Radiochemical Purity & TLC/ITLC QC of Radiopharmaceuticals

Introduction

Radiochemical purity (RCP) is the fraction of a radiopharmaceutical's total radioactivity that is present in the desired, correctly labeled chemical form, and thin-layer chromatography (TLC/ITLC) is the bench method nuclear medicine clinics use to measure it before a dose ever reaches a patient. A kit can contain exactly the right radionuclide and the right amount of activity and still be unfit for use, because some of that activity may be in the wrong chemical form—and only an RCP test catches it. ^{1, 2, 5}

Most diagnostic nuclear medicine runs on technetium-99m (Tc-99m) "cold kits": a vial of freeze-dried ligand and a reducing agent into which the technologist injects Tc-99m pertechnetate eluted from a generator. The labeling reaction is fast and usually efficient, but it is never guaranteed. Two predictable impurities can appear—free (unbound) Tc-99m pertechnetate and hydrolyzed-reduced Tc-99m colloid—and either one degrades the biodistribution the physician is counting on. RCP testing quantifies how much of the activity ended up in the intended form so the preparation can be released or rejected on objective grounds. ^{3, 4, 5}

This guide separates the three distinct purity concepts that are easy to conflate—radionuclidic, radiochemical, and chemical purity—then walks through the TLC/ITLC method step by step: spotting, developing in a solvent system, separation by retention factor, cutting and counting the strip, and computing RCP. It covers why two solvent systems are combined to resolve all three Tc-99m species, the acceptance limits set by the USP and the package insert, and what to do when a test fails. DRPS supports radiopharmacy QC programs as part of its PET/CT and nuclear medicine physics and medical physics consulting services across Florida, Maryland, Virginia, Washington DC, California, and Nevada.

Topic Explanation

Three kinds of purity—and why they differ

A radiopharmaceutical has to pass three independent purity ideas, and they are not interchangeable. Each describes a different way a preparation can be "impure," each has its own failure mode, and each is verified by a different test. ^{1, 2, 5}

Radionuclidic purity is the fraction of total radioactivity that comes from the desired radionuclide. For a Tc-99m eluate, the relevant contaminant is molybdenum-99 (Mo-99) breakthrough—Mo-99 from the generator column appearing in the eluate. Mo-99 is a long-lived, high-energy contaminant that adds unwanted patient dose and degrades images, so it is limited tightly (the familiar Mo-99/Tc-99m ratio limit). Critically, radionuclidic purity is checked separately from RCP, on the generator eluate, not on the finished kit—see Tc-99m generator quality control for the moly-breakthrough and alumina tests. ^{2, 5, 6}
Radiochemical purity (RCP) is the fraction of the radioactivity that is in the desired chemical/labeled form. The radionuclide can be correct (pure Tc-99m) while a portion of that Tc-99m is in the wrong molecular form—unbound or reduced to a colloid. RCP is what TLC/ITLC measures. ^{1, 3, 5}
Chemical purity concerns non-radioactive contaminants. The classic example is aluminum ion (Al³⁺) carried over from the generator's alumina column. Excess Al³⁺ can cause flocculation of sulfur colloid or alter labeling, and it is checked colorimetrically (a colored spot test) against a limit—not by counting radioactivity at all. ^{2, 5, 6}

The takeaway: a Tc-99m kit can have excellent radionuclidic purity and still fail RCP, or pass both and still carry too much aluminum. The three tests answer three different questions, and a clinic needs all three to release product responsibly.

What RCP catches: the two technetium impurities

RCP testing exists to quantify two specific failure products of the Tc-99m labeling reaction. Both arise from the redox chemistry that makes labeling possible, and both ruin where the tracer goes in the body. ^{3, 4, 5}

Free (unbound) Tc-99m pertechnetate ( $TcO_{4}^{-}$ ). This is technetium that was never reduced and bound to the ligand—it remains as pertechnetate, the same chemical form that came off the generator. Pertechnetate localizes in the thyroid, salivary glands, stomach, and choroid plexus and circulates in blood, so free pertechnetate contamination paints those structures onto a scan that was supposed to image bone, kidneys, or myocardium. It typically arises from insufficient reducing agent, oxidation of the kit (air introduced into the vial, expired or oxidized stannous reagent), or too much oxygen in the eluate. ^{3, 4}
Hydrolyzed-reduced Tc-99m (the "colloid," often written as $TcO_{2}$ or reduced-hydrolyzed Tc). Here technetium was reduced but, instead of binding the ligand, hydrolyzed into an insoluble reduced-technetium colloid. This colloid behaves like any small particulate: the reticuloendothelial system clears it, so it deposits in the liver and spleen and at the injection site. It comes from excess or hydrolyzed stannous ion, high pH, or aged reagents. ^{3, 4}

A clean preparation is one in which the great majority of the Tc-99m is bound to the intended ligand, with only small fractions of free pertechnetate and hydrolyzed-reduced colloid. RCP is simply the percentage in the bound form—and the two impurities are exactly what the chromatography strip is designed to pull apart and count. For context on the isotopes and their emission properties, see common PET and radiopharmaceutical-therapy isotopes.

