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How to Get Fractional Abundance From a Mass Spectrum

A mass spectrometer is a type of chemical instrument that can measure the mass of a single atom. Introduction of the unknown compound into a beam of electrons causes the loss of an electron. The instrument detects the resulting ion by using the interaction of the ion with a magnetic field. The magnetic field changes the path of the ion based on the mass/charge ratio. The greater the mass/charge ratio the less the ion path is changed. Because atoms exist in a number of isotopes, the mass spectrum yields a peak for each isotope. The magnitude of the peak is correlated to the abundance of the isotope.

Things You'll Need

Mass spectrum
Ruler
Periodic table
Calculator

Instructions

- 1
  Examine the mass spectrum of an element, which will have peaks corresponding to the various isotopes of the element. Each peak represents one of the naturally occurring isotopes and the height of the peak corresponds to the abundance of that particular isotope.
- 2
  Measure the height of each peak on the spectrum. Identify the position of each peak on the spectrum in your record of peak heights. The position on the x-axis of the plot corresponds to the atomic mass number of the peaks.
- 3
  Sum the total magnitude of the isotope peaks on the spectrum to determine the number of atoms of the element in which you are interested. Calculate the abundance of each isotope by dividing the magnitude of the peak for a particular isotope by the sum of all atoms of that element counted. For example, assume you are analyzing copper. There are two isotopes of copper at atomic weight units of 62.93 and 64.93. The counts for the Cu(63) isotope are 250 and the counts for the Cu(65) isotope are 111. Therefore the fractional abundance of Cu(63) is 250 / (250 + 111) = 0.693 and the fractional abundance of the Cu(65) isotope is 111 / (250 + 111) = 0.307.
- 4
  Check your answer by calculating the average atomic weight of Cu using the equation average atomic weight of Cu = abundance of 63 x atomic weight of Cu(63) + abundance of 65 x atomic weight of Cu(65) = 0.693 x 62.93 + 0.307 x 65.97 = 43.61 + 20.25 = 63.86. The literature value of the atomic weight of Cu is 63.546, which is less than 0.5 percent error.

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