Evaluation of the α-phenylvinyl cation as a chemical ionization reagent for the differentiation of isomeric substituted phenols in an ITMS

Article abstract of DOI:10.1002/jms.3578

Ion-molecule reactions between the α-phenylvinyl cation and isomeric naturally occurring phenols were investigated using a quadruple ion trap mass spectrometer. The α-phenylvinyl cation m/z 103, generated by chemical ionization from phenylacetylene, reacts with neutral aromatic compounds to form the characteristic species: [M + 103]⁺ adduct ions and the trans-vinylating product ions [M + 25]⁺, which correspond to [M + 103]⁺ adduct after the loss of benzene. Isomeric differentiation of several ring-substituted phenols was achieved by using collision-induced dissociation of the [M + 103]⁺ adduct ions. This method also showed to be effective in the differentiation of 4-ethylguaiacol from one of its structural isomers that displays identical EI and EI/MS/MS spectra. The effects of gas-phase alkylation with phenylvinyl cation on the dissociation behavior were examined using mass spectrometryⁿ and labeled derivatives.

Full text of DOI:10.1002/jms.3578

Journal of
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Research article
Received: 11 October 2014
Revised: 29 January 2015
Accepted: 8 February 2015
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/jms.3578
Evaluation of the α-phenylvinyl cation as a
chemical ionization reagent for the
differentiation of isomeric substituted
phenols in an ITMS
Michela Begala*
Ion–molecule reactions between the α-phenylvinyl cation and isomeric naturally occurring phenols were investigated using a
quadruple ion trap mass spectrometer. The α-phenylvinyl cation m/z 103, generated by chemical ionization from
phenylacetylene, reacts with neutral aromatic compounds to form the characteristic species: [M+ 103]+ adduct ions and the
trans-vinylating product ions [M+ 25]+, which correspond to [M + 103]+ adduct after the loss of benzene. Isomeric differentiation
of several ring-substituted phenols was achieved by using collision-induced dissociation of the [M+ 103]+ adduct ions. This
method also showed to be effective in the differentiation of 4-ethylguaiacol from one of its structural isomers that displays
identical EI and EI/MS/MS spectra. The effects of gas-phase alkylation with phenylvinyl cation on the dissociation behavior were
examined using mass spectrometryn and labeled derivatives. Copyright © 2015 John Wiley & Sons, Ltd.
Additional supporting information may be found in the online version of this article at the publisher’s web site.
Keywords: α-phenylvinyl cation; chemical ionization; ITMS; substituted phenols; 4-ethylguaiacol; isomeric differentiation
isomer specific fragmentation behavior under CID.[9] Therefore,
the discovery of new CI reagent gasses for the differentiation and
Introduction
The differentiation of isomers and their identifications constitute
the subject of continuous interest because different isomers can
have widely varying pharmacological and toxicological properties
and environmental impact. Isomers differentiation can be success-
fully achieved using several techniques, including nuclear magnetic
resonance, infrared and X-ray crystallography. Nevertheless, these
techniques require relatively large amount of samples and are
not applicable to the analysis of complex mixtures or trace
amounts of analytes. Mass spectrometry (MS) is a powerful tech-
nique for the characterization of isomeric compounds.[1] MS offers
several advantages including high sensitivity and capability to per-
form analysis without isolating and purifying the mixture compo-
nents. Chemical reactivity has been coupled to MS to access
structural information such as functional group identification[2]
and isomer distinction.[1] Chemical ionization (CI) MS[3] is an adapt-
able method for performing ion–molecule reactions in the gas
phase. Many CI reagents that show structural specificity upon reac-
tions with analytes display product ion distributions that may di-
rectly provide key information for structure identification.[1–5]
Moreover, the combination of CI with tandem MS techniques[6] in
an ion trap mass spectrometer (ITMS)[7] allows to perform selective
isolation of the desired reagent ions, thus avoiding the occurrence
of other reactions of the neutral analyte with cogenerated reagent
ions.1z In same cases, ion–molecule reactions combined with multi-
stage collision-induced dissociation (CID)[8] have provided differ-
ences in fragmentation useful for the differentiation of isomeric
analytes.1h,f As an example, Giorgi et al. carried out ion–molecule
reactions using ethylamine as reagent gas to derivatize isomeric
analytes into characteristic adducts that showed different and
identification of isomers continues to be an area of considerable
interest.
α-Phenylvinyl cations (α-PVCs), first postulated in 1964,[10] are by
now accepted as reactive intermediates in numerous reactions.
α-PVC has been readily generated in the gas phase by using MS.
Despite this, mass spectrometric investigation of α-PVCs has
received only little attention with most of the emphasis devoted
to the energetics of the formation,[11] the isomerization[12] and
the determination of the gas-phase basicity, i.e. for ring-substituted
phenylacetylenes.[13]
We recently reported[14] the gas-phase reaction of the α-PVC
towards monosubstituted benzenes in an ITMS. It was found that
α-PVC reacts selectively with electron-rich benzenes to form the ad-
duct ion [M + 103]+ and the trans-vinylating product ion [M+ 25]+.
We now report a study aimed to investigate whether the α-PVC
can work as a new CI reagent for isomers differentiation. In particu-
lar, we focused our attention on a certain class of low mass isomeric
compounds where conventional EI/MS and EI/MS/MS do not work
successfully.
