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avatar: RadicalRadon
RadicalRadon
11 posts
Replied to RadicalRadon's post on June 6th, 2019

We recently got a LS6500. Running minivials and the standard size ones. Our Eckert and Ziegler brand new standards (H3 and C14 and BKGND) show that the machine is working perfectly, background counts of 60. When we have a blank vial with just the Meridian Quantum Gold cocktail our background shoots up to about 300 DPM, (not really an issue yet). BUT when we put either a blank swab (Q-tip head) or a blank swipe (standard I think 1 inch circle) the DPM increases to 1400 and 6000 respectively. These are brand new swipes and swabs they don't have any contamination on them. The DPM is the same in all isotope ranges we select to run on the machine. When I cut the swipe down to a more reasonable size the counts go down but I can't figure out why I would be getting super high counts when nothing is there. The base count for a full-size swipe is 6000, cut in half its 1900, in a quarter its 1200, 1200 if i cut it into a circle and stick it on the bottom of the vial. These are still insane numbers of DPM.

The large vials are 20ml and ive been filling them to 10-12 ml, the small ones are 8 and I've been filling them to 4ml

Do I just report that on a blank swipe the background is 6000? and hope that it's fine from there?

Seeing as the EPA free release limits for most non tritium Isotopes is only 1000 DPM I'd really rather not have results that tells me that I have background 6 times that amount.

We did have a tech come out and install and then fix the diaphram that broke on it, he troubleshot and said nothing was wrong with the machine and guessed that our cocktail was a bad batch or something, this doesn't make sense because vial with nothing but cocktail in it gets me a reasonable background (288 DPM) considering the plastic vial and all other noise that might be going on.

The first time I ran a blank swipe test I actually got 14,000 DPM, vertified on multiple ranges on the counter.

Ideas?

avatar: dpkleessr
dpkleessr
1076 posts
Replied to RadicalRadon's post on June 7th, 2019

If your background vial is already reading 60cpm then even that is a little high for a background vial.  In all of my almost 33 years of working in Beckman LS counters, background vials that high would seem to indicate some kind of other issue as I would normally see the counts down in the teens or 20's.

The Q-Tips that you use, are they on wooden sticks or are they on a plastic stick?  Actually, seeing a vial with only cocktail in it and getting 288 DPM is concerning to me too.  Backgrounds should NEVER be that high under any circumstances.

What would be more helpful for me in order to help you is what do the background numbers look like if you only count them in CPM mode with and without H# turned off.  After you get me that information we can go from there but I am definitely concerned with what you are seeing.

Don

avatar: RadicalRadon
RadicalRadon
11 posts
Replied to dpkleessr's post on June 7th, 2019

The Q-tips are wooden sticks. When we do backgrounds at the facility with a G-M pancake probe we do get counts of about 55 per minute. 

 

I just ran a sample with on the Wide setting with H# off and on

The samples were in this order:

Full swipe

2nd full swipe but stuck to the side of the vial

1/4 of the swipe

E&Z BKGND Standard

Blank vial, half full with Meridian Gold Star Quanta cocktail

 

H# on (CPM): 

B1: 7416, H#: 86.6

B2: 4921, H#: 87.6

B3: 1844, H#: 71.1

B4: 75, H#: 1.3

B5: 541, H#: 67.2

 

H# off (CPM)

B1: 7496

B2: 4959

B3: 1852

B4: 64

B5: 598

I took pictures of the data sheets and what the swipes look like:

https://m.imgur.com/a/6gR9feN

Edit: sorry for posting this three times I think the server went down right when I posted it or something.

avatar: dpkleessr
dpkleessr
1076 posts
Replied to RadicalRadon's post on June 7th, 2019

After looking at the data you posted here I believe that you may have a light leak which typically has something to do with the diaphragm not closing properly but just enough to trigger the sensors that it is closed.  There is no huge difference between the results with and without the H# turned on is what leads me to that conclusion.  The one caveat to counting ANY type of swipe or swab  is that unless the swipe is either laying flat on the bottom of the vial or completely emulsified is that the geometry of the how the swipe sits inside of the vial can affect the count rates.  It is what is known as beta self adsorption in that the filter material can actually block some of the counts getting to the PMT's within the 20ns time frame to be considered as coincident counting.  Your counts, however, are just way too high for background counts of anything which again is why I believe that you may have a light leak.  I guess that I also have to assume that this is a relatively new occurrence for you using this LSC.  Kind of looking towards that you are going to need to call someone in to service your counter especially with a background count of 288 with only cocktail in the vial.

