Radical-induced, proton-transfer-driven fragmentations in [b5 − H]˙+ ions derived from pentaalanyl tryptophan

2015 ◽  
Vol 17 (16) ◽  
pp. 10699-10707 ◽  
Author(s):  
Declan Williams ◽  
Justin Kai-Chi Lau ◽  
Junfang Zhao ◽  
Stefanie Mädler ◽  
Yating Wang ◽  
...  
Keyword(s):  
Proton Transfer ◽  
Sequence Scrambling ◽  
Product Ions

Dissociation of [b5 − H]˙+ ions show sequence scrambling with the Trp residue and radical centre always retained in the product ions.

scholarly journals Measurement of atmospheric sesquiterpenes by proton transfer reaction-mass spectrometry (PTR-MS)

2009 ◽  
Vol 2 (1) ◽  
pp. 99-112 ◽  
Author(s):  
S. Kim ◽  
T. Karl ◽  
D. Helmig ◽  
R. Daly ◽  
R. Rasmussen ◽  
...  
Keyword(s):  
Proton Transfer ◽  
Transfer Reaction ◽  
Deciduous Forest ◽  
Proton Transfer Reaction ◽  
Major Product ◽  
Reaction Mass ◽  
Diffusion Method ◽  
Research Station ◽  
Mixing Ratios ◽  
Product Ions

Abstract. The ability to measure sesquiterpenes (SQT; C15H24) by a Proton-Transfer-Reaction Mass Spectrometer (PTR-MS) was investigated. SQT calibration standards were prepared by a capillary diffusion method and the PTR-MS-estimated mixing ratios were derived from the counts of product ions and proton transfer reaction constants. These values were compared with mixing ratios determined by a calibrated Gas Chromatograph (GC) coupled to a Flame Ionization Detector (GC-FID). Product ion distributions from soft-ionization occurring in a selected ion drift tube via proton transfer were measured as a function of collision energies. Results after the consideration of the mass discrimination of the PTR-MS system suggest that quantitative SQT measurements within 20% accuracy can be achieved with PTR-MS if two major product ions (m/z 149+ and 205+), out of seven major product ions (m/z 81+, 95+, 109+, 123+, 135+, 149+ and 205+), are accounted for. Considerable fragmentation of bicyclic sesquiterpenes, i.e. β-caryophyllene and α-humulene, cause the accuracy to be reduced to 50% if only the parent ion (m/z 205+) is considered. These findings were applied to a field dataset collected above a deciduous forest at the PROPHET (Program for Research on Oxidants: Photochemistry, Emissions, and Transport) research station in 2005. Inferred average daytime ecosystem scale mixing ratios (fluxes) of isoprene, sum of monoterpenes (MT), and sum of SQT exhibited values of 15 μg m−3 (4.5 mg m−2 h−1), 1.2 μg m−3 (0.21 mg m−2 h−1), and 0.0016 μg m−3 (0.10 mg m−2 h−1), respectively. A range of MT and SQT reactivities with respect to the OH radical was calculated and compared to an earlier study inferring significantly underestimated OH reactivities due to unknown terpenes above this deciduous forest. The results indicate that incorporating these MT and SQT results can resolve ~30% of missing OH reactivity reported for this site.

scholarly journals Measuring acetic and formic acid by proton-transfer-reaction mass spectrometry: sensitivity, humidity dependence, and quantifying interferences

2015 ◽  
Vol 8 (3) ◽  
pp. 1303-1321 ◽  
Author(s):  
M. Baasandorj ◽  
D. B. Millet ◽  
L. Hu ◽  
D. Mitroo ◽  
B. J. Williams
Keyword(s):  
Mass Spectrometry ◽  
Proton Transfer ◽  
Formic Acid ◽  
Transfer Reaction ◽  
Full Range ◽  
Proton Transfer Reaction ◽  
Reaction Mass ◽  
Humidity Effect ◽  
Product Ions ◽  
Field Deployment

