Medical mass spectrometry is a diagnostic technology that identifies molecules by measuring their mass-to-charge ratio. In medicine, it helps detect drugs, hormones, metabolites, proteins, toxins, and disease markers in blood, urine, or tissue samples. It matters because many diseases change the chemical pattern inside the body before obvious symptoms appear.
By measuring molecules with high sensitivity, clinicians can make faster and more precise decisions.
Key Facts
- Mass spectrometry measures mass-to-charge ratio, written as m/z.
- A sample is ionized so neutral molecules become charged particles that can be moved by electric or magnetic fields.
- For a singly charged ion, m/z is approximately equal to molecular mass in daltons.
- Different ions separate because lighter ions or more highly charged ions respond differently to fields than heavier or less charged ions.
- A mass spectrum is a graph of signal intensity versus m/z, and peaks act like a molecular fingerprint.
- In time-of-flight mass spectrometry, ions with the same charge and kinetic energy follow v = sqrt(2qV/m), so lighter ions reach the detector sooner.
Vocabulary
- Ionization
- Ionization is the process of adding or removing electrons or protons so a molecule becomes electrically charged.
- Mass-to-charge ratio
- Mass-to-charge ratio, or m/z, is the ion mass divided by its charge number and is the main quantity measured in mass spectrometry.
- Mass analyzer
- A mass analyzer is the part of a mass spectrometer that separates ions according to their m/z values.
- Detector
- A detector is the component that records arriving ions and converts their impacts into an electrical signal.
- Mass spectrum
- A mass spectrum is a plot showing the abundance of detected ions at different m/z values.
Common Mistakes to Avoid
- Treating mass spectrometry as measuring mass directly is wrong because the instrument measures m/z, not mass alone. Charge state must be known or inferred to calculate molecular mass.
- Forgetting that samples must be ionized is wrong because neutral molecules do not respond strongly enough to the analyzer fields. Ionization is what lets the instrument guide and separate molecules.
- Assuming the tallest peak is always the most medically important molecule is wrong because peak height depends on abundance, ionization efficiency, and instrument settings. A smaller peak may be the key diagnostic marker.
- Ignoring calibration is wrong because small errors in m/z can lead to incorrect molecule identification. Clinical instruments use known standards to keep measurements accurate.
Practice Questions
- 1 A singly charged ion has a mass of 180 daltons. What is its m/z value?
- 2 An ion has a mass of 600 daltons and a charge of 2+. What m/z value will the mass spectrometer report?
- 3 Two molecules have the same mass, but one forms a 1+ ion and the other forms a 2+ ion. Explain how their positions on a mass spectrum would differ and why this matters for identification.