SPECTROSCOPY

Question 1

Define quantitative and qualitative methods in spectroscopy

Answer 1

Quantitative: Provides info about the quantity of these species, as numerical information. Examples include classical gravimetric and volumetric measurements and most instrumental analyses

Qualitative: Provides info about the identity of an atomic or molecular species. Examples include many classical methods relying on a change in colour, solubility, BP/MP

Question 2

What region of EMR does UV-visible spectroscopy use?

What does it measure?

Is it quantitative of qualitative?

Answer 2

EMR: the UV or visible region

Measure: the absorption of energy by electrons, provides the difference in energy between the highest occupied molecular orbital and the lowest unoccupied molecular orbit

Quantitative: measures the absorption of a solution, the amount of light absorbed is directly proportional to the concentration of analyse

Question 3

Define Beer-Lambert’s Law

Answer 3

A = Elc
A: absorbance (unitless
E: molar absorptivity M^-1 or cm^-1
l: path length (cm)
c: concentration (M)

Question 4

What region of EMR does colorimetry spectroscopy use?

What does it measure?

Is it quantitative of qualitative?

Answer 4

EMR: the visible region of EMR

Measures: the absorbance of a solution, determines the concentration of a coloured analyse in a solution

Quantitative: amount of radiation absorbed is directly proportional to the concentration of the coloured species in the solution

Question 5

What region of EMR does infrared (IR) spectroscopy use?

What does it measure?

Is it quantitative of qualitative?

Answer 5

EMR: Uses the slightly lower than red in visible region/infrared region, wavenumber are proportional to energy

Measures: Informs about the infrared radiation absorbed by a molecule. For the molecules absorbing infrared radiation, the bonds between atoms in the molecule stretch and vibrate, this is then represented by a wave number

Qualitative: informs about the structure of the atoms

Question 6

What does a broad, rounded peak in the region of 2800-3400 mean (IR)

Answer 6

Hydroxyl (OH)

Question 7

What does a sharp, strong peak between 1850-1650 mean (IR)

Answer 7

Carbonyl (C=O)

Question 8

What is nuclear magnetic resonance (NMR) spectroscopy?

Answer 8

The most powerful tool available for organic structure determination, a nucleus with an odd atomic number or an off mass number has a nuclear spin

In H NMR a spinning charged nucleus generates a magnetic field. H nuclei are protons

Question 9

What is mass spectroscopy?

Answer 9

Molecular weight can be determined from a very small sample of a compound

It does not involve the absorption or emission of light (not technically a spectroscopic technique)

A beam of high-energy electrons breaks the molecule apart, the masses of fragments and their relative abundance reveal info about the structure of the molecule

Question 10

Why is chromatography undertaken?

Answer 10

we carry out chromatography so that we can isolate the analyses of interest and detect them individually

Question 11

Define the process of liquid chromatography

Answer 11

Inert gas pushes components through a column
This technique relies on the ability of the analyte of interest to vaporize. Involves equilibrium process between the gas phase and the liquid phase
Volatile compounds (low bp) spend more time in the gas phase and exit the column faster
In both gas chromatography and liquid chromatography, the compounds enter the detector after separation – this is where the magic happens
Detectors include those we have covered today but a total chromatogram allows us to see a graphical representation of the separation of components in the sample