For Malvern Mastersizer Systems and Sample Dispersion Units
Malvern's Quality Audit Standards have been produced to provide users of Mastersizer particle size analysers with a reliable one-shot polydisperse transfer standard to enable them to check the performance of their particle sizers and sample dispersion units.
3 Steps to checking uniformity of performance
The importance of checking sample dispersion units
However good the optical bench of a particle size analyser may be, a successful
measurement will ultimately depend on the good performance of the sample dispersion unit.
This unit must evenly disperse the particles in a sample, keep them in a homogenous
dispersion without agglomeration and pass them through the laser beam of the sizer without
bias or distortion.
Differences in the performance of sample dispersion units are therefore a
critical factor in determining the overall sizing accuracy of a sizer system.
Unlike the optical bench of the Mastersizer, which has few moving parts, sample dispersion
units are electro-mechanical units with a number of moving components which, like all mechanical
devices may be subject to wear.
In addition, operating environments are subject to world-wide variations from cool, clinically clean
and closely controlled to extremely humid, dirty and inhospitable.
It is this very variability which makes it essential to find a way of monitoring changes of state
between units, particularly where intersite agreement is important.
Choice of Quality Standard material – ISO13320 recommendations
Since its introduction in 1999, ISO13320 has become an indispensable guide to the theory and use
of laser diffraction-based (Low Angle Laser Light Scattering - LALLS) particle size analyzers.
It is a guidance document that clearly describes the principles of LALLS, defines terminology and
explains what to expect from an instrument of this type.
Sample handling and dispersion, guidance on the use of dry powder feeders, information on error
diagnosis and a discussion of Mie theory versus the Fraunhofer approximation are all included
together with advice on sizing standards.
A successful Quality Audit Standard material should have the following characteristics:
* At current usage rates
Using Quality Audit Standards to check the performance of your Mastersizer 2000 system…
Yearly in combination with NIST traceable latex samples
Stage 1) Check the optical bench using NIST-traceable latex samples (The bench is now within ± 2% of all other validated benches)
Stage 2) Check performance of the sample dispersion units using Quality Audit Standards. The complete system is now within 2-3% of all other validated systems
Daily/Weekly
1 single simple internal procedure:
Measure a Quality Audit Standard on your sample dispersion unit and check the results against the published pass/fail criteria. If it passes, both the bench and the sample dispersion unit are within specification.
How the pass/fail criteria are set for Malvern Quality Audit Standards.
Bottle-to-bottle uniformity is established
(If a standard was to vary by 5% from sample to sample, it would have insufficient precision for our requirements)
The relative standard deviation (RSD) between the dv50 measurements obtained here was just 0.22%. This means that a deviation from the expected measurement result obtained from a bottle can be attributed either to a variation in the measurement technique employed or to a genuine variation in the performance of the sample dispersion unit. So the next stage is:
Establishing and testing robust measurement protocols.
Once the bottle-to-bottle uniformity of the standards is known, variations in results could be attributed to variations in measurement procedures.
Once robust procedures have been established, thoroughly tested and specified, the margin for error has been reduced to a minimum and variations in results can be reliably attributed to variations in system performance.
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