We examined the 229 nm deep-ultraviolet resonance Raman (DUVRR) spectra of solution and solid-state trinitrotoluene (TNT) and its solution and solid-state photochemistry. Although TNT photodegrades with a solution quantum yield of ϕ ∼ 0.015, the initial photoproducts show DUVRR spectra extraordinarily similar to pure TNT, due to the similar photoproduct enhancement of the -NO2 stretching vibrations. This results in TNT-like DUVRR spectra even after complete TNT photolysis. These ultraviolet resonance Raman spectral bands enable DUVRR of trace as well as DUVRR standoff TNT detection. We determined the structure of various initial TNT photoproducts by using liquid chromatography-mass spectrometry and tandem mass spectrometry. Similar TNT DUVRR spectra and photoproducts are observed in the solution and solid states.
Selected reaction monitoring (SRM) of quinolone drugs showed different sensitivities in aqueous solution vs. biological extract. The authors suggested formation of two singly protonated molecules with different behavior, one undergoing loss of H(2)O and the other loss of CO(2), so that SRM transitions might depend on the ratios of these forms generated by the electrospray. These surprising results prompted us to re-examine several quinolone drugs and some simpler compounds to further elucidate the mechanisms. We find that the relative contributions of loss of H(2)O vs. loss of CO(2) in tandem mass spectrometric (MS/MS) experiments depend not only on molecular structure and collision energy, but also, in certain cases, on the cone voltage. We further find that many product ions formed by loss of H(2)O can reattach a water molecule in the collision cell, whereas ions formed by loss of CO(2) do not. Since reattachment of H(2)O can occur after water loss in the cone region and prior to selection of the precursor ion, this effect leads to the dependence of MS/MS spectra on the cone voltage used in creating the precursor ion, which explains the formerly observed effect on SRM ratios. Our results support the earlier conclusion that varying amounts of two ions of the same m/z value are responsible for problems in the analysis of these drugs, but the origin is in dehydration/rehydration reactions. Thus, SRM transitions for certain complex compounds may be comparable only when monitored under equivalent ion-forming conditions, including the voltage used in the production of the protonated molecules in the electrospray ionization (ESI) source.
A prominent dissociation path for electrospray generated tryptic peptide ions is the dissociation of the peptide bond linking the second and third residues from the amino-terminus. The formation of the resulting b 2 and y n-2 fragments has been rationalized by specific facile mechanisms. An examination of spectral libraries shows that this path predominates in diprotonated peptides composed of 12 or fewer residues, with the notable exception of peptides containing glutamine or glutamic acid at the N-terminus. To elucidate the mechanism by which these amino acids affect peptide fragmentation, we synthesized peptides of varying size and composition and examined their MS/MS spectra as a function of collision voltage in a triple quadrupole mass spectrometer. Loss of water from N-terminal glutamic acid and glutamine is observed at a lower voltage than any other fragmentation, leading to cyclization of the terminal residue. This cyclization results in the conversion of the terminal amine group to an imide, which has a lower proton affinity. As a result, the second proton is not localized at the N-terminus but is readily transferred to other sites, leading to fragmentation near the center of the peptide. eptide ion fragmentation by tandem mass spectrometry has become a routine means of determining peptide sequence for the purpose of protein identification [1]. A prominent dissociation path for electrospray-generated tryptic peptide ions is the breaking of the peptide bond linking the second and third residues from the amino-terminus [2-6]. The formation of the resulting b 2 and y n-2 fragments from diprotonated peptide ions (with n amino acid residues) has been rationalized by specific facile mechanisms [2]. It has been pointed out, however, that certain peptides ions do not fragment preferentially by this route but rather fragment at peptide bonds closer to the center of the peptide [2]. To explain the difference between the two groups of peptides it was suggested that the dominant b 2 ions formed from the first group have a protonated diketopiperazine structure (1) whereas b 2 ions from the other peptides have a protonated oxazolone structure (2).More recent studies, however, indicated that b 2 ions from the first group also have the oxazolone structure [5,6] so that the reason for the difference between the two groups remains unclear. Another recent study of several synthetic (Ala) x His peptide ions indicated that this fragmentation pathway diminished with increasing peptide length [4].In the present study, we attempt to distinguish between these two groups of peptides by searching for correlation between the mode of fragmentation and the amino acid sequence, mainly the amino acids at or near the N-terminus. After finding certain differences through statistical analysis of a large database of peptide MS/MS spectra, we synthesized specific peptides to study their fragmentation as a function of collision Address reprint requests to Dr.
Electronic cigarette (EC) use is gaining popularity as a substitute for conventional smoking due to the perception and evidence it represents a safer alternative. In contrast to the common perception amongst users that ECs represent no risk initial studies have revealed a complex composition of e-cigarette liquids. Conventional cigarette smoking is a known risk factor for developing bladder cancer and prior reports raise concern some of those causative compounds may exist in EC liquids or vapor. Urine samples were collected from 13 e-cigarette using subjects and 10 non e-cigarette using controls. Five known bladder carcinogens that are either present in conventional cigarettes, products of combustion, or solvents believed to be used in some e-cigarette formulations were quantified by liquid chromatography – mass spectrometry (LC-MS). Analysis of e-cigarette user urine revealed the presence of two carcinogenic compounds, o-toluidine and 2-naphthylamine, at a mean 2.3 and 1.3 fold higher concentration (p-value of 0.0013 and 0.014 respectively). Many of these subjects (9/13) were long term nonsmokers (>12 months). Further study is needed to clarify the safety profile of e-cigarettes and their contribution to the development of bladder cancer given the greater concentration of carcinogenic aromatic amines in the urine of e-cigarette users.
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