Spectroscopy

Emission Spectra:

Electrons can only absorb specific amounts of energy that then enable them to “jump” to the next ‘quantised energy level’ = the next electron shell.

  • This jump is said to then be the excited state of an electron

~IF the energy isn’t at the EXACT right wavelength then the light will be reflected not absorbed. (this in of itself is known as the excitation-emission profile)

When going back to its ground state (the lowest energy level of that electron) the excited electron emits the extra energy as a photon(s) of light.

  • when an electron jumps MORE than one shell it’ll ‘skip’ back down to its ground state and emit INDIVIDUAL photons rather than all at once

~The emitted photon(s) will go in RANDOM directions.

So basically…... as a summary:

  • electrons can gain energy and then move electron shells to one with a higher energy state through a process called “excitation”

  • when the electron ‘loses’ this energy (as light) it then drops back down to its ground state

  • meaning that the differences relate to wavelengths of light and therefore electrons can become excited by absorbing light.

3 Ways this can happen:

  1. (the hardest way) = the atom absorbs light.

    • this MUST be the EXACT wavelength that matches perfectly/corresponds exactly with the energy difference between two or more shells (this difference is unique to each individual element.

  2. (the easier way) = heat the object up

    • the KINETIC energy of the atom starts to change and this means that the electrons can then ‘steal’ the extra energy from the atom — not only cooling down the object but also allowing for the electron transition to occur.

  3. (the unheard of way) = electricity passing through it

    • the ELECTRICAL current FORCES the electrons to collide with each other and steal energy this way, meaning that electron transition can occur.

    • it then proceeds to glow (which is how lighting works)

Emission Spectroscopy:

Emission spec is the technique used to determine what TYPE of substance(s) are present — meaning it’s qualitative in nature i,e. it tells you the type NOT the amount.

  • the differences occur because of the differences in core charge/valence attraction

    AS SUCH,

  • it can be used to identify elements based upon their unique set of energy levels meaning that elements will emit light with a unique set of wavelengths as the pattern for 1 element is ALWAYS the same (it’s in this sense as to why it’s often called a fingerprint)

~the MORE complex an atom is the MORE lines will be emitted,

But what’s it used in?

Due to it being in qualitative in nature it’s limited in its diagnostic ability and is mainly used in telescopes.

Atomic Absorption Spec (AAS):

It’s a QUANTITATIVE analysis (meaning it tells us the amount of a particular substance present rather than the type(s) of substances present)

  • the concentration of the sample determines how much of the light will be absorbed by the sample

How it works:

  1. hollow cathode lamp:

    • a ‘torch’ that emits a specific amount of light by heating a filament of the desired substance (to then emit the light that will be absorbed)

  2. burner

    • the flame ‘produces’ atoms of the substance — the more substance = the more absorbed, the less substance = the less absorbed

  3. slit

    • to then only get parallel light (shines through the substance)

  4. diffraction monochromator

    • uses a series of crystals to bend the light

  5. slit

    • to get only parallel light

  6. detector

    • measures how much light passes/shines on it.

      ~ the less detected, the more absorbed therefore there’s more substance

      ~ the more detected, the less absorbed therefore there’s less substance

Mass Spectrometry``:

determines the mass and relative abundance of each isotope — the electrons are used to ‘knock off’ electrons forming ions and these ions are then deflected and detected

the heavier the ion, the less deflection that occurs

Two features that determine how much it’s deflected

1 = how high the charge is — the higher the charge the more it’s deflected

2 = the mass of the cation — the lighter the mass, the more it’s deflected

Steps:

1) injection into a vacuum

neutral atoms get injected into a vacuum where they then vapourise

this vapourisation makes ionisation easier.

2) ionisation

neutral atoms pass through an ionisation chamber in which an electron gun ‘fires’ electrons that then hit atoms’ electron shell essentially making an electron fly away, turning the atom into a cation (charged)

3) acceleration

the cations then pass through charged plates and gain speed/kinetic energy (so deflection can occur) through the electrostatic force of attraction and repulsion acting on the cations - the positive plates repel the cations, and the negative plates attract them.

4) deflection

the accelerated cations then pass through a magnetic field that deflects the cations

the lighter the cation the more it’s deflected

this in itself allows for only one isotope to be detected rather than everything that’s present

5) detection

the cations that were deflected perfectly - ie the cations of the isotope you wanted to measure - are then detected by a detector which tells you how much % of that isotope is present

a window is also used to lessen the chances of getting any unwanted isotope cations detected.