Technical note: Determining chemical composition of atmospheric single particles by a standard-free mass calibration algorithm

Abstract

Abstract. The chemical composition of individual particles can be revealed by single particle mass spectrometers (SPMS). With higher accuracy in the ratio of mass to charge (m/z), more detailed chemical information could be obtained. In the SPMS, the conventional standard-based calibration methods (internal/external) are constrained by the inhomogeneity of ionization lasers and the finite focusing ability of the inlet system, etc., therefore, the mass accuracy is restricted. In this study, we obtained the detailed and trustable chemical composition of single particles utilizing a standard-free mass calibration algorithm. In the algorithm, the characteristic distributions of hundreds of ions were concluded and collected in a database denoted as prototype. Each single-particle mass spectrum was tentatively calibrated by a mass calibration function with specific coefficients. The range of coefficients is constrained by the magnitude of mass deviation to a finite vector space. To find the optimal coefficient vector, the conformity of each tentatively calibrated spectrum to the prototype dataset was assessed. With maximum conformity, the optimal calibrated spectrum was obtained. For more than 98 % ambient particles, a twentyfold improvement in mass accuracy, from ~10,000 ppm (integer) to ~500 ppm (2 decimal places) was achieved. The improved mass accuracy validated the determination of adjacent ions with m/z difference ~0.05 Th. Furthermore, atmospheric trace ions that were poorly studied before are successfully specified. The obtained detailed single-particle-level chemical information could help understand the source apportionment, reaction mechanism, and mixing state of atmospheric particles.