Measuring the mass of an electron: an undergraduate laboratory experiment with high resolution mass spectrometry

Abstract High-resolution mass spectrometry (HRMS) has become increasingly affordable and user-friendly. Its potential spans a wide range of applications and experiments including the measurement of accurate masses, supporting the elucidation of elemental compositions and the identification of unknown compounds. To illustrate the main features of mass spectrometry, and particularly, of HRMS, we have designed and implemented a 3-h laboratory experiment using direct infusion electrospray HRMS analysis of non-steroidal anti-inflammatory drugs (e.g., ibuprofen or naproxen) solutions, acquiring full-scan spectra in both positive and negative ionization modes. The experimental accurate mass measurements (m/z values) of selected characteristic fragment ions -so called twin ions, with common elemental composition in both ionization modes but with different charge, allow the indirect measurement of the mass of an electron with relative errors below 5% with respect to the accepted IUPAC value (0.00055 Da). The experiment demonstrates how powerful and useful HRMS can be for research challenges often encountered during undergraduate or graduate research projects as well as for addressing undergraduate level general chemistry problems that provide the opportunity to discuss aspects related to the Nature of Science in an analytical chemistry context (such as measurement precision and accuracy).