Chlorine Isotope Abundance in Mass Spectrometry

Steps:

• Recognize CH3Cl as the compound, with molecular ion CH3^{35}Cl^+ at m/z = 50 (12 + 3 + 35).
• Identify m/z = 52 as CH3^{37}Cl^+ due to the ^{37}Cl isotope.
• Use chlorine isotopic ratio: ^{35}Cl (75%) and ^{37}Cl (25%), so m/z = 52 intensity is (25/75) = 1/3 ≈ 33% of m/z = 50 intensity.
• Calculate: 0.33 × 18% ≈ 6% relative abundance for m/z = 52.

Why B is correct:

• Chlorine isotopic abundance ratio (^{37}Cl/^{35}Cl ≈ 1:3) means the m/z = 52 peak is ~33% of the m/z = 50 peak, so 0.33 × 18% = 6%.

Why the others are wrong:

• A: 5% underestimates the ratio (actual ~33%, not ~28%).
• C: 18% ignores isotope effect; it would imply equal abundance, which is false.
• D: 54% overestimates; it would require ^{37}Cl dominance, contradicting natural ratios.

Final answer: B