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

Effect of Temperature

๏ Boltzmann Equation

๏ Example 8-2:

- Ratio of Na atoms in the 3p excited state at 2500 K (Acetylene/Air) = 1.72 10-4

- Change in number of excited atoms for 10 K = 4%

kT

E

P

P

N

N jj exp00

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

Sample introduction๏ Nebulization

- 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)

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

Solid Samples๏ Electrothermal

- 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.

๏ Glow Discharge

- low pressure plasma in contact with the sample sputters the sample atoms (~0.1mg/min)

๏ Direct Sample Insertion

- self evident

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

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 temperatureHydrogen-Air 2000-2100 C GC detectorsAcetylene-Air 2100-2400 C Best for non-refractory elementsAcetylene-N2O 2600-2800 C Best for refractory elements

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: 0.002-0.01 ppb

๏ Better because holds atoms in light beam longer

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 2000-3000C

๏ yields a transient signal (<1 second)

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)

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

background correction๏ two line

๏ continuum

๏ Smith-Hieftje (self reversal)

Zeeman background correction

๏ moderate field (10kGauss)

๏ transitions split to satellite and main

๏ absorption of atoms only occurs when polarization is parallel to magnetic field

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?

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)

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)

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