WANG ET AL.
5
S‐β‐CD (DS 12–15) as chiral selector at different BGE pH
are shown in Table 1.
a decreasing tendency of resolution was observed, which
might be due to the decreased opportunity for the interaction
of analytes with chiral selectors. Based on the above results,
we concluded that an increase in pH has a favorable effect on
enantiomeric resolution of the analytes, but only if the
inclusion complex directed toward anode at low pH range
(range of 2.5–4.0). Thus, the best results with respect to
resolution values were all obtained at reversed polarity mode.
Simultaneously, we investigated the effect of buffer pH
on mobility parameters of analytes to provide a better under-
standing of the host–guest interaction. Take CLO, for exam-
ple: Figure 4 shows the variation of μeof, μep, and μapp with
20 g/L S‐β‐CD (DS 7–11), increasing pH from 2.5–8.0.
CLO, as its pKa ~13.6, it could be completely positively
charged over the pH range of 2.5–8.0, and changes in BGE
pH caused a minor effect on its μep. In cases of three nega-
tively charged S‐β‐CDs, as salts of strong acid, their charge‐
state also showed pH independence. Thus, the electrostatic
interaction between the chiral selector and analyte would
remain constant along with the increase of pH. This was also
applicable for the other 11 analytes, according to their chem-
ical properties of quaternary ammonium salts (AMB and
HMB) or basic analytes (pKa ≥9.3 for the other drugs).
When BGE pH varied from 2.5–4.0, the analytes could
only be detected at the anode because electrophoretic mobil-
ity (μep, negative value) was larger than electroosmotic
mobility (μeof, positive value). With a pH increase, both
the migration times and resolution of the enantiomers
tended to increase, mainly due to increasing EOF. However,
SAL was a special case of 12 drugs. Its two enantiomers
had no elution within 40 min using the two randomly
substituted S‐β‐CDs at pH 4.0. Interestingly, similar phe-
nomena were not scarce at pH 5.0. For example, no peak
was observed for SOT, MET, PRO, SAL, and CLE with
HS‐β‐CD as a chiral selector. In the case of two randomly
sulfated CDs, most of the drugs (except TER and CLO)
couldn't be eluted within 40 min, indicating that stronger
electrostatic interaction might be formed between analytes
and two randomly substituted S‐β‐CDs. At higher pH, the
increased EOF might cause a change of apparent mobility
(μapp) toward the cathode (positive value); therefore, over
the pH range of 6.0–8.0, separation was carried out at
normal polarity mode. It was shown that migration times
of analytes were shortened with increasing buffer pH. Also,
