TOF Mass Spectometry Flashcards
What information can a mass spectrometer provide?
- Relative molecular mass of a molecule if a compound is analysed
- Can be used to identify elements/compounds
- Relative atomic mass of an element
- Relative abundance of its isotopes
What are the 5 stages in the process of TOF mass spectrometry?
- Ionisation
- Acceleration
- Ion drift
- Ion detection
- Data analysis
What happens during ionization?
Electrospray ionization- sample is dissolved in a solvent and pushed through a small nozzle at high pressure. A high voltage is applied to the sample, causing each particle to gain a H+ ion. The solvent is then removed, leaving a gas made up of positive ions.
Electron impact ionization- sample is vaporized and an ‘electron gun’ is used to fire high energy electrons at it. This knocks one electron off of each particle, so they become +1 ions.
What happens during acceleration?
The positive ions are accelerated by an electric field. The electric field gives the same kinetic energy to all the ions. The lighter ions experience a greater acceleration- they’re given the same energy as heavier ions, but they’re lighter, so they accelerate more.
What happens during ion drift?
The ions enter a region with no electric field. They drift through it at the same speed as they left the electric field. So the lighter ions will be drifting at higher speeds.
What happens during ion detection?
Because the lighter ions travel faster than the heavier ions through the drift region, they reach the detector faster. The detector detects the current created when the ions hit it and records how long they took to pass through the spectrometer. This data is then used to calculate the mass/charge values needed to produce a mass spectrum.
What does each line represent on a mass spectrum is the sample is an element? What does the height of each line mean?
Each line will represent a different isotope of the element. The height of each line gives the relative isotopic abundance.
On a mass spectrum of an element, what are the values on the x axis and y axis?
The x axis represents the mass/charge ratio (m/z). Since the charge of the ions is mostly +1, it represents the relative isotopic mass.
The y axis represents the relative abundance of the isotopes, usually as a percentage.
Why are the ions produced in a mass spectrometer always positive, even for substances like chlorine, which usually form negative ions?
The mass spectrometer produces ions by removing electrons, so the ions will always be positive.
How can mass spectrometry be used to identify elements?
Elements with different isotopes produce more than one line in a mass spectrum because the isotopes have different masses. This produces characteristic patterns which can be used as ‘fingerprints’ to identify certain elements.
What causes a molecular ion (M+) peak in a mass spectrum?
The molecular ion/M+ is formed when one electron is removed from a molecule in a molecular sample. This gives a peak in the mass spectrum with a m/z value equal to the Mr of the molecule.
