Loss of Linearity

[handlovic]

Hello,

I just installed a new column and have run into an interesting problem. When I make my first injection (6ug/mL) on the calibration I get a typical response with an area of approx. 100,000, but when I make the second injection (3ug/mL) I get an area of approx. 2,000. The tune looks great making me think the MS is fine. I remade the solutions to make sure they weren't an issue but got the same results. Does anyone have any ideas on how to fix this?

Here are further instrumental details for reference:
Temp: 100-220@30Deg Per Minn
Carrier Gas: 1.00mL/Min of hydrogen
Split Flow: 5.00mL/Min
Purge Flow: 8.00mL/Min
Inlet Mode: Splitless for 1.00Min
Inlet Temp:250
MS: SIM
Injection Size: 1.00uL with Autosampler

Sample details:
Solvent: Pyridine
Compound: TMS Derivative of 17a-Ethinylestradiol (CAS#:57-63-6)

[handlovic]

Also, when I repeat the injections I get the same response every time with a relative standard deviation of about 1%. The peaks are symmetrical with the same retention times.

[James_Ball]

Did you get approximately 50,000 on the old column for 3ug/ml?

The new column may have some active sites that have not been capped off yet that were on the old column from either matrix or analyte being loaded onto it. Maybe inject some of the samples and see if the response improves for the low calibrator.

[handlovic]

Did you get approximately 50,000 on the old column for 3ug/ml?

The new column may have some active sites that have not been capped off yet that were on the old column from either matrix or analyte being loaded onto it. Maybe inject some of the samples and see if the response improves for the low calibrator.

I have ran the sample several times and the response does not improve. Yes, on the last column I received a fully linear calibration down to 0.023 ug/mL with an R2 of about 0.998. I am really not sure what could cause this.

[tkubowicz]

Hello

You're using SIM mode to quantify. Are you using SIM-Target Ion or SIM-TIC?
I'd check if qualifier ions response is changed the same way.
For investigation I'd also check SCAN mode to check if it is the same.
Have you tried to run calibration anyway and check if without lowest level curve is ok?

Regards

Tomasz Kubowicz

[handlovic]

Hello

You're using SIM mode to quantify. Are you using SIM-Target Ion or SIM-TIC?
I'd check if qualifier ions response is changed the same way.
For investigation I'd also check SCAN mode to check if it is the same.
Have you tried to run calibration anyway and check if without lowest level curve is ok?

Regards

Tomasz Kubowicz

Hi Tomasz,

I am using SIM-TIC. When I run a full scan I notice the same drop in response. The 3.0ug/mL is only the second point in the curve. On the old column, I used to get a linear response down to about 23ng/mL.

[tkubowicz]

Hello

I think that it can be something with with MS background. It is probably much higher than you had before and it is affecting low levels (it is not changing high concentrations that much).
What I'd do is:
- check electron multiplier voltage EMV. Compare to previous tunings.
- clean ion source
- clean dynode (if it is dirty is typical source of excessive bckground)

Regards

Tomasz Kubowicz

[James_Ball]

Hello

You're using SIM mode to quantify. Are you using SIM-Target Ion or SIM-TIC?
I'd check if qualifier ions response is changed the same way.
For investigation I'd also check SCAN mode to check if it is the same.
Have you tried to run calibration anyway and check if without lowest level curve is ok?

Regards

Tomasz Kubowicz

Hi Tomasz,

I am using SIM-TIC. When I run a full scan I notice the same drop in response. The 3.0ug/mL is only the second point in the curve. On the old column, I used to get a linear response down to about 23ng/mL.

Try using SIM-Target Ion, it will filter out any noise from other masses that may be interfering and giving a higher background.

[handlovic]

The noise does not change. The height just drastically changes from 2.5E6 at 6ug/mL down to about 80,000 at 3ug/mL.