Hydrogen and trimethylsilyl transfers during EI mass spectral fragmentation of hydroxycarboxylic and oxocarboxylic acid trimethylsilyl derivatives

Article abstract of DOI:10.1016/j.jasms.2007.10.014

This paper, describing electron ionization mass spectral fragmentation of some hydroxycarboxylic and oxocarboxylic acid trimethylsilyl derivatives, focuses on the formation of fragment ions resulting from the interactions between the two functionalities of these compounds. These interactions result in the formation of fragment ions at [CH₂=C(OTMS)₂]^+·, [CH₂=CHC(OTMS)=OTMS]⁺, [M-31]⁺, [M-105]⁺, and [M-RCHO]^+· in the case of hydroxycarboxylic acid trimethylsilyl derivatives of formula RCHOTMS(CH₂)_nCOOTMS and at [RC(OTMS)=CH₂]^+·, [RC(=OTMS)CH=CH₂]⁺, and [M - RC(=O)CH₂]⁺ in the case of oxocarboxylic acid trimethylsilyl esters of formula RC(=O)(CH₂)_nCOOTMS. Some of these fragmentations appeared to be sufficiently specific to be used to characterize these compounds. Several fragmentation pathways involving trimethylsilyl and hydrogen transfers were proposed to explain the formation of these different fragment ions and were substantiated by deuterium labeling.

Full text of DOI:10.1016/j.jasms.2007.10.014

Hydrogen and Trimethylsilyl Transfers During
EI Mass Spectral Fragmentation of
Hydroxycarboxylic and Oxocarboxylic Acid
Trimethylsilyl Derivatives
Jean-François Rontania and Claude Aubertb
a Laboratoire de Microbiologie de Géochimie et d’Ecologie Marines (UMR 6117), Centre d’Océanologie de
Marseille (OSU), Campus de Luminy, Marseille, France
b Laboratoire de Pharmacocinétique et Toxicocinétique (UPRES 3286), Faculté de Pharmacie, Marseille, France
This paper, describing electron ionization mass spectral fragmentation of some hydroxycar-
boxylic and oxocarboxylic acid trimethylsilyl derivatives, focuses on the formation of fragment
ions resulting from the interactions between the two functionalities of these compounds. These
interactions result in the formation of fragment ions at [CH2 ϭ C(OTMS)2]ϩ·, [CH2
ϭ
CHC(OTMS) ϭ OTMS]ϩ, [M Ϫ 31]ϩ, [M Ϫ 105]ϩ, and [M Ϫ RCHO]ϩ· in the case of
hydroxycarboxylic acid trimethylsilyl derivatives of formula RCHOTMS(CH2)nCOOTMS and
at [RC(OTMS) ϭ CH2]ϩ·, [RC( ϭ OTMS)CH ϭ CH2]ϩ, and [M Ϫ RC( ϭ O)CH2]ϩ in the case
of oxocarboxylic acid trimethylsilyl esters of formula RC( ϭ O)(CH2)nCOOTMS. Some of these
fragmentations appeared to be sufficiently specific to be used to characterize these compounds.
Several fragmentation pathways involving trimethylsilyl and hydrogen transfers were pro-
posed to explain the formation of these different fragment ions and were substantiated by
deuterium labeling. (J Am Soc Mass Spectrom 2008, 19, 66–75) © 2008 American Society for
Mass Spectrometry
ilylation is by far the major derivatization proce-
dure for GC/MS analyses [1–3]. Silyl derivatives
are formed when active proton displacement (in
previously observed in the case of trimethylsilyl deriv-
atives of some steroid compounds [8, 10–13]. The
capability of the trimethylsilyl group to migrate over a
wide range of distances was also demonstrated in the
case of semirigid or rigid molecules [5]. Recently, a
significant transfer of trimethylsilyl group towards the
carbonyl group was also observed during EI fragmen-
tation of the trimethylsilyl derivatives of long-chain
␻-hydroxycarboxylic and ␻-dicarboxylic acids [14],
showing that such migrations may also occur in the case
of nonrigid structures.
