Reproducibility of results, both from run-to-run on a single instrument, and from instrument-to-instrument, is clearly one of the most desirable features an instrument line can offer. For those who have a need to compare results from different company sites, and results from a new instrument, good reproducibility is important by eliminating instrument variability as a possible source of error.
In the field of laser diffraction particle sizing, most instruments cannot provide an instrument to instrument coefficient of variation for mean size of much better than 2-4% under ideal conditions. Much of this is due in part to the inherent asymmetric nature of a laser beam, lack of sensitivity to beam misalignment, and alignment drift. Beckman Coulter Particle Characterization was the first to address this issue with the introduction of auto-laser alignment.
Automated Alignment ensures that the beam is always aligned. The LSô Series instruments perform an alignment routine automatically if two hours or more have elapsed since the last sample analysis. The reticle is automatically moved into beam; the alignment routine calculates the displacement of the beam and makes adjustments accordingly. (Below are examples of Flux vs. Angle diagrams)
On one of these two runs, made with a generic laser diffraction instrument, the laser beam is misaligned. Which one is it?
Because the detectors on all conventional laser diffraction particle size analyzers (except the Beckman Coulter LS Series) are placed above the center diffraction pattern, itís impossible to tell when your instrument is vertically misaligned.
Beckman Coulter Instrument
The Beckman Coulter LS Series has offered the Auto-Alignment feature since 1989. In the past, competitors (Malvern & Horiba) have always stated that we auto-align because of constant laser drift. Just recently Malvern began offering this feature on their NEW (Malvern 2000), and Horiba on their NEW (Horiba LA920) Particle Size Analyzers. So once again we can say "Follow the Leader".
When measured on Beckman Coulterís exclusive, patented, X-shaped detector array, (detectors above and below the center of the Aires disk), a diffraction pattern appears in a distinctive step format when the laser beam is misaligned (left). Compare this pattern to that produced by a correctly aligned instrument (right). Automatic realignment minimizes the danger of alignment drift.