Manual vs Autosampler Injection Metrohm 881 Compact Pro

[koifreak]

I'm am currently using the Metrohm 881 Compact Pro IC fitted with the autosampler. In the last few months we found that there are differences between the peak area obtained using manual injection vs. autosampler injection, despite being injected from the same sample / standard solutions. The difference is in fact more than 50 % (Peak area from Autosampler injection being higher than manual injection). We also found that while the peak area from autosampler injection remained consistent, the peak area from manual injection of similar standard concentrations have reduced by half compared to data obtained 3 months ago. Our standard is sialic acid standard solutions 1.25 - 20 ppm.

We have tested the following:
1) Checked the pump flow. It will flow 10mL of eluent after 10mins with 1mL/min pump rate.
2) Changed the amp body cell, Au electrode and reference electrode.

Recently we have been advised to check the column (Metrosep Carb1-150). But I highly doubt the problem could arise from the column since the difference is observed between the injection methods. We injected full set of standard solutions using both the manual and autosampling and observed similarly consistent differences between manual and autosampling injection peak areas (manual injections' peak areas 50 % lower than autosampling injections' peak areas).

We've been stumped for awhile now and really hope anyone could help advise on the possible source of problem the soonest possible. Thank you.

[Markus Laeubli, Metrohm]

Dear koifreak

Do you use the same injector for both manual and automatic injection?
What is the loop size?
Are you applying full loop injection?

[James_Ball]

Dear koifreak

Do you use the same injector for both manual and automatic injection?
What is the loop size?
Are you applying full loop injection?

As stated above, are you injecting say, 10ul into a 10ul loop for a manual injection or are you using an over fill method of injecting 50ul into a 10ul loop? Also do you use a syringe volume larger than what you are actually putting through the loop so there is no chance of air bubbles before or after the injection aliquot. The autosampler will work on the technique that it over fills and flushes the loop with sample and there is still sample behind that aliquot coming into the loop at the time the valve switches. If you inject manually and there is a small air bubble near the syringe plunger and you push the plunger all the way down as you inject that small air bubble can enter the loop and you are actually injecting less during a manual injection.

[koifreak]

Thank you for your replies!

We made sure the injection procedures were correct. We have managed to find the source of the problem being different settings in the potential profile for the automation and manual injection methods. Once rectified, we got matching peak areas again.