Natural oxygen in the atmosphere contains approximately 0.204 % 18O

CHEMISTRY · Single correct · AP · Chemistry · Atomic Structure and Properties · Atomic Structure and Properties

Problem

Natural oxygen in the atmosphere contains approximately 0.204 % 18O. A mass spectrum of oxygen obtained from melted Antarctic glacier ice shows the peak at m/z = 20 (18O+) to be only 0.10 % of total intensity. Which of the following explanations best accounts for the decreased abundance of 18O in the ice sample? (a) During formation of the ice, lighter

^{16}\text{O}

was preferentially incorporated into the liquid ocean water, leaving the vapor phase—and thus the snowfall—relatively enriched in

^{18}\text{O}

. (b) The mass spectrometer used could not reliably detect ions with

m/z = 20

, so the peak for

^{18}\text{O}^+

appears artificially small. (c) Fractionation during evaporation favored water molecules containing

^{16}\text{O}

; therefore the snow that formed the glacier was relatively depleted in

^{18}\text{O}

. (d) Over the long period since the ice formed, the

^{18}\text{O}

present in the water has slowly decayed to

^{16}\text{O}

, lowering its measured abundance.

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