Differentiation of isomeric cresols by silylation in combination with gas chromatography/mass spectrometry analysis

Article abstract of DOI:10.1002/rcm.8576

Rationale: m-Cresol is listed as a priority controlled contaminant in many countries, but it is very difficult to accurately determine isomeric cresols due to their incomplete chromatographic separation on commercially available chromatographic columns and their nearly identical mass spectra. Methods: Silylation of isomeric cresols was carried out by treatment with N-methyl-N-(trimethylsilyl)trifluoroacetamide. The formed trimethyl(tolyloxy)silanes were analyzed by gas chromatography/mass spectrometry (GC/MS). Theoretical calculations were carried out with the Gaussian 03 program using the density functional theory (DFT) method at the B3LYP/6-311 + G(2d,p) level. Results: The derivatives of three isomeric cresols and six isomeric xylenols have been completely separated on an HP-5MS capillary column within a GC run of only 10 minutes. In addition, the derivative o-cresol can be very easily differentiated from its isomers due to its characteristic base peak ion at m/z 91 in electron ionization (EI)-MS. DFT calculation results indicated that the formation of the abundant fragment ion at m/z 91 is attributed to a facile dissociation pathway involving the shift of a neighboring phenylmethyl hydrogen atom in EI-MS of trimethyl(o-tolyloxy)silane. Conclusions: Silylation provides a promising solution for simultaneous determination of isomeric cresols and isomeric xylenols.

Full text of DOI:10.1002/rcm.8576

Differentiation of isomeric cresols by silylation in combination with GC-MS
analysis
Jiaxiang Xua, Guohua Zhub, Huarong Zhanga*, Jinsong Liub, Kezhi Jianga*
a. Key Laboratory of Organosilicon Chemistry and Material Technology, Hangzhou Normal University,
Hangzhou, 311121, China
b. Zhejiang Province Environmental Monitoring Center, Hangzhou, 310015, China
Address Correspondence to:
Huarong Zhang, Dr., Email: zhanghuaxy99@hznu.edu.cn
Kezhi Jiang, A.P., Dr., Email: jiangkezhi@hznu.edu.cn
Rationale: m-Cresol is listed as the priority controlled contamination in many countries, but it is very
difficult to accurately determine isomeric cresols due to the incomplete chromatographic separation of
them on a commercially available chromatographic column and their nearly identical mass spectra.
Methods: Silylation of the isomeric cresols was carried out by treatment with
N-methyl-N-(trimethylsilyl)trifluoro acetamide (MSTFA). The formed trimethyl(tolyloxy)silanes were
analyzed by gas chromatography/mass spectrometry (GC/MS). Theoretical calculations were carried out
on the Gaussian 03 program by using the density functional theory method at the B3LYP/6-311+G(2d,p)
level.
Results: The derivatives of three isomeric cresols and six isomeric xylenols have been completely
separated on an HP-5MS capillary column within a GC run of only 10 mins. In addition, the derivative of
o-cresol can be very easily differentiated from its isomers due to its characteristic base peak ion at m/z 91
in EI-MS. DFT calculation results indicated that formation of the abundant fragment ion at m/z 91 is
attributed to a facile dissociation pathway involving the shift of a neighboring phenylmethyl H atom in
EI-MS of trimethyl(o-tolyloxy)silane.
Conclusions: Silylation provides a promising solution for simultaneous determination of isomeric isomeric
cresols and isomeric xylenols.
Keywords: cresol, GC-MS, silylation, isomeric differentiation, DFT
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lead to differences between this version and the Version of Record. Please cite this article as
doi: 10.1002/rcm.8576
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1. Introduction
Phenols have extensively used as raw materials for manufacturing plastics, dyes,
pesticides, pharmaceuticals, and other industrial chemicals. Meanwhile, some volatile
phenols have been confirmed as biotoxic compounds with teratogenesis, mutagenesis and
carcinogenic effect [1,2]. To protect the environment and human oneself, several volatile
phenols, including m-cresol and 2,4-xylenol, are listed as the priority controlled
contamination in many countries. For now, gas chromatography-mass spectrometry is the
first choice for determination of these volatile phenols [3-8]. However, m-cresol / p-cresol and
2,4-xylenol / 2,6-xylenol cannot easily be separated each other on a commercial available GC
column [4, 6]. What’s worse, these isomers exhibit the nearly identical electron ionization mass
spectra. Thus, it is very challenging to accurately determine these isomers at the same time.
Derivatization has been verified as a much effective pre-treatment method in analytical
chemistry, which provides a promising strategy for solution of many analytical problems [9,10]
Silylation is an effective characteristic derivatization method for alcohols and phenols, which
has been used to improve analytes’ thermal stability, volatility and detection sensitivity [11,12]
.
