Beckman Coulter LS 13 320, User Manual

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PN B05577AC C-1
APPENDIX C
Optical Models
Introduction
The Fraunhofer optical model preprogrammed in your system is a general optical model that can be
used for any sample (and suspension fluid) on the LS 13 320.
The LS 13 320 also lets you make optical models using the complete Mie theory of the scattering of
light by spherical particles. The Mie theory takes into account the complex refractive indices of both
the particles and the suspension fluid. For particles larger than 100 microns in diameter, the
Fraunhofer model will produce results almost identical to those obtained with an optical model
generated for the material and fluid. Between 10 and 100 microns, the difference will be observable
but slight, and is generally considered negligible. If a sample contains a significant fraction of
material below 10 microns, however, the correct optical model will produce significantly more
accurate size distributions than the Fraunhofer model.
Please note that for an optical model to be “correct”, the model must match the sample. Since the
Mie theory assumes spherical particles with a homogeneous refractive index, if a material deviates
significantly from these conditions (for example needles or rods, or birefringent materials), the
resulting size distribution may not be accurate.
Refractive Index
The complex refractive index consists of a real part and an imaginary part. The real part is what is
generally thought of as the refractive index; the imaginary part represents the absorption
coefficient of the material. The real part of the refractive index of most samples and suspension
fluids are listed in reference books such as the Merck Index or the CRC Handbook of Chemistry and
Physics. This number must be known to within 0.01 to 0.03 units. The real part of the refractive
index for most samples ranges from 1 to 3; the real part for most suspension fluids ranges from
1 to 2.
The imaginary or absorptive part of the refractive index for a sample is hard to find in any reference.
Use the guidelines below to enter the imaginary part of the refractive index. This does not affect the
results substantially unless the value of this parameter changes by a factor of greater than
approximately 3.
Guidelines to use as an estimation of the imaginary part of the refractive index:
•
White or transparent powders - 0 to 0.1
•
Clear materials, glass, clear polymers - <0.001
•
Latex, translucent materials, quartz, polymer resins, crystallization processes - <0.01
•
Lightly colored translucent material - 0.01 to 0.1
•
Gray or lightly pigmented materials, metal oxides, highly-colored materials - 0.1 to 1