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, 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 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 1800’s - 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 (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