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Journal of Chromatographic Science Advance Access published April 25, 2013
Journal of Chromatographic Science 2013;1– 6
doi:10.1093/chromsci/bmt039
Article
Determination of Ephedrine and Pseudoephedrine by Field-Amplified Sample Injection
Capillary Electrophoresis
Dongli Deng1,2, Hao Deng2, Lichun Zhang2 and Yingying Su2*
1
Engineering Research Center of Chemical Zero Discharge & Department of Chemical Engineering, Chongqing Industry Polytechnic
College, Chongqing 401120, China, and 2Analytical & Testing Center, Sichuan University, Chengdu 610064, China
*Author to whom correspondence should be addressed. Email: [email protected]; [email protected]
Received 18 July 2012; revised 7 March 2013
A simple and rapid capillary electrophoresis method was developed
for the separation and determination of ephedrine (E) and pseudoephedrine (PE) in a buffer solution containing 80 mM of NaH2PO4
( pH 3.0), 15 mM of b-cyclodextrin and 0.3% of hydroxypropyl methylcellulose. The field-amplified sample injection (FASI) technique was
applied to the online concentration of the alkaloids. With FASI in the
presence of a low conductivity solvent plug (water), an approximately 1,000-fold improvement in sensitivity was achieved without any
loss of separation efficiency when compared to conventional sample
injection. Under these optimized conditions, a baseline separation of
the two analytes was achieved within 16 min and the detection
limits for E and PE were 0.7 and 0.6 mg/L, respectively. Without
expensive instruments or labeling of the compounds, the limits of
detection for E and PE obtained by the proposed method are comparable with (or even lower than) those obtained by capillary electrophoresis laser-induced fluorescence, liquid chromatography –tandem
mass spectrometry and gas chromatography– mass spectrometry.
The method was validated in terms of precision, linearity and accuracy, and successfully applied for the determination of the two alkaloids in Ephedra herbs.
Introduction
Ephedrine (E) and pseudoephedrine (PE) are two major bioactive components in the herbal medicine Ephedra sinica (1).
Because of their diaphoretic, antipyretic, respiratory, antitussive
and antiasthmatic effects, they are usually used for the clinical
treatment of common cold, sinusitis, hay fever, bronchitis, rhinitis and upper respiratory allergies (2). Therefore, studies on E
and PE are of great and continuing interest in drug analysis and
preparation. In pharmacokinetic studies, the separation and determination of E and PE at the low mg/L level, or even lower, is
widely desired (3).
Gas chromatography (GC) (4 –6) and high-performance liquid
chromatography (HPLC) (7 –10) have been presented for determination of E derivatives. Capillary electrophoresis (CE), with
high efﬁciency, short analysis time, low operational cost and
very low sample consumption (usually a few nanoliters),
has also been introduced as a powerful complementary technique to HPLC and GC for the separation of E and/or PE in
Ephedra herbs, preparations, foods or human urine (11– 16).
Unfortunately, due to the relatively small sample size used in CE,
sensitivity is often a limitation. As a result, some sensitive extraction (17–19) or detection techniques have been developed,
such as laser-induced ﬂuorescence (LIF) and mass spectrometry
(MS), for the sensitive determination of E and PE in Ephedra
herbs, preparations or human urine (20 –22). Despite the good
sensitivity, MS and LIF are expensive for routine analysis.
Moreover, extraction techniques and derivatization of the
analytes by ﬂuorescent reagents are time-consuming and
laborious.
Due to its simplicity and practicality, ﬁeld-ampliﬁed sample injection (FASI) has been shown to be of great value and practical
applicability for CE (23 –25). In FASI, by using electromigration
injection, the sample solution of low concentration is added into
the capillary, which is ﬁlled with a buffer of high concentration.
When the sample ions pass the boundary between the sample
matrix and the buffer, with high voltage applied, the ions experience a signiﬁcant decrease in velocity because of the dramatic
decrease of the local electric ﬁeld, thus stacking at the boundary.
Recently, a sensitive method has been developed for the determination of E derivatives by using headspace solid-phase microextraction (SPME) with a novel ﬁber, followed by FASI-CE (18).
