Chromatography Forum

Hello
Maybe someone will give an idea what is the source of this strange noise increase
Multiresidue method LC-MS/MS (Agilent 6410A), ESI+
Example sequence (samples):
1/2/3/4
5 - noise increase
6/7
8 - noise increase
9/10
11 - noise increase
12/13
14 - noise increase
15/16
17 - noise increase
...
- noise increase occurence independent on matrix (the same phenomena for samples/calibrants/reagent blanks etc.)
- area/hight for analyte signal stable
- noise increase like 2x
http://snag.gy/UbRdD.jpg

Printscreen of TIC for sample 38 (black line,noise increase) and reinjection 38A (red line,normal signal)
http://snag.gy/qAYkg.jpg

A. You mention that this is a multi-residue LC-MS/MS analysis, and show the noise for the m/z 327 to 77 transition.
1) is there a similar behavior for all transitions ?

B. It is very obvious that the behavior occurs at every 3rd injection, AFTER the first 5 injections in the example shown
1) is this behavior observed for every sample set ?
2) Does your analysis sequence go,
Blank/ low level to high level calibn stds/Blank/ QC Sample/ Blank/Sample 1/Sample 2 etc. ?
3) is the behavior observed if blanks are regularly inserted in the sequence ?

C. The appearance of the TIC suggests that the m/z 327 analyte is bleeding slowly into the source, rather than being a distinct chromatographic peak. This suggests a possible hang-up of the analyte in the LC lines and/or ion source. Is there any chance that this is column bleed from a Phenyl-type column.
1) does your HPLC method include a high-% organic wash at end of individual analysis ?
2) what column are you using ?
3) what is mobile phase ?

JMB

Hello JMB,
Thanks for reply.
Yes its MRM method. Noise increase is similar across all transitions. The 327->77 is just for example that peak area is stable (its my ISTD - TPP).
Every 3rd injection after the first increase of signal (its random from 1 to 6...trying to find the rule of it)
Sequence: reagent blank/low level to high level calibrants/QC sample/blank/samples 1...i dont insert any blanks in sequence just calibrant on LOQ level.
I wash HPLC/nebulizer with high% organic (9:1 ACN/H2O and the end of each batch).
Column: Zorbax Eclipse Plus 100x2.1 1.8um
Mobile phase:
A: H2O 0.01% Formic acid, 5mM Ammonium formate
B: 9.5:0.5 ACN/H2O 0.01% Formic acid, 5mM Ammonium formate

I elongate organic stage in gradient ... and set few (10) no injection at the end of sequence will check results tomorrow.

I wash HPLC/nebulizer with high% organic (9:1 ACN/H2O and the end of each batch).

Is yours a gradient HPLC method, and does it include,

a) a high-% organic wash after each INJECTION ?
and
b) a needle wash/rinse ?

JMB