Detecting and measuring metal elements in pharmaceutical products is an important consideration in finished-product pharmaceutical manufacturing. Possible sources of metal-object contamination include ingredient contamination, mixing-process errors, and machinery failures in the tablet- and capsule-forming and filling process. In these instances, a food-processing metal detector can play an important role in detecting magnetic or conductive metals. The author discusses new detection-system technologies that can improve performance and provides key criteria to consider when selecting or upgrading a system for pharmaceutical use.
A typical detector of metal foreign objects contains a transmitter antenna that sends out a radiofrequency signal ranging from 300 kHz to 1 MHz (see Figure 1). Two receiver antennas sit on each side of the transmitter at equal distance from the transmitter. When the system is balanced and there is nothing magnetic or conductive inside the aperture of the metal detector, the difference of the two signals is zero, signifying that no metal is present. When metal is present and traveling through the detector, a detectable imbalance is created. The accurate performance of this system depends on three factors:
Figure 1: Pharmaceutical metal-detection system showing detail of the rejection system (APEX 500 Rx Metal Detection System, Thermo Fisher Scientific). The system is placed at the outlet of the tablet press, deduster, or capsule filler. Every product coming off the production line passes through its aperture and is metal detected. ALL FIGURES ARE COURTESY THERMO FISHER SCIENTIFIC
The receivers' signals are digitized and analyzed by a digital-signal processor (DSP) that filters the signals. The DSP uses signal-processing algorithms to increase the probability of an accurate detection. The signals have two components: one is magnetic (X), and one is conductive (R). These components enable the system to detect metal foreign objects that are mainly conductive and have a small amount of magnetism such as in 316-alloy stainless steel. As a result, detection of conductive metal objects relies on a different signal analysis compared with a ferrous metal containing iron. Most metals exhibit both magnetic and conductive behaviors; these behaviors can change with the size of the metal. An off-line laboratory instrument such as an X-ray fluoresence spectrometer is necessary to determine full metal composition.
- The closeness of the metal that is being detected to the antennas or coils (i.e., the aperture or opening size)
- The effectiveness of the fields created by the transmitter (i.e., the transmit-antenna design)
- The frequency of the signals used (the higher the frequency, the better the detection of conductive, nonmagnetic metals).
In some applications, the ability of the system to ignore the signals that can be caused by the uncontaminated product passing through the metal detector is crucial. For example, some products may have a chemical composition that appears to be slightly magnetic or conductive to the metal-detector fields. This type of product effect can be ignored by the system by first learning the magnitude of the product's X and R signals. During production, the system creates a region where any combination of X and R signals with the same ratio and similar magnitude are thereby ignored. This process, called phasing, is typically only required in pharmaceutical applications using products that have high concentrations of iron or other metallic elements.
Figure 2: Multicoil architecture in a metal detector results in higher sensitivity. RF is radiofrequency.
The pharmaceutical industry has used metal-detection systems for more than a decade to detect metallic contaminants, and the systems are evolving to offer improved performance and ease of use. One new technology is the inclusion of multicoil arrangements in metal detectors to improve the signal viewed by the receiver. Compared with a single transmitter with two receivers, multicoil arrangements can improve detection performance (as measured by the diameter of the metal sphere size that can be detected) of the instrument by up to 20%. Scientists at the Thermo Fisher Scientific Minneapolis, Minnesota, site for example, uses electromagnetic field simulation software to optimize the number and placement of such coils or antennas, which make it easier to detect smaller pieces of metal foreign objects without seeing higher levels of false rejections (see Figure 2).