Hydrogen transfers between distant functionalities
during EI fragmentation processes are also well known
[6, 15, 16], for this reaction hydrogen activation is
needed and is caused by groups such as keto, amino,
ether, trimethylsilyloxy, as well as by chain branching
and unsaturation.
In the present work, which is a part of a more general
study of the synergistic effects during EI mass fragmen-
tation of polyfunctional compounds [14, 17–19], we
describe EI mass spectral fragmentation of several hy-
droxycarboxylic and oxocarboxylic acid trimethylsilyl
derivatives, which involves trimethylsilyl and hydro-
gen transfers.
S
ϪOH, ϪSH, or ϪNH groups) by an alkylsilyl group
occurs. The most common silylation procedure is trim-
ethylsilylation. Trimethylsilyl derivatives combine ther-
mal stability and high volatility. In general, electron
ionization (EI) mass spectra of trimethylsilyl ethers or
esters exhibit a significant [M Ϫ 15]ϩ ion formed by loss
of a methyl group bonded to silicon, which is very
useful in determining the molecular mass. It is very
important to note that EI mass spectra of trimethylsilyl
derivatives may be employed not only for molecular
weight determinations, but also for structural deduc-
tions [4].
Trimethylsilyl group migrations during EI fragmen-
tation process are common [5, 6] and take place by
means of ion-neutral complexes [7]. These rearrange-
ments are generally considered useful for deducing
structures of unknown spectra [8]; however, in some
cases, misleading conclusions may be drawn. Though
trimethylsilyl group migrations usually occur through
five- to eight-member-ring transition states [9], intense
long-range migration of the trimethylsilyl group was
The characterization of such compounds appears to
be of great environmental importance [20, 21]. Indeed, it
was previously demonstrated that autoxidative pro-
cesses could induce a bias during alkenone-based pa-
Address reprint requests to Dr. J.-F. Rontani, Laboratoire de Microbiologie
de Géochimie et d’Ecologie Marines (UMR 6117), Centre d’Océanologie de
Marseille (OSU), Campus de Luminy–case 901, 13288 Marseille, France.
E-mail: jean-francois.rontani@com.univmed.fr
Published online October 30, 2007
© 2008 American Society for Mass Spectrometry. Published by Elsevier Inc.
1044-0305/08/$32.00
doi:10.1016/j.jasms.2007.10.014
Received August 31, 2007
Revised October 22, 2007
Accepted October 22, 2007

J Am Soc Mass Spectrom 2008, 19, 66–75
EIMS OF HYDROXY- AND OXOACID TMS DERIVATIVES
67
(a)
Abundance
(b)
Abundance
117
131
m/z 387
m/z 387
OTMS
90
90
80
70
60
50
40
30
20
10
0
OTMS
80
70
COOTMS
COOTMS
73
m/z 117
m/z 131
60
[M – 31]+
.
.
[M – CH3CHO]+
[M – CH3CH2CHO]+
[M - CH3 - TMSOH]+
50
40
30
20
10
0
[M - 31]+
[M - CH3 - TMSOH]+
[M - CH3]+
[M - CH3CH2]+
75
371
358
387
217
204
204
217
[M - CH3]+
.
M+
385
358
387
117
55
55
40
147
311
297
95
94
401
402
40
80
120 160 200 240 280 320 360 400
80
120 160 200 240 280 320 360 400
m/z-->
m/z-->
m/z 285
m/z 331
OTMS
COOTMS
COOTMS
(c)
(d)
OTMS
m/z 233
Abundance
Abundance
Abundance
m/z 215
73
147
233
73
331
[M – CH3(CH2)7CHO]+
215
90
80
70
60
50
40
30
20
10
0
90
80
70
60
50
40
30
20
10
0
[M - CH3 - TMSOH]+
[M – 31]+
[M - CH3]+
[M – 31]+
[M - CH3]+
129
204
189
285
217
217
401
385
429
130
339
56
204
117
413
M+
101
147
302
359
171
444
60 100 140 180 220 260 300 340 380 420
50
100 150 200 250 300 350 400 450
m/z-->
m/z-->
Figure 1. EI mass spectra of (a) 14-hydroxypentadecanoic, (b) 14-hydroxyhexadecanoic, (c) 10-
hydroxyoctadecanoic, and (d) 3-hydroxyhexadecanoic acid trimethylsilyl derivatives.
