Atomic spectroscopy ch 20

Atomic Spectroscopy (Chapter 21)

Used for qualitative and quantitative determinationsWidely used in industrial settingsDetection limits: ppm, ppb

Convert sample to atomsDetermine atom conc. by measuring A or IA = kC I=kC

Three classes:absorptionemissionfluorescence

Atomic Spectroscopy (Chapter 21)

GS

ES

GS

ES

thermal

AAS AES

Atomic Spectroscopy (Chapter 21)

Step 1: Atomization

Volatilize, decomp. to form atomic gas

1. Convert solution to a mist2. Carry into source

CaCl2(soln) CaCl2(gas) Ca0(gas) + 2Cl0(gas)

Excitation: Ca0(gas) Ca*

(gas)

Emission: Ca*(gas) Ca0

(gas)

AA: measure ground state AE: measure excited state

3. Evaporate solvent4. Vaporize5. Decompose to atoms

Why are spectraso narrow?

Atomic Spectroscopy (Chapter 21)

Atomization Sources1. Flame (AA, AE)2. Furnace (AA)3. Plasma (AE)

A. Flamepneumatic nebulizerburnerflame temperature is impt (1700 – 3100 C)

T ~ 2500 KSmaller volume (L)

Greater sensitivityLonger residence timesLess reproducible, more complicated

why?

Very high Temperature (6000 - 10,000K)Large ES populationReduced interference

C. Plasma

ICP

why?

Atomic Spectroscopy (Chapter 21)

Temperaturevery importantdegree at which sample breaks downdetermine the percent of atoms in GS, ES, or ionized

N* / No = g* / go exp (-E/kT)

If T = 2500 K ( = 500 nm) N*/No = 10-5

If T = 6000 K N*/No = 10-2

AE: I depends of ES populationchange T by 10 K, change N* by 4%

AA: A depends of GS populationChange T by 10K, little change in N

Instrumentation

SourceSample cellMonochromatorDetector

Beers Law: “monochromatic”molecule spectra – much broader than bandwidthatomic spectra – much narrower than bandwidth

(0.001 – 0.01 nm)

1. Need a special source: Hollow cathode lampelement specific

why?

2. Background Subtraction Methodsremove flame signal from atom

Interferencesspectralchemicalionization

Spectraoverlap of analyte signal with other signals (molec – element); chose another , change T

Chemicalchemical reactions decrease conc of of analyteexample: Ca2+ in presence of SO4

2-

add a releasing agent (EDTA) or change temp

Ionizationionization of atoms decrease conc. of neutral atomsex. Kg K+ + 1e-

Add an ionization supressor (Csg Cs+ + e-)

Detection Limits

Depends on element and method

Element ICP Flame E Flame A FurnaceCa 0.02 0.1 1 0.02Fe 0.3 30 5 0.005Na 0.20 0.1 2 0.0002

units: ng/mL

What is better, big number or small number?