MCAT Chemical and Physical Foundations of Biological Systems Flashcards: 4e Mass Spectrometry Atomic Identification

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This deck focuses on 4e Mass Spectrometry Atomic Identification, giving you a quick way to review the definitions, rules, and examples that matter most for MCAT Chemical and Physical Foundations of Biological Systems.

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Work through these flashcards in short sessions. Try to answer each prompt before flipping the card, then revisit any cards you miss until the explanation feels automatic.

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

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

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

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

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

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

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

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

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

Answer: 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.

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

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

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

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

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

Answer: 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.

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

Answer: 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.

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

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

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

Answer: 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.

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

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

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

Answer: 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.

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

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

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

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

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

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

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

Answer: 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.

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

Answer: 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.

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

Answer: 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.

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

Answer: $m/z = \frac{M + z}{z}$ (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.

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

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

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

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

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

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

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

Answer: 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.

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

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