Topic 17.7 Technical Analysis

Question 1

Combustion analysis

Answer 1

Known mass of a substance is burned and the masses of carbon dioxide ……….

Question 2

Working out the formula of a compound

Answer 2

picture of notes

Question 3

High resolution in mass spec

Answer 3

Modern mass spectrometry can measure a relative isotopic mass to four decimal places so the relative atomic mass can be found:
1H: 1.0078
14N: 14.0031
16O: 15.9949
12C: 12.0000

Question 4

Mass spec

Answer 4

-A gaseous sample is injected into the mass spectrometer
-It is bombarded by high energy electrons from an electron gun
-A molecular ion is produced
-If a covalent bond in the molecular ion breaks, fragmentation happens
-Fragmentation produces an ion and a free radical
-Only the ion is attracted to the charged plates in the mass spectrometer and detected subsequently

Question 5

What causes fragmentation

Answer 5

A covalent bond in the molecular ion may break causing fragmentation.

Question 6

High Performance Liquid Chromatography (HPLC)

Answer 6

-Stationary phase is solid packed with silicon molecules (non-polar)
-Forced through column at high pressure
-Retention time: time taken from injection to detection
The retention times depend on several variables:
-the nature of the solvent (polarity)
-the pressure used
-the temperature

Question 7

The process of HPLC

Answer 7

-The solvent is forced through a metal tube under high pressure
-The particle size of the stationary phase is much smaller which leads to better separation
-The sample is injected into the column
-The components are detected after passing through the column (usually by their absorption of UV radiation)
-The whole process is automated and the results are quickly available on a computer display

Question 8

Gas chromatography

Answer 8

-The metal tube can be several meters long and is coiled to save space
-The stationary phase: solid or liquid coated on the inside of the tube
-The mobile phase: inert carrier gas (often nitrogen and helium)
-The column is fitted inside a thermostatically controlled oven
-The sample is injected into the column
-The gas stream will carry the sample through and out of the column
-The components passing through the column are detected as they leave (a device called a flame ionisation detector is often used)
-The whole process is automated and the results are quickly available on a computer display
-Each component in the sample is represented by a peak in the chromatogram

Question 9

Uses if HPLC-MS

Answer 9

-Seperation
-Identification
-Quantification
-Elucidate the structure and chemical properties of different molecules

Question 10

Applications of GC-MS

Answer 10

-Drug detection
-Fire investigation
-Environmental analysis
-Explosives investigation
-Food and favour analysis
-Identification of unknown samples

Question 11

NMR basic principles

Answer 11

-Powerful analytical technique for determining the structures of organic molecules
-Atoms with an odd number of nucleons (protons + neutrons) have a property called spin
-These nuclei behave like tiny magnetic fields

Question 12

Magnetic fields

Answer 12

-Nuclei with spin behave as if they were tiny bar magnets (they respond to an applied magnetic field)
-They can align with the magnetic field
-Nuclei aligned with a magnetic field are in a relatively low energy state
-Nuclei with spin can also align against the magnetic field
-Nuclei aligned against the magnetic field are in a higher energy state than nuclei aligned with a field
-Energy is needed to move a nucleus to the higher energy state. The amount of energy needed depends upon the chemical environment of the atom. The electromagnetic radiation absorbed is deleted as different ‘signals’ at different frequencies.

Question 13

Obtaining a NMR spectrum

Answer 13

-The compound is dissolved in a deuterated (one in which H atoms have been replaced by deuterium, 2H).
-A standard reference compound of tetramethylsilane (TMS) is added. TMS has a single, strong, sharp absorption peak well away from the peaks produced by the sample being analysed.
-TMS is unreactive so will not react with the sample being analysed.

Question 14

13C NMR

Answer 14

13C NMR makes up only about 1% of all naturally occurring atoms (enough signal to be detected in an NMR spectrometer)
-The number of peaks a 13C NMR spectrum corresponds to the number of carbon environments in the molecule studied.

Question 15

13C NMR for ethanol

Answer 15

Contains 3 peaks:
-TMS at 0ppm (usually removed from the spectrum for clarity)
-Two other peaks corresponding to the two different carbon environments in the molecule (amount of carbon environments = the amount of peaks).

Question 16

Chemical shift ranges

Answer 16

Tables showing likely values of chemical shifts fo 13C atoms in different environments have been produced.

Question 17

Carbon environments

Answer 17

Eg. Molecule is symmetrical so symmetrical parts are the same environment
So has 2 carbon environments therefore 2 peaks.

Question 18

Hydrogen environments

Answer 18

….

Question 19

The information that a proton NMR spectrum gives about a molecule

Answer 19

-The different non-equivalent hydrogen atoms in the molecule (the different peaks)
-The ratios of the number of equivalent hydrogen atoms (the areas under the peaks)
-The chemical environment of the equivalent hydrogen atoms (the position of a peak)
-The number of adjacent hydrogen atoms that are non-equivalent (peak splitting in high-resolution spectra)

Question 20

TMS is used to provide the reference peak because:

Answer 20

-It has 12 equivalent hydrogen atoms (they are in the same chemical environment)
-Its protons are highly shielded (its NMR peak is upfield of most other peaks)
-It is non-toxic and chemically inert (it will not react with the sample).