Polarizable polysiloxane stationary phase containing a cyano unit attached to an aromatic side group for highly selective separation of H-bonding and aromatic analytes

Article abstract of DOI:10.1039/c6ra20397c

A new polarizable polysiloxane stationary phase containing a cyano unit attached to an aromatic side group, called CPPP [14.6% 3,4-bis(4-cyanophenyl)-2,5-diphenyl phenyl polysiloxane], was synthesized and used as a stationary phase for capillary gas chrom

Full text of DOI:10.1039/c6ra20397c

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Polarizable polysiloxane stationary phase
containing a cyano unit attached to an aromatic
side group for highly selective separation of H-
bonding and aromatic analytes
Cite this: RSC Adv., 2016, 6, 109786
*
Xue Han, Xinxin He, Bing Wang and Bo Wu
A new polarizable polysiloxane stationary phase containing a cyano unit attached to an aromatic side group,
called CPPP [14.6% 3,4-bis(4-cyanophenyl)-2,5-diphenyl phenyl polysiloxane], was synthesized and used as
a stationary phase for capillary gas chromatography (GC). The chemical components of CPPP were
1
characterized by H nuclear magnetic resonance spectroscopy. The stationary phase exhibited excellent
film-forming ability and high column efficiency (3852 plates per m) for naphthalene at 120 ꢀC. The
thermal stability of CPPP was evaluated by thermogravimetric analysis and column bleeding. Results
revealed that CPPP could endure high temperatures up to 370 ꢀC. Moreover, the selectivity of the CPPP
stationary phase for H-bonding and aromatic analytes, including Grob test mixtures, fatty alcohols and
substituted benzenes, was evaluated. Relying on sensitive dipole–induced dipole, H-bonding and p–p
interactions with specific solutes, CPPP showed advantages over current stationary phases for
separations of polycyclic aromatic hydrocarbons and aromatic aldehydes. In conclusion, the new CPPP
stationary phase with high thermal stability and unique selectivity exhibits great potential for GC separations.
Received 12th August 2016
Accepted 11th November 2016
DOI: 10.1039/c6ra20397c
www.rsc.org/advances
molecular differences in similar structures compared with
dipole–dipole interaction. For instance, Lee et al.4,5 developed
1 Introduction
polysiloxanes with improved polarizability by using biphenyl,
dicyanobiphenyl, and naphthyl. The crosslinked stationary
phases exhibit stronger dipole–induced dipole interaction than
phenyl/diphenyl substituted polysiloxanes and possess
moderate thermal stabilities, improved polarity, and unique
selectivity for structural isomers, such as polycyclic aromatic
amines.
Numerous polysiloxanes containing phenyl or diphenyl
substituents are widely used as commercial stationary phases
for capillary gas chromatography (GC) because of their high
column efficiency, good thermal stability, and high polarity
compared with dimethyl polysiloxanes. Among these phenyl/
diphenyl substituted polysiloxanes, the content of the
diphenyl group reaches 75%,1,2 resulting in increased selectivity
for polar compounds. However, polysiloxanes with high phenyl/
diphenyl content are crystalline solids, instead of gums or
highly viscous liquids, and thus exhibit weakened coating
ability during column preparation.
The polarity and selectivity of phenyl/diphenyl substituted
polysiloxanes should be increased while maintaining their
forms as gums or liquids. This goal can be achieved by intro-
ducing polar substituent groups, such as methoxyl and cyano
group, to the phenyl ring.3 The content of phenyl/diphenyl
group should also be decreased to prevent polysiloxanes from
forming solid. Based on the enhanced dipole–dipole interaction
with solutes, the modied stationary phase is expected to exert
improved selectivity for polar compounds.