Why chromatography works here

Thin-layer chromatography separates molecules by how they distribute between a stationary phase (the strip) and a mobile phase (the solvent), and because each Tc-99m species has a different chemistry, each migrates a different distance. Free pertechnetate is small, soluble, and unbound; the labeled product is a defined complex; the hydrolyzed-reduced colloid is an insoluble particulate that essentially does not move. Choosing the right strip-and-solvent combination makes those differences show up as physical separation along the strip, which a detector can then quantify. ^{1, 7, 8}

The instant-TLC (ITLC) variant uses glass-fiber strips impregnated with silica gel (ITLC-SG) instead of conventional silica-on-glass plates. ITLC develops in a few minutes rather than tens of minutes, which is why it dominates in clinical radiopharmacy: the result has to come back fast enough to release a short-half-life dose on schedule. ^{7, 8}

Key Technical Principles

The retention factor

The retention factor $R_{f}$ describes how far a chemical species travels up the strip relative to the solvent front, and it is the quantitative language of chromatographic separation. It is defined as the distance the spot of interest migrates from the origin divided by the distance the solvent front migrates from the origin:

R_{f} = \frac{d _{spot}}{d _{solvent front}}

An $R_{f}$ near 0 means the species stayed at the origin (the spotting point); an $R_{f}$ near 1 means it moved with the solvent front to the top of the strip. In a Tc-99m system, the goal is to choose a solvent in which the species you want to measure sits at a known end of the strip, separated cleanly from everything else. For example, in a solvent that carries free pertechnetate to the front, pertechnetate shows $R_{f} \approx 1$ while the labeled complex and the colloid stay near the origin ( $R_{f} \approx 0$ ).

Two solvent systems resolve three species

A single strip-and-solvent combination usually cannot separate all three Tc-99m species at once, so the standard approach runs two complementary systems and combines the results. The principle is to design each system so that exactly one impurity is isolated at a known location while everything else moves together. ^{7, 8}

System for free pertechnetate. In an organic solvent such as acetone (methanol is also used), free pertechnetate migrates with the solvent front ( $R_{f} \approx 1$ ) while the labeled product and the hydrolyzed-reduced colloid remain at the origin ( $R_{f} \approx 0$ ). Counting the front region of this strip gives the fraction that is free pertechnetate.
System for hydrolyzed-reduced technetium. In saline (0.9% NaCl), both the labeled product and free pertechnetate move up the strip while the insoluble hydrolyzed-reduced colloid stays at the origin ( $R_{f} \approx 0$ ). Counting the origin region of this strip gives the fraction that is hydrolyzed-reduced technetium.

The labeled (desired) product is then obtained by difference: the activity that is not free pertechnetate (from the first system) and not hydrolyzed-reduced colloid (from the second system). The exact solvent, strip, and $R_{f}$ assignments are specified in each product's package insert and USP monograph and must be followed for that product; the description here is the general principle, not a substitute for the validated method. ^{5, 7, 8}

Computing radiochemical purity

Once each strip is developed, it is cut into segments and each segment is counted in a dose calibrator or well counter; the counts give the percentage of activity in each chemical form. The radiochemical purity is the activity in the desired form as a percentage of the total:

RCP (%) = \frac{A _{desired form}}{A _{total}} \times 100 = (1 - \frac{A _{free} + A _{hydrolyzed}}{A _{total}}) \times 100

In practice each impurity is measured on its own strip and expressed as a percent of that strip's total counts:

% free pertechnetate = \frac{A _{front (acetone strip)}}{A _{total (acetone strip)}} \times 100

% hydrolyzed-reduced = \frac{A _{origin (saline strip)}}{A _{total (saline strip)}} \times 100

RCP (%) = 100 - % free pertechnetate - % hydrolyzed-reduced

A few measurement points matter. Each strip must be counted in a consistent geometry (the same well or dose-calibrator setup), background must be handled, and for the short-lived Tc-99m the two strips should be counted close together in time so decay does not distort the ratio—because RCP is a ratio of activities measured at essentially the same time, the decay correction cancels, which is one reason the method is robust. The dose calibrator used to count the segments should itself be under a QC program; see dose calibrator quality control.

