Determination of Relative Atomic Masses of Isotopes by Mass Spectrometry

Overview

Mass Spectrometry Technique:
Mass spectrometry involves volatilizing a substance, ionizing it with high-energy electrons, and separating the resulting positive ions based on their mass-to-charge ratio (m/e). These ions are then recorded as peaks in a mass spectrum, where (m/e) is plotted on the x-axis and the relative number of ions on the y-axis.

Isotopic Abundance Calculation:
The relative atomic masses of isotopes can be determined by analyzing the relative abundance of isotopes in a sample. By calculating the average atomic mass using the percentages of each isotope present, the fractional atomic mass of the element can be obtained. This process allows for the precise measurement of the exact masses of different isotopes of an element.

Questions and Answers

1. How are isotopes separated and identified in mass spectrometry?

   In mass spectrometry, isotopes are separated and identified based on their mass to charge ratio (m/e).

2. Why do many elements have fractional atomic masses?

   Many elements have fractional atomic masses because atomic masses are the average masses of isotopes proportional to their relative abundance.

3. What is the purpose of a mass spectrometer in determining relative atomic masses of isotopes?

   A mass spectrometer is used to measure the exact masses of different isotopes of an element.

4. How is the mass spectrum plotted in a mass spectrometer?

   In a mass spectrometer, the mass spectrum is plotted with (m/e) on the x-axis and the relative number of ions on the y-axis.

5. How are positive ions of isotopes accelerated in a mass spectrometer?

   Positive ions of isotopes are accelerated in a mass spectrometer by applying a potential difference (E) of 500-2000 volts between perforated accelerating plates.

Multiple-Choice Questions

1. What is the instrument used to measure the exact masses of different isotopes of an element?

   a) Mass Spectrometer
   b) Spectrograph
   c) Electrometer
   d) Ionizer
   Answer: a) Mass Spectrometer

2. What does a mass spectrum plot in a mass spectrometer show?

   a) Relative abundance of isotopes vs. temperature
   b) Relative abundance of isotopes vs. pressure
   c) Relative abundance of isotopes vs. mass number
   d) Relative abundance of isotopes vs. color
   Answer:c) Relative abundance of isotopes vs. mass number

3. What is plotted on the abscissa (x-axis) in a mass spectrum plot?

   a) Relative number of ions
   b) Mass to charge ratio (m/e)
   c) Temperature
   d) Pressure
   Answer: b) Mass to charge ratio (m/e)

4. What is the mathematical relationship for (m/e) in a mass spectrometer?

   a) m/e = Hr/2E
   b) m/e = E/Hr
   c) m/e = r/2EH
   d) m/e = 2Hr/E
   Answer:a) m/e = Hr/2E

5. How are ions accelerated in a mass spectrometer?

   a) By Heat
   b) By Light
   c) By Magnetic Field
   d) By Electrical Field
   Answer:d) By Electrical Field