R. S. Houk

Attenuation of metal oxide ions in inductively coupled plasma mass spectrometry with hydrogen in a hexapole collision cell

Some strongly bound metal oxide ions (MO+) can be attenuated in a hexapole collision cell with a mixture of helium and hydrogen gas. Various metal oxide ions with different dissociation energies, such as ZrO+, CeO+, LaO+, SmO+, HoO+, YbO+ and WO+, have been chosen to study the attenuation effect. By adjusting the collision conditions, especially the composition and flow rate of the collision gas and the hexapole dc bias voltage, the MO+/M+ signal ratio can be suppressed by a factor of up to 60 for CeO+ and LaO+ while maintaining ≈20% of the original signal for atomic analyte ions (M+). For the species studied, collisions with H2 improve the MO+/M+ signal ratio more extensively for oxide ions with higher dissociation energies. The same collision conditions also serve to remove most of the ArO+, ArN+ and Ar2+ from the background spectrum.

An ion trap-ion mobility-time of flight mass spectrometer with three ion sources for ion/ion reactions

This instrument combines the capabilities of ion/ion reactions with ion mobility (IM) and time-of-flight (TOF) measurements for conformation studies and top-down analysis of large biomolecules. Ubiquitin ions from either of two electrospray ionization (ESI) sources are stored in a three dimensional (3D) ion trap (IT) and reacted with negative ions from atmospheric sampling glow discharge ionization (ASGDI). The proton transfer reaction products are then separated by IM and analyzed via a TOF mass analyzer. In this way, ubiquitin +7 ions are converted to lower charge states down to +1; the ions in lower charge states tend to be in compact conformations with cross sections down to ∼880 Å2. The duration and magnitude of the ion ejection pulse on the IT exit and the entrance voltage on the IM drift tube can affect the measured distribution of conformers for ubiquitin +7 and +6. Alternatively, protein ions are fragmented by collision-induced dissociation (CID) in the IT, followed by ion/ion reactions to reduce the charge states of the CID product ions, thus simplifying assignment of charge states and fragments using the mobility-resolved tandem mass spectrum. Instrument characteristics and the use of a new ion trap controller and software modifications to control the entire instrument are described.

Atmospheric pressure laser desorption/ionization of plant metabolites and plant tissue using colloidal graphite

Colloidal graphite is a promising matrix for atmospheric pressure laser desorption/ionization mass spectrometry. Intact [M+H](+) and [M-H](-) ions are readily produced from a wide range of small molecule plant metabolites, particularly anthocyanins, fatty acids, lipids, glycerides, and ceramides. Compared with a more traditional organic acid matrix, colloidal graphite provides more efficient ionization for small hydrophobic molecules and has a much cleaner background spectrum, especially in negative ion mode. Some important metabolites, e.g., fatty acids and glycosylated flavonoids, can be observed from Arabidopsis thaliana leaf and flower petal tissues in situ.

Diagnostic studies of a low-pressure inductively coupled plasma in argon using a double Langmuir probe

The electron temperature and electron number density of a laboratory-built low-pressure inductively coupled plasma (LP-ICP) were measured using a double Langmuir probe. The probe was composed of two 0.5 mm diameter tungsten wires and a magnesia sleeve shielding the wires that could be linearly moved in the central channel of the plasma by a vacuum linear-motion feedthrough. The role of parameters was studied at a height of 25 mm above the load coil, which yielded an electron temperature of 3–6 eV and an electron number density of 2.5–5 × 1013 cm−3. Less dependence on operating factors was noticed at higher observation heights, and when water loaded on the low-pressure plasma was increased, the electron temperature and electron number density were reduced, unlike with an atmospheric ICP.

Theoretical Investigation of Small Polyatomic Ions Observed in Inductively Coupled Plasma Mass Spectrometry: HxCO+ and HxN2+ (x = 1, 2, 3)

Two series of small polyatomic ions, HxCO+ and HxN2(+) (x = 1, 2, 3), were systematically characterized using three correlated theoretical techniques: density functional theory using the B3LYP functional, spin-restricted second-order perturbation theory, and singles + doubles coupled cluster theory with perturbative triples. On the basis of thermodynamic data, the existence of these ions in inductively coupled plasma mass spectrometry (ICP-MS) experiments is not surprising since the ions are predicted to be considerably more stable than their corresponding dissociation products (by 30-170 kcal/mol). While each pair of isoelectronic ions exhibit very similar thermodynamic and kinetic characteristics, there are significant differences within each series. While the mechanism for dissociation of the larger ions occurs through hydrogen abstraction, the triatomic ions (HCO+ and HN2(+)) appear to dissociate by proton abstraction. These differing mechanisms help to explain large differences in the abundances of HN2(+) and HCO+ observed in ICP-MS experiments.

