SCYM Cell Sorting 3 — Questions and Answers
Question 1: Which parameter is most directly controlled by changing the nozzle drive frequency on a cell sorter?
- Laser power at the interrogation point
- Droplet formation rate and size (Correct answer)
- Sheath fluid pressure
- PMT voltage sensitivity
Correct answer: Droplet formation rate and size
The nozzle drive frequency determines how rapidly the jet breaks up into droplets, directly controlling droplet size and formation rate.
Question 2: What is the 'breakoff point' in a droplet-based cell sorter?
- The location where the laser beam intersects the sample stream
- The point at which the continuous liquid jet separates into individual droplets (Correct answer)
- The voltage threshold that triggers the sort decision
- The point at which sorted cells impact the collection vessel
Correct answer: The point at which the continuous liquid jet separates into individual droplets
The breakoff point is where the continuous liquid stream transitions into discrete droplets, and its stability is critical for accurate and consistent sorting.
Question 3: If a cell sorter's breakoff point is unstable, what is the most likely consequence?
- Increased fluorescence background from the sheath fluid
- Inaccurate delay timing, leading to poor sort purity and yield (Correct answer)
- Loss of forward scatter signal from small cells
- Increased laser reflection into the PMTs
Correct answer: Inaccurate delay timing, leading to poor sort purity and yield
An unstable breakoff point means the actual droplet containing the target cell may not align with the calculated sort delay, causing the wrong droplet to be charged and deflected.
Question 4: What role does the 'sort delay' (or 'drop delay') calibration play in electrostatic sorting?
- It sets the time the laser fires relative to the cell's entry into the flow cell
- It defines the time between laser interrogation of a cell and the moment that cell reaches the droplet breakoff point (Correct answer)
- It controls the duration of the electrical charge applied to deflection plates
- It regulates the pressure differential between sample and sheath
Correct answer: It defines the time between laser interrogation of a cell and the moment that cell reaches the droplet breakoff point
Sort delay is the number of drops between laser interrogation and droplet breakoff; correct calibration ensures the right droplet is charged at the precise moment it separates from the jet.
Question 5: Which type of collection vessel is preferred when sorted cells must remain viable for downstream culture?
- Polystyrene test tube coated with ethanol
- Conical tube pre-loaded with culture medium or collection buffer (Correct answer)
- Dry glass capillary tube
- Vacuum-sealed microcentrifuge tube
Correct answer: Conical tube pre-loaded with culture medium or collection buffer
Pre-loading collection tubes with culture medium or protein-containing buffer cushions the impact of sorted cells and maintains viability for downstream applications.
Question 6: During a multicolor sort, spectral spillover into the sort trigger channel can cause which problem?
- False triggering events that inflate the event count and reduce effective sort purity (Correct answer)
- Loss of scatter signals used for cell sizing
- PMT overload leading to instrument shutdown
- Incorrect sheath fluid pH
Correct answer: False triggering events that inflate the event count and reduce effective sort purity
Spillover into the trigger channel can generate spurious trigger pulses from non-target events, causing the sorter to attempt to sort events that are not cells or are not targets.
Question 7: A technician notices that sorted cell purity is consistently lower than the gated population purity. What should be checked first?
- Laser power stability
- Sort delay calibration accuracy (Correct answer)
- PMT gain settings
- Sheath fluid temperature
Correct answer: Sort delay calibration accuracy
If sort purity is lower than gate purity, the most common cause is an incorrect sort delay that causes the wrong droplet to be deflected.
SCYM Cell Sorting 3
Which parameter is most directly controlled by changing the nozzle drive frequency on a cell sorter?