[RASMB] XL/I absorbance problem

[Tom Laue]

Hi Andrew-
I agree with the preceding remarks, and would only add that the lamp & 
its power supply may also cause spikes due to triggering differences and 
variable arc quality. If you swap lamps between machines, does the 
problem follow the lamp? If so, you've nailed the problem. To my 
knowledge, it is rare for the power supply to be a problem, but you may 
wish to swap those between machines to see if the problem follows the 
power supply.
Best wishes,
Tom

John Philo wrote:
> Andrew,
>
> I agree with Virgil that the slit assembly should be checked for smooth
> motion and absence of dirt, but the fact that the incident data shows spikes
> too suggests this may be a lamp (or lamp power supply) problem.
>
> Inside the monochromator there is a partially-reflective surface that
> directs about 10% of the light coming from the diffraction grating to the
> "incident" detector. That detector measures the intensity of each flash and
> is used to normalize each flash. Thus the graph of the incident intensity
> tells you how constant your lamp output is from flash to flash. In principle
> lamp variation is completely compensated, but I think in practice big
> variations do add to the noise.
>
> Since you see this when scanning an empty hole this is not the problem in
> your case, but many users are not aware that one thing that can fool the
> incident detector and cause extra noise is shiny surfaces on the cell. Those
> blackened areas on the window holders and the screw rings really do need to
> be black for optimal signal/noise and linearity! Shiny surfaces can
> re-direct light back up into the monochromator and onto the incident
> detector, causing false readings. So take a permanent black marker and fill
> in the chips and scratches on the black anodization.
>
> John
>
> -----Original Message-----
> From: rasmb-bounces at rasmb.bbri.org [mailto:rasmb-bounces at rasmb.bbri.org] On
> Behalf Of Leech, AP
> Sent: Wednesday, June 13, 2007 7:03 AM
> To: rasmb at server1.bbri.org
> Subject: [RASMB] XL/I absorbance problem
>
>
> Hello all,
>
> I'm not rally happy with the noise I'm seeing on my XL/I's absorbance scans,
> and I have a few queries. I'd be grateful for any comments.
>
> 1. Noise seems excessive even on reasonable samples. Scans on an empty hole
> at 280 nm give an overall flat profile (in A280) but peak noise is +/-0.05.
> More peculiar is that the noise seems to be spikes, randomly distributed
> along radius and not in the same place on successive scans. (For comparison
> our old XL/A has peak to peak noise +/- 0.015 under the same conditions)
>
> 2. Using the request data intensity option, the overall profile has a dip at
> r=6.3, but also numerous spikes, mostly upwards of about 600 counts on a
> background of 6000. Cleaning if anything has made this worse. (Cf about 3000
> for the XL/A which does need
> cleaning.)
>
> 3. A wavelength intensity profile looks normal though the maxima are not
> great (16000 at 230 nm after cleaning, 11000 before).
>
> 4. What does the "request data incident" option in the GUI do? For the XL/A
> this produces very flat lines with some spikes upward or downward, of
> constant size; for the XL/A reasonably flat lines with random noise
> superimposed (4000 +/-100). As I don't know what this is measuring, I'm not
> sure what to expect.
>
> I'll try and post some graphs later.
>
> Many thanks,
>
> Andrew
>
>   

-- 
Department of Biochemistry and Molecular Biology
University of New Hampshire
Durham, NH 03824-3544
Phone: 603-862-2459
FAX:   603-862-0031
E-mail: Tom.Laue at unh.edu
www.bitc.unh.edu
www.camis.unh.edu