Fourier Transform Ion Cyclotron Resonance (FT-ICR) Mass Spectrometry & Phenylketonuria (PKU) Screening

Download this Lesson

Phenylketonuria (PKU)
  • Genetic disorder characterized by the inability to metabolize phenylalanine, an essential amino acid
  • Recessively inherited
  • Caused by partial or complete deficiency of phenylalanine hydroxylase
  • Enzyme that metabolizes phenylalanine into tyrosine
  • Accumulation of phenylalanine leads to neurological damage
  • Mental retardation, seizures

  • Diet low in phenylalanine
    • Many nutrition labels include phenylalanine content
  • Avoid high protein foods (meat, fish, poultry, eggs, cheese, milk, dried beans, peas)

  • Every US state requires blood screening for all newborns
  • Fourier transform mass spectrometry often used
  • A high phenylalanine:tyrosine ratio (above 2:1) results in a positive screening

Molecular Structure of

Mass Spectrometry

Ion Source
Mass Analyzer

Fourier Transform-Ion Cyclotron Resonance (FT-ICR)  Mass Spectrometry
  1. Ions are trapped in an ion trap
  2. Magnetic and electric fields used to make ions cycle
  3. Ions excited by energy
  4. Causes ions to move in packets
  • Ions perpendicular to a magnetic field move in a circular path with a radius & frequency:

r = mv/eB                f = eB/pm

where m is mass (kg), v is velocity (m/s), B is the strength of the electric field (T), e is elementary charge (C) and p is charge of a proton (C).
  • When ions pass by receiver plate, causes electrical signal called an image current
  • Signal is free induction decay
  • Consists of many superimposed sine waves
  • Fourier Transform converts time signal to frequency domain, where mass can be calculated.
Diagram of Typical FT-ICR Mass Spectrometer.  Ions enter ion trap in
which they cycle at characteristic radius and frequency.  As ions pass
receiver plate, and electrical signal is induced and converted to a
time-domain signal. 

Fragmentation of Phenylalanine

Typical Fragments of Phenylalanine as a result of energy from the ion source.

Mass Spectrometry Results for L-Phenylalanine


Mass Spectrum of Phenylalanine with characteristic fragments highlighted in red and blue. 


Experimental Study

GC FT-MS chromatographs of a PKU newborn before treatment (above) with a phenylalanine: tyrosine ratio
of 2.30 and after three months of treatment (below) with a ratio of 1.02 (Mesaros et. al. 2010). 


C. Mesaros, A. Iordache, O. Cozar, C. Cosma, M. Culea.  "Diagnosis by Mass Spectrometry." Romaninan Journal of Biophysics (2010).  20: 71-82.