Douglas C. Duckworth

Examination of the potential of ionic liquids for gas separations

Abstract: Ionic liquids have received increasing interest in recent years for “green” synthesis and separations because they have essentially no vapor pressure. We have begun an investigation of the potential of ionic liquids for gas separations, including the removal of carbon dioxide from stack gas generated in coal-fired power plants. In this paper, we report results from measurements of the permeance of nitrogen and carbon dioxide in supported ionic liquid membranes. Preliminary results for a porous alumina membrane saturated with l-butyl-3-methyl imidazolium bis[trinuoromethylsulfonyl] amide yielded a CO2 : N2 selectivity of 127. Using previously reported measurements of CO2 solubility in ionic liquids (1) and the measured membrane transport characteristics, a preliminary economic analysis of a separation process based on supported ionic liquid membranes has been performed. A comparison of cost estimates for this membrane-based separation to cost estimates reported for carbon dioxide removal using a conventional amine scrubbing operation shows that, with continued technology development, an ionic liquid membrane process may potentially be economically competitive with amine scrubbing. A preliminary cost estimate for an ionic liquid scrubber indicates that an ionic liquid absorption process shows less favorable economics than a supported ionic liquid membrane or an amine scrubber. However, results indicate that a more comprehensive technical and economic assessment is warranted.

Forensic glass analysis by ICP-MS: a multi-element assessment of discriminating power via analysis of variance and pairwise comparisons

Glass fragments from 81 automobile side windows were collected and analyzed by the FBI Laboratory using ICP-AES in 1991. The FBI selected 9 elements (Al, Ba, Ca, Fe, Mg, Mn, Na, Sr and Ti) to use for discrimination among the glass samples. This multi-element discrimination showed a significant improvement in the discrimination statistics over using only refractive index (RI) measurements. Oak Ridge National Laboratory (ORNL) recently analyzed fragments from 76 of the original side window fragments using inductively coupled plasma mass spectrometry (ICP-MS). The ICP-MS analyses measured 45 elements using a hierarchical sampling scheme to estimate variances due to sampled population (VP), variance due to sample dissolution and within sample heterogeneity (VD), and variance due to replicate measurements (VM). The between-to-within ratio [B/W = VP/(VD + VM)] afforded a measure of the variance within the population to that in the analytical measurement, providing a first approximation of the discriminating power of each element. Florida International University updated the RI measurements on 72 available glass fragments. These RI measurements along with ICP-AES and ICP-MS elemental analyses were used for pairwise comparisons of all possible pairs of the 72 glasses that had a complete set of measurements. The pairwise comparisons used Tukeys HSD method to compare RI and element-by-element discrimination potential of ICP-AES and ICP-MS for analyzing glass in forensic casework.

Inter-Laboratory note. Direct insertion probe for radiofrequency powered glow discharge mass spectrometry

The design of a direct insertion probe for the direct solids elemental analysis of both electrically conductive and non-conductive materials by radiofrequency powered glow discharge mass spectrometry is described. The probe allows convenient sample interchange and utilizes small sample sizes (<0.5 cm). Spatial variations of the populations of the various ionic species within the discharge are demonstrated, indicating the ability to position judiciously the discharge in relation to the sampling orifice and the need for such positioning.

Analysis of soils by glow discharge mass spectrometry

The analysis of soils by conventional solution-based techniques, such as inductively coupled plasma and thermal ionization mass spectrometry, is complicated by the need for sample dissolution or the combination of a solids atomizer with an auxiliary ionization source. Since time is an important consideration in waste remediation, there exists a need for a method of rapidly analysing many soil samples with little sample preparation; glow discharge mass spectrometry (GDMS) has the potential to meet this need. Because GDMS is a bulk solids technique, sample preparation is simplified in comparison to other methods. It appears that, even with the most difficult samples (geological materials, such as soils and volcanic rock), all that is required is grinding, drying and mixing with a conducting host material prior to electrode formation. As a first test of GDMS for soil analysis, a National Institute of Standards and Technology (NIST) Standard Reference Material (SRM) was analysed by direct current GDMS. Fifty-one elements were quantified from a single cathode using ion beam ratios and ‘standard’ relative elemental sensitivity factors (RSF). Average errors for the suite of elements were less than a factor of 4 and 1.4 for uncorrected and corrected values, respectively. User-generated RSF values were applied to the analysis of several elements in NIST SRM 2704 Buffalo River Sediment. In the absence of isobaric interferences, accuracies ranging from 0.6 to 73% were observed, demonstrating the potential of the technique for the determination of many elements. The presence of entrained water and inhomogeneity resulting from cathode preparation is thought to affect matrix-to-matrix reproducibility. While further success depends on developing means of circumventing mass spectral interferences and addressing factors affecting plasma chemistry, the immediate goal of developing a screening method for priority metals in soils was met.