TABLE 1 Separation results of 12 analytes with three S‐β‐CD at pH 2.5–8.0
β‐blockers
Phenethylamines
Anticholinergic agents
SOT
BEV
MET
PRO
TER
SAL
CLE
CLO
ATP
AMB
HOM
HMB
t1/min
t1/min
t1/min
t1/min
t1/min
t1/min
t1/min
t1/min
t1/min
t1/min
t1/min
t1/min
Analyte
Rs
Rs
Rs
Rs
Rs
Rs
Rs
Rs
Rs
Rs
Rs
Rs
HS‐β‐CD
2.5a 11.9 5.1 8.6 1.7 14.8 1.3 12.5 1.9 4.5 4.0 15.3 6.7
8.5 9.6 7.5 5.6
8.3 2.0
6.5 2.3
7.8 2.6 11.3 7.4
9.9 2.7 16.4 8.5 14.3 11.2
9.8 6.2
8.7 7.9
9.7 9.1
3.0a 13.3 6.6 10.0 2.0 17.2 1.4 14.3 2.3 4.8 5.3 27.4 8.5 9.6 10.1 8.7 6.4 10.5 2.5
4.0a 14.3 7.7 15.6 2.3 21.6 1.6 19.6 2.5 6.5 6.3 36.8 10.5 15.8 12.0 12.3 8.6 14.6 3.7
5.0a
nd 18.7 2.5
nd
nd 9.3 6.6
nd
8.4 3.1 12.4 8.6 15.3 8.7 10.1 3.4 16.5 2.8 15.5 7.9 12.5 9.2
6.2 2.5
4.8 2.3
nd 20.6 9.8 24.8 4.2 15.2 3.0 22.3 9.7 18.9 11.8
6.0 11.3 5.9 9.7 1.9 16.7 1.4 5.2 1.9 15.2 2.8
7.0 8.5 4.6 7.8 1.4 12.1 1.3 12.4 1.8 10.9 2.3
8.0 5.6 3.9 6.8 1.2 10.2 1.1 9.8 1.5 7.5 1.8
9.8 6.5 10.6 6.5
7.6 5.3 7.4 4.3
7.8 3.0 12.1 2.2 11.9 5.4
5.6 2.7 9.8 1.8 9.8 3.3
8.9 7.8
7.1 5.7
S‐β‐CD pH 2.5a 13.2 6.1 13.5 1.8 14.5 1.3 11.3 2.0 5.8 4.0 14.4 8.1 15.6 19.8 7.0 1.4 10.4 10.2 12.2 10.8 19.8 14.2 14.7 12.8
(DS 7–11)
3.0a 16.1 7.0 15.3 2.1 18.6 1.5 12.9 2.2 7.0 5.2 20.8 10.1 9.6 21.4 8.8 1.5 15.8 11.8 14.8 14.6 18.9 16.1 18.4 14.6
4.0a 25.9 7.5 18.6 2.4 25.4 1.6 14.6 2.4 8.3 6.2
nd 35.8 28.6 10.4 1.6 28.6 12.5 20.8 18.9 35.6 18.5 26.7 18.2
5.0a
nd nd nd nd 13.1 5.6 nd nd 20.6 1.7 nd nd nd nd
6.0 14.9 5.2 16.5 2.3 14.5 1.4 13.5 2.3 10.8 3.4 10.5 3.4 9.8 15.3 12.4 1.5 15.3 10.6 15.5 14.0 22.7 12.3 21.7 13.5
7.0 10.8 4.3 12.4 1.8 12.4 1.3 10.9 2.2 8.8 2.4
8.0 7.9 4.0 9.8 1.7 10.1 0.9 8.8 1.9 6.7 2.0
8.3 3.0 7.3 10.2 10.5 1.4 10.4 6.3 10.5 9.4 16.6 9.8 16.5 9.8
6.3 2.4
6.5 8.2 8.4 1.3
7.3 4.1
7.6 6.3 12.4 8.6 10.5 7.9
S‐β‐CD
2.5a 14.5 6.6 13.0 1.8 16.7 1.4 12.8 2.0 5.1 4.3 25.4 9.0 14.5 16.2 6.4 1.3 13.6 12.7
8.7 8.0 18.5 10.1 14.2 12.3
(DS 12–15) 3.0a 18.4 7.6 16.7 2.2 21.7 1.6 14.9 2.3 6.5 4.6 32.1 10.5 17.9 18.3 8.6 1.5 15.6 14.1 10.9 9.3 22.9 13.3 16.4 15.8
4.0a 27.3 7.9 17.8 2.3 25.6 1.6 18.5 2.4 7.2 6.2
5.0a
nd nd nd nd 10.7 5.8
6.0 15.4 6.0 20.1 2.0 18.9 1.4 16.7 2.1 12.6 3.5
7.0 12.7 5.1 14.6 1.5 15.2 1.3 13.3 2.0 9.7 3.0
8.0 8.5 3.6 10.8 1.3 12.0 1.2 10.2 1.8 7.9 2.3
nd 28.7 20.5 10.8 1.6 30.6 15.8 15.6 10.2 36.8 15.9 20.5 18.7
nd nd 16.4 1.7 nd nd nd nd
8.7 2.9 11.4 13.6 10.2 1.5 15.8 9.3 16.3 9.8 16.4 9.8 15.6 13.3
6.4 2.3
5.9 2.2
9.2 9.7 8.6 1.4 12.1 7.2 12.3 6.4 12.2 7.4 12.4 10.8
7.8 7.0 6.8 1.3 6.9 4.8 9.9 4.5 10.0 5.5 9.9 8.2
BGE condition: 20 g/L chiral selectors, 30 mM phosphate; voltage: 20 kV.
aDetected at reverse polarity.
Nd, not detected in 40 min.