Indian Journal of Chemical Technology Vol. 8, May 200 1 , pp. 1 9 1 - 1 94

Simultaneous determination of mefenamic acid and paracetamol in pharmaceutical preparations by IH-Nudear Magnetic Resonance Spectroscopy

Sajid Husain, M Kifayatullah & R Sekar* Analytical Chemistry Division, Indian Institute of Chemical Technology. Hyderabad 500 007, India.

Received 02 May 2000; accepted 1 7 January 2001

A simple, specific and rapid nuclear magnetic resonance (NMR) spectroscopic method for the determination of mefenamic acid and paracetamol in pharmaceutical preparation is described. Samples are dissolved directly in DMSO-d6 and their spectra are recorded on a Gemini-200 MHz NMR spectrometer instrument. The results obtained by the proposed method is agreed closely with those found by the method of British Pharmacopoeia (BP). No interference due to excipients is also observed.

Mefanamic acid (N-2, 3-xylyl anthranilic acid) I, analgesic for mild to moderate pain including primary dysmenorrhea I , Paramacetamol (N -acety 1-p-amino­phenol) II, is one of the most popular non-prescriptive analgesic, antipyretic drug2. Their combination is highly useful for the treatment of moderate pains connected with rheumatoid arthritis. Several methods have been reported for the determination of I and II in a single dosage form3. ' 2 and the majority of these are spectrophotometric. These methods, however, are not selective and they are time-consuming. Most of the spectrophotometric methods for the determination of II are based on deacylation of II to p-aminophenol and subsequent oxidation or diazotization and coupling. These methods have the disadvantage that the starting material and degraded product of II, the p-amino­phenol, interfere. The methods described in BpI 3 for the determination of II requires 60 min refluxing. The cerium (IV) method 14 reported in literature requires 90 min of heating. The BpI5 describes a titrimetric method for the determination of I. The application of 'H-NMR for determination of I and II has been reported 16. 17, when only p�esent as a single active ingredient. This methodology, however, could not be applied for the simultaneous determination of I and II in mixtures. There are two spectrophotometric methods3, ' 8 available in literature for the simultaneous determination of I and II mixtures, one of which involves separation before spectrophotometric mea­surement where as the other (Vierordt' s method) is

*For correspondence

based on selection of two points on the wavelength scale where the ratios of absorptivities are maximum. Moreover, the poor solubility of mefenamic acid 1 9 in conventional solvents may pose problems in developing a method for the simultaneous determi­nation of I and II in mixtures. Therefore, a simple, rapid and accurate 'H-NMR method is developed for the simultaneous determination of I and II in pharma­ceutical preparations.

Experimental Procedure

Materials and methods

Reagents-Mefenamic acid was obtained from Parke Davis,India. The purity was checked by JR, NMR and Mass spectrometry. Paracetamol was purchased from Sigma Chemicals and used as obtained, DMSO-d6 99.9 atom% D and tert-butyl alcohol were purchased from Aldrich Chemicals.

Drug fonnulations-Commercial tablets of mefenamic acid (Ponston), paracetamol (Crocin, Pacimol, Paracin, Premimol, Pyrigesic and combined formulation of mefenamic acid and paracetamol (Lanagesic) were purchased from different local markets.

Internal standard (tert-butyl alcohol solution)­tert-Butylalcohol (200 mg) was weighed in 1 .0 mL volumetric flask, diluted to volume with DMSO-d6 and mixed well .

Instrumentation-Varian Gemini 200 MHz, Fourier Transform NMR spectrometer.

192 INDIAN J. CHEM. TECHNOL., MAY 200l

a)

9

b)

1

i 10

i 9

COOH si-HH� W b)={a H3C CH3

M e fenamic acid

8 7 6 Cj

HO-o-HHCOCH� Para cetamol

i i i i 8 1 6 5

�H3 H� -7-0H CH3

tert. butyl alcahol ( IS I

b a

4 3 2

i i 4 2

b a

IS

TMS

i 0

TMS

i 0

TMS

I o

Fig. l -IH-NMR spectra of (a) Mefenamic acid (b) Paracetamol (c) Lanagesic tablet formulation (Label claim, Mefenamic acid 250 mg+Paracetamoi 500 mg) 1 3.4 mg with internal standarad, tert-butyl alcohol ( 1 .8 mg), DMSO-d6

Operation conditions-Proton resonance frequency 200 MHz; spectral width 2000Hz; pulse width 1 8 f..ls (90° pulse); pulse repetition time l Os; acquisition time 4.096s; data points 1 6K; spectral resolution 0.2Hz; probe temp. 2rC; chemicals shifts were referenced to in 8 (ppm) with respect to tetramethylsilane (TMS) as an internal reference.

