CONTAMINATION CONTROL - Metal Particles | Meddling Metals Can Contaminate Biopharmaceuticals
By Mary Stellmack and Kent Rhodes, PhD
FIGURE 1. Stainless steel corrosion particles on filter surface, oblique illumination, 95X.
Metal particles are common contaminants in the drug manufacturing process. The most common source is processing equipment that generates wear particles, but metal particles can also originate from contaminated raw materials. To protect consumers from metal contamination and other defects that can compromise the safety of a drug product, biopharmaceutical companies abide by stringent quality control (QC) standards regulated by the U.S. Food and Drug Administration (FDA). If an in-house QC inspector can identify contaminants, then steps can immediately be taken to correct problems in the manufacturing process.
The trained eye of a QC inspector can detect contaminants and defects as small as 50 µm, but microscopic metal contaminants, as well as other defects and particulates, are often in the sub-visible range and may go unnoticed, except for a hazy or “twinkling” appearance in a liquid product. Isolating and analyzing sub-visible metal particles may require specialized technical skills and analytical instrumentation that a biopharmaceutical company’s QC laboratory does not possess.
Many biopharmaceutical organizations seek the expertise of independent analytical or microanalysis laboratories that have the experience, skill, and instrumentation to identify contamination and its source(s). Several current techniques and analytical methods are available to detect, isolate, and identify such impurities. These laboratories have discovered that, despite such advances, the chemical nature of metal corrosion sometimes makes it impossible to identify the source of metal contamination.
FIGURE 2. Tablet with metal wear particles, 15X.
Identification Through IsolationSamples sent to an independent laboratory undergo examination, particle isolation, and preparation in a cleanroom to eliminate the chance of cross-contamination. Solid and liquid pharmaceuticals are typically examined first with the naked eye and then with a stereomicroscope. The most common physical signs of a metal contaminant are the appearance of shiny metal flakes or dark, brittle particles ranging in color from red or orange to brown or black. The particle must be isolated to identify an unknown metal.
If visible particles are present in a vial of liquid sample, a magnet can be drawn along the vial wall to collect susceptible particulates. If the particles follow the movement of the magnet, the liquid sample likely contains metal particles. The absence of a response to the magnet does not eliminate the possibility of metal contamination, however. The liquid sample can be filtered on a polycarbonate membrane filter—typically 0.2 µm or 0.4 µm pore size—in a vacuum filtration apparatus. The smooth, shiny surface of these filters allows an analyst to see the microscopic metal particles (see Figure 1, p. 22) and remove them from the filter surface for analysis with a bit of adhesive on a tungsten needle.
Metal particles in solid tablets may appear as discrete chunks that can be removed in one piece from the tablet. Particles as small as several hundred micrometers, if lodged at the tablet surface, can often be removed with forceps for analysis. Smaller discrete metal particles may need to be freed from their surroundings by applying a few micro-drops of water or other suitable solvent to the tablet surface. This softens or dissolves the surrounding tablet material, freeing the metal particle, which can then be lifted with a tungsten needle.
In some cases, solid tablets exhibit gray or brown stains that, on further examination, provide evidence of metal contamination (see Figure 2, p. 22). These stained areas commonly contain sub-visible metal or metal corrosion particles that are 10 µm and smaller, mixed with normal tablet materials and sometimes machine oil. A tungsten needle is used to remove and transfer a portion of the stained material to a glass slide. A micro-drop of hexane or other suitable solvent is applied to the stained material, and any oils present are extracted and identified using infrared spectroscopy. The remaining insoluble materials, which include the metal particles and tablet materials, are divided into two portions. One portion is analyzed with infrared spectroscopy to verify the presence of the normal tablet materials, and the other portion is submitted for energy dispersive X-ray spectrometer (EDS) analysis to confirm the presence of metal or metal corrosion particles.
Identification of the Metal ContaminantDiscrete metal particles or clumps of tablet material mixed with suspected metal debris are mounted either on a beryllium or carbon stub with a small amount of adhesive or on conductive carbon tape. Once properly prepared, the samples are submitted for analysis in a scanning electron microscope (SEM) equipped with an EDS detector. SEM and EDS provide two kinds of information: a high quality morphological image showing the physical features of a sample with the SEM, and an X-ray spectrum with the EDS that shows the elements in the sample that can be processed to determine their quantitative amounts.
The SEM uses electrons instead of light to form an image. The sample is bombarded with electrons, and the atoms in the sample interact with the electrons to produce secondary electrons and backscattered electrons. These electrons are collected by a detector and used to produce high-resolution images of the surface of the sample. Metal particles can be easily distinguished from organic materials by their backscattered electron signal.
The electron beam of the SEM also generates X-rays from the sample. Each element has a unique X-ray pattern, and the EDS detector is used to collect the X-rays and analyze their energies. The spectrum of X-ray energies is used to identify the elements present in the sample and to determine their amounts using quantitative analysis. The EDS method can detect elements with the atomic number of carbon (12) or greater, and it has a detection limit on the order of 0.1% wt/wt for most elements. Because most elements are heavier than carbon, EDS is an ideal detection method for metals and other inorganic materials, such as glass fragments or minerals.
For a pure metal particle like iron or aluminum, the EDS spectrum will display a composition showing close to 100% iron or aluminum. If the particle is somewhat oxidized or corroded, the EDS spectrum will display the presence of oxygen in some amount and a proportionately lower amount of the metal. For example, a sample of oxidized iron might produce an EDS spectrum with 70 wt% iron and 30 wt% oxygen. For real world samples, metal corrosion particles often display small amounts of multiple elements because the source metals are typically alloys and not pure metals, and they may have been exposed to various environments prior to analysis.
FIGURE 4. Energy dispersive X-ray spectrometer spectrum of type 304 stainless steel corroded by acid mixture.
Identifying the SourceThe most common source of metal contaminants in biopharmaceutical products is processing machinery that generates wear particles. Stainless steel is the type of metal found most frequently in liquid injectables or over-the-counter pharmaceuticals.
Stainless steels comprise a family of many commercially available steel alloys. Stainless steel contains at least 10% chromium, as well as varying amounts of other alloying elements. There are over 150 grades of stainless steel, each distinguished by the composition of alloying elements. The EDS data from a metal particle can be compared to published databases of common stainless steel compositions. In some cases, possible sources for the particle can be narrowed down to a few likely choices and traced to the type of steel used in certain pieces of manufacturing equipment.
Corrosion Complicates AnalysisUnfortunately, finding a solution is not always that easy. The majority of metal particles isolated from biopharmaceutical products are slightly corroded or oxidized due to exposure to air, water, bacteria, or chemicals found in high chloride or sulfate environments. During the corrosion process, the ratios of iron and alloying elements may change, and the amount of oxygen in the metal sample may increase. This changes the elemental composition to the extent that many times it is not possible to match the particle to any known source (see Figures 3 and 4 p. 23). Fortunately, the presence or absence of certain alloying elements can sometimes provide clues about the origin of the particles. For example, because molybdenum does not exist in all types of stainless steel, the presence or absence of this element narrows the list of likely sources.
The natural wear and tear of manufacturing machinery cannot be avoided. Biopharmaceutical companies must continue to diligently monitor not only the quality of the product but also the condition of the manufacturing machinery. Working in conjunction with independent analytical laboratories will help to prevent metal-contaminated pharmaceuticals from reaching store shelves and the consumer.