Tim M. Brewer

Online mercury speciation through liquid chromatography with particle beam/electron ionization mass spectrometry detection

The ability of particle beam/electron ionization-mass spectrometry (PB/EI-MS) to provide elemental and molecular information for a sample solution has been evaluated for the speciation of inorganic and organic mercury compounds. Specifically, the EI process yields mass spectra which reflect the chemical species eluting from the chromatographic column, either atomic or molecular. A detailed evaluation of source temperature and electron energy parameters has been performed for the PB/EI-MS method. Preliminary studies demonstrated that the approach can be utilized for the determination of mercury at μg mL−1 levels. A non-linear response at low concentrations (0.1–1 μg mL−1) of mercury was observed due to poor transport of small particles through the PB interface. The use of KCl as a carrier agent to enhance particle transport was investigated. The analyte responses for mercury species showed higher sensitivity with good linearity upon KCl addition, providing limits of detection on the ng mL−1 level (sub-ng absolute). The three mercury species (inorganic, phenyl- and methylmercury) were separated using a reversed phase (C18) column with a total elution time of less than 11 min and the species were detected using PB/EI-MS. Post-column addition of the KCl carrier to the mobile phase was accomplished with a HPLC pump and a tee connection. It is believed that the PB/EI-MS technique is well suited not only for speciation of mercury, but also for obtaining comprehensive speciation information via atomic and molecular mass spectral information of diverse species, and thus can be used to solve a number of speciation challenges.

Analysis and Mechanisms of Cyclotrimethylenetrinitramine Ion Formation in Desorption Electrospray Ionization

The general ion chemistry of the explosive molecule cyclotrimethylenetrinitramine (RDX) was studied with an atmospheric pressure ionization mass spectrometer (API-MS) fitted with a desorption electrospray ionization (DESI) source. Explosive molecule chemistry within trace detection techniques such as ion mobility spectrometry (IMS) is an area of intense interest because of the widespread deployment of IMS-based explosive detectors for counterterrorism efforts. As in IMS, the DESI-MS experiments analyze material that starts in the solid phase and is detected in the gas phase. Using the unique chemical characterization inherent in mass spectrometry, information pertinent to the atmospheric ionization of RDX is obtained in order to help explain the behavior of explosive molecule signatures observed within IMS experiments. Qualitative and quantitative information was obtained over 3 orders of magnitude of deposited mass (nanograms to greater than micrograms). A method was developed to use the relative integrated mass spectral peak intensities of RDX monomer and dimer chloride adducts to determine the amount of explosive present on a surface. The ratio of RDX dimer chloride adduct to monomer chloride adduct ranged from 0.1 for 15 ng to 1.0 for 1.5 microg of deposited explosive. The results are explained in terms of mechanisms reported in the literature for electrospray ionization (ESI), as well as by simple solution dynamics and the interaction chemistry between RDX molecules. On the basis of all available data, the RDX dimer chloride adduct becomes disproportionately favored over the monomer chloride adduct at larger amounts of explosive because of effects related to desorbed droplet charge, solvent declustering, and the strong intermolecular forces between RDX molecules in the solid, liquid, and gas phases. Additionally, considerations for optimization of the DESI-MS process are described in order to increase the practicality for this technique as an explosives detection tool in the public domain.

Primary and secondary droplet and charge transmission characteristics of desorption electro-flow focusing ionization

We present the investigation of droplet charging and charge transmission characteristics of an electro-flow focusing nozzle for desorption-based ambient ionization mass spectrometry. The electro-flow focusing geometry utilizes a concentrically flowing gas to focus a charged solvent stream through a small orifice, generating a steady liquid jet and charged droplet stream that impinges and ionizes the analyte and surface. Transmitted current measurements and a scaling analysis were incorporated to decouple analyte desorption and ionization from secondary droplet charging and to identify the regimes of operation, secondary droplet charge transport characteristics, and parameters limiting transmitted charge relevant for ambient ionization mass spectrometry.

