Atomic Spectroscopy

Question 1

What is atomic spectroscopy?

Answer 1

Exciting valence electrons for absorption and emission spectroscopy

Question 2

How to obtain atomic spectra?

Answer 2

Atomic spectra through a prism, light source and prism, prism splits up spectrum, some stronger lines, some diffused lines, each elements has its own pattern s, p, d, f lines, fingerprint lines for quantitative analysis of mixtures

Question 3

Qualitative atomic spectra?

Answer 3

Pick out which elements are there

Question 4

Quantitative atomic spectra?

Answer 4

Use standard and compare

Question 5

Typical spectra of atoms?

Answer 5

Narrow, discrete lines rather than peaks
Fewer peaks
Higher resolution
No chemical bonds because of atoms it happens inside them (not molecules) so no rotations or vibrations
Only electronic energy levels to consider

Question 6

Atomic emission spectroscopy AES background noise?

Answer 6

Zero background low noise, so much better choice

Question 7

Atomic absorption spectroscopy AAS background noise?

Answer 7

Bright background (high noise)
Measure intensity change 
More signal than emission 
Trace detection possible with suitable lamp (eg use Na lamp for Na detection to avoid noise from other colours)

Question 8

How to decide which atoms AES is good for?

Answer 8

Population of excited states, we have to calculate the population to predict absorption and emission intensities, single is proportion to number of atoms
AES - low noise (background)
AAS - high signal

Question 9

Where are most atoms in the distribution?

Answer 9

Most atoms in ground state, getting to excited state is difficult, very high population of ground state signal is very strong and reliable

Question 10

Energy gap for emission and absorption?

Answer 10

The energy gap for emission is exactly the same as for absorption (contrast to fluorescence) all systems are more stable at lower energy even in the flame most of the atoms will be in their lowest energy state

Question 11

What is the effect of temperature?

Answer 11

Only for very high temperature are the line affected, position of line are temperature independent only population changes, ie at very high temperatures the pattern may be cleaner but very small effect.

Question 12

How does the Rydberg equation fit the spectra pattern?

Answer 12

Always same pattern, can be explained by equation, two numbers one identified as lower quantum number and higher quantum number experimentally show equation fits pattern

Question 13

Atomic spectroscopy in astronomy?

Answer 13

The visible spectrum offer a window into space based on atomic emission and absorption spectroscopies

Question 14

Example of spectroscopy in astronomy?

Answer 14

New elements ‘Nebelium” postulated in 1911, later recognised as O2+ forbidden transitions become possible for long lives states, no collisions or quenching in empty space so both fluoresce and phosphorescence are possible and visible

Question 15

Set up for AES and AAS?

Answer 15

Dilute solution of ions is sprayed at constant rate into a flame, sample is atomised and some is excited, intensity of emission is measured

Question 16

AES intensity?

Answer 16

AES monitors emission from the excited state, but only a few percent of atoms is ever excited, hence emission is not very intense (except in a few cases)

Question 17

How to get round that AES is not intense?

Answer 17

An alternative way is to analyse the ground state atoms, spray the atoms from solution into a flame, irradiate the flame with the correct wavelength for the species, measure the absorption of radiation AAS

Question 18

Temperature for AES?

Answer 18

Needs to be constant temperature and as hot as possible
constant conditions = steady reading
Temperature effects - quantify later

Question 19

Different temperatures of different conditions?

Answer 19

Gas/air (eg Bunsen) ~ 1500K
C2H2/air ~ 2500K
C2H2/O2, N2O2 ~ 3000K
Plasma 6000 - 10000K (highest temperature a reaction can go under lab conditions, excite many more atoms with these temperatures)

Question 20

What is plasma?

Answer 20

A hot partially ionised gas - the sun is a giant plasma

Question 21

What is inductively couples plasma AES?

Answer 21

Much more sensitive

Applicable to many more elements

Question 22

How does intensity depend on Boltzmann distribution?

Answer 22

The intensity of the absorption peak depends on the number of molecules that absorb the radiation ie the number in the energy level - the population

Question 23

ICP-AES detection limits?

Answer 23

Lower values are better - minimum amount that can reliably be detected

Question 24

What does intensity of emission depend on?

Answer 24

The intensity of emission depends on the number of atoms in the excited state, don’t see ant emission at room temperature

Question 25

What temperature is needed to get emission?

Answer 25

Need to heat sodium atoms at 1500K, at the temperature of a gas air flame only around 1 atom in 4 million is in the upper energy level from which emission takes place, at 2500K the ratio is 1,71 x 10-4 or about 1 excited atom in 6000 - higher temperature gives stronger signal (higher temperature more emission because more populated hence why plasma is useful)

Question 26

Limit of plasma?

Answer 26

A 10K fluctuation increases the number of excited atoms by about 4% - limits the accuracy and precision of the method, negligible effect on the ground state, therefore AAS less sensitive to fluctuations, need perfect plasma that doesn’t fluctuate and create noise

Question 27

AES how to perfumed practical measurements?

Answer 27

Practical measurements performed by simple calibration with standard solution, measure emission at standard concentrations and plot graph, measure emission from ‘unknown’ and read off the concentration from the graph

Question 28

What is atomic spectroscopy?

Answer 28

Atomic spectroscopy is based on atomic energy levels that are filled with electrons

Question 29

What does atomic emission require?

Answer 29

Atomic emission requires filling of excited states and dropping back of the electron into lower levels. Best at high temperatures for example in plasmas

Question 30

What does the Boltzmann distribution allow?

Answer 30

The Boltzmann distribution allows emission intensities to be estimated from wavelength (easier in the red than in the blue)

Question 31

What does atomic absorption require?

Answer 31

Atomic absorption requires a lamp to excite the atoms and can be much more sensitive at lower temperatures

Question 32

What does atomic fluorescence require?

Answer 32

Atomic fluorescence employs a lamp to excite but the fluorescence emission is measured as a very highly selective and sensitive method

Question 33

What does each method suffer from?

Answer 33

Each of the method suffers from signal stability issues and interferences