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Double-Beam AAS. Single-Beam. I 0. Double-Beam. I sample. Skoog Principles of Instrumental Analysis. What is the problem with just measuring I sample /I 0 ?. Background. Sources of Background: scattering or molecular emission Background Correction: With blank sample
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Double-Beam AAS Single-Beam I0 Double-Beam Isample Skoog Principles of Instrumental Analysis What is the problem with just measuring Isample/I0?
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.
Background Correction with Zeeman Effect Lines differ by ~0.01 nm Ingle and Crouch, Spectrochemical Analysis
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
Background Correction with Zeeman Effect DC on atomizer At Ab AC on atomizer DC on source Ingle and Crouch, Spectrochemical Analysis
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.
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]
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?
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.
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
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.
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.
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
ICP-AF Spectrometer Ingle and Crouch, Spectrochemical Analysis
LODs for AFS Ingle and Crouch, Spectrochemical Analysis