NMR checkweighing

Valerie Scott discusses BOC Edwards' new non contact check- weigher, which uses nuclear magnetic resonance (nmr) as a tool for measuring the weight of the passing sample

The Generics Group and BOC Edwards Pharmaceutical Systems have delivered the first magnetic resonance (MR) check-weighing technology for a pharmaceutical packaging application to a major US pharmaceutical company.The technology, licensed on a royalty basis by Generics, has been used by BOC Edwards Pharmaceutical Systems to successfully develop an innovative non-contact check-weighing system.

Check-weighing is typically used to provide a certain level of quality control, but existing technologies cannot always meet the industry's demanding requirements for accuracy and throughput, and in today's industrial environment there is a continuous need to improve efficiency, quality and yield.

Demands on check-weighing systems vary but, in general, there is now a need to perform 100%, non-contact inspection with a high degree of accuracy while maintaining fast production rates. The BOC approach to in-line non-contact check-weighing is a major development to suit the FDA's PAT initiative, and Generics MR technology created the opportunity for an innovative and highly effective check-weighing solution.

Traditional methods required samples to be removed from the production lines and weighed on precision balances, both before and after filling, a process that was time consuming and resulted in inspection rates of only a few percent. Generics decided on a radical new approach to the problem by using (NMR) methods for check-weighing.

In NMR, the magnetic properties of atomic nuclei are investigated using a magnetic field and radio frequency (RF) energy. The technique only measures materials which contain nuclei with a magnetic moment. These include ¹H, ¹³C, ¹⁹F, ²³Na and ³¹P. This 'limitation' of NMR proves to be an advantage for check-weighing where the product is packaged in glass containers. Hydrogen nuclei (protons) are most commonly measured, as hydrogen is present in many substances. In a proton NMR system, glass contributes no signal, thereby enabling a single-step, tare-less mass measurement.

Recent advances in electronics, magnet and RF coil design have enabled the development of smaller, cheaper and more reliable magnetic resonance systems. In BOC Edwards Pharmaceutical Systems' first order, a non-contact check-weigher was integrated into a pharmaceutical filling line designed to fill 2-l00ml vials at rates of up to 400 vials per minute, and performs fill level verification on every single vial.

The system works because almost all types of pharmaceuticals, in both solid and liquid forms, will exhibit some level of magnetic resonance signal. This is proportional to the number of atoms of interest contained in the sample being inspected, and it is this correlation that enables the technique to be used for check-weighing.

The sample is not 'weighed' in the conventional sense; the signal amplitude is calibrated against mass thus enabling 'weighing' to be performed. The advantages of MR check-weighing are shown in table 1.

Generics' early work in this field demonstrated the feasibility of performing MR check-weighing on a variety of pharmaceutical products, in both powder and liquid form.

Product weights below 1g were measured with an accuracy of better than ±5% at rates consistent with manufacturing throughput. These measurements were made with products in their final packaging, which was either glass or plastic.

An accuracy of better than ±1% can be achieved in production systems, as has been demonstrated in the first commercial application of the technology.

Dr Paul Stewart, director of technology at BOC Edwards said: 'We are delighted to have worked with Generics on this revolutionary system. The order is of major importance to BOC Edwards; it demonstrates the success of this innovation in check-weighing, the most significant change in over fifty years.'