The molar mass of a compound can be directly determined from its mass spectrum by identifying the peak with the highest mass-to-charge ratio (m/z) value, assuming the charge (z) is +1.
Here's a breakdown of the process:
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Understand the Mass Spectrum: A mass spectrum plots the relative abundance of ions detected against their mass-to-charge ratio (m/z). Each peak represents an ion with a specific mass.
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Identify the Molecular Ion Peak (M+): The molecular ion peak, often denoted as M+, represents the ion formed when the original molecule loses an electron. This peak corresponds to the molar mass of the original molecule. It's the peak farthest to the right on the spectrum, assuming the charge is +1.
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Look for the Highest m/z Value: Find the peak with the highest m/z value on the spectrum. This peak should correspond to the molecular ion (M+).
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Assume a Charge of +1: In most mass spectra, the ions are singly charged (z = +1). Therefore, the m/z value of the molecular ion peak directly corresponds to the molar mass of the compound. Molar Mass = m/z.
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Consider Isotope Peaks (M+1, M+2, etc.): Be aware of isotope peaks. For example, a molecule containing carbon-13 will appear one mass unit higher than the molecule containing only carbon-12. The molecular ion peak (M+) is typically from the most abundant isotope combination. The peak at the highest m/z may be a very minor isotope peak, and therefore not represent the molar mass of the main compound.
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If M+ is Not Visible: Sometimes the molecular ion peak is very small or absent. This happens when the molecular ion is unstable and fragments easily. In such cases, you need to analyze the fragmentation pattern to deduce the molar mass. This is a more complex process.
Example:
Suppose a mass spectrum shows a peak at m/z = 100, and it's the highest m/z value observed. Assuming the charge is +1, the molar mass of the compound is 100 g/mol.
In Summary:
To find the molar mass from a mass spectrum, identify the molecular ion peak (M+, if visible) with the highest m/z value. Assuming the ion has a charge of +1, this m/z value equals the molar mass of the compound. Be aware of isotope peaks and the possibility of a missing or very small molecular ion peak.
