Cris L. Lewis

Consideration of a millisecond pulsed glow discharge time-of-flight mass spectrometer for concurrent elemental and molecular analysis

The glow discharge ionization source operated in the pulsed (or modulated) power mode offers unique characteristics not available from its steady state counterpart. It has been well established that higher instantaneous power densities are obtainable without compromising the sample integrity when pulsed plasmas are implemented. This operating parameter affords higher sputter yields and lower limits of detection relative to the steady state plasmas. Of special interest are the discrete temporal regions associated with the modulated plasma. The presence of these time regimes offers temporal selectivity, allowing the collection of analytical data in a region where the contribution from background and contaminant species is minimized. These regions are characterized by strikingly different ionization mechanisms. Acquisition of data during each of these temporal regimes provides both molecular and elemental information. In this work the potential use of the pulsed glow discharge for collecting concurrent molecular and elemental information was explored. This task was accomplished using time-of-flight mass spectrometry (TOFMS). TOFMS has a significantly high throughput and duty cycle, making it ideally suited for rapid acquisition of spectra. This characteristic allows data acquisition during each of these temporal regions for each discharge pulse power cycle, affording concurrent elemental and molecular detection. p-Xylene was used as a test molecule for these studies.

Inductively Coupled Plasma Mass Spectrometry for Elemental Speciation: Applications in the New Millennium

Abstract The current status of elemental speciation using inductively coupled plasma‐mass spectrometry (ICP‐MS) as the method of detection is examined in this paper. Manuscripts that describe specific speciation applications using on‐line hyphenation of gas chromatography (GC), liquid chromatography (LC), capillary electrophoresis (CE), and field flow fractionation (FFF) with ICP‐MS detection are presented. This review covers the application papers published between January 2000 and December 2003. Evaluation of the current literature shows that LC is the most prevalent separation technique for elemental speciation followed by CE, GC, and FFF, respectively.

Influence of discharge parameters on real-time chemical speciation for gas chromatography pulsed glow discharge plasma time-of-flight mass spectrometry

A millisecond pulsed glow discharge ionization source was employed for the determination of aromatic and chlorinated hydrocarbons. Chemical speciation employed four separate dimensions that incorporate analyte volatility, elemental constituents, chemical structure, and molecular weight. Analyses were performed in real time using a gas chromatograph that was interfaced to a pulsed glow discharge time-of-flight mass spectrometer. This instrument allowed for the sequential collection of the complete mass spectrum at three different time regimes occurring during the glow discharge pulse cycle. This report focuses on discharge parameters such as pressure and sampling distance that enhance or suppress different ionization processes. Increasing the ion source pressure resulted in a decrease in analyte ionization and sampling efficiency. Ions sampled in close proximity to the cathode surface demonstrated the best elemental sensitivity. Sampling ions at increasing distance, in the negative glow, produced structural information in the form of fragmentation patterns similar to conventional 70 eV electron impact spectra. Molecular weight information obtained after power termination was ascertained from M+ and MH+ ions. At very small distances, the most dominant molecular peak was the M+ ion produced by the Penning process. At increased sampling distances molecular peaks were dominated by MH+ ions; the ionization mechanism is attributed to proton transfer reactions from background species such as ArH+.

Determination of 40Ca+ in the Presence of 40Ar+: An Illustration of the Utility of Time-Gated Detection in Pulsed Glow Discharge Mass Spectrometry

The glow discharge ionization source operated in the pulsed, or modulated, power mode affords a number of distinct advantages over its steady-state counterpart. It is well-known that pulsed plasma operation permits the application of higher instantaneous powers by allowing time for the sample to cool. This minimizes sample overheating while effecting higher sputtering yields and lower limits of detection. The presence of discrete time regimes affords the added advantage of temporal selectivity. Such selectivity allows the observation of analyte ions during a time regime in which their signal is at a maximum while that of electron ionized background species is declining. Significantly, time regimes are found when no background argon ion signals are observable but analyte ion signals remain. This means that discrimination against isobaric interferences arising from the discharge gas is possible. A prime example of the utility of this advantage arises in the determination of calcium with an argon glow discharge. Both the major argon and calcium isotopes are found at a nominal m/z of 40. Time-gated mass spectrometeric detection during the afterpeak time regime enables the ready determination of (40)Ca(+) in samples at the ppm level. A linear calibration curve is obtained that also demonstrates the elimination of the (40)Ar(+) signal from mass spectra obtained with either a dc or rf glow discharge ion source.

