FTMS PKU

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

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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

Treatment

Diet low in phenylalanine

Many nutrition labels include phenylalanine content

Avoid high protein foods (meat, fish, poultry, eggs, cheese, milk, dried beans, peas)

Testing

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
Phenylalanine

Mass Spectrometry

Ion Source

Compound vaporized into gas phase
Energy added to ionize the compound
Excess energy may also fragment compound

Mass Analyzer

Fragments sorted based on mass/charge ratios (m/z)
Ions detected
Ion cyclotron resonance (ICR) in an ion trap is a type of mass analyzer

Fourier Transform-Ion Cyclotron Resonance (FT-ICR) Mass Spectrometry

Ions are trapped in an ion trap
Magnetic and electric fields used to make ions cycle
Ions excited by energy
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.
source: http://www.chem.ucsb.edu/~devries/groupsite/labicr.htm

Fragmentation of Phenylalanine

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

Mass Spectrometry Results for L-Phenylalanine

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Mass Spectrum of Phenylalanine with characteristic fragments highlighted in red and blue.
source: http://webbook.nist.gov/cgi/cbook.cgi?ID=C63912&Units=SI&Mask=1E9F#Mass-Spe

Advantages

Extremely High Resolution (>500,000)
Very Good Accuracy (<1 ppm)

Disadvantages

Expensive
Requires Superconducting Magnet

Experimental Study
bhjk

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).

References

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

http://www.files.chem.vt.edu/chem-ed/ms/ftms.html

http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/MassSpec/masspec1.htm

http://www.medhelp.org/lib/pku.htm