Characterisation of four alkyl-branched fatty acids as ...
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Original Paper
Characterisation of four alkyl-branched fatty acids as methyl, ethyl, propyl and
butyl esters using gas chromatography-quadrupole time of flight mass
spectrometry.
Peter J. WATKINS†
CSIRO Agriculture and Food, 671 Sneydes Road, Werribee, Victoria, 3030, Australia
† To whom correspondence should be addressed.
E-mail: [email protected]
Analytical SciencesAdvance Publication by J-STAGEReceived September 20, 2019; Accepted October 30, 2019; Published online on November 8, 2019DOI: 10.2116/analsci.19P349
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Abstract
Branched fatty chain fatty acids (BCFAs) are associated with the ‘mutton flavour’ found
with the aroma resulting from cooked older sheep meat with three BCFAs,
4-methyloctanoic (MOA), 4-ethyloctanoic (EOA) and 4-methylnonanoic (MNA) acids
as the main compounds responsible for ‘mutton flavour’. Usually, BCFA analysis is
done by gas chromatography (GC) with the use of quadrupole mass spectrometry (qMS)
becoming predominant. 2-Butyloctanoic acid (2BO) has been used in this facility using
as an internal standard to determine BCFA content in sheep fat. In this present work,
GC-qMS, along with GC-quadrupole-time of flight MS (GC-QTOF-MS), have been
deployed to characterise alkyl esters (as methyl, ethyl, propyl and butyl) for MOA,
EOA, MNA and 2BO. This work presents, for the first time, the mass spectral
characterisation of 2BO for these alkyl esters using GC-qMS and GC-QTOF-MS.
Keywords: GC-QTOF-MS, fragmentation mechanism, mutton flavour, alkyl esters
Analytical SciencesAdvance Publication by J-STAGEReceived September 20, 2019; Accepted October 30, 2019; Published online on November 8, 2019DOI: 10.2116/analsci.19P349
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Introduction
Branched fatty chain fatty acids (BCFAs) have historically been associated with the
‘mutton flavour’ found with the aroma resulting from cooked sheep meat, particularly
that taken from older animals1. Considerable attention has been given to BCFAs since
their presence can result in lower consumer acceptance of the meat product2. Three
BCFAs, 4-methyloctanoic (MOA), 4-ethyloctanoic (EOA) and 4-methylnonanoic
(MNA) acids, have been implicated as the main compounds responsible for ‘mutton
flavour’ in cooked sheep meat. Usually, the BCFA content of sheep has been analysed
using methyl esterification for measurement by gas chromatography using flame
ionisation detection3-5. Butyl esterification has also been reported for sheep fat analysis
due to the reported low volatility of the butyl esters compared to their methyl
equivalents5. Sweep co-distillation, originally developed for pesticide residue recovery
from meat and dairy products6, has also been used to recover BCFAs from sheep fat
which, after derivatisation as trimethylsilyl esters, were measured using gas
chromatography-mass spectrometry (GC-MS)7-9.
Direct esterification of sheep fat to produce methyl esters (MEs) from BCFAs
and quantitation by GC-quadrupole MS (GC-qMS) with single ion monitoring and the
use of a suitable internal standard has been used by other workers10-12. In this laboratory,
2-butyloctanoic acid (BOA) has been used as an internal standard for the measurement
of BCFAs in sheep fat using GC-MS13. As far as the author is aware, the mass spectral
characterisation of BOA as an ester has not been previously done and the mass spectrum
of BOA as methyl, ester, propyl and butyl esters are reported here for the first time.
Proposed fragmentation mechanisms are proposed for these compounds, based on the
use of collision ion dissociation with GC-quadrupole time-of-flight MS
(GC-QTOF-MS). This was also done for MOA, EOA and MNA as well. Tandem MS
Analytical SciencesAdvance Publication by J-STAGEReceived September 20, 2019; Accepted October 30, 2019; Published online on November 8, 2019DOI: 10.2116/analsci.19P349
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has previously been used for the structural characterisation of longer chain BCFAMEs,
particularly C17:0 and above14,15 but has not been reported for the esters of the shorter
chain BCFAs such as MOA, EOA, MNA or BOA. Using QTOF-MS as a tandem MS,
this approach was investigated to ascertain if it was suitable for the structural
characterisation of the shorter chain BCFAMEs.