.
Several isomeric chlorophenols have been simultaneously determined by silylation and
GC-MS analysis [5,6,13]. Silylation has also been used for differentiation of cucurbic acid and
its 6,7-stereoisomers by EI-MS [14]. In this work, we try to differentiate these challenging
isomeric cresols and isomeric xylenols by silylation and GC-MS.
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2. Experimental
2.1 Reagents and Sample Preparation
The analytical reagents of cresols and xylenols were purchased from Aladdin (Beijing,
China). MSTFA was purchased from TCI (Japan). Methanol (HPLC-grade) was purchased
from Sigma-Aldrich (St Louis, MO, USA), and water (HPLC-grade) was generated using a
Milli-Q system (Millipore, Bedford, MA, USA).
2.2 Derivatization
Silylation of cresols was carried out by a classical method (Scheme 1) [12]. Every cresol
was mixed with MSTFA in dichloromethane, and the resulting mixture was kept at 40 °C for
30 min. The resulting mixture was directly analyzed by GC-MS or GC-MS/MS.
2.3 GC-MS analysis
The GC-MS experiments were performed using a Trace 2000 GC/DSQ MS instrument
(Thermo-Fisher Scientific, Waltham, MA, USA) equipped with an HP-5ms capillary column
(30 m long, 0.25 mm id, 0.25 μm film thickness; Agilent Technologies, Santa Clara, CA,
USA). Helium (>99.999%) was used as the carrier gas at a constant flow of 1.0 mL min-1.
The column temperature was maintained at 80 oC for 2 min, programmed at 3 oC min-1 to 110
oC, and then programmed at 25 oC min-1 to 260 oC, which was maintained for 5 min. The GC
inlet temperature was 260 oC and the transfer line temperature was 250 oC. Mass spectra were
acquired by EI-MS under normal conditions: the ion-source temperature was 200 oC, the
electron energy was 70 eV, the scan rate was 2 scans s-1, and the mass range was 33-600 amu.
Xcalibur software (Version 1.4) was used to control the GC-MS instrument and to acquire
and process the data.
The GC-MS/MS experiments were performed on a Shimadzu GCMS-TQ8040
instrument (Shimadzu, Japan) equipped with a DB-5ms capillary column (30 m long, 0.25
mm id, 0.25 μm film thickness). The parameters of the GC were similar to the GC-MS
experiments. The parameters of MS/MS experiments were set as the following: the
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ion-source temperature was 220 oC, the collision energy was 10 eV, and the voltage of the
detector was 1.1 kV.
2.4 Theoretical calculations
Theoretical calculations were carried out on the Gaussian 03 program by using the
density functional theory (DFT) method at the B3LYP/6-311+G(2d,p) level [15]. The
optimized structures for precursor ions, intermediates and products were identified as a true
minimum in energy by the absence of imaginary frequencies. The optimized structures were
shown by Gauss View (Version 3.09) software. The energies discussed here were the sum of
electronic and thermal free energy. The optimized geometry data were available in the
supplementary data.
3. Results and Discussion
3.1 GC separation
Fig. 1-a shows the total ion current chromatogram (TIC) of the three isomeric cresols,
but only two peaks are observed on the TIC. The peak at tR 6.13 min corresponds to o-cresol
by comparison with the retention time of its standard, while m- and p-cresols co-elute at tR
6.51 min on the TIC. Thus, it fails to accurately determine m-cresol, a precedent-controlled
pollutant, at conventional conditions, which might give rise to the false-positive result in
analysis of the real samples [4,6]. Similarly, it is also very difficult to separate the priority
controlled 2,4-xylenol from 2,6-xylenol on a common chromatographic column. Silylation is
[11,12]
a widely-used derivatization method for determination of alcohols and phenols
.
Fortunately, the silylation products of the three isomeric cresols are completely separated on a
common HP-5MS capillary column under the conventional condition (Fig. 1-b). The three
components at tR 6.34 min, tR 6.60 min and tR 6.74 min in the TIC are corresponding to the
silylation derivatives of o-, m- and p- cresols, respectively. Furthermore, a complete
separation of the derivatives of three isomeric cresols and six isomeric xylenols has been
achieved within a GC run of only 10 mins. A complete chromatographic separation on a
commercial available chromatographic column brings a bright prospect for accurate
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determination of the three isomeric cresols and six isomeric xylenols.
