threshold on an Aria

Marty Bigos mbigos at
Wed Jan 9 12:37:19 EST 2008


I duplicated your described setup on our LSR-II:
		SSC in log
		Threshold on SSC
		Threshold set at 1000

I varied the PMT voltage and was able to sweep the bead peak  
(displayed on SSC area) from 1000 to the top of the scale. It did not  
disappear anywhere on-scale.

Based on this, my conclusion is that either your instrument is bad or  
something is wacky in how you have the Aria set up (although I cannot  
think of anything that would result in your described behavior). You  
can check the instrument by thresholding on another parameter (FITC  
for example) and repeating your test. If you don't see the same  
behavior, then this suggests you have a bad digital card.

Marty Bigos, Director
Gladstone Flow Core

J. David Gladstone Institutes
1650 Owens Street
San Frnacisco CA 94158

(office) 415-734-4821
(mobile) 415-845-8450


On Jan 7, 2008, at 12:25 PM, markG at wrote:

> I’m having a problem with event triggering on my Aria. When I  
> choose a threshold
> parameter and value, a dead spot appears in that parameter at  
> approximately 10X
> the threshold value.
> E.g. I set the threshold as side scatter with a value of 1000 in  
> log mode. I
> load some beads and see a tight population on a side scatter	
> histogram. If I
> adjust the side scatter PMT voltage, the population moves around
> correspondingly, and of course disappears when it drops below the  
> threshold at
> 1000. In a narrow range near 10,000, however, it also disappears,  
> only to
> reappear once the voltage is moved either up or down. Oddly, when  
> the beads
> fall in this dead spot, the event rate doesn’t change, although no  
> other PMTs
> detect anything.
> I’ve talked to a couple different people at BD and they keep	
> telling me that
> this is normal and has to do with flooding the PMT at a high	
> voltage. I’m
> confused by this because the PMT works when the voltage is  
> increased to move
> the population even higher, just not in that one spot. Also, the  
> dead spot
> moves if the threshold value is changed.  Can someone please	
> explain this
> phenomenon to me better than the BD technical support? Or if this  
> shouldn’t be
> happening, what is likely failing?
> Thanks for any help you can give this first-time poster,
> Mark Gebhard
> Graduate Research Assistant
> University of Texas at Austin

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