1. announcements and reminders
Homework #2 due Wednesday September 27
problems:
chapter 7(15(a,c,f and g) , 19 and 22)
chapter 8(4,5,7 and 8)
last lecture: optical instruments intro atomic spectroscopy
today’s lecture: atomic spectroscopy and start 1st inst: AAS

2. Effect of Temperature Boltzmann Equation Example 8-2:
Ratio of Na atoms in the 3p excited state at K (Acetylene/Air) =
Change in number of excited atoms for 10 K = 4%

3. Band and Continuum Spectra
Band spectra – the additional vibrational and rotation energy levels of simple molecules (e.g., CaOH) result in broad (a few nm) spectral features
Continuum Spectra – particulate matter within flame acts as a source of black body radiation

4. Sample introduction Nebulization Electrothermal Vaporizer
sample is converted into a mist of finely divided droplets (aerosol)
Electrothermal Vaporizer
sample is vaporized and atomized within a closed tube that is electrically heated at temperatures up to 3000 K (page 203)

5. Chemical conversion
convert sample into a volatile molecular species that is more efficiently atomized.
hydride generation (page 203)
As-cmpds + NaBH4 → AsH3
cold vapor AAS
Hg-cmpds and ions + SnCl2 → Hg0 US EPA Method 1631

6. Solid Samples Electrothermal Ablation
sample is vaporized and atomized within a closed tube that is electrically heated at temperatures up to 3000 K (page 203)
Ablation
high energy discharge (electrical spark or arc; laser pulse) causes a plume of particulate and vaporized matter to be ejected from a solid surface.

7. Direct Sample Insertion
Glow Discharge
low pressure plasma in contact with the sample sputters the sample atoms (~0.1mg/min)
Direct Sample Insertion
self evident

8. AAS Flame AAS: 1% precision possible (2-3% typical)
Useful for ~60 metals and metalloids
Detection limits 1-20 ppb typical, depends on element
Linear Dynamic Range: ~one order of magnitude
Matrix effects: significant
~$30,000

9. flame processes Oxidation: atoms metal oxides
(secondary combustion zone; yellow)
Ionization: neutral atoms ions
(interzonal region; measurement zone)
Excitation - atom is thermally excited into a higher energy state
Dissociation: molecules atoms (primary combustion zone; blue)
Volatilization: salt particles gaseous molecules
Desolvation: spray salt particles
Relative importance of these processes depends upon:
element
flame temperature
Hydrogen-Air C GC detectors
Acetylene-Air C Best for non-refractory elements
Acetylene-N2O C Best for refractory elements

10. electrothermal AAS
sample is vaporized and atomized within a closed tube that is electrically heated to 3000 K
add-on to flame AAS
Precision: 5-10%
Matrix effects: severe
Sample required: L
Detection limits: ppb
Better because holds atoms in light beam longer

11. electrothermal process
Drying- removes solvent
eg, 125C for 20 s
Charring (Ashing) – destroy organic matter
smoke cause strong signal (not related to concentration)
eg., 1400 C for 60 s
Atomize - rapidly vaporize sample and break it down to atoms
rapid heating to C
yields a transient signal (<1 second)

12. Chemical conversion
convert sample into a volatile molecular species that is more efficiently atomized
e.g., hydride generation
As-cmpds + NaBH4 ➔ AsH3
continuous signal (although the magnitude and temporal dependence will depend upon the reaction kinetics)

13. hollow cathode lamp
emits sharp atomic lines characteristic of the element from which the cathode is made
simplicity of HCL spectra means that a low resolution monochromator can be used

14. background correction
two line
continuum
Smith-Hieftje (self reversal)

15. Zeeman background correction
moderate field (10kGauss)
transitions split to satellite and main
absorption of atoms only occurs when polarization is parallel to magnetic field

16. Questions about AAS Question: why is monochromator after the flame?
Question: What about flame light that is at the same wavelength as is being measured?

17. AAS summary Advantages of AAS: very sensitive (ppb detection limits)
useful for a wide range of metals
relatively inexpensive (~$30,000)
When is AAS the preferred technique?
Low concentration of metal ions in aqueous solution
If you are interested in only a few metals (6)
If sample volume is very small (<1 mL), electrothermal atomic absorption is best
Disadvantages of AAS: limited dynamic range (need to know conc you are looking for)
Subject to numerous types of intereference (flame and furnace)

18. basic properties quantification of nearly 70 elements
ppb - weight percent
microliter or microgram (furnace) milliliter (flame)
biomedical, environmental, steel and metal, pharmacy, food, pollution and industrial monitoring
solid liquid or gas
minimal sample prep
sample prep 0sec to 24hrs, measurement (seconds (flame) minutes (furnace))
no chemical info (only elemental composition)
destructive
more appropriate for few elements at a time (not easily applied to multielement problems)
accuracy (homogeneous solution 5-10 x detn limit (accuracy ~1%)
(direct solids accuracy ~5-10%)
linear dynamic range usually 2 orders of magnitude (sometimes 3)

19. AAS costs simple no bkgd corr ($10k) full package ($100k)
lamps (single element($150-$400) multielement($300-$400)) last ~200hrs of continuous emission
gases typically run $120/year
graphite tubes ($20-$50/ea) last ~200 firings
heavily used system may cost $1-$2000/year for operations
full service contract typically will run $6000/yr