Presentation on theme: "Atomic Spectroscopy Energy Level Diagrams Sample Introduction"— Presentation transcript:
1Atomic Spectroscopy Energy Level Diagrams Sample Introduction Sources for Atomic AbsorptionHollow Cathode LampsSources for Atomic EmissionFlamesPlasmasFurnacesWavelength SeparatorsComparison of TechniquesFAAS vs. ETAAS vs. ICP-AESI would encourage you to read the following on reserve in Milne.Inductively Coupled Plasma and Its Applications 1-28, 71-92An Introduction to Analytical Atomic Spectrometry 1-47,
7Sample Introduction Venturi Effect and Atomization Pneumatic Nebulizers (for ICP techniques)Break the sample solution into small droplets.Solvent evaporates from many of the droplets.Most (>99%) are collected as wasteThe small fraction that reach the plasma have been de-solvated to a great extent.
10Flame AAS/AES Spray Chamber/Burner Configurations Samples are nebulized (broken into small droplets) as they enter the spray chamber via a wire capillaryOnly about 5% reach the flameLarger droplets are collectedSome of the solvent evaporatesFlow spoilersCheaper, somewhat more ruggedImpact beadsGenerally greater sensitivity
12In 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.
15ElectroThermal AAS (ETAAS, GFAAS) The sample is contained in a heated, graphite furnace.The 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)There is no nebulziation, etc. The sample is introduced as a drop (usually uL)The furnace goes through several steps…Drying (usually just above 110 deg. C.)Ashing (up to 1000 deg. C)Atomization (Up to deg. C)Cleanout (quick ramp up to 3500 deg. C or so). Waste is blown out with a blast of Ar.The 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.
17Sources (for FAAS and ETAAS) Hollow Cathode Lamps (HCL) are the main source.These are element specific, constructed of the same element you are analyzing.A 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.Since emission/absorption is quantized, this is the same wavelength of light that the analytes will absorb!Multielement lamps are available.Limited lifespan, treat carefully, do not exceed specified maximum current!
19FAAS and ETAAS Considerations… Temperature level and consistency are key.Alignment of the source light is important.Since temperatures are relatively low, refractory species and excessive amounts of complexes can form in the flameHinder ATOMIC absorptionAnalytes may also be lost by volatilization prior to the absorption of light.Matrix modifiers may overcome these two barriers.Reduce oxide and oxyhydroxide formationReduce sample loss from volatilizationComplex with interfering species (molecular)Ammonium chloride, palladium nitrate, magnesium nitrate, for example
20ICP-AES (ICP-OES)Inductively coupled plasmas are at least 2X as hot as flames or furnaces.The Ar plasma is the result of the flow of Ar ions in a very strong, localized radio field.K are common plasma temperatures.Hot enough to excite most elements so they emit light.Hot enough to prevent the formation of most interferences, break down oxides (REEs) and eliminate most molecular spectral interferences.The way to do atomic emission spectroscopy today.
26Wavelength Separators for Atomic Spectroscopy Must be able to separate light that might be only a fraction of nm from the next nearest wavelength of lightAtomic spectra are complex!Usually Czerny-Turner configuration or a modification of it.Need to incorporate background correctionAtomic spectra are complex with many possible spectral interferencesLamp intensities may fluctuateFlame composition may fluctuateOver time on the same sampleFrom sample to sample
27Double Beam AAS Instruments account for instability in the source 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.
28Deuterium Background Correction: A D2 lamp (continuum lamp) alternately passes light through the sample with the HCL. Analytes don’t absorb much D2 radiation since it is a continuum source. The light absorbed with the HCL light passes through the sample minus the light absorbed when the D2 radiation passes through is the signal that is measured.
29Wavelength Separators for ICP-AES Must have greater resolving power than those for AAS methods.Plasmas are hotter, therefore the spectra are more complex.ICP techniques usually cover a wider range of wavelengths190 – 900 nmDifferent detectors for different wavelengthsPMTs still #1 choice, but CCD arrays also common.The light emitted by an ICP is also more intense! Slits must have greater adjustability…..Sequential (scanning) ICP-AES instruments are now less common than simultaneous (like a diode array UV-VIS) instruments.