Atomic structure - Mass spectrometry Flashcards
What is mass spectrometry?
A chemical technique used to identify the amount and type of chemical in a given sample
What are the four stages of a time-of-flight spectrometer?
Ionization, Acceleration, Drifting, Detection
What are the two types of ionization?
Electron impact
Electrospray ionization
Describe the three stages of electron impact
1.Sample is vaporized
2.High energy electrons are fired from an “electron gun” and hit the sample
3. Electrons are knocked off, producing a 1+ charge
What is a disadvantage of electron impact?
Can cause molecule fragmentation in smaller molecules.
What is ionizing a sample?
Given a positive charge
Describe the four stages of electrospray ionization
- Sample is injected into machine and ionized by the electrospray.
2.Sample is dissolved in volatile solvent and injected through a hypodermic needle to give a fine mist.
3.High voltage is applied to the sample, causing each particle to gain a 1+ charge.
4.This produces a sample of gas made up of positive ions.
What are the ions accelerated by?
By an electric field
What does acceleration rely on?
The particles being charged
(this is why they are made positive)
What attracts the ions?
A negatively charged plate on the front to attract them, and a positively charged back to repel the ions further down the M.S
What happens after acceleration in regards to KE?
Ions all have the same kinetic energy
What does velocity of an ion depend on?
Its mass and kinetic energy
So, lightest particles will move faster than heavier particles.
What is drifting?
Ions are not deflected by an electric field so they pass through the tube.
Lighter ions drift faster.
Which ions are detected first and why?
Lighter ions because they have a shorter drift time
How are ions separated?
Based on mass - controls the velocity
How is a mass spectrum generated?
The detector uses an electric field to detect the charged ions and forms a graph (M.S)
How is a current formed after the ions hit the detector?
Ions gain an electron to become neutral.(from the negatively charged plate)
This creates a current which is detected.
What does a large current mean?
Lots of ions hit the plate at a specific time.
What is the size of the current proportional to?
The number of ions.
Which peak shows the highest mass of an atom?
The molecular ion peak.
What is electron impact usually used for?
Molecules with low molecular mass.
What is the electron gun?
A hot wire filament which emits electrons as current runs through it.
What is the equation for electron impact?
X (g) → X+ (g) + e-
