Basic research on ion-trap mass spectrometry (IT/MS) in the Environmental Molecular Sciences Laboratory (EMSL) at Pacific Northwest National Laboratory has resulted in a new patented design for an asymmetric ion trap instrument. This advancement, primarily supported by DOE with leveraged funding from other sources, greatly increases potential applications of this approach for analysis of environmental samples.
Advances in ionization techniques have enabled ionization of almost any type of sample for mass spectrometric analysis. Considerable work in the EMSL's Chemical Structure and Dynamics (CS&D) research group has focused on laser ablation/desorption and matrix-assisted laser desorption/ionization, as well as electrospray ionization techniques. The challenges inherent in the analysis of nanoparticles and aerosols also have been considered in this development effort. The development of this asymmetric ion-trap technology addresses some of the major instrumental challenges of environmental mass spectroscopy, such as sample inhomogeneity, the design of field-deployable instruments, and containment of instrument cost.
Solids analysis has been assisted greatly by being able to observe both the cation and anion spectra from the components in a sample. In many sophisticated instruments, these data can be obtained from the same analyte by sequential data acquisition. In field-deployable systems, however, it is difficult to ensure that the analyte remains unchanged from measurement to measurement; therefore, simultaneous mass spectra of both charge signs and the highest precision possible are needed. Some workers, notably in the single-particle aerial analysis research community, have addressed this problem by combining two time-of-flight mass spectrometers with a single source region, thus producing an instrument that, while effective, is large and cumbersome.
The Paul or radio-frequency (RF) trap first stores ions in an RF field. Typically, ions are detected by either resonant or instability ejection through an endcap electrode to a detector. The conditions for storage and ejection depend on only the absolute value of the mass-to-charge ratio, not the sign. In recent years, many manufacturers of IT/MS have added a feature that allows users to select either positive or negative ion detection, but not both, because the shot-to-shot jitter in the signal requires considerable signal averaging to give good-quality spectra. Much of this jitter can be traced to the symmetrical electrode design that is almost universally used. It has also been recognized that synchronizing ion production with the RF field improves the signal reproducibility, but the question remains whether resonant or unstable ions exit through the endcap toward or away from the detector. In a "perfect" symmetrical trap, the exit direction depends on the phase of the ion. Significant improvement was realized by J. Franzen and coworkers at PNNL, who introduced higher-order "odd" terms into the potential through intentional distortion of the hyperbolas. With this modification and the incorporation of their secular resonance ejection scheme, major improvements have been reported.
Workers at PNNL undertook a careful mathematical analysis of the nature of the trapping field as a function of trapping voltage and geometry.
They determined that the trap designer could specify certain independent parameters to produce a harmonic trapping well. Within this design latitude, they also discovered that moving the exit or detection endcap nearer to the center of the trap while retaining the harmonic character of the trapping field greatly increases the likelihood that ions will be ejected from the trap toward the detector without intentionally adding anharmonicities. The simplicity of this design and its electronic requirements make it particularly attractive, and the development of a prototype led to the filing of a patent claim that was granted without comment from the examiner.
A prototype instrument based on these concepts was constructed under the EMSL project, building on the CS&D team's extensive experience with RF ion traps, laser techniques, and practical analysis using mass spectroscopy. The prototype was built around a Teledyne 3DQ IT/MS, a commercial instrument that was particularly attractive because it is designed to be easily modified, it is price-competitive, and the manufacturer was willing to cooperate in the development of modifications. This cooperation included providing access to source-code software, detailed drawings of the trap, and spare vacuum vessels. The prototype instrument, including pumping and control electronics, is about the size and weight of a standard desktop computer, and can be easily controlled by a PC.
Tests were made with both electron impact ionization of the fluorocarbon FC-43 and laser desorption/ionization of trichloroethane from a ceramic rod. An example of these spectra is shown in Figure 2; Figure 1 shows a typical spectrum with a standard ion trap. An unexpected result of these prototype tests was the observation of unusually narrow spectral line widths, which is a natural consequence of the asymmetric design.