Chemiluminescence System of Permanganate …...capillary electrophoresis with laser-induced fluorescence detection, e gas chromatography, f CL: chemiluminescence A. Mokhtari & M. Aaghamohammadhasan

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Eurasian Journal of Analytical Chemistry ISSN: 1306-3057 2017 12(2):61-74

DOI 10.12973/ejac.2017.00154a

© Authors. Terms and conditions of Creative Commons Attribution 4.0 International (CC BY 4.0) apply.

Correspondence: Ali Mokhtari, Department of Science, Golestan University, Gorgan, I.R. Iran.

[email protected]

Chemiluminescence System of Permanganate-Sulfite for Simple Determination of Zolpidem

Ali Mokhtari Golestan University, IRAN

Mohsen Aaghamohammadhasan Young Elite Sponsors Institute, IRAN

Received 19 September 2016 ▪ Revised 4 November 2016 ▪ Accepted 4 November 2016

ABSTRACT

Zolpidem is an imidazopyridine agent for short term treatment of insomnia with a favorable

adverse effect profile. A novel chemiluminescence (CL) method has been proposed for

simple determination of zolpidem. The method is based on the fact that zolpidem can

produce CL emission in the system of acidic KMnO4 (permanganate) and Na2SO3 (sulfite).

On the optimized conditions of chemical variables, CL intensity was proportional to

concentration of zolpidem in the range 2.5-295.1 ng mL-1. Limit of detection was 1.6 ng mL-

1 (S/N=3). The percent of relative standard deviation was 4.5% for 98.4 ng mL-1 zolpidem.

The proposed method was satisfactorily applied for the determination of zolpidem in

pharmaceutical formulations and human plasma samples. Sampling rate of the method was

calculated about 30 samples h-1. CL mechanism has been proposed using UV-Vis and CL

spectra.

Keywords: zolpidem, chemiluminescence, permanganate, plasma, pharmaceuticals

INTRODUCTION

Zolpidem is an imidazopyridine agent with a favorable adverse effect profile [1]. It is indicated

for the short term (≤4 weeks) treatment of insomnia. Zolpidem is rapidly absorbed; a mean

maximum plasma concentration of 121 ng mL-1 is reached 1.6 hours after a 10 mg dose [2]. An

overdose of zolpidem may cause excessive sedation, pin-point pupils, or depressed

respiratory function, which may progress to coma [1], and possibly death [3]. Combined with

alcohol, opiates, or other CNS depressants, it may be even more likely to lead to fatal

overdoses. Zolpidem may provide short-lasting but effective improvement in symptoms of

aphasia present in some survivors of stroke [4].

Various analytical methods have been proposed for the determination of zolpidem such

as liquid chromatography with mass spectrometry detection [5-11], high performance liquid

chromatography with spectrophotometric detectors [12-14], capillary electrophoresis [15], gas

chromatography [16-17], spectrophotometry [18, 19], potentiometry [20], voltammetry [21]

and Radioimmunoassay [22].

mailto:[email protected]

A. Mokhtari & M. Aaghamohammadhasan

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Chemiluminescence (CL) methods because of their intrinsic advantages, such as high

sensitivity, wide linear dynamic range and simple instrumentation have become an important

and valuable detection method in analytical chemistry. CL relying on the effects related to the

chemical reaction only, i.e. without the need of external energy supply, has been found to be

more advantageous than other luminescence methods [23].

Acidic potassium permanganate is one of the most important oxidants applied in CL

reactions [24]. Reaction between potassium permanganate and sulfite ions produce a week CL

emission which can enhance in the presence of some analytes [25-27]. It is proposed that sulfite

acts as a reductant to produce an excited molecule of sulfur dioxide, which emits radiation in

the range of 450 - 600 nm [26].

The aim of this work is to develop a simple and rapid method for the determination of

zolpidem that does not require complex instruments but gives results better or comparable to

those obtained by the previously methods. The method described is based on the sensitizing

effect of zolpidem in the reaction of sulfite with acidic permanganate. This method was

applied to the determination zolpidem in pharmaceuticals and human plasma. To our best

knowledge, this report describes the first application of chemiluminescence to the

determination of zolpidem.

