Selected Ion Flow Tube Mass Spectrometry as a Tool to Understand Hydride Atomization and the Fate of Free Analyte Atoms in an Externally Heated Quartz Tube Atomizer.

Hydride atomization and the fate of free analyte atoms in an externally heated quartz tube atomizer (QTA) were investigated employing selected ion flow tube mass spectrometry (SIFT-MS). SIFT-MS proved to be ideally suited to study water concentration in gases leaving the atomizer. This made it possible to quantify the oxygen "contaminant" flow rate to QTA as 0.04-0.05 mL min-1. This is valid for typical conditions of hydride generation. Most significantly, studies of temperature influence on water concentration resulted in detailed insight into hydrogen radical-forming reactions between oxygen and hydrogen. Minimum QTA temperatures required to generate hydrogen radicals under a variety of different flow rates and compositions of the QTA atmosphere were found to be in the range between 585 and 800 °C. The ability of SIFT-MS to detect extremely low concentrations of arsane and selane was employed to quantify the fraction of As and Se removed from the QTA in the form of hydride in dependence on QTA temperature under typical conditions of hydride generation. It was found that free As atoms formed by atomization of arsane decay to different species than to arsane. In the case of selane under typical atomization conditions, the efficiency of the decay of free Se atoms to selane was between 50 and 100% in dependence on actual flow rates and compositions of the QTA atmosphere.

Atomization of As and Se volatile species in a dielectric barrier discharge atomizer after hydride generation: Fate of analyte studied by selected ion flow tube mass spectrometry.

Atomization of hydrides and their methylated analogues in a dielectric barrier discharge (DBD) plasma atomizer was investigated. Selected ion flow tube mass spectrometry (SIFT-MS) was chosen as a detector being capable of selective detection of non-atomized original volatile species allowing thus direct quantification of atomization efficiency. Selenium hydride (SeH2) and three volatile arsenic species, namely arsenic hydride (AsH3), monomethylarsane (CH3AsH2) and dimethylarsane ((CH3)2AsH), were selected as model analytes. The mechanistic study performed contributes to understanding of the atomization processes in atomic absorption spectrometry (AAS). The presented results are compatible with a complete atomization of arsenic hydride as well as its methylated analogues and with atomization efficiency of SeH2 below 80%. Using AsH3 as a model analyte and a combination of AAS and SIFT-MS detectors has revealed that the hydride is not atomized, but decomposed in the DBD atomizer in absence of hydrogen fraction in the carrier gas. Apart from investigation of analyte atomization, the SIFT-MS detector is capable of quantitative determination of water vapor content being either transported to, or produced in the atomizer. This information is crucial especially in the case of the low-power/temperature DBD atomizer since its performance is sensitive to the amount of water vapor introduced into the plasma.

Ligand Switching Ion Chemistry: An SIFDT Case Study of the Primary and Secondary Reactions of Protonated Acetic Acid Hydrates with Acetone.

A study was performed of the reactions of protonated acetic acid hydrates, CH3COOHH+(H2O)n, with acetone molecules, CH3COCH3, using a selected ion flow-drift tube (SIFDT). The rationale for this study is that hydrated protonated organic molecules are major product ions in secondary electrospray ionization mass spectrometry (SESI-MS) and ion mobility spectrometry (IMS). Yet the formation and reactivity of these hydrates are only poorly understood, and kinetics data are only sparse. The existing SIFDT instrument in our laboratory was upgraded to include an octupole ion guide and a separate drift tube by which hydrated protonated ions can be selectively injected into the drift tube reactor and their reactions with molecules studied under controlled conditions. This case study shows that, in these hydrated ion reactions with acetone molecules, the dominant reaction process is ligand switching producing mostly proton-bound dimer ions (CH3COCH3)H+(CH3COOH), with minor branching into (CH3COCH3)H+(H2O). This switching reaction was observed to proceed at the collisional rate, while other studied hydrated ions reacted more slowly. An attempt is made to understand the reaction mechanisms and the structures of the reaction intermediate ions at the molecular level. Secondary switching reactions of the asymmetric proton-bound dimer ions lead to a formation of strongly bound symmetrical dimers (CH3COCH3)2H+, the terminating ion in this ion chemistry. These results strongly suggest that, in SESI-MS and IMS, the presence of a polar compound, like acetone in exhaled breath, can suppress the analyte ions of low concentration compounds like acetic acid thus compromising their quantification.

