Mass Spectrometry and Atomic Identification (4E) - Chem & Physics

1. What is the key difference between the molecular ion peak and a fragment peak?

   $M^+$ is intact molecule; fragments are smaller ions from cleavage. Molecular ion corresponds to the whole molecule, while fragments result from bond breakage during ionization.

2. What does the y-axis of a typical mass spectrum display?

   Relative abundance (relative intensity) of ions. The y-axis indicates how abundant each ion is relative to the base peak.

3. Calculate $m/z$ for $[M+H]^+$ when $M = 180$ and $z = 1$.

   $181$. For $[M+H]^+$, $m/z = M + 1$ since one proton adds mass 1 and charge 1.

4. If an ion has $z = +1$, what is the relationship between its $m/z$ and its mass $m$?

   $m/z = m$ (numerically equal when $z = 1$). For singly charged ions, $m/z$ equals the ion's mass since charge is unity.

5. What does a peak at $M+2$ most strongly suggest when it is unusually prominent?

   A significant $+2$ isotope such as $^{37}\text{Cl}$ or $^{81}\text{Br}$. Unusually strong $M+2$ suggests elements with significant isotopes two units heavier, like Cl or Br.

6. Which isotope pattern indicates one bromine: $M:M+2 \approx 1:1$ or $3:1$?

   $M:M+2 \approx 1:1$ indicates one bromine. Bromine's $^{79}$Br:$^{81}$Br ratio of 1:1 yields $M:M+2 \approx 1:1$ for one Br atom.

7. Which isotope pattern indicates one chlorine: $M:M+2 \approx 3:1$ or $1:1$?

   $M:M+2 \approx 3:1$ indicates one chlorine. Chlorine's $^{35}$Cl:$^{37}$Cl ratio of 3:1 produces $M:M+2 \approx 3:1$ for one Cl atom.

8. Identify the element indicated by an $M$ and $M+2$ pair with intensities near $3:1$.

   Chlorine (from $^{35}\text{Cl}$ and $^{37}\text{Cl}$). Chlorine's isotopes $^{35}$Cl and $^{37}$Cl have 3:1 abundance, causing 3:1 $M:M+2$ intensity.

9. Identify the element indicated by an $M$ and $M+2$ pair with intensities near $1:1$.

   Bromine (from $^{79}\text{Br}$ and $^{81}\text{Br}$). Bromine's isotopes $^{79}$Br and $^{81}$Br have nearly equal natural abundances, leading to 1:1 intensity ratio.

10. In EI mass spectrometry, what is the typical charge $z$ of most detected ions?

    $z = +1$ for most ions. EI typically produces singly charged ions by ejecting one electron from the molecule.

11. What does $m/z$ represent on the x-axis of a typical mass spectrum?

    Mass-to-charge ratio of detected ions. In mass spectra, the x-axis displays $m/z$, which is the mass of an ion divided by its charge.

12. If a compound has two chlorines, what is the approximate $M:M+2:M+4$ pattern?

    $9:6:1$. For two Cl atoms, the isotope pattern follows binomial expansion of $(3+1)^2$, giving 9:6:1.

13. In ESI, what does a series of peaks at different $m/z$ values for one analyte usually indicate?

    Multiple charge states ($z > 1$) of the same molecule. ESI can protonate molecules multiple times, producing ions with varying $z$ and thus different $m/z$ for the same mass.

14. If a compound has two bromines, what is the approximate $M:M+2:M+4$ pattern?

    $1:2:1$. For two Br atoms, equal isotope abundances produce a 1:2:1 pattern via $(1+1)^2$ expansion.

15. Which peak is most useful for estimating a compound's molar mass in EI-MS?

    The molecular ion peak ($M^+$), if present. The $M^+$ peak represents the mass of the intact molecule, allowing direct estimation of molar mass.

16. Identify the number of chlorines if the isotope cluster shows $M:M+2:M+4 \approx 9:6:1$.

    Two chlorines. The 9:6:1 ratio matches the binomial distribution for two Cl atoms with 3:1 isotope ratio.

17. What is the molecular ion peak ($M^+$) in electron ionization mass spectrometry?

    Peak from the radical cation of the intact molecule. In EI-MS, the molecular ion forms when the molecule loses an electron, creating a radical cation with the full molecular mass.

18. What does a mass spectrometer primarily measure to help identify an unknown compound?

    Ion $m/z$ values and their relative abundances. Mass spectrometry identifies compounds by detecting ions' mass-to-charge ratios and their intensities, forming a unique fingerprint.

19. What does a peak at $M+1$ most commonly indicate for organic molecules?

    Presence of $^{13}\text{C}$ isotopes in the molecule. The $M+1$ peak mainly arises from the natural 1.1% abundance of $^{13}$C in carbon-containing molecules.

20. What is the formula relating observed $m/z$ to $M$ and $z$ for $[M+zH]^{z+}$ ions?

    $m/z = \frac{M + z}{z}$ (using $m_H \approx 1$). The formula accounts for the added mass of $z$ protons divided by the charge $z$, approximating proton mass as 1.

21. Identify $M$ if an ESI peak at $m/z = 500.5$ corresponds to $[M+2H]^{2+}$.

    $M = 999$. Rearranging $m/z = (M + z)/z$ gives $M = m/z \cdot z - z = 500.5 \cdot 2 - 2$.

22. Calculate $m/z$ for $[M+2H]^{2+}$ when $M = 1000$ and $z = 2$.

    $501$. For $[M+2H]^{2+}$, $m/z = (M + 2)/2$ due to two protons adding mass 2 and charge 2.

23. Identify the ionization method that is considered a 'soft' technique and often preserves $M^+$.

    Electrospray ionization (ESI). ESI is a soft ionization method that minimizes fragmentation, often yielding intact molecular ions.

24. What does the base peak in a mass spectrum represent?

    The most intense peak, set to $100\%$ relative intensity. The base peak is the strongest signal, normalized to 100% to scale other peaks' intensities for comparison.

25. In positive-mode ESI, what common ion form corresponds to adding a proton?

    $[M+H]^+$. Positive ESI commonly adds a proton to the molecule, forming a singly charged cation.

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