Mass Spectrometry and Relative Masses of Atoms, Isotopes and Molecules Flashcards
what is the relative atomic mass of an element compared to in order to calculate its value
hydrogen
why are the relative atomic masses of elements compared to hydrogen and what was hydrogens value
- 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
using hydrogens RFM as a scale, what would oxygens RFM be if it was 16 times heavier than hydrogen
1 x 16 = 16
what was decided in 1961 due to the discovery of isotopes
that the isotope of carbon 12 was the standard in which the isotope of other elements would be measured
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
- it is relative to the mass of a carbon 12 atom
- so the value is not likely to be whole number
what is the true definition of relative atomic mass
- 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)
what is the definition of relative isotopic mass
- 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
what is a mass spectrometer
a device that measures the masses of atoms, molecules and fragments of molecules
what does a mass spectrometer do
- it produces positive ions
- that are deflected by a magnetic field in accordance to their mass to charge ratio (m/z)
what does a mass spectrometer calculate
- the relative abundance of each positive ion
- which is displayed as a percentage
what are the three types of positive ions you could have
- positively charged atoms
- positively charged molecules
- positively charged fragments of molecules
when are fragments of molecules tested when using mass spectrometry
when dealing with organic compounds
what data do you need to calculate the exact value of the relative masses of isotopes
- the mass spectrum of the element
- along with the abundance of each isotope
how do you obtain the relative molecular masses of an element through a m/z graph
by observing the peaks with the largest m/z ratio
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
- RAM = (mass1 x percentage1) + (mass2 x percentage2) / 100
- (35 x 3) + (37 x 1) = 142
- 3
142/ 4 = 35.5
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
- 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
why do you get peaks of 70, 72 and 74 on the chart (specifically those numbers)
- 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
using the data from the mass spectrum, why does the abundance of chlorine molecules decrease as the RMM increases as shown on the graph
- 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
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
- 35Cl and 35Cl = 9
- 37Cl and 35Cl = 6
- 37Cl and 37Cl = 1
why are the ratio of molecules of chlorine like so? (the math behind it)
- 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
how would you calculate the relative molecular mass (Mr) of the chlorine from the data you have
- 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
why do you divide by 16 for this calculation rather than 100
- 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
why do you have to be careful when reading the m/z values of organic compounds
- 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
where would you only be seeing carbon-13 peaks
- with organic compounds with large masses
- as the percentage of carbon-13 isotopes becomes significant
why wouldnt you see an m + 1 peak for an element with a small mass
- as the peak would often be missing
- or just too insignificant to even read
