Hello Dr. Pete,

Thanks for your reply.

The instrument was working well with poor signal. Then, I cleaned the heated capillary and once I put it back, there is no signal in the negative ion mode but worked perfect in the positive ion mode (even lot better what it was before cleaning heated capillary).

Today, I checked all the connection of the ESI source and heated capillary and everything is good. According to the previously discussion of Rupert Derler (https://www.researchgate.net/post/Has_anyone_experienced_the_problem_that_you_only_get_an_MS_signal_in_positive_ion_mode_bot_none_in_negative_ion_mode_What_could_be_the_problem), I tried calibration and tuning by following the user manual (Thermo). The calibration solution contains caffeine, MRFA, ultramark, acetic acid in methanol and water. For the calibration, i used positive ion mode and worked well. However, afterwards, I tried the negative ion mode. But, no signal or one signal in every 10 seconds was found. The NL value of the negative mode is usually zero but sometimes it is e4. I did not see any noise in the negative mode but positive mode. I used direct infusion.

Let me know if any more information is needed.

Thanks a lot again and I appreciate your concern and help.

Regards,

Babu

On LCQ Deca In negative mode try using a lower current on the ESI probe instead of 4.5 to 5kv use 2.0 to 3.5kV.

Hello Dr. Levison,

I tried it along with all types of tuning and calibration to resolve the problem and have gotten no signal/peak in the positive ion mode even. Now, I have no signal in both positive and negative ion modes. Would you please give me some suggestion to get the signals back? Let me know if you have anything to know. Thanks a lot for your suggestion and concern.

Regards,

Sabyasachy (Babu) Mistry

Purdue University

It seems the info is still incomplete to understand the question and provide suggestions. Please clarify for us.

I do not see any information in your responses which would tell us what your mobile phase is (or the liquid that you are infusing the sample in). This is very important information as some solutions will suppress ionization, esp in negative mode. BTW: Some samples just do not make measurable amounts of negative ions at all. This is normal. What flow rate are you using? When you change flow rate, does the signal change? Did you tune in negative mode to confirm the instrument is working properly?

Am I correct in assuming that you only had signals in positive ion mode and never had any nice clean signals in Negative mode while infusing your sample into the MS? Or did you once have a very nice negative mode signal with this specific sample and now it is gone?

Answers to these questions would certainly help me, and perhaps others, to understand what your real question is and what you have observed and done so far.

Check the manual for the LCQ Deca (see link) and make sure that the probe position on the ESI manifold is correct for the flow rates you are using. Also make sure the locking ring on the API stack High Voltage (HV) cable (on the right side of the source) is secured (turned all the way clockwise).

I suggest running through the diagnostics test in Appendix A of the manual to narrow down the issues you are having with the instrument. As Bill and John said we need more information to help us answer your question. 

https://conquerscientific.com/wp-content/ad_images/2014/04/finnigan-lcq-deca-hardware-manual.pdf

Check visually that the spray is formed. If not, check the readbacks in diagnostics and, if they are correct, modify the parameters of the probe until you obtain a well formed spray. If the spray is OK, then clean the API stack, at least to the skimmer. (And obviusly make sure that your sample contains compounds that ionize in negative mode and that the phase in which you infuse it favors the negative ionization). Additionally, you could try passivating the heated capillary (a metal needle, I supose) . 

It does sound as if you are having intermittent signal.  If you visually notice arcing at the sprayer tip that seems to correlate to the presence of signal, then you will most likely need to clean the instrument at least as far as the skimmer, as Juan has suggested, but not as far as the ion trap. Arcing is damaging to your instrument, so don't continue to run like that because the signal is present.