Atomic Fluorescence - Analytical Chemistry II - Lecture Slides, Slides of Analytical Chemistry

Download Atomic Fluorescence - Analytical Chemistry II - Lecture Slides and more Slides Analytical Chemistry in PDF only on Docsity! Atomic Absorption & Atomic Fluorescence Spectrometry • Sample Atomization • Atomic Absorption (AA) • Atomic Fluorescence (AF) - Both AA and AF require a light source - Like Molecular Absorption & Fluorescence, in AA high intensity is NOT required, in AF high intensity results in greater sensitivity docsity.com AA occurs in flame, plasma, etc. Molecular emission is generally unwanted & can cause interference AE AE docsity.com docsity.com Graphite YY Internal gas flow f furnace Window To spectro- photometer Light beam External gas flow (a) Graphite tube Platform (b) Figure 9-6 (a) Cross-sectional view of a graphite furnace. (Courtesy of the Perkin-Elmer Cor- poration, Norwalk, CT.) (b) The L'vov platform and its position in the graphite furnace. B Sample introduction for solutions: 1) Pneumatic nebulizers 2) Ultrasonic nebulizers 3) Electrothermal vaporizers 4) Hydride generation 3 BH4- + 3 H+ + 4 H3AsO3
3 H3BO3 + 4 AsH3 + 3 H2O 5) Cold vapor generation Hg2+ + Sn2+
Hgo + Sn4+ docsity.com There are many possible variations for the hydride generation apparatus docsity.com Normal or idealized situation for the absorption of source radiation in AA Problems arise if the sample absorption is interfered with by a matrix component - sloped baseline - overlapping molecular band Background correction is then needed docsity.com Normally assume baseline is flat not structured. In the absence of peak would have flat baseline Peak height easily measured What if baseline is sloped? How is peak height measured? Need measurement of baseline here & here What if the baseline is really a mess? Use Background Correction docsity.com The AA source (HCL or EDL) tells us the absorbance at the λ of interest Using another light source will allow us to determine the background absorbance Typically we are interested in points on either side of the peak docsity.com The Zeeman effect splits the absorption peak in a magnetic field & shifts absorption to higher & lower wavelength. The new absorption peaks interact differently with polarized light allowing analyte & background absorbance to be measured docsity.com The Smith-Hieftje technique splits the HCL line Self-reversal or Self-absorption docsity.com Another type of matrix interference not alleviated by background correction involves variable amounts of analyte ionization in flames or plasmas docsity.com TABLE 9-3 Detection Limits (ng/mL)* for Selected Elements} AASE AASS§ AESE AES AFSE Element Flame Electrothermal Flame ICP Flame Al 30 0,005 2 2 5 As 100 0,02 0.0005 40 100 Ca 1 0.02 0.1 0.02 0.001 Cd 1 0.0001 800 2 0.01 Cr 3 0.01 4 0.3 4 Cu 2 0.002 10 0.1 1 Fe 5 0.005 30 03 8 Hg 500 0.1 0.0004 1 20 Mg 0.1 0.00002 5 0.05 1 Mn 2 0.0002 3 0.06 2 Mo 30 0.005 100 0.2 60 Na 2 0.0002 0.1 0.2 oad Ni 5 0.02 20 04 3 Pb 10 0.002 L100 Zz, 10 Sn 20 0.1 300 30 50 Vv 20 0.1 10 0.2 70 Zn 2 0.00005 0.0005 2 0.02 *Nanosram/milliliter = 10-3 ue/mL = 1073 ppm. docsity.com Chapter 10: Emission Spectroscopy Using Plasmas, Arcs or Sparks • Inductively Coupled Plasma (ICP) • Direct Current Plasma (DCP) • Arcs and Sparks docsity.com Sample aerosol or vapor in argon Radio-frequency induction coil Tangential argon plasma support flow Inductively Coupled Plasma (ICP) docsity.com ICP Temps. The viewing area for each element is typically reported as mm above the load coil docsity.com Direct Current Plasma (DCP) Torch Lab will involve use of DCP for multielement analysis docsity.com Aperature plate Computer Echelle adj. source grating mirror Plane mirror Entrance slit f Source Figure 10-7 Schematic of an echelle polychromator system. Collimating docsity.com Concave diffraction _ grating a Secondary optics Photomultipler detectors “Vet Movable , x “ks primary | = Measuring electronics Dedicated System ICP source electronics Gas flow regulation computer Instrument control electronics Conditioning Sample introduction Figure 10-8 Schematic of an ICP polychromator. (Courtesy of Thermo Jarrell Ash Corp.) docsity.com Parabolic collimator Visible Schmidt prism cross-dispenser docsity.com Visible detector UV detector 4 ICP torch Figure 10-11 An echelle spectrometer with segmented array of charge-coupled devices. (From T. W. Bamard et al., Anal. Chem., 1993, 65, 1232. With permission.) @  Characterization of the Detection Power of ICP-AES Detection limit (ng/mL) Number of lines Bo _|1-2 oe] 3-6 (i) 10-30 30-100 100-300 11-16 H He Li EN} G F |.Ne Na may) P | S| Cl-Ar i Ga} Ge|As| Se) Br| Kr | Ro Nb Te | Ru] Rh| Pd | Ag In} Sno Sbi.Te I | Xe Cs Ir | Pt Hg | Tl | Pb | Bi | Po} At| Rn aus Fr | Ra] Ac” | Ce | pr | Na Th | Pa Kay! Figure 10-13 Periodic table characterizing the detection power and number of useful emission lines of ICP by employing a pneumatic nebulizer. The degree of shading indicates the range of detection limits for the useful lines. The area of shading indicates the number of useful lines. docsity.com & docsity.com Counter Electrodes for electrodes holding sample Figure 10-16 Some typical graphite electrode shapes. Narrow necks are to reduce thermal conductivity. a - 3 , “ Reflecting N 7 i prism “ Long 7 : wavelengths Photographic plate or film Short wavelengths | mp \e@@ Slit | | Lens Eagle Mount Source docsity.com  {TMI oi Jyporr mus NaZaNa ZnAg CuGeSnCdinCu Figure 10-18 Projected spectra by a comparator-densito- meter: (a), (b), and (c) spectra of sample at three different exposures; (d) iron spectrum on the sample plate; (e) and (f) iron spectra on the master plate. docsity.com ABLE 10-2 Effect of Standardization Frequency on Precision of ICP Data* Relative Standard Deviation, % Concentration Multiple above Detection Limit docsity.com Frequency of Recalibration, hr 10! to 107 10? to 103 10% to 104 104 to 105 05 3-7 1-3 1-2 1.5-2 2 5-10 2-6 1.5-2.5 2-3 8 8-15 3-10 3-7 48 from: R. M. Bares, in Applications of Inductively Coupled Plasmas to Emission Spectroscopy, R. M. Barnes, Ed., p. 16. Philadelphia: The Franklin Institute tess, 1978. With permission. docsity.com E 10-3 Comparison of Detection Limits for Several Atomic Spectral Methods* Number of Elements Detected at Concentrations of <1 ppb 1-10 ppb 11-100 ppb 101-500 ppb >500 ppb coupled plasma emission 9 32 14 6 0 nic emission 4 12 19 6 19 nic fluorescence 4 14 16 4 6 ic absorption 1 14 25 3 14 = a limit ‘correspond to a signal that is twice as great as the standard deviation for the background noise. Data abstracted with permission from V. A. Fassel ley, Anal. Chem., 1974, 46(13), 11114, Copyright 1974 American Chemical Society.