Determination of phosphorus and carbon in phosphorylated deoxynucleotides via particle beam/hollow cathode glow discharge optical emission spectroscopy (PB/HC-OES)

A new approach to the qualitative and quantitative determination of nucleic acids is presented. Particle beam hollow cathode optical emission spectroscopy (PB/HC-OES) has been applied to determinations of five nucleotides (AMP, ADP, dATP, dCTP, and dGTP) by monitoring P I 219.9 nm and C I 193.0 nm optical emission. A high level of correlation exists between the phosphorus emission response and the degree of phosphorylation, demonstrating the technique’s ability to produce vital qualitative and quantitative information for specific nucleotides, with detection limits on the single-nanogram level for phosphorus (9 ng) and picogram level for carbon (20 pg), respectively, with RSD <12% for triplicate injections over a concentration range of 10−4–101 μg mL−1. The stoichiometric response suggests that it is possible to identify the various nucleotides by the amount of phosphate present within the nucleotide. Empirical formula calculations on P I and C I emission response ratios demonstrate the potential of the technique as an element specific detector for liquid chromatography (LC).

On-line separation and identification of inorganic and organic arsenic species in ethanolic kelp and bladderwrack extracts through liquid chromatography/particle beam-electron ionization mass spectrometry (LC/PB-EIMS).

Ion-pair, reversed-phase high-performance liquid chromatography (IP-RP-HPLC) and ion exchange chromatography (IEC), coupled to particle beam-electron ionization mass spectrometry (PB-EIMS), are employed for the separation and identification of inorganic and organic arsenic compounds (As (III) chloride, arsenobetaine (AB), dimethylarsinic acid (DMA) and an arsenosugar (oxo-arsenosugar-glycerol, As 328) in commercial ethanolic kelp and bladderwrack extracts. The reversed-phase separation was performed on a C18 column using an isocratic mobile phase composition of water:methanol (96:4) containing 0.1% of trifluoroacetic acid (TFA) as an ion-pairing agent. The flow rate was optimized at 0.7 mL min−1 and the separation accomplished in less than 8 minutes. Baseline resolution was achieved in an elution window of about 4 minutes. The ion-exchange separation was performed on an anion-exchange column using a gradient mobile phase composition of 0.5 mM nitric acid containing 2% methanol and 50 mM nitric acid. The separation was accomplished in less than 8 minutes at a flow rate of 0.9 mL min−1. The eluted species were detected and identified using a PB-EIMS system providing species-specific information. The influence of various instrument parameters was evaluated to achieve optimal mass spectrometric response for the test compounds. The absolute detection limits of arsenic species with HPLC/PB-EIMS were 0.03, 0.05, 0.008 ng and 0.005 (5 µL injection) in the SIM mode and 0.1, 0.14, 0.04 and 0.01 ng in the TIC mode for As (III), DMA, AB and As 328 respectively. These investigations revealed that the majority (90–95%) of the arsenic present in the ethanolic extract samples is in the form of inorganic arsenic, with minor amounts of As (5–10% of the total As) present in the form of DMA. There were no detectable levels of AB and arsenosugars. The total arsenic content in the ethanolic extracts was verified by ICP-OES and validated by analyzing NIST SRM 3241 Ephedra sinica Stapf Native Extract and SRM 3243 Ephedra-Containing Solid Oral Dosage Form. These investigations indicate that PB-EIMS is a viable approach for comprehensive speciation of various botanical extracts.

