Low-mobility-pass filter between atmospheric pressure chemical ionization and electrospray ionization sources and a single quadrupole mass spectrometer: computational models and measurements.

Rationale: Mixtures of ions produced in sources at atmospheric pressure, including chemical ionization (APCI) and electrospray ionization (ESI) can be simplified at or near ambient pressure using ion mobility based filters.

Methods: A low-mobility-pass filter (LMPF) based on a simple mechanical design and simple electronic control was designed, modeled and tested with vapors of 2-hexadecanone in an APCI source and with spray of peptide solutions in an ESI source. The LMPF geometry was planar and small (4 mm wide × 13 mm long) and electric control was through a symmetric waveform in low kHz with amplitude between 0 and 10 V.

Results: Computational models established idealized performance for transmission efficiency of ions of several reduced mobility coefficients over the range of amplitudes and were matched by computed values from ion abundances in mass spectra. The filter exhibited a broad response function, equivalent to a Bode Plot in electronic filters, suggesting that ion filtering could be done in blocks ~50 m/z units wide.

Conclusions: The benefit of this concept is that discrimination against ions of high mobility is controlled by only a single parameter: waveform amplitude at fixed frequency. The effective removal of high mobility ions, those of low mass-to-charge, can be beneficial for applications with ion-trap-based mass spectrometers to remove excessive levels of solvent or matrix ions.