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What limits the carbon-counting method at high mass?

  • A.Isotope peaks overlap and multiply
  • B.The molecular ion becomes too weak to measure
  • C.Carbon-13 abundance changes with molecular size
  • D.The instrument's resolution falls at high mass

Answer: A

With many carbons the chance of two heavy isotopes rises, so M plus two and beyond become significant and the simple proportion breaks down. Abundance is a constant of nature, and molecular ion strength and resolution are separate issues. An approximation valid when a probability is small must fail when it is not, and here the failure sets in around thirty or forty carbons.

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