Method-About twenty tablets were finely powdered and in the case of capsules, they were carefully emptied out into a small container. The average weight per tablet and capsule was calculated from the contents of 20 tablets or capsules. Two

methods were followed for further preparation of the sample for NMR analysis. Samples containing 10 to 25 mg of mefenamic acid, paracetamol and their mixtures were placed in 5 mrn 0.0. NMR tubes to each of which was added 0.3 mL of OMSO-d6. Each NMR tube was stoppered, shaken for 1 min at 65°C then allowed to cool to room temperature. To each sample tube tert-butyl alcohol (IS) solution ( 1 .0 to 4.0 mg) was added using a microsyringe, following by 0.2 mL of OMSO-d6 and the contents were mixed well. 'H-NMR spectra were recorded 3 min after mixing.

Alternatively, 20-50 mg of sample was accurately weighed into a 5.0 mL calibrated centrifuge tube. OMSO-d6 ( 1 .0 mL) was added, thoroughly mixed, and the tube was kept in a waterbath at 65°C for 2 min. After cooling the sample to room temperature, 1 .0 to 4.0 mg of tert-butyl alcohol was added from the tert-butyl alcohol internal standard solution, mixed well and centrifuged. The clear solution of 0.5 mL was transferred into a 5 .0 mm 0.0. NMR sample tube and NMR spectra were recorded.

The quantities of mefenamic acid and paracetamol are calculated from the following equations:

Mefenamic acid (%) = W( X EWm x

1m x 100 Ws Ew( I(

W Ew I Paracetamol (%) = _t X --

p x --..E.. x l 00

Ws Ew( It

. . . ( 1 )

. . . (2)

where, W,=Weight of tert-butyl alcohol added; Ws=Weight of sample taken; Ewm=Equivalent weight of mefenamic acid (242.28/6=40.2 1 ) ; EWp=Equivalent weight of paracetamol ( 1 5 1 . 16/3=50.39); Ew(= Equi­valent weight of tert-butyl alcohol (74. 12/9= 8.24); 1m, Ip and I( are the integral values of mefenamic acid, paracetamol and tert-butyl alcohol (IS) respectively. The equivalent weight (Ew) is calculated as follows:

E = molecular weight of the sample

W number of protons involved in the integration

Results and Discussion Tert-Butyl alcohol is chosen for the internal

standard because it offers several advantages (a) readily available in pure form, (b) soluble in most of the organic solvents, and (c) the integral value is large. OMSO-d6 is used as NMR solvent because the analytes and internal standard are freely soluble in it.

HUSAIN et al.: DETERMINATION OF MEFENAMIC ACID AND PARACETAMOL 193

Table I -Results of assay of mefenamic acid and paracetamol synthetic mixtures by IH-NMR Spectroscopy

Sample Internal Mefenamic acid Paracetamol No std, mg Added Founda Rec Added Founda Rec,

mg mg % mg mg %

I 1 .02 6.07 5.97 98.3 8.56 8.4 1 98.3 2 2.04 7.76 7.56 97.4 1 0.87 1 0.78 99.2 3 1 .63 1 0. 1 1 1 0 . 1 0 98.9 1 1 .74 1 1 .52 98.2 4 3 .06 9.25 9.22 99.7 7.63 7 .35 96.6 5 4.08 1 3 .43 1 3 .47 1 00.3 1 5 .4 1 1 5 . 1 5 98.3 6 1 .43 1 3.72 1 3.28 96.8 1 5 . 8 1 1 5 . 4 1 97.5 7 1 .63 1 0.65 1 0.38 97.5 1 2.54 1 2.53 99.9 8 3.06 1 0.92 10.77 98.6 1 6.26 1 6.39 1 00.8 9 4.08 1 1 .72 1 1 .86 99.7 1 7.55 17.34 98.8 10 4.08 9.74 9.57 98.3 17 .80 1 7 .76 99.8 I I 1 .22 9.30 9.20 99.0 20. 1 0 20.44 1 0 1 .2 1 2 2.22 1 1 . 1 8 10.99 98.3 19.3 1 1 8.04 96.9

