An experimental study of organic matters that cause isobaric ions interference for boron isotopic measurement by thermal ionization mass spectrometry

Abstract

The isobaric ions interference induced from organic matter is one of the major problems for the measurement of boron isotopic composition by thermal ionization mass spectrometry (TIMS). However, the exact compounds (or structure) of organic matter are still unknown so far. The experimental characterization on organic matters in natural biocarbonate samples has been performed with Fourier transform infrared spectra (FTIR), Laser Raman and TG-DSC-MS in this work. From the results of infrared and Raman spectra, the organic matter with amide group is detected initially, and the thermal decomposition characteristic patterns of biocarbonate samples from TG-MS confirm that possible organic functional groups existing in the natural samples are amide group (i.e., CONH2), and amide acid group (i.e., COORNH2), which is in very good agreement with the results of FTIR and Laser Raman analysis. In the measurement of TIMS, two reagents, acetamide and EDTA that contain carbonyl group (CO) and amide group (NH2), are designed to add into NIST SRM 951 solution, and the results indicate that the presence of acylamino(CONH2) could bring negative shifts for 11B/10B ratios and lower δ11B value, and the ratios are gradually close to the real value when the organic matters in samples are consumed on heating filament in TIMS. Our experiments have shown that the acylamino group (CONH2) indeed exists in the biocarbonate samples and interferes in the measurement of boron isotopic composition by TIMS.