Harris Quantitative Analysis, Instrumental Analysis

1. Atomic Spectroscopy (Chapter 21)
Used for qualitative and quantitative determinations
Widely used in industrial settings
Detection limits: ppm, ppb
Convert sample to atoms
Determine atom conc. by measuring A or I
A = kC I=kC
Three classes:
absorption
emission
fluorescence

2. Atomic Spectroscopy (Chapter 21)
GS
ES
GS
ES
thermal
AAS AES

4. Atomic Spectroscopy (Chapter 21)
Step 1: Atomization
Volatilize, decomp. to form atomic gas
1. Convert solution to a mist
2. 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 solvent
4. Vaporize
5. Decompose to atoms

5. Why are spectra
so narrow?

6. Atomic Spectroscopy (Chapter 21)
Atomization Sources
1. Flame (AA, AE)
2. Furnace (AA)
3. Plasma (AE)
A. Flame
pneumatic nebulizer
burner
flame temperature is impt (1700 – 3100 C)

10. T ~ 2500 K
Smaller volume (µL)
Greater sensitivity
Longer residence times
Less reproducible, more complicated
why?

12. Very high Temperature (6000 - 10,000K)
Large ES population
Reduced interference
C. Plasma
ICP
why?

13. Atomic Spectroscopy (Chapter 21)
Temperature
very important
degree at which sample breaks down
determine the percent of atoms in GS, ES, or ionized
N*
/ No = g*
/ go exp (-∆E/kT)
If T = 2500 K (λ = 500 nm) N*
/No = 10-5
If T = 6000 K N*
/No = 10-2
AE: I depends of ES population
change T by 10 K, change N*
by 4%
AA: A depends of GS population
Change T by 10K, little change in N

14. Instrumentation
Source
Sample cell
Monochromator
Detector
Beers Law: “monochromatic”
molecule spectra – much broader than bandwidth
atomic spectra – much narrower than bandwidth
(0.001 – 0.01 nm)

15. 1. Need a special source: Hollow cathode lamp
element specific
why?

16. 2. Background Subtraction Methods
remove flame signal from atom

17. Interferences
spectral
chemical
ionization
Spectra
overlap of analyte signal with other signals
(molec – element); chose another λ, change T
Chemical
chemical reactions decrease conc of of analyte
example: Ca2+
in presence of SO4
2-
add a releasing agent (EDTA) or change temp
Ionization
ionization of atoms decrease conc. of neutral atoms
ex. Kg → K+
+ 1e-
Add an ionization supressor (Csg → Cs+
+ e-
)

18. Detection Limits
Depends on element and method
Element ICP Flame E Flame A Furnace
Ca 0.02 0.1 1 0.02
Fe 0.3 30 5 0.005
Na 0.20 0.1 2 0.0002
units: ng/mL
What is better, big number or small number?