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

What this deck covers

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.

How to use these flashcards

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 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 2: 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 3: 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 4: 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 5: 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 6: 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 7: 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 8: 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.

Flashcard 9: 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 10: 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 11: 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 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: 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 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: 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 16: 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 17: 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 18: 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 19: 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 20: 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 21: 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 22: 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 23: 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 24: 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 25: 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.