Answer to Question #13913 Submitted to "Ask the Experts"

I'm trying to understand differences in how radioactivity and isotopes are detected. If I understand correctly, liquid scintillation counting (LSC) measures radioactivity, so it is used to determine radioactive isotopes. Liquid chromatography—mass spectrometry (LCMS) measures mass/charge. For radioactive isotopes, you should be able to use LSC or LCMS for detection. For non-radioactive, you can only use LCMS. Is this accurate? If not, can you explain? Also, what is considered radioactive? For instance, is deuterium considered radioactive? Can it be detected by LSC or LCMS?

Indeed, both methods that you cite have been used for analysis of radionuclides. LSC, a technique in which an organic scintillator is dissolved in an appropriate solvent system and into which the radioactive sample is introduced, has been used for many years for measurements of radiations from many different radionuclides. It has found a special niche for assessing beta-emitting radionuclides. It is well suited to this task because it has high efficiency for beta radiation, even for very low energy beta particles such as those from tritium decay. The bio-triad that is very commonly measured via LCS is tritium (³H), maximum beta energy of 0.0186 Me), carbon-14 (¹⁴C), maximum beta energy of 0.156 MeV, and phosphorus-32 (³²P), maximum beta energy of 1.71 MeV. With modern equipment these three can be quantitatively assessed simultaneously. The LSC technique has been used for many other radionuclides, but it is a low-resolution technique, and if more than one beta-emitting radionuclide is present and the beta energies are not very different, the technique will not resolve the output pulses in a satisfactory manner to allow evaluation of the individual radionuclides. In such cases, quantitative analysis would require separating the radionuclides and counting them individually. Other radiations, such as alpha and gamma (especially low energy) can also be effectively measured with LSC.

When it comes to measuring radionuclides that have long half-lives, such as uranium (e.g., ²³⁵U half-life 7.1 × 10⁸ y; ²³⁸U 4.5 × 10⁹ y) and plutonium (e.g., ²³⁹Pu 2.4 × 10⁴ y; ²⁴⁰Pu 6.6 × 10³ y), the specific activities—i.e., activity per unit mass are often low and the type of radiation, often alpha, may be difficult to measure with adequate sensitivity; quantities as prepared for analysis may not be sufficient and/or in an acceptable physical configuration to measure the radiations of concern with required precision. Additionally, the radiation measurement process becomes more complicated and sometimes impossible if one is interested in quantifying different isotopes of such elements; even if different isotopes emit different radiations some isotopes may be present in much smaller amounts than other isotopes. It is in these kinds of instances that mass spectrometry, (MS), has proven especially useful.

A fairly common technique has been inductively coupled plasma—mass spectrometry (ICP-MS). While I have not personally used these techniques, high performance liquid chromatography (HPLC) has been used as a separative technique and combined with appropriate ionization and measurement techniques, including ICP-MS, sometimes with multi-collector (MC), e.g., HPLC-MC-ICP-MS. There have also been reports of usage of HPLC with thermal ionization and mass spectrometry (HPLC-TIMS). Most of such applications have involved uranium and transuranic element and isotope evaluations. In theory, it is possible to do analyses of other lighter and shorter-lived radionuclides by similar techniques, but it is often not practical because (1) many such radionuclides can be analyzed more efficiently, often with higher sensitivity, and more economically by conventional radiological methods and (2) when dealing with relatively short-lived radionuclides the mass quantities represented by significant amounts of radioactivity may be extremely small. For example, 100 becquerel (Bq), or 100 atoms decaying per second of cobalt-60 (⁶⁰Co) is a quantity easily measured by conventional radiation measurement techniques; however, this amount of activity represents only about 2.4 × 10^-12 g of ⁶⁰Co, or about 4 × 10^-14 moles. Depending on what matrix this was in and how it was separated, ionized, and fed to an MS system, analysis could still fail or not be as precise, fast, or economical as conventional radiological methods.

In reference to the last part of your question, a nucleus is considered radioactive if it is energetically unstable. Radioactive decay is a nucleus' attempt to achieve a stable configuration by altering its configuration. It does this by any of four major decay processes that result in changes to the neutron-to-proton ratio in the nucleus: (1) alpha emission (a nuclear particle containing 2 neutrons and 2 protons), (2) beta (minus) particle emission (a nuclear neutron transforms to a proton plus a conventional negative electron, and the electron gets ejected and is called a beta particle, (3) beta (plus) emission (a nuclear proton transforms to a neutron plus a positively charged "electron," often referred to as a positron, which gets ejected from the nucleus), and (4) electron capture (an inner shell electron gets captured by the nucleus, transforming a proton into a neutron). A lesser decay mode, significant primarily in heavy nuclei is spontaneous fission in which the nucleus splits into two fragments, usually with one of significantly lower mass than the other. Gamma radiation emission is not actually a decay process, but it is a deexcitation process by which a nucleus can release its excitation by doing a slight reshuffling of its nucleons (kind of analogous to the way characteristic x rays are emitted as electrons drop from a higher to a lower electron shell in the atom). Gamma emission frequently follows immediately after decay events in order to allow deexcitation of a nucleus left in an excited state after the decay event.

While deuterium that you mention is an isotope of hydrogen it is completely stable from an energetics standpoint. It has been used in some nuclear applications, most notably in which it has been used as fuel in the fusion process, sometimes being fused with radioactive tritium, the third isotope of hydrogen. Nuclear fusion produces a large amount of energy per unit mass of reactants and holds promise as a power source, although its practical development has been very slow. Deuterium cannot be detected by LSC since it emits no radiation to excite the scintillator. It can be detected by various LC-MS and HPLC-MS techniques that you can find by doing a quick internet search. 

I hope this addresses your concerns.

George Chabot, PhD, CHP