In general gamma-ray spectrometry is used to obtain the characteristic spectrum of a radionuclide that emits gamma rays such as positron emission radionuclides. The gamma spectrum is unique to that nuclide and is characterized by the number of photons of particular energies emitted according to decay scheme. This property contributes to the identification of radionuclides present in a radiopharmaceutical preparation and to their quantification. Furthermore, gamma spectroscopy allows the estimation of the degree of radionuclidic impurity by detecting peaks other than those expected.

Radiochemical purity is defined as the ratio, expressed as a percentage, of the activity of the radionuclide concerned which is present in the radiopharmaceutical preparation in stated chemical form, to the total radioactivity of that radionuclide present in the radiopharmaceutical preparation. Since the radiochemical form of the radiopharmaceutical determines its biodistribution, the control of radiochemical impurities is important to avoid the patient unnecessary radiation exposure because the radiochemical impurities could have a different biodistribution and not specific uptake which may affect and invalidate the clinical outcome of PET imaging studies. The nature of contaminants will depend on the radionuclide production and on the production process of the radiopharmaceutical as well as the impurity profile of the starting material employed. Radiochemical impurities may also originate from the chemical changes of radiopharmaceutical during storage. Measurement of radiochemical purity requires the use of a method to separate the different labeled chemical species which may be present in the radiopharmaceutical preparation. Chromatographic methods are commonly used in radiochemical determination because they can effectively separate and quantify the radioactive species in the sample. For ¹⁸F-FDG preparation, radiochemical impurities include ¹⁸F⁻ fluoride ion or radiolabeled intermediates and by-products derived from the synthesis process, such as ¹⁸F-2-fluoro-2-deoxy-D-mannose (¹⁸F-FDM) and partially or fully acetylated derivatives of ¹⁸F-FDG.

The radionuclidic purity is defined as the ratio, expressed as a percentage, of the radioactivity of the radionuclide concerned to the total radioactivity of the radiopharmaceutical preparation. Radionuclidic impurities can contribute significant effects on the patient’s overall radiation dose as well as impact the image quality. Radionuclidic impurities can originate from secondary nuclear reactions during the target irradiation as a result of isotopic impurities present in the irradiated material as well as in the target body. The specific monographs prescribe the required radionuclidic purity and may set limits for specific radionuclidic impurities. The most generally used method for radionuclidic purity testing is the gamma spectrometry. For example, the control of radionuclidic purity in ¹⁸F-radiopharmaceutical EP monographs prescribes the determination of the amount of ¹⁸F and radionuclidic impurities with a half-life longer than 2 hours. The preparation to be examined is retained for at least 24 hours to allow the ¹⁸F to decay to a level that permits the detection of such impurities.

Chemical purity addresses to not radioactive materials in the PET radiopharmaceutical, including by-products, solvents, and other residual components used in the production process. not radioactive materials (e.g., stabilizers, additives, etc.) that are intentionally added to the PET radiopharmaceutical may also be included in this category. Chemical impurities are considered all not radioactive substances that can either affect radiolabeling or directly produce adverse biological effects in patients. In monographs, chemical purity is controlled by specifying limits on chemical impurities.

To characterize and determine the quantity of potential chemical contaminants in the final product, several methods may be used, including gas chromatography (GC), HPLC, or TLC. Using the example of ¹⁸F-FDG, a colorimetric test for the detection of Kryptofix 2.2.2 has been developed. With this test, one can interpret whether the level of Kryptofix 2.2.2 is within the acceptable regulatory limits in the EP monograph for ¹⁸F-FDG. Another example of chemical purity determination is in EP monograph of ¹¹C-methionine where is reported the HPLC method for testing the chemical impurities L-homocysteine thiolactone and from DL-homocysteine which derive from the synthesis of this radiopharmaceutical.

Enantiomers are either of a pair of optical isomers that are mirror images of each other. Enantiomers exhibit stereoisomerism because of the presence of one or more chiral centers. In the clinical use of chiral radiopharmaceutical, high enantiomeric purity is important since the biological inactive enantiomer could become a disturbing background source of radiation that interferes with interpretation of the signal of active enantiomer. For this reason the enantiomeric purity has to be verified where appropriate. The limit for each enantiomeric impurity is reported in the pharmacopoeia monograph of the radiopharmaceutical: for example, the limit for D-¹¹C-methionine is 10 % of the total radioactivity of the ¹¹C-methionine batch.

PET radiopharmaceutical preparations must be prepared using precautions designed to exclude microbial contamination and to ensure sterility. The test for sterility [3] is carried out under aseptic conditions by inoculating the sample in two culture media Soybean-Casein Digest Medium and Fluid Thioglycollate Medium and by incubating for not less than 14 days. Soybean-Casein Digest Medium is a culture media for aerobic bacteria and fungi, while Fluid Thioglycollate Medium is primarily intended for the culture of anaerobic bacteria. Growth promotion tests should be performed by incubating reference bacteria in the two media. Bacterial growth should be visible within the period of incubation. Results of the growth promotion would indicate that both Soybean Casein Digest Medium and Fluid Thioglycollate Medium are capable of supporting bacterial growth; hence, results of the sterility test are reliable. A system suitability test should be performed using two methods: by transferring the solution to be tested in a membrane and by adding to the final portion of sterile diluents used to rinse the filter an inoculum of a small number of viable microorganism (the same used for growth promotion test) (membrane filtration) or by transferring the solution to be tested to the culture medium and by adding to the medium an inoculum of a small number of viable microorganism (the same used for growth promotion) (direct inoculation). The solutions are then incubated for not more than five days. If clearly visible growth of microorganism is obtained after incubation, visually comparable to that in the control vessel without product, the product possesses no antimicrobial activity under the conditions of the test so the test for sterility may then be carried out without further modifications. If clearly visible growth is not obtained, the conditions should be modified in order to eliminate antimicrobial activity of the product, and the system suitability should be repeated.

Bacterial endotoxins (pyrogens) are polysaccharides from bacterial gram-negative membranes which are water-soluble, heat stable, and filterable. Their presence in a radiopharmaceutical preparation can cause fever and also leukopenia in immunosuppressed patients. To minimize the presence of pyrogens, it is important that preparations are manufactured and dispensed under aseptic conditions. The test for bacterial endotoxins is used to quantify endotoxins, and it is based on the blood-clotting reaction that occurs when bacterial endotoxins activate the blood cell component Limulus amebocyte lysate (LAL) derived from horseshoe crabs (Limulus polyphemus or Tachypleus tridentatus). The test may be conducted with one of the following techniques reported in the EP chapter for bacterial endotoxins [4]:

The limit for bacterial endotoxins is indicated in the individual monograph.

Chromatographic separation methods are commonly employed in the quality control of PET radiopharmaceuticals. Chromatography is an analytical technique based on the separation of two or more compounds by the distribution between two phases, a stationary and a mobile phase. These two phases can be solid–liquid, liquid–liquid, or gas–liquid. Chromatographic separation methods are used to separate radioactive or not radioactive species in the sample followed by their identification and quantification and are commonly employed in radiochemical, chemical, and enantiomeric purity determinations. The most used chromatographic methods are high-performance liquid chromatography (HPLC), thin-layer chromatography (TLC), and gas chromatography (GC) (Table 5.1).