In Table 1 some analytical characteristics are compared for the methods proposed for

the determination of zolpidem. Compared to chromatographic or electrophoretic methods,

this CL method has the advantages of simplicity, rapidity, use of non-expensive

instrumentation and compared to other spectrophotometric or electrochemical methods, this

CL method has a better sensitivity. CL methods have found extensive applications in many

interesting areas, but their main disadvantages are usually related, in general, with their poor

selectivity [25]. Some ultrasensitive methods also have potential for determining the target

molecules [28-34].

EXPERIMENTAL

Materials and methods

All chemicals were analytical grade. Deionized water was used in all experimens. Stock

solution of zolpidem (3.27×10-3 mol L-1, 1500 µg mL-1) was prepared by dissolving 0.1500 g

zolpidem tartrate (Tehran Darou Co., Iran) in 5.0 mL methanol (Dr. Mojallali Co., Iran) and

dilution with water in a 100-mL volumetric flask. The stock solution was stored in a

refrigerator and kept from the light. Working solutions were obtained by serial dilution of

stock solution. permanganate solution (0.001 mol L-1) was daily prepared by dissolving 0.0160

g of KMnO4 (Chem lab, Belgium) in calculated volume of 1.0 mol L-1 sulfuric acid (Merck,

Germany) solution and diluting with water in a 100-mL volumetric flask. Stock solution of

sulfite (1.0 mol L-1) was prepared by dissolving 6.30 g of Na2SO3 (Chem lab, Belgium) in water

and diluting to mark in a 50-mL volumetric flask. The working solutions were prepared with

Eurasian J Anal Chem

63

a series of dilutions of the main solution with deionized water. Zolpidem tablets (5 mg/tablet

and 10 mg/tablet) were provided from local drug stores.

CL Instrument

We used a lab-made CL analyzer. The light emitted by the CL reaction was detected with

no wavelength discrimination with a head on photomultiplier tube (PMT) located inside a

darkroom. Reaction cell was a 0.50-cm path length quartz cell. The block diagram of the

instrument is shown in Figure 1.

Preparation of real samples

Five tablets were weighed and powdered. An accurately weighed portion of the powder,

including active ingredients equivalent to one tablet dosage, was transferred into a 250.0-mL

volumetric flask containing 20.0 mL H2O and 5.0 mL methanol. The mixture was sonicated for

10 minutes. Then the volume was adjusted to 100.0 mL with water and the suspension was

filtered. An appropriate volume of the sample solution was further diluted with water so that

the final concentration was in the working range.

For plasma samples only a deproteination process was carried out by using acetonitrile

as a sample pretreatment and extraction procedure was not necessary [35, 36]. The standard

Table 1. Analytical features of the methods proposed for the determination of zolpidem

Method LDR

(ng mL-1)

LOD

(ng mL-1)

Sample Speed

(h-1)

Ref.

LC/MS a 1-200 N/A Blood [5]

LC/MS 1-250 N/A Blood [6]

UPLC/MS/MS b 0.1-600 0.05 Blood, Urine 15 [7]

LC/MS/MS 0.1-0.5 0.01 Saliva [8]

LC/MS/MS 1-5, 100-200 0.5 Saliva [9]

LC/MS/MS 2.5-300 N/A Plasma [10]

UHPLC/MS/MS c N/A 0.09 Blood [11]

HPLC 1-400 N/A Plasma 7 [12]

HPLC N/A N/A N/A 5 [13]

HPLC/FL 1.8-288 N/A N/A [14]

CE/FL d N/A 2.0 Urine 6 [15]

GC e 5-200 N/A N/A [16]

GC N/A LOQ: 1.0 Plasma [17]

Spectrophotometry (5-50)×103 N/A Tablet [18]

Spectrophotometry (2-16)×103 0.038 Tablet [19]

Potentiometry 30.7-30.7×103 N/A Tablet [20]

Voltammetry 153-3070 61.4 Tablet [21]

Radioimmunoassay N/A LOQ: 0.1 Urine, Serum [22]

CL f 2.5-295.1 1.6 Tablet, Plasma 30 This work a LC/MS: liquid chromatography-mass spectrometry, b UPLC: ultra-performance liquid chromatography, d CE/FL:

capillary electrophoresis with laser-induced fluorescence detection, e gas chromatography, f CL: chemiluminescence


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addition method was used for the determination of zolpidem in the plasma samples.

