Sodium trifluoroacetate as a tune/calibration compound for positive- and negative-ion electrospray ionization mass spectrometry in the mass range of 100–4000 Da

We have identified aqueous:acetonitrile solutions of alkali–metal trifluoroacetate compounds as tune/calibration standards for both positive- and negative-ion electrospray ionization mass spectrometry (ESI/MS). Each alkali–metal trifluoroacetate solution in water and acetonitrile (50:50, v/v) yields evenly spaced, singly charged peaks in the mass range of 100–3500 Da. Intense peaks are formed either by infusing the solution using a syringe pump, by infusing the solution into a stream of liquids [such as a high-performance liquid chromatography (HPLC) mobile phase] flowing to the electrospray needle, or by injecting the salt-containing solution into an HPLC mobile phase containing trifluoroacetic acid. The advantages of these compounds include: (i) generation of singly charged ions in both positive and negative ionization modes in the mass range of approximately 100–3500 Da, (ii) formation of evenly spaced peaks with similar intensity across the entire mass range, (iii) the most abundant isotope in each mass cluster is the lowest mass peak (monoisotopic mass), which is free from variation in natural isotope distribution, (iv) commercial availability, (v) they easily dissolve in common liquid chromatography solvents, and (vi) lack of any long-lasting memory effects or background problems.