Worked example

Consider a Tc-99m kit assayed by the two-strip ITLC method, with a $\geq 95%$ RCP acceptance limit from the package insert. The technologist spots the preparation near the origin of two ITLC-SG strips, develops one in acetone and one in saline, then cuts each at the midpoint and counts the halves.

Acetone strip (resolves free pertechnetate; pertechnetate runs to the front):

Solvent-front half (free $TcO_{4}^{-}$ ): 1,800 counts
Origin half (everything else): 58,200 counts
Strip total: 60,000 counts

% free pertechnetate = \frac{1 , 800}{60 , 000} \times 100 = 3.0%

Saline strip (resolves hydrolyzed-reduced colloid; colloid stays at the origin):

Origin half (hydrolyzed-reduced colloid): 900 counts
Solvent-front half (labeled product + pertechnetate): 59,100 counts
Strip total: 60,000 counts

% hydrolyzed-reduced = \frac{900}{60 , 000} \times 100 = 1.5%

Radiochemical purity:

RCP = 100 - 3.0% - 1.5% = 95.5%

At 95.5%, the preparation is just above a $\geq 95%$ limit and passes—it may be released for patient use, and the result, strip counts, solvent systems, and lot information are documented. Had the colloid fraction been 3.0% instead of 1.5%, RCP would be $100 - 3.0 - 3.0 = 94.0%$ , which fails a 95% limit; the vial would be rejected and not injected.

Clinical Impact

Biodistribution and image quality

RCP failures do not produce a blank scan—they produce a misleading one, which is more dangerous. Free pertechnetate adds uptake in the thyroid, salivary glands, stomach, and blood pool; hydrolyzed-reduced colloid adds uptake in the liver, spleen, and injection site. On a bone scan, a myocardial perfusion study, or a renal study, those extra hotspots are abnormal findings that the tracer was never supposed to create—they reduce target-to-background contrast and can mimic or mask disease. A study read off a low-RCP preparation can send a patient toward an unnecessary follow-up or, worse, obscure a real lesion. ^{3, 4}

Patient dose and repeat studies

A failed or unrecognized RCP problem wastes the patient's radiation dose. If a preparation is injected and the resulting images are nondiagnostic because of impurity uptake, the study often has to be repeated—a second radiopharmaceutical dose, a second appointment, and a second round of radiation exposure that a pre-injection RCP check would have prevented. The few minutes of chromatography are a direct patient-safety control, not paperwork. ^{3, 5}

Quantitative and therapy applications

For quantitative imaging and radiopharmaceutical therapy, RCP is part of the dose-accuracy chain. When the administered activity is used for SUV quantification, SPECT-based dosimetry, or theranostic dose planning, the assumption is that the activity is in the intended molecular form going to the intended target. Impurity activity that localizes elsewhere is, in effect, mis-targeted dose—a concern that grows as programs move into Lu-177 theranostics and dosimetry, where labeling-yield and RCP verification of the therapy agent is integral to delivering the prescribed absorbed dose. RCP, the uptake-time protocol, and scanner calibration are links in one quantitative chain. ^{3, 4}

Practical Optimization Tips

A dependable RCP program is built on disciplined technique and clear acceptance rules.

1. Follow the package-insert method exactly

Use the exact strip, solvent system, spotting volume, and development distance specified in the product's package insert or USP monograph—solvent and $R_{f}$ behavior are product-specific and not interchangeable between kits.
Use the acceptance limit stated for that product; do not assume a generic 95% applies to every kit.

2. Protect the spotting and developing steps

Spot a small, well-defined drop near the origin and let it dry as directed; an oversized or smeared spot blurs the separation.
Keep the origin spot above the solvent level in the developing chamber so the sample is not washed off into the solvent.
Develop in a covered, solvent-saturated chamber to keep migration reproducible, and mark the solvent front promptly when development is complete.

3. Count consistently and correctly

Count strip segments in a consistent geometry (same well counter or dose-calibrator setup) and subtract background.
Count the paired strips close together in time so Tc-99m decay does not bias the ratio.
Beware of very high count rates that can cause dead-time losses on the counter and distort the apparent ratio; dilute or adjust geometry if needed.

4. Build failure handling into the SOP

Define the pass/fail limit and the action on failure (reject, do not inject, prepare a fresh vial) in writing.
Record the RCP result, the strip counts, solvent systems, kit lot, and operator for every preparation, and retain the records for inspection.