Therefore, each time, 0.4 mL of plasma sample was transferred into a centrifuge tube including

2 mL of acetonitrile and the mixture centrifuged at 4000 r/min for 15 min. The protein-free

supernatant was transferred into a small conical flask and evaporated to dryness under a

stream of nitrogen at room temperature. The dry residue was transferred into a 25.0 mL flask

using double distilled water, then the standard solution was added into the flask and the

mixture was diluted to the mark.

Analytical procedure

Acidic potassium permanganate (400 µL) along with 400 µL zolpidem solution were

transferred into the reaction cell using a calibrated sampler. Then, the cell was placed at its

location in the darkroom and in front of photomultiplier tube (PMT). After a few seconds 200

µL of sodium sulfite solution was injected into the cell using a microsyringe and a needle. The

time profile of CL emission was recorded by a computer. The data information was collected

automatically into an Excel file.

RESULTS AND DISCUSSION

Time profile of CL reaction

The CL reaction of zolpidem in the system of permanganate-sulfite is very fast. Typical

CL peaks of zolpidem are shown in Figure 2. Maximum CL intensity appeared about 0.3

second from reagent mixing and then CL intensity declined to base after about 0.5 second.

Figure 1. Schematic block diagram of the CL instrument


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Figure 2. Typical CL profiles for some concentrations of zolpidem including: a) 2.45 b) 6.1 c) 24.6 d) 98.4 e) 295.1 and f) 983.7 ng mL-1

Modes of determination

To achieve highest sensitivity, two different modes of detection were examined for the

determination of zolpidem. In the first mode, zolpidem was mixed with sulfite in the reaction

cell and then acidic permanganate was injected into the cell to start CL reaction. The CL

response recorded from this mode is shown in Figure 3 (peak b). In second mode, sulfite

solution was injected into the cell containing zolpidem and permanganate solutions, (peak c

in Figure 3). No background was seen from the blank in the second mode (peak a). First mode

yielded a weak CL signal (with good reproducibility) and second mode had a good sensitivity

Figure 3. CL time profiles obtained from two modes of detection. peak a: background from reaction

between sulfite and acidic permanganate in second mode, peak b: CL from injecting permanganate

solution into the cell containing zolpidem and sulfite solutions (first mode), peak c: CL from injecting

sulfite solution into the cell containing permanganate and zolpidem solutions after 35 s (second mode).

permanganate: 1.0×10-3 mol L-1, H2SO4: 4.5×10-2 mol L-1, sulfite: 0.1 mol L-1, zolpidem: 98.4 ng mL-1


66

but the time spent after mixing the zolpidem solution with permanganate is important and it

has to be identical in all experiments to obtain reproducible results. Second mode was adopted

because it had better S/N in comparison with first mode.

As can be seen in Figure 4, the shorter time of mixing results in higher sensitivities.

Therefore, sulfite solution was injected into the cell containing zolpidem and permanganate

solutions after shortest possible time (35 s). This mixing time was identical for all samples and

standards.

Figure 4. Effect of mixing time on the sensitivity. Mixing time is: the time after mixing the MnO4- and

zolpidem solutions in the reaction cell just before injecting sulfite solution. Conditions: permanganate

(1.0×10-3 mol L-1), sulfite (0.04 mol L-1), H2SO4 (0.1 mol L-1) and zolpidem (98.4 ng mL-1)

Optimization of chemical variables

To study the effect of chemical variables, influence of KMnO4, H2SO4 and Na2SO3

concentrations on the CL intensity were investigated.

The influence of concentration of KMnO4 on the sensitivity was studied in the range

4.0×10-5-4.0×10-3 mol L-1. KMnO4 solutions were prepared in 0.1 mol L-1 of H2SO4. As can be

seen in Figure 5 the CL signal increase with increasing KMnO4 concentration to 1.0×10-3 mol

L-1 and then decrease. So, concentration of 1.0×10-3 mol L-1 was selected as the optimum

concentration of KMnO4.


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Figure 5. Effect of permanganate concentration on the sensitivity. Conditions: sulfite (0.04 mol L-1),

H2SO4 (0.1 mol L-1) and zolpidem (98.4 ng mL-1)

The effect of Na2SO3 concentration on the CL sensitivity was studied in the range 4.0×10-

3 to 0.3 mol L-1. For this variable sensitivity increased with increasing the concentration to 0.1

mol L-1 and then decreased slowly at higher concentrations (Figure 6). So, concentration of 0.1

mol L-1 of Na2SO3 was selected as the optimum concentration for further investigations.