Av., % 98.6 98.8 SD 1 .04 1 .48 CV 1 .06 1 .50

a=average of triplicate anayses

Table 2-Comparison of IH-NMR with BP method for the assay of individual tablets from different suppliers

Sample Mefenamic acid a

No. NMR BP

Rec., Rec., %

1 1 00.3 1 0 1 .2 2 98.6 1 00.2 3 97.5 99.9 4 99.2 98.2 5 98.7 1 00.3 6 1 0 1 .3 1 00.6

Av% 99.3 1 00. 1 SD 1 .34 1 .02

a= average of 4 determinations

Table 3-Determination of content uniformity for mefenamic acid and paracetmol in commercial Lanagesic tablet by

I H-NMR

Sample Mefenamic acid " Paracetamol" No Rec. , % Rec. , %

I 1 0 1 .5 97. 8 2 1 02. 1 98.6 3 100.3 99.5 4 1 00.3 98.3 5 1 0 1 .3 99.2 6 99.6 98.2 7 1 00.2 97.9 8 100.4 98.8 9 98.7 96.3

AV., % 1 00.5 98.3 SD 1 .02 0.92

CV % 1 .0 1 0.94

a= Average of triplicate analyses Declared content of the tablets: Mefenamic acid 200 mg+paracetamol 500 mg

Paracetamola

NMR BP

Rec., % Rec. , %

1 0 1 .4 1 0 1 .2 98.8 99.2 99.3 1 00.9 98.6 1 00. 1 98.3 1 00.2 96.5 98.6 98.7 1 00. 1 1 .36 1 .0 1

The spinning rate is adjusted to 30 revolution per second, so that no spinning side bands occur in the region of integral signal of the analytes and internal standard. The IH-NMR spectra of mefenamic acid, paracetamol and the mixture are shown in Fig. 1 . It is seen from the figure that mefenamic acid exhibits two singlets 8 (2. l 6, 2.32 ppm) due to its two non­equivalent methyl groups, whereas the paracetamol shows a methyl proton 8 (2.08 ppm) each of which is very intense and well separated. As their integral values are large, they can be measured with sufficient accuracy. The integral of the selected signals are compared with the integral for the tert-butyl protons 8 ( 1 .23 ppm) of tert-butyl alcohol (IS). Other signals appearing at lower signals appearing at lower field due to OH, NH, and aromatic protons are of much less importance for this analytical study.

194 INDlAN J. CHEM. TECHNOL., MAY 2001

The spin lattice relaxation time TJ . determined by use of inversion recovery pulse sequence ( 1 80° -'t-

90°)20 for the selected signals is 0. 1 8s and O. I Ss for methyl protons at 2. 1 6 and 2.32 ppm of mefenamic acid respectively. The Tl value for paracetamol is 0.75s at 2.08 ppm and for the tert-butylalcohol, it is 1 .08s. The pulse repetition time (PR) and pulse width (PW) are selected to get the best possible SIN ratio within a reasonable accumulation time21 .22• With mefenamic acid and paracetamol of 1 2 mg/mL con­centration, a SIN ratio > 10 is obtained from 10 pulses. The percentage of mefenamic acid and paracetamol obtained by NMR in standard mixtures are calculated by using Eqs ( 1 ) and (2). Table 1 records the results obtained for standard mixtures. It can be observed that the average percentage assay is 98.6 and 98.8 with standard deviation of 1 .04 and 1 .48 for mefenamic acid and paracetamol respectively.

The proposed method is applied to the determi­nation of mefenamic acid and paracetamol in commercial tablets/capsules. The results are compared with those obtained by BP method and are shown in Table 2. The results are in good agreement. The developed method is then utilised for analysing the content uniformity of Langesic tablets. The results are given in Table 3 and are found to be in good agreement with the declared quantities of mefenamic acid and paracetamol.

In conclusion, the proposed IH-NMR method is simple, rapid and there is no need for extensive sample preparation.

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