The molecular mass of ethylene glycol (Problem 3.125) is...

The molecular mass of ethylene glycol (Problem 3.125$)$ is 62.0689 amu when calculated using the atomic masses found in a standard periodic table, yet the molecular mass determined experimentally by high-resolution mass spectrometry is 62.0368 amu. Explain the discrepancy.

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Key Concepts

Atomic Mass Calculation

This concept refers to the method of calculating the molecular mass by summing the atomic masses of the individual atoms, where those atomic masses are typically taken from standard periodic tables and represent the weighted averages of all naturally occurring isotopes. These values might include rounding and approximations that can differ slightly from the exact masses of specific isotopes present in a given sample.

Isotopic Composition

The isotopic composition of an element is the relative abundance of its isotopes naturally occurring in a sample. Because the standard atomic masses are averages based on these natural abundances, any deviation in the exact isotopic makeup of a particular sample can lead to differences between the calculated molecular mass and the mass observed experimentally, especially when using high-resolution mass spectrometry.

High-Resolution Mass Spectrometry and Calibration

High-resolution mass spectrometry measures the masses of ions with great precision, often detecting small deviations that are not predicted by using standard average atomic masses. Instrument calibration, intrinsic precision, and the ability to resolve specific isotopic peaks can all contribute to minor discrepancies between the theoretical molecular mass and the experimentally observed mass.

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