In this work, a concentration factor of 120-fold compared to the
hydrodynamic injection model was achieved by FASI.
This work aims to establish a simple, inexpensive, rapid and
sensitive method for the separation and determination of E and
PE by using capillary zone electrophoresis (CZE) – ultraviolet
(UV) detection, which is available in a majority of laboratories.
Coupled with the online concentration technique, FASI, an approximately 1,000-fold improvement was achieved in sensitivity
without any loss of separation efﬁciency, compared with hydrodynamic injection. Furthermore, without expensive instruments
or labeling of the compounds, the limits of detection (LODs) for
E and PE obtained by the present method are comparable with
(or even lower than) those obtained by CE –LIF (20 –22), liquid
chromatography– tandem mass spectrometry (LC –MS-MS) (9)
and GC –MS (4, 5). Using Ephedra sinica as a real sample, the approach was evaluated and validated for quantitative analysis.
Combined with certain extraction techniques, it is promising for
use in pharmacokinetic studies for E and PE at the low mg/L
level or even lower.
Experimental
Chemicals and reagents
Analytical grade reagents, high-purity substances and double distilled water (DDW) were used throughout. Standards of E and PE
as hydrochloride salts were obtained from the National Institute
for the Control of Pharmaceutical and Biological Product
(Chengdu, China). Hydroxypropyl methylcellulose (HPMC) and
b-cyclodextrin were purchased from Sigma-Aldrich (Steinheim,
# The Author [2013]. Published by Oxford University Press. All rights reserved. For Permissions, please email: [email protected]
Germany) and Fluka (Buchs, Switzerland), respectively. All other
reagents were obtained from Kelong Chemical Reagents Factory
(Chengdu, China). The herb sample, Ephedra sinica, was
obtained from a local pharmaceutical store.
The stock solutions of E and PE at a concentration of
1.0 mg/mL, 80 mM of NaH2PO4 and 3% of HPMC were prepared
in DDW and stored at 48C in a refrigerator. The standard solutions were prepared by stepwise dilution of the stock solutions.
The buffer solutions were adjusted to obtain the desired pH by
1.0 M phosphoric acid or sodium hydroxide.
Equipments
The experiments were performed on a lab-constructed CE
system with a CL 101A high voltage supply and a CL 1020 UV detector from Cailu Scientiﬁc Instrument Company (Beijing, China).
A fused-silica capillary of 75 mm i.d. was obtained from Yongnian
Photoconductive Fiber Factory (Hebei, China). The total length
of the 75 mm i.d. capillary for separation was 40 cm and the
effective length from the inlet to the detection point was 32 cm.
The new capillary was sequentially conditioned with 1 M of HCl,
1 M of NaOH and double distilled water (DDW) for 15 min.
Between runs, the separation capillary was ﬂushed with 1 M of
NaOH, water and buffer for 1 min, with the aid of two syringes.
Sample preparation
The herbal medicine Ephedra sinica was crushed into ﬁne
powder by a disintegrator, and 10.0 mg of plant powder was
weighed and extracted with 25 mL of water in an ultrasonic bath
for 1 h. The extractant solution was removed from the ultrasonic
bath and cooled to room temperature, and was ﬁltered through a
0.45 mm membrane and stocked for further experiments. For
the determination of E and PE in the plant extract, a 0.1 mL
stock solution was diluted properly with water as a sample solution for experiments.
CE separation and detection
All separations were run in a buffer of 80 mM of NaH2PO4
( pH 3.0), 15 mM of b-cyclodextrin and 0.3% of HPMC. The
detection wavelength was set at 214 nm. In non-stacking mode,
the injection of the samples was conducted by hydrodynamic
injection for 10 s by using gravity generated with a 14 cm height
difference. For FASI stacking mode, ﬁrst, the water plug was
loaded using the non-stacking mode into the injection end of
the capillary in 10 s; thereafter, two ends of the capillary were
both transferred into sample vials with 1.5 mL of solution, and
the analytes were electro-injected into columns by a high
voltage of 9 kV for 40 s. Finally, during the separation process, a
9 kV positive polarity of separation voltage was applied with the
background electrolyte (BGE) at two ends of the column.