Increasing the number of rings of polycyclic aromatic
hydrocarbons (PAHs) graed to the backbone of polysiloxanes
can enhance the average molecular polarizability. According to
ref. 6, the average molecular polarizability values of benzene,
biphenyl, triuoro-substituted tetraphenyl-phenyl and cyano-
substituted tetraphenyl-phenyl are 10.40, 20.05, 46.90, and
3
˚
64.91 A , respectively. Increased polarizability can strengthen
the dipole–induced dipole interaction between the graed
polysiloxanes and polar compounds, thereby improving the
selectivity for isomers.7,8 Moreover, the new 3,4-bis(4-
cyanophenyl)-2,5-diphenyl phenyl polysiloxane (CPPP) phase
exhibits blends of polarizable as well as polar characteristics
that make CPPP have excellent separation performances for
analytes having conjugated p-electrons. The participant of
cyanophenyl-substituted functional group can produce various
of interactions (e.g., dispersive, acid–base, and dipole–induced
dipole), therefore, CPPP can be applied to separate samples
covering a broad polarity range.
Dipole–induced dipole interaction between the stationary
phase and solutes is “soer” and more sensitive to slight
College of Chemistry and Chemical Engineering, Shandong University, China. E-mail:
wubo@sdu.edu.com
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All reagents and other testing compounds, except aromatic
aldehydes (industrial grade), are of analytical grade. Poly-
ethylene pyrolysis products were prepared as described in ref. 9.
The PAHs and aromatic aldehydes were prepared by mixing
various chemically pure reagents together.
2.2 Synthesis of 3,4-bis(4-bromophenyl)-2,5-
diphenylcyclopenta-2,4-dien-1-one (Cp-Br)10
3,4-Bis(4-bromophenyl)-2,5-diphenylcyclopenta-2,4-dien-1-one
was synthesized through the cyclization reaction between 1,2-
bis(4-bromophenyl)ethane-1,2-dione (12.1 g, 0.033 mol) and
dibenzyl ketone (6.9 g, 0.033 mol) in 50.0 mL of absolute
ethanol under nitrogen atmosphere. When the solution was
heated at approximately 75 ꢀC, KOH (1.0 g) in 5.0 mL of ethanol
was added dropwise into the solution for more than 1 h under
stirring at 78 ꢀC. The reaction was allowed to continue for
30 min, and the mixture was ltered aer cooling. The crude
product was re-crystallized from ethanol/toluene (v/v ¼ 1 : 1) to
obtain pure 3,4-bis(4-bromophenyl)-2,5-diphenylcyclopenta-
2,4-dien-1-one (14.3 g, 80.0%) as bright black crystals.
Fig. 1 Reaction scheme for preparation of CPPP.
2.3 Synthesis of 3,4-bis(4-cyanophenyl)-2,5-
diphenylcyclopenta-2,4-dien-1-one (Cp-CN)
Based on above points, a new polarizable stationary phase
with 14.6% cyano-substituted tetraphenyl-phenyl functional
groups was synthesized. The details of the method for synthesis
of CPPP are presented in Fig. 1. Chromatographic properties,
such as column efficiency, polarity, thermal stability, repeat-
ability, and selectivity of CPPP-coated fused silica capillary
columns, were systematically investigated. The fabricated
capillary columns provided excellent selectivity and high sepa-
ration efficiency for solutes with similar chemical structures,
such as substituted benzenes, PAHs, and aromatic aldehydes.
The compound Cp-CN was synthesized according to ref. 9.
Briey, compound Cp-Br (2.00 g, 3.68 mmol), K4Fe(CN)6 (0.62 g,
1.48 mmol), Pd(OAc)2 (0.008 g, 0.04 mmol), P(t-Bu)3 (0.32 mL,
0.12 mmol) and Na2CO3 (0.16 mg, 1.48 mmol) were dissolved in
40.0 mL of N-methyl pyrrolidone under N2 atmosphere. The
ꢀ
mixture was stirring for 8 hours at 140 C. Aer removing the
solvent under reduced pressure, the crude product was ob-
tained. Then the crude product was puried by column chro-
matography on silica gel to obtain compound Cp-CN as dark
1
crystals (0.77 g, 48.1%). H NMR (400 MHz, CDCl3, dppm): 7.09
2 Experiment
2.1 Reagents and apparatus
(d, 4H); 7.12–7.20 (m, 4H); 7.34–7.42 (m, 6H); 7.62 (d, 4H).
