Gregory D. Schilling

Characterization of Direct-Current Atmospheric-Pressure Discharges Useful for Ambient Desorption/Ionization Mass Spectrometry

Two relatively new ambient ionization sources, direct analysis in real time (DART) and the flowing atmospheric-pressure afterglow (FAPA), use direct current, atmospheric-pressure discharges to produce reagent ions for the direct ionization of a sample. Although at a first glance these two sources appear similar, a fundamental study reveals otherwise. Specifically, DART was found to operate with a corona-to-glow transition (C-G) discharge whereas the FAPA was found to operate with a glow-to-arc transition (G-A) discharge. The characteristics of both discharges were evaluated on the basis of four factors: reagent-ion production, response to a model analyte (ferrocene), infrared (IR) thermography of the gas used for desorption and ionization, and spatial emission characteristics. The G-A discharge produced a greater abundance and a wider variety of reagent ions than the C-G discharge. In addition, the discharges yielded different adducts and signal strengths for ferrocene. It was also found that the gas exiting the discharge chamber reached a maximum of 235 °C and 55 °C for the G-A and C-G discharges, respectively. Finally, spatially resolved emission maps of both discharges showed clear differences for N2+ and O(I). These findings demonstrate that the discharges used by FAPA and DART are fundamentally different and should have different optimal applications for ambient desorption/ionization mass spectrometry (ADI-MS).

Evaluation of a 512-channel Faraday-strip array detector coupled to an inductively coupled plasma Mattauch-Herzog mass spectrograph.

A 512-channel Faraday-strip array detector has been developed and fitted to a Mattauch-Herzog geometry mass spectrograph for the simultaneous acquisition of multiple mass-to-charge values. Several advantages are realized by using simultaneous detection methods, including higher duty cycles, removal of correlated noise, and multianalyte transient analyses independent of spectral skew. The new 512-channel version offers narrower, more closely spaced pixels, providing improved spectral peak sampling and resolution. In addition, the electronics in the amplification stage of the new detector array incorporate a sample-and-hold feature that enables truly simultaneous interrogation of all 512 channels. While sensitivity and linear dynamic range remain impressive for this Faraday-based detector system, limits of detection and isotope ratio data have suffered slightly from leaky p-n junctions produced during the manufacture of the semiconductor-based amplification and readout stages. This paper describes the new 512-channel detector array, the current dominant noise sources, and the figures of merit for the device as pertaining to inductively coupled plasma ionization.

Evaluation of a fourth-generation focal plane camera for use in plasma-source mass spectrometry

A fourth-generation focal plane camera containing 1696 Faraday-strip detectors was fitted to a Mattauch-Herzog mass spectrograph and characterized for its performance with inductively coupled plasma ionization. The camera provides limits of detection in the single to tens of ng L−1 range for most elements and has a linear dynamic range of at least nine orders of magnitude. Isotope-ratio precision better than 0.02% has also been achieved with this device, and this fourth-generation system features the broadest simultaneous mass range obtainable to date with this family of focal plane camera detectors.

Use of an ambient ionization flowing atmospheric-pressure afterglow source for elemental analysis through hydride generation

An ambient mass spectrometry ionization source based on an atmospheric-pressure flowing afterglow has been coupled to a Mattauch-Herzog mass spectrograph capable of simultaneous acquisition of a range of mass-to-charge values by means of a Faraday-strip array detector. The flowing afterglow was used as the ionization pathway for species produced by hydride generation. This ionization strategy circumvents problems, such as discharge instabilities or memory effects, induced by introducing the gaseous sample into the discharge. The generated spectra show both atomic and molecular peaks; calibration curves were calculated for both peak types with limits of detection for arsenic below 10 ppb. This study demonstrates the ability to use an ambient mass spectrometry source, commonly used for molecular analyses, for the detection of gas phase elemental species with the possibilty of performing speciation by coupling with a separation technique.

Optimization of Ag isotope-ratio precision with a 128-Channel array detector coupled to a Mattauch-Herzog mass spectrograph

Isotope-ratio measurements are necessary in a wide range of applications. The precision of these measurements is of utmost importance because it governs the ability to distinguish differences between samples. Often, simultaneous isotope detection is necessary to reach the precision values needed for an analysis to be conclusive. Furthermore, the more isotope ratios that can be precisely determined at once, the better the chances of distinguishing between samples. Therefore, detector arrays, able to simultaneously monitor a broad range of isotopes, are attractive. Such an array detector, termed the focal plane camera (FPC), has been shown to be capable of achieving impressive precision values (0.02% RSD) within relatively short integration times (200 s). However, because the channels of the FPC array detector are inherently discrete, optimization of peak integration methods is important. This paper compares isotope-ratio precision values based on different peak integration methods of raw and zero-filled interpolated data. Also, problems associated with peak drift are explored and the use of flat-topped peak shapes for improved isotope-ratio precision levels are investigated.

Development and characterization of an electrostatic quadrupole extraction lens for mass spectrometry

A new ion optical element for mass spectrometry has been developed to increase ion-beam transmission into the high-vacuum region of a mass spectrometer. The optic, termed the electrostatic quadrupole extraction lens (EQEL), incorporates quadrupole-like focusing fields into a conventional extraction element. These fields have been established by purposefully positioning openings in an extraction optic and surrounding the optic with a solid, cylindrical barrel unit. Modeling of the EQEL optic shows several regions of high ion beam transmission while, experimentally, the EQEL provides a 35-fold improvement over a conventional extraction optic when used with a glow discharge Mattauch–Herzog geometry mass spectrograph.