leotemperature estimations [20]. The allylic hydroper-
oxyketones thus formed are cleaved homolytically and
heterolytically in seawater, affording several (␻-1)- and
(␻-2)-oxocarboxylic acids [21]. These compounds and
the corresponding hydroxyacids produced after subse-
quent NaBH4-reduction were proposed as specific
markers of alkenone autoxidative alterations in seawa-
ter [21]. It is thus essential to be able to differentiate
these compounds from isobaric fatty acids or diols,
respectively.
11-enoic, 10-hydroxyoctadecanoic, 11-hydroxyoctade-
canoic, 12-hydroxyoctadecanoic, 9-oxooctadecanoic, 10-
oxooctadecanoic, and 9-oxooctadec-10-enoic acids were
prepared according to previously described procedures
[22]. 9-Oxodecanoic acid resulted from hydration and
retro-aldolization (alkaline hydrolysis) of 9-oxooctadec-
10-enoic acid. NaBD4 reduction of 10-oxooctadecanoic
acid afforded (10-D1)-10-hydroxyoctadecanoic acid.
Deuteration of 10-oxooctadecanoic acid in the ␣-posi-
tions relative to the carbonyl group was carried out
according to classic deuterium exchange procedure
To generalize our conclusions, shorter hydroxycar-
boxylic and oxocarboxylic acids derived from the oxi-
dation of fatty acids were also investigated.
[23]. Production of
a mixture of (11-D1)-12-hy-
droxyoctadecanoic and (12-D1)-11-hydroxyoctade-
canoic acids in low yields required two steps: (1)
epoxidation of vaccenic acid with 3-chloroperoxyben-
zoic acid in dry methylene chloride and (2) subsequent
reduction of the foregoing epoxide with excess of
NaBD4 in diethyl ether/methanol.
Experimental
Chemicals
3-Hydroxyhexadecanoic, 3-hydroxyoctadecanoic, and
6-oxoheptanoic acids were obtained from Sigma-Al-
drich. (␻-1)- and (␻-2)-Oxocarboxylic acids ranging
from C14 to C16 were obtained by heterolytic cleavage of
autoxidation products of C37 and C38 alkenones [20, 21].
NaBH4 reduction of these oxoacids then afforded the
corresponding (␻-1)- and (␻-2)-hydroxycarboxylic
acids. 8-Hydroxyoctadec-9-enoic, 10-hydroxyoctadec-
Silylation
Compounds (1 mg) to be silylated were taken up in 300
␮L of a mixture of pyridine and BSTFA (N,O-Bis(trim-
ethylsilyl)trifluoroacetamide, Supelco Inc., St Quentin
Fallavier, France) (2:1, vol/vol) and allowed to react at

68
RONTANI AND AUBERT
J Am Soc Mass Spectrom 2008, 19, 66–75
50 °C for 1 h. After evaporation to dryness (to eliminate
pyridine), the residue was dissolved in ethyl acetate (2
mL/mg) and BSTFA (0.1 mL) (to avoid desilylation)
and analyzed by gas chromatography/mass spectrom-
etry (GC/MS).