Experimental
Reagents
MOA (>= 98 %), MNA (>97 %) and 2-BOA (96 %) were purchased from
Sigma-Aldrich. EOA (97 %) was purchased from Alfa Aeser GB. All other chemicals
were of analytical reagent grade or better.
Preparation of alkyl BCFA esters
BCFA standard solutions were prepared by diluting approx. 100 mg (accurately
weighed) to 20 mL with hexane. An aliquot of each standard (50 µL), tetrahydrofuran (1
mL), ROH (1 mL, R = CH3, CH3CH2, CH3(CH2)2 or CH3(CH2)3) and 40 µL H2SO4
were added to a 100 mL Kimax tube and then heated at 80 °C for 2 hr. After cooling,
hexane (2 mL) and saturated NaCl solution (1 mL) was added to the tube. The organic
layer was removed and washed with 5% NaHCO3 solution (2 mL). After standing for 20
min, the organic layer was removed, ready for separation and characterisation using
GC-MS.
Separation and characterisation of alkyl BCFA esters
GC-qMS
The separation and characterisation were performed using an Agilent Model 6890 GC
interfaced to an Agilent Model 5793 Mass Selective Detector (MSD) with a CombiPAL
autosampler. Separations of the alkyl BCFA esters (1 µL) were made using a DB-5
capillary column (J&W, length = 30 m, i.d. = 0.32mm, film thickness = 0.32 µm). The
Analytical SciencesAdvance Publication by J-STAGEReceived September 20, 2019; Accepted October 30, 2019; Published online on November 8, 2019DOI: 10.2116/analsci.19P349
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column oven temperature was initially held at 80 °C for 3 min, heated to 160 °C at a
rate of 8 °C min-1 and then heated to 300 °C where it was held for 2.5 min. The injector
was heated at 250 °C and operated in split mode (20:1). Helium was used as the carrier
gas (2.0 mL min-1). The transfer line was held at 280 °C. The mass spectrometer was
operated in scan mode (m/z range from 35 to 250 Da) with the detector set to 2200 V.
The detector response was not recorded beyond 12 min. The data was collected in
GC-MSD Chemstation format and later converted using software provided by Agilent
for use with the Agilent Masshunter suite of software. Each mass spectrum was
normalised to the base peak.
GC-QTOF-MS
The separation and characterisation were performed using an Agilent Model 7890B GC
interfaced to an Agilent Model 7200 Accurate Mass Quadrupole Time of Flight Mass
Spectrometer integrated with an Agilent GC Sampler 120. Separations of the alkyl
BCFA esters were performed using a DB-5ms capillary column (Agilent, length = 30 m,
i.d. = 0.25 mm, film thickness = 250 µm). The column oven temperature was initially
held at 100 °C for 3 min, heated to 210 °C at a rate of 8 °C min-1 and then heated to
300 °C where it was held for 2.5 min. The multimode inlet was heated at 250 °C and
operated in split mode (25:1). Helium was used as the carrier gas (1.2 mL min-1). The
mass spectrometer was operated in two modes; MS and tandem MS (ie MS/MS). For
MS mode, the quadrupole was operated in electron ionisation (EI) mode with the ion
source at 230 °C with an electron voltage of 70 eV and current of 25 µA operating in 2G
mode. The TOF analyser scanned the m/z range from 35 to 250 Da, acquiring the
accurate mass spectral data. For MS/MS analysis, the precursor ion, taken as the
molecular ion [M+], was identified from the TOF analysis, and the quadrupole was
operated in EI mode as described above in order to isolate the precursor ion (M+) for the
Analytical SciencesAdvance Publication by J-STAGEReceived September 20, 2019; Accepted October 30, 2019; Published online on November 8, 2019DOI: 10.2116/analsci.19P349
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respective alkyl BCFA ester. Table 1 shows the precursor ions used for tandem MS.