Don

avatar: RadicalRadon
RadicalRadon
11 posts
Replied to dpkleessr's post on June 10th, 2019

There was a diaphram error that required a tech to come out and replace our diaphram sensors, and we got these counts both before and after this work was done.

avatar: RadicalRadon
RadicalRadon
11 posts
Replied to dpkleessr's post on June 10th, 2019

We just ran a test with our standards, theyre pretty much right on the money so I don't believe that its a light leak issue. Additionally when I cut a swipe down to size so that it lays on the bottom of the vial we still get counts in the thousands range, I don't remember exactly how many but it was about 2,000 and I threw that sample out already.

 

 

avatar: dpkleessr
dpkleessr
1076 posts
Replied to RadicalRadon's post on June 10th, 2019

OK, I would like you to try the following. Make up some blanks with your filter supports and swabs placed in a rack with a USER # card in place as if you were going to count the samples in CPM mode only without H#. Place them inside the sample changer base with the lid closed for overnight. First thing in the morning count the samples WITHOUT opening the lid and report back to me the results. I believe that I know what's happening but I need you to do this in order to verify my assumption.

Don

avatar: RadicalRadon
RadicalRadon
11 posts
Replied to dpkleessr's post on June 11th, 2019

I did that overnight, or at least with the blacks that I had been using. There wasn't a huge change. It did go down slightly, but a blank vial with only cocktail got 540 CPM and both I ran with blank swipes still have ~10,000 counts. 

 

I can try making new ones and seeing if that changes overnight?

avatar: RadicalRadon
RadicalRadon
11 posts
Replied to dpkleessr's post on June 14th, 2019

I made up 4 samples last night, a minivial with only cocktail, one with a swab head in it, a large vial with only cocktail, and a large vial with a swipe in it.

Last night the counts were

Mini with cocktail: 188 CPM, 56.5 H#

Mini with swab: 429 CPM. 63.5 H#

Large with cocktail: 506 CPM, 68.5 H#

Large with swipe: 2146 CPM, 86.9 H#

 

This morning my counts were

 

Mini with cocktail: 192 CPM, 62.7 H#

Mini with swab: 431 CPM, 62.7 H#

Large with cocktail: 497 CPM, 69 H#

Large with swipe: 2126 CPM. 86.5 H#

avatar: dpkleessr
dpkleessr
1076 posts
Replied to RadicalRadon's post on June 14th, 2019

Statistically, there is no difference between the numbers. Seeing background samples as high as you are observing falls into a few categories: 1.) a light leak, albeit a small one; 2.) a chemiluminescence event going on. Typically the counts die out after some time but so far that does not appear to be the case with your system; 3.) You may have a noisy HV power supply or a problem with the front end in general which could include your PMT's, their sockets which are also preamps, and the Coincidence board. Really tough to diagnose this from afar but the only other thing I would ask you to do is keep the samples in the dark and count them WITHOUT the H# turned on, in other words, CPM mode, no H# and then get back to me. That may help me figure out where to go next with this problem. I worked on LS counters for over 32 years and there were times where they were real hair pulling situations in order to figure out what's going on.

Don

avatar: RadicalRadon
RadicalRadon
11 posts
Replied to dpkleessr's post on June 14th, 2019

Without the H# on.

Minivials:

just cocktail: 528 CPM

with swab: 559 CPM

 

Large vials:

just cocktail: 562 CPM

With swipe: 2336 CPM

 

I did have the door to the room that the counter is in open a bunch today and had to mess around with the vials to get them in the correct order so they may have had a little light exposure. I can run them again on monday morning and see what happens.