Correcting the fundamental ion mobility equation for field effects.

Area of Science:

• Analytical Chemistry
• Physical Chemistry
• Chemical Physics

Background:

• The fundamental ion mobility equation relates collision cross section to drift velocity and electric field.
• The zero-field approximation is inaccurate for high ion drift velocities common in modern IMS-MS.
• This inaccuracy leads to erroneously large collision cross section calculations.

Purpose of the Study:

• To develop correction factors for the zero-field ion mobility equation.
• To improve the accuracy of collision cross section calculations in high-performance IMS-MS.
• To compare the efficacy of momentum-transfer (MT) and two-temperature (2T) theories for correcting ion mobility data.

Main Methods:

• Developed correction factors using improved momentum-transfer (MT) theory and two-temperature (2T) theory.
• Compared corrected and uncorrected equations by recovering known hard-sphere cross sections from mobility data.
• Evaluated performance based on ion drift velocity relative to thermal speed and ion-to-gas mass ratios.

Main Results:

• Both MT and 2T expressions significantly improve accuracy when ion drift velocity exceeds ~4% of thermal speed.
• The MT expression is more accurate than 2T theory when the ion-to-gas mass ratio is greater than four.
• The zero-field equation yields erroneously large cross sections at higher drift velocities.

Conclusions:

• Corrected ion mobility equations are essential for accurate collision cross section determination in modern IMS-MS.
• The improved MT theory offers superior accuracy for typical analytical applications involving heavier ions.
• These findings enhance the quantitative capabilities of ion mobility spectrometry coupled with mass spectrometry.