Nitrogen was used as the collision gas (1.5 mL min-1) to produce post-source spectrum
in a linear hexapole collision cell using collision energies ranging from 5 to 20 eV. The
TOF analyser was set to scan the m/z range from 35 to (M+10) Da, depending on the
compound. The detector response was not recorded beyond 16.75 min. Each mass
spectrum was normalised to the most abundant peak.
Results and Discussion
Characterisation of mass spectra of alkyl BCFA esters
GC-qMS
Fig. 1 shows the mass spectrum of methyl 2-butyloctanoate (MeBOA) as well as methyl
4-methyloctanoate (MeMOA), methyl 4-ethyloctanoate (MeEOA), methyl
4-methylnonanoate (MeMNA) obtained using EI with a single quadrupole mass
spectrometer. As far as this author is aware, the EI mass spectrum for MeBOA (Fig. 1d)
has not previously reported and presented here for the first time. In this case, the
molecular ion (m/z = 214) was in low abundance and the base peak was m/z = 87.
Unlike typical mass spectra of fatty acid methyl esters which are characterised by a base
peak at m/z = 74 resulting from the McLafferty rearrangement14, this peak was absent
from the mass spectrum. Two other ions were present, m/z = 130 and m/z = 158, which
are believed to be McLafferty rearrangement products arising from the butyl and hexyl
groups, rather than a methyl group. The ion of m/z = 130 arises when the butyl group is
in the β position relative to the carbonyl moiety while the ion m/z = 158 arises when
hexyl group is located at this position and Fig 2a shows the proposed McLafferty
rearrangement for the butyl group which results in the ion m/z = 130 and a neutral
compound C6H12, MW = 84. When the adjacent group is hexyl, the rearrangement
results in the formation of two products; an ion of m/z = 158 along with the neutral
Analytical SciencesAdvance Publication by J-STAGEReceived September 20, 2019; Accepted October 30, 2019; Published online on November 8, 2019DOI: 10.2116/analsci.19P349
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compound C4H8, MW = 56 and Fig 2B shows the proposed McLafferty rearrangement.
The remaining peaks in the mass spectrum are due to the periodic carbomethoxy ion
series [(CH2)nCOOCH3]+ at m/z = 87, 101, 115, 129, etc14.
For MeMOA, the molecular ion (M, m/z = 172) was present in low abundance
(Fig 1a). The most abundant ions were m/z = 87, formed by β cleavage between C-3 and
C-4, and m/z = 74 formed from the McLafferty rearrangement. It was evident that there
was also β cleavage between C4 and C5 (m/z = 115) but in less abundance. It was also
apparent that the CH3O- and CH3OCOCH2- moieties were lost from the molecular ion
with m/z = 141 (i.e. [M–31]+) and m/z = 99 (i.e. [M–73]+) present in the mass spectrum.
Similar explanations were applicable for the mass spectra of MeMOA (Fig 1b) and
MeMNA (Fig 1c) and a summary of the mechanisms for each mass spectrum is shown
in Table 1.
In the case of the ethyl (Et) esters, an extra methylene group is introduced into
the structure, and the proposed mechanisms responsible for the mass spectrum of these
compounds are similar to those for the methyl esters. The related mass spectra can be
found in the Supplementary Material. In the case of EtMOA, like MeMOA, the
molecular ion (m/z = 186) was low in abundance and the most abundant ions were m/z =
101 resulting from β cleavage between C-3 and C-4, and m/z = 88, the McLafferty
rearrangement ion (Fig S1a to S1c in Supplementary Material). For EtBOA, similar
rearrangement mechanisms as for MeBOA can be used to explain the mass spectrum for
this compound; i.e. the base peak arises from the series [(CH2)nCOOCH2CH3]+ with two
McLafferty-type rearrangements occurring (Fig S1d in Supplementary Material).