Results
Linearity, precision and detection enhancement
Under the optimized conditions, calibration curves, detection
limit and precision were obtained. The analytical performance
data are listed in Table I. As shown in Table I, satisfactory linearity
2 Deng et al.
of E and PE was achieved in a range of 1.00 –100 mg/L with correlation coefﬁcients of 0.9980 and 0.9976, respectively. The detection limits of E and PE were 0.7 and 0.6 mg/L, respectively,
and the sensitivity was sufﬁcient for determination without
using expensive instruments, such as LIF (20–22) or MS (4, 5, 9).
The precision of the method was investigated by determining
the intra-day and inter-day relative standard deviations (RSDs) of
the migration time and peak area for ﬁve repeated injections. All
results are listed in Table II; RSDs for E and PE migration times
varied from 0.49 to 2.53%, and RSDs of peak areas for the two
compounds fell in the range of 3.81 –7.64%.
The enhancement ability of FASI-CE was evaluated by a comparison with the hydrodynamic injection of E and PE mixed
standard solution at 50,000 mg/L for 10 s (Figure 1A) and the
FASI injection of E and PE mixed standard solution at 50.0 mg L-1
(Figure 1B). As shown in Figure 1, peak heights were basically
the same, but the concentrations of the two standard solutions
showed a difference of 1,000 times, indicating the remarkable
stacking efﬁciency of FASI-CE for the selected template.
Sample analysis
To demonstrate the applicability of this method, it was used to
determine the E and PE in an actual sample of the herbal medicine Ephedra sinica. The Ephedra extract was diluted 200 and
1,000 times, respectively, and determined under the optimal
experimental conditions. In FASI-CE, the qualitative and quantitative analyses of E and PE were completed by the two parameters of migration time and addition of spiked standard. The
diluted samples were directly determined and the recoveries for
E and PE in the spiked samples (spiked levels of analytes were 5
and 20 mg/L, respectively) were in the range of 86.2 –102.6%
(Table III). The electropherograms of the diluted samples and
spikes are shown in Figure 2. The results demonstrated the
potential of trace analyses of a sample by FASI-CE after efﬁcient
cleanup.
Table I
Linear Ranges and LODs of FASI-CE for the Determination of E and PE*
Compound
E
PE
LOD‡ (mg/L)
Linearity
Range (mg/L)
Calibration curve†
r
1 – 100
1 – 100
Y ¼ 79.323X – 21.180
Y ¼ 89.343X – 40.119
0.9980
0.9976
0.7
0.6
*Note: A series of standard solutions of E and PE (1, 5, 10, 20, 50 and 100 mg/L) were prepared
for the calibration curves.
†
Detection limits were estimated on the basis of 3:1 signal-to-noise ratios.
‡
Y and X represent peak area (mV/s) and analyte concentration (mg/L), respectively.
Table II
Reproducibility of Migration Time and Peak Area for E and PE
Compound
RSD (%)
Inter-day (4 n 10)
E
PE
Intra-day (5 days)
Migration time
Peak area
Migration time
Peak area
0.49 –1.26
0.50 –1.25
6.68
6.79
2.53
2.45
3.81
7.64
Figure 2. Electropherograms of the diluted and spiked samples: sample diluted 1,000
times (A); spiked concentrations of E and PE at 5 mg/L for the sample diluted 1,000
times (B); sample diluted 200 times (C); spiked concentrations of E and PE at 20 mg/L
for the sample diluted 200 times (D).
Figure 1. Electropherograms of two injection modes: hydrodynamic injection of E and
PE mixed standard solution at 50,000 mg/L for 10 s (A); the FASI injection of E and PE
mixed standard solution at 50.0 mg/L using the best injection parameters (B).