Abstract. We present a detailed investigation of the factors governing the quantification of formic acid (FA), acetic acid (AA), and their relevant mass analogues by proton-transfer-reaction mass spectrometry (PTR-MS), assess the underlying fragmentation pathways and humidity dependencies, and present a new method for separating FA and AA from their main isobaric interferences. PTR-MS sensitivities towards glycolaldehyde, ethyl acetate, and peroxyacetic acid at m/z 61 are comparable to that for AA; when present, these species will interfere with ambient AA measurements by PTR-MS. Likewise, when it is present, dimethyl ether can interfere with FA measurements. For a reduced electric field (E/N) of 125 Townsend (Td), the PTR-MS sensitivity towards ethanol at m/z 47 is 5–20 times lower than for FA; ethanol will then only be an important interference when present in much higher abundance than FA. Sensitivity towards 2-propanol is <1% of that for AA, so that propanols will not in general represent a significant interference for AA. Hydrated product ions of AA, glycolaldehyde, and propanols occur at m/z 79, which is also commonly used to measure benzene. However, the resulting interference for benzene is only significant when E/N is low (&amp;lesssim;100 Td). Addition of water vapor affects the PTR-MS response to a given compound by (i) changing the yield for fragmentation reactions and (ii) increasing the importance of ligand switching reactions. In the case of AA, sensitivity to the molecular ion increases with humidity at low E/N but decreases with humidity at high E/N due to water-driven fragmentation. Sensitivity towards FA decreases with humidity throughout the full range of E/N. For glycolaldehyde and the alcohols, the sensitivity increases with humidity due to ligand switching reactions (at low E/N) and reduced fragmentation in the presence of water (at high E/N). Their role as interferences will typically be greatest at high humidity. For compounds such as AA where the humidity effect depends strongly on the collisional energy in the drift tube, simple humidity correction factors (XR) will only be relevant for a specific instrumental configuration. We recommend E/N ~ 125 Td as an effective condition for AA and FA measurements by PTR-MS, as it optimizes between the competing E/N-dependent mechanisms controlling their sensitivities and those of the interfering species. Finally, we present the design and evaluation of an online acid trap for separating AA and FA from their interfering species at m/z 61 and 47, and we demonstrate its performance during a field deployment to St. Louis, USA, during August–September of 2013.

scholarly journals Measuring acetic and formic acid by proton transfer reaction-mass spectrometry: sensitivity, humidity dependence, and quantifying interferences

2014 ◽  
Vol 7 (10) ◽  
pp. 10883-10930 ◽  
Author(s):  
M. Baasandorj ◽  
D. B. Millet ◽  
L. Hu ◽  
D. Mitroo ◽  
B. J. Williams
Keyword(s):  
Mass Spectrometry ◽  
Proton Transfer ◽  
Formic Acid ◽  
Transfer Reaction ◽  
Proton Transfer Reaction ◽  
Reaction Mass ◽  
Hydrated Product ◽  
Product Ions ◽  
Humidity Dependence ◽  
Significant Interference

Abstract. We present a detailed investigation of the factors governing the quantification of formic acid (FA), acetic acid (AA) and their relevant mass analogues by proton transfer reaction-mass spectrometry (PTR-MS), assess the underlying fragmentation pathways and humidity dependencies, and present a new method for separating FA and AA from their main isobaric interferences. PTR-MS sensitivities towards glycolaldehyde, ethyl acetate and peroxyacetic acid at m/z 61 are comparable to that for AA; when present, these species will interfere with ambient AA measurements by PTR-MS. Likewise, when it is present, dimethyl ether can interfere with FA measurements. On the other hand, for E/N = 125 Townsend (Td), the PTR-MS sensitivity towards ethanol at m/z 47 is 5–20× lower than for FA; ethanol will then only be an important interference when present in much higher abundance than FA. Sensitivity towards 2-propanol is <1% of that for AA, so that propanols will not in general represent a significant interference for AA. Hydrated product ions of AA, glycoaldehyde, and propanols occur at m/z 79, which is also commonly used to measure benzene. However, the resulting interference for benzene is only significant when E/N is low (

scholarly journals Measurement of atmospheric sesquiterpenes by proton transfer reaction-mass spectrometry (PTR-MS)

