Same standard, different calibration curve

Hello

Can you describe how you prepare STDs (dilutions, pipettes etc)? It looks like concentration problem (preparation or calculation). Have you got good RSD% on each level?

Regards

Tom

I used 1,5-3-7,5-15-37,5 microliters of every standard (x10) and transferred into a 1,5 mL vial to the volume of 1,5 mL (with the ISTD) to make 0.01-0.02-0.05-0.1-0.250 ppm standards. First, I tried with an automatic pipette (first time it appears the issue) and then with a manual 10 ul syringe to make sure to pick the correct volume (first though with the automatic pipette was the non-correct volume). i'm pretty sure about the volume picked with the manual syringe because I paid a lot of attention so i don't think that i could be the problem. At the end, the vials had all the same volume so I don't think that it can be a problem of incorrect volumes.

How can i visualize the RSD%?

Here is the calibration curve with the ISTD

1. You have 5 different levels. But how many injections you did for the same level? Normally you should have at least 3 injection per level. So if you have good RSD for each level it means that injection precision is not an issue.

2. You have ISTD calibration curve. Check ESTD calibration (without IS). Perhaps you're problem is IS response.

3. HPLC vials are not calibrated glassware (volumetric) so you shouldn't assume that mark 1.5ml is actually true. and you shouldn't use them to prepare STDs. At least topping them up to mark - It is not proper way.

1. You have 5 different levels. But how many injections you did for the same level? Normally you should have at least 3 injection per level. So if you have good RSD for each level it means that injection precision is not an issue.

2. You have ISTD calibration curve. Check ESTD calibration (without IS). Perhaps you're problem is IS response.

3. HPLC vials are not calibrated glassware (volumetric) so you shouldn't assume that mark 1.5ml is actually true. and you shouldn't use them to prepare STDs. At least topping them up to mark - It is not proper way.

I did 1 injection for level actually. do you suggest 3 injection?

How do you know without any precision test (multiple injection of the same STD) that your sampler is working fine and there is no issues with it? If you have problems with correct injection volume from injection to injection (syringe, loop, leak etc.) you will end up with scattered results. 

Check first injection precision.

ok! Will check that with 3 injection per standard!

So... update: the results of three injections are exactly the same and I still have the problem even with a little change in my standard preparation. I was thinking about dirt MS source or head of the column or inlet so probably tomorrow I'll clean the source, cut 20 cm from the head of the column and change the inlet.