Chapter 17 Spectroscopy

Question 1

How does mass spectrometry work?

Answer 1

A vaporised organic sample is accelerated through the mass spectrometer where it becomes ionised (loses an electron) and forms a positive ion.

The spectrometer measures the mass to charge ratio (m/z) of the molecular ion (M+) and since the mass of the electron is negligible the m/z of the M+ represents the molecular mass of the sample.

Question 2

Where is the M+ peak located on the mass spectrometer?

Answer 2

The peak at the highest m/z value on the right hand side of the mass spectrum. This also represents the molecular mass of the sample.

Question 3

What is the M+1 peak?

Answer 3

A small peak which can be identified following one unit from the M+ peak due to the 1.1% of Carbon present in the Carbon-13 isotope.

Question 4

What are the other peaks in a mass spectrum known as?

Answer 4

Fragment ions

Question 5

How are the fragment ions formed?

Answer 5

Fragmentation as a result of excess energy due to the ionisation which then transfers to the molecular ion making it fragment into smaller pieces.

Question 6

What are the products of fragmentation?

Answer 6

A smaller positive ion and a free radical
The smaller positive ion will be detected by the mass spectrometer however the uncharged radical will remain undetected.

Question 7

What is the possible positive fragment ion with the m/z value of 15?

Answer 7

CH3+

Question 8

What is the possible positive fragment ion with the m/z value of 17?

Answer 8

OH+

Question 9

What is the possible positive fragment ion with the m/z value of 29?

Answer 9

C2H5+

Question 10

What is the possible positive fragment ion with the m/z value of 43?

Answer 10

C3H7+

Question 11

What is the possible positive fragment ion with the m/z value of 57?

Answer 11

C4H9+

Question 12

Write the equation that represents ethanol passing through the mass spectra?

Answer 12

CH3CH2OH (g) —-> CH3CH2OH+ (g) + e-

Question 13

How does infrared spectroscopy work?

Answer 13

The organic sample is placed in the infrared spectrometer
A beam of infrared radiation in the range of 200-4000cm^-1 is passed through the sample
The molecules will absorb some of the infrared frequencies
The emerging beam of radiation can then be analysed and compared to data sheets to identify what frequencies have been absorbed

Question 14

What is the region below 1500 cm^-1 within infrared spectroscopy called?

Answer 14

Fingerprint region

Question 15

How can infrared spectroscopy be applied?

Answer 15

Pollutants can be identified by their IR frequencies, vehicle emissions can be used to detect and measure carbon monoxide, carbon dioxide and hydrocarbons.

Infrared- based breathalysers can be used to detect the IR absorbance of the compounds in the breath, such as ethanol.