Hyperbolic Rods Research Articles

Thermometry in a Multipole Ion Trap

We present a characterization of the ions’ translational energy distribution in a multipole ion trap. A linear mapping between the energy distribution of the trapped ions onto the ions’ time-of-flight (TOF) to a detector is demonstrated. For low ion temperatures, a deviation from linearity is observed and can be attributed to the emergence of multiple potential minima. The potential landscape of the trapped ions is modeled via the finite element method, also accounting for subtleties such as surface-charge accumulation. We demonstrate the validity of our thermometry method by simulating the energy distribution of the ion ensemble thermalized with buffer gas using a Molecular Dynamics (MD) simulation. A comparison between the energy distribution of trapped ions in different multipole trap configurations—i.e., with hyperbolic rods, cylindrical rods, and cylindrical wires—is provided. With these findings, one can map the temperature of the trapped ions down to the Kelvin regime using their TOF distributions. This enables future studies on sympathetic cooling and chemical reactions involving ions in multipole traps.

Comparing the performance of hyperbolic and circular rod quadrupole mass spectrometers with applied higher order auxiliary excitation

This work applies higher order auxiliary excitation techniques to two types of quadrupole mass spectrometers (QMSs): commercial systems and spaceborne instruments. The operational settings of a circular rod geometry commercial system and an engineering test-bed for a hyperbolic rod geometry spaceborne instrument were matched, with the relative performance of each sensor characterized with and without applied excitation using isotopic measurements of Kr+. Each instrument was operated at the limit of the test electronics to determine the effect of auxiliary excitation on extending instrument capabilities. For the circular rod sensor, with applied excitation, a doubling of the mass resolution at 1% of peak transmission resulted from the elimination of the low-mass side peak tail typical of such rod geometries. The mass peak stability and ion rejection efficiency were also increased by factors of 2 and 10, respectively, with voltage scan lines passing through the center of stability islands formed from auxiliary excitation. Auxiliary excitation also resulted in factors of 6 and 2 in peak stability and ion rejection efficiency, respectively, for the hyperbolic rod sensor. These results not only have significant implications for the use of circular rod quadrupoles with applied excitation as a suitable replacement for traditional hyperbolic rod sensors, but also for extending the capabilities of existing hyperbolic rod QMSs for the next generation of spaceborne instruments and low-mass commercial systems.

Homotopy analysis method to study a quadrupole mass filter

The homotopy analysis method (HAM) is applied to study the behavior of a hyperbolic rods of quadrupole mass filter and a sinusoidal potential form V(ac) cos(Ωt). Numerical computation method of a 20th-order HAM is employed to compare the physical properties of the confined ions with fifth-order Runge-Kutta method. Also, comparison is made for the first stability region, the ion trajectories in real time, the polar plots, and the ion trajectory in x - y plan. The results show that the two methods are fairly similar; therefore, the HAM method has potential application to solve linear and nonlinear equations of the charge particle confinement in quadrupole field.

Higher order parametric excitation modes for spaceborne quadrupole mass spectrometers

This paper describes a technique to significantly improve upon the mass peak shape and mass resolution of spaceborne quadrupole mass spectrometers (QMSs) through higher order auxiliary excitation of the quadrupole field. Using a novel multiresonant tank circuit, additional frequency components can be used to drive modulating voltages on the quadrupole rods in a practical manner, suitable for both improved commercial applications and spaceflight instruments. Auxiliary excitation at frequencies near twice that of the fundamental quadrupole RF frequency provides the advantages of previously studied parametric excitation techniques, but with the added benefit of increased sensed excitation amplitude dynamic range and the ability to operate voltage scan lines through the center of upper stability islands. Using a field programmable gate array, the amplitudes and frequencies of all QMS signals are digitally generated and managed, providing a robust and stable voltage control system. These techniques are experimentally verified through an interface with a commercial Pfeiffer QMG422 quadrupole rod system. When operating through the center of a stability island formed from higher order auxiliary excitation, approximately 50% and 400% improvements in 1% mass resolution and peak stability were measured, respectively, when compared with traditional QMS operation. Although tested with a circular rod system, the presented techniques have the potential to improve the performance of both circular and hyperbolic rod geometry QMS sensors.