Mass Spectrometry
Analyses by gas chromatography/electron impact
mass spectrometry were performed with a Hewlett
Packard HP 5890 series II plus gas chromatograph
connected to a HP 5972 mass spectrometer (Agilent
Technologies, Massy, France). The following opera-
SiMe
3
CH
2
O
C
(CH )
2 n-1
OSiMe
3
CH
2
CH
2
d , [M - R-CHO]
R
CH
CHOSiMe
2
3
SiMe
3
O
C
O
C
(CH )
2 n-1
(CH )
2 n
OSiMe
3
OSiMe
CH
3
2
CH
2
CH
2
CH
M
2
- R-CHO
R
R
HC
HC
O
O
SiMe
SiMe
3
3
O
C
O
H
(CH )
(CH )
2 n
2 n
C
OSiMe
OSiMe
3
3
CH
CH
2
CH
2
CH
2
␣
␣
a
R
R
CHOH
HC
O
SiMe
SiMe
3
3
O
C
O
C
␣
(CH )
(CH )
2 n
2 n
OSiMe
OSiMe
3
3
CH
CH
2
CH
2
CH
2
SiMe
3
O
OSiMe
OSiMe
3
H C CH
2
C
H C
2
C
OSiMe
3
3
+
c , m/z 217
, m/z 204
b
Scheme 1. Proposed formation pathways of ions b؉· (at m/z 204),
؉·
c؉ (at m/z 217), and d؉· [M Ϫ RCHO]
involving trimethylsilyl
transfer.

J Am Soc Mass Spectrom 2008, 19, 66–75
EIMS OF HYDROXY- AND OXOACID TMS DERIVATIVES
69
m/z 187
D
(a)
COOTMS
Abundance
TMSO
m/z 360
187
+ TMS
m/z 331
90
360
73
80
70
[M - CH3 - TMSOH]+
60
50
.
40
[M - CH3(CH2)5CHO]+
[M – CH4 - CH3]+
[M - CH3]+
30
129
217
204
117
95
20 55
10
340
331
414
430
146
140
0
60
100
180
220
260
300
340
380
420
m/z -->
+ TMS
m/z 316
m/z 345
TMSO
(b)
COOTMS
Abundance
345
D
m/z 202
202
.
90
80
70
60
50
40
30
20
10
0
[M - CH3(CH2)5CHDCHO]+
[M - CH3 - TMSOH]+
73
[M – CH4 - CH3]+
[M - CH3]+
430
117
129
55
217
316
340
414
146
140
95
60
100
180
220
260
300
340
380
420
m/z-->
(c)
m/z 216
Abundance
COOTMS
73
D
TMSO
90
80
70
60
50
40
30
20
10
0
+ TMS
m/z 302
m/z 332
332
[M - CH3(CH2)7CDO]+
216
[M - CH3 - TMSOH]+
[M – CH4 - CH3]+
[M - CH3]+
217
129
55
50
302
300
414
430
204
200
104
100
340
350
147
150
250
m/z-->
400
Figure 2. EI mass spectra of (a) (11-D1)-12-hydroxyoctadecanoic, (b) (12-D1)-11-hydroxyoctade-
canoic, and (c) (10-D1)-10-hydroxyoctadecanoic acid trimethylsilyl derivatives.
tive conditions were employed: 30 m ϫ 0.25 mm (i.d.)
Results and Discussion
capillary column coated with SOLGEL-1 (SGE; film
thickness, 0.25 ␮m); oven temperature programmed
from 60 °C to 130 °C at 30 °C minϪ1 and then from
130 °C to 300 °C at 4 °C minϪ1; carrier gas (He)
pressure was maintained at 1.04 bar until the end of
the temperature program and then programmed from
1.04 bar to 1.5 bar at 0.04 bar minϪ1; injector (on
column with a retention gap) temperature, 50 °C;
electron energy, 70 eV; source temperature, 170 °C;
mass range, 50–700 Th; cycle time, 1.5 s.
Fragmentation of Hydroxycarboxylic Acid
Trimethylsilyl Derivatives
We could previously demonstrated [14] that mass spec-
tra of ␻-hydroxycarboxylic and ␻-dicarboxylic acid
trimethylsilyl derivatives exhibited some fragmenta-
tions that are lacking in mass spectra of compounds
containing only an ester or an ether group. Most of the
fragments ions thus formed resulted from trimethylsilyl

70
RONTANI AND AUBERT
J Am Soc Mass Spectrom 2008, 19, 66–75
and hydrogen transfers between the two functionalities
of the considered molecules.