Common pitfalls to avoid

Introducing air/oxygen into the kit vial, which oxidizes the stannous reducing agent and drives up free pertechnetate.
Using expired or improperly stored kits or reagents, where the reducing capacity has degraded.
Letting the spot dip into the solvent, washing the sample off the origin and invalidating the run.
Mixing up the two solvent systems or counting the wrong strip region for the species of interest.
Skipping documentation—an undocumented RCP result is, for inspection purposes, no result at all.
Assuming one good batch covers the day—RCP can vary vial to vial; test per the product's required frequency.

Regulatory Considerations

Radiochemical purity testing lives at the intersection of NRC/Agreement-State byproduct-material rules, USP standards, and the FDA-approved package insert, and it functions as a release criterion: a preparation that fails its RCP limit must not be administered. ^{5, 9, 10}

Key frameworks to reference:

USP General Chapter <821> Radioactivity and the radiopharmaceutical compounding/preparation chapter, <825> Radiopharmaceuticals—Preparation, Compounding, Dispensing, and Repackaging, set the standards framework for radioactivity measurement and for the preparation and quality control of radiopharmaceuticals; individual product monographs specify the RCP method and limit. ^{5, 7}
FDA-approved package insert for each kit—the legally controlling source for that product's RCP test method, solvent systems, and acceptance limit. The insert, not a generic value, governs whether a given preparation passes. ^{3, 5}
10 CFR Part 35 — Medical Use of Byproduct Material, the NRC rule governing authorized users, the authorized nuclear pharmacist and radioactive-drug framework, written directives, and the Radiation Safety Officer's responsibilities. Quality control of the radiopharmaceutical before administration sits inside this program. ⁹
10 CFR 35.60 and related provisions require instrumentation to measure activity before administration; the dose calibrator or well counter used to count RCP strip segments must itself be calibrated and under QC. ¹⁰

Agreement States administer their own equivalent programs. Of the states DRPS serves, Florida, Maryland, Virginia, California, and Nevada are NRC Agreement States that license the medical use of byproduct material under their own radiation-control rules, while Washington, DC is regulated directly by the NRC. A facility must verify which authority issues its license and which QC, documentation, and recordkeeping requirements apply, and the Radiation Safety Officer owns the RCP QC program—the procedures, acceptance limits, training, and records that an inspector will review. ^{9, 10}

Frequently Asked Questions (FAQs)

What is radiochemical purity (RCP) of a radiopharmaceutical?

Radiochemical purity is the fraction of the total radioactivity in a sample that is present in the desired, correctly labeled chemical form. For a Tc-99m kit it is the percentage of Tc-99m bound to the intended ligand, as opposed to free Tc-99m pertechnetate or hydrolyzed-reduced technetium. RCP is measured before administration and typically must meet a 90 to 95 percent minimum.

How is radiochemical purity different from radionuclidic and chemical purity?

The three are distinct. Radionuclidic purity is the fraction of total radioactivity from the desired radionuclide, such as Tc-99m versus Mo-99 breakthrough. Radiochemical purity is the fraction of that activity in the desired chemical form. Chemical purity concerns non-radioactive contaminants such as aluminum ion. Each is checked by a different test.

How does thin-layer chromatography measure radiochemical purity?

A small drop of the radiopharmaceutical is spotted near the origin of a chromatography strip, the strip is developed in a solvent, and the labeled product and impurities migrate different distances based on their retention factor Rf. The strip is then cut into segments and each segment is counted in a dose calibrator or well counter. The activity in each region gives the percentage of each chemical species.

Why are two solvent systems used for Tc-99m radiochemical purity?

A single TLC strip and solvent usually cannot separate all three Tc-99m species at once. One solvent system, often acetone or a similar organic solvent, moves free pertechnetate with the solvent front while the labeled product stays at the origin. A second system, often saline, moves the labeled product and pertechnetate while leaving hydrolyzed-reduced technetium colloid at the origin. Combining the two resolves all three components.

What is an acceptable radiochemical purity limit before injection?

Acceptance limits are set by the USP monograph or the manufacturer package insert for each specific product and commonly require radiochemical purity of at least 90 to 95 percent. A preparation that fails its limit must not be administered to a patient. Always use the limit stated in the current package insert or USP monograph for the exact product.

What does a failed radiochemical purity test mean and what do you do?

A failed test means too much of the activity is free pertechnetate or hydrolyzed-reduced technetium, which would degrade biodistribution and image quality. The preparation must be rejected and not injected. The technologist documents the result, investigates likely causes such as a kit error, expired or oxidized reagents, air introduced into the vial, or generator and saline issues, and prepares a fresh vial that passes before any patient is dosed.

