[RASMB] Lamp Diagnostics

[John Philo]

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