General Quality Control Procedures
Most radiopharmaceutical kits are prepared using Tc-99m. The Tc-99m becomes attached to a substrate molecule in the kit, called a ligand, designed to localize in a specific organ system. Most of the Tc-99m should tag to the ligand for the radiopharmaceutical to be efficient. Very little free Tc-99m should be present in the final product. Hydrolyzed reduced Tc-99m, another byproduct of the tagging process, should also be present in low levels. Both free and hydrolyzed reduced Tc-99m can give artifacts on scans, which may mislead diagnosis or make assessing scans difficult.
The USP XXIII has set minimum tagging efficiency standards for most radiopharmaceuticals. Each radiopharmaceutical compounded must be tested for radiochemical purity before use in patients. Unless the radiopharmaceutical is efficiently tagged, the accuracy of the patient diagnosis may be compromised.
MCA or SCA or Scaler with scintilation well
The solvent used represents the mobile phase of the chromatography procedure. As the solvent moves up the media (stationary phase), the different species within the sample can be separated. Selection of a solvent for a particular radiopharmaceutical purity test is based on many considerations. These include the solubility of all the species in the sample and the polarity of the chromatography media. A particular solvent and media combination may yield the most accurate purity testing. Alternative solutions are often suggested in literature. It is mandatory that any alternatives be validated before routine use.
Listed below are some of the more commonly used solvents in Tc-99m chromatography.
Acetone, reagent grade
Ethanol, anhydrous USP
*Solvents specific to a chromatography procedure are listed in that procedure.
Solvent Storage & Handling:
The purity of solvents used is important to ensure reliability of test results. Contaminants, such as water or other chemicals, can alter the system and give inaccurate results. Solvents can evaporate, absorb moisture from the air, or become contaminated with other solvents.
Stock bottles should be dated when opened. Discard opened solvents that are 6 months old. Keep a sealed stock bottle in inventory as back-up. All flammable solvents should be stored in a fire safe cabinet that meets regulatory requirements. Corrosives, such as HCl, should be stored separated. Have chemical data sheets for each solvent in an easily accessible area known to all staff. Solvents must be stored in tightly stoppered containers. All glassware and equipment used should be clean and dry.
The media, representing the stationary phase, supports the sample being tested. Several types of media are used in routine chromatography testing. These include paper (Whatman), instant thin layer sheets (ITLC Gelman), and separation cartridges (Sep-Pak). A particular media and solvent combination may yield the most accurate purity testing. Alternative media is often suggested in literature. It is mandatory that any alternatives be validated before routine use.
Listed below are some of the more commonly used stationary phases in Tc-99m chromatography.
Aluminum Oxide Plates
Solvent Saturation Pads
*Media specific to a chromatography procedure are listed in the procedure.
Media Storage and Handling:
Media should be handled gently. Plates and sheets and crack or chip. Paper can absorb moisture and tear. The media should be marked lightly. Use a felt-tipped pen to mark the front. The origin and cut-line should be lightly drawn with a very soft lead pencil. Media should be stored in a well-closed container with a desiccant.
Commercially available chromatography kits contain pre-cut strips of the appropriate size and media. Ans alternative is to prepare in-housestrips from large sheets of media using a template.
1. Using a paper cutter or very sharp scissors, cut 8 cm long and 1 cm wide strips from a large sheet.
2. Using a soft lead (No. 1) pencil, lightly draw a line 1 cm from the edge of the strip. This is the origin where the sample will be spotted.
3. Draw a second line with the pencil 3 cm from the bottom of the strip. This is the "cut" line after the strip has been developed.
4. Using a solvent soluble ink, lightly dot the ink 7 cm from the bottom of the strip. This is the solvent front. The step can be done at time of sample testing.
5. Store prepared strips in a tightly closed container over a dessicant.
*Alternative strip preparations are listed in textbooks and other references. These may be used as appropriate.
1. Place enough solvent in the developing chamber to just cover the bottom (less than 1 cm high). An evacuated empty 20 ml vial with the rubber septum removed works nicely.
2. Using a syringe with a small needle (such as a 1 ml syringe, tuberculin syringe, or insulin syringe), spot the sample on the pencil origin line. The spot should be about 2 microliters.
3. Immediately develop the strip it the solvent. Allow the solvent to migrate to the ink spot/line. Do NOT allow the solvent to reach the end of the strip.
4. Cut the strip at the cut line, producing a "bottom" section, containing the origin, and a "top" section, containing the front.
5. Place each section in a clean glass tube (small test tube works well). Count each section in a well counter and record.
6. Using the formula listed for the procedure, calculate the percentage of impurities and record final results.
1. Use fresh solvents and strips.
2. Check tweezers, tongs, scissors, tubes and vials for contamination.
3. Assure background counts on the well counter are insignificant.
4. Use a counting rate between 10,000 and 200,000 counts per minute.
5. Develop strips immediately after spotting. Spot with a very small amount.
6. Label tubes and vials to distinguish between solvent types and strip sections.
7. Date opened stock bottles and discard after 6 months.