Measurement of onset potentials for proton transfer reactions with a triple quadrupole instrument

Problems with the treatment of data from the study of both exothermic and endothermic ion‐molecule reactions are discussed. Recommendations are (i) to extrapolate data from an obviously linear region in the plot of ratios of secondary to primary ion intensity which appears ∼1–3 V above the value of the onset potential; (ii) to use ionization methods that give low internal energies to primary ions; (iii) to set for moderately high resolution; (iv) to check the stability of the primary ion current frequently; (v) to pay special attention heed to source and collision region pressure stability; and (vi) to keep collision region pressures below 0.6 mPa.