Internal energies and energy distributions were studied using the 'survival yield' method developed previously. In addition to conventional benzylpyridinium salts, protonated esters (fragmenting by rearrangement) and protonated leucine enkephalin were also used, extending the validity of the technique. Fragmentation processes were studied in the cone voltage region and modeled by the RRKM-based MassKinetics program. The results show that the shapes of the energy distributions are similar to thermal distributions. The mean internal energies are very similar for all compound classes studied, and show a linear increase with collision energy in the 10-50 eV region.

The present article reviews the mass spectrometric fragmentation processes and fragmentation energetics of leucine enkephalin, a commonly used peptide, which has been studied in detail and has often been used as a standard or reference compound to test novel instrumentation, new methodologies, or to tune instruments. The main purpose of the article is to facilitate its use as a reference material; therefore, all available mass spectrometry-related information on leucine enkephalin has been critically reviewed and summarized. The fragmentation mechanism of leucine enkephalin is typical for a small peptide; but is understood far better than that of most other compounds. Because ion ratios in the MS/MS spectra indicate the degree of excitation, leucine enkephalin is often used as a thermometer molecule in electrospray or matrix-assisted laser desorption ionization (ESI or MALDI). Other parameters described for leucine enkephalin include collisional cross-section and energy transfer; proton affinity and gas-phase basicity; radiative cooling rate; and vibrational frequencies. The lowest-energy fragmentation channel of leucine enkephalin is the MH(+)  → b(4) process. All available data for this process have been re-evaluated. It was found that, although the published E(a) values were significantly different, the corresponding Gibbs free energy change showed good agreement (1.32 ± 0.07 eV) in various studies. Temperature- and energy-dependent rate constants were re-evaluated with an Arrhenius plot. The plot showed good linear correlation among all data (R(2)  = 0.97), spanned over a 9 orders of magnitude range in the rate constants and yielded 1.14 eV activation energy and 10(11.0)  sec(-1) pre-exponential factor. Accuracy (including random and systematic errors, with a 95% confidence interval) is ±0.05 eV and 10(±0.5)  sec(-1), respectively. The activation entropy at 470 K that corresponds to this reaction is -38.1 ± 9.6 J mol(-1)  K(-1). We believe that these re-evaluated values are by far the most accurate activation parameters available at present for a protonated peptide and can be considered as "consensus" values; results on other processes might be compared to this reference value.

Reported here is a study of the effects of liquid helium cooling on the fragmentation of ions formed by electron impact mass ionization. The molecules of interest are picked up by the helium nanodroplets as they pass through a low pressure oven. Electron impact ionization of a helium atom in the droplet is followed by resonant charge transfer to neighboring helium atoms. When the charge is transferred to the target molecule, the difference in the ionization potentials between helium and the molecule results in the formation of a vibrationally hot ion. In isolation, the hot parent ion would undergo subsequent fragmentation. On the other hand, if the cooling due to the helium is fast enough, the parent ion will be actively cooled before fragmentation occurs. The target molecule used in the present study is triphenylmethanol (TPM), an important species in synthetic chemistry, used to sterically protect hydroxyl groups. Threshold PhotoElectron PhotoIon COincidence (TPEPICO) experiments are also reported for gas-phase TPM to help quantify the ion energetics resulting from the cooling effects of the helium droplets.

Uric acid is a potent antioxidant and scavenger of singlet oxygen and other radicals in humans. Allantoin, the predominant product of free radical-induced oxidation of uric acid is efficiently excreted in the urine and has potential as a biomarker of oxidative stress. We developed a rapid and specific assay for urinary allantoin using ultra performance liquid chromatography-tandem mass spectrometry suitable for high-throughput clinical studies. The method required minimal sample preparation, was accurate (mean error 6%), precise (intra- and inter-day imprecision: <8%) and sensitive (limit of detection: 0.06 pmol). Allantoin levels measured in control samples were comparable to literature values.

F2-isoprostanes are useful biomarkers of oxidative status in humans. We developed an ultraperformance liquid chromatography-tandem mass spectrometric (UPLC-MS/MS) method to quantify 2,3-dinor-8-iso prostaglandin F 2α , an urinary metabolite of 8-iso-prostaglandin F 2α. Urine was purified by solid phase extraction and analyzed by UPLC-MS/MS with negative ion-electrospray ionization. The method was robust with a mean inaccuracy of 9%, interday and intraday imprecision of 7.5% or lower, and a lower limit of quantification of 0.5 μg/L, equivalent to 0.04 pmol injected onto the column. An analysis time of 6 minutes was shorter than previously published methods and amenable to large studies. Keywords Oxidative damage biomarker; UPLC-MS/MS; F2-isoprostanesVariability in the capacity of antioxidant defense systems to protect against reactive oxygen species (ROS) may contribute to individual differences in susceptibility to chronic diseases [1]. F2-isoprostanes (iPF 2α ), non-enzymatic products of arachidonic acid peroxidation, are considered sensitive and specific biomarkers of oxidative status. Among them, iPF 2α -III (8-iso-PGF 2α ) and iPF 2α -VI have been identified in human urine [2]. Increased iPF 2α -III levels have been reported in patients with cardiovascular disease [3], Alzheimer disease [4], and type 2 diabetics [5] and increased iPF 2α -VI in patients with Down syndrome [6]. Both were elevated in patients with lung disorders [7] and in heavy smokers [8;9]. 2,3-dinor-8-iso-prostaglandin F 2α (2,3-dinor-iPF 2α -III) is a metabolite of 8-iso-PGF 2α in humans with greater urinary excretion than the parent compound. It is useful as a biomarker for the overall formation of 8-iso-PGF 2α and by extension, lipid peroxidation in vivo. Traditional analytical approaches for the quantification of isoprostanes include immunological methods (ELISA and RIA) and mass Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. . The signal ratio of the primary transitions was used to quantify the analyte and the secondary transitions were used as purity checks. The peak area ratio of the primary to secondary transition was determined for each specimen and compared with a pre-determined value derived from calibration samples. This ratio remained consistent over time in calibration samples (mean ratio: 10.3±1.51; RSD =15%, n=49, over 18 weeks). Deviation from this ratio would suggest the presence of an interfering compound. SRM chromatograms corresponding to the primary and secondary transitions for 2,3-dinor-iPF 2α -III and iPF 2α -III-d 4 are shown in Fig 1A (standard mix) and 1B (contro...