John E. Bartmess

Empirical methods for determination of ionization gauge relative sensitivities for different gases

Abstract The relative sensitivities of a Bayard-Alpert ionization gauge for various organic molecules have been measured. There is a good correlation with total ionization cross section at 75 eV. For monofunctional compounds a correlation with number of electrons is seen with different functional groups on different lines. The best general correlation is with the polarizability, α, with R x =0.36 α +0.30, where R x is the chemical sensitivity relative to N 2 =1.00. Alkanes and the noble gases have slightly larger R x values than predicted by this equation.

Ionization Mechanism of Negative Ion-Direct Analysis in Real Time: A Comparative Study with Negative Ion-Atmospheric Pressure Photoionization

The ionization mechanism of negative ion-direct analysis in real time (NI-DART) has been investigated using over 42 compounds, including fullerenes, perfluorocarbons (PFC), organic explosives, phenols, pentafluorobenzyl (PFB) derivatized phenols, anilines, and carboxylic acids, which were previously studied by negative ion-atmospheric pressure photoionization (NI-APPI). NI-DART generated ionization products similar to NI-APPI, which led to four ionization mechanisms, including electron capture (EC), dissociative EC, proton transfer, and anion attachment. These four ionization mechanisms make both NI-DART and NI-APPI capable of ionizing a wider range of compounds than negative ion-atmospheric pressure chemical ionization (APCI) or negative ion-electrospray ionization (ESI). As the operation of NI-DART is much easier than that of NI-APPI and the gas-phase ion chemistry of NI-DART is more easily manipulated than that of NI-APPI, NI-DART can be therefore used to study in detail the ionization mechanism of LC/NI-APPI-MS, which would be a powerful methodology for the quantification of low-polarity compounds. Herein, one such application has been further demonstrated in the detection and identification of background ions from LC solvents and APPI dopants, including water, acetonitrile, chloroform, methylene chloride, methanol, 2-propanol, hexanes, heptane, cyclohexane, acetone, tetrahydrofuran (THF), 1,4-dioxane, toluene, and anisole. Possible reaction pathways leading to the formation of these background ions were further inferred. One of the conclusions from these experiments is that THF and 1,4-dioxane are inappropriate to be used as solvents and/or dopants for LC/NI-APPI-MS due to their high reactivity with source basic ions, leading to many reactant ions in the background.

Ionization mechanism of positive-ion direct analysis in real time: a transient microenvironment concept.

A transient microenvironment mechanism (TMEM) is proposed to address matrix effects for direct analysis in real time (DART). When the DART gas stream is in contact with the sample, a transient microenvironment (TME), which can shield analytes from direct ionization, may be generated through the desorption of the matrix containing the analyte. The DART gas stream can directly ionize the matrix molecules, but the analytes will be ionized primarily through gas-phase ion/molecule reactions with the matrix ions. Experimental results showed that as little as 10 nL of liquid or 10 microg of solid was able to generate an efficient TME. Generated TMEs were able to control the ionization of an analyte below an analyte-to-matrix ratio that was dependent on the DART temperature and the boiling points of the analyte and matrix. TMEs generated by common solvents were studied in detail. The ionization of both polar and nonpolar compounds, present in a solvent or another analyte below a ratio of 1:100, were found to be mainly controlled by the generated TMEs at a DART temperature of 300 degrees C.

Liquid chromatography/negative ion atmospheric pressure photoionization mass spectrometry: a highly sensitive method for the analysis of organic explosives

Gas chromatography/mass spectrometry (GC/MS) is applied to the analysis of volatile and thermally stable compounds, while liquid chromatography/atmospheric pressure chemical ionization mass spectrometry (LC/APCI-MS) and liquid chromatography/electrospray ionization mass spectrometry (LC/ESI-MS) are preferred for the analysis of compounds with solution acid-base chemistry. Because organic explosives are compounds with low polarity and some of them are thermally labile, they have not been very well analyzed by GC/MS, LC/APCI-MS and LC/ESI-MS. Herein, we demonstrate liquid chromatography/negative ion atmospheric pressure photoionization mass spectrometry (LC/NI-APPI-MS) as a novel and highly sensitive method for their analysis. Using LC/NI-APPI-MS, limits of quantification (LOQs) of nitroaromatics and nitramines down to the middle pg range have been achieved in full MS scan mode, which are approximately one order to two orders magnitude lower than those previously reported using GC/MS or LC/APCI-MS. The calibration dynamic ranges achieved by LC/NI-APPI-MS are also wider than those using GC/MS and LC/APCI-MS. The reproducibility of LC/NI-APPI-MS is also very reliable, with the intraday and interday variabilities by coefficient of variation (CV) of 0.2-3.4% and 0.6-1.9% for 2,4,6-trinitrotoluene (2,4,6-TNT).

