The Importance of Field Strength in the Low Field Portion of a Differential Ion Mobility Spectrometry Waveform Jesus I. Martinez-Alvarado, Brandon G. Santiago, Gary L. Glish University of North Carolina, Chapel Hill, 27599 Introduction: Differential ion mobility spectrometry (DIMS) is a gas-phase separation technique that can be coupled to mass spectrometry to improve signal-to-noise ratios. DIMS separations use the dependence of ion mobility on electric field strength to separate ions. Through application of an asymmetric waveform alternating between high and low electric field strengths, the difference between high and low field mobilities is sampled. Addition of solvent vapors to the DIMS carrier gas has been shown to alter the differential ion mobility of ions and enhance the selectivity of DIMS separations. The primary explanation in the literature to explain solvent effects is the dynamic cluster-decluster model. This model rationalizes the change in differential ion mobility, and thus the compensation field required to pass the ion through DIMS, by stating that an ion travels in a solvated state during a low field portion of the separation waveform and then undergoes rf heating during the high field portion of the waveform, causing it to travel as a bare ion. Herein, we investigate the dynamic cluster-decluster model by varying the amount of time spent in the low field portion of the DIMS waveform. Experimental Methods: Experiments were performed using a Bruker HCT Quadrupole Ion Trap mass spectrometer for detection. The DIMS device consists of two planar stainless steel electrodes 4 mm wide and 1 mm long, separated by a.3 mm gap. DIMS scans were performed using a dispersion field strength of 37.5 kv/cm, with the waveform parameters as described below. The electrospray desolvation gas was used as both desolvation gas and carrier gas through DIMS by diverting the flow through the housing of the DIMS assembly. The desolvation gas flow was set to 5. L/min and either 2 or 3 C in the instrument control software. Solvent vapor from LC-MS grade methanol was introduced at a rate of.3 ml/min to the desolvation gas via a Swagelok tee connected to the output of a Hitachi LC 2 pump. Electrospray ionization was used to ionize.1% triethylamine in 9/1 MeOH/EtOH at a flow rate of 2 μl/min. Results and Discussion: The dynamic cluster-decluster model is predicated around the idea that the ion travels in a solvated state during the low field portion of the separation waveform, and after undergoing rf heating travels as a bare ion during the high field portion of the waveform. In DIMS literature, low field is typically described as fields <1 kv/cm in strength (although some claim this value to be as low as 2.5 kv/cm). Due of the power requirements of using a rectangular waveform at the required voltages for DIMS, other
alternatives are typically used. One such alternative is a bisinusoidal waveform, generated by coupling two sinusoidal waveforms. A drawback of using this type of waveform is that as the amplitude of the high field portion of the waveform is raised, the absolute amplitude of the low field portion of the waveform is also increased. It was observed that at field strengths where the low field portion of the waveform was >1 kv/cm the addition of methanol vapor still affected the differential ion mobility of various analyte ions. To further examine this previous observation the form parameter of the bisinusoidal waveform was varied, with example waveforms shown in Figure 1. The form parameter is defined as the amplitude of the first harmonic divided by the summed amplitude of the two harmonics, with greater form parameter yielding less time spent in the low field. At a desolvation gas setting of 3 C, it can be observed in Figure 2a that methanol changes the required compensation field to pass triethylamine ions through the DIMS device at all form parameters. A form parameter of.7 yields no net time in the low field at a dispersion field of 37.3 kv/cm, yet it is apparent that the differential ion mobility has been changed based on the shift in required compensation field. This shift is also observed at a desolvation gas setting of 2 C (Figure 2b). Additionally, Figure 2 shows that at both temperature settings form parameters generating greater time in the low field yield greater changes in the required compensation field. As expected, this effect is more prominent at 2 C where the lower effective temperature of the ions makes clustering more favorable. Further manipulation of the time spent in the low field was accomplished through varying the phase shift of the two sinusoidal waveforms relative to each other. Figure 3 depicts the changes to the waveform shape upon varying the phase shift and how time in the low field was increased. Although drawing conclusions from how the phase shift affected the required compensation field is beyond the scope of this work, it can be observed that at both temperatures and all phase shifts the use of methanol led to a change in required compensation field. As with form parameter, a greater change in required compensation field was observed at a temperature setting of 2 C, presumably due to the more favorable clustering kinetics present at lower ion effective temperatures. Summary and Conclusions: The form parameter and phase shift of a bisinusoidal waveform used for DIMS separations were varied to manipulate the duration of the waveform that could be described as low field. At form parameters and phase shifts that gave no or minimal net time in the low field, it was observed that methanol still changed the required compensation field. This result would not be expected based on the dynamic clusterdecluster theory. However, in agreement with dynamic cluster-decluster theory is the fact that form parameters yielding increased time in the low field showed greater changes in required compensation field. It was also shown that lower temperatures increased the shift in required compensation field caused by dopant vapors, a change that could be explained by more favorable ion kinetics for clustering at lower
temperatures. These results suggest another factor beyond the dynamic clusterdecluster theory plays a role in how solvent vapors affect differential ion mobility. Figure 1. Representative waveforms depicting how changing the form parameter changes the amount of time spent in the low field. Here the form parameter is changed from.57 (left) to.77 (right), and time in the low field is highlighted in yellow
Compensation Field (V/cm) Compensation Field (V/cm) 4 3 (a) 3 2 2.55.6.65.7.75.8 Triethylamine 3 No MeOH Triethylamine.3 ml/min MeOH 2 (b) Form Paramter 2.55.6.65.7.75.8 Triethylamine with no methanol Form Parameter Triethylamine Figure 2. Plots with depicting.3 ml/min the effects methanol of form parameter on required compensation field with no methanol is used (red) and when methanol vapor is added to the carrier gas (blue) at a dispersion field of 37.3 kv/cm. Experiments were performed at desolvation gas temperature settings of 3 C (a) and 2 C (b)
Figure 3. Representative waveforms depicting how changing the phase shift changes the amount of time spent in the low field. Time spent in the low field increases from 1 (left) to 4 (right), and is highlighted in yellow
Compensation Field (V/cm) Compensation Field (V/cm) 4 3 (a) 3 2 2 1 2 3 4 5 Triethylamine with no methanol Waveform Triethylamine 3 with.3 ml/min methanol 2 2 (b) 1 2 3 4 5 Triethylamine with no methanol Waveform Trielthylamine with.3 ml/min methanol Figure 4. Plots depicting the effects of phase shift on required compensation field with no methanol is used (red) and when methanol vapor is added to the carrier gas (blue) at a dispersion field of 37.3 kv/cm. Experiments were performed at desolvation gas temperature settings of 3 C (a) and 2 C (b)