David W. Koppenaal

Collision and reaction cells in atomic mass spectrometry: development, status, and applications

The development and utilization of collision and reaction cells in atomic mass spectrometry is reviewed. These devices have been used for decades in fundamental studies of ion–molecule chemistry and have only recently been applied in the GD-MS and ICP-MS fields. Such cells are used to promote reactive and non-reactive collisions, with resultant benefits in interference reduction, isobar separation, and thermalization/focusing of ions in ICP-MS. Novel ion–molecule chemistry schemes, using a variety of reaction gas reagents selected on the basis of thermodynamic and kinetic principles and data, are now designed and empirically evaluated with relative ease. These chemical resolution techniques can avert interferences requiring mass spectral resolutions of >600000 (m/Δm). Purely physical ion beam processes, including collisional dampening and collisional dissociation, are also employed to provide improved sensitivity, resolution and spectral simplicity. Collision and reaction cell techniques are now firmly entrenched in current-day ICP-MS technology, enabling unprecedented flexibility and freedom from many spectral interferences. A significant body of applications has now been reported in the literature. Collision/reaction cell techniques are found to be most useful for specialized or difficult analytical needs and situations, and are employed in both single- and multi-element determination modes.This contribution is a guest editorial statement and technical assessment for a special issue of the Royal Society of Chemistry journal entitled Journal of Analytical Atomic Spectrometry (JAAS). The editorial introduces the subject area of collision and reaction cells in atomic mass spectrometry, reviews current literature and commercial instrumentation trends, and previews four perspective and numerous research articles contained in the special journal issue.

Communication. Selective removal of plasma matrix ions in plasma source mass spectrometry

We report a new method for selective removal of argon ions and other plasma matrix ions in plasma source MS. The method consists of sampling the plasma and reacting the sampled plasma and analyte ions with hydrogen gas. Reactions have been studied in three instruments: in the ion trap of a plasma source ion trap (PSIT) mass spectrometer and in the post-skimmer region of both a conventional ICP mass spectrometer and a second PSIT. In the ion trap, the reaction between Ar+ and H2 proceeds at nearly the collisional rate whereas reaction of most other atomic ions is four to five orders of magnitude slower. For modest H2 pressures and reaction times in the ion trap [10–4 Torr (1 Torr = 133.322 Pa) and 10 ms], the Ar+ signal is reduced by six orders of magnitude. We have examined reactions of H2 with 33 different atomic ions; the only ions for which a reaction was evident were N+, O+, Cl+, and Ar+. The decrease in Ar+ occurs by a sequence of fast reactions resulting in charge transfer from Ar+ to form the low m/z ions H2+ and H3+, which are rapidly ejected from the ion trap. The net effect is the selective removal of Ar+ chemically, not by virtue of its mass-to-charge ratio only, as in resonant ion ejection methods. In the conventional ICP-MS experiments the reaction time is short, limiting the decrease in Ar+ to about 40-fold in preliminary and unoptimized experiments. However, the reaction is still selective: simple scattering by H2 reduces the 45Sc+ signal at only 5% of the rate of reactive loss of Ar+. Production of H2+ and H3+ is observed directly in the conventional ICP-MS experiments, indicating that the chemistry in the post-skimmer region is consistent with that observed in the ion trap. We discuss methods by which the magnitude of Ar+ reduction observed in the ion trap might be realized in conventional ICP-MS, thus possibly allowing a greater analyte ion transmission efficiency and reduced space-charge effects.

