LC-MS: Benzaldehyde not detecting

[nick_b_hall_99]

Hi all, I'm very new to LC-MS and am trying to understand why benzaldehyde isn't flying / ionising in my system.

I've run a few other samples and they work fine, including the structurally similar cinnamaldehyde.
I don't think it's a suppression issue as a solution of 50:50 benzaldehyde:cinnamaldehyde still shows cinnamaldehyde nicely, so it's not some stabiliser in my benzaldehyde sample suppressing the signal.

Help greatly appreciated!
Conditions below.

Sample:
1 mg/mL benzaldehyde in ACN

Column:
4.6x150mm C18 5µm

Method:
Solvent A - Water (LC-MS grade) + 0.1% FA
Solvent B - ACN (LC-MS grade) + 0.1%FA
Gradient from 95A:5B to 5A:95B over ~50mins with appropriate switches etc.

System:
Agilent 1200 series HPLC
Agilent 6120 Quadrupole LC-MS

MS conditions:
ESI, 3000V
Gas temp 250°C, 12L/min
Nebuliser pressure 35psig
Quadrupole temp 100°C
We've tried a variety of fragmentor voltages and not seen much effect, perhaps some benzaldehyde fragments at higher voltages.

[mburleson]

Hi all, I'm very new to LC-MS and am trying to understand why benzaldehyde isn't flying / ionising in my system.

I've run a few other samples and they work fine, including the structurally similar cinnamaldehyde.
I don't think it's a suppression issue as a solution of 50:50 benzaldehyde:cinnamaldehyde still shows cinnamaldehyde nicely, so it's not some stabiliser in my benzaldehyde sample suppressing the signal.

Help greatly appreciated!
Conditions below.

Sample:
1 mg/mL benzaldehyde in ACN

Column:
4.6x150mm C18 5µm

Method:
Solvent A - Water (LC-MS grade) + 0.1% FA
Solvent B - ACN (LC-MS grade) + 0.1%FA
Gradient from 95A:5B to 5A:95B over ~50mins with appropriate switches etc.

System:
Agilent 1200 series HPLC
Agilent 6120 Quadrupole LC-MS

MS conditions:
ESI, 3000V
Gas temp 250°C, 12L/min
Nebuliser pressure 35psig
Quadrupole temp 100°C
We've tried a variety of fragmentor voltages and not seen much effect, perhaps some benzaldehyde fragments at higher voltages.

Before analyzing the benzaldehyde in its matrix of your sample, I'd first attempt to optimize the source conditions if possible. 3 kV does seem a little low - most ESI's I operate or have operated are about 4.5 - 5 kV for positive mode. I've used your voltage of 3 kV for negative ion detection. ESI responds really well with at least 20% water in your sample.

So, what I would do is make a 50 ppm solution of just benzaldehyde in 50:50 water:ACN (assuming it dissolves) and directly infuse (syringe pump) that to your source for optimizing the parameters. Since ESI is soft ionization, you'll most likely see the M+H peak at 107 m/z.