Presentation on theme: "Atomic Spectroscopy ZEnergy Level Diagrams ZSample Introduction ZSources for Atomic Absorption ZHollow Cathode Lamps ZSources for Atomic Emission ZFlames."— Presentation transcript:
Atomic Spectroscopy ZEnergy Level Diagrams ZSample Introduction ZSources for Atomic Absorption ZHollow Cathode Lamps ZSources for Atomic Emission ZFlames ZPlasmas ZFurnaces ZWavelength Separators ZComparison of Techniques ZFAAS vs. ETAAS vs. ICP-AES I would encourage you to read the following on reserve in Milne. Inductively Coupled Plasma and Its Applications 1-28, 71-92 An Introduction to Analytical Atomic Spectrometry 1-47, 115-125
Sample Introduction (common)
Sample Introduction ZVenturi Effect and Atomization ZPneumatic Nebulizers (for ICP techniques) ZBreak the sample solution into small droplets. ZSolvent evaporates from many of the droplets. ZMost (>99%) are collected as waste ZThe small fraction that reach the plasma have been de-solvated to a great extent.
Flame AAS/AES Spray Chamber/Burner Configurations Flame AAS/AES Spray Chamber/Burner Configurations Samples are nebulized (broken into small droplets) as they enter the spray chamber via a wire capillary Samples are nebulized (broken into small droplets) as they enter the spray chamber via a wire capillary Only about 5% reach the flame Only about 5% reach the flame Larger droplets are collected Larger droplets are collected Some of the solvent evaporates Some of the solvent evaporates –Flow spoilers »Cheaper, somewhat more rugged –Impact beads »Generally greater sensitivity
In FAAS, a key consideration is the height above the burner that the analyte absorption is measured at (burner positions are adjustable)! Temperature value (adjusted using different fuel/oxidant ratios) and consistency are important.
ElectroThermal AAS (ETAAS, GFAAS) ZThe sample is contained in a heated, graphite furnace. ZThe furnace is heated by passing an electrical current through it (thus, it is electro thermal). To prevent oxidation of the furnace, it is sheathed in gas (Ar usually) ZThere is no nebulziation, etc. The sample is introduced as a drop (usually 10-50 uL) ZThe furnace goes through several steps… ZDrying (usually just above 110 deg. C.) ZAshing (up to 1000 deg. C) ZAtomization (Up to 2000-3000 deg. C) ZCleanout (quick ramp up to 3500 deg. C or so). Waste is blown out with a blast of Ar. ZThe light from the source (HCL) passes through the furnace and absorption during the atomization step is recorded over several seconds. This makes ETAAS more sensitive than FAAS for most elements.
Sources (for FAAS and ETAAS) ZHollow Cathode Lamps (HCL) are the main source. ZThese are element specific, constructed of the same element you are analyzing. ZA current is passed through the lamp, exciting the element of interest. As it returns to the ground state, it emits light which is focused through the sample. ZSince emission/absorption is quantized, this is the same wavelength of light that the analytes will absorb! ZMultielement lamps are available. ZLimited lifespan, treat carefully, do not exceed specified maximum current!
FAAS and ETAAS Considerations… ZTemperature level and consistency are key. ZAlignment of the source light is important. ZSince temperatures are relatively low, refractory species and excessive amounts of complexes can form in the flame ZHinder ATOMIC absorption ZAnalytes may also be lost by volatilization prior to the absorption of light. ZMatrix modifiers may overcome these two barriers. ZReduce oxide and oxyhydroxide formation ZReduce sample loss from volatilization ZComplex with interfering species (molecular) ZAmmonium chloride, palladium nitrate, magnesium nitrate, for example
ICP-AES (ICP-OES) ZInductively coupled plasmas are at least 2X as hot as flames or furnaces. ZThe Ar plasma is the result of the flow of Ar ions in a very strong, localized radio field. Z6000-10000 K are common plasma temperatures. ZHot enough to excite most elements so they emit light. ZHot enough to prevent the formation of most interferences, break down oxides (REEs) and eliminate most molecular spectral interferences. ZThe way to do atomic emission spectroscopy today.
Wavelength Separators for Atomic Spectroscopy ZMust be able to separate light that might be only a fraction of nm from the next nearest wavelength of light ZAtomic spectra are complex! ZUsually Czerny-Turner configuration or a modification of it. ZNeed to incorporate background correction ZAtomic spectra are complex with many possible spectral interferences ZLamp intensities may fluctuate ZFlame composition may fluctuate ZOver time on the same sample ZFrom sample to sample
Double Beam AAS Instruments account for instability in the source. Other techniques are added to account for scattering if light in the sample and the absorption of light by non-atomic species in the sample. In the example shown here, P/Po is alternatively recorded by the instrument to cancel out short-term lamp fluctuations.
Deuterium Background Correction: A D 2 lamp (continuum lamp) alternately passes light through the sample with the HCL. Analytes dont absorb much D 2 radiation since it is a continuum source. The light absorbed with the HCL light passes through the sample minus the light absorbed when the D 2 radiation passes through is the signal that is measured.
Wavelength Separators for ICP-AES ZMust have greater resolving power than those for AAS methods. ZPlasmas are hotter, therefore the spectra are more complex. ZICP techniques usually cover a wider range of wavelengths Z190 – 900 nm ZDifferent detectors for different wavelengths ZPMTs still #1 choice, but CCD arrays also common. ZThe light emitted by an ICP is also more intense! Slits must have greater adjustability….. ZSequential (scanning) ICP-AES instruments are now less common than simultaneous (like a diode array UV-VIS) instruments.
A Modern Sequential ICP with a Monochromator
A Modern Simultaneous ICP Design (most instruments sold now are simultaneous)
An Earlier Polychromator Design used in ICP Instruments