Structures, heats of formation, ionization energies, and proton affinities of selenium, argon dimer, argon-chlorine, and their hydrides (Se, SeH, SeH2, ArH, ArH2, Ar-2, Ar2H, Ar2H2, ArCl, and ArHCl) are estimated by quantum chemistry calculations using G3, G4, and W1 composite methods and coupled cluster approach at the CCSD(T)/aug-cc-pVTZ levels. Thermochemistry of the reactions between ions A(+) = Se center dot+, SeH+, SeH2 center dot+, SeH3+, Ar-2(center dot+), Ar2H+, Ar2H2 center dot+, Ar2H3+, ArCl+, ArClH center dot+, and ArClH2+ with various neutral gas G commonly used in dynamic reaction chamber-inductively coupled plasma-mass spectrometry (DRC-ICP-MS) (G = H-2, CH4, NH3, O-2, CO, CO2, NO, and N2O) has been investigated.