17.1-mass Spectrometry

Question 1

What is mass spectra used to identify ?

Answer 1

The molecular mass of an organic compound and to gain further information about its structure

Question 2

What happens when an organic product is placed in a mass spectrometer ?

Answer 2

It loses an electron and forms a positive ion - a molecular ion

Question 3

What does the mass spectrometer detect ?

Answer 3

Mass to charge ratio of the molecular ion which gives the molecular mass of the compound

Question 4

What is the M+ ion ?

Answer 4

The molecular ion M+ is the positive ion formed when a molecule loses an electron

Question 5

How are molecular ions created in a mass spectrometer ?

Answer 5

• when a compound is analysed in a mass spectrometer vaporised molecules are bombarded with a beam of high speed electrons
• these knock Off an electron from some of the molecules creating a molecular ion

Question 6

What is the peak with the highest m/z value ?

Answer 6

The molecular ion M+ peak which give’s information about the molecular mass of the compound

Question 7

What is the value of m/z equal too ?

Answer 7

The relative molecular mass of the compound

Question 8

How do you find out the molecular mass ?

Answer 8

• the molecular ion peak M+ peak has to be located
• molecular ion peak is the clear peak at the highest m/z value on the right hand side of the mass spectrum

Question 9

What is the M+1 peak ?

Answer 9

The smaller peak which is due to the natural abundance of the isotope of carbon-13

Question 10

What does the height of the M+1 peak depend on ?

Answer 10

• Depends on how many carbon atoms are present in that molecule
• the more carbon atoms = the larger the M+1 peak

Question 11

What is fragmentation ?

Answer 11

In the mass spectrometer some molecular ions break down into smaller pieces known as fragments in a process called fragmentation

Question 12

What are other peaks on the spectra caused by ?

Answer 12

By fragment ions formed from the breakdown of the molecular ion

Question 13

What occurs in simple fragmentation ?

Answer 13

Breaks a molecular ion into two species - a positively charged fragment ion and a radical

Question 14

What is fragmentation analysis ?

Answer 14

Different compounds may have the same molecular mass and same molecular ion peak the fragment ions found in the spectrum may be different

Question 15

What is the m/z value of CH3+ ?

Answer 15

15

Question 16

What is the m/z value or C2H5+ ?

Answer 16

29

Question 17

What is the m/z value or C3H7+ ?

Answer 17

43

Question 18

What is the m/z value of C4H9+ ?

Answer 18

57

Question 19

What is the formula to find the number of carbon atoms using the M peak ?

Answer 19

Number of carbon atoms = height of M+1 peak / height of M peak x 100

Question 20

Explain why there sometimes a peak in the spectrum after the molecular ion peak ?

Answer 20

The M+1 peak arises because there is a small percentage of carbon -13 I the sample and this adds one to the molecular mass of the molecular ion