A mass spectrometer is an instrument used to measure the mass, or more specifically the mass to charge ratio, of ionized atoms or electrically charged particles. Mass spectrometers help determine the composition of an unknown sample by isolating ionized atoms based on their mass-to-charge ratio, measured in Atomic Mass Units per charge (AMU/q). Mass spectrometers find widespread application in the basic sciences, medicine, and space-based research. Two common space-related applications of mass spectrometry are the study of the composition of planetary atmospheres and the monitoring of air quality on manned space missions. Although mass spectrometry has been used in space-related applications for many years, usage in space presents unique design challenges, both in terms of detection sensitivity and logistical considerations such as weight and power requirements.
In early mass spectrometers, atoms or molecules were ionized by a hot filament and accelerated through the instrument under the influence of voltage gradients. The ions followed a semi-circular trajectory through the instrument, which utilized a strong magnetic field to selectively direct ions of a specific mass towards a detector. By controlling the strength of the magnetic field and the accelerating voltage, ions of different mass/charge ratios could be selectively guided towards the detector. These early mass spectrometers suffered from numerous deficiencies and drawbacks, most significantly the difficulty in achieving and maintaining a stable magnetic field.
Quadrupole mass spectrometers (QMS) eliminated the need for magnetic fields. Similar to its predecessor, a QMS employs a hot filament to ionize the atoms or molecules. Ionization results from the conversion of normally neutral atoms or molecules to electrically charged particles. The ions are accelerated through a mass filter having four parallel metal rods, referred to as the quadrupole. DC and RF (frequency Ω) voltages are applied to opposing pairs of these rods with opposite polarities to create an electric field inside the rod assembly. For a given DC and RF voltage, only ions of a certain mass-to-charge ratio will pass through the quadrupole filter, while all other ions are thrown out of their original path. The stability region is defined by ion trajectories that are periodic and bounded. A detector placed at the end of the rod assembly opposite the ionizer measures those ions that pass through the quadrupole filter. A mass spectrum is obtained at the detector by measuring the ions passing through the quadrupole filter as the voltages on the quadrupole rods are varied. The mass resolution of the QMS is the maximum atomic mass that can be distinguished. The maximum attainable resolution is determined by both the fidelity of the electronics and the overall tolerances of the instrument design. Generally, the voltages employed in QMS systems are of the order of a few thousand volts to obtain a mass resolution of a few hundred Daltons.
The quadrupole rods can be a circular or hyperbolic. Circular rods are easier to manufacture and consequently cheaper. However, the quadrupole electric field produced with circular rods is slightly distorted, which can reduce the maximum attainable mass resolution of the instrument. Consequently, in applications requiring high mass resolution, the more difficult and expensive to manufacture hyperbolic rods are employed as quadrupole rods.
To improve resolution, the electric field generated by the quadrupole rods can be perturbed by introducing an excitation RF signal at an auxiliary frequency (ω) different from the fundamental frequency (Ω). This perturbation causes the original stability region to break into smaller regions termed islands, including an ‘upper stability island.’ The result of this auxiliary frequency is the creation of bands of instability in the previously stable regions of the electric field. Charged particles having a mass within a certain range that previously passed through a stable region of the electric field may now be thrown off trajectory as they coincide with these islands of instability. In this way, the use of an auxiliary frequency to drive the quadrupole rods allows a QMS to operate with improved resolution. A QMS driven under an auxiliary frequency excitation is able to better differentiate between charged particles having close, yet different masses, or mass-to-charge ratios. The size and shape of the upper stability island is determined by the auxiliary frequency used and the amplitude of the excitation RF signal. To create an island of appropriate size, for example, the auxiliary frequency (ω) inserted into the QMS system needs to be near an integer multiple of the fundamental RF frequency (i.e., ω=0-0.1Ω, 0.9-1.1Ω, 1.9-2.1Ω, etc.). Employing an excitation RF signal in one of these auxiliary frequency ranges in conjunction with the DC voltage U and the RF voltage V allows for improved resolution and discrimination between ions with small differences in their mass-to-charge ratio. In general, the auxiliary signal amplitude required for appropriate island formation increases with auxiliary signal frequency.
Unfortunately, this use of auxiliary frequency excitation presents problems in constrained applications, such as space-based applications, where it is advantageous for the QMS to have increased sensitivity and enhanced resolution to better detect and differentiate between complex molecules with higher masses. Having to excite the quadrupoles with an excitation RF signal at an auxiliary frequency in order to create islands of stability/instability requires higher power and increased complexity of the voltage control system. Additionally, the excitation RF signal must be driven at an amplitude that corresponds to a few hundred volts (˜10% of the fundamental RF signal amplitude), to create islands of the appropriate size. However, in space-based applications, power and size is at a premium.
Other factors that affect the resolution and accuracy of the measurement made by the QMS are imperfections in the rods and limitations of the electronics. Furthermore, electronic component values drift with temperature and time, which can have the material effect of shifting the operating point of the quadrupole sufficiently to degrade the detected mass spectrum.
Thus there exists a need to enable a QMS to resolve species of heavy, complex molecules in a power efficient manner, but also to improve the tolerance of the instrument to variations in electronic component values.