HRMS (EI) calcd for C31H18N2O [M+], 434.1419; found, 434.1428.
Measurements were conducted on GC-2014C (Shimadzu,
Tokyo, Japan) equipped with a capillary split injection system
2.4 Procedure for preparation of CPPP
and a ame-ionization detector. Thermogravimetric analysis Methyl vinyl polysiloxanes with 18% vinyl group were prepared
(TGA) was employed on an LCT-2 TGA instrument (Beijing using the procedure reported by Feng et al.11 CPPP was
Optical Instrument Factory, Beijing, China). 1H nuclear synthesized through the Diels–Alder reaction of poly-
magnetic resonance (NMR) spectra were recorded on a DPX 300 methylvinylsiloxane and Cp-CN. Sufficient amounts of Cp-CN
spectrometer (Bruker Analytische Messtechnik, Bremen, Ger- (3.9 g, 9.0 mmol) and methyl vinyl polysiloxane (4.1 g, con-
many). High-resolution mass spectrometry (HRMS) was ob- taining 8.8 mmol vinyl) were dissolved in 30.0 mL of diphenyl
tained with a magnetic sector type analyzer using EI method. ether and heated at 220 ꢀC for 48 h under a continuous stream
Fused silica capillary tubes were produced in our laboratory of nitrogen. The color of the solution changed from dark brown
from ber-level (SiO2; 99.999% purity) raw tubes by using a self- to yellow with a large number of bubbles. Polymerization
made drawing machine. SiO2 (99.999% purity) was purchased reaction was terminated when no bubble was observed. The
from Beijing Shengbogaotai Optical Technology Co., Ltd. The solution was divided into two layers aer cooling, and the upper
external coating of the fused silica columns was polyamide. All diphenyl ether layer was removed to obtain a high viscosity
GC separations in this work were performed on column I (30 m polymer. For purication, the polymer was dissolved in toluene,
ꢁ 0.25 mm i.d.; lm thickness, 0.50 mm), except the separation precipitated with methanol three times, and then vacuum dried
of polyethylene pyrolysis products, which was performed on at 50 ꢀC to obtain a clear, pale yellowish gum. The yield was
1
column II (10 m ꢁ 0.25 mm i.d.; lm thickness, 0.50 mm). about 50.4%. H NMR (400 MHz, CDCl3, dppm): 6.92 (m, 4H);
Commercial DB-5 and DB-17 capillary columns (30 m ꢁ 0.25 7.06 (t, 2H); 7.20 (dd, 4H); 7.39 (m, 4H); 7.42 (m, 4H); 7.61 (s,
mm i.d.; lm thickness, 0.50 mm) were purchased from Agilent 1H). Peaks at 5.89 and 0.079 ppm were respectively attributed to
Technologies.
vinyl and C–H on Si atoms in the skeleton of polysiloxane.
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Table 2 Abraham system constants of the CPPP stationary phasea
2.5 Column preparation
T (ꢀC)
c
e
s
a
b
l
n
R2
Before coating, the fused silica capillary tubes were rinsed with
5 mL of dichloromethane and purged with nitrogen at 250 C
ꢀ
for 3 h.12,13 The column was statically coated with 0.80% solu-
tion (w/v) of CPPP in dichloromethane [containing dicu-
mylperoxide at about 4% (w/w) of the weight of the stationary
phase] as a free radical initiator14 with 0.50 mm-thick lm.15
Aer coating, the column was conditioned under nitrogen ow
80
100
120
ꢂ3.046
ꢂ3.315
ꢂ3.482
ꢂ0.004
0.021
0.799
0.721
0.672
0.730
0.642
0.528
0
0
0
0.692
0.665
0.629
38
38
38
0.99
0.99
0.99
0.048
a
e, s, a, b, l, n, and R2 stand for p–p/n–p interactions, dipole/polarizable
interactions, H-bond basicity, H-bond acidity, dispersive interactions,
number of solutes used in multiple linear regression, and coefficient
of determination, respectively.
