Mass spectrometer Flashcards
What is the first stage of mass spectrometer ?
ionisation
Why does a sample need to be ionised before entering a mass spectrometer ?
mass spectrometry measures the mass of charged particles, only ions will be detected,
What are the two methods of ionisation in mass spectrometer?
Electron impact and electrospray ionisation
how does electrospray ionisation work ?
— in this method the sample is dissolved in a solvent and pushed through a small nozzle at high pressure. A high voltage is applied to it, causing each particle to gain an H+ ion. The solvent is then removed, leaving a gas made up of positive ions.
How does electron impact ionisation work ?
— in this method, the sample is vaporised andan ‘electron gun’ is used to fire high energy electrons at it. This knocks
one electron off one each particle, so they become +1 ions.
What is the 2nd stage of mass spectrometery ?
Accelaration
Explain acceleration in mass spectrometer ?
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 as much energy as the heavier ions,
but they’re lighter, so they accelerate more.
what is the eqaution for electron impact ionisation ?
X(g) + e– -> X(g) + 2e–
when is electron impact ionisation used ?
low formula mass
what is the equation for electrospray impact?
X(g) + H+ -> XH+(g)
when is electropray ionisation used ?
for high formal mass
What is the formula for kinetic energy ?
KE =𝟏/𝟐𝒎𝒗^𝟐
KE = kinetic energy of particle (J)
m = mass of the particle (kg)
𝑣 = velocity of the particle (m s–1
)
how would you rearrange the kinetic energy formula to obtain velocity ?
𝒗 = √𝟐𝑲𝑬/m
what is the third stage of mass spectrometer ?
Ion drift /flight tube
what happens in ion drift?
Next, the ions enter a region with no electric field. They drift through itat the same speed as they left the electric field. So the lighter ions will be drifting at higher speeds.
give the two equations and the units for time ?
𝒕 =𝒅/𝒗
𝒕 = 𝒅√𝒎/𝟐𝑲E
t = time of flight (s)
d = length of flight tube (m)
𝑣 = velocity of the particle (m s–1
)
m = mass of the particle (kg)
KE = kinetic energy of particle (J)
What is the fourth stage of mass spectrometry ?
detection
how does detection work ?
Because lighter ions travel through the drift region at higher speeds, they
reach the detector in less time than heavier ions. 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 is a detector ?
a negatively charged electric plate
why may their be small peaks in a mass spectra ?
electron impact ionisation was used and fragments have fragmented
what does the hight of each peak in a mass spectrometer mean ?
the relative isotopic abundance
what is the y axis in a mass spectrum ?
% abundance
what is a x axis in a mass spectrum?
mass/charge ratio
how do you identify a element from a mass spectrum ?
Step 1: For each peak, read the % relative isotopic abundance from they-axis and the relative isotopic mass from the x-axis. Multiply them together to get the total relative mass for each isotope.
Step 2: Add up these totals.
Step 3: Divide by 100 (since percentages were used).
this will give you an Ar which you can fond the mass from
how do you identify the element from the mass spec if the relative abundance is not in percent ?
don’t divide by 100
how do u find the Mr of a molecule on a mass spectra?
the mass/charge ratio of its peak
How do u calculate distance
Velocity xtime