Substituted isomeric phenols, i.e. methoxyphenols, dihydro-
xybenzenes and xylenols, have been chosen as model compounds
*
Correspondence to: Michela Begala, Unit of Drug Sciences, Department of Life and
Environmental Sciences, University of Cagliari, Via Ospedale 72, 09124 Cagliari,
Italy. E-mail: michelabegala@unica.it
Unit of Drug Sciences, Department of Life and Environmental Sciences, University
of Cagliari, Via Ospedale 72, 09124, Cagliari Italy
J. Mass Spectrom. 2015, 50, 693–702
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M. Begala
as they are widely used in many field, such us cosmetic, tanning,
1,4-Dimethoxy-2-methyl benzene (10) and its deuterium-
labeled analog 10d6 were synthesized by the reaction of
methylhydroquinone (Sigma-Aldrich) with iodomethane and
iodomethane-d3, respectively.[16]
dye, chemical and pharmaceutical industries, and as
a
consequence, they are often encountered as environmental
pollutants.[15] In order to rationalize the experimental results,
the mechanisms of formation of the most significant ions were
tentatively proposed based on MSn experiments carried out on
the adduct ions using labeled derivatives when synthetically ac-
cessible or commercially available. Therefore, the possible com-
petition of other mechanisms affording the same loss cannot
be ruled out.
Results and discussion
Methoxyphenols
The tree sets of phenolic isomers have quite similar EI and CID mass
spectra of EI-generated M+. ions.18,1g Ortho (1) and para (3)
methoxyphenols behave identically as they both dissociate pre-
dominantly via loss of methyl radical (m/z 109). The meta isomer
(2) displays a different fragmentation behavior characterized by
loss of formaldehyde and, to a lesser extent, by loss of methyl
radical.1g Unlike the molecular ions M+. generated under EI condi-
tion, however, the α-PVC addition products [M + 103]+ at m/z 227
show quite different dissociation patterns for the three positional
isomers 1–3 (Fig. 1).
Although the CID mass spectra of the reaction products (m/z 227)
from isomeric derivatives 1–3 show two common fragment ions
with similar abundances, namely, the reformation of the starting
PVC (m/z 103) and the trans-vinylating product[14] (m/z 149), how-
ever, the spectra yield same unique behaviors for individual
regioisomers that allowed their differentiation. The meta isomer
can be distinguished from the ortho and para isomers because of
the formation of a unique ion at m/z 125 (Fig. 1(b)), reasonably
protonated meta-methoxyphenol. The para isomer displays a
unique ion at m/z 197 (Fig. 1(c)). The ortho isomer can be distin-
guished from the para because of the lack of ions at m/z 197 and
121 (Fig. 1(a)).
Moreover, three ions present in each spectrum, i.e. ions at
m/z 212, 199 and 195, show marked differences in their relative
intensities. The ions at m/z 212 and 199 are highly abundant for
para isomer (100%, 98%), less abundant for the meta (27%, 25%)
and they are nearly absent (m/z 199, 4%) or of very low abundance
(m/z 212, 15%) in the CI–CID spectrum from ortho isomer. On the
contrary, the ion at m/z 195 is very abundant for the ortho isomer
(b.p.), it is of lower intensity for the para isomer (48%), whereas
for the meta isomer, which again behaves somewhere between
the ortho and para isomers, it is 68%.
In the case of the ortho isomer (Fig. 1(a)), it is reasonable to as-
sume that the ion at m/z 195 may be generated by methanol loss
from two distinct structures, i.e. the ring-alkylation product (struc-
ture a, Scheme 1) and/or the oxygen-alkylation product (struc-
ture b). However, when ring labeled o-methoxyphenol (1d4) was
used, in the CID mass spectrum of the resulting adduct ion with
α-PVC (m/z 231) the ion observed at m/z 195 in Fig. 1(a) mainly
shifted to m/z 199 (Fig. 2). This CID pattern indicates that, in the case
of ortho isomer, the methanol loss stems from a proton migration
from the ortho hydroxyl group (ortho effect), rather than a deute-
rium from the ring (Scheme 1). This implies that b is the structure
that contributes most to the methanol loss, reasonably activated
by the attack of the α-PVC on the hydroxyl group. On the contrary,
in the case of the m and p isomeric methoxyphenols, only the ring-
alkylated reaction products can contribute to the methanol loss,
thus reflecting the lower intensity of the m/z 195 ions in their CID
mass spectra.
Experimental
Ion–molecule reaction were examined with a Varian Saturn 2000
ITMS, operating under CI conditions, coupled with a Varian 3800
gas chromatograph (Varian, Walnut Creek, CA, USA). A 3 ml of a
mixture of phenylacetylene and methanol (molar ratio 1 : 0.1)
were introduced in the CI reservoir bulb and used as reagent
gas. Details on the use of the Saturn ion trap for ion–molecule
reaction have been reported previously.1z The CI parameters
for the generation of the CI reagent gas are described in detail
elsewhere.[14]
The reaction of α-PVC towards the aromatic compounds 1–11
was performed by mass selecting the reagent ion at m/z 103 and
storing it inside the IT. Isolation of the m/z 103 ions was achieved
by using the standard MS/MS isolation function available via the
SATURN GC/MS WORKSTATION version 5.41 software and keeping the CID
energy at 0 V in order to avoid the activation of decomposition
processes.[14] Each reaction product, obtained from the reaction
of the neutral analyte with the reagent gas, was isolated and then
collisionally activated prior to mass analysis. CIDs experiments were
carried out by using helium as the collision gas [gas purity (He) was
99.999%]. For MSn experiments, the collision energy was varied in
such a way that the relative abundance of the surviving precursor
ions was 10–25%. An excitation time of 20ms and an isolation
width of 2 m/z for the precursor ion were used. Each MS/MS spec-
trum was an average of five scans.