QMS in the Third Stability Zone with a Transverse Magnetic Field Applied

We report here a study using a quadrupole mass spectrometer (QMS) in which a static magnetic field is applied transversely to the body of the mass filter operating in stability zone 3. Significant improvement in QMS performance was obtained under certain magnetic field conditions, and these have been explained in terms of our theoretical model. The theoretical approach assumed in the model is that the QMS contains hyperbolic rods as electrodes and that the magnetic field acts over the full length of the mass filter assembly. Our latest analysis also predicts for what values of operating parameters an enhancement of the quadrupole resolution is achieved when a transverse magnetic field is applied. The model predicts instrument resolution R > 5000 for Ar with a 100 mm long mass filter and R > 3500 for a HT and D(2) mixture with a 200 mm long mass filter via application of a transverse magnetic field.

Distribution into the quadrupole mass filter with round rods

Abstract Potential field distribution of a quadrupole mass filter with circular cross-section electrodes is described. At first, using superposition principle, we calculate potential around a round rod which is subjected to a given potential. By standard separation method, we then able to obtain the potential distribution into the quadrupole mass filter with circular rods. The results are compared with those obtained with a conventional hyperbolic rod set. Also, the results show that, for the same equivalent operating point in two stability diagrams (having the same β y ) the associated modulated secular ion frequencies behavior are the same.

Comparison of quadrupole mass filters with hyperbolic and cylindrical rods working in the third stability zone

The working point of a quadrupole mass filter is generally set in the first zone of stability. For applications that require higher resolutions other stability zones can provide a suitable alternative and in this article it is investigated the third zone in particular for applications in the low mass range. Ion trajectory simulations for 4He + have been carried out for cylindrical as well as hyperbolic rods and the results have been compared. For cylindrical rods the best mass resolution has been obtained with a ratio between rod radius and field radius of 1.120. For this value further simulations have been performed to determine the dependence of transmission on the injection regions and injection angle in the quadrupole field. The analysis of the transmission regarding low and high mass tails has shown that it is possible to get rid of the low mass tails for cylindrical rods by limiting the injection region, instead the high mass tails, which affect the transmission for both cylindrical and hyperbolic rods, can be reduced by increasing the frequency of the radio frequency applied to the rods. Therefore, although the mass resolution at 50% of the peak height for hyperbolic rods is always greater than for cylindrical ones, with a suitable (reduced) ion injection region, the maximum transmission and the baseline resolution of a filter equipped with cylindrical rods can be made to approach that of a hyperbolic rod filter.

Effect of an Axial Magnetic Field on the Performance of a Quadrupole Mass Spectrometer

We consider the case of a quadrupole mass spectrometer (QMS) in which a static magnetic field is applied axially in the z-direction along the length of the mass filter. The theoretical approach assumed in the model is that the QMS contains hyperbolic rods as electrodes and that the magnetic field acts over the full length of the mass filter assembly. Initial experimental results with argon and helium for a low-resolution instrument confirm the predicted theoretical trends. The analysis also predicts for which values of operating parameters an enhancement of the instrument resolution is achieved when an axial magnetic field is applied. The model predicts instrument resolution R >3000 for a QMS with a 200 mm long mass filter via application of an axial magnetic field.

Liquid chromatography/tandem mass spectrometry (highly selective selected reaction monitoring) for the analysis of isopropylthioxanthone in packaged food

