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  6. Beckman DU650 Sudden change in photometric accuracy

Discussion Category:  UV/VIS Spectrophotometers

Beckman DU650 Sudden change in photometric accuracy

I had a problem with wavelength cal on a Beckman DU650. I aligned the internal slit and that problem went away. In checking calibration I found that the photometric accuracy  has become much worse. Prior to my work , checking the %T accuracy with  NIST SRM2031 90% nominal, showed error at 500nm to be about 0.5%T. After my work the error is approx 2%T. I have run wavelength cal and set scan gain numerous times.   Things I did before this showed up; -replaced lamp -aligned lamp -minor smudges cleaned on filter wheel -removed and cleaned sensor -aligned #2 slit -moved grating mirror beam up to center on slit at 550nm (screw adjust on mirror base)   I have run the performance tests, all look good. Absorbance noise;  0.000027 avg SD,   spec <=0.0002A Wavelength Diff:  0 Repeatability diff;  0 Resolution  1.7390nm  (spec<=1.8nm) Baseline flatness, RMS 0.0004   (spec <=0.001A) Stability at 340nm , 60 min. delta abs -0.00143   spec <0.003A   I tried undoing most of what I did, including; -reinstalling the old halogen lamp -moving the grating mirror beam back to half down off the #2 slit -several setscan gain -several power on/off   Why is this suddenly 4 times worse than before? Where do I look to fix? Thanks!
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gr
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avatar placemarkReply by
dpkleessr
Sure would have been nice if you put your plans out for review before you implemented them but sorry to say it but you pretty much screwed the pooch with what you did. There is a correct way and more than a few wrong ways to correct wavelength errors and I guess you chose poorly. By the way, it has nothing to do with your lamps unless they are very old or otherwise show signs of aging. I need to look at everything you did and didn't do to come up with an action plan for you so this might take me a little time. Don
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gr

Thanks Don! I am taking a guess that my photometric problems have something to do with touching the sensor? I look forward to your reply.

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avatar placemarkReply by
dpkleessr

Actually, I believe that the problems got totally out of hand when you messed with the slit.  Is there anyway that you can tell where its original position was?  If so, return it to that position for starters.

Don

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gr

Back in 2012 I had problems and found the slit had come loose and the white light at zero was 2 or 3 mm to the right of the slit, so I didn't have much choice but to re-attach it. Was tape the original mounting method?

I have a photo of the slit as I found it back then (slit is shown centered near the bottom of the casting notch). I will re-attach in this position.

Thanks!

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gr

I have moved slit #2 back to the original position I found it in (in 2012). The zero white light is now 4 to 5 mm to the right of the slit. I ran setscan gain, but wavelength cal fails with zero order.

It looks like to move the white to the slilt #2 position either the slit #1 must be moved or the grating stepper zero position must be changed????

Thanks Don!

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dpkleessr

First, you NEVER mess with parts that are physically glued in place and yes, that slit was originally glued in place with the tape only there to insure it didn't move if the glue dried out.  The issue is moving the grating position and that is not a simple thing to do without prior training and experience.  I will try to find the instructions I sent someone else that needed to move the grating position so please hang loose for a bit while I find that.

Don

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gr

I see in an older question that a user came up with a way to slip the cable using a spring clip spreader tool to move the beam back to where it belongs. Do you think this is a good idea?

 

http://www.labwrench.com/?community.posts/threadNo/70179/subject/Light-Path-Fail-Issue/

Which says:

Beckman DU: "Light Path Failed" fix as a follow-up to my September 17, 2013 post question> The alignment of the Beckman DU grate is actually relatively easy, with the right tool being used! Personally, I used a Vaco "C" washer spreader tool to relieve just enough tension off the string that is so tightly wound around the D pulley located immediately under, and connected to, the grate. Set the Vaco tool to spread (not squeeze) the tool's two individual head prongs apart while the two handles are being hand-squeezed. Stick the two relaxed prongs into the tension spring, one right at the begining and at the other at the end of the spring. Then just spread the tension spring open a very, very slight bit and gently rotate the D pulley a fraction at a time. Move the tool along in the rotation direction to keep even "hold open" tension on the spring.  Reset the machine and see if your new alignment is "spot on" or not. (The beam may move a bit until the new string positioning sets in). If not, repeat until the beam zero's in on the slit. It's really that easy; however, do not try to forcibly rotate the D pulley without relieving the string tension as the abrasive coating on the D pulley can mess up the string's gripping surface, just like sandpaper on clear plastic would! My alignment method is probably not by the Beckman book, but it works. Got two Beckmans up and running this way. Solution by Don Ladanyi

 

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dpkleessr
I see nothing implicitly wrong with using the tool he did and his comments about the pulleys and cables are spot on so please adhere to them. I would add one additional thing to do, place a piece of tape on the pulley and cable closest to you so that if something unfortunate happens then you will not have the entire grating cable fall off the pulley and that would create a whole new problem. Getting the white light repositioned this way is probably going to be a multi step process. Make an adjustment, try to recalibrate and repeat until you are dead on with the white light on the slit. Don't be surprised if in the process of trying to "slip" the cable on the pulley that you go back and forth across the slit opening. It is a very narrow opening and even those of us that were factory trained experienced the frustration of trying to make this adjustment. By the way, the Beckman Coulter DU800 has the same optical bench so anything you do with the DU600 series optics applies to that spec too. Good luck and let me know how it goes. Don Klees
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