Analytical Spectroscopy

Question 1

What is the wave description of light?

Answer 1

Light is composed of an electric field and a magnetic field which oscillate at right angles to one another.

Question 2

What is the wave equation?

Answer 2

C = V Lambda where:
C = speed of light
V = frequency
Lambda = wavelength

Question 3

What is the particle description of light?

Answer 3

Light is composed of discrete entities known as photons.
The energy of a photon is calculated by E = hv where:
E = energy
H = Plancks constant
V = frequency

Question 4

Which equation is used to calculate the energy of light?

Answer 4

E = hc/lambda where:
H = Plancks constant
C = speed of light 
lambda = wavelength

Question 5

describe the absorption of light by a molecule?

Answer 5

energy is transferred from a photon to a molecule causing it to move into a higher orbital/excited state.

Question 6

Describe the emission of light energy from a molecule?

Answer 6

the molecule releases energy in the form of a photon and moves back to a lower orbital.

Question 7

X-ray radiation causes which electron transitions?

Answer 7

Inner electron transitions; can lead to ionisation or photolysis (bond breaking).

Question 8

Ultra violet radiation and visible light cause which electron transitions?

Answer 8

Outer (valence) electron transitions

i.e outer electrons move to a higher orbital.

Question 9

The difference in what corresponds to the transitions between different energy states in atoms or molecules?

Answer 9

Difference in wavelength.

Question 10

Infra-red radiation causes?

Answer 10

Vibration of bonds within molecules.

Question 11

Microwave radiation causes?

Answer 11

Rotation of bonds within molecules.

Question 12

Radiowave radiation causes?

Answer 12

Nuclear spin transitions.

Question 13

Spectroscopy analysis can be either?

Answer 13

Quantitative or Qualitative

Question 14

Qualitative analysis is described as?

Answer 14

Substances are IDENTIFIED based on the wavelength(s) that the absorb or emit.

Question 15

Quantitative analysis is described as?

Answer 15

Concentrations of analytes are MEASURED.

Question 16

Accuracy

Answer 16

The closeness of a measurement to the true value.

Question 17

Precision

Answer 17

The closeness of a set of measurements to one another.

Question 18

Selectivity

Answer 18

The degree to which the method responds to the selected analyte and not to the other matrix components. Ideally this is high.

Question 19

Detection Limit

Answer 19

The lowest concentration we can detect with statistical certainty.

Question 20

UV-Visible Spectrometry Steps:

Answer 20

1. Shine light of the correct wavelength (corresponding to the change in energy in the analyte) through the solution.
2. measure the proportion of light absorbed.

Question 21

What equation is used to calculate the absorbance of an analyte?

Answer 21

A = log (Io/I) Where:
Io = incident radiation
I = transmitted radiation
N.B. absorbance has no units

Question 22

what is the Beer Lambert Law?

Answer 22

The Beer Lambert law is the equation used to calculate the absorption of a substance and relates absorbance to concentration.
A = e(epsilon) x C x l where:
e= molar absorptivity
C = concentration
l = path length

Question 23

Molar absorptivity relates to the probability of a particular electron transition occuring i.e?

Answer 23

Large molar absorptivity = intense colours

Small molar absorptivity = weak colours

Question 24

What is a chromophore?

Answer 24

A chromophore is a particular functional group within a molecule that absorbs light, different chromophores have different absorption wavelengths and molar absorptivitys.

Question 25

List some common chromphores.

Answer 25

1. double or triple bonds and conjugated systems (benzene)
2. Lone pairs of electrons on O, N or S
3. Metal ion complexes.