For the propyl (Pr) analogues, there were similarities in the mass spectra that
were observed for the methyl and ethyl esters. For example, the mass spectrum of
PrMOA (Fig 3a) revealed that the molecular ion (m/z = 200) was low and the ions
Analytical SciencesAdvance Publication by J-STAGEReceived September 20, 2019; Accepted October 30, 2019; Published online on November 8, 2019DOI: 10.2116/analsci.19P349
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resulting from β cleavage were also evident; between C-3 and C-4 (m/z = 115) and, to a
lesser extent, between C-4 and C-5 (m/z = 143). The loss of the propoxy moiety
(CH3CH2CH2O-, [M-59]+, m/z = 141) was also evident yet the ion resulting from the
loss of the CH3CH2CH2COCH2- group ([M-101]+, m/z = 99) was low in abundance. The
presence of these ions can be explained in a similar way as for the methyl and ethyl
esters. There were differences though between these and the other esters. Firstly, there
was no evidence of a McLafferty-type rearrangement with this compound which, if it
had have occurred, would have resulted in the formation of an ion m/z = 102, which was
not apparent in the mass spectrum. The second difference was the presence of the ion,
m/z = 159, in the mass spectrum. It is believed that this ion was formed as a result of a
double hydrogen rearrangement producing an ion [M-41]+ (Figure 2-b). This reasoning
was based on an earlier report where the analogous mechanism was described for butyl
esters of BCFAs16. For PrEOA and PrMNA, similar mechanisms are applicable for the
mass spectrum of these compounds (Fig S1e and S1f in Supplementary Material). In the
case of PrBOA (m/z = 242, Fig 4b), the mechanisms for the fragmentation of this
compound are similar to that of MeBOA and EtBOA; that is, the mass spectrum result
from two McLafferty type rearrangements with the butyl and hexyl groups located in
the β position relative to the carbonyl group (m/z = 158 and 176 respectively), and the
ion m/z = 115 resulting from the loss of neutral aliphatic radicals. As for the other
BCFAs, the double hydrogen rearrangement was also evident with the presence of an
ion m/z = 201, i.e. [M-41]+.
As noted above, the characterisation of the mass spectrum resulting from
butyl (Bu) esters of MOA, EOA and MNA has been previously described16. Ha and
Lindsay reported that the general fragmentation pattern for these esters could be
explained by (i) the loss of the butoxy radical (CH3CH2CH2CH2O-) resulting in the
Analytical SciencesAdvance Publication by J-STAGEReceived September 20, 2019; Accepted October 30, 2019; Published online on November 8, 2019DOI: 10.2116/analsci.19P349
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formation of the ions m/z = 73 and [M–73]+, (ii) α cleavage of the alkyl group, R, to
form ions m/z = 101 (CH3CH2CH2CH2OCO-) and [M–101]+, (iii) double hydrogen
rearrangement to produce ion m/z = [M–55]+ and (iv) site-specific McLafferty-type
rearrangement to produce ions m/z = 56 and 4116. In the case of BuMOA (Fig 3c), as for
all the alkyl BCFA esters, the abundance of the molecular ion (m/z = 214) was low, and
the ions described by Ha and Lindsay16 were present; i.e. m/z = 73, 101, 141 [M–73]+
and 158 [M–55]+. The ion expected from [M–101]+ (m/z = 113) was in low abundance
while the ion m/z = 56 was the base peak. Similar mechanisms can be used to account
for the mass spectrum of BuEOA and BuMNA (Fig S1g and S1h in Supplementary
Material). For BuBOA (m/z = 256, Fig 3d), the ion resulting from the double hydrogen
rearrangement was most abundant (m/z = 201, [M–55]+). There were also ions present
resulting from i) the loss of the butoxy radical, m/z = 73 and 183, ii) α cleavage of the
alkyl group, m/z = 101 and 155, and iii), as for the related esters of 2-BOA,
McLafferty-type rearrangement products arising when the butyl and hexyl groups are
located at the β position relative to the carbonyl group; m/z = 172 and 200, respectively.
GC-QTOF-MS
The accurate mass spectra of the Me, Et, Pr and Bu esters of MOA, EOA, MNA and
2-BOA were measured after electron ionisation (70 eV) using the TOF mass analyser
(Figure S2 in Supplementary Material). The molecular ion was identified in each
spectrum and used as a precursor ion to investigate the behaviour of each compound
with collision induced dissociation (CID, Table 1). Figure 4 shows the TOF mass
spectrum resulting from CID at 5, 10, 15 and 20 eV after electron ionisation (70 eV) of
the molecular ion (m/z = 172.20) for MeMOA. With increasing voltage, higher collision
rates can be induced which generate further fragmentation of the molecular species.