2008 ◽  
Vol 1 (1) ◽  
pp. 401-433
Author(s):  
S. Kim ◽  
T. Karl ◽  
D. Helmig ◽  
R. Daly ◽  
R. Rasmussen ◽  
...  
Keyword(s):  
Proton Transfer ◽  
Transfer Reaction ◽  
Deciduous Forest ◽  
Proton Transfer Reaction ◽  
Major Product ◽  
Reaction Mass ◽  
Diffusion Method ◽  
Research Station ◽  
Mixing Ratios ◽  
Product Ions

Abstract. The ability to measure sesquiterpenes (SQT; C15H24) by a Proton-Transfer-Reaction Mass Spectrometer (PTR-MS) was investigated with SQT standards, prepared by a capillary diffusion method, and the estimated mixing ratios, derived from the counts of product ions and proton transfer reaction constants were intercompared with measured mixing ratios, measured by a complementary Gas Chromatograph (GC) coupled to a Flame Ionization Detector (GC-FID). Product ion distributions due to soft-ionization occurring in a selected ion drift tube via proton transfer were measured as a function of collision energies. Results after the consideration of the mass discrimination of the PTR-MS system suggest that quantitative SQT measurements within 20% accuracy can be achieved with PTR-MS if two major product ions (m/z 149+ and 205+) out of seven major product ions (m/z 81+, 95+, 109+, 123+, 135+, 149+ and 205+) are accounted for. Bicyclic sesquiterpenes, i.e. β-caryophyllene and α-humulene, showed considerable fragmentation causing the accuracy of their analysis to be reduced to 50% if only the parent ion (m/z 205) is considered. These findings were applied to a field dataset collected above a deciduous forest at the PROPHET (Program for Research on Oxidants: Photochemistry, Emissions, and Transport) research station in 2005. Inferred Average daytime ecosystem scale mixing ratios (fluxes) of isoprene, sum of monoterpenes (MT), and sum of SQT exhibited values of 15 μg m−3 (4.5 mg m−2 h−1), 1.2 μg m−3 (0.21 mg m−2 h−1) and 0.0016 μg m−3 (0.10 mgm−2 h−1) respectively. A range of MT and SQT reactivities with respect to the OH radical was calculated and compared to an earlier study inferring significantly underestimated OH reactivities due to unknown terpenes above this deciduous forest. The results indicate that MT and SQT can resolve ~30% of missing OH reactivity, reported from this site.

Collision-Induced Carbanion Rearrangements. The Elimination of Ethene From the 3,3-Dimethylheptan-4-one Enolate Ion

1987 ◽  
Vol 40 (8) ◽  
pp. 1365 ◽  
Author(s):  
GJ Currie ◽  
MB Stringer ◽  
JH Bowie ◽  
JL Holmes
Keyword(s):  
Proton Transfer ◽  
Major Product ◽  
Negative Ion ◽  
Deuterium Labelling ◽  
Ethyl Group ◽  
Mechanisms Of Formation ◽  
Product Ions ◽  
Rearrangement Processes ◽  
Methane Elimination ◽  
Collision Activation

The mechanisms of formation of the major product ions produced by collision activation of the enolate negative ion of 3,3-dimethylheptan-4-one have been studied by deuterium labelling. H2 loss involves 1,2-elimination from the 6- and 7-positions. Methane elimination is complex involving three competitive rearrangement processes. Ethene loss produces the most abundant fragment ion and occurs by elimination of the C1-C2 ethyl group with concomitant proton transfer from the 1-position. In the case of the enolate ion from CD3CH2CMe2COPr, the expected elimination of C2H2D2 competes with loss of C2H4 from positions 6 and 7. Other decompositions are as follows: loss of C3H8 involves the methyl and ethyl substituents at positions 7, 1 and 2 respectively; loss of C4H8 occurs by two processes, ( i ) successive losses of two C2H4 units (from the 1, 2 then 6, 7 positions), and (ii) loss of CH2=CMe2 (from the 2 and 3 positions); and loss of C5H12 produces Et-C=C-0-. .

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