The Elements of an Effective Solid-Dosage Metal Detection Program

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.

Metal detectors

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:

• 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).

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.

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).

Selecting a detection system

There are many criteria to consider when evaluating metal-detection systems for tablet and capsule processing. Perhaps the most important is the system's sensitivity (i.e., how small a metal fragment can be detected [the typical range for pharmaceutical applications is less than 0.5 mm]) and how immune the system is to outside influences such as vibration and electromagnetic noise, which can cause false rejects (see Table I). When selecting the size of the system's aperture, one should consider performance as well as throughput and size to avoid product jams during production.

Table I: Typical detectable metal diameter versus metal detector's aperture size.

Sensitivity. One subtle, but important, factor to consider when evaluating different systems is what, if any, effect a product might have on the metal detector. Most tablets and capsules look like "dry" products to the metal detector, but it is possible that they may trigger a false reject if they contain significant concentrations of metal elements such as iron. If a false reject continually occurs, the detector's operating threshold can be desensitized, but this change may reduce the system's performance. Before selecting a system, therefore, it is important to test the complete system on all of the pharmaceutical products that may be used to determine how the system will perform in a production setting.

Once it is determined that a system can detect the smallest fragments of metal possible with no chance of false rejection, there are other aspects of the system to consider to ensure that it can be successfully used in a particular production process. Table II provides a list of key questions to ask when evaluating a system.

Table II: Key criteria for selecting a pharmaceutical metal detection system.

Quality control. One key consideration is how the detection system is audited. Typically, manufacturers audit detection systems every 1 to 2 hours during production to check for detection sensitivity (see Table I). Most of these audits involve simple pass/fail checks. An additional automatic early-warning system has been developed (Thermo Scientific AuditCheck) that can be set to periodically (i.e., based on user-defined preferences) pass a metal piece through the detector's fields to measure the two primary signals previously defined (X and R) (see Figure 3). These signals are calibrated during set-up, and any changes from the expected values can generate warnings or alarms to the user. An early warning allows the user to take immediate action such as cleaning the detector of foreign material or adjusting the detection threshold. As a result, production volume and time can be maximized. In addition, the automation of the audit can help to reduce labor resources.

Figure 3: AuditCheck (Thermo Fisher Scientific) block diagrams. The diagram on the left is a side view of the metal detector, displaying where the product and AuditCheck device passes through the electromagnetic field. The diagram on the right shows how the magnetic (X) and conductive (R) signals are calibrated with AuditCheck and how warning and alarm limits are set.

Operation and deployment

After selecting and implementing a new or upgraded metal-detection system, a short on-line trial test is advised to further determine how the system is performing during production. In addition to the criteria outlined in Table II, one should consider the following:

1. Did the system reliably detect the metal sizes and types in question

2. Were there false rejects and, if so, why? Can they be avoided or minimized?

3. Did the system jam?

4. Can the system be adjusted easily to the production process with no loss of material?

5. Was set-up easy and how much operator intervention is needed to run the system?

6. How is the system audited and documented?

7. Can the system be cleaned easily? Can it be disassembled and reassembled quickly?

8. Did the system produce any errors? If so, how did it react?

Conclusion

Foreign-metal-object detection systems can help to prevent product contamination. Selecting a system requires a trial-and-error approach that takes into consideration the system's sensitivity, ease of use, and audit ability. Once implemented on a production scale, a metal-detection system can help to maximize product volume and quality.

Bob Ries is a lead product manager at Thermo Fisher Scientific, tel. 763.783.2500, bob.ries@thermofisher.com.