Randall Urdahl

Determination of stimulants using gas chromatography/high-resolution time-of-flight mass spectrometry and a soft ionization source

RATIONALE The aim of this study was to investigate the mass spectral fragmentation of a small set of stimulants in a high-resolution time-of-flight mass spectrometer equipped with a soft ionization source using vacuum ultraviolet (VUV) photons emitted from different plasma gases. It was postulated that the use of a plasma gas such as Xe, which emits photons at a lower energy than Kr or Ar, would lead to softer ionization of the test compounds, and thus to less fragmentation. METHODS A set of nine stimulants: cocaine, codeine, nicotine, methadone, phenmetrazine, pentylenetetrazole, niketamide, fencamfamine, and caffeine, was analyzed by gas chromatography/time-of-flight mass spectrometry (GC/TOFMS) in positive ion mode with this soft ionization source, using either Xe, Kr, or Ar as plasma gases. Working solutions of the test compounds at 0.1 to 100 ng/μL were used to establish instrument sensitivity and linearity. RESULTS All test compounds, except methadone and pentylenetetrazole, exhibited strong molecular ions and no fragmentation with Xe-microplasma photoionization (MPPI). Methadone exhibited significant fragmentation not only with Xe, but also with Kr and Ar, and pentylenetetrazole could not be ionized with Xe, probably because its ionization energy is above 8.44 eV. The Kr- and Ar-MPPI mass spectra of the test compounds showed that the relative intensity of the molecular ion decreased as the photon energy increased. CONCLUSIONS When coupled to a TOF mass spectrometer this soft ionization source has demonstrated signal-to-noise (S/N) ratios from 7 to 730 at 100 pg per injection (depending on the compound), and a dynamic range of three orders of magnitude (100 pg to 100 ng) for some of the test compounds.

Properties of microplasmas excited by microwaves for VUV photon sources

Microplasma sources typically take advantage of pd (pressure × size) scaling by increasing pressure to operate at dimensions as small as tens of microns. In many applications, low pressure operation is desirable, which makes miniaturization difficult. In this paper, the characteristics of low pressure microplasma sources excited by microwave power are discussed based on results from experimental and computational studies. The intended application is production of VUV radiation for chemical analysis, and so emphasis in this study is on the production of resonant excited states of rare gases and radiation transport. The systems of interest operate at a few to 10 Torr in Ar and He/Ar mixtures with cavity dimensions of hundreds of microns to 1 mm. Power deposition is a few watts which produces fractional ionization of about 0.1%. We found that production of VUV radiation from argon microplasmas at 104.8 nm and 106.7 nm saturates as a function of power deposition due to a quasi-equilibrium that is established between the electron temperature (that is not terribly sensitive to power deposition) and the population of the Ar(4s) manifold.

Investigation of LTE Condition inside a Microwave Induced Nitrogen Plasma Torch

Optical emission spectroscopy is employed to determine the Fe I excitation temperature (Texc) and NH rotational temperature (Trot) inside the microwave excitation region of an atmospheric-pressure, microwave-induced plasma (MIP) torch operating in nitrogen. The plasma is operated at 2.45 GHz at an input power of 900 W with 50 W reflected, and inner and annular auxiliary nitrogen flow rates of 1 slpm each, and annular outer gas nitrogen flow rates of 13 and 25 slpm. Iron is introduced to the inner flow at a concentration of 200 ppm in a 1% HNO3 solution. Measured relative emission intensities of Fe I emission lines in the band between 370.56 and 376.72 nm are used to determine Texc using a fitting method assuming Boltzmann equilibrium. NH radical A( 3 Π)–X( 3 Σ ) spectra are also employed (in the band between approximately 335 and 343 nm) to determine Trot by comparing measured spectra with spectra calculated using the Specair software. The presence of overlapping spectral features from the N2 (C-B) second positive band near 337 nm are accounted for by activating these transitions in Specair in addition to the NH transitions. Good agreement between the Texc of Fe and Trot of NH is observed, indicating a local thermodynamic equilibrium (LTE) condition. These temperatures range from approximately 5000 K near the inner gas flow injector to approximately 4000 K at a distance of 25 mm downstream of the injector. The temperatures do not change significantly as a function of the outer gas flow rate.