Table III
Results of the Determination of Alkaloids in Ephedra sinica and Recovery Values (n ¼ 3)
Sample
E
PE
Added
content
(mg/L)
Diluted
Ephedra
(200
times)
Spiked
sample
Diluted
Ephedra
(1,000
times)
Spiked
sample
Determined
content (mg/
L + RSD, %)
Recovery
(%)
Added
content
(mg/L)
40.8 + 3.1
20.0
61.3 + 2.7
13.1 + 2.4
Recovery
(%)
19.7 + 1.9
102.6
20.0
8.4 + 1.8
5.0
Determined
content (mg/
L + RSD, %)
37.9 + 2.2
5.0
8.5 + 1.3
Effect of BGE pH
The pH value of the BGE plays an important role in the separation of compounds. On one hand, the inherent electroosmotic
ﬂow (EOF) of CE will change when the pH increases or
decreases; on the other hand, the ionization efﬁciency of compounds maintains a close link with the pH value of the buffer. In
this work, the research was conducted from pH 3.0 to 6.0 by
adjusting 80 mM of NaH2PO4 with 1 mM of NaOH and 1 mM of
HCl. The result in Figure 3 showed that with a decreased pH
value, the analysis time was extended and the resolution of E and
PE was increased. Therefore, the pH at 3.0 was the ﬁnal choice.
90.9
4.2 + 0.9
94.3
NaH2PO4, or adding borate or an organic solvent such as acetonitrile. To solve the problem of the separation of E and PE,
b-cyclodextrin (b-CD) was added to the buffer solution, in accordance with the previous work (26, 27). The effect of different amounts of b-CD was also investigated, from 0 to 20 mM.
With an increased b-CD concentration, the retention time of E
was more advanced than that of PE, and the resolution of E and
PE was improved. The 15 mM amount was ﬁnally chosen for the
following experiment.
86.2
Discussion
Optimization of separation conditions for the alkaloids
Selection of BGE
Phosphate buffer is one of the most commonly used BGEs in the
contemporary practice of CZE. Thus, NaH2PO4 was chosen as
the BGE in this work. Unfortunately, because of structural similarity, E and PE could not be completely separated in such a
buffer solution, even after changing the concentration of
Effect of HPMC
The addition of an EOF modiﬁer can result in the reduction of
the reaction between analytes and the inner wall of the capillary
to obtain ﬁne separation efﬁciency in a biological ﬁeld (28).
Cellulose and its derivations, such as HPMC, are often applied to
the buffer solution with the purpose of decreasing the migration
velocity of EOF (29). This experiment also found the same phenomenon with the addition of HPMC. As a consequence, a series
of buffers with different concentrations of HMPC were chosen
to study on the inhibitory effect of EOF. With increased concentrations of HMPC in the buffer solution, the migration times of E
and PE gradually became longer, which resulted in a smaller
EOF. The neutral molecule thiourea was added into sample as
the indicator under the buffer with 0.3% of HMPC, but it was not
detected until 90 min after injection. The results show that the
EOF could be satisfactorily suppressed by a low concentration of
Determination of Ephedrine and Pseudoephedrine by Field-Ampliﬁed Sample Injection Capillary Electrophoresis 3
Figure 4. Effect of water plug injection time on FASI stacking. Sample: 0.05 mg/mL
of E and PE; buffer solution: 80 mM NaH2PO4, 15 mM b-CD and 0.3% HMPC; injection
voltage: 6 KV; electrokinetic injection time: 30 s; separation voltage: 9 KV.
Figure 3. Effect of buffer pH on the separation of analytes. Sample: 10 mg/mL of E
and PE; buffer solution: 80 mM NaH2PO4 and 15 mM b-CD; injection mode:
hydrodynamic injection; injection time: 10 s; separation voltage: 9 KV.
HMPC. Therefore, 0.3% of HMPC was considered to be appropriate for further investigation.
Development of injection parameters for FASI
Effect of buffer conductivity
Buffer solution conductivity (solution concentration) is an important factor for the stacking effect. A higher concentration of
the buffer solution could achieve a larger electrical conductivity,
which resulted in better enrichment effects and peak proﬁles of
the compounds. However, a higher concentration of the buffer
solution produced a greater ionic strength and a higher current
in the electrophoretic running, and generated more Joule heat,
which led to instability of the baseline and broadening of peaks.
