2. Experimental
2.1. Reagents and materials
Tetramethoxysilane (99%) and γ-choloropropyl-trimethoxysilane (98%) were
purchased from Shanghai Bore Chemical reagent Co. (Shanghai, China). Polyethylene
glycol (Mn = 10 000) was obtained from Sinopharm Chemical Reagent Co. (Shanghai,
China).
DL-Histidine
(98%) was purchased from Aladdin Chemistry Co. Ltd.. Fused
silica capillaries (i.d. = 75 μm) were purchased from Hebei Ruifeng Instrumental
Company (Hebei, China). HPLC-grade ACN from Merck (Darmstadt, Germany) was
used for the preparation of mobile phases. All other reagents used in the experiment
were of analytical grade and were used without further purification. All solutions were
passed through 0.45 μm filter prior to use.
2.2. Instrumentation
Separations were performed on an Agilent CE system (Agilent Technologies,
Germany) equipped with a diode array detector. An Agilent CE Chemstation (Rev.A
09.03) was used for the data analysis and instrumental control. The columns were
heated in a thermostat (Binder BD, Germany). pH values of the mobile phases were
measured using a Sartourius PB-10 pH meter (Beijing, China). Scanning electronic
microscopic (SEM) images were obtained by a S-4800 scanning electron microscope
(JEOL, Japan).
2.3. Column preparation
2.3.1. Pretreatment of capillary
A bare capillary was preconditioned prior to use by flushing with methanol for 20
min, H2O for 20 min, 1 M NaOH for 1.5 h and kept overnight, and then flushed with
H2O for 20 min, 1 M HCl for 2 h, H2O for another 20 min in sequence until the pH
value of the outlet solution was 7.0, and finally, rinsed with methanol for 20 min.
Then the pretreated capillary was dried by purging nitrogen gas at 120 ℃ for further
use.
2.3.2. Preparation of organic-silica hybrid monolith
240 mg of polyethylene glycol was dissolved in 2.5 ml of 0.01 M acetic acid in a
glass
vial,
and
then
0.9
ml
of
tetramethoxysilane
and
0.3
ml
of
γ-chloropropyl-trimethoxysilane were respectively added dropwise. The solution was
stirred at 0 ℃
for 4 h until a homogenous solution was obtained. The
precondensation mixture was sonicated for 3 min at 0 ℃, and then was introduced
into the pretreated capillary to an appropriate length. Afterwards, the capillary was
placed in an incubator at 55 ℃ for 12 h with both ends sealed to complete the
condensation reaction. The resultant chloropropyl modified silica (CP-silica) hybrid
monolith was then flushed with H2O and methanol to remove the polyethylene glycol
and other residuals.
2.3.3. Preparation of histidine-based hybrid monolith
An aqueous solution of histidine (10 mg ml-1) was pumped through the CP-silica
hybrid monolithic column for 30 min. Afterwards, the column was heated at 75 ℃
for 24 h with both ends sealed in an incubator. Finally, the histidine-modified silica
(HS) hybrid monolithic column was flushed with H2O to remove the unreacted
residue. The monolithic column was then cut to an effective length of 25 cm with a
total length of 33.5 cm.
2.4. Electrochromatography
Prior to CEC, the prepared column was placed in a CE cartridge and was
preconditioned with the mobile phase for at least 30 min. After that, the column was
equilibrated by applying a voltage of -20 kV until a stable current and baseline were
achieved. NaH2PO4 solution (100 mM), which was used as the phosphate buffer, was
adjusted with NaOH and HCl to different pH values. The mobile phases were
composed of ACN with various concentrations of phosphate buffer. Working solutions
were prepared by dissolving sample in the mobile phase at suitable concentrations.
Separations were performed at 25 ℃ at different applied voltages (stated in the
figure captions). Samples were injected by flushing for 0.01 min. Thiourea was
selected as EOF marker. The electroosmotic mobility (EOF) and retention factor (𝑘)
were calculated by the following equations: 𝜇𝐸𝑂𝐹 = 𝐿𝑒 ∙ 𝐿𝑡 /(𝑉 ∙ 𝑡0 ) and 𝑘 =
(𝑡𝑅 − 𝑡0 )/𝑡0, respectively. Where 𝐿𝑒 and 𝐿𝑡 are the effective length and total length
of column, respectively. 𝑉 is the applied voltage, 𝑡𝑅 and 𝑡0 represent the retention
time of analytes and thiourea, respectively.
Figures
Fig. S-1. SEM images of the HS hybrid monolithic column. Magnification: (A) 1 000
×, (B) 3 000 ×, (C) 5 000 × and (D) 15 000 ×.
Fig. S-2. (A) Influence of mobile phase pH on the EOF of the IAS hybrid monolithic
column (a) and the HS hybrid monolithic column (b). Conditions: 10 mM NaH2PO4
buffer at different pH values; applied voltage, ±20 kV; detection wavelength, 214 nm.
EOF marker, thiourea. Effects of ACN content (B) and salt concentration (C) on the
EOF velocity of the HS hybrid monolithic column. Conditions: (B) 10 mM NaH2PO4
at pH 3.0 with different ACN contents; (C) NaH2PO4 (2.5-12.5 mM, pH 3.0). Applied
voltage, -20 kV; detection wavelength, 214 nm.
Fig. S-3. Dependence of the plate height of thiourea on the linear velocity of mobile
phase on the HS hybrid monolithic column. Conditions: 10 mM NaH2PO4 buffer
containing 4% ACN at pH 3.0; applied voltage from -2 kV to -23 kV; detection
wavelength, 214 nm.
Fig. S-4. Electrochromatograms of five amides on the on the HS hybrid monolithic
column. Conditions: 10 mM NaH2PO4 buffer containing various ACN content at pH
3.0. Applied voltage, -20 kV; detection wavelength, 214 nm. Peak identification: 1,
formamide; 2, acrylamide; 3, DMF; 4, N,N-dimethyl acetamide; 5, caprolactam.
Fig. S-5. Electrochromatograms of five amides on the on the HS hybrid monolithic
column. Conditions: various NaH2PO4 buffer concentrations at pH 3.0. Applied
voltage, -20 kV; detection wavelength, 214 nm. Peak identification: 1, formamide; 2,
acrylamide; 3, DMF; 4, N,N-dimethyl acetamide; 5, caprolactam.