Synechococcus elongatus UTEX 2973, a fast growing cyanobacterial chassis for biosynthesis using light and CO2

Photosynthetic microbes are of emerging interest as production organisms in biotechnology because they can grow autotrophically using sunlight, an abundant energy source, and CO2, a greenhouse gas. Important traits for such microbes are fast growth and amenability to genetic manipulation. Here we describe Synechococcus elongatus UTEX 2973, a unicellular cyanobacterium capable of rapid autotrophic growth, comparable to heterotrophic industrial hosts such as yeast. Synechococcus UTEX 2973 can be readily transformed for facile generation of desired knockout and knock-in mutations. Genome sequencing coupled with global proteomics studies revealed that Synechococcus UTEX 2973 is a close relative of the widely studied cyanobacterium Synechococcus elongatus PCC 7942, an organism that grows more than two times slower. A small number of nucleotide changes are the only significant differences between the genomes of these two cyanobacterial strains. Thus, our study has unraveled genetic determinants necessary for rapid growth of cyanobacterial strains of significant industrial potential.

The Structure of a Cyanobacterial Bicarbonate Transport Protein, CmpA

Cyanobacteria, blue-green algae, are the most abundant autotrophs in aquatic environments and form the base of the food chain by fixing carbon and nitrogen into cellular biomass. To compensate for the low selectivity of Rubisco for CO2 over O2, cyanobacteria have developed highly efficient CO2-concentrating machinery of which the ABC transport system CmpABCD from Synechocystis PCC 6803 is one component. Here, we have described the structure of the bicarbonate-binding protein CmpA in the absence and presence of bicarbonate and carbonic acid. CmpA is highly homologous to the nitrate transport protein NrtA. CmpA binds carbonic acid at the entrance to the ligand-binding pocket, whereas bicarbonate binds in nearly an identical location compared with nitrate binding to NrtA. Unexpectedly, bicarbonate binding is accompanied by a metal ion, identified as Ca2+ via inductively coupled plasma optical emission spectrometry. The binding of bicarbonate and metal appears to be highly cooperative and suggests that CmpA may co-transport bicarbonate and calcium or that calcium acts a cofactor in bicarbonate transport.

Laser ablation inductively coupled plasma mass spectrometry

Abstract Laser ablation inductively coupled plasma mass spectrometry (LA-ICP/MS) has been successfully deployed by researchers at the Pacific Northwest Laboratory for the analysis of Hanford nuclear waste. This effort required research to determine the relative importance of the numerous parameters involved in LA-ICP/MS. These factors include laser parameters and sample matrix variables. The precision in relative concentrations was determined as a function of laser wavelength. Ablation at 355 or 266 nm gave precision of 5–10%, while the value for 532 nm was 25% and for 1064 nm was 40%. Analysis of the data for relative fractionation between elements showed large differences in the IR for volatile vs. refractory elements. The UV ablation gave results that were much less dependent on matrix effects and showed minimal or no fractionation. Measurements of the particle distribution produced by laser ablation prior to transfer to the ICP/MS were made as a function of laser wavelength and pulse energy. These results show that the laser wavelength and pulse energies that provide the best precision and accuracy for analysis of glass or tank waste simulant materials produce particle size distributions with the majority of ablated material present as particles from 0.1–1.0 μm in diameter. This size range is important for quantitative transport to the ICP/MS and for complete digestion of the particles in the plasma. The particle counts were used to normalize the mass spectra of glasses having widely different opacities and laser ablation efficiencies resulting in quantitative analysis of these samples.

Liquid Sampling-Atmospheric Pressure Glow Discharge Ionization Source for Elemental Mass Spectrometry

A new, low power ionization source for elemental MS analysis of aqueous solutions is described. The liquid sampling-atmospheric pressure glow discharge (LS-APGD) operates by a process wherein the surface of the liquid emanating from a 75 μm i.d. glass capillary acts as the cathode of the direct current glow discharge. Analyte-containing solutions at a flow rate of 100 μL min(-1) are vaporized by the passage of current, yielding gas phase solutes that are subsequently ionized in the <5 W (maximum of 60 mA and 500 V), ~1 mm(3) volume, plasma. The LS-APGD is mounted in place of the normal electrospray ionization source of a Thermo Scientific Exactive Orbitrap mass spectrometer system without any other modifications. Basic operating characteristics are described, including the role of discharge power on mass spectral composition, the ability to obtain ultrahigh resolution elemental isotopic patterns, and demonstration of potential limits of detection based on the injection of aliquots of multielement standards (S/N > 1000 for 5 ng mL(-1) Cs). While much optimization remains, it is believed that the LS-APGD ion source may present a practical alternative to high-powered (>1 kW) plasma sources typically employed in elemental mass spectrometry, particularly for those cases where costs, operational overhead, simplicity, or integrated elemental/molecular analysis considerations are important.