Based on this, 80 mM of NaH2PO4 was a reasonable choice
in this work.
Effect of water plug
In FASI mode, as long as the conductivity of the sample solution
is lower than the conductivity of the background buffer solution,
the ﬁeld ampliﬁcation effect could be generated. The sample
ions were introduced by electric injection mode and generated
aggregation in the capillary end, which changed the interface
strength of the electric ﬁeld and weakened the ﬁeld ampliﬁcation effect. The introduction of a low conductivity zone may
4 Deng et al.
enhance the ﬁeld ampliﬁcation places and improve the enrichment effect (30). In this work, it was found that the introduction
of a short plug of water before sample electrokinetic injection
provided a high electric ﬁeld strength from the beginning of the
injection, thus yielding an improvement in concentration factor.
The relationship between the water plug length and stacking efﬁciency of the analytes is shown in Figure 4. All peak heights of
the two compounds showed the same increasing trend with the
water plug injection time from 0 to 10 s. However, above 10 s,
a signiﬁcant loss of efﬁciency was observed. Therefore, a relatively moderate injection time of 10 s was a good choice.
Effect of electrokinetic injection time and voltage
Injection time is another important condition of FASI-CE.
In electrokinetic injection mode, the injection voltage directly
plays the decisive role in the sampling volume of the analyte,
thus affecting the enrichment effect. The peak heights of E and
PE were sharply enhanced in a certain range of injection voltage
(5 to 30 s), and they were slowly increased by the injection time
at 40 s (Figure 5). The longer the electrokinetic injection time,
the more analytes were introduced into the columns. When the
sampling time was extended to 50 and 60 s, a decline appeared
of peak height and peak broadening for E and PE. Because of the
boundary between the low conductivity zone and the buffer solution of high conductivity, the analyte still showed electromigration behavior. Although a water plug can inhibit migration in
a certain period of time, if the injection time is too long, the previously stacked ions might move across the pseudostationary
boundary from the water zone into the buffer area, resulting in
an imperfect sample stacking process and peak broadening.
Thus, the electrokinetic injection time of 40 s was considered.
Similarly, sampling voltage also affected the amount of sampling and showed the same changing trend as injection time.
The effect of sample injection voltage on peak heights was
Figure 5. Effect of sample electrokinetic injection time in FASI on the peak heights of
analytes. Sample: 0.05 mg/mL of E and PE; buffer solution: 80 mM NaH2PO4, 15 mM
b-CD and 0.3% HMPC; injection time of water plug: 10 s; injection voltage: 6 KV;
separation voltage: 9 KV.
investigated in the range of 6.0 –12.0 kV. When the sampling
voltage was 9.0 kV, the best stacking effect was obtained.
Conclusions
The present work demonstrated the strong potential for the detection of E and PE in trace compounds. Inclusion compounds
were formed by b-CD with E and PE, which changed the properties of the template molecules and made the separation complete. Using a combination of FASI and CZE, a highly efﬁcient
concentration was obtained with over a 1,000-fold increase in
sensitivity. For its sensitivity, repeatability and wide linear range,
this method can be applied to trace analyses of E and PE. On the
other hand, coupling with some extraction techniques, the high
sensitivity and the low analysis costs make this method suitable
for large-scale applications.
Acknowledgments
Financial support from the National Natural Science Foundation
of China (No. 21105067) and Scientiﬁc Research Foundation of
Chongqing Industry Polytechnic College (GZY201110-YK) are
gratefully acknowledged.
References
1. New Medical College of Jiangsu; Dictionary of Chinese traditional
medicines. People’s Publisher of Shanghai, Shanghai, China, (1977),
pp. 2222–2225.
2. Josefson, D.; Herbal stimulant causes US deaths; British Medical
Journal, (1996); 312: 1378–1379.
3. Macek, J., Ptacek, P., Klıma, J.; Rapid determination of pseudoephedrine in human plasma by high-performance liquid chromatography;
Journal of Chromatography B, (2002); 766: 289–294.