This is useful for the study of molecular fragmentation patterns. This is one advantage
Analytical SciencesAdvance Publication by J-STAGEReceived September 20, 2019; Accepted October 30, 2019; Published online on November 8, 2019DOI: 10.2116/analsci.19P349
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of QTOF-MS compared to qMS as the technique provides structural information. An
additional advantage is the increased sensitivity of MS/MS compared to qMS. Under
low energy conditions (5 eV, Fig 4a), the base peak was 115.0750 which, after
comparison with the EI mass spectrum (Fig 1a), was believed to result from cleavage
between C-4 and C-5 of this compound. The Mass Calculator tool, available in the Mass
Hunter software suite provided with the GC-QTOF-MS, suggested an ion of empirical
formula [C6H11O2]+ which, with a molecular weight of 115.0759, would substantiate
this speculation. An alternative explanation for the presence of this ion has been
presented elsewhere15 and is described below.
Increasing the CID energy allows for the determination of the ion structures
and the analytical identification of compounds with high specificity15. The increase in
CID energy causes further fragmentation of the product ions from the molecular ion
which can be used to identify potential fragmentation pathways. This one advantage of
QTOF-MS comparted to qMS as the technique provides structural information. An
addition advantage is the increased sensitivity of MS/MS compared to qMS. In the case
of MeMOA, increasing the CID energy reduced the abundance of two ions, m/z =
115.705 and 101.059 (e.g. cf Fig 4a with 4d). These ions respectively have empirical
formula of [C6H11O2]+ and [C5H9O2]
+. Additionally, other ions increased in abundance
with the decrease of these two ions with the proposed empirical formula shown in
brackets; namely, m/z = 83.048 ([C5H7O]+), 73.066 ([C4H9O]+), 59.049 ([C3H7O]+) and
55.055 ([C4H7]+). It is believed that these are formed from subsequent neutral loss
fragmentation of the [C6H11O2]+ ion (Figure 5). With the loss of a methylene moiety
from this ion, an ion of empirical formula [C5H9O2]+ (m/z = 101.059) is formed which,
with the loss of H2O and CO, forms the ions [C5H7O]+ (m/z = 83.048) and [C4H9O]+
(m/z = 73.066). The latter ion can undergo further dissociation with the respective
Analytical SciencesAdvance Publication by J-STAGEReceived September 20, 2019; Accepted October 30, 2019; Published online on November 8, 2019DOI: 10.2116/analsci.19P349
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neutral loss of CH2 and H2O to form [C3H7O]+ (m/z = 59.049) and [C4H7]+ (m/z =
55.055).
As for MeMOA, increasing the CID energy after electron ionisation of the
molecular ion decreased the abundance of the main ions at lower energy found for the
other esters as well. For example, at 5 eV for MeEOA the main ions were m/z = 115.07
([C6H11O2]+), 129.09 ([C7H13O2]
+) and 157.12 ([C11H22O2]+, Fig S3a) which were either
no longer abundant or evident when the CID energy was increased to 20 V where the
main ions were 55.05 ([C4H7]+), 59.05 ([C3H7O]+), 69.07 ([C5H7O]+) and 73.06
([C4H9O]+, Fig S3d), where the formula in brackets indicates the empirical formula
assigned for each ion. A proposed fragmentation pathway for collision induced
dissociation of the molecular ion for MeEOA is shown in Figure S10a. The pathway for
MeEOA is comparatively more complex compared to that of MeMOA. As noted above,
at the lower energy, the main ions were [C11H22O2]+, most likely formed by the loss of
an Et moiety (at C-4) from the molecular ion, as well as [C7H13O2]+ and [C6H11O2]
+
formed from the loss of a C4H9 moiety from the parent ion with subsequent neutral loss
of CH2, respectively (Fig S3d in in Supplementary Material). Here, it was assumed that
the ion m/z = 115.07 resulted from the loss of the C4H9 moiety. The fragmentation
pathways were identified for the main ions found at the higher CID energy levels;
namely, [C4H7]+, [C3H7O]+, [C5H7O]+ and [C4H9O]+. The details on the formation of
these ions are shown in the proposed fragmentation pathway (Fig S10a in
Supplementary Material). Similar arguments can also be given for the remaining esters.