Determination of “free” iron and iron bound in metalloproteins via liquid chromatography separation and inductively coupled plasma-optical emission spectroscopy (LC-ICP-OES) and particle beam/hollow cathode-optical emission spectroscopy (LC-PB/HC-OES) techniques

The abilities of liquid chromatography-particle beam/hollow cathode-optical emission spectrometry (LC-PB/HC-OES) and liquid chromatography-inductively coupled plasma-optical emission spectroscopy (LC-ICP-OES) techniques are demonstrated for the quantitative determination of free iron and bound iron in metalloproteins. Separations were performed in the size exclusion and reversed-phase chromatographic modes. Myoglobin, holo-transferrin and iron(II) chloride were separated by size exclusion chromatography and the eluent species were detected by both PB/HC-OES and ICP-OES through the Fe I 259.9 nm and S I 180.7 nm optical emission. Sulfur optical emission was monitored as means of protein identification through the Fe/S empirical formula differences, with the absence of S emission used to identify the “free” Fe. Both techniques demonstrated detection limits for triplicate injections on the nanogram level for iron (0.9 ng mL−1 ICP-OES and 41.9 ng mL−1 PB/HC-OES) over the concentration range of 0.1 ng mL−1–100 μg mL−1 for iron and iron metalloproteins. The corresponding values for sulfur in the proteins were more similar for the two techniques, being of the order of 20 ng mL−1. The SEC retention times and peak shapes of the three analyte peaks for both techniques are similar to those determined by UV absorbance at 220 nm (Fe-containing metalloproteins) and conductivity (inorganic iron), demonstrating the ability of both techniques to preserve the integrity of the separation. While the LODs in “bulk” sampling were lower with ICP-OES, the signal-to-noise values were comparable for both techniques when sampling chromatographic eluents in real-time. In addition, a reversed-phase separation of the same analyte mixture was carried out with Fe I 259.9 nm and S I 180.7 nm optical emission detection by the PB/HC-OES, demonstrating the compatibility of the PB/HC-OES with a wide range of solvent polarities, its ability to detect non-metals and act as a protein specific detector for liquid chromatography of proteins.

Plasma parameter and film casting optimization for the determination of particulate matter in a sol–gel matrix by radiofrequency glow discharge optical emission spectrometry (rf-GD-OES)

Solids analysis has typically suffered from the lack of true analytical blanks and the inability to study particulate solids in their native state. Reported here is an approach to the analysis of particulate matter entrapped in a sol–gel matrix by radiofrequency glow discharge optical emission spectroscopy (rf-GD-OES). Sol–gel glasses are synthesized by acid-catalyzed condensation of methyltrimethoxysilane (MTMOS) and spun cast onto glass substrates. Slurries of powdered NIST standard reference materials (SRMs) 1884a Portland Cement and 1648 Urban Particulate Matter were incorporated into the films and analyzed for both main and trace element components. Cast films were analyzed to determine the discharge operating conditions and effects of particle size and deposited layer thickness while monitoring optical emission lines of a number of analytes. Using the sol–gel method, analytical blanks were obtained by use of an undoped sol–gel. Detection limits were determined for minor elements using background/blank subtraction and were found to be in the range of 1–12 μg g−1 in Portland Cement for elements Al, Ca, P, Mn and Fe, which corresponds to single-nanogram absolute detection limits.

Ambient low temperature plasma etching of polymer films for secondary ion mass spectrometry molecular depth profiling.

The feasibility of a low temperature plasma (LTP) probe as a way to prepare polymer bevel cross sections for secondary ion mass spectrometry (SIMS) applications was investigated. Poly(lactic acid) and poly(methyl methacrylate) films were etched using He LTP, and the resulting crater walls were depth profiled using time-of-flight secondary ion mass spectrometry (ToF-SIMS) to examine changes in chemistry over the depth of the film. ToF-SIMS results showed that while exposure to even 1 s of plasma resulted in integration of atmospheric nitrogen and contaminants to the newly exposed surface, the actual chemical modification to the polymer backbone was found to be chemistry-dependent. For PLA, sample modification was confined to the top 15 nm of the PLA surface regardless of plasma exposure dose, while measurable change was not seen for PMMA. The confinement of chemical modification to 15 nm or less of the top surface suggests that LTP can be used as a simple method to prepare cross sections or bevels of polymer thin films for subsequent analysis by surface-sensitive molecular depth profiling techniques such as SIMS, X-ray photoelectron spectroscopy (XPS), and other spatially resolved mass spectrometric techniques.