Compounds 1–11 (1μl aliquots of 1.0 × 10À5 M solutions in chlo-
roform) were introduced into the trap through the gas chromato-
graph inlet line. A FactorFour Low Bleed/MS capillary column
(30 m, 0.25 mm i.d., 0.25 μm film thickness) was used. The oven
temperature was programmed from 40 °C (held for 5 min) to
250 °C at 20°C/min (held for 2 min). The transfer line was main-
tained at 250°C and the injector port (30/1 split) at 280 °C.
Phenylacetylene, methanol, o-methoxyphenol (1), m-metho-
xyphenol (2), p-methoxyphenol (3), 1,2-dihydroxybenzene (4),
1,3-dihydroxybenzene (5), 1,4-dihydroxybenzene (6), 2,6-dime-
thylphenol (7), 2,4-dimethylphenol (8), 3,5-dimethylphenol (9)
and 4-ethylguaiacol (11) were purchased from Sigma-Aldrich
(Sigma-Aldrich S.r.l., Milan, Italy) and used without further
purification.
o-Methoxyphenol-d4 (1d4) was prepared starting from
1,2-dihydroxybenzene-d4 (3d4) (Toronto Research Chemicals Inc.,
Toronto, Canada) in the presence of Cs2CO3 in acetonitrile using
iodomethane (Sigma-Aldrich) as alkylating agent.[16]
2-Phenyl-benzofurane was prepared starting from pheny-
lacetylene and ortho-iodophenol as reported in literature.[17]
o-Methoxyphenol-d3 (1d3), m-methoxyphenol-d3 (2d3) and
p-methoxyphenol-d3 (3d3) were prepared using iodomethane-d3
starting from 1,2-dihydroxybenzene, 1,3-dihydroxybenzene and
1,4-dihydroxybenzene, respectively.[16]
A possible structure for the fragment ion at m/z 195 that origi-
nated from the ortho-O-alkylated reaction product b can be
envisioned as the open structure c (Scheme 1). Nevertheless, con-
sidering the high abundance of this ion, more stable cyclisized
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Differentiation of isomeric phenols using α-PVC
Figure 1. Collision-induced dissociation of adduct ions at m/z 227 obtained from the CI (α-phenylvinyl cation) of isomers (a) 1, (b) 2 and (c) 3.
Scheme 1. Elimination of neutral methanol from the reaction product of α-phenylvinyl cation with o-methoxyphenol (1) (m/z 227) (R = H4) and labeled
derivative 1d4 (m/z 231) (R = D4).
structures, such as indenic, anthracene or benzofurane, cannot be
ruled out. However, as the authentic standard was available only
for the benzofurane structure d, the proposed structure in Scheme 1
should be regarded as tentative. As can be seen, the CI–CID (meth-
anol) spectrum of protonated 2-phenyl-benzofurane (Fig. 3(a)),[17]
used as model compound, appeared very similar to that obtained
from CID experiments on the m/z 195 fragment ion (Fig. 3(b)), as
they are both predominantly characterized by CO elimination
(m/z 167).
Ion at m/z 199, which is particularly abundant in the CID spectra
from meta and para isomers (Fig. 1(b) and (c) and Scheme 2), elim-
inates under MS3 experiments a water molecule to form the ion at
m/z 181 (Fig. 4(a)). This loss implies that, in the case of meta and
para-metoxy isomers, the α-PVC attack occurs preferentially on
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Figure 2. Collision-induced dissociation of the reaction product (m/z 231) from α-phenylvinyl cation and labeled o-methoxyphenol (1d4).
Figure 3. (a) Chemical ionization–collision-induced dissociation (methanol) spectrum of protonated 2-phenyl-benzofurane and (b) mass spectrometry3
spectrum of the m/z 195 fragment from the reaction product of α-phenylvinyl cation with o-methoxyphenol (1).
the ring. As a consequence, the initial loss of 28 Da in the transition
227 > 199 most likely corresponds to the elimination of C2H4 from
the ring-alkylated product rather than of CO. This fragmentation
behavior is surprising because it must involve extensive rearrange-
ment of the reaction product. A possible structure for this ion is
depicted in Scheme 2 for the para isomer. Ions at m/z 209, again ob-
served only for meta and para isomers, correspond to the water loss
from the ring-alkylated [M + 103]+ ion, thus confirming that for
meta and para metoxybenzenes, the attack on the ring is favored.
MS3 experiments revealed that also the m/z 209 ion is a direct pre-
cursor to m/z 181, again attributed to C2H4 loss (Fig. 4(b) and
Scheme 2). Considering that the m/z 181 ion was also observed
for compound 1 (70%) (Fig. 1(a)), a similar behavior could be hy-
pothesized starting from the ring-alkylation product of 1 with PVC.
