mass spectrometry

Question 1

what does a mass spectrometer do

Answer 1

separates molecules according to their charge and mass

Question 2

what type of molecules can mass spectrometry separate

Answer 2

ions of slightly different masses and can easily distinguish between different isotopes for a given element

Question 3

how can we identify a sample chemical

Answer 3

by measuring its molecular weight (formula mass)

Question 4

what is relative mass

Answer 4

the mass of one atom compared to the mass of a different atom (carbon-12)

Question 5

what is relative atomic mass

Answer 5

the average mass of an atom of an element compared with 1/12 of the mass of an atom of carbon-12

Question 6

what is relative isotopic mass

Answer 6

mass of an isotope of an atom compared with 1/12 of the mass of an atom of carbon-12

Question 7

what is relative molecular mass

Answer 7

the average mass of a molecule or formula on a scale where an atom of carbon-12 is 12

Question 8

what can you use mass spectrometry for

Answer 8

it can tell you the relative atomic mass, relative molecular mass, relative isotopic abundance and molecular structure of an atom

Question 9

what are the three main parts of a mass spectrometer

Answer 9

the ionisation source
the analyser
the detector

Question 10

ionisation source..

Answer 10

..the sample if ionised, forming positively charged ions

Question 11

analyser..

Answer 11

..the ions produced are separated according to their mass

Question 12

detector..

Answer 12

..the ions cause the detector to produce an electric current that computer system converts into a mass spectrum

Question 13

describe the process of mass spectrometry (time of flight mass spectrometry)

Answer 13

• liquids and gases are injected but solids are heated to vaporise them into a gas
• a heated element produces high energy electrons which bombard atoms or molecules and knockjed electrons out to form cations
• an electric field accelerates the ions to give them the same KE
• placed in a vacuum so the ions don’t collide with any air molecules
• lighter ions will reach detector first because they all have the same KE
• computer detects the lighter ions first and converts them into a mass spectrum

Question 14

what is a mass spectrum

Answer 14

the mass/charge (m/z) ratios of the ions present in a sample plotted against their intensities (% abundance)

Question 15

what does the y-axis of the mass spectrum tell us

Answer 15

the abundance of ions often as a %.

Question 16

what does the height of each peak tell us

Answer 16

the relative isotopic abundance

Question 17

what does each line represent if the sample is an element

Answer 17

each line will represent a different isotope of the element

Question 18

what does the x-axis of the mass spectrum represent

Answer 18

the mass/charge ratio or simply the relative mass

Question 19

what can we calculate from the mass spectrum

Answer 19

the relative atomic mass

Question 20

how do we calculate relative atomic mass from a mass spectrum

Answer 20

1) multiply the relative %relative isotopic abundance from the y-axis and the relative isotopic mass from the x axis
2) add up the totals
3) divide by 100

Question 21

what happens if relative abundance in the mass spectrum is not given as a %

Answer 21

you would divide by the total relative abundance instead of 100

Question 22

why are there many peaks in a mass spectrum

Answer 22

fragments of the original molecule ions are formed in the ionisation chamber. if the fragments have a positive charge it will be accelerated by an electric field and detected.

Question 23

what does the highest/intense peak refer to

Answer 23

the base peak which means its the most stable fragment that is made the most often and the most detectable

Question 24

what is the name for the ion with the higest m/z ratio

Answer 24

molecular ion/original parent molecule.

Question 25

what is the relative formula mass on the mass spectrum

Answer 25

the number of the molecular parent ion