4. Marchei, E., Pellegrini, M., Paciﬁci, R., Zuccaro, P., Pichini, S.; A rapid
and simple procedure for the determination of ephedrine alkaloids
in dietary supplements by gas chromatography– mass spectrometry;
Journal of Pharmaceutical and Biomedical Analysis, (2006); 41:
1633– 1641.
5. Li, H.X., Ding, M.Y., Lv, K., Yu, J.Y.; Separation and determination of
ephedrine alkaloids and tetramethylpyrazine in Ephedra sinica
Stapf by gas chromatography-mass spectrometry; Journal of
Chromatographic Science, (2001); 39: 370–374.
6. Wang, M., Marriott, P.J., Chan, W.H., Lee, A.W.M., Huie, C.W.;
Enantiomeric separation and quantiﬁcation of ephedrine-type alkaloids in herbal materials by comprehensive two-dimensional gas
chromatography; Journal of Chromatography A, (2006); 1112:
361–368.
7. Imaz, C., Navajas, R., Carreras, D., Rodrıguez, C., Rodrıguez, A.F.;
Comparison of various reversed-phase columns for the simultaneous
determination of ephedrines in urine by high-performance liquid
chromatography; Journal of Chromatography A, (2000); 870:
23– 28.
8. Pellati, F., Benvenuti, S.; Determination of ephedrine alkaloids in
Ephedra natural products using HPLC on a pentaﬂuorophenylpropyl
stationary phase; Journal of Pharmaceutical and Biomedical
Analysis, (2008); 48: 254– 263.
9. Gray, N., Museng, A., Cowan, D.A., Plumb, R., Smith, N.W.; A simple
high pH liquid chromatography–tandem mass spectrometry method
for basic compounds: Application to ephedrines in doping control
analysis; Journal of Chromatography A, (2011); 1218: 2098– 2105.
10. Bagheri, H., Khalilian, F., Ahangar, L.E.; Liquid–liquid–liquid microextraction followed by HPLC with UV detection for quantitation of
ephedrine in urine; Journal of Separation Science, (2008); 31:
3212–3217.
11. Chen, H.L., Chen, X.G., Pu, Q.S., Hu, Z.D., Zhao, Z.F., Hooper, M.;
Separation and determination of ephedrine and pseudoephedrine by
combination of ﬂow injection with capillary electrophoresis;
Journal of Chromatographic Science, (2003); 41: 1 –5.
12. Li, G.B., Zhang, Z.P., Chen, X.G., Hu, Z.D., Zhao, Z.F., Hooper, M.;
Analysis of ephedrine in Ephedra callus by acetonitrile modiﬁed
capillary zone electrophoresis; Talanta, (1999); 48: 1023–1029.
13. Jiang, T.F., Lv, Z.H., Wang, Y.H.; Chiral separation of ephedrine alkaloids from Ephedra sinica extract and its medicinal preparation
using cyclodextrin-modiﬁed capillary zone electrophoresis; Journal
of Analytical Chemistry, (2007); 62: 85 –89.
14. Li, F., Ding, Z.T., Cao, Q.E.; Separation and determination of ephedrine and pseudoephedrine in Ephedrae Herba by CZE modiﬁed
with a CUOO-L-lysine complex; Electrophoresis, (2008); 29:
658– 664.
15. Huang, H.Y., Hsieh, Y.Z.; Determination of puerarin, daidzein, paeoniﬂorin, cinnamic acid, glycyrrhizin, ephedrine, and [6]-gingerol in
Ge-gen-tang by micellar electrokinetic chromatography; Analytical
Chimica Acta, (1997); 351: 49– 55.
16. Borst, C., Holzgrabe, U.; Comparison of chiral electrophoretic separation methods for phenethylamines and application on impurity analysis; Journal of Pharmaceutical and Biomedical Analysis, (2010);
53: 1201–1209.
