[RASMB] Lamp Diagnostics

Fri Apr 4 08:57:17 PDT 2008

It is worth noting that Beckman has a service manual for the XLA XLI that discusses all of these features and tests and specs.  The spec for linearity is 20% when measured at 280 nm on a windowless cell (or an empty hole) at 3000 rpm using OD values 1 cm apart.  The old manual specified 6.0 and 7.0 cm.  With the old lamp rotation of the lamp strongly influenced the intensity across the cell; the newer lamp holder is now symmetric and do not vary as much with rotation, in my experience.  The old part # for this manual is 679045 but my copy is dated 1/92.  The newest version I am aware of is called Proteome Lab XLA & XLI Service Manual dated (10/3).  Sorry I do not have a part # - the relevant chapter is section 3.  It describes wavelength calibration and accuracy checks, Intensity checks, cleaning the optics, radial position calibration, etc, etc.  Obviously depends upon the software you have as well.   In my opinion it is well worth buying or getting a copy for your service guy - in the long run it saves both of you time.  Why this isn't standard operation procedure with Beckman is a very good question.

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Dr. John J. "Jack" Correia
Department of Biochemistry
University of Mississippi Medical Center
2500 North State Street
Jackson, MS  39216
(601) 984-1522                                 
fax (601) 984-1501                             
email address: jcorreia at biochem.umsmed.edu     
homepage location: http://biochemistry.umc.edu/correia.html 
dept homepage location:    http://biochemistry.umc.edu/ 
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>>> On 4/4/2008 at 10:18 AM, in message <C7B1B01A69FC4164B0213B9168C3CBAA at 79B7XD1>, "John Philo" <jphilo at mailway.com> wrote:
Mitra,

To my knowledge there is no specification for intensity variation across the
cell, but in my experience few instruments would vary as little as 10%.
Probably 20-30% is more typical. It is a common mistake to think that the
absolute intensities have a strong effect on the signal/noise. It is
doubtful that you would notice the effect of even a 50% drop in intensity
(assuming you are starting from a normal level). Further, positioning the
lamp to give the maximum possible intensity will not necessarily give the
best overall performance---quite often the maximum intensity position also
gives high stray light and compromises the linearity. 

John

-----Original Message-----
From: mitrana at mail.utexas.edu [mailto:mitrana at mail.utexas.edu] 
Sent: Thursday, April 03, 2008 6:00 PM
To: jphilo at mailway.com 
Cc: 'RASMB'
Subject: RE: [RASMB] Lamp Diagnostics

John
I feel better already.

What I meant in 1. is that I take a wavelength scan at 5.9, 6.5, and 7.1 cm
and compare the 230 nm peaks. I guess a radial scan is definitely a better
way to check intensity variability across a cell. How much variability is
considered acceptable - somehow 10% seems stuck in my head.

Mitra

Quoting John Philo <jphilo at mailway.com>:

> Mitra,
>
> Points 2 and 3 are normal. Every time the monochromator moves there is 
> an uncertainty of +/- 1-2 nm (I believe the official specification is 
> actually
> +/- 4 nm).
>
> Also for wavelength scans the monochromator doesn't actually take data 
> at the exact wavelength interval you ask for. So sometimes it will 
> essentially step over the sharp peaks (if you double-click on the line 
> graph and ask it to mark the data points with symbols you can more 
> easily see there are no points where the peak should be).
>
> I'm not sure I understand your point 1, but I think the intensity 
> variability may just be a consequence of the wavelength variability 
> discussed above. Normally one would try to adjust the lamp alignment 
> using radial scans, not wavelength scans, and typically the wavelength 
> intensity scans to check the strength of the 230 nm peak or wavelength 
> calibration are run at 6.5 cm.
>
> John
>
> -----Original Message-----
> From: rasmb-bounces at rasmb.bbri.org 
> [mailto:rasmb-bounces at rasmb.bbri.org] On Behalf Of 
> mitrana at mail.utexas.edu 
> Sent: Thursday, April 03, 2008 2:58 PM
> To: 'RASMB'
> Subject: [RASMB] Lamp Diagnostics
>
> Hello Spinners
> Our AUC started giving some problems recently. Did a wavelength scan 
> and here's a summary of the results -
>
> 1. Lamp intensity varies at the 5.9 cm position although at the 'sweet
spot'
> obtained from the lamp alignment this variation is about 9-10%.
>
> 2. Sometimes it seems the peaks are cut-off and I get a plateau 
> instead of a peak with of course a lower intensity count.
>
> 3. The peak oscillates 1-2 nm (227-230 nm and 526-528 nm) on 
> successive scans at a single radial position.
>
> It seems like a monochromator problem to me but I'd like to hear some 
> comments from experts here. I am particularly worried about the peaks 
> getting cut-off.
> Our facility did not renew the service contract this year (I know, 
> extremely bad idea but too many size exclusion column huggers around) 
> and I'd rather know the problem instead of being told by the Beckman 
> Service person that it's within specs while charging $290.00 per hour.
>
> Mitra
>
> --
> Mitra S. Rana
> Graduate Student
> Institute for Cellular and Molecular Biology 2500 Speedway, UT-Austin 
> Austin, TX-78712 _______________________________________________
> RASMB mailing list
> RASMB at rasmb.bbri.org 
> http://rasmb.bbri.org/mailman/listinfo/rasmb 
>

--
Mitra S. Rana
Graduate Student
Institute for Cellular and Molecular Biology 2500 Speedway, UT-Austin
Austin, TX-78712

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