The effect of H2SO4 concentration on the CL intensity was studied in the range 0.01 to

0.12 mol L-1. The CL response increased with increasing the concentration of H2SO4 to 0.045

mol L-1 and then decreased. Therefore, 0.045 mol L-1 of H2SO4 was selected for further studies.

The results are shown in Figure 7.

Figure 6. Effect of sulfite concentration on the CL intensity.

Conditions: permanganate (1.0×10-3 mol L-1), H2SO4 (0.1 mol L-1) and zolpidem (98.4 ng mL-1)


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Figure 7. Effect of H2SO4 concentration on the CL intensity.

Conditions: permanganate (1.0×10-3 mol L-1), sulfite (0.1 mol L-1) and zolpidem (98.4 ng mL-1)

Analytical features

It was found that, CL response of zolpidem is linear for the concentration range between

2.5 and 295.1 ng mL-1 (y = 0.0452x + 0.0433, R² = 0.9971). The limit of detection (LOD) was

calculated 1.6 ng mL-1. For calculating LOD, the equation 3s/m was used in which s and m are

standard deviation in blank signal and the slope of calibration curve, respectively. The

standard deviation in blank signal was determined based on peak to peak noise (6s=0.15 mV).

Reproducibility was investigated and the percent of relative standard deviations (n=11) for

98.4 ng mL-1 zolpidem was 4.5%. Samplenig rate of the method was calculated about 30

samples h-1.

Interference study

The selectivity and direct application of proposed method for analyzing zolpidem was

studied by analyzing zolpidem in presence of some ions and excipients in pharmaceutical

preparations without any prior separation or isolation. The effect of these foreign species

under optimized CL conditions was determined by analyzing the standard solution of

zolpidem (61.4 ng mL-1) in presence of these compounds. The tolerance of each substance was

taken as the largest concentration of substance yielding an error of less than 3σ in the analytical

signal of 61.4 ng mL-1 of zolpidem (σ is the standard deviation in the response obtained from

11 times repeating determination of 61.4 ng mL-1 zolpidem) [32]. The results showed that no

interference could be detected when the sample include up to a 100 fold Al3+, Fe2+, K+, Ca2+,

Mg2+ Na+, NO3-, Starch, 10 folds of sucrose, and uric acid.

Analysis of real samples

The CL method was used for the determination of zolpidem in tablets and plasma

samples using standard addition method. Table 2 shows the analytical recoveries from real

samples.


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Table 2. Determination of zolpidem in real samples

Sample Added

(ng mL-1)

Found

(ng mL-1) a Recovery (%)

Tablet 1 (5 mg)

10.0 9.42±0.75 94.2

30.0 31.89±1.99 106.3

60.0 58.91±3.22 98.2

90.0 90.04±3.48 100.0

Tablet 2 (10 mg)

10.0 10.11±0.55 101.1

30.0 29.05±2.18 96.8

60.0 63.82±5.11 106.4

90.0 95.32±7.33 105.9

Plasma

100.0 108.4±14.27 108.4

200.0 187.6±23.31 93.8

400.0 395.9±18.60 99.0 a mean values of three replications

The matrix of each tablet has been diluted about 250000 times. Zolpidem concentration

in the ultimate diluted solution of the tablet was about 20-40 ng mL-1. High sensitivity of the

method allowed us to use this dilution factor (250000). Concentration of inactive ingredients

in tablet after 250000 times dilution is very low. As shown in Table 2, interference effect from

inactive ingredients is negligible (average recovery for tablet analysis is 101.1%).

Possible mechanism

The light emitted in the CL reactions of permanganate can be grouped as: manganese

ions, singlet oxygen and oxidation products of analyte. No detectable CL intensity observed

for the mixture of permanganate and zolpidem. This suggests that oxidation products and

manganese species or singlet oxygen are not main emitters. When a fluorescent species exists

in the reaction, it can receive energy from an excited intermediate and emit light with different

energy. Excited sulphur dioxide molecules are often considered as CL emitters when sodium

sulfite reacts with potassium permanganate [37]. As can be seen in Figure 8, the spectrum

which is corresponded to permanganate (spectrum a) inhibits in presence of sulfite (spectrum

c). It is due to reduction of permanganate by sulfite ions.


