Flexibility of IMS Methods Provided by Gas Selection

For light ions (i.e., not in the Rayleigh limit), fi depends on m and a change of M multiplies K of nonisobaric ions by different factors, which could materially affect the separation. However, the choice of gas matters in other cases too the value of fl always depends on such properties of gas molecules as size, shape, electrical polarizability (ap), and dipole moment (pm) The interplay of those dependences is complex, but qualitatively many trends are understood. With respect to size, for 

FIGURE 1.11 2D IMS/MS spectra of (1+) tryptic peptide ions from rabbit muscle aldolase generated using matrix-assisted laser desorption ionization (MALDI). (From Ruotolo, B.T., McLean, J.A., Gillig, K.J., Russell, D.H., J. Mass Spectrom., 39, 361, 2004.) Except for the systematic shift of absolute mobility, the separations in four gases are broadly similar. 

FIGURE 1.12 The effect of gas molecule size on the collision cross section depends on the ion dimensions. The difference between 11 of an ion (unshaded) in a gas of large (grey) and small (black) molecules is larger for small ions (a) than for large ions (b) and for elongated ions (c, two views) than for their spherical isomers (a). 

Collision Cross Sections of Protonated Oligoglycine and Neurotensin Ions with He, Ar, N2, and CO2 (at 250 °C) Measured Using Conventional IMS Absolute Values for He and Relative Values for Other Gases 

FIGURE 1.13 Collision cross sections for conformers of protonated bovine ubiquitin ions (z = 6-13) produced using ESI, measured in He (a) and N2 (b) gases. (From Sbvartsburg, A. A., Li, F., Tang, K., Smith, R.D., Anal. Chem., 78, 3304, ibid 8575, 2006.) 

---