Who regulates radiopharmaceutical purity testing in nuclear medicine?

Radiopharmaceuticals are byproduct material regulated by the NRC or an Agreement State under 10 CFR Part 35, and the products and tests follow USP standards and the FDA-approved package insert. Radiochemical purity testing functions as a release criterion before administration, and the Radiation Safety Officer and authorized users own the quality-control program and its records.

Key Takeaways

Three purity concepts, three tests. Radionuclidic purity (desired radionuclide; Mo-99 breakthrough) is checked on the eluate, radiochemical purity (desired chemical form) is checked by TLC/ITLC, and chemical purity (Al³⁺ and other non-radioactive contaminants) is checked colorimetrically—they are not interchangeable.
RCP catches two impurities. TLC/ITLC quantifies free Tc-99m pertechnetate (wrong-target uptake in thyroid, stomach, blood pool) and hydrolyzed-reduced colloid (liver, spleen, injection site), both of which corrupt biodistribution.
The method is spot, develop, separate by $R_{f}$ , cut, count. $R_{f} = d_{spot} / d_{solvent front}$ quantifies migration, and two complementary solvent systems are combined to resolve all three species.
RCP is a ratio computed from strip counts. $RCP = 100 - % free - % hydrolyzed$ , measured at essentially one time so decay cancels; the limit (commonly 90–95%) comes from the package insert or USP monograph.
It is a release criterion. A preparation below its RCP limit is rejected, never injected; the result and counts are documented, and a fresh vial is prepared.
The RSO owns the program. Within 10 CFR Part 35, USP, and FDA-insert frameworks, RCP QC—procedures, limits, training, and records—is a Radiation Safety Officer and authorized-user responsibility, verified per the facility's license.

Conclusion

Radiochemical purity testing turns an invisible chemistry question—"is the Tc-99m actually bound to the right molecule?"—into a number a clinic can act on before a patient is injected. The TLC/ITLC method is elegant precisely because it is simple: a drop near the origin, a few minutes in the right solvent, a cut, and a count, repeated across two complementary systems to pull apart the labeled product, free pertechnetate, and hydrolyzed-reduced colloid. The arithmetic—a ratio of activities measured at the same instant—is robust, and the decision rule is unambiguous: meet the package-insert limit or reject the vial.

For the RSO and the medical physicist, RCP belongs in the same connected QC system as generator quality control, dose-calibrator QC, and the rest of the radiopharmacy program—with defined methods, defined limits, and retrievable records aligned to the facility's license. A clinic that runs RCP testing well protects its patients from misleading scans and wasted dose, and can defend every administered preparation during an inspection.

How DRPS Can Help

Diagnostic Radiation Physics Services helps nuclear medicine clinics and radiopharmacies build, run, and document defensible radiopharmaceutical QC programs—including radiochemical purity procedures. This may include writing TLC/ITLC SOPs aligned to each product's package insert and USP monograph, setting acceptance limits and failure-handling rules, coordinating RCP with generator and dose-calibrator QC, training technologists on spotting, development, and counting technique, and reviewing records for inspection readiness, all as part of PET/CT and nuclear medicine physics, medical physics consulting, and Radiation Safety Officer support.

DRPS supports facilities across our service locations, including Florida, Maryland, Virginia, Washington DC, California, and Nevada. To discuss a radiopharmacy QC review, contact our team.

Strong radiochemical purity QC is not paperwork—it is the assurance that every dose you inject goes where the physician intends.

Related Resources

References

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Cherry SR, Sorenson JA, Phelps ME. Physics in Nuclear Medicine. 4th ed. Elsevier/Saunders; 2012. elsevier.com
International Atomic Energy Agency. Quality Control in the Production of Radiopharmaceuticals (IAEA-TECDOC-1856). IAEA; 2018. iaea.org
International Atomic Energy Agency. Technetium-99m Radiopharmaceuticals: Manufacture of Kits (Technical Reports Series No. 466). IAEA; 2008. iaea.org
United States Pharmacopeial Convention. USP General Chapter <825> Radiopharmaceuticals—Preparation, Compounding, Dispensing, and Repackaging. USP. usp.org
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U.S. Nuclear Regulatory Commission. 10 CFR Part 35: Medical Use of Byproduct Material. ecfr.gov
U.S. Nuclear Regulatory Commission. 10 CFR 35.60: Possession, use, and calibration of instruments used to measure the activity of unsealed byproduct material. ecfr.gov