EI mass spectra of various long-chain hydroxycar-
boxylic acid trimethylsilyl derivatives of formula
RCHOTMS(CH2)nCOOTMS were investigated (Fig-
ure 1 and Table 1) and appeared to be dominated by
fragment ions resulting from ␣-cleavages relative to
the ionized trimethylsilyl ether group. In the case of
10-hydroxyoctadec-11-enoic and 8-hydroxyoctadec-
9-enoic acid trimethylsilyl derivatives, ␣-cleavage
only acts on the side of the trimethylsilyl ether group
affording an alkyl radical since cleavage on the other
side leads to the formation of an unstable and un-
likely vinyl radical. These mass spectra also exhibited
some other fragment ions, which are lacking in mass
spectra of simple trimethylsilyl esters or ethers, and
thus result from interactions between the two deri-
vatized functionalities of the molecules. This is the
case notably for the ions at m/z 204 and 217, which
were previously observed in EI mass spectra of
␻-hydroxycarboxylic and ␻-dicarboxylic acid tri-
methylsilyl derivatives [14] and whose the formation
was attributed to an initial trimethylsilyl transfer
from the ether group to the ionized ester group. In the
case of hydroxycarboxylic acid trimethylsilyl deriva-
tives of formula RCHOTMS(CH2)nCOOTMS, this
transfer affords the distonic ion aϩ· (Scheme 1), which
may then either undergo ␤-cleavage giving the radi-
cal ion b؉· at m/z 204, or ␥-cleavage after transfer of a
hydrogen atom from the carbon in ␣ position relative
to the ionized ester group to the residual alkoxyl
radical affording the ion c؉ at m/z 217. The formation
؉·
of ions corresponding to [M Ϫ RCHO]
(Figure 1
and Table 1) can be attributed to ␤-cleavage of the
alkoxyl radical of the ion a؉· giving the cyclic ion d؉·
(Scheme 1). The involvement of such a pathway is
well supported by the presence of fragment ions at
m/z 331, 316, and 302 in EI mass spectra of (11-D1)-
12-hydroxyoctadecanoic, (12-D1)-11-hydroxyoctade-
canoic and (10-D1)-10-hydroxyoctadecanoic acid tri-
methylsilyl derivatives, respectively (Figure 2). It is
interesting to note that the ions [M Ϫ RCHO]؉· and
b؉· are isobaric (m/z 204) in the case of 3-hydroxycar-
boxylic acid trimethylsilyl derivatives (Figure 1d,
Table 1).
Mass spectra of the hydroxycarboxylic acid trimeth-
ylsilyl derivatives investigated also exhibit peaks corre-
sponding to [M Ϫ 105]؉ and [M Ϫ 31]ϩ (Figure 1 and
Table 1), which are lacking in mass spectra of com-
pounds containing only an ester or an ether group. The
formation of the peak at [M Ϫ 105]ϩ may be simply
attributed to successive losses of a neutral molecule of
trimethylsilanol and a methyl radical from the molecu-
lar ion. In contrast, the formation of the peak at [M Ϫ
31]ϩ appears to be more difficult to explain. To explain
the formation of a similar ion in mass spectra of
␻-hydroxycarboxylic and ␻-dicarboxylic acids trimeth-
ylsilyl derivatives, we previously proposed some path-
ways involving initial hydrogen transfer from the ether

J Am Soc Mass Spectrom 2008, 19, 66–75
EIMS OF HYDROXY- AND OXOACID TMS DERIVATIVES
71
R
R
CH
CH
CH
CH
- CH
3
O
C
O
C
(CH )
2 n
(CH )
2 n
OSiMe
O
3
Si
+
+
h
i , [M - 105]
- TMSOH
R
R
R
CH
O
H
SiMe
2
O
SiMe
H
CH
O
Si
CH
3
CH
3
- CH
4
O
C
O
C
O
(CH )
2 n
(CH )
2 n
(CH )
2 n
C
OSiMe
OSiMe
OSiMe
3
3
3
CH
CH
CH
M
e
R
R
CH
CH
O
O
- CH
3
(CH )
2 n
(CH )
2 n
Si
Si
O
O
CH
C
CH
f
C
O
OSiMe
3
Si
+
+
g , [M - 31]
Scheme 2. Proposed formation pathways of ions g؉ (at [M Ϫ 31]ϩ) and i؉ (at [M Ϫ 105]ϩ) involving
hydrogen transfer.
