Get the system serviced and maintained by a service engineer. Check the results after calibration for its reproducibility with a reference or two. Meticulous calibration using standard protocol is a prerequisite. Calibration at regular interval of time is required depending upon the performance analysis of the system.

Mr. Ahmadi,

Problems with mass spectrometer?! Is your apparatus own instrumental scheme?

Because of, if it is not - this is analytical instrumentation for absolute determination in quantitative analytical and chemometric therms. In thi scontext, as Mr. Kulkarni, properly has underlined there is better to use service engineer.

Since, you have mentioned ''calibration'' in your posting I should mention that the classical standard protocos do not provide the actual for the analytical instrumentation method performances. The quantification should be carried out by means of the most recent stochastic dynamic approach: Per span of scan time; instead of, over the whole time of a measurement. (Consider work [1].) According to this latter method, the chemometric analysis shows coefficients of linear correlation between the MS outcome intensity expressed via the so-called stochastic dynemic diffusion coefficients and the concentrations of the analyte in solution within /r/ = 0.99996 - 1. The method has been tested up to concentration level ng.(mL)-1.

[1] Steroids, 164 (2020) 108750

Stochastic dynamic mass spectrometric quantification of steroids in mixture — Part II, Bojidarka Ivanova, Michael Spiteller

If you have any questions regarding the method in work [1], then do not hesitate to ask in this forum, because of the equations, therein, are own authored ones. Simply speaking, we shall provide to you the most accurate information about the method itself, its application and employment, routinely.

Dear Hamid

In your case it appears that the degree of ionization depends on the composition of the gas introduced to the MS.

Ar has a lower ionization energy and a larger cross section than He.

It is thus likely that NO2 ionization efficiency and thus sensitivity is lower in the presence of Ar.

I assume you use electron impact ionization, so you might be able to increase the electron current to mitigate the suppression during calibration with Ar. Or you establish calibrations for NO2 in different concentrations of Ar and use the one matching your actual sample composition.

Eventually you may also lower the gas flow into the ion source to reduce matrix effects.

I hope this helps,

Bodo