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2. Materials and methods
2.1. Apparatus
Fluorescence signals were measured on a Cary Eclipse fluores-
cence spectrophotometer (VARIAN, USA) equipped with a xenon
lamp. All the measurements took place in a standard 10 mm path-
length quartz cell, with 2.5 nm bandwidths for the emission and
excitation monochromators.
A Lambda Bio 40 UV–vis spectrophotometer (PerkinElmer, USA)
was used for the absorbance measurements.
The 1H NMR spectrometer was a Bruker DRX300, 300.13 MHz
(Bruker, Switzerland), used for the changes of 1H spectrum.
An Escalab 250 X-ray photoelectron spectroscopy (VG, UK) was
used for recording the binding energy spectrum.
2.2. Reagents
All solvents used were of analytical reagent grade. Suppli-
ers were as follows: acetone and acetonitrile (Nanjing Chemical
Reagent Co., Ltd., China), DMSO and d6-DMSO (Beijing Chem-
ical Plant, China), methanol (Nanjing Chemical Reagent No.1
Plant, China), ethanol (Tianda Chemical Reagent Plant, Tianjin,
China), chloroform (Kemiou Chemical Reagent Research Center,
Tianjin, China), water used in all experiment was redistillation
product.
TBBQ (Aldrich Chemical Co., USA) was prepared as
1.0 × 10−3 mol/l in methanol, solution was found to be stable
for at least 1 week at room temperature. OFL, NOR and CIP of drug
standard samples were kindly provided by Shanxi Provincial Insti-
tute for Drug Control. A standard stock solution of 100 g mL−1
was prepared by dissolving three drugs standard samples in
methanol as needed. A standard working solution was prepared
by diluting a standard stock solution with methanol. The stan-
dard stock solution was stable even after several weeks at room
temperature.
2.3. Pharmaceutical formulation
The following commercial dosage forms were subjected to
the analytical procedure: Ofloxacin tablets (Lizhu Pharmaceutical
Co., Ltd., Zhuhai, China), labeled as containing 100 mg ofloxacin
per tablet; Norfloxacin capsules (Shanxi Haisi Pharmaceutical Co.,
Ltd., Jincheng, China), labeled as containing 100 mg norfloxacin
per grain. Ciprofloxacin hydrochloride tablets (Tianfang Pharma-
ceutical Co., Ltd., Henan, China), labeled as containing 250 mg
ciprofloxacin per tablet.
Fig. 1. The molecular structure of quinolone antibacterial agents.
subject drugs of known concentrations. The amount of drugs
in the sample solutions was determined from their calibration
graphs.
2.4. Procedures
2.4.1. General analytical procedures
2.4.2. Analysis of tablets and capsules
A suitable amount of OFL, NOR and CIP standard stock solu-
tions were transferred to three 10 mL calibrated flasks. 0.5 ml,
0.6 ml and 0.4 ml of TBBQ solution was added for OFL, NOR
and CIP, respectively, and each solution was diluted to 10 ml
with methanol (for OFL and NOR) and with the mixed sol-
vent (volume ration of methanol to ethanol is 4:6, for CIP) and
thoroughly mixed. The solutions were placed in a 50.0 0.5 ◦C
(for OFL and NOR) and 40.0 0.5 ◦C (for CIP) water bath for
30 min. Then the flasks were cooled to room temperature and
the fluorescence intensities of the charge-transfer complexes
of OFL, NOR and CIP were measured at 487 nm, 437 nm and
435 nm using an excitation wavelength of 286 nm, 274 nm and
276 nm against a blank solution, respectively. The calibration
graph was constructed in the same way with solutions of the
Ten tablets or capsules of each drug were weighed and pow-
dered or evacuated. An amount of the powdered tablet equivalent
to 100 mg of each drug was weighed accurately and transferred
into a 100 ml calibrated flask, dissolved in 2 ml of methanol, swirled
and sonicated for 2 min, the solution was diluted to volume with
methanol. The first 10 ml of the filtrate was discarded, and the next
10 ml of sample solution was diluted to 100 times volume with
methanol. After the solution was adequately mixed, the procedure
described in Section 2.4.1 was followed.
2.4.3. Solutions for 1H NMR measurements
7.22 mg, 6.38 mg and 7.72 mg of OFL, NOR and CIP were accu-
rately weighed and were dissolved in 0.5 ml d6-DMSO, respectively,