group to the ionized ester group and subsequent losses
of a neutral molecule of methane and a methyl radical
[14]. Indeed, the loss of methane is often observed in EI
mass spectra of trimethylsilyl derivatives [24, 25]. The
presence of a peak at m/z 414 (corresponding to [M Ϫ
31]ϩ) in the EI mass spectrum of (10-D1)-10-hy-
droxyoctadecanoic acid trimethylsilyl derivative (Fig-
ure 2c) leads us to discard such a possibility. Indeed, the
transfer of the deuterium atom from the ether group to
the ionized ester group would have resulted in a shift of
this peak to m/z 413 [M Ϫ CH3D Ϫ CH3]ϩ. We thus
propose the mechanisms described in the Scheme 2 to
explain the formation of these ions. This pathway
involves initial hydrogen transfer from the carbon in ␣
position relative to the ester group to the ionized
oxygen atom of the ether group. The resulting distonic
ion eϩ· (where the radical is stabilized by conjugation)
may then loose a neutral molecule of methane by way
of a four-membered hydrogen rearrangement with
charge retention [6] to give after cyclization the radical
ion fϩ·. Subsequent loss of a methyl radical by this ion
fϩ· yields the ion g؉ at [M Ϫ 31]ϩ stabilized by
conjugation. The involvement of such a pathway is in
good agreement with the presence of a peak at m/z 414 in
the EI mass spectrum of (10-D1)-10-hydroxyoctadecanoic
acid trimethylsilyl derivative (Figure 2c) and is also well
supported by the presence of a weak peak corresponding
to the fragment ion fϩ· at [M Ϫ CH4]·ϩ in EI mass spectra
of some hydroxycarboxylic acid trimethylsilyl derivatives
(Table 1). The subsequent losses of a neutral molecule of
trimethylsilanol and a methyl radical by the ion e؉·
(Scheme 2) constitute potential sources of the ion at [M Ϫ
105]ϩ; however, this ion can also be formed from the
molecular ion without hydrogen transfer.
Fragmentation of Oxocarboxylic Acid
Trimethylsilyl Esters
EI mass spectra of oxocarboxylic acid trimethylsilyl esters
of formula RC( ϭ O)(CH2)nCOOTMS (Figure 3, Table 2)
exhibit fragment ions at m/z 117, 129, 145, and [M Ϫ CH3]ϩ
resulting from classical fragmentation of the TMS ester
groups [14]. Classical ␥-hydrogen rearrangement of the
keto group also affords the ion [RC(OH) ϭ CH2]ϩ·. If the
keto group possesses ␥-hydrogen atoms in both chains
consecutive rearrangements yielding the ion [CH3C(OH)
ϭ CH2]؉· can occur. The presence of a significant peak at
[M Ϫ 33]ϩ can be attributed to successive losses of a
neutral molecule of H2O and a methyl radical. Indeed, the
loss of water is often observed in EI mass spectra of
long-chain ketones [6].

72
RONTANI AND AUBERT
J Am Soc Mass Spectrom 2008, 19, 66–75
(a)
O
C
Abundance
75
COOTMS
90
80
70
60
50
40
30
20
10
0
+ H
m/z 271
m/z 58
[M – CH3 – H2O]+
[M – CH3]+
271
313
117
129
130
143
55
58
98
145
163
295
.
M+
328
255
260
199
60
100
140
180
220
300
m/z-->
O
C
(b)
COOTMS
Abundance
[M – CH3 – H2O]+
[M – CH3CH2]+
75
+ H
m/z 271
m/z 72
90
80
70
60
50
40
30
20
10
0
57
271
[M – CH3]+
327
144
129
145
157
72
117
313
309
95
111
.