Protonated water and protonated methanol cluster decompositions in a quadrupole ion trap

Abstract Electrospray mass spectra of water and methanol have been studied with a quadrupole ion trap operated with background gas pressures of 2 × 10 −5 −2 × 10 −3 Torr. In each case, the most abundant ions observed consist of protonated clusters of varying size, e.g. (H 2 O) n H + in which n ranges up into the twenties. For clusters of n > 6 or so, rapid desolvation is observed. For intermediate values of n , the desolvation rate constants can be readily measured in the typical time frame of a quadrupole ion trap experiment. The internal temperature of the cluster ions can be estimated from the known thermochemical parameters and the measured decomposition rate constants. In all cases, the temperature exceeds that of the bath gas but is significantly less than 1000 K. It is also noted that the ion temperature is inversely proportional to the logarithm of the bath gas pressure.

Equilibrium and double resonance in the unquenched mode in trapped ion cyclotron resonance spectrometry

Abstract If a MeIver-type trapped ion cyclotron resonance (ICR) cell is used without a quenching pulse, a steady-state abundance of ions is achieved, based on a balance between production and coIlisional ion loss to the walls. The intensity of mass signals in this unquenched mode is shown to be related to the kinetic and equilibrium parameters involved, although not in a linear fashion. Double resonance signals have meaning in certain cases. All transient intermediates and many slowly formed products are observed when using this mode.

The gas-phase acidities of long chain alcohols

Abstract The gas-phase acidities of the C5-C9 normal alcohols, along with some more highly branched alcohols, have been measured by the equilibrium method in an ICR spectrometer. The acidities obtained for the 1-alkanols are consistently weaker than those obtained by the kinetic method, implying that some effect is altering the structure or dynamics of the transition state in the kinetic method.

Quantitative Real-Time Monitoring of Chemical Reactions by Autosampling Flow Injection Analysis Coupled with Atmospheric Pressure Chemical Ionization Mass Spectrometry

Although qualitative and/or semiquantitative real-time monitoring of chemical reactions have been reported with a few mass spectrometric approaches, to our knowledge, no quantitative mass spectrometric approach has been reported so far to have a calibration valid up to molar concentrations as required by process control. This is mostly due to the absence of a practical solution that could well address the sample overloading issue. In this study, a novel autosampling flow injection analysis coupled with an atmospheric pressure chemical ionization mass spectrometry (FIA/APCI-MS) system, consisting of a 1 μL automatic internal sample injector, a postinjection splitter with 1:10 splitting ratio, and a detached APCI source connected to the mass spectrometer using a 4.5 in. long, 0.042 in. inner diameter (ID) stainless-steel capillary, was thus introduced. Using this system together with an optional FIA solvent modifier, e.g., 0.05% (v/v) isopropylamine, a linear quantitative calibration up to molar concentration has been achieved with 3.4-7.2% relative standard deviations (RSDs) for 4 replicates. As a result, quantitative real-time monitoring of a model reaction was successfully performed at the 1.63 M level. It is expected that this novel autosampling FIA/APCI-MS system can be used in quantitative real-time monitoring of a wide range of reactions under diverse reaction conditions.

Triphenylcyclopropenide Anion in the Gas Phase

Abstract The triphenylcyclopropenide anion has been generated in the gas phase, and the approximate gas phase acidity of triphenylcyclopropene is reported. The dimethylphenylcyclopropenide anion has also been formed, but it appears to undergo a rearrangement that is induced by the acids used to probe its base strength.

Gas-phase ion-molecule chemistry of borate and boronate esters.

Borate esters B(OR)3 and boronate esters RB(OR)2 undergo ion-molecule reactions to yield both addition products (by an implied radiative emission mechanism), ligand exchange, and proton transfer products, in both positive and negative ion modes. Although an acidity for CH3B(OR)2 could not be determined, HOB(OR)2 has an acidity between acetaldehyde and nitromethane. In light of the negligible polar electron acceptor properties of the -B(OR)2 group, that functionality must therefore be one of the best resonance electron acceptor groups known, almost half again as effective as the nitro group.