Gas-phase reactions of bare and oxo-ligated actinide and lanthanide cations with pentamethylcyclopentadiene studied in a quadrupole ion trap mass spectrometer

Abstract Reactions of bare and oxo-ligated monopositive ions of uranium and thorium with 1,2,3,4,5-pentamethylcyclopentadiene, C 10 H 16 , (HCp ∗ ) were examined in a quadrupole ion trap (QIT) mass spectrometer. Representative lanthanide ions, Ln + and LnO + , and tantalum ions, Ta + and TaO + , were studied for comparison. The product branching ratios for both primary and secondary reactions of the actinide ions demonstrated gas-phase organoactinide chemistry that is quite disparate from organolanthanide chemistry under comparable conditions for this neutral reactant. Particularly revealing were product distributions for ThO + and UO + , which indicated chemical behavior similar to that of bare Sm + . We conclude that at least one valence electron at the metal center of the actinide oxide ions must remain chemically active. In the case of UO + , this provides evidence for the chemical engagement of the quasi-valence 5f electrons, which is in distinct contrast to the inert character of the 4f electrons of the lanthanides in both Ln + and LnO + . Mass-selective chemistry of two primary products, UC 10 H 10 + and UC 9 H 8 + , also showed behavior similar to that of Sm + and UO + , implying that there are two covalent organouranium bonds in these complex ions. In comparing the QIT results for the lanthanides with those from a low-pressure ion cyclotron resonance (ICR) mass spectrometry study [Organometallics 16 (1997) 3845], qualitative agreement was found, but significant quantitative differences were apparent. Based on results from collision-induced dissociation and effects of variations in bath gas pressure in the QIT, we conclude that the discrepancies arise from the very different pressure regimes in the ICR and QIT. Evidently, the QIT can be operated over a range of pressures that manifest effects of collisional cooling for some reactions. For the lowest pressure QIT experiments, the high degree of fragmentation is reminiscent of the ICR results. We propose that the QIT bath gas can essentially act as an inert “solvent,” which serves to mediate high-energy processes due to energy transfer from nascent hot intermediate products via energy-dissipating collisions.

Direct Measurement of Uranium Isotopic Ratios in Soils by Glow Discharge Mass Spectrometry

Since the current methodology mandated by the environmental protection agency (EPA) for the determination of isotope ratios of priority metals in sediments, sludges, and soils is both time consuming and labor intensive, it would clearly be of great value in environmental surveys if there were a procedure capable of direct analysis of these materials with little or no sample preparation. A method that holds promise in this regard is glow discharge mass spectrometry (GDMS). GDMS has several characteristics that make it worthy of evaluation; essentially all elements are amenable to analysis, preliminary results indicate isotopic biases are small, and the determination of isotopic ratios at the sub-ppm level is a reasonable hope since detection limits are in the sub-ppb range. Furthermore, previous studies have indicated that GDMS is a fairly precise method for providing isotopic information with good precision; precisions better than 0.05% relative standard deviation (RSD) have been obtained.

Electrochemically-Induced Reactions of Hexafluorophosphate Anions with Water in Negative Ion Electrospray Mass Spectrometry of Undiluted Ionic Liquids

The influence of water on the observed gas-phase population of negative ions in electrospray mass spectrometry was studied for the undiluted ionic liquid 1,3-butyl-methyl-imidazolium hexafluorophosphate (BMIM+PF6−). During the electrospray process, electrolytic reduction of water enhances the production of tetrafluorophosphate (F4PO−), which undergoes further reactions to produce difluorophosphate (F2PO2−) anions. These anions are observed in addition to the pre-existing hexafluorophosphate anion. The apparent substitution of two fluorine atoms with one oxygen is attributed to a series of reactions initiated by hydrolysis of hexafluorophosphate. This hydrolysis reaction was enhanced by the addition of hydroxide, formed via the hydrolysis of water or through the addition of ammonium hydroxide. The formation of FxPOy− was studied as a function of the electrospray current and solution flow rate. The mass spectral response shows a quantitative logarithmic relationship between ΣFxPOy− signal intensities (adjusted for mole equivalents of H2O required) and the amount of water present, against which the water content could be rapidly assessed. Results were found to be comparable to Karl Fischer titration data.

Factors Influencing the Quantitative Determination of Trace Elements in Soils by Glow Discharge Mass Spectrometry

The influence of oxygen content, conducting host matrix, and soil composition on the quantitative determination of trace elements in soils by glow discharge mass spectrometry was examined. Oxygen content and conducting host matrix identity influenced relative sensitivity factors employed in the quantitative interpretation of the glow discharge mass spectra. Soil composition did not influence these relative sensitivity factors. Unknown soils taken from the field were analyzed with the use of a set of relative sensitivity factors obtained from standard soils with certified compositions. The quantitative results from glow discharge mass spectrometry of these unknowns compared favorably with quantitative results from inductively coupled plasma atomic emission spectrometry and laser ablation solid sampling inductively coupled plasma mass spectrometry.

Electrospray mass spectrometry of undiluted ionic liquids

Ionic liquids have been analyzed in undiluted form using electrospray mass spectrometry (ES-MS); results indicate that signal-to-noise ratios for minor constituents are comparable to those observed in conventional, diluted ES-MS and that this approach could be readily applied for mass spectrometric analysis of ionic liquids and ionic impurities/additives dissolved therein, especially those that are solvent reactive.

Analysis of solution residues by glow discharge mass spectrometry.

A technique for the analysis of microliter volumes of solution by glow discharge mass spectrometry (GDMS) has been successfully demonstrated. Cathode preparation involves mixing an aliquot of the sample solution with a pure conducting powder, followed by drying and pressing before conventional GDMS analysis. The analyte signal at the 100-ppm level was observed to be stable to better than 5% for the duration of the analysis (30–45 min). Internal and external reproducibilities were better than 5%, and the ion signal intensity was linear with concentration over at least four orders of magnitude. Quantification was demonstrated by means of user-defined relative sensitivity factors. Relative standard deviations were better than 15% for the elements investigated, with no preconcentration of the analyte.