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Temperature Calibration Methods
Background
Before OptiMlet is put into operation for the first time, it must be calibrated using
appropriate reference substances with melting points that are known exactly. In
addition, the OptiMelt temperature calibration must be checked, and readjusted if
necessary, every six months (minimum) to maintain the instrument within its
factory-certified accuracy specifications.
A fresh calibration of the OptiMelt must be performed every 6 months.
Analytical quality control (QC) Laboratories, are required to test their melting
point instrumentation on a regular basis, against Certified Reference Standards
(CRSs), to determine the acceptability of their instruments according to specific
QC requirements set forth by Local, National and International Standards and
Pharmacopoeia Laboratories.
Temperature Calibration Methodology
Most modern melting point instruments include temperature calibration hardware
and procedures to adjust their temperature readings. The calibration
methodologies applied by commercial instruments can be divided into three main
categories:
Method 1. Using a Calibration Bath
The thermometer is removed from the heating block and
immersed in a liquid bath with a well known temperature (i.e. ice
bath) . The temperature readings are adjusted to match the
temperature of the bath. The thermometer is then reinserted into
the heating block.
Method 2. Using a Reference Thermocouple and Electronic Thermometer.
A calibrated thermocouple or Pt/RTD thermometer is inserted into
one of the sample slots of the heating block. The melting point
aparatus is preheated to a fixed Start temperature. Once the
requested Start temperature becomes stable, the readings from
the calibrated thermometer are used to correct the temperature
display of the instrument.
Method 3. Calibration with a Melting Point Standard
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The melting point(s) of one or more CRSs are measured and then
compared with their certified values. If the two sets of numbers
(i.e. measured vs. expected) deviate from one another beyond the
accuracy of the instrument the temperature scale is readjusted
accordingly.
The aging of Pt/RTD resistor thermometers has been well documented in the
engineering and scientific literatures and has been identified as the leading
cause for loss of accuracy in most modern Melting Point (MP) instrumentation.
Calibration methods relying on the use of reference thermometers or calibration
baths (Methods 1 & 2), though adequate for checking the temperature accuracy
of the block's thermometer by itself, are static measurements that do not take
into consideration the thermal lag between the thermometer and the capillary
samples during the heating process.
Calibrating the temperature readings against CRSs (Method 3) accounts not
only for drifts in the thermometer readings but also for the unique heat
conducting properties of each metal-oven/capillary assembly. As a result,
Method 3 is the ONLY calibration method acceptable for the OptiMelt system.
The calibration procedure listed in the OptiMelt's Operations Manual is the only
methodology accepted by local, national and international pharmacopoeias for
certification and determination of acceptability of melting point instrumentation.
The calibration of the thermometer readings according to Methods 1 & 2 is a
convenient way to check the proper operation of the block's thermometer but
does not lead to a certifiable calibration procedure according to GLP and GMP
practices.
OptiMelt Calibration
Calibration of the OptiMelt is very straightforward: The melting points of three
CRSs are measured and then compared with their certified values. If the two
sets of numbers (measured vs. expected) deviate from one another beyond the
accuracy of the instrument (i.e.