ꢀ
ꢀ
at 50 C for 0.5 h, temperature-programmed at a rate of 1 C
minꢂ1 to 160 ꢀC (maintained for 3 h to facilitate crossꢂ-l1inking),
ꢀ
ꢀ
and then to 370 C (maintained for 24 h) at 2 C min
.
cannot be used at low temperature; as such, the CPPP stationary
phase with 14.6% functional groups was further studied.
2.6 Column evaluation
High purity nitrogen was used as carrier gas, with an injection
split ratio of 30 : 1. The efficiency of the column was evaluated
by measuring the number of plates per meter at 120 ꢀC for
naphthalene. The polarity of the stationary phase was charac-
terized using McReynolds constants; the Abraham solvation
parameter model was used to quantitatively evaluate individual
inter-molecular interactions between CPPP and solutes.16–18
3.3 McReynolds constants and Abraham system constants
The polarity and molecular interactions with solutes of CPPP are
represented by McReynolds constants19 and then compared with
DB-17 (50%-phenyl, 50%-methyl polysiloxanes)20 (Table 1). CPPP
exhibited medium polarity. By contrast, the overall polarity of
the CPPP stationary phase was lower than that of DB-17 but
similar to that of 42%-diphenyl, 58%-dimethyl polysiloxanes
according to the reference graph (CP-index versus diphenyl
content of dimethyl diphenyl-polysiloxanes).9 Based on the
structure of CPPP, the content of functional group was about
14.6% in the polymer, thereby indicating that a tetraphenyl-
phenyl group acts as more than two diphenyl groups in chro-
matographic separation. Analysis of the specic values of
McReynolds constants indicated that CPPP exhibited high Z0, U0,
and S0 values. This nding suggested the strong dipole–induced
dipole, and H-bond donor/acceptor interactions of CPPP with
solutes. Therefore, the CPPP stationary phase is suitable for
separation of polar and easily polarizable compounds.
3 Results and discussion
3.1 Characterization of the stationary phase by NMR
spectroscopy
Chemical composition of 14.6% cyano-substituted tetraphenyl-
phenyl group in CPPP was estimated from the signal integrals of
the 1H NMR spectrum. The content of the remaining vinyl
groups was 3.3%. During CPPP synthesis, 3,4-bis(4-
cyanophenyl)-2,5-diphenylcyclopenta-2,4-dien-1-one with huge
steric hindrance did not react completely with vinyl groups in
methyl vinyl polysiloxane, leading to a small percent of vinyl
retained in each polymer mixture for crosslinking.
The solvation parameter model developed by Abraham et al.
was used to characterize the intermolecular interactions
between CPPP and solutes.21,22 Table 2 lists the Abraham system
3.2 Column efficiency
The efficiency of the column was evaluated using the number of
plates per meter at 120 ꢀC for naphthalene at a liner velocity of
8 cm sꢂ1. The column efficiency was 3852 plates per meter and
exhibited an coating efficiency of 79.7%. Thus, this new
stationary phase exerts good lm-forming ability and coating
performance. We also synthesized other polysiloxane contain-
ing 18.0% cyano-substituted tetraphenyl-phenyl group,
increasing the content of tetraphenyl-phenyl group to 18.0% in
the polymer generated a solid, instead of a gummy product.
This result is consistent with those previously reported by Lee
et al.4 Therefore, similar to biphenyl polymers, the column
Table 1 McReynolds constants and the CP-index of CPPP and DB-17
Fig. 2 Chromatogram of Grob test mixtures on the CPPP column.
