The most common approach for identifying unknown analytes using mass spectrometry combines fragmentation rules with circular reasoning - working backwards from the answer. But you no longer have to work backwards from the answer. Using this new Rational Numbers Excel Add-In described in this book and accurate-mass fragmentation data, the mathematical and graphical powers of Excel can be directly applied to analyzing mass spectral data obtained using LCMS. This Excel Add-In is designed to help identify small molecules from less-than-perfect mass spectral data. All data must be accurate mass fragmentation data with a minimum accuracy of +/- 5 mDa, which has become common recently. This book is not concerned with the technical or experimental aspects of ion formation, mass selection or mass measurement; it deals primarily with the analysis of accurate mass fragmentation data of to identify small molecules . In this book, there are no fragmentation rules, chemical drawings of hypothetical fragment ions, or quantum mechanics. This book is basically the author's perspective about a new tool that can help chemists, not just mass spectrometrists, to identify analytes quickly from LCMS data generated using electrospray ionization.