ESLD Woes

[greybeard]

I've been performing carbohydrate profiling on raw materials and finished product (so trace sugar content) for 15 years. Recently our refractive index died finally. So we decided to upgrade to an ESLD detector. We figured the switch should be basically pretty smooth. That's about 50% correct. For finished product, I still run analysis (less sample required due to the extra sensitivity of ESLD over RI) with no real problem. My issue is with raw materials. I have been getting at random 20- 30% more detected or the reverse 20-30% less detected as should be. I've conducted different standard additions a few times and get back what I should. It feels like --ok its broken, but when you show someone or you do extra work to find out what is wrong ---everything runs fine---but then back to normal work and bang, back to broken again.

75% acetonitrile mobile, run the optimization of nitrogen spray to noise, sugars of interest (fructose, dextrose, sucrose, maltose) - use erythritol as internal, 5 ul loop, use ~ 0.1 g/100 mL dilution of sugar or less, room temperature.

These are the same conditions as the finished product (though sample prep is 1 g/100mL due to less sugars).

Suggestions or thoughts?

[Sallybeetle]

Have you tried doubling your run times in case a late eluting peak is under your standard or sample injection analyte peak? Are the standard peak areas consistent with previous runs when this happens?

I do not remember having quite this same problem, but I have had peaks appreciably reduce in height for a few runs to a few months then all of the sudden go back to normal peak height. Have you tabulated reagent lot #'s to see if it can be traced to certain mobile phase or sample prep reagent?

[greybeard]

No, there is no late eluting peak. When we developed the RI method, we adjusted flows, etc to insure no issue as this. When we swapped out detectors from RI to ESLD, then we get this variance that has random outcomes for all samples that day - not just one sample.

I have not done any tracing on lots of reagents. However, I have gone through different lots of them and the variance is too random to point to the reagents.

I've also looked over the different ESLD application notes including the two articles by Dolan. The thing I got from them all was issue with particle spray. I wonder sometime if this causes some of the problem? Could pH influence the outcome or particle spray?

[zheyin]

No, there is no late eluting peak. When we developed the RI method, we adjusted flows, etc to insure no issue as this. When we swapped out detectors from RI to ESLD, then we get this variance that has random outcomes for all samples that day - not just one sample.

I have not done any tracing on lots of reagents. However, I have gone through different lots of them and the variance is too random to point to the reagents.

I've also looked over the different ESLD application notes including the two articles by Dolan. The thing I got from them all was issue with particle spray. I wonder sometime if this causes some of the problem? Could pH influence the outcome or particle spray?

Don't know if the vacuum strength for your ELSD is constant and not too strong...
That could affect the response consistency.

[greybeard]

The discharge tube from the excess solvent is well above the waste level. The exhaust tube from the back side is stretched over to a hood as expressed by the manufacturer.

I have heard that back pressure can cause issues. I have checked the flow test within the instrument and it passes. Is there another way to check the "vacuum" condition you refer too?

Also we have noted some poor performance with our nitrogen source, we have gone through two oxygen filters on a nearby GC. The ELSD uses the same nitrogen source. Can this impact the response issue?

[zheyin]

The discharge tube from the excess solvent is well above the waste level. The exhaust tube from the back side is stretched over to a hood as expressed by the manufacturer.

I have heard that back pressure can cause issues. I have checked the flow test within the instrument and it passes. Is there another way to check the "vacuum" condition you refer too?

Also we have noted some poor performance with our nitrogen source, we have gone through two oxygen filters on a nearby GC. The ELSD uses the same nitrogen source. Can this impact the response issue?

The N2 flow rate is defined by your ELSD and has a guage to regulate, I belive its strength should stay constant during runs. There should be no significant particles in N2 to affect your result, either.

Do you have a meter to constantly monitor the vacuum strenth connecting to the ELSD?
Maybe the hood's sucking strength fluctuates...

When I used ELSD, I didn't have hood, but vacuum jack. The strength is not adjustable and too strong for ELSD, all I got was noise.
So I connected a buffering filtering flask in between, and it worked.