Double-Beam AAS

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 200-380 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, Sxs, is obtained.

cx (SxVscs)/Sxs(VxVs) SxVx
if Vs ltlt Vx
cx (SxVscs)/(Sxs 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 0.420 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
0.580. 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, 2691-2712
  (Atomic Spectroscopy)

• Late 1800s - Physicists observe fluorescence
  from Na, Hg, Cd, and Tl
• 1956 - Alkemade uses AFS to study chemistry in
  flames
• 1964 - AFS recognized as an analytical tool

www.andor.com
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, 555-557.
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
  (ArltH2ltH2OltN2ltCOltO2ltCO2)
• 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