1. Double-Beam AAS Single-Beam I0 Double-Beam Isample
Skoog Principles of Instrumental Analysis
What is the problem with just measuring Isample/I0?

2. Background Sources of Background: scattering or molecular emission
Background Correction:
With blank sample
Ac = At - Ablank
Deuterium lamp (arc in deuterium atmosphere;
continuum nm)
 absorption of deuterium lamp represents Abackground
 absorption of HCL radiation represents At
Advantage over blank sample: observe fluctuations in flame
Culver et al, Anal. Chem., 47, 920, 1975.

3. Background Correction with Zeeman Effect
Lines differ by ~0.01 nm
Ingle and Crouch,
Spectrochemical Analysis

4. Background Correction with Zeeman Effect
Unpolarized light from HCL (A) passes through the rotating polarizer (B)
Light is separated into perpendicular and parallel components (C)
The light enters the furnace with an applied magnetic field, producing
3 absorption peaks (D)
Either analyte or analyte + matrix absorb light (E)
A cyclical absorbance pattern results (F)
Subtract absorbance during perpendicular half of cycle from absorbance
during parallel half of cycle to get the background-corrected value
Skoog, Principles of Instrumental Analysis

5. Background Correction with Zeeman Effect
DC on atomizer At Ab
AC on atomizer
DC on source
Ingle and Crouch, Spectrochemical Analysis

6. AAS: Figures of Merit
Linearity over 2 to 3 concentration decades (can be problem for
multielement analysis)
Probability for line overlap is small  Resolution not as critical
as for AES
Precision: Typically a few % for graphite furnace
0.3 to 1.0% for flame
Accuracy: Largely determined by calibration with standards
Applicability: Limited for certain elements for which flame or
furnace is not hot enough (e.g. W, Ta, Nb).
Flow rates of flame are compatible with HPLC flow rates.
Speed: Multielement analysis with multiple HCLs may require lamp exchanges to select desired elements. This is tedious and also costs light because of beam splitters.

7. Method of Standard Additions
In order to quantitate the element of interest in a sample, it is necessary to calibrate with the method of standard additions.
The analytical signal for the sample, Sx, is obtained (after measuring the blank signal).
A small volume, Vs, of a concentrated standard solution of known concentration, cs, is added to a relatively large volume, Vx of the analytical sample.
The analytical signal for the standard addition solution, Sx+s, is obtained.
cx = (SxVscs)/[Sx+s(Vx+Vs) – SxVx]
if Vs << Vx
cx = (SxVscs)/[(Sx+s – Sx)Vx]

8. Are you getting the concept?
The determination of Pb in a brass sample is done with AAS. The 50.0 mL original sample was introduced into the instrument and an absorbance of was obtained. To the original solution, 20.0 mL of a 10.0 mg/mL Pb standard was then added. The absorbance of this solution was Find the concentration of Pb in the original sample. What assumption(s) has been made in order to use a single standard addition?

9. AAS: Figures of Merit
Detection limits: * Generally lower LOD for very volatile elements
* Higher LOD for carbide-forming elements (e.g. Ba, B, Ca, Mo, W, V, Zr)
* Concentration in GF up to 1000 times higher than in flame; much lower LOD for GF.
* Lower LOD for GF-AAS than ICP-AES unless
atomization requires high temperature
* Generally similar LOD for flame-AAS and ICP-AES
* Improve LOD by adding ethanol or methanol to decrease droplet surface tension during nebulization
Chemical * HCl often avoided as acid in GF-AAS because
interferences: metal chlorides are more volatile than sulfates or
phosphates.
* Addition of Cs salt to sample suppresses ionization.
* La precipitates phosphate, facilitating Ca analysis.
* Proteins may clog burners and are precipitated with
trichloroacetic acid.

10. mA: Concentration giving rise to 1% absorption.

11. Atomic Fluorescence Spectroscopy (AFS)
See also: Fundamental reviews in Analytical Chemistry
e.g. Bings, N. H.; Bogaerts, A.; Broekaert, J. A. C. Anal Chem. 2002, 74, (“Atomic Spectroscopy”)
Late 1800’s - Physicists observe fluorescence from Na, Hg, Cd, and Tl
Alkemade uses AFS to study chemistry in flames
AFS recognized as an analytical tool

12. Fluorescence
Radiative transition between electronic states with the same multiplicity.
Almost always a progression from the ground vibrational level of the 1st excited electronic state.
10-10 – 10-6 sec.
Occurs at a lower energy than excitation.
Skoog, Hollar, Nieman, Principles of Instrumental Analysis, Saunders College Publishing, Philadelphia, 1998.

13. Types of Atomic Fluorescence
Resonance (a)
Excited State Resonance (b)
Stokes/Anti-Stokes Direct Line (c-f)
Stokes/Anti-Stokes Stepwise Line (g-l)
Sensitized (m)
Two-Photon Excitation (n)
Omenetto, N. and Windfornder, J. D., Applied Spectroscopy, 26(5), 1972,

14. Instrumentation Sources: HCL, laser (cw or pulsed), ICP, Xe arc lamp
stable
extremely high radiance at excitation wavelength
Atomizer: flame, plasma, furnace
high nebulization/atomization efficiency
for flame, minimize quenching (Ar<H2<H2O<N2<CO<O2<CO2)
Wavelength Selection: monochromator, filters
low dispersion monochromator or filter with line source
Detector: PMT

15. ICP-AF Spectrometer
Ingle and Crouch, Spectrochemical Analysis

16. LODs for AFS
Ingle and Crouch, Spectrochemical Analysis