The radical ion at m/z 212 was initially attributed to the loss of
Scheme 2. Formation of ion at m/z 181 under mass spectrometry3
CH3 from the methoxy group. However, when OCD3 derivatives
were used, i.e. o-methoxyphenol-d3 (1d3), m-methoxyphenol-d3
experiments starting from compound 3 (R = CH3) and the labeled analog
3d3 (R = CD3).
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Differentiation of isomeric phenols using α-PVC
Figure 4. Mass spectrometry3 of (a) m/z 199 and (b) m/z 209 from the reaction product (m/z 227) arising from α-phenylvinyl cation and para
methoxyphenol 3.
Figure 5. Collision-induced dissociation of adduct ions at m/z 230 obtained from the chemical ionization (α-phenylvinyl cation) of labeled isomers (a) 1d3,
(b) 2d3 and (c) 3d3.
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(2d3), p-methoxyphenol-d3 (3d3) (Fig. 5(a–c)), these ions partially
shifted to m/z 215, thus indicating that the CH3 loss could involve
also another group than OCH3. An alternative source for the methyl
elimination could be envisioned as the methylene group from the
α-PVC portion,[14] reasonably subjected to hydrogen transfer
(Scheme 3). It must be pointed out that the elimination of the
methyl radical from the electron-even reaction product is an
exception to the even-electron rule,[19] and therefore, it most likely
lead to a particularly stable structure which presumably contains
higher degree of conjugated unsaturation (Scheme 3). Neverthe-
less, the proposed structures remain tentative, as no authentic stan-
dard was available.
Dihydroxybenzenes
The EI[18] and CID mass spectra of dihydroxybenzenes 5 and 6 are
very similar. Only ortho isomer (4) can be distinguished from the
other isomers on the bases of the formation of ion at m/z 92 due
to loss of water molecule.
When α-PVC is used as reagent gas, the CID spectra of the adduct
ions at m/z 213 from isomeric dihydroxybenzenes showed some
common ions (Fig. 6), i.e. m/z 195 due to water loss, m/z 185 due
to the loss of 28 Da, m/z 167 (vide infra) and the trans-vinylating
ion at m/z 135. The ortho isomer shows greater dehydration prod-
uct (m/z 195; R.A. % 98%) thus indicating that the proximity effect
of the OH group together with the derivatization with the α-PVC
have an effect on the favorability of this decomposition channel.
Sequential CID experiments reveal that the ion at m/z 195 behaves
identically to the m/z 195 ion observed in the CID spectrum of
o-methoxyphenol, i.e. through the loss of CO to form the ion at
m/z 167 (Fig. 3(b)).
Scheme 3. Mass spectrometry2 experiments on the adduct ions at m/z 231
obtained from the reaction of α-phenylvinyl cation with labeled
p-methoxyphenol (3d3) (the site of the α-phenylvinyl cation attack on the
ring is for illustrative purpose only).
Figure 6. Collision-induced dissociation of adduct ions at m/z 213 obtained from the chemical ionization (α-phenylvinyl cation) of isomeric
dihydroxybenzenes (a) 4, (b) 5 and (c) 6.
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Differentiation of isomeric phenols using α-PVC
The meta (5) isomer can be distinguished from the para (6) on
the bases of the different intensity of ions at m/z 185, 167 and
135 that are 5%, 20% and 28% for meta isomer and 35%, 100%
and 80% for the para isomer. Moreover, the formation of ion at
m/z 107 only for ortho and para isomers allowed their differentia-
tion from the meta isomer.
1,4-Dimethoxy-2-methylbenzene (10) is a precursor[20] in the
synthetic routes to the antimicrobic agent enokipodins[21] and to
the hallucinogenic drug 2,5-dimethoxy-4-methylamphetamine.[22]
4-Ethylguaiacol (11) is a volatile phenol produced by yeast in red
wine that, at high concentrations, can be undesirable.[23]
Although the 1,4-dimethoxy-2-methylbenzene and 4-ethyl-
guaiacol contain different functional groups, their EI mass spectra
are practically indistinguishable, as both exhibit intense molecular
ion at m/z 152, the loss of methyl radical (m/z 137 b.p.) and minor
peaks at m/z 122 and 109.[18] Surprisingly, even the MS/MS spectra
of the molecular ions from both functional isomers showed an
identical fragmentation behavior, consisting in the formation of
the ion at m/z 137.
Dimethylphenols
With conventional EI and EI/CID MS, it is not possible to distinguish
between dimethylphenols isomers 7, 8 and 9.[18] On the contrary,
the CI–CID spectra of the adduct ions with α-PVC (m/z 225) are qual-
itatively different (Fig. 7). The CI–CID spectrum of the adduct ion at
m/z 225 from the 2,6-dimethyl isomer (7) shows a characteristic loss
of a methyl radical generating ions at m/z 210. The 2,4-dimethyl iso-
mer (8) shows a unique ion at m/z 123, whereas the 3,5-dimethyl
isomer (9) displays both this ions with a very low intensity.
2,6-Dimethyl isomer can be further distinguished from the other
two on the basis of the low intensity of the ion at m/z 197, attrib-
uted to the loss of 28Da.
On the contrary, the CI–CID spectra on the adduct ion at m/z 255,
obtained using the α-PVC as reagent gas, are quite different, thus
allowing their differentiation (Fig. 8(a) and (b)).