3.2 EI-MS analysis
Further EI-MS analysis of the three silylation derivatives has been carried out for
isomeric differentiation. As shown in Fig. 2, fragmentation of the trimethyl (o-tolyloxy)silane
radical ion (M+., m/z 180) mainly produces the dimethyl (methylphenolic)silyl cation (I165,
m/z 165) in path-1, and I165 undergoes further dissociation to yield the fragment ions of I149
(m/z 149), I135 (m/z 135) and I91 (m/z 91), through the loss of CH4, C2H6, and O=Si(CH3)2,
respectively. The above fragmentation pathways have been witnessed by MS/MS analysis of
the ion I165 (See the supplementary Figure S1). Alternatively, breakage of the O-C bond in
M+. leads to methylphenyl cation (I91, m/z 91) in path-2. These proposed fragmentation
pathways are given in Scheme 2.
To our interest, silylation derivative of o-cresol can be easily differentiated from that of
the other two isomeric cresols by EI-MS analysis, and the corresponding MS data are
summarized in the supplementary Table S1. The fragment ion at m/z 165 is the base peak ion
in the EI-MS of trimethyl(m-tolyloxy)silane and trimethyl(p-tolyloxy)silane. Whereas, the ion
at m/z 91 (100%) rather than the ion at m/z 165 (87.5%) becomes the base peak ion in the
EI-MS of the ortho- isomer, which is distinctively higher in the relative abundance than the
corresponding mI91 (m/z 91, 28.1%) and pI91 (m/z 91, 27.1%). In addition, the product ions
of oI149 (m/z 149, 17.1%) and oI-135 (m/z 135, 56.9%), originating from dissociation of
oI165, are also much more abundant than the corresponding mI149 (m/z 149, 6.9%) and
mI135 (m/z 135, 9.2%), or pI-149 (m/z 149, 6.5%) and pI135 (m/z 135, 9.5%), respectively.
The above results have also been supported by the MS/MS of the ion I-165 from the three
isomeric molecular ions (See the supplementary Figure S2), indicating that oI165 is more
feasible to undergo dissociation than mI165 and pI165. The more instability of oI165 in EI
mass spectrum can be explained as the result of the ortho-effect [16-19] (Scheme 3), in which a
neighboring phenylmethyl H atom participates in the fragmentation pathways and thus
promotes these reactions.
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3.3 DFT calculation
To further probe the mechanistic ortho-effect in EI-MS of the three isomers [16-19], DFT
calculations were carried out at the ub3lyp/6-311+g(2d,p) level, and the calculated results
were summarized in the supplementary Table S2 and Table 1.
As expected, the lone pair electron on the O atom in the structure of the derivatives is the
most preferred to be ionized upon electron impact ionization. Due to the hyper conjugation
effect of the methyl moiety, the radical cation pM+. is located at 12.4 kJ/mol below the meta-
isomer mM+. in free energy. Whereas, no significant difference in free energy change (Table 1)
was obtained for the fragmentation channels among the meta- and the para- isomers. Thus,
there is no distinctive difference in EI-MS between the meta- and the para-isomers (Figure
2).
As for the ortho- isomer, another fragmentation channels (path-1d1 and path-3 in
Scheme 3) was proposed to be responsible for the distinctively high abundance for the ion at
m/z 91. Optimized structures involved in fragmentation of oM are shown in Figure 3. Due to
the suitable steric conformation, one of the neighboring phenylmethyl hydrogen atoms of
oI165 can be easily transferred to the Si atom in the form of hydride in path-1d1 [20-23], which
results in an isomeric ion oI165b. This process needs to overcome an energy barrier of 84.6
kJ/mol (oI165-TS), and the formed oI165b is located at 34.1 kJ/mol above oI165. The
subsequent elimination of dimethylsilicone from oI165b results in the benzyl ion (oI91b) at
m/z 91 via migration of the H atom from Si atom the ipso carbon on the phenyl ring. This is
the key step in the fragmentation pathway with the accumulated energy barrier of 362.8
kJ/mol (oI165b-TS) relative to oM, which is also 41.9 kJ/mol lower in free energy than the
products (oI91 and the trimethylsiloxy radical) of path-2, indicating a more feasible
dissociation pathway due to the participation of a neighboring phenylmethyl H atom in the
fragmentation processes.
Alternatively, in the fragmentation pathway of path-3, one of the neighboring
phenylmethyl H atom can be directly transferred to the O atom in the molecular ion oM
through a five-membered ring, which overcomes an energy barrier of 180.1 kJ/mol (oM-TS)
and leads to an isomeric radical ion oM2. The subsequent 1,2-H transfer in oM2 via the
transition state of oM2-TS results in another isomeric radical ion oM3, which is located at
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39.5 kJ/mol above oM2. Then, oM3 undergoes dissociation to give oI91b at m/z 91 by the
loss of trimethylsiloxy radical. Overall, the 1,2-H transfer via oM2-TS is the key step in
path-3, with the accumulated energy barrier of 288.0 kJ/mol.