Peaks: (1) 2,3-butanediol; (2) decane; (3) undecane; (4) 1-octanol; (5)
nonanal; (6) 2-ethylhexanoic acid; (7) 2,6-dimethylaniline; (8) 2,6-
dimethylphenol; (9) methyl decanoate; (10) methyl undecanoate; (11)
dicyclohexylamine; and (12) methyl dodecanoate. Conditions:
temperature was increased from 40 ꢀC to 160 ꢀC at 3 ꢀC minꢂ1. Carrier
gas velocity: 13.5 cm sꢂ1. The injection and detector temperatures
were both 220 ꢀC.
Column
X0
Y0
Z0
U0
S0
Sum
CP-indexa
CPPP
DB-17 (ref. 20)
111
119
171
158
157
162
226
243
169
174
834
856
19.8
20.3
a
CP-index which was calculated from the sum of the rst ve
Rohrschneider–McReynolds constants divided by the sum of the
Rohrschneider–McReynolds constants of OV-275 multiplied by 100.
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constants of the CPPP stationary phase at 80 ꢀC, 100 ꢀC, and
120 ꢀC. Data indicated that CPPP showed strong intermolecular
interactions with solutes via dipole–induced dipole interaction
(s), dispersive interactions (l), and H-bond basicity (a).23 This
result is in good agreement with that of McReynolds constants.
As such, we used this new stationary phase to separate
substituted benzene isomers, PAHs, and aromatic aldehydes in
following sections. The nal separation results exceeded our
expectations.
3.4 Selectivity for Grob test mixtures
The prepared stationary phase exhibited good wettability on the
inner surface of the capillary tube as shown in the chromato- Fig. 4 Chromatograms of the polyethylene pyrolysis products on the
gram of the Grob test mixtures24 (Fig. 2). The mixtures were well
resolved and showed symmetrical peaks except 2,3-butanediol
(peak 1). The tailing factor of 2,3-butanediol on the CPPP
column was close to 1.09, indicating slight reversible adsorp-
tion. The elution order of Grob test mixtures remained mostly
unaltered compared with that of SOP-50 (50%-diphenyl, 50%-
dimethyl polysiloxanes).25 Irreversible acid–base adsorption was
indicated by the reduced peak areas of the acid (peak 6) and
amine (peak 11) on the chromatographic column. Notably, 2,6-
dimethylphenol (peak 8) was eluted aer 2,6-dimethyl aniline
(peak 7), thereby indicating that the new column more strongly
retained acidic analytes than alkaline ones, which was quite
different from the polarizable stationary phases studied in our
previous works.8 Moreover, the three homologous fatty acid
methyl esters (methyl decanoate, methyl undecanoate and
methyl dodecanoate) were baseline separated on the CPPP
column, TZE1 and TZE2 were calculate to be 38.5 and 36.4
respectively, demonstrating the good separation efficiency of
the CPPP column.
CPPP column (10 m ꢁ 0.25 mm i.d.). Conditions: temperature was
increased from 100 ꢀC (maintained for 1 min) to 370 ꢀC (maintain for 5
min) at 15 ꢀC minꢂ1. Carrier gas velocity: 15 cm sꢂ1. The injection and
detector temperatures were both 400 ꢀC.
CPPP column was further evaluated by GC separation of the
polyethylene pyrolysis products aer the column was condi-
tioned at 370 ꢀC for 24 h. As shown in Fig. 4, no obvious baseline
dri was observed on the CPPP column and it still exhibited
ꢀ
excellent separation performance for the analytes until 370 C.
The good thermal stability of the CPPP phase demonstrated its
potential for practical applications.
3.6 Separation performance and retention behaviour
3.6.1 Separation performance of CPPP for H-bonding
analytes. Based on the obtained data in Section 3.3, a mixture
of alcohols and n-alkanes was separated on CPPP to investigate
its separation performance for H-bonding analytes.