M+
342
255
260
181
180
60
100
140
220
300
340
m/z-->
+ H
m/z 272
m/z 257
m/z 58
(c)
Abundance
215
COOTMS
90
80
70
60
50
40
30
20
10
0
C
O
73
+ H
m/z 215
m/z 156
m/z 58
129
[M – CH3 – H2O]+
[M - CH3]+
55
58
130
143
355
117
97
141
228
241
272
.
M+
370
171
257
199
337
340
156
60
100
140
180
220
m/z-->
260
300
Figure 3. EI mass spectra of (a) 14-oxopentadecanoic, (b) 14-oxohexadecanoic, and (c) 10-oxoocta-
decanoic acid trimethylsilyl esters.
These EI mass spectra also show strong fragment
130 (Figure 3c). Transfer of a hydrogen atom from the
ions resulting from the interactions between the keto
and ester functionalities ([RC(OTMS) ϭ CH2]؉·, [RC( ϭ
OTMS)CH ϭ CH2]ϩ and [M Ϫ RC( ϭ O)CH2]ϩ) (Figure
3, Table 2). These fragment ions are lacking or present in
very weak proportions in EI mass spectra of ketones [6]
and TMS esters [14].
To explain the formation of the ions [RC(OTMS) ϭ
CH2]؉·, we propose the pathways described in Scheme
3 involving an initial trimethylsilyl transfer from the
ester group to the ionized ketone. The distonic ion j؉·
thus formed may then undergo ␤-cleavage giving the
radical ion [RC(OTMS) ϭ CH2]؉· (k؉·). When R- has
more than two carbon atoms, subsequent ␥-hydrogen
rearrangement of the ion k؉· affords the ion l؉· at m/z
carbon in ␣-position relative to the ionized keto group
of ion j؉· to the carboxyl radical and subsequent
␥-cleavage may afford the ion [RC( ϭ OTMS)CH ϭ
CH2]ϩ (m؉). When R- is longer than ethyl, the ion m؉
can then undergo ␥-hydrogen rearrangement giving the
ion n؉ at m/z 143 (Figure 3c). The shift of the ions k؉·,
l؉·, m؉, and n؉ to m/z 232, 134, 244, and 146, respec-
tively, in the mass spectrum of (9-D2,11-D2)-10-oxoocta-
decanoic acid trimethylsilyl ester (Figure 4) is consistent
with such pathways.
We propose the fragmentation pathways described
in the Scheme 4 to explain the formation of the intense
fragment ions at [M Ϫ RC( ϭ O)CH2]ϩ in the mass
spectra of oxocarboxylic acid trimethylsilyl esters.

J Am Soc Mass Spectrom 2008, 19, 66–75
EIMS OF HYDROXY- AND OXOACID TMS DERIVATIVES
73
CH
2
OH
C
(CH )
2 n
R
O
CH
2
C
O
SiMe
3
CH
2
CH
CH
2
H
CH
2
R
O
C
C
O
H
SiMe
C
O
SiMe
3
3
(CH )
CH
2 n
CH
O
CH
2
CH
2
m
CH
2
CH
2
n , m/z 143 (R > CH CH )
3
2
R
C
R
C
O
O
SiMe
3
CH
2
CH
2
CH
2
CH
2
SiMe
3
O
C
O
C
(CH )
(CH )
2 n
2 n
O
O
CH
2
CH
j
2
CH
CH
2
2
M
R
C
O
SiMe
3
CH
2
CH
2
CH
2
O
C
O
(CH )
2 n
(CH )
2 n
C
O
CH
2
O
CH
2
CH
2
CH
2
CH
C
2
R
C
SiMe
O
H
3
SiMe
O
k
3
CH
2
CH
2
l
, m/z 130 (R > CH CH )
3
2
Scheme 3. Proposed formation pathways of ions l؉· and n؉ involving trimethylsilyl transfer.