RADIONUCLIDE DETECTION BY INDUCTIVELY COUPLED PLASMA MASS SPECTROMETRY: A COMPARISON OF ATOMIC AND RADIATION DETECTION METHODS

Radionuclide detection by mass spectrometric techniques offers inherent advantages over conventional radiation detection methods. Since radionuclides decay at variable rates (half-lives) and via various nuclear transformations (i.e. emission of alpha-, beta-, and/or gamma-radiation) their determination via radiation detection depends not only on decay systematics but also on detector technology. Radionuclide detection by directatom measurement, however, is dependent only on technique sensitivity and is indifferent to decay mode. Evaluation of inductively coupled plasma mass spectrometry (ICP/MS) indicates this method to be superior to conventional radiation detection techniques for many radionuclides. This work discusses factors which influence detection by both methods. Illustrative applications of ICP/MS to the ultra-trace determination of several radionuclides, including129I, are presented.

Diurnal rhythms result in significant changes in the cellular protein complement in the cyanobacterium Cyanothece 51142.

Cyanothece sp. ATCC 51142 is a diazotrophic cyanobacterium notable for its ability to perform oxygenic photosynthesis and dinitrogen fixation in the same single cell. Previous transcriptional analysis revealed that the existence of these incompatible cellular processes largely depends on tightly synchronized expression programs involving ∼30% of genes in the genome. To expand upon current knowledge, we have utilized sensitive proteomic approaches to examine the impact of diurnal rhythms on the protein complement in Cyanothece 51142. We found that 250 proteins accounting for ∼5% of the predicted ORFs from the Cyanothece 51142 genome and 20% of proteins detected under alternating light/dark conditions exhibited periodic oscillations in their abundances. Our results suggest that altered enzyme activities at different phases during the diurnal cycle can be attributed to changes in the abundance of related proteins and key compounds. The integration of global proteomics and transcriptomic data further revealed that post-transcriptional events are important for temporal regulation of processes such as photosynthesis in Cyanothece 51142. This analysis is the first comprehensive report on global quantitative proteomics in a unicellular diazotrophic cyanobacterium and uncovers novel findings about diurnal rhythms.

The development of a micro-Faraday array for ion detection

Abstract A micro-Faraday array detector was evaluated for use as an ion detector for mass spectrometry. This charge-integrating detector was based upon the merging of technologies from the fields of CCDs and infrared (IR) multiplexers. Measurements were performed by exposing the detector to an Ar+ ion beam of low flux. The array detector responds to both positive and negative charges and preliminary results indicate a detection limit of 100 ions. Current data indicate that the linear dynamic range of the device is over five orders of magnitude. The capability of the device to perform specialized charge read out modes could theoretically both lower the detection limit by a factor of seven and increase the linear dynamic range to nine orders of magnitude using non-destructive read outs.

Performance of an inductively coupled plasma source ion trap mass spectrometer

A unique system incorporating features of inductively coupled plasma (ICP) and ion trap mass spectrometry (ITMS) is described. The combined ICP-ITMS system is characterized instrumentally and the initial performance features are presented. Bare atomic ions are observed for all elements, including previously problematic high oxygen bond strength elements that produced high levels of MOn+ and M(OH)n+ ions in early ICP-ITMS experiments. Approximate, unoptimized detection limits of 10–500 pg ml–1 are reported. Other advantages of the system include the complete destruction of typical ICP-MS polyatomic ions and the near complete neutralization of plasma Ar+, both of which contribute to much simpler spectra below 80 m/z.