A
commercial DB-5 column was also provided for reference. As
shown in Fig. 5, all components were baseline resolved with
nice peak shapes on both columns. However, the retention
behaviour of CPPP was differ greatly from the DB-5 stationary
phase. On the DB-5 column, the components were eluted
according to their increasing boiling points. While on CPPP, the
alcohols were eluted aer corresponding alkanes though the
alcohol has a lower boiling point in each pair. This nding
3.5 Column bleeding and operating range
The thermal stability of CPPP polymer was analyzed via TGA
ꢂ1
ꢀ
ꢀ
ꢀ
from 100 C to 700 C at 10 C min under helium. The TGA
curve for CPPP was shown in Fig. 3. The onset of thermal
decomposition of CPPP was about 400 ꢀC and the polymer then
signicantly degraded at 410 ꢀC. The thermal stability of the
Fig. 5 Chromatograms for separation of the mixture of n-alkanes and
alcohols on CPPP and DB-5 columns. Peaks: (1) n-undecane; (2) 1-
octanol; (3) n-dodecane; (4) 1-nonanol; (5) n-tridecane; (6) 1-decanol;
(7) n-tetradecane; (8) 1-undecanol; (9) n-pentadecane; and (10) 1-
dodecanol. Conditions: temperature was increased from 100 ꢀC
(maintained for 5 min) to 160 ꢀC at 5 ꢀC minꢂ1. Carrier gas velocity:
12 cm sꢂ1. The injection and detector temperatures were both 220 ꢀC.
Fig. 3 TGA analysis for CPPP.
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suggested that the H-bonding interaction between CPPP and H- demonstrated the excellent selectivity of CPPP for aromatic
bonding analytes played a signicant function in separating analytes with subtle differences in their structures.
polar compounds.
3.6.3 Separation performance of CPPP for PAHs. For PAHs,
3.6.2 Separation performance of CPPP for substituted the sensitive dipole–induced dipole interaction and concen-
benzenes. The aromatic structure of CPPP is conducive for trated p–p interaction are extremely important in distinguish-
enhancing p–p interactions among analytes of the p-system. ing CPPP from the traditional polysiloxane stationary phases in
Consequently, CPPP was employed to separate the substituted terms of the retention behavior and resolving ability (Fig. 7).
benzenes. As shown in Fig. 6, the mixture containing 23 The baseline separation of all components was achieved, and
different di- and tri-substituted benzenes was effectively sepa- good peak shapes were obtained on the CPPP column. On the
rated and sharp symmetrical peaks were obtained on CPPP. DB-17 column, the peak pairs of 8-hydroxyquinoline (peak 8;
ꢀ
ꢀ
Peak pairs, such as 2-chlorotoluene (peak 1; b.p., 158.5 ꢀC)/3- b.p., 267 C)/acenaphthylene (peak 9; b.p., 268 C) and phen-
ꢀ
ꢀ
chlorotoluene (peak 2; b.p., 159.8 C) and 1,3-dichlorobenzene anthrene (peak 16; b.p., 340 C)/anthracene (peak 17; b.p., 342
(peak 8; b.p., 173.0 ꢀC)/1,4-dichlorobenzene (peak 9; b.p., 173.4 ꢀC) were co-eluted. Phenanthrene and anthracene are known
ꢀC) were baseline resolved on the CPPP column despite minor critical pairs in GC analysis because of their relatively similar
differences in their boiling points. The above mentioned results boiling points.24 However, this pair can be successfully sepa-
rated on the CPPP column. In terms of the retention behavior,
Fig. 6 Chromatograms of substituted benzenes on the CPPP and DB-
17 columns. Peaks: (1) 2-chlorotoluene; (2) 3-chlorotoluene; (3) 4- Fig. 7 Separation of polycyclic aromatic hydrocarbons on the CPPP
