By Hyun Joo on Tuesday, July 6, 2004 - 07:20 am:

Hi all,

I have experienced extremely low probe current (e.g., 0.1 - 0.2 uA)during ESI optimization, even though I set up probe voltage as 4.5 kV. For the optimization, I injected certain amount of standard compounds through manual injector.

But, there was no changes of probe current values, which means no ion response. I tried acidified water/acettonitrile or methanol combination. I could not have any significant ion response.

However, during the tuning with PEG 400 or 600, I got satisfactory ion response as well as increased probe current values, such as 40-45 uA at 4.5 kV of probe voltage.

I am not sure if this means mismatched mobile phase combination or voltage discharge etc. during the ionization process. I want to know if this instrumental or chemistry problem.

Thanks a lot in advance.
Hyun

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By MG on Tuesday, July 6, 2004 - 10:30 am:

What kind of compounds are these? What are their concentrations? What type of mass analyzer and what scan mode and polarity?

On instruments that read back probe current, I have used it as a diagnostic tool to tell me if there is spray, or if I have an arc or corona discharge. It will change with sample or mobile phase solvent, modifiers, flow rate and source parameters. I've never used it to indicate the presence or absence of sample (not to say that this can't be done, I've just never heard of it).

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By Hyun Joo on Tuesday, July 6, 2004 - 11:57 am:

Thanks MG for your response.
I used 3-phenoxybenzyl alcohol and 3-phenoxybenzaldehyde with Shimadzu QP8000 single Quad MS at electrospray positive. I understand that these compounds could be analyzed with GC/MS. However, I wanted to analyze the compounds with LC/MS.

Firstly, I tested the compounds with Atmospheric Pressure Chemical Ionization (APCI, +). I could not detect any M+1 peak ions for the compounds.

And then I tested the compounds with electrospray ionization (ESI, +) mode. Concentrations for the compounds ranged from 10 to 50 ppm. Mobile phase compositions were 0.1% formic acid in water/acetonitrile or methanol (10/90, v/v). Flow rate was 0.2 mL/min. If these chemicals are not compatible with LC/MS, I understand that.

However, I am still confused about the role of probe current. If the probe current values are almost zero during the ESI, does this mean no or little electrospray?

Hyun

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By MG on Thursday, July 8, 2004 - 11:18 am:

On probe current, the values will depend on the design of your ion source. As I have not run a Shimadzu instrument, I don't know exactly what your values mean. But to answer your question, zero ESI current happens when there is no spray, or the voltage is turned off or set very low, or you are attempting to spray a nonpolar solvent without any polar modifiers (not applicable to your situation).

I suspect the problem is that these will be more difficult compounds for LC/MS, but they may work if you can find the optimum conditions. For the alcohol, I suggest you also try APCI(-). I have used that mode for alcohols successfully. I have not had good luck with aldehydes. In any case, I recommend trying to infuse your compound and if you see nothing, do SIM on the expected M+H or M-H ion and ramp your up-front lens voltage(s) (cone voltage, fragmentor, or whatever it is called on your instrument). Choose the value at signal maximum, then repeat the full scan experiment at that value.

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By Hyun Joo on Thursday, July 8, 2004 - 11:53 am:

Thank you MG. I really appreciate your response.

Hyun