Particle characterisation
Disclosed herein is a method of characterizing particles in a sample. The method comprises illuminating the sample in a sample cell with a light beam, so as to produce scattered light by the interaction of the light beam with the sample; obtaining a time series of measurements of the scattered light from a single detector; determining, from the time series of measurements from the single detector, which measurements were taken at times when a large particle was contributing to the scattered light; determining a particle size distribution, including correcting for light scattered by the large particle.
1. A method of characterising particles in a sample, comprising:
illuminating the sample in a sample cell with a light beam, so as to produce scattered light by the interaction of the light beam with the sample;
obtaining a time series of measurements of the scattered light;
determining, from the time series of measurements, which measurements were taken at times when a large particle was contributing to the scattered light, wherein determining which measurements were taken at times when a large particle was contributing to the scattered light comprises: dividing the time series up into a plurality of shorter sub-runs, performing a correlation on each sub-run, and then determining which of the sub-runs comprise measurements with a scattering contribution from a large particle;
determining a particle characteristic from the time series of measurements, including correcting for light scattered by the large particle, comprising excluding, or analysing separately, sub-runs taken at times when the large particle was contributing to the scattered light.
2. The method of claim 1 , wherein determining a particle characteristic from the time series of measurements, including correcting for light scattered by the large particle further comprises: analysing separately each sub-run that has not been excluded as including a contribution of scattered light from a larger particle, and then using an average of the sub-run analyses to determine the particle characteristic.
3. The method of claim 1 , wherein determining a particle characteristic comprises performing a dynamic light scattering correlation operation on the time series of measurements.
4. The method of claim 1 , wherein determining which measurements were taken at times when a large particle was contributing to the scattered light comprises detecting and/or removing a low frequency variation of less than 10 Hz in the time series of measurements, the low frequency variation corresponding with a large particle.
5. The method of claim 1 , wherein determining which of the sub-runs comprise measurements with a scattering contribution from a large particle comprises evaluating a parameter for each sub-run, and comparing the parameter with a threshold value.
6. The method of claim 5 , wherein the parameter is selected from: intensity, polydispersity index, and Z average.
7. The method of claim 5 , wherein the threshold value is derived from a distribution of the parameter values calculated from each sub-run.
8. The method of claim 7 , wherein the threshold value is derived from an average value of the parameter and a standard deviation of the parameter.
9. The method of claim 1 , wherein determining which of the sub-runs comprise measurements with a scattering contribution from a large particle comprises:
determining a particle characteristic for each sub-run by dynamic light scattering;
wherein the processing of the time series of measurements so as to correct for light scattered by the large particle comprises correcting for light scattered by the large particle only in the sub-runs in which a large particle was identified by the correlation.
10. The method of claim 1 , wherein each sub-run has a duration of: 5 seconds or less; 2 seconds or less; or 1 second or less; 0.5 seconds or less; or 0.1 seconds or less.
11. The method of claim 1 , wherein the large particle has a diameter greater than 100 nm.
12. The method of claim 1 , wherein a criteria for identifying a scattering contribution from a large particle is based on a parameter determined from the time series of measurements of scattered light.
13. The method of claim 1 , wherein a criteria for identifying a scattering contribution from a large particle is based on a parameter input by a user.
14. The method of claim 1 , wherein the particle characteristic is at least one of a particle size distribution, an electrophoretic mobility, a magnetophoretic mobility, a molecular weight, an interaction parameter and a microrheological property.
15. An apparatus for characterising particles, comprising: a light source, a sample cell, a detector and a processor; wherein
the light source is operable to illuminate a sample within the sample cell with a light beam so as to produce scattered light by interactions of the light beam with the sample;
the detector is configured to detect the scattered light and produce a time series of measurements;
the processor is configured to:
receive the time series of measurements, and
determine, from the time series of measurements, which measurements were taken at times when a large particle was contributing to the scattered light, wherein determining which measurements were taken at times when a large particle was contributing to the scattered light comprises: dividing the time series up into a plurality of shorter sub-runs, performing a correlation on each sub-run, and then determining which of the sub-runs comprise measurements with a scattering contribution from a large particle;
determine a characteristic from the time series of measurements, including correcting for light scattered by the large particle, comprising:
excluding, or analysing separately, sub-runs taken at times when the large particle was contributing to the scattered light, and
analysing separately each sub-run that has not been excluded as including a contribution of scattered light from a larger particle, and then using an average of the sub-run analyses to determine the particle characteristic.
16. The apparatus of claim 15 , wherein the detector comprises a photon counting detector.
17. The apparatus of claim 15 , wherein the detector is configured to detect backscattered light.
18. The apparatus of claim 15 , further comprising an optical fibre that provides an optical path between the detector and a scattering volume that is illuminated by the light source.
19. The apparatus of claim 15 , comprising a plurality of detectors configured to detect the scattered light.
20. The apparatus of claim 15 , wherein the particle characteristic is at least one of a particle size distribution, an electrophoretic mobility, a magnetophoretic mobility, a molecular weight, an interaction parameter, and a microrheological property.