MCQ Detailed View

Mass spectrometry (MS) is a powerful analytical technique that works on the principle of measuring the mass-to-charge ratio (m/z or e/m ratio) of ions. While many students assume it directly measures “mass,” the fundamental property obtained in mass spectrometry is actually the e/m ratio, which later allows the calculation of the molecular mass.

In a mass spectrometer, the sample is first ionized, converting molecules or atoms into charged species. These ions are then accelerated in an electric field and passed through a magnetic or electric analyzer. Because charged particles are deflected according to their charge and mass, the instrument essentially records their mass-to-charge ratio. This ratio is what generates the peaks on the mass spectrum.

For example, if a particle has a charge of +1 (one proton equivalent), the m/z value corresponds directly to its mass. However, if the ion has a charge of +2 or more, the observed m/z will be lower than its actual molecular mass, emphasizing the importance of charge in mass spectrometry.

Thus, the primary measurement in mass spectrometry is not the mass alone, but the e/m ratio. The actual molecular mass is then derived from these values, depending on the charge state of the ion.

Applications of e/m ratio measurements in mass spectrometry include:

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  Identifying unknown compounds by analyzing their ion fragments.

  
  


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  Determining molecular weights of organic and inorganic compounds.

  
  


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  Studying isotopic abundances, since isotopes have different e/m ratios.

  
  


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  Structural elucidation of complex biomolecules (proteins, nucleic acids).

  
  


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  Forensic and environmental analysis, where very small amounts of compounds can be detected.

  
  

In short, while mass spectrometry is commonly described as a tool for measuring “masses,” it is technically accurate to say that it measures the e/m ratio of ions. From this fundamental property, chemists calculate molecular weights and study chemical structures.

Therefore, the correct answer is Option 2: e/m ratio.