Ion chemistry of phthalates in selected ion flow tube mass spectrometry: isomeric effects and secondary reactions with water vapour.

Phthalates are widely industrially used and their toxicity is of serious environmental and public health concern. Chemical ionization (CI) analytical techniques offer the potential to detect and monitor traces of phthalate vapours in air or sample headspace in real time. Promising techniques include selected ion flow tube mass spectrometry (SIFT-MS), proton transfer reaction mass spectrometry (PTR-MS) and ion mobility spectrometry (IMS). To facilitate such analyses, reactions of H3O+, O2+ and NO+ reagent ions with phthalate molecules need to be understood. Thus, the ion chemistry of dimethyl phthalate isomers (dimethyl phthalate, DMP - ortho; dimethyl isophthalate, DMIP - meta; dimethyl terephthalate, DMTP - para), diethyl phthalate (DEP), dipropyl phthalate (DPP) and dibutyl phthalate (DBP) was studied by SIFT-MS. Reactions of H3O+, O2+ and NO+ with these phthalate molecules M were found to produce the characteristic primary ion products MH+, M+ and MNO+, respectively. In addition, a dissociation process forming the (M-OR)+ fragment was observed. For phthalates with longer alkyl chains, mainly DPP and DBP, a secondary dissociation channel triggered by the McLafferty rearrangement was also observed. However, this is dominant only for the more energetic O2+ reactions with phthalates, additionally resulting in a recognisable formation of the protonated phthalate anhydride. For the NO+ reagent ions, the McLafferty rearrangement makes only a minor contribution and for H3O+, it was not observed. Experiments on the effect of water vapour on this ion chemistry have shown that protonated DMIP and DMTP efficiently associate with H2O forming the DMIP·H+H2O, DMIP·H+(H2O)2 and DMTP·H+H2O cluster ions, whilst the protonated ortho DMP isomer as well as other ortho phthalates DEP, DPP and DBP does not associate with H2O. The results indicate that the degree of hydration can be used to identify specific phthalate isomers in CI.

Chemical ionization of glyoxal and formaldehyde with H3O+ ions using SIFT-MS under variable system humidity.

Glyoxal (C2H2O2) is a highly reactive molecule present at trace levels in specific gaseous environments. For analyses by chemical ionization mass spectrometry, it is important to understand the gas-phase chemistry initiated by reactions of H3O+ ions with C2H2O2 molecules in the presence of water vapour. This chemistry was studied at variable humidity using a selected ion flow tube, SIFT. The initial step is a proton transfer reaction forming protonated glyoxal C2H3O2+. The second step, in the presence of water vapour, is the association forming C2H3O2+(H2O) and interestingly also protonated formaldehyde CH2OH+. Hydrated protonated formaldehyde CH2OH+(H2O) was also observed. Relative signals of these four ionic products were studied at the end of the flow tube where the reactions took place during 0.3 ms in helium carrier gas (1.5 mbar, 300 K) as the water vapour number density varied up to 1014 cm-3. The data were interpreted using numerical kinetics modelling of the reaction sequences and the mechanisms and kinetics of the reaction steps were characterised. The results thus facilitate SIFT-MS analyses of glyoxal in humid air whilst drawing attention to ion overlaps with formaldehyde products.

Dissociation of dicyclohexyl phthalate molecule induced by low-energy electron impact.

