Signal intensity increased for the second injection, why?

[OwenYoung]

I injected calibrators twice in my method before and after unknown samples. I noticed the signal intensities for the second injection were 30 ~ 40 % higher than that of the first injection. What are the reasons to cause the intensity increase?
I used ESI negative mode and MRM scan. Plasma sample was extracted using MTBE. Retention times were stable and chromatography were good for the first and second injection.
Thanks.

[Uwe Neue]

what happens if your third injection is a blank?

[OwenYoung]

what happens if your third injection is a blank?

Thanks for your reply. This two injections were not consecutive. this two injections are same calibration sample but injected twice before and after unknown samples. There are about 10 - 20 unknown sample between the first injection and the second injection. I did twice calibration sample injection in my method to monitor the stability of instrument. I found the signal intensity of the second injections were much higher than that of the first injection. I checked carry over and carry over was less than 0.1 %. thanks

[dr_Pyrex]

I used API3000 (ESI , negative mode) and I observed the same phenomena. In my opinion lower sensivity on the begining fo sequece is due to instability of heater temperature in the ESI interface. I always try to obtain stable conditions by long (2-3 hours) equlibraion of the system (pumping moblile phase with proper heater tempertature and gases flow).

[OwenYoung]

I used Thermo Finnigan TSQ Quantum Ultra with heated electrospray ionization (H-ESI) probe. I did not use the heat function on the probe. The ion transfer capillary tube temperature is 275 oC.
I also noticed that the third injection was just a little bit increase comparing to the second injection. Look like the the system reached equilibrium after the second injection. I am still wondering what factor cause the signal intensity increased?

[Noser222]

I've noticed something similar. I have an Agilent 1100 Single Quad.

I haven't read anything about this phenomenon, but someone with more experience than I have said that you can have "active sites" from the spray champer to the detector that may bind the analyte until you reach a saturation point and get a steady signal. When you clean the spray champer, you wash it away from these sites and have to start again. You probably notice a larger variation the higher in concentration you get.

Anyway, it's the best explanation I've come across...

[OwenYoung]

Looks like this phenomenon was compound dependent. I had more than 10 compounds in this study but only one compound had this problem. Maybe you are right, the instrument has some "active sites" for this compound because I did not see same phenomenon in my old Thermo Finnigan TSQ quantum with classic ESI source. For this new Finnigan TSQ ultra with H-ESI, I had this problem. My question is that how can I find the active sites and block them???

[Noser222]

I don't think it is possible to block them, other than what you are doing, which is to have a few "equilibration injections" at the beggining of a series.

[Kostas Petritis]

You can probably block them through passivation. This can be achieved either with nitric acid (but you can not use it with chromatographic columns on line) or with 40 mM of EDTA 2Na.

[SIELC_Tech]

My guess is that it is somthing like any of the "dronic" acids (alendronic, residronic, etc.) or compounds with some strong affinity to metal inclusion in silica and SS surface of your instruments. Compounds with a lot of nitrogens/oxygens/ acidic groups. You need to deactivate these sites.

Regards,

Vlad