Atomic Spectroscopy Flashcards
What is atomic spectroscopy?
Exciting valence electrons for absorption and emission spectroscopy
How to obtain atomic spectra?
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
Qualitative atomic spectra?
Pick out which elements are there
Quantitative atomic spectra?
Use standard and compare
Typical spectra of atoms?
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
Atomic emission spectroscopy AES background noise?
Zero background low noise, so much better choice
Atomic absorption spectroscopy AAS background noise?
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)
How to decide which atoms AES is good for?
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
Where are most atoms in the distribution?
Most atoms in ground state, getting to excited state is difficult, very high population of ground state signal is very strong and reliable
Energy gap for emission and absorption?
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
What is the effect of temperature?
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.
How does the Rydberg equation fit the spectra pattern?
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
Atomic spectroscopy in astronomy?
The visible spectrum offer a window into space based on atomic emission and absorption spectroscopies
Example of spectroscopy in astronomy?
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
Set up for AES and AAS?
Dilute solution of ions is sprayed at constant rate into a flame, sample is atomised and some is excited, intensity of emission is measured
AES intensity?
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)
How to get round that AES is not intense?
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
Temperature for AES?
Needs to be constant temperature and as hot as possible
constant conditions = steady reading
Temperature effects - quantify later
Different temperatures of different conditions?
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)
What is plasma?
A hot partially ionised gas - the sun is a giant plasma
What is inductively couples plasma AES?
Much more sensitive
Applicable to many more elements
How does intensity depend on Boltzmann distribution?
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
ICP-AES detection limits?
Lower values are better - minimum amount that can reliably be detected
What does intensity of emission depend on?
The intensity of emission depends on the number of atoms in the excited state, don’t see ant emission at room temperature
What temperature is needed to get emission?
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)
Limit of plasma?
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
AES how to perfumed practical measurements?
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
What is atomic spectroscopy?
Atomic spectroscopy is based on atomic energy levels that are filled with electrons
What does atomic emission require?
Atomic emission requires filling of excited states and dropping back of the electron into lower levels. Best at high temperatures for example in plasmas
What does the Boltzmann distribution allow?
The Boltzmann distribution allows emission intensities to be estimated from wavelength (easier in the red than in the blue)
What does atomic absorption require?
Atomic absorption requires a lamp to excite the atoms and can be much more sensitive at lower temperatures
What does atomic fluorescence require?
Atomic fluorescence employs a lamp to excite but the fluorescence emission is measured as a very highly selective and sensitive method
What does each method suffer from?
Each of the method suffers from signal stability issues and interferences