Experimental investigation of electron ionization (EI) of and electron attachment (EA) onto dicyclohexyl phthalate (DCHP) was carried out using a crossed electron and molecular beam technique. Formation of positive and negative ions by EI and EA with the corresponding dissociation processes was studied and discussed. Due to a low ion yield of the parent positive ion, we were not able to estimate the ionization energy of DCHP. However, we estimated the appearance energies for the protonated phthalate anhydride (m/z 149) to be 10.5 eV and other significant ionic fragments of m/z 249 [DCHP-(R-2H)]+, m/z 167 [DCHP-(2R-3H)]+, and m/z 83 [C6H11]+. The reaction mechanisms of the dissociative ionization process were discussed. In the case of negative ions, we estimated the relative cross sections for a transient negative ion (TNI) and for several detected ions. At low electron energies (close to 0 eV), the TNI of DCHP molecules was the dominant ion, with products of dissociative EA dominating in broad resonances at 7.5 and 8.5 eV.

Electron interaction with copper(II) carboxylate compounds.

In the present study we have performed electron collision experiments with copper carboxylate complexes: [Cu2(t-BuNH2)2(µ-O2CC2F5)4], [Cu2(s-BuNH2)2(µ-O2CC2F5)4], [Cu2(EtNH2)2(µ-O2CC2F5)4], and [Cu2(µ-O2CC2F5)4]. Mass spectrometry was used to identify the fragmentation pattern of the coordination compounds produced in crossed electron - molecular beam experiments and to measure the dependence of ion yields of positive and negative ions on the electron energy. The dissociation pattern of positive ions contains a sequential loss of both the carboxylate ligands and/or the amine ligands from the complexes. Moreover, the fragmentation of the ligands themselves is visible in the mass spectrum below m/z 140. For the studied complexes the metallated ions containing both ligands, e.g., Cu2(O2CC2F5)(RNH2)+, Cu2(O2CC2F5)3(RNH2)2+ confirm the evaporation of whole complex molecules. A significant production of Cu+ ion was observed only for [Cu2(µ-O2CC2F5)4], a weak yield was detected for [Cu2(EtNH2)2(µ-O2CC2F5)4] as well. The dissociative electron attachment processes leading to formation of negative ions are similar for all investigated molecules as the highest unoccupied molecular orbital of the studied complexes has Cu-N and Cu-O antibonding character. For all complexes, formation of the Cu2(O2CC2F5)4-• anion is observed together with mononuclear DEA fragments Cu(O2CC2F5)3-, Cu(O2CC2F5)2- and Cu(O2CC2F5)-•. All dominant DEA fragments of these complexes are formed through single particle resonant processes close to 0 eV.

Dissociative electron attachment to hexafluoroacetylacetone and its bidentate metal complexes M(hfac)2; M = Cu, Pd.

Beta-diketones are a versatile class of compounds that can complex almost any metal in the periodic table of elements. Their metal complexes are found to be fairly stable and generally have sufficient vapor pressure for deposition techniques requiring volatile metal sources. Motivated by the potential role of low energy electrons in focused electron beam induced deposition, we have carried out a crossed electron∕molecular beam study on the dissociative electron attachment and non-dissociative electron attachment (NDEA) to hexafluoroacetylacetone (HFAc) and its bidentate metal complexes: bis-hexafluoroacetylacetonate copper(II), Cu(hfac)2 and bis-hexafluoroacetylacetonate palladium(II), Pd(hfac)2. The relative ion yield curves for the native precursor to the ligand as well as its stable, 16 valence electron Pd(II) complex and open shell, 17 valence electron Cu(II) complex, are presented and compared. For HFAc, the loss of HF leads to the dominant anion observed, and while NDEA is only weakly pronounced for Pd(hfac)2 and loss of hfac(-) is the main dissociation channel, [Cu(hfac)2](-) formation from Cu(hfac)2 dominates. A comparison of the ion yield curves and the associated resonances gives insight into the role of the ligand in the attachment process and highlights the influence of the central metal atom.