In particularly, beside the common fragment ions at m/z 223, due
to methanol loss, and m/z 195, due to the subsequent loss of 28 Da,
the adduct ion from compound 10 exhibits some unique fragment
ions at m/z 240, m/z 227 and m/z 213 (Fig. 8(a)). The nature of these
ions was elucidated by deuterium labeling. CI–CID spectrum of the
deuterium-labeled 1,4-[OCD3]2 reaction product at m/z 261 ob-
tained from the reaction of (10d6) and α-PVC (Fig. 9) showed that
ion at m/z 240 shifted to m/z 243 and to m/z 246, thus suggesting
that radical CD3 and CH3 were lost. It follows that the methyl loss
might originate from the two methoxy groups at position 1 and 4
and/or the two further methyl groups, i.e. at position 2 and the
1,4-Dimethoxy-2-methylbenzene and 4-ethylguaiacol
As
a material for further investigations, we selected two
functional isomers, i.e. 1,4-dimethoxy-2-methylbenzene (10) and
4-ethylguaiacol (11), chosen because of their chemical importance.
Figure 7. Collision-induced dissociation of adduct ions at m/z 225 obtained from the chemical ionization (α-phenylvinyl cation) of isomeric dimethylphenols
(a) 7, (b) 8 and (c) 9.
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Figure 8. Collision-induced dissociation of adduct ions at m/z 255 obtained from the chemical ionization (α-phenylvinyl cation) of 1,4–dimethoxy-2-methyl
benzene (10) (a) and of 4-ethylguaiacol (11) (b); MS3 of ions at m/z 213 (c) and m/z 209 (d) from the reaction product (m/z 255) arising from 10 and 11,
respectively.
Figure 9. Collision-induced dissociation of adduct ions at m/z 261 obtained from the chemical ionization (α-phenylvinyl cation) of 1,4–dimethoxy-2-methyl
benzene (10d6).
methyl group generated by H transfer on the methylene of the
α-PVC portion (see Scheme 3, referred to compound 3d3). The ion
Conclusions
The potential of the α-PVC as CI reagent for isomer differentiation
was illustrated for substituted phenols. CID of the adduct ions yields
a large number of fragments that serve as highly effective finger-
print useful for isomer differentiation and identification. In the spe-
cific case of methoxyphenols, the use of labeled derivatives put in
evidence that the site of the attack of PVC on neutral isomers direct
the dissociation reactions. In particularly, the most significant diag-
nostic ions for isomer differentiation posses particular stable struc-
tures arising from the condensation of the α-PVC portion with the
benzene ring of the analyte, thus reflecting an active role of the
α-PVC moiety in the activation of unique fragmentation routes.
These results demonstrate that the α-PVC can be considered as a
suitable reagent gas under CI condition for the differentiation and
identification of isomeric organics.
at m/z 227, which formally correspond to the loss of 28 Da, shifted
to m/z 233, thus confirming that it was generated by neutral loss
of C2H4. The ion at m/z 213, generated by loss of 42 Da, was shifted
to m/z 217 revealing that a CD3 group is involved in the fragmenta-
tion process. This loss would actually appear as C2H3O or C3H6. MS3
experiments of the addition product clarified the nature of the
neutral loss. The fragment ion at m/z 213 was isolated and sub-
jected to dissociation, resulting in the elimination of water molecule
(m/z 185) (Fig. 8(c)). This process is not feasible with the neutral loss
of C2H3O. Conversely, although the adduct ion does not have a uni-
fied group of three carbon and six hydrogens, however, the loss of
C3H6 can be rationalized based on the migration of methyl group to
the methylene moiety of the α-PVC.
The adduct ion from 4-ethylguaiacol (11) and the α-PVC
(m/z 255) displays under CID experiments a unique ion at m/z 209
due to the loss of 46 Da (Fig. 8(d)). MS3 experiments reveal that this
ion fragments further by the elimination of methanol to generate
ion at m/z 177 (Fig. 8 (c)). This data suggest that the initial loss of
46 Da does not affect the methoxy group, indicating it is reasonably
due to the elimination of a water molecule and 28 Da.
Acknowledgements
This work was supported financially by the University of Cagliari
(Fondi d’Ateneo ex 60%) and Fondazione Banco di Sardegna.
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Differentiation of isomeric phenols using α-PVC
L. M. Amundson, E. Archibold, B. E. Winger, H. I. Kenttämaa.
Differentiation of protonated aromatic regioisomers related to lignin
by reactions with trimethylborate in a Fourier transform ion cyclotron
resonance mass spectrometer. J. Am. Soc. Mass Spectrom. 2011, 22,
1040; (y) L. Trupia, N. Dechamps, R. Flammang, G. Bouchoux,
M. T. Nguyen, P. Gerbaux. Isomeric recognition by ion/molecule
reactions: the ionized phenol-cyclohexadienone case. J. Am. Soc. Mass
Spectrom. 2008, 19, 126; (z) A. A. Mosi, W. R. Cullen, G. K. Eigendorf.
Ion–molecule reactions of halocarbon cations with polycyclic
hydrocarbons in a quadrupole ion trap. Part I – differentiation of
structural isomers. J. Mass Spectrom. 1997, 32, 864.
