[RASMB] Photomultiplier tube disturbances?

[Borries Demeler]

> 
>   Hi Troy-
> I think John has the problem identified. I wish AOS could change the 
> gains, but they are set by the XLA, and inaccessible the external computer.
> Best wishes,
> Tom

Tom, I have to disagree here.

The intensity changed significantly around scan 25. This caused the jump,
not the DNA which only has about 0.35 OD (0.2 for the DNA and 0.15 for the 
baseline OD). This is not enough to cause a gain reset - as long as the intensity
is constant, which it clearly is not. So that is the problem here. Even running
against water wouldn't help in that case.

Ours is much more constant and tolerates concentrations up to 0.5 OD without
any rescaling issues.

On two other instruments I have seen the same problem and in each case 
Beckman fixed it satisfactorily by replacing a bad component.

If you look carefully at this data you see that it is NOT a rescaling issue, but
slides 3-4, and 6 show clearly that this is just a baseline jumping around,
not a rescale. Rescaling is what you would see if the reference had too much
absorbance and the gain resets. I have attached an image that shows that for
comparison (look at scans 1-4 and compare them to the rest).

So that's not what's going on here. Experimental design is fine, instrument is not.

-b.

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