Nick Lockyer Flashcards
Name the three components that are essential to a mass spectrometer.
An ion source, a mass analyser(s) and an ion detector
What regions of the mass spectrometer must be under high vacuum and why?
The mass analyser and ion detector. Decreasing the pressure increases the mean free path of the ions and thus allows them to reach the detector
Give the equation for the mean free path.
L = kT/ (2)^-1/2 * p * σ
where σ = π*d^2
Define the mass resolution.
The ability to separate two ion signals from each other in a mass spectrum.
Using two peaks: R=M1/(M1-M2)
Using one peak: R=M1/ width at FWHM
What is the mass accuracy?
The difference between the theoretical mass and the measured mass.
Briefly describe how a magnetic sector mass analyser works.
Under an applied magnetic field, B, ions of the same charge adopt different, curved trajectories depending on their momentum. If the B field is scanned, ions with various m/q values will be detected sequentially.
What is a double-focusing instrument?
A mass spectrometer containing a magnetic and electrostatic sector. Ions with the same m/q ratio but different kinetic energy will follow different trajectories. Thus the two sectors are positioned so that the dispersion energy of one sector is corrected by the convergence of another.
Briefly explain how a quadrupole works.
A quadrupole consists of a parallel arrangement of four electrodes with opposite electrodes electrically connected. Potentials of opposite polarity are applied to adjaect rods. The potentials consist of a RF component superimposed on a direct component. Ions entering the quadrupole experience oscillations in the x and y directions due to forces induced by the applied potentials. A positive ion will be accelerated towards a negative pole. However, if the polarities are switched before the ion collides with the electrode, it will be drawn towards another electrode. Ions can be made to follow an oscillatory path through the quadrupole in this manner.
What is V and what is U?
V = Radiofrequency component U = Direct component
How are stable ion trajectories determined in quadrupole mass analysers?
Solving the Mathieu equations allows diagrams of stability to be produced, which show triangular regions that indicate values of U and V for which ions of different masses follow stable trajectories to the detector.
What is a ToF analyser and how does it work?
A Time-of-Flight analyser accelerates a defined ‘packet’ of ions towards a field-free region where they are allowed to traverse a distance d to the detector. Their flight time is then measured. Smaller ions travel faster than larger ions, and vice versa.
What does ICR stand for? How does this type of mass analyser work?
Ion cyclotron resonance. A high intensity magnetic field, B, is used to trap low velocity ions in a stable, low radius circular trajectory of frequency v(c) (=zB/m).
By measuring v(c) we can determine the m/z ratio.
