relative mass and the mass spectrometer

Question 1

• define the relative atomic mass

Answer 1

the relative atomic mass is the average mass of one atom of an element, taking account of its isotopes, relative to 1/12 the relative mass of an atom of Carbon-12

Question 2

• define relative isotopic mass

Answer 2

the mass of a particular atom of an isotope compared to the value of the unified atomic mass unit.

Question 3

• define the relative molecular mass

Answer 3

the relative molecular mass, Mr, is the average mass of that molecule compared to 1/12 of the relative molecular mass

Question 4

• what can the mass spectrometer do

Answer 4

it can tell you:
the relative atomic mass,

relative molecular mass,

relative isotopic abundance

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Question 5

• what happens when a sample is squirted into a time of flight mass spectrometer

Answer 5

ionisation

acceleration

ion drift 

detection

Question 6

• what are the two ways of ionising your sample

Answer 6

electrospray ionisation

electron impact ionisation

Question 7

• explain how to ionise your sample with electrospray ionisation

Answer 7

the sample is dissolved and pushed through a small nozzle at high pressure.

a high voltage is applied to it, causing the particle to gain an H+ ion.

the sample is turned into a gas made up of positive ions

Question 8

• explain how to ionise your sample using electron impact ionisation

Answer 8

the sample is vaporised and an electron gun is used to fire high energy electrons at it.

this knocks one electron off each particle, so they become +1 ions

Question 9

• explain the accleration stage when a sample is squirted into a time of flight mass spectrometer

Answer 9

the positively charged ions are accelerated by an electric feild so that they all have the same kinetic energy.

this means that the lighter ions will end up moving faster than the heavier ones

Question 10

• explain the ion drift stage when a sample is squirted into a time of flight mass spectrometer

Answer 10

the ions enter a region with no electric feild, so they just drift through it

lighter ions will drift through faster than heavier ions

Question 11

• explain the detection stage when a sample is squirted into a time of flight mass spectrometer

Answer 11

when ions with the same charge arrive at the detector the lighter ones are first as they have higher velocities.

the flight times are recorded

the positive ions pick up an electron from the detector which causes a current to flow

Question 12

• what is a mass spectrum

Answer 12

a mass spectrum is mass/charge plotted against abundance

Question 13

• what is at the y-axis of a mass spectrum

Answer 13

the Y-axis gives the abundance of ions.,Often as a percentage for an element, the height of each peak gives the relative isotopic abundance

Question 14

• what is at the x-axis of a mass spectrum

Answer 14

the x-axis units are given as a mass/charge ratio.

 since the charge on the ions is mostly 1+ you can usually assume the x-axis is simply the relative isotopic mass

Question 15

• what are the steps to calculating the relative atomic mass from a mass spectrum

Answer 15

for each peak, read the % relative isotopic abundance from the y-axis and the relative isotopic mass from the x axis.

multipily them together to get the total mass for each isotope 

add up these totals 

divide by 100

Question 16

• what do we do if the relative abundance is not given as a percentage

Answer 16

if the relative abundace is not given as a percentage the total abundance may not add up to 100.

in this cas do steps one and two and divide by the sum of the relative abundances

Question 17

• what happens with elements with differnt isotopes in a mass spectrometry

Answer 17

elements with differnet isotopes produce more than one line in a mass spectrum because the isotopes have different masses.

this produces characteristic patterns which can be used as ‘fingerprints’ to identify certain elements

Question 18

• how do we identify elements with one element in a mass spectrum

Answer 18

they can be identified in a mass spectrum by looking for a line at their relative atomic mass

Question 19

• what else can Mass spectrometry be used for

Answer 19

mass spectrometry can be used to identify molecules

Question 20

• explain the steps to getting a mass spectrum for a molecular sample

Answer 20

a molecular ion, M+, is formed in the mass spectrometer when one electron is removed from the molecule

this gives a peak in the spectrum with a mass/charge ratio equal to the relative molecular mass of the molecule 

this can be used to help identify an unknowm compound