Mass Spectrometry and Relative Masses of Atoms, Isotopes and Molecules

Question 1

what is the relative atomic mass of an element compared to in order to calculate its value

Answer 1

hydrogen

Question 2

why are the relative atomic masses of elements compared to hydrogen and what was hydrogens value

Answer 2

• they were compared to hydrogen because it was the lightest element due to it having the lightest atom
• which led to scientists giving it the relative atomic mass of 1

Question 3

using hydrogens RFM as a scale, what would oxygens RFM be if it was 16 times heavier than hydrogen

Answer 3

1 x 16 = 16

Question 4

what was decided in 1961 due to the discovery of isotopes

Answer 4

that the isotope of carbon 12 was the standard in which the isotope of other elements would be measured

Question 5

what is the relative isotopic mass of en element in regards to carbon and what does this mean for the value of the calculated relative isotopic mass

Answer 5

• it is relative to the mass of a carbon 12 atom

- so the value is not likely to be whole number

Question 6

what is the true definition of relative atomic mass

Answer 6

• the weighted average mass of an element compared to 1/12 the mass of a carbon-12 atom
• which has a mass of 12 (so 1)

Question 7

what is the definition of relative isotopic mass

Answer 7

• the mass of an atom of the isotope of the element compared to 1/12 the mass of a carbon-12 atom
• which has a mass of 12

Question 8

what is a mass spectrometer

Answer 8

a device that measures the masses of atoms, molecules and fragments of molecules

Question 9

what does a mass spectrometer do

Answer 9

• it produces positive ions

- that are deflected by a magnetic field in accordance to their mass to charge ratio (m/z)

Question 10

what does a mass spectrometer calculate

Answer 10

• the relative abundance of each positive ion

- which is displayed as a percentage

Question 11

what are the three types of positive ions you could have

Answer 11

• positively charged atoms
• positively charged molecules
• positively charged fragments of molecules

Question 12

when are fragments of molecules tested when using mass spectrometry

Answer 12

when dealing with organic compounds

Question 13

what data do you need to calculate the exact value of the relative masses of isotopes

Answer 13

• the mass spectrum of the element

- along with the abundance of each isotope

Question 14

how do you obtain the relative molecular masses of an element through a m/z graph

Answer 14

by observing the peaks with the largest m/z ratio

Question 15

on a mass spectrum for chlorine, there are two peaks corresponding to the isotopic masses of 35 and 37. the ratio of these peaks is approximately 3;1 (or 75% and 25%). using this info, calculate the relative atomic mass of this sample of chlorine

Answer 15

• RAM = (mass1 x percentage1) + (mass2 x percentage2) / 100
• (35 x 3) + (37 x 1) = 142
• 3
  142/ 4 = 35.5

Question 16

on the same mass spectrum graph for chlorine, there are 3 peaks of chlorine’s relative isotopic mass decreasing in size in the order of 70, 72 and 74. state why these values are much higher than the calculated RAM of chlorine

Answer 16

• chlorine in its stable form exists as a diatomic molecule
• meaning two chlorine atoms have covelantly bonded together
• giving the total RMM to be about double the calculated RAM for one atom of chlorine

Question 17

why do you get peaks of 70, 72 and 74 on the chart (specifically those numbers)

Answer 17

• because there are two isotopes of chlorine that could be covelantly bonded together as was previously detected by the mass spectrometer
• Cl(35) and Cl(35) = RMM of 70
• Cl(37) and Cl(35) = RMM of 72
• Cl(37) and Cl(37) = RMM of 74

Question 18

using the data from the mass spectrum, why does the abundance of chlorine molecules decrease as the RMM increases as shown on the graph

Answer 18

• the graph shows that there is a larger abundance of Cl(35) isotopes than Cl(37)
• this also means that Cl molecules containing one Cl(37) would be less common than molecules containing the more abundant Cl(35)s twice
• and Cl molecules containing two Cl(37)s would be even less abundant

Question 19

if the ratio of 35Cl and 37Cl is 3:1 what would a table of the different types of Cl molecules and their ratio of molecules look like

Answer 19

• 35Cl and 35Cl = 9
• 37Cl and 35Cl = 6
• 37Cl and 37Cl = 1

Question 20

why are the ratio of molecules of chlorine like so? (the math behind it)

Answer 20

• for 35Cl and 35Cl, theres a 3/4 chance of coming across both
• so 3/4 x 3/4 = 9/16
• for 37Cl and 35Cl theres a 3/4 chance and 1/4 chance for the 37Cl
• so 3/4 x 1/4 = 3/16
• but you can reverse the order in which they bond (to 35Cl and 37Cl) which gives you another 3/16
• 3/16 + 3/16 = 6/16
• for 37Cl and 37Cl, the 1/4 chance twice is 1/4 x 1/4 = 1/16
• giving you 9/16 : 6/16 : 1/16 = 9:6:1

Question 21

how would you calculate the relative molecular mass (Mr) of the chlorine from the data you have

Answer 21

• you would use the ratios as the percentages or abundance, but its still similar to the mass1 x percentage1 formula
• (9 x 70) + (6 x 72) + (1 x 74) / 16 = 71

Question 22

why do you divide by 16 for this calculation rather than 100

Answer 22

• because you are dividing by the sum of the ratios
• it would be 100 if you had percentages
• as then you would be dividing by the sum of the percentages of each isotopic molecule

Question 23

why do you have to be careful when reading the m/z values of organic compounds

Answer 23

• because there is a small chance of the carbon-13 isotope being present in the compound
• which can lead to an m + 1 peak
• where you have a very small abundance of an isotope with 1 more mass than the original element is even meant to have

Question 24

where would you only be seeing carbon-13 peaks

Answer 24

• with organic compounds with large masses

- as the percentage of carbon-13 isotopes becomes significant

Question 25

why wouldnt you see an m + 1 peak for an element with a small mass

Answer 25

• as the peak would often be missing

- or just too insignificant to even read