As shown in Table 1, formation of the product ion at m/z 91 (I91) in path-2 is
endoenergic by 413.6 kJ/mol for oM, 451.6 kJ/mol for mM and 471.4 kJ/mol for pM,
respectively. By contrast, the accumulated energy barrier for the fragmentation pathway to
the product ion at m/z 91 (oI91-b) is only 362.8 kJ/mol (oI165b-TS) in path-1d1 and only
288.0 kJ/mol in path-3, respectively. As can be seen, both path-3 and path-1d1 are kinetically
more feasible than fragmentation of oM (mM or pM) in path-2. Both path-3 and path-1d1 are
unique dissociation pathways to the ortho- isomer, and thus the ortho-isomer has the
significantly more abundant fragment ion at m/z 91 (base peak) than the other two isomers in
EI-MS.
In addition, due to the ortho effect, dissociation of the oM can lead to a more stable
fragment ion of oI135b with a penta- cyclic ring in structure (Scheme 3), which is 99.9
kJ/mol lower in free energy than the corresponding oI135 (Table 1). Thus, the product ion at
m/z 135 (56.9%) in EI-MS of oM is also much more abundant than the corresponding mI-135
(m/z 135, 9.2%), or pI-135 (m/z 135, 9.5%). Similar result was obtained for the fragment ion
at m/z 149 (Table 1).
4. Conclusion
In this work, three isomeric cresols and six isomeric xylenols were differentiated from
one another by silylation in combination with GC/MS analysis. These cresols and xylenols
were easily converted into the corresponding trimethyl(tolyloxy)silane or
(dimethylphenoxy)trimethylsilane, and these derivatives were completely separated on a
common HP-5MS capillary column within a GC run of only 10 mins. In addition, the
silylation derivative of o-cresol can be very easily differentiated from its isomers due to its
significantly abundant fragment ions at m/z 91, m/z 135 and m/z 149. DFT calculation results
indicated that formation of these abundant fragment ions is attributed to the ortho effect in the
EI-MS, in which a neighboring phenylmethyl H atom participates in the fragmentation
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processes for the o- isomer. These results provided a promising solution for simultaneous
determination of isomeric cresols and xylenols, and further works in this field will be carried
out.
Acknowledgments:
The authors gratefully acknowledge financial support from Scientific Research Projects
of Zhejiang Education Department (No.Y201533091), the Natural Science Foundation of
Zhejiang Province (No. LQ17E030001). Computer time was made available on the SGI Aktix
450 sever at Computational Center for Molecular Design of Organosilicon Compounds,
Hangzhou Normal University.
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Table 1. Free energy (Hartree) and relative free energy (kJ/mol) of the key species in
fragmentation of the trimethyl(o-tolyloxy)silane radical ions.
Rel G (kJ/mol)
Structure
ortho-
8.9
meta-
12.4
159.7
468.5
-
para-
0.0*
163.3
469.1
-
M+.
I-165 + .CH3
168.8
468.4
177.5
379.9
280.0
413.6
I149 + .CH3 + CH4
I149b + .CH3 + CH4
I135 + .CH3 + C2H6
I135b + .CH3 + CH4
I91 + Me3SiO.
374.3
-
385.2
-
464.0
471.4
* Relative free energy (kJ/mol) of pM+. was set at zero.
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BSTFA
Si
HO
O
Scheme 1. Silylation of cresol
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_
CH3
Path-1
Si
Si
O
O
I-165
M
m/z 165
m/z 180
_
_
O Si(CH3)2
Path-1a
CH4
_
_
OSi(CH3)3
(CH3)2
Path-1c
Path-1b
Path-2
SiH
O
Si
O
I-149
m/z 149
I-135
I-91
m/z 91
m/z 135
Scheme 2. The proposed fragmentation pathways of trimethyl(tolyloxy)silane radical ion
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_
CH3
H-shift
Si
Si
Si
Si
O
O
Path-1
O
Path-3
O
H
oM-2
oI-149b
m/z 149
oM
oI-165
m/z 165
m/z 180
m/z 180
_
CH4
Path-1d3
Path-1d
H-shift
_
_
OSi(CH3)3
O Si(CH3)2
Path-1d1
_
H
(CH3)2
Path-1d2
SiH
Si
O
Si
O
O
oI91-b
m/z 91
oI-135b
m/z 135
oI-165-b
m/z 165
oM-3
m/z 180
Scheme 3. The proposed fragmentation pathways of trimethyl(o-tolyloxy)silane radical ion
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Figure. 1 TIC of isomeric cresols (a) and the silylation derivatives of three isomeric cresols and six
xylenols (b)
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Figure 2. EI-MS of the isomeric trimethyl(o-tolyloxy)silanes
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Figure 3. Optimized structures involved in fragmentation of oM
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Figure 4. Potential energy diagram for the fragmentation pathways of o-M+. to the ions at m/z 91
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