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Greetings Vincent.
We run a Coulter ELITE here and I have had similar problems.
As to the cause of this instability, my bet is sheath temperature (see (3)
Now I'm sure you do all this stuff but it might be worth setting
out the following tips for others new to the wonderful world of sorting
1) Set up for sorting using fluorescent beads to get a 'ballpark'
value and *confirm this with the sample to be sorted.*
2) Periodic monitoring of the sort recovery by spitting 20 or so
sorted particles onto a slide and counting them by eye -I do this every
10-15 minutes or so and find that we can maintain good sorts over the time
period of which you speak
3) Maintain constant sheath temperature -on the ELITE I leave the
door to the sheath/waste bottles open - even in a temperature controlled
environment there may be local temperature changes around the sheath
container (on the ELITE it is proximal to the compressor assembly)
4) Minimise sample debris/aggregates -over long periods I suspect
that the region around the sort nozzle becomes contaminated with 'gunge'
5) (On the ELITE) - sort at reduced sheath pressure - at 9-10 p.s.i
the whole system seems to be more stable, with better recovery and purity
(I use 9 p.s.i)
6) Strive to reduce 'spray' -by this I refer to that fine mist that
is sometimes seen around the main sort streams, I believe that this
contributes to deflection plate contamination with concomitant irregularity
in the sort trajectories as well as contamination of the sort tip -this is
a tricky one; some days it's there and some days it's not, but the main
culprits seem to be:
i) dirty/damaged sample insertion rod (check the tip of this under a
stereo mike if you have one).
ii) high crystal drive (in any event, if you find that you need
more than about 40% drive on the ELITE with the standard tip ,
the crystal/flow tip assembly has probably been contaminated with
sheath fluid at some point).
iii) loose fitting sort electrode clip -now, I know that at several
thousand volts the resistance of this connection should not
make much difference, but in practice it does seem to. -I give the
electrode lead a turn before clipping it onto the sample insertion
rod -this torque seems to help stability!
I hope this is some help.
Arnold.
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