A universal relationship between optimum drift voltage and resolving power

Abstract

The drift voltage is one of the key experimental parameters of any drift tube ion mobility spectrometer. In this work, we show that a universal relationship between optimum drift voltage and the resolving power reached at this point exists, governed only by temperature and ion charge state. With these two quantities known, the measured optimum drift voltage and resolving power combination can be used to estimate the ideality of the drift conditions inside a drift tube, since any deviation from the theoretical values must be caused by non-idealities in the ions' drift. Analyzing drift voltage sweeps from nine different ion mobility spectrometers, a continuous increase in drift tube ideality over the past is observed, reaching from less than 50% thirty years ago to 99% for a current design based on printed circuit boards. Furthermore, possible causes for the observed non-idealities are discussed.

Flexible Drift Tube for High Resolution Ion Mobility Spectrometry (Flex-DT-IMS)

“…The experimental data closely follows the theoretical trendline from eq 4 , with good agreement ( R 2 = 0.9240). The ideality eq ( eq 5 ), as derived by Kirk et al, 13 is given as, where R opt, meas is the measured resolution, T is the drift temperature and U opt, meas is the optimal drift voltage to acquire the optimal resolution. The measured resolution at the optimal drift voltage of 3159 V was 81.7 ( Figure 4 ) which, compared to the ideal value of 85.2, indicates that the system has an ideality factor of 95.9%.…”

“…This result is in agreement with the well-known effect of increasing signal intensity when operating above the optimum drift voltage. 13 The raw data is depicted in the Supporting Information, Figure S6 .…”

“…For IMS, the resolution or resolving power, R P , can be defined as the drift time, t d , divided by the peak width at half-maximum, w 0.5 , and can be broadly thought of as the ability of the instrument to separate chemical species of similar mobilities ( eq 3 ), More detailed equations for IMS resolution have been derived previously. 12 One such expression for estimating resolving power, R P , is given in eq 4 , which places two fundamental limitations on the resolving power of an IMS instrument as described by Kirk et al, 13 where w min , is the minimum injection width of the ion packet into the drift tube; K , is the mobility of the ion under investigation; U D , is the drift tube voltage; L , is the drift tube length; T , is the drift tube temperature, and, k B , z , and e are constants: Boltzmann, ion charge state (typically +1) and electron charge. The left-hand denominator in eq 4 is the contribution from the “minimum-width-limit” and the right-hand denominator relates to “diffusion-limited” resolving power.…”

Plate-height model of ion mobility-mass spectrometry

“…Although diffusion appears as the most important among the intrinsic dispersion processes, it is not the only factor that contributes to zone spreading in IMS. 24,[31][32][33][34][35][36][37][38][39][40][41] Consequently, the diffusion limit of resolving power is essentially never reached in practice, and eqn ( 5)-( 7) only express the contribution of this process to total zone spreading.…”

“…Instead, it has a maximum, corresponding to the operating condition where resolving power is the highest, while further increase in the field strength results in lower resolving power due to the increased contribution of the initial ion package size. 31,39,40 Finally, it is important to point out that among all intrinsic and extrinsic sources of dispersion, the injection pulse-width is the only one where the corresponding theoretical plate and resolving power limits depend on pressure, because they depend on the absolute value of a single transport property. In other words, the implicit dependence of N inj on p is due to its inverse proportionality to K 2 .…”