4. Pipetting System
Photometers and photometric test kits possess spe-
cifications that are adhered to and above all else also
documented by the manufacturer.
The
certificate for the photometer
enclosed with
each device documents the quality of the measuring
device.
5. Analytical Quality Assurance (AQA)
5.1 Quality Control at the Manufacturer
The objective of analysis must always be to determi-
ne the true content of the analyte in question as
accurately and precisely as possible.
Analytical Quality Assurance represents a suitable
and indispensible method by which the quality of the
user's own work can be assessed, errors in the mea-
surement system diagnosed, and the comparability
with the results obtained using the respective refe-
rence methods demonstrated.
Details regarding the necessity of AQA can be found
in the memorandum M 704 of the German “Abwasser-
technische Vereinigung” (ATV, Wastewater-technical
Association) and in the corresponding self-con-
trol/self-monitoring regulations of the German federal
states (available in english).
Causes for errors can include:
●
the working materials used;
●
the handling; and/or
●
the sample under investigation.
These errors have effects on both the accuracy and
precision of the results obtained.
Positive-displacement pipettes permit
●
an exact dosage of the sample volume, and
●
a precise measurement of sample and reagent
volumes and of the volumes of water for dilution
purposes.
Pipettes of varying volumes and also ones with a
fixed volume are available.
Sources of error and hints on how to avoid
them:
●
Closely follow the instructions for use contained
with the pipette in question.
●
Check the pipetted volumes by weighing using
analytical scales (weighing accuracy ±1 mg),
1 ml of water at 20 °C = 1.000 g ±1 mg.
●
Check the pipetted volume using Spectroquant
®
PipeCheck; this is a photometric check of the
pipette, and scales are not necessary (see section
“AQA”).
●
Avoidance of spread effects by rinsing the pipette
several times with the solution to be pipetted.
●
Always exchange the pipette tip.
●
Draw up the liquid slowly and depress piston
completely to discharge the liquid.
XI
Release 07/03