These pathways involve initial hydrogen transfer from
the carbon in ␥-position relative to the keto group to the
ionized ester group and subsequent ␤-cleavage relative
to the keto group affording the ion o؉ and the radical p·
stabilized by conjugation. These pathways are well
supported by the presence of a peak at m/z 215 in the
mass spectrum of (9-D2,11-D2)-10-oxooctadecanoic acid
trimethylsilyl ester (Figure 4) corresponding to [M Ϫ
CH3(CH2)6CD2C( ϭ O)CD2]ϩ.
alone. Examination of EI mass spectra of hydroxycar-
boxylic and oxocarboxylic acid trimethylsilyl deriva-
tives allowed us to confirm this assertion.
Although mass spectra of hydroxycarboxylic acid tri-
methylsilyl derivatives of formula RCHOTMS
(CH2)nCOOTMS are dominated by fragment ions result-
ing from classic ␣-cleavage relative to the trimethylsily-
loxy ether group, they also exhibit fragment ions at m/z
204, 217, [M Ϫ 31]ϩ, [M Ϫ 105]ϩ, and [M Ϫ RCHO]ϩ·
resulting from trimethylsilyl and hydrogen transfers be-
tween the two functionalities of the molecule. These
fragment ions could be very useful to characterize these
compounds and notably to differentiate them from iso-
baric diol trimethylsilyl derivatives.
Conclusions
It is generally considered that a molecule containing
two functional groups can undergo EI mass fragmenta-
tions that are not found for those with either group
Mass spectra of oxocarboxylic acid trimethylsilyl

74
RONTANI AND AUBERT
J Am Soc Mass Spectrom 2008, 19, 66–75
R
C
O
CH
2
CH
2
CH
2
CH
CH
H
R
H
C
O
O
C
O
C
+
CH
2
(CH )
(CH )
2 n
2 n
p
O
O
CH
CH
2
2
SiMe
SiMe
3
3
CH
o
CH
2
2
M
Scheme 4. Proposed formation pathways of ion o؉ at [M Ϫ RC ( ϭ O)CH2]ϩ involving hydrogen
transfer.
m/z 259
D
D
D
D
COOTMS
C
O
m/z 215
+ H
m/z 276
215
Abundance
[M - CH3]+
359
90
80
70
60
50
40
30
20
10
0
73
129
134
276
.
[M]+
55
117
232
244
62
143
259
260
97
146
199
340
340
315
374
380
60
100
140
180
220
m/z-->
300
Figure 4. EI mass spectrum of (9-D2,11-D2)-10-oxooctadecanoic acid trimethylsilyl ester.
Biological Samples, with Special Attention to Drugs of Abuse and
Doping Agents. J. Chromatogr. B 1998, 713, 61–90.
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Rockford, IL, 1982; p 35.
3. Evershed, R. P. Handbook of Derivatives for Chromatography, 2nd ed;
Wiley: Chichester, 1993; p 51.
4. Goad, L. J.; Akihisa, T. Analysis of Sterols; Blackie Academic and
Professional: London, 1997; p 156
esters of formula RC( ϭ O)(CH2)nCOOTMS show frag-
ment ions at [RC(OTMS) ϭ CH2]؉·, [RC( ϭ OTMS)CH
ϭ CH2]ϩ, and [M Ϫ RC( ϭ O)CH2]ϩ also resulting from
trimethylsilyl and hydrogen transfers between the two
functionalities of the molecule. These specific fragment
ions could be very useful in differentiating these com-
pounds from isobaric fatty acid trimethylsilyl esters.
5. Kingston, D. G. I.; Bursey, J. T.; Bursey, M. M. Intramolecular Hydrogen
Transfer in Mass Spectra. II. The McLafferty Rearrangement and
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in the Mass Spectra of Trimethylsilyl Ethers of Cholesterol Oxidation
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Spectrometry. J. Am. Soc. Mass Spectrom. 1993, 4, 327–335.
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Acknowledgments
The authors acknowledge support for this work by grants from
the Centre National de la Recherche Scientifique (CNRS) and the
Université de la Méditerranée.
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