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Analytical Chemistry –Atomic absorption Spectroscopy

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1 Analytical Chemistry –Atomic absorption Spectroscopy

2 IIntroduction to Atomic Spectroscopy
AAtoms exhibit interactions with EM radiation. We will discuss some of the spectroscopic methods used in their determinations. An absorption of a photon causes an excited state, where an atom 's electron goes from a lower energy state to one of higher energy. As in molecular spectroscopy, we can monitor the difference in the absorption or monitor the subsequent photon emission. Let's look at the excitation of an atom by energy absorption. As review from Freshman Chemistry: Remember the energy levels allowed in an atom: 1s,2s,2p,3s,3p,4s,3d,4p, etc.   Shorthand Notation 1H 1s1 6C 1s2 2s2 2p2 28Ni 1s2 2s2 2p6 3s2 3p6 4s2 3d8 Filled Shell Configuration 6C [He] 2s2 2p2 28Ni [Ar] 4s2 3d8

3 Every element has its own signature of energy levels where certain quantized energies are required to cause excitations, i.e. the energy spacings are different. Excited atoms may lose energy and emit a photon of light and return to a lower energy state. An atom's emission of the quantized wavelengths of light provide a series of narrow bands termed line spectra. Atomic instrumental methods typically monitor either the atomic specie's absorption or emission of light energy. the emission spectra for Na and H in the visible region are shown.

4 Figures of Merit · Precision: 1-2%
Figures of Merit ·  Analyte concentrations: part per million (ppm) to part per trillion (ppt) level ·  Precision: 1-2% ·  Because spectrum is so sharp(narrow), little overlap of different elements (can do multiple analysis). ·   Able to Measure over 60 elements in a sample. Atomic spectra produce optical spectra of gaseous atoms with bandwidth ~0.001 nm; whereas molecular species have optical spectra with bandwidth ~100 nm,

5 Two methods used in atomic spectroscopy.
i) Atomic absorption spectroscopy (AAS) AA Instrumentation: 1) hollow-cathode lamp 2) monochromator 3) flame or graphite furnace(sample cell) 4) Beam chopper {not shown (modulation)} 5) detector 6) read out display

6 Ar°--->Ar+--->M°--->M*--->M° + 
The flame converts the sample to atomic vapor and the atoms absorb light produced from the Hollow cathode lamp. The sample cell can be either the burner flame or a graphite furnace! The monochromator allows the selection of the wavelength of light that reaches the detector. There is no reference cell, the resonance line light is modulated using a mechanical chopper which alteranetly blocks the light beam reaching the detector. Thus the burner flame light reaching the detector can be compensated. The detector typically is a PMT. A working calibration curve involving standards is used to determine analyte concentrations. Can use a dual beam instrument, where the lamp source light is split. The sample light beam is directed through and reference beam around the sample cell. In this setup, the baseline should be more stable due to decrease in source intensity fluctuations of the instrument. The AAS light source: Hollow cathode Lamp: Since atomic absorption is a Quantized process, we need light at specific narrow line wavelength. Therefore, the HC lamp must contain the element we are trying to determine. The lamp consists of inert gas, an anode and a cathode sealed in quartz glass. The gas is ionized, which collide with the metal and produce a sputtering effect. These excited metal atoms give the light energy, which the sample analyte can absorb. [-]  e-  [+] ---> cathode anode Ar°--->Ar+--->M°--->M*--->M° + 


8 1) Flame burner head cell path--
AA sample cells:




12 ·        A liquid sample is introduced into an atomizer whose temperature is 2000–3000K.
·        Sample, oxidant, and fuel are combined and nebulized–broken into small droplets ·        Droplets entering the flame lose their water through evaporation; then the remaining sample vaporizes and decomposes into atoms ·        The most common fuel-oxidizer combination is acetylene and air (flame temp: 2400–2700K) ·        Hotter flames are needed for refractory elements (those with high vaporization temperatures) or to decompose species such as metal oxides or hydroxides. ·        When a hotter flame is required, the acetylene–nitrous oxide combination is usually used (flame temp: 2900–3100K) ·        Sample amounts: minimum needed is 1–2 mL. Graphite furnace cell path--

13 · The GF Instrumental configuration is difficult to operate.
Furnaces Graphite furnace offers greater sensitivity than a flame and requires a smaller volume of sample. A graphite furnace (at ~2000K) confines the atomized sample in the optical path for a residence time of several seconds resulting in high sensitivity. Sample amounts: minimum needed is 1–2 µL. ·        The GF Instrumental configuration is difficult to operate. ·        Monochromator: wavelength selection ~ diffraction grating Detector: photomultiplier tube

14 Standards Std1  1.0 ppm Abs 0.046 Std2  3.0 ppm Abs 0.168
Data: Calibration Curve: Usually make a "best fit" plot using different concentrations of standards and their absorbances. For example: Copper in sample: using standards and linear regression curve. Standards Std1  1.0 ppm Abs 0.046 Std2  3.0 ppm Abs 0.168 Std3  5.0 ppm Abs 0.281 Std4  7.5 ppm Abs 0.401 Std5  10 ppm Abs 0.529 unknown reading: Abs=  Conc = ? What if we had taken an absorbance reading of 0.769? Is this a problem? What would you suggest we do?

15 ii. Emission, Inductively Coupled Plasma

16 N*/No = (g*/go) e-E/RT
(ii) Atomic emission spectroscopy (AES): overview Similar to AAS but no light source (HC) is needed. Some of the atomized atoms are promoted to excited electronic states by collisions with other atoms. The excited atoms emit their characteristic radiation as they return to their ground state. ·        an emission technique ·        Allows simultaneous analysis of different species. ·        Several methods of excitation are possible: flame, a spark, or Inductively coupled Plasma (ICP). The other methods for excitation are not as efficient in exciting the analyte (atoms) as ICP. The analyte signal increases as the number of same excited atoms increases. As According to the Boltzman Distribution; N*/No = (g*/go) e-E/RT COMPARISON OF AAS vs AES Atomic absorption (AAS) an absorption technique one analyte at a time cheaper analysis than AES requires analyte specific HC lamp source can use a flame or graphite furnace to heat sample works with metals and a few metalloids some matrix effects less signal variation due to temperature effects for a graphite furnace, it confines the sample to light path for longer times giving enhanced signal, greater sensitivity, less sample requirements

17 Atomic Emission (AES) AES is more expensive ICP instrumentation somewhat more complicated to operate AES require less sample preparation and gives simultaneous analysis doesn't require different lamps( light sources). Uses PDA or a series of detectors to monitor different wavelengths (different element ID's)

18 Interferences in Atomic Absorption Spectroscopy
The detection limit is the concentration of an element that gives a signal equal to twice the peak-to-peak noise level of the baseline. IInterference is any effect that changes the signal when analyte concentration remains unchanged. Types of Interferences 1. Spectral – unwanted signals overlapping analyte signal 2. Chemical – chemical reactions decreasing the concentration of analyte atoms 3. Ionization – ionization of analyte atoms decreases the concentration of neutral atoms What about interferences? How do we over come interferences? Chemical interference -- most common in AA, due to a thermally stable compound present, which influences the analyte's energy absorption. Two ways of controlling: use hotter flame or add a releasing agent (something that ties up the competing species). Ionization interference-- flame too hot causes ionization of analyte. Add alkali metals, which have lower ionization potentials to suppress analyte ionization. Matrix interferences -- due to nature of sample, the junk in it! Dilute the sample if possible to reduce effects or use "method of standard addition", the sample is spiked with a standard, readings plotted and the concentration is extrapolated from plot.

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