Brittany D. M. Hodges

Collision-induced dissociation of intact duplex and single-stranded siRNA anions.

A tandem mass spectrometry approach is demonstrated for complete sequencing of a model small interfering RNA (siRNA) based on ion trap collisional activation of intact single-stranded anions. Various charge states of the siRNA duplex and the individual strands were generated by nanoelectrospray (nano-ESI). The siRNA duplex anions were predominantly dissociated into the sense and antisense strands by collisional activation. The characteristic fragment ions (c/y- and a-B/w-ion series) from both strands were observed when higher activation amplitude was applied and when beam-type collisional activation was examined; however, the coexistence of fragment ions from both strands complicated spectral interpretation. The effect of precursor ion charge state on the dissociation of the individual sense and antisense strand siRNA anions was studied using ion trap collision-induced dissociation under various activation amplitudes. Through the activation of relatively low charge state precursor ions at relatively low excitation energy, selective backbone dissociation predominantly via the c/y channels was achieved. By applying relatively high excitation energy, the a-B/w channels also became prominent; however, the increase in spectral complexity made complete peak assignment difficult. In order to simplify the product ion spectra, proton-transfer reactions were applied, and complete sequencing of each strand was achieved. The application of tandem mass spectrometry to intact single-stranded anions demonstrated in this study can be adapted for the rapid identification of other noncoding RNAs in RNomics studies.

Inorganic speciation in static SIMS: a comparative study between monatomic and polyatomic primary ions.

This paper addresses the use of static SIMS (S-SIMS) for the speciation of inorganic compounds. Specifically, the use of SF5+ and ReO4− polyatomic primary ions for the analysis of binary salts is compared to bombardment with Ga+. The features of the mass spectra are discussed with respect to the gain in secondary ion intensity and the increased production of molecule-specific adduct ions and structural fragments when using polyatomic instead of monatomic ion bombardment.

Ion-molecule reactions of gas-phase chromium oxyanions: CrxOyHz− + O2

Chromium oxyanions, CrxOyHz−, were generated in the gas-phase using a quadrupole ion trap secondary ion mass spectrometer (IT-SIMS), where they were reacted with O2. Only CrO2− of the Cr1OyHz− envelope was observed to react with oxygen, producing primarily CrO3−. The rate constant for the reaction of CrO2− with O2 was ∼38% of the Langevin collision constant at 310 K. CrO3−, CrO4−, and CrO4H− were unreactive with O2 in the ion trap. In contrast, Cr2O4− was observed to react with O2 producing CrO3− + CrO3 via oxidative degradation at a rate that was ∼15% efficient. The presence of background water facilitated the reaction of Cr2O4− + H2O to form Cr2O5H2−; the hydrated product ion Cr2O5H2− reacted with O2 to form Cr2O6− (with concurrent elimination of H2O) at a rate that was 6% efficient. Cr2O5− also reacted with O2 to form Cr2O7− (4% efficient) and Cr2O6− + O (2% efficient); these reactions proceeded in parallel. By comparison, Cr2O6− was unreactive with O2, and in fact, no further O2 addition could be observed for any of the Cr2O6Hz− anions. Generalizing, CrxOyHz− species that have low coordinate, low oxidation state metal centers are susceptible to O2 oxidation. However, when the metal coordination is >3, or when the formal oxidation state is ≥5, reactivity stops.

Static secondary ion mass spectrometry (S-SIMS) for the characterization of surface components in mineral particulates

The feasibility of static secondary ion mass spectrometry (S-SIMS) for the detection of molecule specific information from complex materials, such as natural clay and soil samples, has been investigated. Ion trap (IT), as well as triple quadrupole (TQ) instruments, have been used for mass analysis. Secondary ion images have been acquired using time-of-flight (TOF) S-SIMS. The generation of molecular adduct ions from thin and thick layers on the mineral substrates has been investigated using KBr as a simple model system. Results show that molecular adducts of KBr can be indeed detected from the spiked materials. However, the concentrations of the spiking solutions have to be significantly larger than expected from the surface area measured by gas adsorption techniques. In addition imaging analysis has evidenced that the detection of adduct ions in the mass spectra directly relates to the presence of local micro-crystallites.