Temporal emission characteristics of millisecond pulsed radiofrequency and direct current glow discharges

Interpretation of optical emission spectra reveals the primary excitation mechanisms for discharge gas, argon and sputtered analyte, copper, species in glow discharge plasmas operated with millisecond pulses of radiofrequency or direct current power. There is negligible difference between the two power sources. During the applied power pulse, plasma processes include ion and atom excitation through electron excitation, asymmetric charge exchange and Penning ionization. Fast ion and atom excitation processes, characterized by monitoring argon emission at 811.5 nm, occur within 2 mm of the cathode surface. Electron excitation, for both discharge gas and sputtered species, maximizes 3 mm from the cathode surface. Asymmetric charge exchange between ground state sputtered atoms and discharge gas ions, characterized by Cu II emission at 224.7 nm, occurs at 5 mm from the cathode surface. Upon power termination, the recombination of ions with thermal electrons yields excited atoms and argon metastable species. At this time, emission monitored at 811.5 nm maximizes 6–7 mm from the cathode surface, corresponding to an increase in the metastable population and, hence, Penning ionization.

Characterization of a Pulsed Glow Discharge Laser Ablation System Using Optical Emission

Investigations involving laser-based sampling of copper into an auxiliary pulsed glow discharge for ionization and excitation are presented. The interaction of the ablated copper with the auxiliary glow discharge was studied by monitoring the copper atom emission signal at 368.744 nm. Results demonstrate the ability to time ablation appropriately to access specific temporal regions of the pulsed plasma. More specifically, laser-ablated material was introduced into the glow discharge negative glow during the afterpeak. Ionization and excitation was accomplished by collisions with a metastable argon population produced by the glow discharge (Penning ionization) followed by recombination to yield excited-state Cu atoms. The work presented investigates parameters that affect the atomic emission signal intensity of the ablated material, including cathode-to-target distance, discharge gas pressure, and relative timing of discharge and ablation. Results demonstrate that decreasing the glow discharge working gas pressure increases the transport efficiency of laser-ablated material into the negative glow. These investigations are part of an ongoing series of studies on sample introduction schemes that utilize different ionization and excitation mechanisms found in pulsed glow discharge plasmas.

Practical considerations when using radio frequency-only quadrupole ion guide for atmospheric pressure ionization sources with time-of-flight mass spectrometry

Construction details and performance evaluation of a radio frequency (rf)-only quadrupole ion guide for use with an electrospray ionization time-of-flight mass spectrometer is presented in this paper. Angiotensin III and cytochrome c were used in these experiments to investigate the ion transmission properties of the rf-only quadrupole for different m/z species. In addition, influence of ion kinetic energies along with the characteristic fragmentation due to collision induced dissociation (CID) were studied. These experiments demonstrate that the transmissions of different m/z ions were not only dependent on the frequency and magnitude of the rf waveform, which is similar to a high vacuum rf-only quadrupole ion guide, but also on the pressure inside the quadrupole chamber. For the pressure range tested, low m/z ions are better focused with increasing pressure. As expected, transmission of ions are subject to space charge limitations when significant numbers of ions are focused on the axis of the quadrupole. It is also observed that CID results are related to transverse motion and longitude motion of ions inside the quadrupole region. Consequently, CID is useful for fragmentation of linear peptides and it is not effective (in present configuration) for large bulky proteins. The kinetic energy of ions that enter the repelling region of the TOFMS is ultimately determined by the ensemble effect resulting from the dc bias potential of the quadrupole (the dominant factor), skimmer-2, pressure inside the quadrupole chamber, and jet expansion. While this system is tested with an ESI source, the operational principle and design criteria are directly applicable for improving other atmospheric pressure ionization sources with time-of-flight mass analyzers such as an inductively coupled plasma ion source.

Ion formation in millisecond pulsed glow discharge plasmas

Ion intensity profiles, for both discharge gas (40Ar2+, 40Ar+, 40Ar+2) and sputtered species (63Cu+), have been measured for a series of parameters including sampling distance, pulse power, discharge gas pressure, pulse width, and duty cycle in a millisecond pulsed direct current (dc) glow discharge plasma using time-gated detection with a time-of-flight (ToF) mass spectrometer. Throughout these experiments constant power was maintained for comparative profile measurements. Intensity profiles for both discharge gas and sputtered material were constructed using the intensity values from a compilation of over 100 mass spectra. Ion signals from analytically important (sputtered) species differed in their response to changes in sampling distance, discharge gas pressure, pulse width, and duty cycle than those of discharge gas species. These intensity profiles provide insight into the effects of discharge conditions on the time-dependent behavior of different ions in the plasma. In addition to using time-gated acquisition, it is possible to further influence ion formation within the plasma through the careful selection of these plasma parameters; in doing so, one can maximize sputtered ion signals while suppressing ion signals from discharge gas species. In this research, we conduct a comparative investigation of ion signal temporal profiles through the variation of discharge parameters to better refine the method and to gain a better understanding of the processes taking place in the discharge.

A fluorescence detection scheme for ultra large molecules after gas phase separation

A system was proposed to remove the upper mass limitation of mass spectrometry. In present study, ultra large molecules were separated in the gas phase by mass analyzer after electrospray ionization. Instead of conventional detection with electron multiplier, a laser-induced-fluorescence detection scheme was applied. The instrument sensitivity is independent of molecular weight, but related to the spectroscopic properties of the fluorophores presented by the large biomolecules.