Massspectrometry is more often applied to molecules than to atoms. We will see in Chapter 3 that the molecular weight of a molecule is the sum of the atomic weights of the atoms in the molecule. The mass spectrum of H₂ is taken under conditions that prevent decomposition into H atoms. The two naturally occurring isotopes of hydrogen are ¹H (atomic mass = 1.00783 amu; abundance 99.9885%) and ²H (atomic mass = 2.01410; abundance 0.0115%). (a) How many peaks will the mass spectrum have?

Key Concepts

Mass Spectrometry

Mass spectrometry is an analytical technique used to measure the mass-to-charge ratio of ions. It provides information about the molecular weight and structure of compounds by generating a mass spectrum, which displays the abundance of ions at different mass-to-charge ratios. In the context of molecules, it helps identify the presence of different isotopes and molecular fragments.

Isotopes

Isotopes are variants of a chemical element that have the same number of protons but different numbers of neutrons, resulting in different atomic masses. For hydrogen, the two isotopes are protium (¹H) and deuterium (²H). The presence of isotopes in a sample affects the mass spectrum, as each isotope will produce a distinct peak corresponding to its mass.

Mass Spectrum Peaks

In a mass spectrum, each peak represents a different ion detected by the mass spectrometer, corresponding to specific mass-to-charge ratios. The number of peaks is influenced by the isotopes present in the sample and their relative abundances. For the hydrogen molecule (H₂), the mass spectrum will show peaks for both isotopes, indicating the molecular composition and the presence of H₂ and its isotopic variants.