(B)Chemical Ionization: Many modern MS instruments can perform chemical ionization in addition to El EI ionization in excess (analyte 10-100 ppm)of reactant gas Most common reactant gas is methane EI ionization of methane produces CH4++CH4→CH5++CH3 CH3++CH4→C2H5++H2 These ions react with analyte: CHs++A>CH+AH+proton transfer C2Hs+A>C2H4+AH+proton transfer C2Hs+A>C2H6+(A-H)*hydride elimination analyte most common ions (M+1)+and (M-1)+ sometimes (M+17)+(addition of CHs+)or (M+29)+(addition of C2H5+) CEM 333 page 18.9 (B) Chemical Ionization: • Many modern MS instruments can perform chemical ionization in addition to EI EI ionization in excess (analyte 10-100 ppm) of reactant gas Most common reactant gas is methane EI ionization of methane produces CH4 + + CH4 ® CH5 + + CH3 CH3 + + CH4 ® C2H5 + + H2 These ions react with analyte: CH5 + + A ® CH4 + AH+ proton transfer C2H5 + + A ® C2H4 + AH+ proton transfer C2H5 + + A ® C2H6 + (A - H)+ hydride elimination analyte • most common ions (M+1)+ and (M-1)+ • sometimes (M+17)+ (addition of CH5 +) or (M+29)+ (addition of C2H5 +) CEM 333 page 18.9