LC/MS/MS : Problem with repeatability

[laiwong]

Hie guys,

I'm new into this forum, and this forum was highly recommended to me.

I am currently attempting to quantify a sugar derivative (A) by LC-MS/MS with an internal standard (IS), but am having a very difficult time getting good repeatability. A run consisting of 6 continuous injections is giving me a %RSD of 36 % (the ratio of A peak area/IS peak area), with MS/MS on both compound A(400ng/mL) and IS(200ng/mL). Individual %RSD value of A and the IS is about 25% and 32% respectively.

Does anyone has any ideas as to why I am getting such bad repeatability? I am at my wits end now, and would appreciate advice and suggestions. Thanks

[Alp]

What method of ionization are you using? ESI, APCI...?
Mobile phase components and percentage? (High water content??)

Column? Sample makeup?
Flow rate?

Are the 6 injections from the same sample?

Without knowing more one can only guess where the problem is.

I will suggest you make sure your nebulizer is nebulizing properly and voltage connection for your ion source is good.

[Fri_nick1]

Hi,

I am new here. I thought I could give you ideas for running sugar derivatives. I did that long time ago using LCQ classic(i know its old but it was the only one avalaible). The source was APCI because ESI did not work, and Phenomenex Razex Ca2+ column gave me good result.
Good Luck

[gxch43]

just for your reference:

1. please check the retention time, if the retention time is not stable, different organic solvent % can lead to different responses.
2.check the sample you were injecting, if the sample is viscous, and the injection speed is very fast.
3. check the curtain plate of the instrument,if the curtain plate is dirty,

hope this help

[Alp]

gxch43 brought up good points that you might want to investigate.
Is your syringe pulling the same amount of sample each injection? Is your syringe seal leaking?