Quality of Analytical Data

Question 1

Systematic Error

Answer 1

Quantified by Bias
Low systematic error = Low Bias
eg. Instrument miscalibration, positive or negative uncorrected blank values. errors in standards.

Question 2

What do systematic errors affect and how are they measured?

Answer 2

Affect the mean set of results.
Measured by analysing a sample with a known concentration of the analyte. The difference between the true value and the measured value gives the bias.

Question 3

Random errors

Answer 3

Affect the precision of an analysis
Low random error= High precision
Cause the spread of results above and below the mean.

eg. Instrument noise, variations in the analytical technique, variations in calibration of items, variations in concentration.

Question 4

How are random errors determined?

Answer 4

By analysing replicates of the same sample to obtain the mean results of the measurement.

Question 5

What is Bias quantified by?

Answer 5

The analysis of a reference material with a known concentration of the analyte.
By comparison of the analytical method with another method known to be accurate in the analysis of similar samples.

Question 6

Normal Distribution & Standard deviation

Answer 6

68.3% results lie within 1 standard deviation of the mean
95.5% results lie within 2 standard deviations of the mean
99.7% lie within 3 standard deviations of the mean.

Question 7

Accuracy, Trueness & Precision

Answer 7

Accuracy refers to an individual analytical result.
If the result is close to the true value, it is accurate.
Trueness refers to a set of results of an analytical method
If an analytic method produces results which are close to the true value, it shows good trueness, thus have small bias.
To give accurate results, the method must be precise

Question 8

Error (Standard deviation) in the Mean

Answer 8

The error in the mean decreases with the number of measurements
The error becomes negligible with approx 30 measurements

Question 9

Relative SD

Answer 9

RSD is the ratio of the SD to the mean
(coefficient of variation)
RSD increases markedly when the analyte is a very small fraction of the total mass

Question 10

Measurement Uncertainty

Answer 10

Uncertainty exists in the result of any analysis due to systematic and random errors.
Measurement uncertainty (MU) is a quantitative estimate of the limits within which the true value of a measurand is expected to lie, to a specified level of confidence.
A measurand is any parameter that is measured.

Question 11

Individual uncertainties are obtained from:

Answer 11

Standard deviations from repeat analyses.
Data from manufacturers of reference materials, glassware, instrumentation, etc.