⁹⁹Tc bioassay by inductively coupled plasma mass spectrometry (ICP-MS)

A means of analyzing ⁹⁹Tc In urine by inductively coupled plasma mass spectrometry (ICP-MS) has been developed. Historically, ⁹⁹Tc analysis was based on the radiometric detection of the 293 keV E_Max. beta decay product by liquid scintillation or gas flow proportional counting. In a urine matrix, the analysis of ⁹⁹Tc is plagued with many difficulties using conventional radiometric methods. Difficulties originate during chemical separation due to the volatile nature of Tc₂O₇ or during radiation detection due to color or chemical quenching. A separation scheme for ⁹⁹Tc detection by ICP-MS is given and is proven to be a sensitive and robust analytical alternative. A comparison of methods using radiometric and mass quantitation of ⁹⁹Tc has been conducted in water, artificial urine, and real urine matrices at activity levels between 700 and 2200 dpm/L. Liquid scintillation results based on an external standard quench correction and a quench curve correction method are compared to results obtained by ICP-MS. Each method produced accurate results, however the precision of the ICP-MS results is superior to that of liquid scintillation results. Limits of detection (LOD) for ICP-MS and liquid scintillation detection are 14.67 and 203.4 dpm/L, respectively, in a real urine matrix.

In order to determine the basis for the increased precision of the ICP-MS results, the detection sensitivity for each method is derived and measured (k vs. λ). The detection sensitivity for the ⁹⁹Tc isotope by ICP-MS is 2.175 x 10^-7 +/- 8.990 x 10^-9 and by liquid scintillation is 7.434 x 10^-14 +/- 7.461 x 10^-15. A difference by seven orders of magnitude between the two detection systems allows ICP-MS samples to be analyzed for a period of 15 s compared to 3600 s by liquid scintillation counting with a lower LOD.