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Why is high-resolution mass spectrometry useful for distinguishing ions with nearly identical masses?

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Figure 1: An electron bombardment ion source in cross section. An electron beam is drawn from the filament and accelerated across the region in which the ions are formed and toward the electron trap. An electric field produced by the repeller forces the ion beam from the source through the exit slit. — mass spectrometry Figure 1: *An electron bombardment ion source in cross section.* An electron beam is drawn from the filament and accelerated across the region in which the ions are formed and toward the electron trap. An electric field produced by the repeller forces the ion beam from the source through the exit slit.

High-resolution mass spectrometry is essential for distinguishing ions with nearly identical masses because it allows for extremely precise measurements of an ion's mass, often to an accuracy of one part in one million.1

While conventional mass spectrometers may struggle to resolve ions with similar mass-to-charge ratios, high-resolution instruments can differentiate between them by identifying their exact mass differences. For example, the ions $C_{6} H_{12}^{+}$ and $C_{4} H_{4} O_{2}^{+}$ have nominal masses that are very close, but their accurate masses—84.0939 and 84.0211, respectively—are distinct enough to be easily separated by high-resolution analysis.21

By providing this level of precision, high-resolution mass spectrometry serves as the best index for determining the exact molecular formula of an unknown compound. Once the molecular formula is established, chemists can deduce the total number of rings and double bonds in a structure, which is a critical step in identifying complex organic molecules.21