References
[1] (a) W. A. Tao, F. C. Gozzo, R. G. Cooks. Mass spectrometric quantitation
of chiral drugs by the kinetic method. Anal. Chem. 2001, 73, 1692; (b)
L. Wu, E. C. Meurer, B. Young, P. Yang, M. N. Eberlin, R. G. Cooks.
Isomeric differentiation and quantification of alpha, beta-amino acid-
containing tripeptides by the kinetic method: alkali metal-bound
dimeric cluster ions. Int. J. Mass Spectrom. 2004, 231, 103; (c)
A. E. P. M. Sorrilha, F. C. Gozzo, R. S. Pimpim, M. N. Eberlin. Multiple
stage pentaquadrupole mass spectrometry for generation and
characterization of gas-phase ionic species. The case of the PyC2H5
+.
isomers. J. Am. Soc. Mass Spectrom. 1996, 7, 1126; (d) J. S. Splitter,
F. Turecek (Eds). Applications of Mass Spectrometry to Organic
Stereochemistry. VCH Publishers: New York, 1994; (e) J. Y. Salpin,
J. Tortajada. Structural characterization of hexoses and pentoses
using lead cationization. An electrospray ionization and tandem mass
spectrometric study. J. Mass Spectrom. 2002, 37, 379; (f) M. L. Gross,
P. H. Lin, S. J. Franklin. Analytical applications of ion-molecule
reactions. Identification of C5H10 isomers by ion-cyclotron resonance
spectrometry. Anal. Chem. 1972, 44, 974; (g) T. Donovan,
C. J. Brodbelt. Examination of ortho effect in the collisionally activated
dissociation of closed-shell aromatic ions. Org. Mass Spectrom. 1992,
27, 9; (h) R. H. Staley, R. R. Corderman, M. S. Foster, J. L. Beauchamp.
Nucleophilic-attack on protonated oxiranes in gas-phase.
Identification of C2H5O+ isomeric ions corresponding to protonated
ethylene oxide. J. Am. Chem. Soc. 1974, 96, 1260; (i) F. Kjeldsen,
K. F. Haselmann, E. S. Sorensen, R. A. Zubarev. Distinguishing of
Ile/Leu amino acid residues in the PP3 protein by (hot) electron
capture dissociation in Fourier transform ion cyclotron resonance
mass spectrometry. Anal. Chem. 2003, 75, 1267; (j) F. F. Hsu, J. Turk,
M. L. Gross. Structural distinction among inositol phosphate isomers
using high-energy and low-energy collisional-activated dissociation
tandem mass spectrometry with electrospray ionization. J. Mass
Spectrom. 2003, 38, 447; (k) P. Dwivedi, C. Wu, L. M. Matz,
B. H. Clowers, W. F. Siems, H. H. Hill. Gas-phase chiral separations by
ion mobility spectrometry. Anal. Chem. 2006, 78, 8200; (l) M. Fu,
P. Duan, S. Li, S. C. Habicht, D. S. Pinkston, N. R. Vinueza,
H. I. Kenttämaa. Regioselective ion–molecule reactions for the mass
spectrometric differentiation of protonated isomeric aromatic
diamines. Analyst 2008, 133, 452; (m) M. Fu, P. Duan, J. Gao,
H. I. Kenttämaa. Ion–molecule reactions for the differentiation of
primary, secondary and tertiary hydroxyl functionalities in protonated
analytes in a tandem mass spectrometer. Analyst, 2012, 137, 5720; (n)
L. A. B. Moraes, A. A. Sabino, E. C. Meurer, M. N. Eberlin. Absolute
configuration assignment of ortho, meta, or para isomers by mass
spectrometry. J. Am. Soc. Mass Spectrom. 2005, 16, 431; (o)
M. Benassi, Y. E. Corilo, D. Uria, R. Augusti, M. N. Eberlin. Recognition
and resolution of isomeric alkyl anilines by mass spectrometry. J. Am.
Soc. Mass Spectrom. 2009, 20, 269; (p) L. L. da Rocha, R. Sparrapan,
R. Augusti, M. N. Eberlin. Direct assignment of positional isomers by
mass spectrometry: ortho, meta and para acyl and amidyl anilines
and phenols and derivatives. J. Mass Spectrom. 2004, 39, 1176; (q)
Y. E. Corilo, M. N. Eberlin. Recognizing α-, β- or γ-substitution in
pyridines by mass spectrometry. J. Mass Spectrom. 2008, 43, 12; (r)
M. Benassi, M. N. Eberlin. Absolute assignment of constitutional
isomers via structurally diagnostic fragment ions: the challenging
case of α-and β-acyl naphthalenes. J. Am. Soc. Mass Spectrom. 2010,
21, 2041; (s) R. S. Thompson, L. P. Guler, E. D. Nelson, Y.-Q. Yu,
H. I. Kenttämaa. Mechanistic study of stereoselective gas-phase
reactions of phosphenium ions with cis- and trans-
1,2-diaminocyclohexanes. J. Org. Chem., 2002, 67, 5076; (t)
V. V. Mancel, N. Sellier. Stereochemical differentiation of cyclic glycols
by ion-molecule reactions in a quadrupole mass spectrometer using
dimethyl ether acetonitrile and 2-S-pyrrolidinemethanol. Rapid
Commun. Mass Spectrom. 2000, 14, 80; (u) C. Bure, N. Sellier,
D. Lesage, F. Fournier, J. C. Tabet. Ion/molecule reactions with
dimethyl ether and dimethyl-d6 ether: differentiation of four isomers
contained in patchouli essential oil. Rapid Commun. Mass Spectrom.
