Mass spectrometry

Question 1

what happens when an organic compound is placed into a mass spectrometer?

Answer 1

it loses an electron and forms a positive ion, molecular ion. The mass spectrometer detects the mass to charge ratio (m/z) of the molecular ion which gives the molecular mass

Question 2

how do you find the molecular mass from a mass spectrum?

Answer 2

the molecular ion peak (M+ peak) must be located. It is the clear peak at the highest m/z value on the right of the mass spectrum.

Question 3

what is the peak next to the molecular ion peak?

Answer 3

It is referred as the M+1 peak. It exists due to 1.1 percent of carbon present as carbon-13 isotope

Question 4

what is fragmentation?

Answer 4

in the mass spectrometer, some molecular ions break down into fragments. Other peaks are caused by fragment ions.

Question 5

What does the simplest fragmentation form? and how does it affect the mass spectrum?

Answer 5

two species formed- positively charged fragment ion and a radical. Any positive ion formed will be detected not uncharges radicals.

Question 6

why do all molecules fragment differently?

Answer 6

depending on their structures. So this allows us to identify molecules

Question 7

what are the main features of a mass spectrum?

Answer 7

• molecular ion peak
• M+1 peak
• a number of fragment ion peaks

Question 8

what happens when covalent bonds absorb infrared radiation?

Answer 8

they bend or stretch more

Question 9

what happens when covalent bonds absorb infrared radiation?

Answer 9

they bend or stretch more. Stretch meaning rhythmic movement along line between atoms so distance between two atomic centres increases and decreases. A bend results in a change in bond angle.

Question 10

what does the amount that a bond stretches or bend depend on?

Answer 10

• the mass of the atoms in bond. Heavier atoms vibrate more slowly.
• the strength of the bond. Stronger bonds vibrate faster.

Question 11

Since frequency values are very large, chemists use what more convenient scale?

Answer 11

wavenumber which is proportional to frequency.

Question 12

Describe the wavelength of radiation which is re-emitted from earth’s surface?

Answer 12

longer wavelength

Question 13

What absorb these longer wavelength radiation?

Answer 13

greenhouse gases e.g carbon dioxide, water vapour, methane as it has the same frequency as the natural frequency of their bonds. Vibrating bonds in these molecules re-emit energy as radiation to the Earth, increasing temp-global warming

Question 14

Chemists use infrared spectroscopy to identify functional groups in organic molecules. How does this work?

Answer 14

• sample placed in IR spectrometer
• beam of radiation (200-4000cm-1) is passed through sample
• molecule absorbs some IR radiation. Emerging beam of radiation is analysed to identify frequencies that have been absorbed by the sample

Question 15

what are the number of peaks called in an infrared spectrum?

Answer 15

fingerprint region. Unique peaks are used to identify molecules

Question 16

where is the peak of the C-H bond?

Answer 16

2850-3100cm-1

Question 17

Describe the infrared spectrum of an alcohol?

Answer 17

broad peak within 3200-3600cm-1 caused by O-H bond. There is also a peak between 1000-1300cm-1 caused by C-O bond.

Question 18

Describe the infrared spectrum of an alcohol?

Answer 18

broad peak within 3200-3600cm-1 caused by O-H bond in alcohol. There is also a peak between 1000-1300cm-1 caused by C-O bond.

Question 19

Describe the infrared spectrum of an aldehyde/ketone

Answer 19

has a key absorbance peak between 1630-1820cm-1 caused by C=O bond.

Question 20

Describe the infrared spectrum of a carboxylic acid

Answer 20

key absorbance peak between 1630-1820cm-1 caused by C=O bond. A very broad peak between 2500-3330cm-1caused by O-H bond in carboxylic acid. Peak at 1000-1300cm-1 for the C-O bond which is nor always reliable as it is in the fingerprint region.

Question 21

describe applications of infrared spectroscopy

Answer 21

• remote sensors analyse IR spectral fingerprints of vehicle emissions e.g carbon dioxide, carbon monoxide to monitor local pollution.
• IR-based breathalysers can detect ethanol in the breath

Question 22

what is the typical sequence of identification of compounds?

Answer 22

• elemental analysis
• mass spectrometry
• infrared spectroscopy

Question 23

what is elemental analysis?

Answer 23

use of percentage composition data to determine the empirical formula

Question 24

what is mass spectroscopy?

Answer 24

use of the molecular ion peak from a mass spectrum to determine the molecular mass; use of fragment ions to identify sections of a molecule

Question 25

what is mass spectroscopy?

Answer 25

use of absorption peaks from an infrared spectrum to identify bonds and functional groups present in the molecule