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Figure 8. UV-Vis spectrum of a) permanganate b) permanganate-zolpidem c) permanganate-

sulfite d) permanganate-sulfite-zolpidem e) sulfite conditions: permanganate (1.7×10-4 mol L-1), sulfite

(2.5×10-2 mol L-1), zolpidem (3.75 µg mL-1)

In this work, it was found that zolpidem could produce an intense CL in the system of

potassium permanganate and sulfite. The mechanism could be due to energy transfer from

some oxidation products of zolpidem to SO2 molecules when collisions occurs. Then, excited

state molecules of SO2 could return to ground state with emitting light. SO2* molecules emit

light in the range 450-600 nm [38]. The reactions in this system explained by Meixner and

Jaeschke [39] as follows:

𝑀𝑛𝑂4− + 𝐻+ + 𝑆𝑂3

2− → 𝑀𝑛𝑂42− + 𝐻𝑆𝑂3

●

2𝐻𝑆𝑂3● → 𝑆2𝑂6

2− + 2𝐻+

𝑆2𝑂62− → 𝑆𝑂4

2− + 𝑆𝑂2*

𝑆𝑂2*→ 𝑆𝑂2 + ℎ𝜐

UV-Vis spectra showed that permanganate reacts slowly with zolpidem. As can be seen

in Figure 8, absorption spectrum of permanganate (spectrum a) is decreased at presence of

zolpidem (spectrum b).

The CL and fluorescence spectra were obtained with a spectrofluorimeter (Spectrolab,

model Spectro-96) with turned off excitation lamp. No useful and interpretable results

obtained using fluorescence spectrum of various mixtures. So these results put aside. The CL

reaction was very fast. Therefore, a fast scan (15000 nm min-1) using batch mode was used for

taking CL spectra. No detectable CL intensity was seen for the reaction between permanganate

and sulfite in absence of zolpidem. Compared to intense peaks obtained by the CL instrument,

spectrofluorimeter detected only very weak CL intensity for mixture of permanganate-sulfite-

zolpidem. It is might be due to lower sensitivity of the fluorescence instrument. The CL

spectrum (shown in Figure 9) is similar to emission spectrum of SO2* molecule. Based on above

discussions, following mechanism is proposing for this CL reaction [40, 41].


71

𝑍𝑜𝑙𝑝𝑖𝑑𝑒𝑚 + 𝑀𝑛𝑂4− + 𝐻+ + 𝑆𝑂3

2− → 𝐻𝑆𝑂3● + 𝑝𝑟𝑜𝑑𝑢𝑐𝑡𝑠

2𝐻𝑆𝑂3● → 𝑆2𝑂6

2− + 2𝐻+

𝑆2𝑂62− → 𝑆𝑂4

2− + 𝑆𝑂2*


𝑍𝑜𝑙𝑝𝑖𝑑𝑒𝑚 + 𝑀𝑛𝑂4− → [𝑜𝑥𝑖𝑑𝑎𝑡𝑖𝑜𝑛 𝑝𝑟𝑜𝑑𝑢𝑐𝑡𝑠]*

[𝑜𝑥𝑖𝑑𝑎𝑡𝑖𝑜𝑛 𝑝𝑟𝑜𝑑𝑢𝑐𝑡𝑠]*+𝑆𝑂2 → 𝑆𝑂2*


Zolpidem undergoes oxidation by N-demethylation, N-oxidation, decarboxylation, and

oxidation of methyl groups and hydroxylation of a position on the imidazolepyridine ring

[42]. Oxidation products in weak oxidative medium are oxozolpidem, zolpaldehyde,

zolpyridine [43] and in strong oxidation medium, the products are acid derivatives [44].

CONCLUSIONS

This method is simple and less expensive in comparison to the existing techniques for

the determination of zolpidem. This method offers a good accuracy, high speed and precision

and has been used to determine zolpidem in tablets and plasma samples. It can be a basis for

the development of an HPLC-CL method for the determination of zolpidem in biological

fluids.

ACKNOWLEDGEMENTS

The authors are grateful to the Campus of Golestan University for supporting this work.

Figure 9. CL spectrum of permanganate- sulfite-zolpidem.

Conditions: permanganate (1.0×10-3 mol L-1), sulfite (0.1 mol L-1), zolpidem (983.7 ng mL-1). The peak is

smoothed using Excel functions


72

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