2000, 14, 872; (v) H. F. Wu, Y. J. Chuan. Isomer differentiation by
combining gas chromatography, selective self-ion/molecule reactions
and tandem mass spectrometry in an ion trap mass spectrometer.
Rapid Commun. Mass Spectrom. 2003, 17, 1030; (w) E. P. Burrows.
Dimethyl ether and dimethyl-d6 ether chemical ionization mass
spectrometry of polynuclear aromatic hydrocarbons. J. Mass
Spectrom. 1995 30, 312; (x) J. Somuramasami, P. Duan,
[2] (a) J. S. Brodbelt. Analytical applications of ion–molecule reactions.
Mass Spectrom. Rev. 1997 16, 91; (b) J. S. Brodbelt, C.-C. Liou,
T. Donovan. Selective adduct formation by dimethyl ether chemical
ionization in a quadrupole ion trap mass spectrometer and a
conventional ion source. Anal. Chem. 1991, 63, 1205; (c)
T. D. McCarley, J. S. Brodbelt. Structurally diagnostic ion-molecule
reactions
and
collisionally
activated
dissociation
of
1,4-benzodiazepines in a quadrupole ion trap mass spectrometer.
Anal. Chem. 1993, 65, 2380; (d) A. Colorado, J. S. Brodbelt. Class-
selective collisionally activated dissociation/ion–molecule reactions of
4-quinolone antibiotics. Anal. Chem. 1994, 66, 2330; (e) C.-C. Liu,
J. Isbell, H.-F. Wu, J. S. Brodbelt, R. A. Bartsch, J. C. Lee, J. L. Halman.
Structurally-selective gas-phase ion-molecule reactions of dibenzo-
16-crown-5 compounds. J. Mass Spectrom. 1995, 30, 572; (f)
R. G. Cooks, H. Chen, M. N. Eberlin, X. Zheng, W. A. Tao. Polar
acetalization and transacetalization in the gas phase: the Eberlin
reaction. Chem. Rew. 2006, 106, 188; (g) M. N. Eberlin. Structurally
diagnostic ion/molecule reactions: class and functional-group
identification by mass spectrometry. J. Mass Spectrom. 2006, 41, 141;
(h) M. A. Watkins, J. M. Price, B. E. Winger, H. I. Kenttämaa. Ion–
molecule reactions for mass spectrometric identification of functional
groups
in protonated
oxygen-containing
monofunctional
compounds. Anal. Chem. 2004, 76, 964; (i) K. M. Campbell,
M. A. Watkins, S. Li, M. N. Fiddler, B. Winger, H. I. Kenttämaa.
Functional group selective ion/molecule reactions: mass
spectrometric identification of the amido functionality in protonated
monofunctional compounds. J. Org. Chem., 2007, 72, 3159; (j)
J. Somuramasami, B. E. Winger, T. A. Gillespie, H. I. Kenttämaa.
Identification and counting of carbonyl and hydroxyl functionalities
in protonated bifunctional analytes by using solution derivatization
prior to mass spectrometric analysis via ion-molecule reactions. J. Am.
Soc. Mass Spectrom. 2010, 21, 773.
[3] M. S. B. Munson, F. H. Field. Chemical ionization mass spectrometry. I.
General introduction. J. Am. Chem. Soc. 1966, 88, 2621.
[4] A. G. Harrison. Chemical Ionization Mass Spectrometry. 2nd. CRC Pess:
Boca Raton, 1992.
[5] (a) M. Vairamani, U. A. Mirza, R. Srinivas. Unusual positive ion reagents
in chemical ionization mass spectrometry. Mass Spectrom. Rev. 1990,
9, 235; (b) S. Prabhakar, M. Vairamani. Acetone chemical ionization
mass spectrometry. Mass Spectrom. Rev. 1997, 16, 259–281.
[6] (a) K. L. Busch, G. L. Glish, S. A. McLuckey. Mass Spectrometry/Mass
Spectrometry: Techniques and Applications of Tandem Mass
Spectrometry. VCH publishers: New York, 1988; (b) R. A. Yost,
D. D. Fetterolf. Tandem mass spectrometry (MS/MS) instrumentation.
Mass Spectrom. Rev. 1983, 2, 1.
[7] R. E. March, J. F. J. Todd (Eds). Practical Aspects of Ion Trap Mass Spec-
trometry, Vols 1–3. CRC Press: Boca Raton, 1995.
[8] (a) K. Levsen, H. Schwarz. Collisional activation mass spectrometry: a
new probe for determining the structure of ions in the gas phase.
Angew. Chem. Int. Ed. 1976, 15, 509; (b) D. D. Fetterolf, R. A. Yost,
J. R. Eyler. The use of reactive collisions in tandem mass spectrometry
for the differentiation of isomeric structures. Org. Mass Spectrom.
1984, 19, 104.
[9] G. Giorgi, L. Salvini, F. Ponticelli. Gas phase reactivity of isomeric
arylglicosides towards amines.
A
chemical ionization mass
spectrometry and tandem mass spectrometry study. J. Am. Soc. Mass
Spectrom. 2004, 15, 244.
[10] C. A. Grob, G. Cseh. Die solvolyse von α-bromstyrolen Substitution
am ungesättigten trigonalen Kohlenstoffatom. Helv. Chim. Acta
1964, 47, 194.
