Elemental composition calculations are used to calculate all possible elemental compositions that have calculated masses falling within a specified error tolerance of a measured mass. This calculation is usually (but not always) done for accurate masses obtained from a. In this Chapter, we will introduce a general notion of spectroscopy as a method and of its most basic type of data, a spectrum. We will also introduce the most basic features of each spectroscopic signal (spectral line or resonance): position, intensity and width/lineshape. Techniques like infrared (IR) and nuclear magnetic resonance (NMR) spectroscopy are particularly useful for detailed molecular analysis.