Atomic Spectroscopy Flashcards
What is atomic spectroscopy
We measure what elements are in the sample and how much (thier ratios)
It sensitive (detects things even at very low concentrations like ppm and also sub ppb)
Why is the monochromater placed AFTER the sample for atomic spec
The sample is first turned to atoms using a flame atomizer
The flame also emits light so the monochromator needs to be after the sample and the flame
What is a line source and why do we use it
The atomic absorption bandwidths are very narrow so to make sure the indecent light bandwidth is less than the absorbance bandwidth we use a lines source
Ex hollow cathode lamp
What is a hollow cathode lamp
It has a hollow cathode that’s made of the same metal we are analyzing
This make it so that is generated the same wavelength that the metal absorbs
A voltage to the lamp ionizes the gas in the lamp and the ions hit the cathode and split off the metal from it and hit the glass
What are the 3 main ways to do atomization (get sample in instrument and into atoms)
Flame
Furnace
Plasma
What is flame atomization
The liquid sample is sent through a nebulizer, the biggest droplets fall out
air carry’s the remaining droplets across the bent path to the flame
The solution gets evaporated then goes into the flame and gets vaporized then decomposed then oxidized then ionized
Wasteful as 5% of sample is lost to the flame
What’s the hottest part of the flame
The interconal layer (1200 degrees Celsius)
What is the stociometric flame
Best flame for absorption
Has a double cone
No soot or excess O2
What is used as the oxidizer for flame atomization
Oxy-acetylene
or NO2
What are graphite furnaces
Less precise than flame, but they are more sensitive
The atoms spend more time in the optical path (more of them get measured)
The sample isn’t lost to nebulization (doesn’t get drained)
Solids can be added directly (not just liquids)
What are the step to how a graphite furnace works
Drying (drying the sample at 125 degrees for 20s)
Charring (decomposing the sample)
Atomizing (atomized the sample, higher temp better)
Cleaning (cleans out remaining residue)
For graphite furnaces, when do you actually measure the analytical signal
The absorbance is measure during the atomizing step
What is inductively coupled plasma
Helps with making ionized gas (like Ar and Ar+)
The flame it at a temp close to the temp of the suns surface
Best for emmision because the high temp atomizes and the remaining energy is used for excitation
How is the Ar plasma made
The sample goes in through a sample aerosol inlet,
cooling gas inlet inserts cool air flow and keeps the plasma in the middle of the tube and insulates the rest of the torch
The radio frequency load coils (made of hollow cu tubes) send in photons and RF to kick an electron out of Ar to make Ar + + e-
These ions hit other Argon’s and form more ions to eventually make plasma
Which atomizer is best for atomic absorbance
We want atoms in the ground stat so a cooler flame or furnace atomizer
Which atomizer is best for atomic fluorescence
Want atom to start in ground state
Flame or furnace but flame is best
Which atomizer is best for atomic emmision
Either a hot (N2O - acet flame)
Or inductively coupled plasma (BEST)
What is the Doppler affect for atomic line widths
The particles travel and emit light but the light compressed near/at the direction the particle is traveling
What is pressure broadening for atomic line widths
Increased collisions between gasses decrease the lifetime of the atoms excites state and change/share their velocities
What do the Doppler effect and pressure broadening help with
They give measurable line widths for atomic lines
What detector do you use for single element analysis in atomic absorption
A photomultiplier tube which helps cause it has a most sensitivity
What detector do you use for multi element analysis in atomic emmision
A photodiode array to let simultaneous measurement of many wavelengths
What are the types of interferences in in atomic spec
Spectral interference
Chemical
Ionization
What is spectral interference
The atoms with overlapping absorbance have their peaks too close together to analyze
What is chemical interference
The formation of oxides or refractory (non breaking down) compounds affect the signal
Can use releasing against to stop this
What is ionization interference
The formation of ions at a high temp reduces the amount of neutral atoms
What is x ray fluorescence
- Use an X-ray to kick electron out of the core shell of the atom
- The core is now unstable so the electron for the out shells (K alpha nearest and beta second nearest) go to centre and release a photon as they come in
Works best on heavier elements
What is a bremsstrahlung (broad hump)
Point on the x ray fluorescence graph where everything is accessible (all ions aplha and beta)
