3.1.1.2 Mass spectrometer (TOF)

what is a mass spectrometer

. measure the mass of atoms and molecules and it is very accurate
. it can identify a particular element

what is the ar

average mass of 1 atom compared to 1/12 the mass of a carbon 12 atom

what is the mr average mass of 1 molecule to 1/12 mass of carbon 12 atom

average mass of 1 molecule to 1/12 the mass of carbon 12 atom

why do we use carbon 12 for comparison

it has a high isotopic purity meaning 99% of carbon is carbon 12

describe the process of the mass spectrometer

1.) ionisation; ions are made into positive ions
2.)acceleration; the ions gain kinetic energy they all gain the same K.E. meaning ions that are lighter accelerates faster, there is a negative plate that the positive electrons are attracted too
3.) iron drift; when the ions go through a small hole. As on the negative plate there is a small hole
4.)detection; the ions hit a detector the lightest ions arrive first and the heavier ions arrive last.

what is the function of the vacuum in the mass spectrometer

to suck out all the air so the ions dont collide with the air.

what is the function of the detector

the detector sends a reading to a pc which gives a display

Describe how the molecules are ionised using electrospray ionisation.

(Sample is) dissolved (in a volatile solvent) Allow named solvent (eg water/methanol)
(Injected through) needle/nozzle/capillary at high voltage/positively charged
Each molecule/particle gains a proton/H+

Give an equation, including state symbols, for the process that occurs during the ionisation using an electrospray ionisation

X + H+ ----> XH+(g)

Describe how the molecules are ionised uisng the electron gun

electrons get fired to a positively charged plate through the source of a hot filament wire.
then the gaseous sample is passed through a tube wjilst the electrons are fired so the electrons knock off an electron from the particles of the sample

Give an equation, including state symbols, for the process that occurs during the ionisation using an electron gun

1.) X----> X+ + e-
or
2.) X + e- ------>X+ + 2e-

what is inside the detector and what happens

electrons are in the detector.
so the positve ions pick up electrons from the detector; which causes a current flow
therefore if there is a larger current= there is a greater abundance in the sample of that ion

what does it mean if the peak is the furthest to the right

it mans that its the heaviest peak

what else does the mass spectra show

it tells us the mr of the mz ratio

will the abundance will always be a perecentage

no it can also be a ratio

what does mz mean

it means mass; charge ratio, however when a proton is gained (h+) or an electron is knocked off the charge of the ion is always 1. Therefore since its mass; 1 mz always means just mass

for the mass spectra for an electron spray what is the actual mr

it is the mr-1 because an proton is added

can two atoms of the same element have the same peak

no as there can be different masses

what equations are used for mass spectrum calculations

v=d/t
and KE= 1/2mv^2

how do you fine the mass of an ion

1.) divide it by 1000 to get into kg
2.) divide by avogadros constant 6.022 x 10^23
we divide by avogadros constant as that is how many atoms there is in one mole and we need the mass of the atom
we can use the mass of an atom to find the mass of an ion because even though an electron is lost it has a very small mass so the mass of an atom is the same as the mass of an ion

what is the reasonableness check

its to see if your answer seems right always remember that the time of flight is ---> 't heavier> t lighter'

what is the short cut calculation of mass spectrum calculations

Ar(1)/t(1)^2= Ar(2)/t(2)^2

In a TOF mass spectrometer, ions are accelerated to the same kinetic energy (KE).
The kinetic energy of an ion is given by the equation
Where: KE = 1/2mv^2
kinetic energy / J m = mass / kg v = speed / m s-1
In a TOF mass spectrometer, each 84Kr+ ion is accelerated to a kinetic energy of 4.83 × 10−16 J and the time of flight is 1.72 × 10−5 s
Calculate the length, in metres, of the TOF flight tube. The Avogadro constant, L = 6.022 × 1023 mol−1

1.44 (m)
if you dont understand watch a video

53Cr+ ion travels along a flight tube of length 1.25 m
The ion has a constant kinetic energy (KE) of 1.102 × 10-13 J m = mass of the ion / kg v = speed of ion / m s-1
Calculate the time, in s, for the 53Cr+ ion to travel down the flight tube to reach the detector.
The Avogadro constant, L = 6.022 × 1023 mol-1

8.8. x 10-26 kg
if you dont get it watch a video

how do you find the relative atomic mass

(abundance x ar) + (abundance x ar)/ total abundance
the total abundance isnt always 100 it can be the total of the ratio like 3:4 so the total is 7

how could you find the abundance if they didnt tell you the abundance method 1

draw a number line
and then find the percentage by counting the distance
look at notes if you dont understand
however this method can be used spefically for two isotpes

how could you find the abundance if they did tell you the abundance method 2

using algerbra solve for x
once youve found x you can find the other missing/ unknown abundance
this can be used for more than 2 isotopes