Surface charge measurement
The invention relates to methods and apparatus for determining properties of a surface. Embodiments disclosed include an apparatus for measuring a surface charge of a sample, comprising: a sample holder having an opposed pair of electrodes and configured to hold a sample in position in a measurement volume between the electrodes such that a planar surface of the sample is aligned orthogonal to the electrode surfaces; a measurement chamber for containing a measurement liquid and having an open end configured to receive the sample holder to position the electrodes in a preset orientation; a laser light source positioned and configured to direct a laser beam through the measurement chamber between the electrodes and parallel to the planar surface of the sample when the sample holder is received in the measurement chamber; and a detector positioned and configured to detect scattered light from the measurement volume, wherein the apparatus is configured to allow for detection of the scattered light by the detector over a range of distances from the surface of the sample.
1. A zeta potential measurement apparatus, comprising:
a measurement chamber having an open top that is capable of receiving a dip cell for holding a sample in the measurement chamber;
a laser light source positioned and configured to direct a laser beam through the measurement chamber;
a detector positioned and configured to detect scattered light from the measurement chamber; and
a processor responsive to the detector and operative to calculate a zeta potential of a surface of the sample from a mathematical relationship that relates a plurality of different detection signals from a plurality of different locations relative to the sample surface to the zeta potential of the sample surface in the measurement volume.
2. The apparatus of claim 1 wherein the processor is operative to calculate the zeta potential based on movement of tracer particles detected by the detector.
3. The apparatus of claim 2 wherein the processor is operative to determine tracer particle movement by Doppler analysis of the scattered light.
4. The apparatus of claim 1 wherein the apparatus is responsive to the detector to perform an autocorellating PALS detection.
5. The apparatus of claim 1 wherein the apparatus is further operative to apply an alternating voltage to an opposed pair of electrodes.
6. The apparatus of claim 5 wherein the alternating voltage comprises half cycles of 75 ms or longer in duration.
7. The apparatus of claim 6 wherein the half cycles are between 75 ms and 1200 ms in duration.
8. The apparatus of claim 1 wherein the apparatus is further operative to electrically actuate an actuation mechanism to adjust a sample location relative to the laser beam.
9. The apparatus of claim 1 wherein the laser light source is configured to provide multiple light beams through the measurement chamber.
10. The apparatus of claim 1 wherein the detector is configured to detect scattered light from multiple detection points over the range of distances from the sample surface.
11. The apparatus of claim 1 wherein the processor is operative to receive signals from steps of manually adjusting an accessory to obtain the plurality of detection signals from a plurality of different locations.
12. The apparatus of claim 1 wherein the processor is operative to receive signals from steps of automatically adjusting an accessory to obtain the plurality of detection signals from a plurality of different locations.
13. The apparatus of claim 1 wherein an open-topped removable cuvette defines the measurement chamber.
14. The apparatus of claim 1 wherein the processor is operative perform an extrapolation based on the different detection signals in calculating the zeta potential.