What is the difference between the molecular ion peak and the base peak in a mass spectrum?

The molecular ion peak ($M^+$) represents the entire molecule and gives the relative molecular mass ($M_r$). The base peak is the tallest peak in the spectrum, representing the most abundant (usually most stable) ion fragment, and is assigned a relative abundance of 100%.

How can you distinguish between a compound containing one Chlorine atom and one containing one Bromine atom using mass spectrometry?

A compound with one Chlorine atom will show two molecular ion peaks ($M$ and $M+2$) in a $3:1$ height ratio. A compound with one Bromine atom will show two molecular ion peaks ($M$ and $M+2$) in a $1:1$ height ratio.

When calculating relative atomic mass ($A_r$), why is it incorrect to simply average the mass numbers of the isotopes?

A simple average assumes all isotopes exist in equal amounts. $A_r$ must be a weighted average because isotopes have different natural abundances; an isotope that makes up 90% of the element must influence the $A_r$ much more than one that makes up only 10%.

What is a common mistake when identifying the molecular ion peak in a spectrum containing Carbon?

Students often mistakenly identify the $M+1$ peak as the molecular ion peak. The $M+1$ peak is a tiny peak caused by the $^{13}C$ isotope and should be ignored when determining the $M_r$ of the main molecule.

What error occurs if you use the mass of the most abundant isotope as the relative atomic mass of an element?

This ignores the contribution of other isotopes, leading to an inaccurate $A_r$. While the $A_r$ is often close to the mass of the most abundant isotope, the small contributions from others are essential for precise chemical calculations and stoichiometry.

Why might a student calculate an $A_r$ greater than 100 if they are not careful with the formula?

This usually happens if the student forgets to divide the sum of (mass $\times$ abundance) by the total abundance (usually 100). The $A_r$ must always fall between the masses of the lightest and heaviest isotopes.

Define the 'Molecular Ion' ($M^+$).

The molecular ion is the species formed when a complete molecule loses one electron during ionization in a mass spectrometer. Its mass-to-charge ratio ($m/z$) is equal to the relative molecular mass ($M_r$) of the compound.

What does the $m/z$ ratio represent?

It represents the mass ($m$) of an ion divided by its charge ($z$). In most standard mass spectrometry, the charge is $+1$, so the $m/z$ value effectively tells us the relative mass of the ion.

What is the 'M+1 peak'?

The $M+1$ peak is a small peak occurring at one mass unit higher than the main molecular ion peak. It is caused by molecules that contain a single atom of the $^{13}C$ isotope instead of the more common $^{12}C$.

Why does the height of the M+1 peak vary between different organic molecules?

The height of the $M+1$ peak is proportional to the number of carbon atoms in the molecule. Since there is a fixed probability (approx. 1.1%) of any carbon atom being $^{13}C$, a molecule with more carbons is more likely to contain one $^{13}C$ atom, resulting in a taller $M+1$ peak.

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1. Physical Chemistry

Mass Spectrometry

Summary

Mass spectrometry is a powerful analytical technique used to determine the relative atomic mass of elements and the relative molecular mass of compounds. By ionizing samples and measuring the mass-to-charge ratio of the resulting ions, it provides a 'molecular fingerprint' that reveals isotopic composition and structural information.

1. Definition & Core Concepts

Mass Spectrometry: An analytical method that measures the mass-to-charge ratio ( $m/z$ ) of ions to identify the isotopic composition of elements and the molecular weight of compounds.
Mass-to-Charge Ratio ( $m/z$ ): The fundamental unit of measurement in a mass spectrum, where $m$ is the mass of the ion and $z$ is its charge; since most ions formed have a $+1$ charge, the $m/z$ value usually corresponds directly to the mass of the ion.
Relative Abundance: The percentage or proportion of a specific isotope or ion fragment relative to the total amount of ions detected, typically represented as the height of a peak on the spectrum.

A generic mass spectrum showing the molecular ion peak (M+), a small M+1 peak, and a fragment peak.

2. Underlying Principles: Atomic Mass Calculation

3. Molecular Analysis & The Molecular Ion Peak

4. Isotopic Patterns: The M+1 and Halogen Peaks

5. Key Distinctions: M vs M+1 vs M+2

6. Exam Strategy & Tips