Analytical radiochemistry techniques
Basic Techniques and Instruments
Analytical techniques are used commonly in the development and optimization of radiochemical synthesis processes, and are a major part of the quality control (QC) testing that is required after production before injection into humans. The techniques are identical to what is used in conventional analytical chemistry, except the instruments are fitted with radiation detectors.
A dose calibrator (ion chamber) measures the total radioactivity of a sample. The sample can be liquid in a vial, or can be an object containing radioactivity.
In high performance liquid chromatography (HPLC), the sample to be analyzed is loaded into a loop of an injection valves. When the valve is switched, the contents of the loop are interposed between a stream of the HPLC mobile phase coming from the high pressure pump and the separation column. The column contains the stationary phase, and may consist of packed resin or a porous polymer monolith with certain pore size and chemical functionality. The pump pushes the sample through the column. Depending on the interaction between various species in the sample and the mobile and stationary phases, different species take different amounts of time to pass through the whole column. After traversing the column, species can be detected by their physical and chemical properties, e.g. UV absorbance refractive index (RI), pulsed amperometric detection (PAD) etc.
By also adding a gamma detector to the output of the HPLC column, one can monitor the radioactive species in a sample. Standards can verify identify of peaks by retention time. The result is a chromatogram showing the radiation detected at each timepoint.
Because all radioactive chemical species (at different retention time) are detected in an identical manner, peak areas can be directly compared to estimate the proportion of each radioactive species in the sample. This can be used to estimate the purity of a sample (during QC testing), or to estimate the conversion from one species to another (to assess reaction efficiency).
Because some species can get partially or completely trapped in the HPLC column, there could be some bias in the chromatogram, and other methods (e.g. radio-TLC) may be preferred for accurate quantitation.
In thin layer chromatography (TLC), a sample is spotted on one of a plate coated with a stationary phase such as porous silica. The sample is dried, and the sample end of the plate is immersed in a shallow bath of the mobile phase. The mobile phase creeps up the plate via capillary action, dragging the sample with it. Depending on the interaction of the sample with the mobile and stationary phases, different chemical constituents will travel at different speeds. When the solvent front reaches a predefined point, the TLC plate is removed from the mobile phase and dried. The plate is then scanned.
By scanning with a gamma detector, one can monitor the position of radioactive species on the TLC plate. The result is a chromatogram showing the radiation detected at each position along the plate.
Similar to radio-HPLC, because all radioactive chemical species are detected in the same way, the area under peaks on the chromatogram can be compared to give a quantitative comparison of the proportion of different species. Typically it is used to determine the efficiency of a reaction during synthesis development and optimization. In rare cases it is also an accepted method to measure the radiochemical purity.
Since the entire TLC plate is scanned, there is no bias like in radio-HPLC where the column is not scanned but rather only the molecules that make it all the way through. However, this technique can suffer from a different bias: if any species is volatile, some portion or all of it can be lost from the plate before scanning, thus resulting in an underestimate of its amount in the original sample.
In gas chromatography, a sample is volatilized into a carrier gas stream that flows through the GC column and to the detector. It is often used for the determination of residual organic solvents in formulated radiopharmaceuticals.
A nice animation illustrating sample injection and flame ionization detection (FID) can be found here: https://www.youtube.com/watch?v=PV4NYBUaUrQ