Proposed fragmentation pathways resulting from the collision induced dissociation of
the molecular ion for MeMNA, MeBOA, EtMOA, EtEOA, EtMNA and EtBO are
shown in Figures S10 (b) to (g) with the related ester’s CID mass spectra to be found in
Figures S4 to S9 (Supplementary Material), respectively.
Analytical SciencesAdvance Publication by J-STAGEReceived September 20, 2019; Accepted October 30, 2019; Published online on November 8, 2019DOI: 10.2116/analsci.19P349
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As noted above, MeMOA under low energy conditions (5 eV, Fig 4a) had a
base peak of m/z 115.0750 that was speculated to result from cleavage between C-4 and
C-5 of this compound with an empirical formula of [C6H11O2]+. An alternative
explanation for the formation of this ion has been presented elsewhere15. These authors
proposed that that, due to cleavage of the molecule between C-5 and C-6, the ion
resulted from the loss of an alkyl radical with no rearrangements under the low energy
conditions used for MS/MS15. The ion was present in the mass spectrum of longer chain
saturated BCFAMEs after collision induced dissociation of the molecular ions generated
by EI and appeared either as the result of direct cyclisation or as a resonance stabilised
radical di-cation. The final product was suggested to be a resonance stable oxane
cation15.
Tandem MS has been applied to longer chain branched saturated FAMEs,
where ions are formed as a result of fragmentation occurring at positions α relative to
the branched alkyl group14,15. It has been suggested that this could be used as a
diagnostic tool for the identification of monomethyl branches at the iso, anteiso and
midchain positions for BCFAMEs from C12:0 to C26:015. For larger chain BCFAMEs,
fragmentation occurs at the opposing α position of the branched group and results in
ions of moderate (~50 % of M+) or much greater abundances than that found for the
molecular ion, M+. If applicable for the shorter chain BCFAMEs such as MeMOA using
QTOF-MS, then this would mean that there would have been cleavage between C-3 and
C-4 as well as C-4 and C-5 of MeMOA resulting in the formation of ions m/z = 87 and
115 respectively of moderate abundance. While there was true for the ion m/z = 115.075,
this was not the case for ion m/z = 87.044 where only a small abundance was evident
(Fig 3a). This would suggest that the use of QTOF-MS does not appear to be feasible
for the identification of monomethyl branches in short chain BCFAMEs such as
Analytical SciencesAdvance Publication by J-STAGEReceived September 20, 2019; Accepted October 30, 2019; Published online on November 8, 2019DOI: 10.2116/analsci.19P349
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MeMOA and related esters investigated in this study.
Conclusions
Three branched-chain fatty acids, 4-methyloctanoic (MOA), 4-ethyloctanoic (EOA) and
4-methylnonanoic (MNA) acids, are compounds believed to be responsible for ‘mutton
flavour’. 2-Butyloctanoic acid (2BO) has been used as an internal standard for the
characterisation of these compounds in sheep fat using GC-qMS. GC-qMS, along with
GC-QTOF-MS, have been deployed to characterise alkyl (methyl, ethyl, propyl and
butyl) esters for MOA, EOA, MNA and 2BO with the mass spectrum for the alkyl esters
of 2BO reported for the first time. Proposed fragmentation mechanisms were proposed
for these compounds, based on the use of collision induced dissociation with
GC-QTOF-MS.
Supporting Information
Supplementary material contains supporting mass spectra and related fragmentation
mechanisms. This material is available free of charge on the Web at
http://www.jsac.or.jp/analsci/.