High-resolution mass spectrometry with a multiple pass quadrupole mass analyzer.

The peak shape narrows and the resolution improves if the ions are simply reflected back and forth through a conventional quadrupole mass analyzer. CO(+) and N(2)(+) at m/z = 28 are separated to 50% valley with half of the original signal remaining. These two ions can be resolved to baseline (m/Δm) = 5000 with 1% of the original signal remaining.

A Linear Ion Trap Mass Spectrometer with Versatile Control and Data Acquisition for Ion/Ion Reactions

A linear ion trap (LIT) with electrospray ionization (ESI) for top-down protein analysis has been constructed. An independent atmospheric sampling glow discharge ionization (ASGDI) source produces reagent ions for ion/ion reactions. The device is also meant to enable a wide variety of ion/ion reaction studies. To reduce the instrument’s complexity and make it available for wide dissemination, only a few simple electronics components were custom built. The instrument functions as both a reaction vessel for gas-phase ion/ion reactions and a mass spectrometer using mass-selective axial ejection. Initial results demonstrate trapping efficiency of 70% to 90% and the ability to perform proton transfer reactions on intact protein ions, including dual polarity storage reactions, transmission mode reactions, and ion parking.

Reduction of matrix effects in inductively coupled plasma mass spectrometry by flow injection with an unshielded torch.

Solution samples with matrix concentrations above approximately 0.1% generally present difficulties for analysis by inductively coupled plasma mass spectrometry (ICP-MS) because of cone clogging and matrix effects. Flow injection (FI) is coupled to ICP-MS to reduce deposition from samples such as 1% sodium salts (as NaCl) and seawater (approximately 3% dissolved salts). Surprisingly, matrix effects are also less severe during flow injection, at least for some matrix elements on the particular instrument used. Sodium chloride at 1% Na and undiluted seawater cause only 2 to 29% losses of signal for typical analyte elements. A heavy matrix element (Bi) at 0.1% also induces only approximately 14% loss of analyte signal. However, barium causes a much worse matrix effect, that is, approximately 90% signal loss at 5000 ppm Na. Also, matrix effects during FI are much more severe when a grounded metal shield is inserted between the load coil and the torch, which is the most common mode of operation for the particular ICP-MS device used.

Mass resolution of 11,000 to 22,000 with a multiple pass quadrupole mass analyzer

CO+ and N2+ are separated with resolution of 11,000 [full width half maximum (FWHM)] using a conventional quadrupole mass spectrometer by applying square wave voltages to the entrance and exit lenses to trap or reflect the ions for multiple passes. A resolution of 22,000 (FWHM) with 63% of the total signal remaining is attained using multiple passes when ions are stored between injection pulses. Gated ion extraction also reduces the mass shift and number and intensity of artifact peaks and permits better resolution compared to the performance obtained when the ions are injected continuously.

A secondary discharge intensifies optical emission from a Mach disk extracted from an inductively coupled plasma

The axial channel of a conventional argon inductively coupled plasma (ICP) is extracted through a circular orifice into an evacuated quartz chamber. Emission from the Mach disk region is focused on the entrance slit of an echelle spectrometer equipped with two segmented-array charge-coupled-device detectors. The background pressure in the extraction chamber is 1000 Pa (7.5 torr). At this pressure two barrel shocks and Mach disks are visible. Deliberate use of a mild secondary discharge between the plasma and the sampler enhances emission from the Mach disk for a variety of lines from typical analytes (Ca, Sr, Mg and Mn) by factors of 11 to 25. Detection limits are in the range 0.1–2 µg l–1. Sodium chloride at concentrations up to 10 000 mg l–1 induces only a modest loss (0–6%) of intensity for ion lines, in contrast to the much more severe matrix effects seen in ICP mass spectrometry.