## 17.3 Determination of Molecular Mass by Molecular Ion Peak This section explains how to determine the molecular mass of a compound using its mass spectrum, focusing on the identification and interpretation of key peaks. ### Molecular Ion Peak ($M^+$) The **molecular ion peak** (symbol: **$M^+$ peak**) is the peak in a mass spectrum that represents the molecular ion. This ion is formed when the original molecule loses one electron. * The molecular ion peak is the peak with the largest *mass-to-charge ratio* (m/z), excluding any minor peaks caused by heavier isotopes. * The m/z value of the molecular ion peak is numerically equal to the molecular mass of the compound, as most ions formed in mass spectrometry carry a +1 charge. $$ \text{m/z of } M^+ \text{ peak} = \text{Molecular Mass} $$ ### Base Peak The **base peak** is the most intense peak in the mass spectrum, corresponding to the most abundant (most stable) ion fragment produced during the analysis. * It is assigned a relative abundance of 100%. * All other peaks in the spectrum are scaled relative to the base peak's intensity. * *Note: The base peak is not always the same as the molecular ion peak.* ### Isotope Peaks The presence of natural isotopes in elements gives rise to small peaks at m/z values higher than the molecular ion peak. * **M+1 Peak:** A smaller peak located one m/z unit higher than the molecular ion peak ($M^+$). For organic compounds, this peak is primarily due to the presence of the carbon-13 isotope ($^{13}C$), which has a natural abundance of about 1.1%. * **M+2 Peak:** An even smaller peak located two m/z units higher than the $M^+$ peak. This can be due to the presence of two $^{13}C$ atoms or other isotopes like $^{18}O$, $^{34}S$, or halogens like $^{37}Cl$ and $^{81}Br$. ## Worked Examples ### Example 1: Mass Spectrum of Hexane ($C_6H_{14}$) For hexane, the molecular ion peak ($M^+$) appears at an **m/z of 86**. This value corresponds directly to the molecular mass of hexane. * **Calculation:** $(6 \times 12.01) + (14 \times 1.01) \approx 86 \, \text{g mol}^{-1}$ ### Example 2: Mass Spectrum of Ethene ($C_2H_4$) Let's analyze the provided mass spectrum for the ethene molecule. 1. **Molecular Ion Peak ($M^+$):** The molecular ion peak appears at **m/z = 28**. This is equivalent to the molecular mass of ethene ($2 \times 12 + 4 \times 1 = 28 \, \text{g mol}^{-1}$). 2. **M+1 Peak:** The peak at **m/z = 29** is the $M+1$ peak. It arises from ethene molecules containing one $^{13}C$ atom and one $^{12}C$ atom. 3. **M+2 Peak:** The very small peak at **m/z = 30** is the $M+2$ peak, resulting from ethene molecules containing two $^{13}C$ atoms. This peak is much smaller than the $M$ and $M+1$ peaks due to the low probability of having two $^{13}C$ atoms in one molecule. 4. **Other Peaks (Fragment Ions):** The other peaks at lower m/z values (e.g., 27, 26, 25) are formed by fragment ions created when the ethene molecule breaks apart during ionization. ## Possible Questions and Answers **Q:** What is the significance of the molecular ion peak ($M^+$) in a mass spectrum? **A:** Its m/z value is used to determine the molecular mass of the compound. **Q:** Why is the base peak assigned a relative abundance of 100%? **A:** The base peak corresponds to the most abundant ion fragment. All other peak intensities are measured relative to this most intense peak for standardization. **Q:** An organic compound shows a molecular ion peak at m/z = 78 and an M+1 peak. What is the likely cause of the M+1 peak? **A:** The M+1 peak is most likely caused by the presence of a molecule containing a $^{13}C$ isotope instead of a $^{12}C$ isotope. ## Summary * **Molecular Mass Determination:** The molecular mass of a compound is found from the m/z value of its **molecular ion peak ($M^+$)** in a mass spectrum. * **Molecular Ion Peak ($M^+$):** The peak with the highest m/z ratio, representing the intact molecule with one electron removed. * **Base Peak:** The tallest peak in the spectrum, representing the most stable or abundant fragment ion (relative intensity = 100%). * **Isotope Peaks ($M+1, M+2$):** Small peaks appearing after the $M^+$ peak due to the presence of heavier isotopes (e.g., $^{13}C$). Their relative sizes can help identify the elements present. * **Application:** Mass spectrometry is a powerful analytical technique used in chemistry, forensics, and biology to identify unknown compounds and determine their molecular weight and structure. See also: ## References Derived from FBISE textbook