Ultramark 1621 as a calibration/reference compound for mass spectrometry. II. Positive‐ and negative‐ion electrospray ionization

Electrospray ionization of solutions of Ultramark 1621, a commercially available mixture of fluorinated phosphazenes, in solution of 50:23:23:4 v/v of acetonitrile+water+methanol+acetic acid, produced a series of intense singly charged peaks in the m/z range of 900 to ca . 2200 in both positive‐ and negative‐ionization modes. The chemical compositions and accurate masses of ions in both positive‐ and negative‐ion spectra have been reported. An accuracy of approximately 2 ppm has been achieved for the average molecular weight of bovine insulin using Ultramark 1621 peaks as reference standards.