# Nanoparticle Drift Tube IMS

DT-IMS: Drift Tube Ion Mobility Spectrometry

Our lab uniquely employs an atmospheric pressure, fluid-mechanically gated drift tube coupled to a condensation particle counter (CPC) to characterize the electrical mobilities of particles between 2 nm and 50 nm in mobility diameter.

Principle of operation of the DT-IMS-CPC system:

Aerosolized nanoparticles are charged via bipolar diffusion charging and are introduced to the inlet of the DT-IMS-CPC system by an aspirating flow. Particles cycle in a fountain-like manner into and out of the DT-IMS through coaxial tubing (‘Aerosol In’ and ‘To Vacuum’). Upon application of an electrical potential to the electrodes surrounding the drift tube, positively charged particles are driven down the DT-IMS toward the detector, a condensation particle counter. In the constant electric field, smaller particles experience less drag from the counter-flowing drift gas and hence traverse the drift region in a shorter period of time than their larger counterparts. An arrival time distribution is thus measured by the CPC, and these arrival times can be correlated to particle mobility or size through the Stokes-Millikan equation (discussed in more detail below). The relationship between mobility and arrival time in the DT-IMS-CPC system is linear in the typical range of operation, as seen in Figure 1 (green circles – experiment; black line – fit):

Figure 1b also displays the experimentally-determined resolving powers of the DT-IMS-CPC at various arrival times (green circles – experiment; black line – resolving power of the TSI nanoDMA, another mobility analysis device) compared to that of the TSI nanoDMA, the current state-of-industry mobility analysis device.

Ion Mobility Spectrometry Operation Principles:

Ion mobility spectrometer (IMS), is the main tool used in our lab for nanoparticle and ion analysis.  In it an analyte (in our case, an ion or nanoparticle) is monitored temporally or spatially as it is driven electrostatically through a fluid which provides a counteracting drag force. At low Reynolds ($Re=\frac{\rho Vd_p}{\mu }$, where $\rho$, $V$, and $\mu$  represent the density, free stream velocity, and viscosity of the fluid, and  represents the diameter of the particle) and Mach ($Ma=\frac{V}{c}$; c is the local speed of the sound) numbers, the drag force experienced by a solid-phase analyte or particle is represented by:

$F_{drag}=-f(v_p-u_{gas})$

where f is the friction factor (kg/sec) and  and  are the respective velocities of the analyte/particle and the gas. Different fields have used various forms for f, but attention will be paid here to the form used in aerosol science as well as that used in ion mobility spectrometry-mass spectrometry. In aerosol science, the friction factor has been related to the diameter of a particle via Stokes-Millikan equation:

$f=\frac{3\pi \mu d_p}{C_c(Kn)}$

with the Cunningham Slip correction factor described as:

$C_c(Kn)=1+Kn*(A_1+A_2{\mathrm{exp} \left(\frac{-A_3}{2*Kn}\right)\ })$

The electrostatic force experienced by the particle is described by:

$F_{elec}=neE$

where  $n, e,$ and $E$ represent the number of charges on the particle, the charge of an electron, and the electric field in which the particle is present, respectively. When the electrostatic and drag forces are balanced, the velocity of the particle can be calculated as:

$V=\frac{ne}{f}E=Z_pE$

Where the particle’s electrical mobility, , is defined as the ratio of the particle’s velocity to the applied electrostatic field. Using the Stokes-Millikan representation for drag, electrical mobility can be related to the mobility diameter of the particle:

$Z_p=\frac{neC_c}{3\pi \mu d_p}$

Thus, when driven via an applied electrostatic field, particle motion through a fluid can be related directly to particle size – the basis of ion mobility spectrometry.  We have developed models to describe the mobilities (and hence collision cross sections) of ions accounting for their structures and potential interactions with gas molecules (see Educational Resources to download IMoS) as well as models to predict the mobilities (i.e. drag coefficients) of highly non-spherical nanoparticles at variable temperature and pressure (variable Knudsen number).

