Emission, Absorption, Mass Spectrometry

Question 1

how does emission spectra work

Answer 1

• voltage, heat or light is applied to the element, atoms absorb energy and are excited
• electrons transition to higher energy levels
• atom is unstable and causes electron to return to ground state
• release a photon of energy equal to the energy difference between 2 shells

Question 2

hence, explain what emission lines and spectrum are

Answer 2

emission lines: represent individual photons of light emitted by the electrons returning to ground state
these collectively form an emission spectrum

Question 3

Prove that shells exist bc of emission spectra

Answer 3

• emission spectrum of an element have very specific emission lines
• which represent photons of light released by electrons of
  an unstable atom transitioning from higher energy electron shells to lower energy electron shells
• emission lines correspond to the differences in energy levels between shells
• proves that electrons occupy specific energy levels, hence shells exist

Question 4

explain how ES characteristic of that element

Answer 4

• each element has specific energy levels
• each element has specific differences between energy levels
• each elements electrons release specific photons of energy when transitioning from excited to ground state
• each element has a unique emission spectrum

Question 5

how do flame tests work

Answer 5

insert the sample of metallic compound into a non-luminous flame of a bunsen burner, identify metal by comparing the flame colour with characteristic flame colours produced by metals

Question 6

why flame test works

Answer 6

specific flame colours are caused by specific photons of light emmitted by specific electron transitions from higher energy levels to lower energy levels in atoms of specific elements. Hence you can use the flame test to distinguish between different elements

Question 7

why flame test bad

Answer 7

• qualitative test, people’s perception of colour different
• small range of metals detectable by flame test
• metals in small concentrations are difficult to observe
• mixtures of metals produce confusing results

Question 8

how to improve flame test

Answer 8

• use spectroscope to observe emission lines and compare it to previously recorded emission spectrums
• AES

Question 9

flame test colours for
sodium
strontium
copper
barium
lithium
calcium
potassium

Answer 9

sodium yellow
strontium scarlet
copper green
barium yellow-green
lithium crimson
calcium red
potassium lilac

Question 10

why non-luminous flame?

Answer 10

luminous flame obscures colour emitted by the metal atoms by its orange flame colour
not hot enough to excite metal atoms

Question 11

what is AES used for?

Answer 11

quantitative chemical analysis to identify element based on its characteristic emission spectrum

Question 12

components of AES

Answer 12

excited sample
prism
monochromator
detector
display

Question 13

what does the prism do in AES?

Answer 13

separates light emitted by element into individual colours

Question 14

what does the monochromator do in AES?

Answer 14

allows single wavelength of light to pass through at a time

Question 15

how does AES work?

Answer 15

• element heated, … release photons of energy
• prism
• monochromator
• spectra is then detected and recorded

Question 16

How does absorption spectrum work?

Answer 16

• when white light is passed through a cool sample of gas
• electrons only absorbs photons of energy equal to the energy difference between shells and atom becomes excited
• electrons transition from ground state to higher energy levels

Question 17

hence, explain what absorption lines and absorption spectrum is

Answer 17

absorption lines represent the individual colours of light absorbed by atoms
these collectively form absorption spectrum

Question 18

what do dark regions in AS correspond to?

Answer 18

colours of light that provide the exact quantity of energy required for its electrons to transition to a higher energy level

Question 19

why are there many absorption lines for hydrogen even though it only has 1 electron?

Answer 19

each individual absorption line is caused by the promotion of the electron to the second, third or fourth energy level or higher
the absorption spectrum represents the collective wavelengths of energy absorbed by many individual hydrogen atoms

Question 20

why are there many emission lines for hydrogen even though it only has 1 electron?

Answer 20

there are many ways for an electron to transition from higher energy electron shells back to ground state. The emission spectrum represents the collective emissions of many individual hydrogen atoms

Question 21

what is mass spectrometry?

Answer 21

chemical analysis to calculate the mass and relative abundance of atomic particles

Question 22

what does a mass spectrometer do?

Answer 22

• separates the individual isotopes in a sample of the element
• determines the mass of each isotope relative to the carbon 12 standard
• calculates the relative abundancies of the isotopes in the sample

Question 23

parts of MS

Answer 23

1. vapouriser
2. ioniser
3. particle accelerator
4. deflector
5. detector

Question 24

why must sample be vapourised?

Answer 24

great question

Question 25

MS equation

Answer 25

r = mv/qB
radius
mass
velocity
charge
magnetic field

Question 26

how does the mass spectrometer work? (outline)

Answer 26

• vapourisation
  -ionisation
• acceleration through particle accelerator (electric field)
• deflection in magnetic field
• detection

Question 27

explain vapourisation and ionisation

Answer 27

sample is vapourised. It is then ionised using high energy electrons with an electron gun, creating positive ions only, necessary as only charged particles will interact with the electric and magnetic fields

Question 28

explain particle accelerator

Answer 28

electric field accelerates the ionised particles

Question 29

explain deflection

Answer 29

magnetic field deflects particles according to their mass to charge ratio
↓ mass, ↑ charge, more deflection
↑ mass, ↓ charge, less deflection

Question 30

explain detection

Answer 30

ions that have a particular mass to charge ratio are detected by a device that counts the number of ions that strikes it

Question 31

define mass spectrum

Answer 31

info obtained from a mass spectrometer is presented graphically, relative abundance of each ion plotted against its mass-to-charge ratio

Question 32

x and y axis of a mass spectrum

Answer 32

x: m/z
y: relative abundance

Question 33

what is AAS

Answer 33

chemical analysis to calculate concentration of a known element of a sample

Question 34

components of AAS

Answer 34

same as AES plus hollow cathode lamp

Question 35

how does the hollow cathode lamp work?

Answer 35

hollow cathode lamp is coated with the metal being investigated
hence cathode light that passes through the atomised sample contains the exact wavelengths that can be absorbed by the metal being analysed
amount of cathode light absorbed by sample gives a measure of the concentration of the specific metal in the sample

Question 36

why is the cathode light pulsed?

Answer 36

allows for the detector to distinguish between
- the cathode light left after absorbance
- light naturally emitted by the atomised sample in the flame

Question 37

what is the calibration curve used for?

Answer 37

good question

Question 38

why is AAS better than flame test

Answer 38

qualitative and quantitative test
more than 70 different elements analysed
detect elements in concentrations as low as mg/L
highly selective and regularly used with mixtures