Mass Spectrometry Flashcards
What is mass spec used to do?
Measures molecular mass and abundance.
What are the 4 stages?
Ionisation, acceleration, ion drift and detection.
What happens in electron impact?
-Electrons are fired at the molecules at a high energy.
-Molecules knock of the electrons so it is now positively charged.
What molecules enter electron impact?
Smaller molecules which can easily become a gas.
What state must an element be in for electron impact to happen?
Gas.
What is the equation for electron impact?
M(g)——> M+(g) + e-
What molecules enter electrospray?
Larger molecules which are harder to turn into a gas.
What happens in electrospray?
-Sample is dissolved in a volatile solvent.
-Injected with a needle which has a high voltage and a positive charge.
-Sample then gains H+ ion and becomes positively charged.
What is the equation for electrospray?
M(g) + H+ ——> MH+
What happens in acceleration?
-Positive ions are accelerated by a negative electric field and set to a constant kinetic energy.
Why is an ion positively charged in ionisation for acceleration?
To be accelerated by a NEGITIVE electric field (oppositely charged so attracted)
What happens in ion drift?
-All ions now have the same kinetic energy.
-Heavier ions will move slower.
-Ions are separated by different flight times.
What happens in detection?
–Ions hit the negative detector which is full of negative electrons.
-As they are oppositely charged the ions attract the electrons.
-The ions then gain electron and become a molecule.
-This creates a current by the electrons moving across.
What is the current in detection proportional to?
The abundance.
What is usually the mass; charge ratio in mass spec?
+1
What happens to the m/z when electrospray is done?
It will usually be 1 less as the m/z recorded has previously gained a H+ ion.
What is the equation for kenetic energy?
KE= 0.5 x mass x (velocity)2
What is the equation for the mass of 1 atom?
Mass x mass number x avroagos constant (6.022 x 10^23)
What is the equation for velocity?
distance/time.
How do you work out two unknowns in a mass spec calulation?
-Write down all values from the question.
-Combine KE equation, mass of an atom equation and velocity equation.
-Cancel out all common values.
-Put in values given
-Solve the answer.
