Instant Notes: Analytical Chemistry

Section B – Assessment of data

B1 ERRORS IN ANALYTICAL

MEASUREMENTS

The causes of measurement errors are numerous and their magnitudes are vari-

able. This leads to uncertaintiesin reported results. However, measurement

errors can be minimized and some types eliminated altogether by careful exper-

imental design and control. Their effects can be assessed by the application of

statistical methodsof data analysis and chemometrics (Topic B5). Gross errors

may arise from faulty equipment or bad laboratory practice; proper equipment

maintenance and appropriate training and supervision of personnel should

eliminate these.

Nevertheless, whether it is reading a burette or thermometer, weighing a

sample or timing events, or monitoring an electrical signal or liquid flow, there

will always be inherent variations in the measured parameter if readings are

repeated a number of times under the same conditions. In addition, errors may

go undetected if the trueor acceptedvalue is not known for comparison

purposes.

Errors must be controlled and assessed so that valid analytical measurements

can be made and reported. The reliabilityof such data must be demonstrated so

that an end-user can have an acceptable degree of confidencein the results of

an analysis.

Measurement
errors

Key Notes

All measurement processes are subject to measurement errors that affect

numerical data and which arise from a variety of sources.

An absolute error is the numerical difference between a measured value

and a true or accepted value. A relative error is the absolute error divided

by the true or accepted value.

Also known as systematic errors, or bias, these generally arise from

determinate or identifiable sources causing measured values to differ

from a true or accepted value.

Also known as random errors, these arise from a variety of uncontrolled

sources and cause small random variations in a measured quantity when

the measurement is repeated a number of times.

Where several different measurements are combined to compute an

overall analytical result, the errors associated with each individual

measurement contribute to a total or accumulated error.

Related topic Assessment of accuracy and precision (B2)

Measurement errors

Absolute and

relative errors

Determinate errors

Accumulated errors

Indeterminate errors