Beckman Coulter LS 13 320, User Manual

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PN B05577AC 2-12
Installation
Making Measurements
Making Measurements
In order to obtain the light intensity patterns (diffraction or PIDS) described in previous chapters,
from which the size distribution is calculated, a number of functions have to be performed by the
instrument. These are explained below in the order they are completed during a complete run cycle.
Measuring Offsets
With the laser off, the offset or bias voltages of the amplifier circuit are measured in order to 'zero
out' (or establish a baseline of any electrical noise) at a detector channel. This is necessary because
the signal at a detector channel (a detector channel consists of a light sensitive detector surface and
the associated amplifying electronics) does not necessarily go to zero in the absence of light.
Therefore, to accurately measure the light intensity the offset voltage must be measured and
subtracted from the scattering signal from the sample.
Alignment
During this step the laser beam is automatically aligned at the center of the detector array. In order
to accurately measure the light intensity as a function of scattering angle, the laser beam must be
precisely positioned. In the LS 13 320, the laser beam is aligned within 1-2 microns of the center of
the detector array on the horizontal and vertical axes.
Measure Background
Here the light intensity levels received by the detectors are measured with no sample in the system,
and then subtracted out. This procedure eliminates the signal due to the outer edges of the Gaussian
laser beam, in addition to any light leakage or scattering from dust on the lenses or any other
particulate present in the optical path. The background should always be measured prior to adding
sample to the system.
Measure Loading
This function measures the amount of light scattered out of the beam by the particles so as to
determine an appropriate concentration of sample. Enough sample is needed to provide an
acceptable signal-to-noise level in the detector channels, but if too many particles are present, light
already scattered from one particle will likely be scattered from another, blurring the light intensity
pattern. The obscuration reported on the title bar of the run window, or on the status bar, is the
percentage of light scattered out of the beam by the particles. When sizing particles without using
PIDS an obscuration level of 8% to 12% is appropriate, except when using the Dry Powder Module in
which case an obscuration of 4% to 7% is sufficient. When PIDS is used, a PIDS obscuration of 40% to
60% is recommended.
IMPORTANT
When analyzing materials with wide size distributions requiring the use of PIDS and diffraction
to detect small amounts of large particles. It is recommended that diffraction obscuration is increased
regardless of whether the PIDS obscuration is above the maximum recommended value.