References
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Analytical SciencesAdvance Publication by J-STAGEReceived September 20, 2019; Accepted October 30, 2019; Published online on November 8, 2019DOI: 10.2116/analsci.19P349
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Table 1. Summary of mass spectral fragmentation patterns for alkyl esters of three
branched chain fatty acids (BCFAs)
RA BCFAB M [M-OR]+ [M-CH2CO2R]+ β C-3/C-4 β C-4/C-5 McLafferty Precursor
MOA 172 141 99 87 115 74 172.15
Me EOA 186 155 113 87 129 74 186.16
MNA 186 155 113 87 115 74 186.16
MOA 186 141 99 101 129 88 186.16
Et EOA 200 155 113 101 143 88 200.17
MNA 200 155 113 101 129 88 200.17
MOA 200 141 99 115 143 102 200.18
Pr EOA 214 155 113 115 157 102 214.19
MNA 214 155 113 115 143 102 214.19
MOA 214 141 99 129 157 116 214.16
Bu EOA 228 155 113 129 171 116 228.21
MNA 228 155 113 129 157 116 228.21
AR=alkyl group; Me = CH3-, Et = CH3CH2-, Pr = CH3CH2CH2-, Bu = CH3CH2CH2CH2-
BMOA = 4-methyloctanoic acid, EOA = 4-ethyloctanoic acid, MNA =
4-methylnonanoic acid
Analytical SciencesAdvance Publication by J-STAGEReceived September 20, 2019; Accepted October 30, 2019; Published online on November 8, 2019DOI: 10.2116/analsci.19P349
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Figure Captions
Figure 1 Electron ionisation (70 eV) mass spectra of (a) methyl 4-methyloctanoate, (b)
methyl 4-ethyloctanoate, (c) methyl 4-methylnonanoate and (d) methyl
2-butyloctanoate.
Figure 2. Proposed schemes for McLafferty-type rearrangement for methyl
2-butyloctanoate with (a) butyl group and (b) hexyl group at β position to carbonyl
group, and (c) double hydrogen rearrangement of propyl BCFA esters.
Figure 3. Electron ionisation (70 eV) mass spectra of (a) propyl 4-methyloctanoate, (b)
propyl 2-butyloctanoate, (c) butyl 4-methyloctanoate and (d) butyl 2-butyloctanoate.
Figure 4. Mass spectra of methyl 4-methyloctanoate after collision induced dissociation
(CID) of the molecular ion (m/z = 172.15) at (a) 5, (b) 10, (c) 15 and (d) 20 eV post
electron ionisation (70 eV).
Figure 5. Proposed fragmentation pathways resulting from collision induced
dissociation of the molecular ion of methyl 4-methyloctanoate.
Analytical SciencesAdvance Publication by J-STAGEReceived September 20, 2019; Accepted October 30, 2019; Published online on November 8, 2019DOI: 10.2116/analsci.19P349
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Analytical SciencesAdvance Publication by J-STAGEReceived September 20, 2019; Accepted October 30, 2019; Published online on November 8, 2019DOI: 10.2116/analsci.19P349
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Figure 1
Analytical SciencesAdvance Publication by J-STAGEReceived September 20, 2019; Accepted October 30, 2019; Published online on November 8, 2019DOI: 10.2116/analsci.19P349
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Figure 2.
Analytical SciencesAdvance Publication by J-STAGEReceived September 20, 2019; Accepted October 30, 2019; Published online on November 8, 2019DOI: 10.2116/analsci.19P349
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Analytical SciencesAdvance Publication by J-STAGEReceived September 20, 2019; Accepted October 30, 2019; Published online on November 8, 2019DOI: 10.2116/analsci.19P349
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Figure 3.
Analytical SciencesAdvance Publication by J-STAGEReceived September 20, 2019; Accepted October 30, 2019; Published online on November 8, 2019DOI: 10.2116/analsci.19P349
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Analytical SciencesAdvance Publication by J-STAGEReceived September 20, 2019; Accepted October 30, 2019; Published online on November 8, 2019DOI: 10.2116/analsci.19P349
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Figure 4.
Analytical SciencesAdvance Publication by J-STAGEReceived September 20, 2019; Accepted October 30, 2019; Published online on November 8, 2019DOI: 10.2116/analsci.19P349