17. Wei, F., Zhang, M., Feng, Y.Q.; Combining poly (methacrylic
acid-co-ethylene glycol dimethacrylate) monolith microextraction
and on-line pre-concentration-capillary electrophoresis for analysis
of ephedrine and pseudoephedrine; Journal of Chromatography B,
(2007); 850: 38 –44.
18. Fang, H.F., Liu, M.M., Zeng, Z.R.; Solid-phase microextraction coupled
with capillary electrophoresis to determine ephedrine derivatives in
water and urine using a sol– gel derived butyl methacrylate/silicone
ﬁber; Talanta, (2006); 68: 979– 986.
19. Deng, D.L., Zhang, J.Y., Chen, C., Hou, X.L., Su, Y.Y., Wu, L.;
Monolithic molecular imprinted polymer ﬁber for recognition and
solid phase microextraction of ephedrine and pseudoephedrine in
biological samples prior to capillary electrophoresis analysis;
Journal of Chromatography A, (2012); 1219: 195– 200.
20. Zhang, J.Y., Xie, J.P., Chen, X.G., Hu, Z.D.; Sensitive determination
of ephedrine and pseudoephedrine by capillary electrophoresis
with laser-induced ﬂuorescence detection; Analyst (2003); 128:
369– 372.
21. Zhang, J.Y., Xie, J.P., Liu, J.Q., Tian, J.N., Chen, X.G., Hu, Z.D.;
Microemulsion electrokinetic chromatography with laser-induced
Determination of Ephedrine and Pseudoephedrine by Field-Ampliﬁed Sample Injection Capillary Electrophoresis 5
22.
23.
24.
25.
ﬂuorescence detection for sensitive determination of ephedrine and
pseudoephedrine; Electrophoresis, (2004); 25: 74 –79.
Zhou, L., Zhou, X.M., Luo, Z., Wang, W.P., Yan, N., Hu, Z.D.;
In-capillary derivatization and analysis of ephedrine and pseudoephedrine by micellar electrokinetic chromatography with laserinduced ﬂuorescence detection; Journal of Chromatography A,
(2008); 1190: 383– 389.
Yu, L.S., Xu, X.Q., Huang, L., Lin, J.M., Chen, G.N.; Microemulsion
electrokinetic chromatography coupling with ﬁeld ampliﬁed sample
injection and electroosmotic ﬂow suppressant for analysis of some
quinolizidine alkaloids; Journal of Chromatography A, (2008);
1198–1199: 220–225.
Zinellu, A., Sotgia, S., Campesi, I., Franconi, F., Deiana, L., Carru, C.;
Measurement of carnosine, homocarnosine and anserine by FASI
capillary electrophoresis UV detection: Applications on biological
samples; Talanta, (2011); 84: 931–935.
Santalad, A., Burakham, R., Srijaranai, S., Grudpan, K.; Field-ampliﬁed
sample injection and in-capillary derivatization for capillary
6 Deng et al.
26.
27.
28.
29.
30.
electrophoretic analysis of metal ions in local wines; Microchemical
Journal, (2007); 86: 209– 215.
Aturki, Z., Fanali, S.; Use of beta-cyclodextrin polymer as a chiral
selector in capillary electrophoresis; Journal of Chromatography A,
(1994); 680: 137–146.
Flurer, C.L., Lin, L.A., Satzger, R.D., Wolnik, K.A.; Determination of
ephedrine compounds in nutritional supplements by cyclodextrinmodiﬁed capillary electrophoresis; Journal of Chromatography B,
(1995); 669: 133–139.
Mitnik, L., Salome´, L., Viovy, J.L., Heller, C.; Systematic study of ﬁeld
and concentration effects in capillary electrophoresis of DNA in
polymer solutions; Journal of Chromatography A, (1995); 710:
309– 321.
Tilley, M., Bean, S.R., Tilley, K.A.; Capillary electrophoresis for monitoring dityrosine and 3-bromotyrosine synthesis; Journal of
Chromatography A, (2006); 1103: 368– 371.
Chien, R.L., Burgi, D.S.; On-column sample concentration using ﬁeld
ampliﬁcation in CZE; Analytical Chemistry, (1992); 64: 489A–496A.