[RASMB] Lamp intensity fluctuations

Leech, AP apl3 at york.ac.uk
Tue May 4 09:38:33 PDT 2010


Hi Titus,

I notice the "peak" in your intensities is at different scan numbers
(#5 in the one plot, #25 in the other) - do these points correspond to
the same or different elapsed time (e.g. different number of cells in
the rotor making time between scans different)?

Also, you did not say how the PMT voltage varied - does it go up with
the lamp intensity, or down?

Does the same thing happen on scanning an empty hole?

Best regards,

Andrew


Titus M. Franzmann wrote:
> Hi, 
> I should have been a little bit more precise about my sample.
> I loaded a cell with water (dH20) in the reference channel and a sample of
> 0.5 Abs in the sample channel. I recorded intensities instead of OD values.
> Thereafter I plotted the lamp intensities from the reference channel (water)
> as a function of the scan number using Ultrascan. The same graph can be
> produced using Origin, plotting the raw intensities at any given radial
> position, or the average of it, as a function of the scan number. Basically
> the same profile. 
> Titus
> 
> -----Original Message-----
> From: Leech, AP [mailto:apl3 at york.ac.uk] 
> Sent: Tuesday, May 04, 2010 9:52 AM
> To: RASMB
> Cc: Tom.Laue at unh.edu; Titus M. Franzmann
> Subject: Re: [RASMB] Lamp intensity fluctuations
> 
> Hi Tom, Titus, all
> 
> I thought the "incident" readings come from the reference detector:
> these can be seen using the menu Scan -> Request Data -> Incident
> from the GUI, but they are only available from the last scan and
> do not seem to be collected during a run. The incident data should
> be flat across the scan, slightly noisy and slightly higher than the
> maximum reference sector data (at least, that is how they are on our
> XL/I). We once had a problem with the reference detector which resulted
> in this data being a perfect flat line, solved by replacing the
> monochromator assembly.
> 
> Best regards,
> 
> Andrew
> 
> 
> Tom Laue wrote:
>> Hi Titus, et al.-
>> I am not sure what the intensity reading is you are presenting- is it 
>> the average value across the radial scan? Is it the reading at a 
>> particular radial position? In any case, it is worthwhile getting some 
>> idea of the complexity that is behind the XLA intensity readings.
>> There are two intensity readings taken with each lamp flash- 1) 5 or 10% 
>> (can't recall which) of the lamp flash intensity itself is monitored 
>> using light bounced off of a semi-reflecting mirror and a photodiode 
>> sensor. This signal is used to correct for the inherent ~3% variation in 
>> flash intensity characteristic of the lamp. 2) The light intensity at 
>> the photomultiplier (PMT) after it has passed through the cell and 
>> slit/lens assembly. I do not believe the photodiode intensity readings 
>> are accessible to the outside world, so your readings are from the PMT.
> [...]
> 

-- 
Dr Andrew Leech                   *  Laboratory Head
Technology Facility               *  Molecular Interactions Laboratory
Department of Biology (Area 15)   *  Tel   : +44 (0)1904 328723
University of York                *  Fax   : +44 (0)1904 328804
PO Box 373,  York  YO10 5YW       *  Email : apl3 at york.ac.uk



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