chlorotoluene; (4) 1,3,5-trimethylbenzene; (5) 1,3,5-trimethylbenzene; and DB-17 column. Peaks: (1) decahydronaphthalene; (2) tetralin; (3)
(6) tert-butylbenzene; (7) mercaptobenzene; (8) 1,3-dichlorobenzene; naphthalene; (4) 2-methylnaphthalene; (5) 1-methylnaphthalene; (6)
(9) 1,4-dichlorobenzene; (10) 1,2,3-trimethylbenzene; (11) benzyl biphenyl; (7) dibenzyl ether; (8) 8-hydroxyquinoline; (9) acenaph-
cyanide; (12) a,a-dichlorotoluene; (13) p-nitrotoluene; (14) o-nitro- thylene; (10) dihydroacenaphthylene; (11) naphthyl ethyl ether; (12)
chlorobenzene; (15) p-nitrobromobenzene; (16) diphenylmethane; fluorenone; (13) a-nitronaphthalene; (14) 4-nitrobiphenyl; (15) benzil;
(17) 4-nitrobenzyl chloride; (18) 2,4-dinitrotoluene; (19) 1-chloro-2,4- (16) phenanthrene; (17) anthracene; (18) phenanthrenequinone; (19)
dinitrobenzene; (20) 1-chloro-3,4-dinitrobenzene; (21) 1-fluoro-2,4- 1,8-dihydroxyanthracene-9,10-dione; (20) pyrene; (21) 1,2-benzan-
dinitrobenzene; (22) 1-bromo-2,4-dinitrobenzene; (23) triphenyl- thracene; and (22) chrysene. Conditions: temperature was increased
methane. Conditions: temperature was increased from 60 ꢀC to from 80 ꢀC (maintained for 5 min) to 290 ꢀC at 6 ꢀC minꢂ1. Carrier gas
320 ꢀC at 6 ꢀC minꢂ1. Carrier gas velocity: 10 cm sꢂ1. The injection and velocity: 10 cm sꢂ1. The injection and detector temperatures were
detector temperatures were both 320 ꢀC.
both 320 ꢀC.
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Table 3 Repeatability of retention times on CPPP column
Month-to-month
Column-to-column
(n ¼ 4)
(n ¼ 3)
Analytes
Mean
RSD (%)
Mean
RSD (%)
Heptane
5.552
6.108
6.962
7.988
9.859
11.138
12.051
0.19
0.51
0.26
0.17
0.39
0.63
0.32
5.559
6.127
6.959
7.993
9.856
11.141
12.120
2.82
4.29
2.75
2.36
4.11
3.59
3.14
Butyl acetate
Chlorobenzene
Bromobenzene
Octanol
p-Cresol
Acetophenone
tetraphenyl-phenyl can enhance the inductive capability of the
stationary phases for polar analytes as well as their dispersive
interactions with aromatic analytes. Therefore, based on these
results, CPPP can be applied in environmental, water, and
petroleum analyses.26,27
3.6.4 Separation performance of CPPP for aromatic alde-
hydes. A complex mixture of 39 different aromatic aldehydes
was separated on the CPPP column and on DB-17 column under
same condition (Fig. 8). The resolution of the dense peaks
separated by the CPPP column was mostly superior to that of
the DB-17 column because of the sensitive dipole–induced
dipole and p–p interaction between the CPPP phase and
aromatic aldehydes as compared with DB-17. In terms of the
elution order, the retention time of several analyteꢀs with a linear
alkyl substitute, e.g., cis-citral (peak 20; b.p., 117 C under 2.67
kPa) and trans-citral (peak 23; b.p., 118 ꢀC under 2.67 kPa), were
evidently increased on CPPP, thereby suggesting the specic
shape selectivity of CPPP and its different interactions with
analytes as compared with the conventional phase. Another
interesting phenomenon was that the halogenated aromatic
aldehydes, e.g., 2,4-dichlorobenzaldehyde (peak 24; b.p., 233 ꢀC)
and 3,4-dichlorobenzaldehyde (peak 25; b.p., 247 ꢀC), were
retained longer on CPPP. This phenomenon can be explained by
the induced polarization of cyano-substituted tetraphenyl-
phenyl groups, which contain mobile p-bonding electrons
and may acquire dipole moments when exposed to polar
compounds, such as halogenated aromatic aldehydes.
