Calibration of a quadrupole ion trap for particle mass spectrometry

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Abstract

A quadrupole ion trap (QIT) is calibrated for microparticle mass spectrometry by confining ϕ=2.02μ m dye-labeled polystyrene microspheres and measuring their secular oscillation frequencies and fluorescence spectra. A particle’s absolute mass and charge are found by measuring its secular oscillation frequencies within the QIT while initiating charge steps through photo-ejection of electrons. The radius of the same microsphere is determined by analyzing the fluorescence emission spectrum, which is dominated by optical cavity resonances, employing Mie theory. The mass of the microsphere is calculated from the radius using the density of bulk polystyrene. For nine particles originating from the same stock sample, the masses obtained from the two methods agrees to within 3% with no systematic deviation. Analysis reveals that small uncertainties in the secular frequency measurements result in significant error in the absolute charges and masses. Nevertheless, excellent agreement between the average masses determined using the two techniques confirm that the value of the trap parameter (z0) obtained from computer modeling is appropriate and that effects of electrode misalignments are small.

Introduction

Accurate mass determination for particles in the 10–10,000 nm size regime is desirable to characterize atmospheric aerosols, viruses, bacteria and advanced particulate materials. However, particles in this size range are usually too large for conventional mass spectrometers yet too small for conventional mass balances. There have been several efforts towards developing techniques for the non-destructive mass spectrometric characterization of single particles, particularly in quadrupole ion traps (QITs) [1]. The foundations of single microparticle QIT mass spectrometry (QIT-MS) were established by Wuerker et al. who demonstrated that the mass-to-charge ratio (m) of a single trapped particle can be determined from its secular oscillation frequencies in a QIT [2]. Subsequently, several groups have applied this strategy to single microparticle mass measurements [3], [4], [5], [6], culminating in the work of Peng et al. who determined the masses of individual biological microparticles including Escherichia coli and human red blood cells [7], [8].

One outstanding problem for single microparticle QIT-MS involves mass calibration of the device for operation in the high mass regime. In principle, once the electrode spacing is known, the mass-to-charge ratio of a charged particle in an ideal Paul trap with infinite hyperbolic electrodes can be ascertained from its secular oscillation frequencies. However, practical traps have electrodes that are non-ideal and which may be misaligned. As pointed out by Schlemmer et al. [5] electrode imperfections and misalignments on the order of tens of microns can lead to systematic errors in m measurements in the percent range.

A recently reported calibration procedure involved mass analysis of single 0.895 μ m diameter polystyrene microparticles originating from a highly monodisperse stock sample [9]. The method entailed visually monitoring star oscillation trajectories and initiating charge steps using electron impact to obtain the particle’s absolute charge and mass. Measured particle masses agreed very well with masses deduced from size and density data specified by the particles’ supplier. One disadvantage of the method is that it relies on size data from the manufacturer. A more desirable procedure would involve ascertaining the size and mass of the same particle in situ. This was attempted some time ago by Davis and Ray [10], who measured the size and mass for a single microsphere (ϕ=2μ m) using an electrodynamic balance (EDB). The particle’s size was obtained from analysis of the angle dependent elastic light scattering and its mass from the electric field required to balance the particle at the EDB centre. Radii deduced using the two methods agreed to within 20%. Recently, Zheng et al. improved this procedure for larger microspheres (ϕ=21μ m) in an EDB, with radii determined through spring point, aerodynamic drag and light scattering measurements agreeing to within 3.9% [11].

In the current work, we characterize single, ϕ=2.02μ m dye-labeled polystyrene microparticles (4.5×1015  kg, 2.7×1012  Da) by obtaining their cavity enhanced fluorescence spectra, and by measuring their secular oscillation frequencies in the QIT. Analysis of optical morphology dependent resonances (MDRs) of a single microsphere allowed the particle’s radius to be determined, and thenceforth for its mass to be ascertained [12]. The m of the same microparticle was measured from observation of its secular frequency in the trap. In turn, by initiating single electron charge steps the particle’s absolute charge and mass were determined. Following a discussion of both techniques, we present data for nine particles, and finally discuss sources of error in the mass determinations.

Section snippets

Materials

The fluorescent polystyrene microparticles used in this study were commercially obtained as an aqueous suspension (Duke Scientific, nominal diameter 2μ m). Each particle contains fluorescent dye molecules (MW=200300  Da) incorporated into the polystyrene matrix. The particles have a broad absorption peaking at 412 nm and an emission envelope extending over 440–550 nm with broad peaks at 445 and 473 nm. The manufacturer’s quoted density for the particles is 1.05 g/cm3, which corresponds to the

Results and discussion

The MDR dominated fluorescence emission from a single polystyrene microsphere is shown in Fig. 4. The MDR peaks are sharp and symmetric indicating that the particle has high sphericity [16]. Furthermore, the MDR spacing reveals that the particle is a single sphere rather than a bisphere or agglomerate. The particle’s radius and refractive index were determined from an analysis of the MDR wavelengths [12] and found to be consistent with a sphere of radius a=1010±1  nm and refractive index mλ=

Summary

A calibration procedure for a microparticle QIT-MS is described. The procedure combines MDR enhanced fluorescence spectroscopy with single particle frequency measurements and charge stepping. The average masses obtained for nine particles using each technique are in excellent agreement (m¯MDR=4.55±0.03×1015  kg and m¯FT=4.54±0.12×1015  kg). This suggests that the trap geometrical parameter (z0=5.89  mm) obtained by computer simulation is appropriate. The large relative errors of the absolute

Acknowledgments

The Australian Research Council and University of Melbourne are acknowledged for financial support. We thank R. J. Mathys for outstanding technical assistance and R. Crowe and C. Turner for assistance in acquiring data. We also thank Drs. H.-C. Chang and W.-P. Peng (IAMS Taiwan) for helpful discussions.

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