Selected References on DT-IMS and Nanoparticle Mobility Analysis:

Buckley, D. T. & Hogan C. J.  (2017) Determination of the Transfer Function of an Atmospheric Pressure Drift Tube Ion Mobility Spectrometer for Nanoparticle Measurements.  Analyst.  142: 1800 – 1812.  10.1039/C7AN00328E

Davidson K. L., Oberreit D. R., Hogan C. J., & Bush M.F. (2017) Nonspecific Aggregation in Native Electrokinetic Electrospray Ionization.  International J. Mass Spectrom.  420: 34-42.   10.1016/j.ijms.2016.09.013

Jeon S., Oberreit D. R., Van Schooneveld G., Gao Z., Bischof J. C., Haynes C. L., & Hogan C. J.  (2016) Ion Mobility based Quantification of Surface Coating Dependent Binding of Serum Albumin to Superparamagnetic Iron Oxide Nanoparticles.  ACS Applied Materials & Interfaces.  8: 24482−24490.  10.1021/acsami.6b09070

Jeon S., Oberreit D. R., Van Schooneveld G., & Hogan C. J.  (2016)  Liquid Nebulization-Ion Mobility Spectrometry Based Quantification of Nanoparticle-Protein Conjugate Formation.  Analytical Chem.  88:7667-7674.  10.1021/acs.analchem.6b01555

Jeon S., Oberreit D. R., Van Schooneveld G., & Hogan C. J.  (2016)  Nanomaterial Size Distribution Analysis via Liquid Nebulization Coupled with Ion Mobility Spectrometry (LN-IMS). Analyst.  141: 1363-1375.  10.1039/C5AN02150B

Ouyang H.., He S., Larriba-Andaluz C., & Hogan C. J.  (2015).  IMS-MS and IMS-IMS Investigation of the Structure and Stability of Dimethylamine-Sulfuric acid Nanoclusters.  The Journal of Physical Chemistry A.  119, 2026−2036.  10.1021/jp512645g.

Gopalakrishnan R., McMurry P. H., & Hogan C. J. (2015).  The Electrical Mobilities and Scalar Friction Factors of Modest-to-High Aspect Ratio Particles in the Transition Regime.  J. Aerosol Sci.  82:24-39.   10.1016/j.jaerosci.2015.01.001

Thajudeen T., Jeon S., & Hogan C. J.  (2015).  The Mobility of Flame Synthesized Aggregates/Agglomerates in the Transition Regime.  J.  Aerosol Sci.  80: 45-57.  10.1016/j.jaerosci.2014.11.003

Kumar A., Kang S-. K., Ouyang H., Larriba-Andaluz C., Hogan C. J., & Sankaran R. M.  Gas-Phase Ligand-Free Ni Nanocluster Formation at Atmospheric-Pressure via Rapid Quenching in a Microplasma Process.  (2014).  Nanotechnology.  25:  385601. 10.1088/0957-4484/25/38/385601.

Oberreit D. R., McMurry P. H., & Hogan C. J.  (2014).  Analysis of Heterogeneous Uptake by Nanoparticles via Differential Mobility Analysis-Drift Tube Ion Mobility Spectrometry.  Phys Chem Chem Phys 16:6968-6979.  10.1039/C3CP54842B

Oberreit D. R., McMurry P. H., & Hogan C. J.  (2014).  Mobility Analysis of 2 to 11 nm Aerosol Particles with an Aspirating Drift Tube Ion Mobility Spectrometer.  Aerosol Sci. Technol. 48:108-118.  10.1080/02786826.2013.861893

Zhang C., Thajudeen T., Larriba C., Schwartzentruber T. E., & Hogan C. J.  (2012).  Determination of the Scalar Friction Factor for Non-Spherical Particles and Aggregates Across the Entire Knudsen Number Range by Direct Simulation Monte Carlo (DSMC).  Aerosol Sci. Technol. 46:1065-1078.  10.1080/02786826.2012.690543