[11] Y. Apeloig, W. Franke, Z. Rappoport, H. Schwarz, D. Stahl. Dissociative
ionization of aryl-substituted vinyl bromide in the gas phase:
experimental and computational evidence for the formation of
stable α-arylvinyl cations both by direct means and spontaneous
J. Mass Spectrom. 2015, 50, 693–702
Copyright © 2015 John Wiley & Sons, Ltd.
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Journal of
MASS
SPECTROMETRY
M. Begala
exothermic isomerization of unstable isomeric ions. Am. Chem. Soc.
1981, 103, 2770.
[12] H. Franke, W. Franke, H. Schwarz, J. Terlouw. Mass spectrometric
investigation of acetylenic compounds. Part 29. On the gas phase
isomerization of phenyl-substituted allyl and vinyl cations. Int. J. Mass
Spectrom. and Ion Processes. 1984, 57, 233.
ion formation in the electron ionization mass spectra of 6-alkyl- and
6-aryldibenzo(d,f)(1,3)dioxepine derivatives. 1. Spirocyclization of the
molecular ions in the gas phase. Rapid Commun. Mass Spectrom.
2007, 21, 1414.
[18] Version of the NIST/EPA/NIH Mass Spectral Library Database (NIST 98),
1998.
[13] (a) F. Marcuzzi, G. Modena, C. Paradisi. Gas-phase basicity of ring-
substituted phenylacetylenes. J. Org. Chem. 1985, 50, 4973; (b)
T. Matsumoto, S. Kobayashi, M. Mishima, Y. Tsuno. Substituent effects
on the gas-phase basicities of 1-arylpropynes and 1-aryl-
3,3-dimethylbutynes effects of the [beta]-alkyl groups on the
resonance demand of vinyl cations. Int. J. Mass Spectrom. Ion Process.
1998, 175, 41.
[14] M. Begala, G. Tocco. Gas-phase ion-molecule reaction of alpha-
phenylvinyl cation towards substituted benzenes in the environment
of an ITMS. J. Mass Spectrom. 2011, 46, 1115.
[15] (a) E. W. Brauer. Safety of over-the-counter hydroquinone bleaching
creams (letter). Arch. Dermatol. 1985, 121, 1239; (b) G. J. Rustin,
M. R. Stratford, A. Lamont, N. Bleehen, P. A. Philip, N. Howells,
R. R. Watfa, J. A. Slack. Phase I study of intravenous 4-hydroxyanisole.
Eur. J. Cancer 1992, 28, 1362; (c) D. W. Everett, T. J. Franz,
T. J. Chando, P. J. Gale, P. A. Lehman, E. H. Schwarzel, P. V. Parab,
C. J. D’Arienzo, K. J. Kripalani. Percutaneous absorption of [3H]
tretinoin and systemic exposure to mequinol after dermal application
of 2% mequinol/0.01% [3H]tretinoin (Solagé®) solution in healthy
volunteers. Biopharm. Drug Dispos. 1999, 20, 301; (d) H. Fiegein.
Cresols and Xylenols in Ullmann’s Encyclopedia of Industrial Chemistry.
Wiley-VCH: Weinheim, 2007.
[19] M. Karni, A. Mandelbaum. The ‘even-electron rule’. Org. Mass Spectrom.
1980, 15, 53.
[20] (a) F. Macedo Jr., C. C. Andrei, D. Campiom, N. K. Ishikawa.
Regioselectivity in the acylation of methylhydroquinone dimethyl
ether: an unprecedented case of indirect steric hindrance.
Tetrahedron Lett. 2011, 52, 1612; (b) M. Saito, S. Kuwahara.
Enantioselective total synthesis of enokipodins A–D, antimicrobial
sesquiterpenes produced by the mushroom, Flammulina velutipes.
Biosci. Biotechnol. Biochem. 2005, 69, 374.
[21] N. K. Ishikawa, K. Yamaji, S. Tahara, Y. Fukushi, K. Takahashi. Highly
oxidized cuparene-type sesquiterpenes from a mycelial culture of
Flammulina velutipes. Phytochemistry 2000, 54, 777.
[22] M. Ono, M. Shimamine, K. Takahashi. Studies on hallucinogens. IV.
Synthesis of 2,5-dimethoxy-4-methylamphetamine (STP) (author’s
transl). Eisei Shikenjo Hokoku 1973, 91, 41.
[23] (a) A. P. Pollnitz, K. H. Pardon, M. A. Sefton. Quantitative analysis of
4-ethylphenol and 4-ethylguaiacol in red wine. J. Chrom. A. 2000,
874 101; (b) P. Caboni, G. Sarais, M. Cabras, A. Angioni.
Determination of 4-ethylphenol and 4-ethylguaiacol in wines by
LC-MS-MS and HPLC-DAD-fluorescence. J. Agric. Food Chem. 2007,
55, 7288.
[16] J. C. Lee, J. Y. Yuk, S. H. Cho. Facile synthesis of alkyl phenyl ethers using
cesium carbonate. Tetrahedron 1995, 25, 1367.
[17] (a) C. G. Bates, P. Saejueng, J. M. Murphy, D. Venkataraman. Synthesis of
2-arylbenzo[b]furans via copper(I)-catalyzed coupling of o-iodophenols
and aryl acetylenes. Org. Lett. 2002, 4, 4727; (b) M. Begala, G. Tocco,
G. Meli, G. Podda, S. A. M. Urru. 2-Substituted benzofuran fragment
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