Mass spectrum identifies elements by isotopic peak positions and intensities

Steps:

• Examine peak positions: major peaks at m/e 35 and 37, with smaller peaks possibly from fragments or impurities.
• Note relative intensities: peak at 35 is ~100 (dominant), at 37 is ~50 (about half), indicating ~3:1 ratio.
• Recall chlorine isotopes: Cl-35 (75.8% abundance) and Cl-37 (24.2%), matching the 3:1 pattern.
• Rule out others: no heavy peaks for tungsten or single dominant peak for arsenic.

Why B is correct:

• Chlorine exhibits two primary isotopes at m/z 35 and 37 with a natural abundance ratio of approximately 3:1, directly matching the spectrum's peak heights.

Why the others are wrong:

• A. Arsenic: primarily As-75 isotope, showing a single dominant peak near m/e 75, not dual peaks at 35/37.
• C. Tungsten: isotopes cluster around m/e 180–186, far heavier than observed peaks.

Final answer: B