Characterization of a High-Power Microwave Induced Plasma inside an MP Torch using Emission Spectroscopy

Atmospheric pressure microwave-induced (MIP or MP) torches are frequently used in industrial and environmental sectors for chemical analysis of various samples. In spite of their significant use in industry, data related to the plasma inside the torch is rare. In this work, we have analyzed an MP torch that is commercially available. The main objective of this work was to gather information related to the plasma dependence on the various gas flow conditions. Investigations were also carried out to understand analyte-plasma interaction and its dependence on gas flow conditions. Experimental results clearly indicate that the physical location of the plasma inside the torch along with the plasma plume length outside the torch is a strong function of the outer gas flow rate. At a higher outer gas flow rate, a “recirculation” zone is created inside the torch that physically “holds” the plasma near the injector exit. The physical movement of this high temperature plasma zone is also confirmed by analyzing the emission spectrum of various nitrogen bands along with OH, NH, and NO bands. High speed imaging of the analyte (1000 ppm Na in 1% HNO3 solution) indicates a central cooler region that is surrounded by a high temperature zone. This was observed by imaging the Na D2 line inside the microwave torch by varying the inner gas flow rate carrying the analyte droplets into the plasma. Further investigation of the analyte plume inside the torch showed that it was strongly affected by the inner gas flow rate. Furthermore, at certain flow conditions, a plasma instability was observed where the analyte plume moved upstream in the torch and the plasma radially expanded in an undesirable fashion.

Thomson scattering diagnostics and computational modeling of a low pressure microwave excited microplasma source

Summary form only given. Low-pressure microwave-excited microplasmas1 are promising sources of VUV photons for a variety of applications, including photoionization for mass spectrometry. A split-ring resonator microstrip architecture can be used to initiate and sustain these microplasmas using the extremely high electric field generated in the sub-millimeter gap between electrodes. The VUV flux, primarily the result of resonance radiation following electronic excitation of rare gas atoms, is a sensitive function of the distribution of electron energies. Direct measurement of the electron energy distribution (EED) could provide critical insight into the physics of the microplasma operation. We present Thomson scattering measurements of the EED in a split-ring resonator argon microplasma operating at 2.5 GHz and approximately 1 Torr. The diagnostic consists of a Q-switched Nd:YAG laser operating at 532 nm. Thomsonscattered light is collected with a high throughput (f/2) triple grating imaging spectrometer and an intensified CCD camera gated to the laser pulses. Stray and Rayleigh-scattered laser light, which can exceed the intensity of the Thomsonscattered light by factors of over 105, is filtered out using a mask placed between the first two gratings, which are operated in subtractive mode. Other available diagnostics include VUV flux measured using a vacuum UV monochromator. Plasma parameters are measured as a function of gas flow rate, absorbed microwave power, and spatial location both within the plasma cavity and in the downstream plume. Experimental results of the EED and VUV flux are compared with computational modeling using the Hybrid Plasma Equipment Model (HPEM).2 In this model, the EED and radiation transport are computed using Monte Carlo simulations, and neutral gas and plasma transport are addressed using fluid techniques. These experimental and computational modeling results provide a means for optimizing the VUV flux produced by the source.

Environmental Applications of Soft Ionization with GC–TOFMS and GC–QTOFMS

Abstract Soft ionization used with gas chromatography time-of-flight mass spectrometry (GC–TOFMS) allows for the selective ionization of aliphatic hydrocarbons and polycyclic aromatic sulfur heterocycles in certified standard reference oil (NIST SRM2721) and coal tar (NIST SRM1597a). Soft ionization is accomplished by photoionization from vacuum ultraviolet resonance lines produced by Xe, Kr, and Ar microplasmas. Molecular ion fragmentation of an aliphatic hydrocarbon C 24 H 50 in a TOF mass spectrometer and a QTOF mass spectrometer using Ar and Kr are compared. The advantage of using soft ionization, as compared with the more commonly used electron ionization, lies in the enhancement of the relative intensity of molecular ions, which can be controlled by the choice of plasma gas. Furthermore, the enhanced selectivity provided by the ability to select the ionization wavelength is demonstrated through the qualitative analysis of a series of polycyclic aromatic sulfur heterocycles including dibenzothiophene, naphthothiophene, benzonaphthothiophene, and various methylated derivatives of these compounds.

