Material since MT3 Flashcards
(73 cards)
What does atomic absorption determine?
It determines the atomic concentrations by measuring absorption or emission at a characteristic wavelength
How does atomic absorption work? what temp is the sample at?
Sample is vaporized at 2400-3400K
What are important characteristics of AA?
It is highly sensitive for most elements
It is able to distinguish one elements from another in complex samples.
It easily analyzes many samples automatically.
Very good precision, 1-2%
What needs to be done prior to analysis in AA?
The unknowns must be diluted down to ppm level if they are in ppb range; called preconcentration
What is the principle of AA?
Quantification of an element in a sample by absorption of radiation by gaseous atoms of the element.
Done by determining absorbance of a series of standards, creating a calibration curve.
It is an application of Beer’s Law
WHat are the components of an AA spectrometer.
Hollow cathode lamp- produces light of particular wavelengths.
Monochrometer- selects the wavelength that was the most intense.
Flame- Creates atomization of sample at temp of 2400-3400K. Where sample is introduced.
Monochromator- a second one removes the unwanted emissions and allows only the chosen wavelength.
Detector- Only one wavelength is received.
Amplifyer- amplifies the signal.
Computer- displays the data.
How is the sample atomized?
The liquid sample is drawn into the nebulizer by rapid flow of oxidant, which has higher pressure. The rapid flow creates a low pressure zone in the nebulizer, creating a suction that pulls the sample up the capillary tube into the nebulizer. The nebulizer creates a mist from the liquid sample. The sample then hits a glass bead, which breaks the small droplets into even smaller particles. Fuel enters here. The sample goes through baffles where large droplets are blocked and mixing of the smaple mist, fuel and oxidant occurs. After the baffles, in the spray chamber, further mixing occurs making the premix burner. The mixture then goes into the burner head and into the flame where an aerosol is created. Only 5% of the initial sample is present.
Looking at the flame, where do the gaseous atoms from?
At the very base of the flame, the darker blue, the solvent evaporates and the sample goes from liquid to gas. In the interconal layer, the hottest part of the flame, the gaseous atoms are formed. The sample goes from a gas to separated gaseous atoms.
Fuel/oxidant ratio and what it means
The ratio influences flame temperature, which affects the atomization efficiency and sensitivity of the analysis.
If fuel/oxidant > 1, reducong flame with a lower temp. This is good for analysis of elements prone to oxidation to avoid formation of stable, heat resistant oxides.
Fuel/oxidant <1, oxidizing flame with higher T. Not desirable bc it leads to formation of stable oxides.
Calcium analysis needs a slightly fuel rich flame to avoid formation of CaO.
Characteristics of a graphite furnace
This can be used instead of a flame.
- electrically heated
- Greater sensitivity
- Less sample needed than flame.
- Improved reproducibility
- More skilled required for operation.
How does the hollow cathode lamp work?
Cathode is negative and the anode is positive. This is a nonspontaneous process that need energy.
A high voltage is applied to ionize filler gas, and the lamp is maintained at a constant current by a lower voltage. The hollow cathode is made up of the element that is being analyzed. The anode is W or stainless steel. The anode and cathode are physically separated by an insulated disk. Filler gas, Ne or Ar are present at low pressure in the lamp and they are attracted by the cathode. The ions hit the cathode and metal ions in the gas phase are knocked out.
There is a glass or quartz window on the end of the lamp where the hv is exerted.
What is the process of sputtering.
High voltage applied to the filler gas, Ar or Ne ionizes them to Ar+ or Ne+. The ions are then attracted to the negative cathode and when they hit the surface of the cathode, metal ions are knocked out in the gas phase. The gaseous metal molecules in ground state then collide with high energy electrons, making them in the excited state. They don’t stay excited long and when they relax to ground state, energy is emitted in the form of light.
Why is the HCL hollow?
The fact that it is hollow gives it more surface area to improve efficiency. Also, because it is hollow, it allows electrons to be trapped inside, which increases the probability of sputtering to occur.
What is the purpose of the monochromator between the HCL and the flame?
To remove the unwanted emission wavelengths from the filler gas and any other unwanted emissions from the metal.
What is the purpose of the monochrometer between the flame and the detector?
It removes the scaterred light from the flame. The aerosol can scater the light.
Removes flame emission.
Removes emission from other unwanted excited species in the flame.
Ensures that the appropriate radiation reaches the detector. 422.67 nm for Ca.
How does the Czerny- Turner monochrometer work?
Polychromatic radiation enters the entrance slit and hits a concave mirror, which collimates the radiation that hits it. focusing it onto a reflection grating, which makes the radiation parallel. The reflection grating diffracts different wavelengths at different angles. The wavelengths are directed to another concave mirror, which focuses each wavelength at a different point on the focal plane. The right wavelength is the one that gets through the exit slit.
How does the beam chopping technique correct for background noise?
Usually comes after the first monochrometer. We see a rotating chopper between the lamp and the flame. light is being passed through to the flame, wwe can measure the emission when both the lamp and flame emission are present- the chopper isnt blocking, and also can meausre the emission from flame only- when the chopper is blocking. We then get a spectrum with two signals, we can subtract the flame emission from the flame and lamp emission, to get the analytical signal.
What are the two types of detectors in AA and give examples of each.
- Detectors based on the photoelectric effect.
Examples: Phototubes and photomultiplier tubes - Silicon-based, solid-state semiconductor devices
Examples: Silicon photodiodes/DAD, Charge-transfer devices, Complimentary Metal oxide semiconductors (CMOS). We used CMOS.
How does the phototube detector work?
Uses the photoelectric effect.
Light hits a photosensitive surface, which is the cathode. For the Ca example, Cs3Sb is an example of the photosensitive surface. When light hits, electrons are released and attracted to the positive anode, which collects the electric current, The current flows through a load resistor, and the voltage acorss the resistor is measured with a voltmeter. The voltage is then amplified and digitalized.
What affects the voltage in a phototube detector?
The more Ca atoms in the sample, the more absorption of light there is by Ca atoms in the flame, meaning less current is produced in the phototube and the digitalized voltage is lower.
Vo is the digitalized voltage when no sample is present. Vo>V always.
How does the photomultiplier tube detector work?
Works based on the photoelectric effect. light enters the tube and hits the photoemissive cathode. Electrons are then emitted from the cathode and a grill focuses the electrons towards the first dynode. More electrons are emitted from the dynode for each electron that hits ot. The electrons travel through the tube, hitting more dynodes, releasing more electrons each time it hits. There are 8-9 dynodes and at the end, the electrons reach the anode. Each dynode is 90 volts more positove than the last.
What is the composition of the dynodes?
usually Mg, Be or Mo coated with Cs2O, which makes it more effective at releasing electrons.
What are the properties of the photomultiplier tube?
The signal processing is done externally- same as phototube.
The current produced at the anode is converted to a voltage and measured. The voltage is then amplified and digitalized.
V is known as the digitalized voltage with sample present in flame
Vo is the digitalized voltage with no sample present in the flame.
Absorbance= log(Vo/V)= EbC
Characteristics of a silicon-based detector
Si is a semiconductor meaning that its conductivity is less than a metal but more than an insulator.