Isopropylthioxanthone (ITX), usually applied as a mixture of 2- and 4-isomers, is a common photo-initiator in UV inks used in paper- or plastic-based packaging materials. In this work a pentafluorophenylpropyl column (HS F5) has been used to achieve the chromatographic separation of the two isomers. A gradient elution with acetonitrile and a 25 mM formic acid–ammonium formate at pH 3.75 are required to provide an R s of 1.3 between the two compounds. The fragmentation pattern of ITX was studied using two mass analyzers, an ion trap (IT) (multi-stage fragmentation) and a triple quadrupole mass analyzer of hyperbolic rods (accurate mass (AM) measurement). The protonated molecule [M+H] + observed in the mass spectrometry (MS) spectrum lost an isopropyl group, [M+H–C 3H 6] +. Later, this ion fragmented, yielding the radical ion [M+H–C 3H 6–CHO] +. The elemental composition of these product ions was confirmed by AM measurement. Electrospray ionization (ESI) was used as an ionization source to couple liquid chromatography (LC) to MS. Instrumental quality parameters of three acquisition modes provided by the triple quadrupole mass analyzer were studied and good run-to-run precision (relative standard deviation, RSD, lower than 10%) and limits of detection (LODs) down to 0.8 pg injected in the LC–MS/MS system were obtained. Finally the LC–MS/MS method using H-SRM Q1 acquisition mode was used to analyze 2- and 4-ITX in a range of food samples. The use of highly selective selected reaction monitoring (H-SRM on Q1) resulted in improved selectivity without sensitivity loss.

Analytical performance of a high-pressure radio frequency-only quadrupole collision cell with an axial field applied by using conical rods

The effect on triple-quadrupole performance of applying an axial field, in an rf-only quadrupole collision cell operated at pressures sufficiently high that collisional focusing is operating, has been investigated. The advantages of such cells have been shown previously to include increased transmission and much improved resolution in fragment ion spectra relative to the performance of collision cells operating at lower gas pressures. The disadvantages of high-pressure collision cells all derive from the relatively long transit times for the ions, which can be long relative to characteristic times for scanning the first mass filter (precursor ion selector) or for switching its setting in multiple reaction monitoring (MRM) cycles. The present work describes experiments on a high-pressure cell in which an axial field is created through use of conical rather than cylindrical or hyperbolic rods. In addition, results of computations of the electric fields within such a cell, and of ion trajectories through it, are presented. It is shown that application of axial fields of the order of 0.1 V/cm can remove all hysteresis effects associated with the long ion transit times, and thus provide excellent performance in quantitation work using MRM, as well as in other scan modes. Furthermore, the advantages of collisional focusing in quadrupole collision cells are shown to be unimpaired by these low axial fields.

A high-resolution focussing “dipole” mass spectrometer

A high-resolution focussing “dipole” mass spectrometer is proposed and its properties are explored by the computer calculation of ion trajectories. The device has a field corresponding to half that of a quadrupole mass filter and consists of two hyperbolic rods and a plane grounded electrode. Low energy ions are injected transversely to the instrument axis. It is shown that by using the focussing properties of quadrupole fields, both a high resolution and a large mass range should be possible while preserving the usual advantages of quadrupole mass spectrometers.

A neutral‐atmosphere composition experiment for the Atmosphere Explorer‐C, ‐D, and ‐E

The neutral‐atmosphere composition experiment instrumentation is designed to obtain in‐situ measurements of neutral thermosphere composition from Atmosphere Explorer‐C, ‐D, and ‐E. The system is based on previously flown OGO‐6 and San Marco‐3 composition instruments. The mass‐spectrometer sensor includes a gold‐plated thermalizing chamber and ion source, a hyperbolic rod quadrupole analyzer, and an off‐axis electron multiplier. Automatic ion‐source sensitivity control and pulse‐counting techniques provide density measurement capability from approximately 125 to 1000 km altitude. The normal operating mode includes measurement at all masses in the range 1 to 44 amu with emphasis on hydrogen, helium, oxygen, nitrogen, and argon. Additional operational modes are optimized for minor constituent studies of any individual gas in this mass range.

Quadrupole Mass Filter: Circular Rods and Peak Shapes

A frequently observed characteristic of quadrupole mass filters is the appearance, at high resolution, of a separate “precursor” peak on the low mass side of each mass peak. The present report shows that such false precursor peaks can result from the use of round rods (rather than hyperbolic rods) that are common in all commercial instruments. Round rods introdnce sixth- and tenth-order terms to the ideal quadrupole field. We have computed the amount of sixth-order distortion introduced by the use of circular rods. The equipotential line produced at the surface of a round rod can be represented quite well by a quadrupole field with a large amount of sixth-order distortion. This distortion introduces a resonance line in the Mathieu stability diagram that at high resolution is near the edge of the peak on the low mass side. The resonance line can distort the peak to produce an apparent precursor peak.