations using these data, what is the percent spike recovery?
A sample is analyzed in duplicate. The two measured values are 0.51 mg/L and 0.49 mg/L. What is the relative percent difference?
Matrix spike samples are used to test for what?
B. Dilution effect.
C. Matrix interferences.
D. Calibration stability.
Control charts are very useful in spotting what type of situation in quality control data?
A. Too many data points.
B. A systemic error or a trend.
C. Expected values for calibration factors.
D. Too few data points.
How is a reagent blank normally defined?
A. A blank laboratory water checked against the calibration standards.
B. A sample that shows no response for the analyte or any other analytical species.
C. A blank laboratory water and all reagents that normally come into contact with the samples during the entire analytical procedure.
D. A sample that has no reagents added to it.
The method detection level (MDL) for a given method is determined to be 0.05 mg/L. According to Standard Methods, if the analyst later measures the concentration of a sample to be 0.04 mg/L, how should the result be reported?
A. Not detected.
B. <0.04 mg/L.
C. Below MQL (minimum quantitation limit).
D. 0.04 mg/L.
The MDL and the lower level of detection (LLD) differ mainly in what way?
A. The LLD is determined using 7 points, while the MDL requires 17 points.
B. The MDL does not take instrument drift into account, while the LLD does.
C. The LLD is always 10 times larger than the MDL.
D. The MDL is determined with samples carried through the complete analysis, while the LLD does not carry samples through the complete analysis.
To check the calibration for a nitrate–nitrogen analysis, a check standard is analyzed. The check standard has a known value of 2.0 mg/L, and the measured value is 1.9 mg/L. What is the percent recovery for this check standard?
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Questions were developed by Steve Rayburn, director of training and validation at Accelerated Technology Laboratories Inc. (West End, N.C.), and reviewed by the Association of Boards of Certification (Ames, Iowa) Validation and Examination Committee.