Laser diffraction is typically used to measure highly diluted samples in order to avoid errors from multiple scattering which occurs when probe radiation passing through the measurement zone experiences more than one scattering event prior to reaching the detectors. The operator can usually control the sample concentration in order to perform analyses in an optimum scattering range as defined by the percent of the original laser intensity reaching the detectors – or %T in the LA series software. A 90 %T indicates that 10% of the original laser intensity has been obscured by the presence of the sample.
The optimum %T depends on factors including the size and scattering strength of the particles, but is typically between approximately 98-75 %T. For larger particles the %T of the red light source is used to determine optimum concentration while the %T of the blue light source may be more useful for submicron samples. The LA-960 particle size analyzer software includes a unique adjustment to the scattering algorithm in order to reduce the impact of multiple scattering on the calculated results.
The Whitehouse PS-202 polydisperse particle size standard was tested on the LA-960 laser diffraction particle size analyzer at concentrations varying from 85-16% T. The certified results, pass/fail criteria, and LA-960 results are shown below.
Note that the D50 result remains within specification down to 27.7 %T while the D10 falls out of specification at 58.1 %T. This exemplifies how multiple scattering effects smaller particles more dramatically. The second peak below 1 micron is an artifact of multiple scattering.
The graph below shows TiO2 results as measured wet on the LA-960 at concentrations ranging from 88-83 %T.
Note how consistent the results remain across the range of concentrations. This lack of concentration dependence facilitates daily operation since the method can allow a wide concentration window without changing reported results.
Instruments without this multiple scattering correction can be more difficult to operate since slight changes in concentration can create large changes in results. The graph below was generated by a customer investigating the effect of concentration on their sample measured on the LA-960 vs. a competitive instrument.
Notice how dramatic the effect of concentration is on the competitive system as compared to the LA-960.
The LA-960 includes a Chi square error calculation that increases when there are fluctuations of light intensity on the detectors during the measurement. The Chi square value can be used to help choose the optimum concentration as show in the graph below. When the concentration is too low (high %T) the signal/noise ratio is high – creating a higher Chi square calculation. Once the %T falls below about 95 the Chi square value stabilizes, as does the reported D50 shown on the right Y axis.
The Method Expert software tool contains a wizard that guides the user through a structured approach to determine the optimum concentration for an unknown sample. The screen shots below show how the concentration wizard is used to design the required experiments, perform the measurements at varying concentration, and select the optimum range.
Note this was an extremely difficult emulsion sample where results tend to be more concentration dependent than other samples possibly due to dilution effects. But the Method Expert wizard makes the correct concentration choice quite easy, even for the new user of laser diffraction.