mass spectrometer Flashcards
What is mass spectrometer used for
-Identify an unknown compound
-Find the relative abundance of each isotope of an element
-Determine structural information about molecules
what is mass spectrum
-The results from a mass spectrometer
-A mass spectrum is a complex bar graph and gives information about the abundance of ions present in the sample and their mass to charge ratio
What are the 4 key stages of mass spectrometry
-Ionisation
-Acceleration
- Ion drift
- Detection
What are the conditions for mass spectrometry
Vacuum → so ions do not collide with air molecules (might stop them reaching the detector)
Gaseous State → solids are vaporised before being injected
Details of ionisation
-Electron Impact:low Mr compounds:
Beam of electrons knocks electrons from atoms or molecules in the sample.Nearly all lose just one electron (~5% will lose two)
-Electrospray ionisation: Used for high Mr compounds:
Sample dissolved in volatile solvent forced through a fine hollow needle connected to +ve terminal of high voltage.This produces tiny positively charged droplets – gained a proton solvent evaporates leaving positive ions
Details of Acceleration
-The ions are accelerated using an electric field so that they all have the same kinetic energy (attracted
towards the negative plate)
Details of Ion drift
The ions pass through a hole in the negatively charged plate, forming a beam and travel along a tube.
The ions are then separated due to their ratio of mass to charge
(m/z), where z is the charge (usually +1)
-Heavier ions travel slower than lighter ones
Details of Detection
-Ions pick up electron from the detector which causes the current to flow, this current is proportional to the abundance of each ion
-The ions are then distinguished by their different flight times.
-lighter ions are detected first as they have higher velocities.
what do the axes of mass spectrum show
x- relative isotopic mass
y- abundance
What is a molecular ion,M+
-The positive ion formed in mass spectrometry when a molecule loses and electron.
Explain how ions are detected and relative abundance is measured in a TOF mass spectrometer.
-Ion hits the detector / negative plate and gains an electron
-Abundance is proportional to the current
Describe how the molecules are ionised using electrospray ionisation.
Sample is dissolved in a volatile solvent (eg water/methanol)
-Injected through needle at high voltage
-Each molecule gains a proton
Explain why it is necessary to ionise molecules when measuring their mass
in a TOF mass spectrometer.
-Ions, not molecules, will interact with and be accelerated by an
electric field
-Only ions will create a current when hitting the detector
