Spectroscopy & UV Flashcards
(40 cards)
What is spectroscopy?
The study of interaction of matter and electromagnetic radiation
What is electromagnetic radiation (EMR)?
The radiant energy that displays the properties of both particles and waves
There are different types of EM waves that make up the electromagnetic spectrum (EMS)
Example of EMR
Visible light
What range is visible light?
400 to 700 nm
What are gamma rays?
Emitted from the nuclei of some radioactive elements and because of their high energy, can severely damage biological organisms
What are X-rays?
Less harmful than gamma except in high doses
What is UV light?
Responsible for sunburn
Repeated exposure leads to skin cancer by damaging DNA molecules in skin cells
What is infrared radiation?
Felt as heat
What are radio waves?
Have the lowest energy (frequency)
E.g. Radio, TV
Also used in NMR and in magnetic resonance imaging (MRI)
Nature & properties of electromagnetic radiation
- Has 2 components = an electric field and a magnetic field which are in planes at right angles to each other
- A given point in space experiences a periodic disturbance in electric and magnetic fields as the wave from radiation passes by
- EM waves can travel through a vacuum
What is frequency (v) in EMR?
The number of crests of the wave that passes a point per second (number of cycles)
Unit is s-1 also known as Hz
What is wavelength (λ) in EMR?
Distance between 2 adjacent crests
Unit is in metres but nm can be used
What is velocity (c) in EMR?
In a vacuum, EM waves travel with the same velocity
This is ALWAYS 2.998 x 10^8 ms-1
What equation relates frequency, velocity and wavelength?
C = λ x v
What is a particle of EMR called?
A photon
How is the energy (E) of a photon calculated?
E = h x v
OR
E = h x c/ λ
h is planks constant = 6.626 x 10-34 J s
Key things about energy of a photon
Directly proportional to frequency (v)
Inversely proportional to wavelength (λ)
Units is Joules
What is the wavenumber?
1/λ —> Ṽ
Units is m-1
Describe UV spectroscopy
- UV provides radiation
- Monochromator selects a single wavelength from the wide range provided by the light source
- i0 is incident light = light going into the sample
- i is the light not absorbed by the sample
- The cell (cuvette) can be glass or plastic for visible light but must be quartz for UV light (glass and plastic absorb UV)
What factors contribute the amount of absorbance?
- The thicker the sample, the more absorption takes place
- More absorption seen in more concentrated samples (more molecules present to absorb light)
- Some molecules are better than others at absorbing a particular wavelength = an inherent property of the molecule
What does absorbance (A) of a sample depend on?
- Concentration (c) of the absorbing species in mol L-1
- Length of the light path (l) through the cell in cm
- Molar absorption coefficient (E) in L mol-1 cm-1
E relates to the inherent property of a molecule to be better at absorbing particular wavelengths than others
What is the Beer-Lambert law equation?
A = E x c x l
How can the beer-lambert equation be used in measuring incident and transmitted light?
A = E x c x l —> A = log10(I0/I0)
How can transmission be calculated using absorption data?
Transmission (T) = I/I0 SO A = -log10(T)