The distribution of magnesium stearate

Annamaria Szalay^a, Klara Pintye-Hodi^a, Katalin Joo^b and Istvan Eros^a report on a study into the distribution of magnesium stearate using an energy dispersive X-ray fluorescence analyser

Lubricants are commonly included in tablet formulations in order to reduce the die wall friction during both the compaction and the ejection of the tablet. Their presence, however, may cause undesirable changes in tablet properties.

Lubrication itself, in pharmacy, consists in adding a small amount of a lubricant to a powder or granule, this lubricant being a substance capable of eliminating the projections and thereby reducing friction when inserted between two rough surfaces.^1-3

When a lubricant is added to a tabletting blend, it is distributed either as a free fraction or as a surface film on the carrier material. In tablet making, the most commonly used pharmaceutical lubricant is magnesium stearate. There are several direct and indirect methods by which the distribution of magnesium stearate on the surface of lubricated particles can be examined.4-9

The aim of this work was to study the influence of the particle size of sorbitol on the distribution of magnesium stearate in a tablet during compaction. A compact table-top energy dispersive X-ray fluorescence analyser was used to measure the elemental range from sodium (Na) to Uranium (U), with a concentration range from ppm to 100%. The magnesium content of magnesium stearate was measured.

The advantage of this new approach is that hugely expensive equipment, such as TEM or SEM, is not necessary for determination of the elements. Sample preparation is very simple and the measurement is very rapid.

Various fractions of sorbitol (Ph.Eur.) were sorted according to size using a vibration sieve. These consisted of the basic particle size ranges: 250-315 mm; 316-400 mm; 401-630 mm; and 601-1000 mm.

analysis parameters

An analytical sieve with variable sieve apertures of 250, 315, 400, 630 and 1000mm was used to obtain various fractions of the sorbitol sample. The individual sieving time was 5 minutes.

The magnesium stearate (Ph. Eur) was used as lubricant in a quantity of 1%. The specific surface area of the magnesium stearate was 0.69 m²/g (BET).

Powder mixing was performed with a Turbula mixer at 50rpm for 10 mins, and compression was carried out using a Korsch EK0 eccentric tablet machine mounted with strain gauges, and a displacement transducer was also applied. The pressure force and displacement were calibrated. Slightly concave punches, 12mm in diameter were used. Table 1 lists the compression para-meters.

The analysis was carried out using an energy dispersive X-ray fluorescence analyser (MiniPal, Philips Analytical, the Netherlands). The spectrum was evaluated by non-linear least squares fitting, based on the AXIL algorithm developed at the University of Antwerp. The powder pressed solid samples were 12mm in diameter and <40mm in height, and were placed in a 12 position sample changer with removable tray. The equipment parameters are shown in table 2.

Some powder mixtures with different magnesium stearate contents were prepared for the calibration, and the equipment showed excellent (>95%) correlation to the actual Mg concentration.

As mentioned above, the purpose of lubricants is to prevent two solids from making contact with each other, or at least to limit such contact. Lubrication is also a surface phenomenon. For this reason, the upper and lower surfaces of the tablets were also analysed.

The spectrum can be seen in Figure 1 and the data are displayed in Table 3. Practically no difference can be detected in the distribution of magnesium stearate on the upper and lower surfaces of the tablets. It can further be observed that the sorbitol particle size did not influence the distribution of magnesium stearate. It can be stated that magnesium stearate used with a low specific area exerts a lubricating effect on the surface of the tablets.

larger particles

It was also interesting to study the distribution of magnesium stearate inside the tablets. Therefore, microsections with different thicknesses were prepared from tablets of No.2 and No.4 with the aid of abrasive paper.

The thickness was determined with a screw-micrometer and the magnesium stearate concentration was measured on the surface of microsections. The results are displayed in Table 4. It can be seen from the data that the amount of magnesium stearate decreased progressively towards the middle of the tablets.

It can additionally be established that it decreased to a higher degree in the case of tablets No.4. This means that the particle size seems to influence the distribution of magnesium stearate inside the tablets. Its distribution is better inside tablets prepared from smaller particles.

It is suggested that larger particles need more magnesium stearate with a low specific surface area, or another quality of magnesium stearate, for instance a higher specific surface area.

uniform distribution

Finally, it can be concluded, that the X-ray fluorescence analyser is a suitable means of measuring the amount of magnesium stearate on the surface of and inside tablets. The distribution of magnesium stearate with a low specific surface area is uniform on the surface, but it is disturbed by the higher particle size inside the tablets. Measurement with the energy dispersive X-ray spectrometer is very simple and the evaluation of the results is very easy.

Acknowledgement

This work was supported by Testor and by PANalytical. a) Department of Pharmaceutical Technology, University of Szeged, Szeged (Hungary) b) Testor Ltd, Budapest (Hungary)