Fig. 8 Separation of aromatic aldehydes on the CPPP and DB-17
columns. Peaks: (1) 2-furaldehyde; (2) 3,5-difluorobenzaldehyde; (3)
2-fluoro-5-trifluoromethylbenzaldehyde;
(4)
3,4,5-tri-
fluorobenzaldehyde; (5) benzaldehyde; (6) 4-fluorobenzaldehyde; (7)
2-trifluoromethylbenzaldehyde; (8) phenylacetaldehyde; (9) 2-amino-
benzaldehyde;
ybenzaldehyde;
obenzaldehyde;
(10)
(12)
(14)
4-methylbenzaldehyde;
2-ethylbenzaldehyde;
(11)
(13)
2-hydrox-
2-chlor-
4-chlorobenzaldehyde;
(15)
2-
bromobenzaldehyde; (16) 4-bromobenzaldehyde; (17) 2-cyano-
benzaldehyde; (18) 1,2-benzenedialdehyde; (19) 1,4-benzenedialde-
hyde; (20) cis-citral; (21) 3-cyanobenzaldehyde; (22) 4-
methoxybenzaldehyde;
(23)
trans-citral;
(24)
2,4-dichlor-
obenzaldehyde; (25) 3,4-dichlorobenzaldehyde; (26) 2-nitro-
benzaldehyde; (27) 3-nitrobenzaldehyde; (28) 4-nitrobenzaldehyde;
(29) 4-hydroxybenzaldehyde; (30) 3-phenyl-2-propenal; (31) 3,5-
bis(trifluoromethyl)benzaldehyde; (32) 2-chloro-4-fluoro-5-nitro-
benzaldehyde; (33) 4-hydroxy-3-methoxybenzaldehyde; (34) 4-
chloro-3-nitrobenzaldehyde; (35) 2,4-dimethoxybenzaldehyde; (36)
4-dimethylaminobenzaldehyde; (37) 3,4,5-trimethoxybenzaldehyde;
(38) 2,4-dinitrobenzaldehyde; and (39) 4-fluoro-3-phenox-
ybenzaldehyde. Conditions: temperature was increased from 100 ꢀC
(maintained for 5 min) to 170 ꢀC at 4 ꢀC minꢂ1 (maintained for another
3.7 Column repeatability and reproducibility
Column repeatability (month-to-month) and reproducibility
(column-to-column) of CPPP was evaluated by separation of
several analytes of different varieties at 120 ꢀC. The columns
have been in use during storage. As obtained in Table 3, with
relative standard deviations (RSD%) of retention time below
0.63% for month-to-month and 4.29% for column-to-column,
the CPPP column is well repeatable and reproducible.
5 min) and then to 280 ꢀC at 6 ꢀC minꢂ1. Carrier gas velocity: 10 cm sꢂ1
The injection and detector temperatures were both 320 ꢀC.
.
phenanthrenequinone and 1,8-dihydroxyanthracene-9,10-dione
exhibited a reverse elution order on the CPPP column compared
with the DB-17 column. This phenomenon may be attributed to
the polarizable cyano-substituted tetraphenyl-phenyl as
appended functional groups to the polysiloxanes. Compared
with phenyl and biphenyl, the cyano-substituted tetraphenyl-
phenyl used in this study was more polarizable. The increased
average molecular polarizability of cyano-substituted
4 Conclusion
This paper reports that the synthesized CPPP stationary phase
possesses good lm-forming ability, medium polarity, high
column efficiency, high thermal stability and good repeat-
ability. The stationary phase also exhibits good resolution and
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excellent selectivity in separating Grob test mixtures, alcohols, 11 Silicone Polymers and Their Applications, ed. S. Y. Feng, J.
substituted benzenes, PAHs and aromatic aldehydes. The
present work demonstrates that CPPP are highly selective GC
Zhang, M. J. Li and Q. Z. Zhu, Chemical Industry Press,
Beijing, 2004, pp. 79–80.
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Acknowledgements
This study was supported by the National Natural Science
Foundation of China (No. 21275090) and Technology Develop-
ment Plan of Shandong Province (No. 2014GGX107010).
18 C. F. Poole and S. K. Poole, J. Chromatogr. A, 2008, 1184, 254–
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