Characterisation of four alkyl-branched fatty acids as

1

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

mailto:[email protected]

mailto:[email protected]

2

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


3

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


4

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


5

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


6

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


7

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


8

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


9

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


10

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


11

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.


12

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


13

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

1. P.J. Watkins, D. Frank, T.K. Singh, O.A. Young and R.D. Warner, J. Agric. Food

Chem., 2013, 61, 3561.

2. O.A. Young, G.A. Lane, A. Priolo and K. Fraser, J. Sci. Food Ag., 2003, 83, 93.

3. E. Wong, L.N. Nixon and C.B. Johnson, J. Ag. Food Chem., 1975, 23, 495.

4. O.A. Young, G.A. Lane, C. Podmore, K. Fraser, M.J. Agnew, T.L. Cummings and

N.R. Cox, N.Z. J. Agric. Res., 2006, 49, 419.

5. J.K. Ha and R.C. Lindsay, Leben.-Wissen. Und –Tech., 1990, 23, 433.


14

6. J. Tekel’ and S. Hatrík, J. Chromat. A., 1996, 754, 397.

7. L. Salvatore, D. Allen, K.L. Butler, D.Tucman, A. Elkins, D.W. Pethick and F.R.

Dunshea, Aust. J. Exp. Agric., 2007, 47, 1201.

8. P.J. Watkins, G. Rose, L. Salvatore, D. Allen, D. Tucman, R.D. Warner, F.R. Dunshea

and D.W. Pethick, Meat Sci., 2010, 86, 594.

9. P.J. Watkins, G. Kearney, G. Rose, D. Allen, A.J. Ball, D.W. Pethick and R.D. Warner,

Meat Sci., 2014, 96, 1088.

10. S. Kaffarnik, C. Heid, Y. Kayademir, D. Eibler and W. Vetter, J. Agric. Food Chem.,

2015, 63, 469

11. S. Kaffarnik, S. Preuß and W. Vetter, J. Chromatogr. A., 2014, 1350, 92.

12. K.F. Schiller, S. Preuss, S. Kaffarnik, W. Vetter, M. Rodehutscord and J. Bennewitz,

Arch. Anim. Breed., 2015, 58, 159.

13. D. Frank, P. Watkins, A. Ball, R. Krishnamurthy, U. Piyasiri, J. Sewell, J. Ortuño, J.

Stark and R. Warner, , J. Agric. Food Chem. 2016, 64, 6856.

14. J.A. Zirrolli and R.C. Murphy, J. Amer. Soc. Mass Spect., 1993, 4, 223.

15. R.R. Ran-Ressler, P. Lawrence and J.T. Brenna, J. Lipid Res., 2012, 53, 195.

16. J.K. Ha and R.C. Lindsay, J. Food Comp. Anal., 1989, 2, 118.


15

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


16

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.


17


18

Figure 1


19

Figure 2.


20


21

Figure 3.


22


23

Figure 4.


24

Figure 5.


Documents

Characterisation of four alkyl-branched fatty acids as