Determination of Chlorophenoxy Acid Methyl Esters and Other Chlorinated Herbicides by GC High-resolution QTOFMS and Soft lonization

Gas chromatography with quadrupole time-of-flight mass spectrometry (GC-QTOFMS) and soft ionization generated by a rare-gas plasma is described here for the determination of various chlorophenoxy acid methyl esters and a few chlorinated herbicides. This plasma-based, wavelength-selectable ionization source, which can use Xe, Kr, Ar, Ne, or He as the plasma gas, enables ionization of GC-amenable compounds with ionization energies below 8.4, 10, 11.6, 16.5, or 22.4 eV, respectively. The advantages of soft ionization include enhanced molecular ions, reduced fragmentation, and reduced background noise as compared to electron ionization. In the study presented here for two plasma gases, we demonstrate that Kr plasma, which is softer than Ar plasma, yields molecular ions with a relative intensity >60% for 11 of the 16 test compounds. When using this “tunable” plasma to ionize the analytes, there is the possibility for selective ionization and less fragmentation, which may lead to increased sensitivity and may help structure elucidation, especially when using high-resolution mass spectrometry that generates accurate masses within a few parts per million (ppm) mass errors. Data generated with the Ar plasma and real matrices such as a peppermint extract, a plum extract, and an orange peel extract, spiked with 16 test compounds, indicate that the test compounds can be detected at 1–10 pg/μL of extract, and compounds such as menthone, limonene, eucalyptol, pinene, caryophylene, and other C15H24 isomers, which are present in the peppermint and the orange peel extracts at ppm to percent levels, do not appear to interfere with the determination of the chlorophenoxy acid methyl esters or the chlorinated herbicides, although there were matrix effects when the test compounds were spiked at 1–10 pg/μL of extract.

UV emission and probe diagnostics and computational modeling of a low pressure microwave excited microplasma source

Summary form only given. Low-pressure microwave-excited microplasmas have a wide variety of potential applications, including their use as UV photoionization sources for mass spectrometry. Resonant microwave microstrip architectures can be used to initiate and sustain these microplasmas. When operated in a windowless configuration, the plasma plume exiting from an aperture in the plasma confinement structure contains a complex mixture of particle species (UV photons, groundstate neutrals, metastables, and plasma electrons and ions)1. Understanding the makeup of this plume is critical to optimize parameters for photoionization, or any other application where exposure to the plasma plume takes place. The microplasma source under investigation consists of a resonant microstrip pattern on alumina substrate, with an elongated 1.5 mm x 6.5 mm plasma confinement structure. 2.5 GHz microwave power is delivered at up to 5 W. Argon and helium/krypton mixtures are flowed through the confinement region at flow rates up to 10 sccm producing confinement region pressures near 1 Torr, with the plasma plume exiting into high vacuum through a 300μm-by-600μm aperture. The effect of confinement region and microstrip geometries, net absorbed microwave power, and source region pressures on the properties of the plasma in the source region and plume are investigated. Ultraviolet emission and Langmuir probe diagnostics are used to diagnose the plasma. The spatial distribution of the plasma density in the plume is measured for a range of microwave powers and flow rates. We also present computational modeling results of this microplasma using the Hybrid Plasma Equipment Model (HPEM)2 with comparisons to the experiments. The angular distribution of UV flux is observed, revealing a highly directional radiation pattern owing to an elongated source region. This flux magnitude and directionality is additionally a function of source region pressure, due to variation in the spatial distribution of the plasma and resonant UV photon absorption and quenching.

Plasma dynamics of microwave excited microplasmas in a sub-millimeter cavity

Summary form only given. Capacitively coupled microplasmas in dielectric cavities have a range of applications from VUV lighting sources for surface treatment to radical production. Due to the large surface-to-volume ratio of these devices, the wall mediated dynamics of plasma transport are important to the uniformity and confinement of the plasma. For example, there may be applications where a plume of ionized gas is desired from the microcavity - whereas other applications may require a confined plasma emitting only VUV photons.In this paper, we will discuss results from a computational investigation of the plasma dynamics in microwave excited micro plasma VUV lighting sources. A 2dimensional hydrodynamics model, the Hybrid Plasma Equipment Model, has been used in which radiation and electron energy transport are addressed using Monte Carlo techniques. The microdischarges have widths of:: 1 mm and lengths of :: 1 cm, operate at pressures of 1-20 Torr, with microwave power of 2-10s Watt at 2.5 GHz and a flow rate of several sccm. Gases are either pure rare gases or mixtures of rare gases. We found that the plasma operates in a mode that has both normal-glow and abnormal glow characteristics. Under usual operation in argon, plasmas are produced with a peak electron density of 1013 cm-3. The plasma may not fill the microdischarge cavity at low power. As the power is increased, the plasma expands to fill the cavity. In this regard, the plasma operates as a normal glow. The current density, however, increases with increasing power, and so in this regard, the plasma resembles an abnormal glow. The expansion of the plasma will eventually overfill the cavity, at which time a plasma plume is formed. These plasma dynamics are sensitive to gas mixture. The scaling of plasma confinement and VUV production as a function of aspect ratio, power and gas mixture will be discussed.