Diagnosable remotely - YES
Fixable remotely - YES
This is almost always diagnosable remotely, either by taking a description from the customer of the fault, or by logging in remotely to observe with PS100 in the cell using AnyDesk or TeamViewer.
VERTICLE DRIFT
Particles drifting vertically, either upwards or downwards is an issue usually caused either by temperature or by instability of the particles.
Upward drift - the most common cause of this is heat from the laser, or a warm environment around the instrument.
SOLUTION 1 - check that the laser timing is correct. From the main ZetaView software windows, click on the small camera icon and select the "laser timing" tab. The laser should be set to "Camera Frequency, Laser Pulse Duration = Shutter Duration" and NOT to "Continuous". Setting the laser timing to continuous will cause excessive heat inside the cell and result in significant upward drift.
SOLUTION 2 - use the temperature control in the ZetaView software and set to around 21-22 degrees. NOTE (the cell can only cool to around 5 degs below ambient!) Allow the temperature of the cell to stabilise before continuing. Use the "Check Particle Drift" feature in the ZetaView software - as long as the line is green, the drift is not an issue. Note - a small amount of upward drift is normal.
Downward drift - much more uncommon. The most likely cause is instability of particles in the buffer causing them to quickly agglomerate and "drop out". In rare cases, it could be a problem with the measurement cell.
SOLUTION - there isn't much that can be done in this case, as the problem is with the sample. Measuring quickly - short videos, etc, would be recommended, but if the drift causes the line to go red in the "Check Particle Drift" function, the measurements will not be accurate. If the issue is not instability of particles, there could be a problem with the measurement cell. Seek advice from PMX, but the only solution here would be to send the cell back to PMX for repair/replacement. This has happened on one instrument at Imperial College before, instrument 19-419.
HORIZONTAL DRIFT
Horizontal drift is fairly common and usually easily fixable. Some common causes:
System not primed - a common cause is that the fluidic system has not been rinsed/primed with the correct buffer/medium before measurement of a sample. For example, the system is always booted up using water. Before EVs in PBS can be measured, the system should be flushed with PBS first. If it is not, the PBS and water will create turbulence in the measurement cell, carrying the particles around.
SOLUTION - rinse the system thoroughly with the suitable buffer/liquid before injecting the samples. Eg, if measuring EVs in PBS, rinse the system with PBS first.
Not enough sample injected - if the user does not inject enough of their sample into the measurement cell, the particle can start to diffuse out of the field of view. This "edge" of the actual sample should be well clear of the measurement cell to avoid this issue.
SOLUTION - inject at least 1ml of sample into the measurement cell.
Air bubbles - the most common cause of horizontal drift. Air bubbles are usually observed when the user has created an air lock when injecting their sample. Eg, not ensuring the injection port is filled with liquid before inserting the syringe. If an air bubble is present, liquid can move to fill the "gap" carrying the particles with it.
SOLUTION - flush the system with water/buffer, using both a syringe from the front and the internal pumps (if present). If the air bubble still is not removed, use an empty syringe to inject air into the cell, then refill with water/buffer and re-inject the sample.
Failing check valves - if the system is primed, with plenty of sample injected, without any air bubbles and horizontal drift is still observed, the most likely explanation is that one of the purple check valves have failed. This is reasonably common - the check valves are designed to "break" relatively easily, so if there is a build up of pressure, the expensive and critical measurement cell is not damaged.
SOLUTION - replace the purple check valves. This is very easy and customers can do it themselves. The check valves just screw in. Instruments are shipped with several of these valves spare, so there should always be some on site.