SECTION 5
SOLVING PROBLEMS
5.3 External Factors Affecting Precision
Problems with instrument precision have a number of possible
sources. Often, poor reproducibility is caused by external factors
that are entirely independent of the instrument itself. The following
are some of these factors:
•
Incorrect Use of Calibration Standards.
Instrument accuracy, and linearity, depend upon the correct use
of osmolality calibration standards. Refer to Section 2.5, 2.6 and
Appendix E for further information.
•
Sampling Error
Sampling error tends to be amplified when dealing with
specimens of 10 microliters or less. You can prevent errors by
using consistent technique and appropriate methods of transfer-
ring samples. See Section 3.2 for details.
•
Micropipettor-Caused Errors
Unlike the maintenance-free micropipettor supplied by Wescor,
many micropipetting devices require routine maintenance.
Without proper maintenance, micropipetting devices can
exhibit significant volumetric error (in excess of 50%)
and cause corresponding variations in indicated osmolality.
Positive displacement micropipettors are not recommended
as an alternative to the Wescor micropipettor, except when
dealing with samples of very high viscosity.
•
Poor Precision
1
Determine whether the problem is with the instrument or is
caused by external factors, such as the micropipettor.
2
Check the location of the osmometer for possible sources of
thermal disturbance, as outlined in Section 2.3.
Use AUTO REPEAT to evaluate the precision of the osmometer.
Run the instrument with 1000 mmol/kg standard in AUTO REPEAT
to determine if the instrument can repeat well. If it does, consider
the possibility of loading errors causing poor repeatability.
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