April04_2005_2pp

April04_2005_2pp - 1 April 4, 2005 ¾ Todays Office Hour:...

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Unformatted text preview: 1 April 4, 2005 ¾ Todays Office Hour: only until 1:50 pm 2 Still More “Why Lasers?” ¾ Temporal Resolution use pulsed lasers (μs – ps) • temporal discrimination between scatter and fluorescence • measure fluorescence lifetimes ¾ Spatial Resolution • focus laser to μm spot size • efficient measurements with microsamples • spatial mapping of sample surfaces 3 Ultra-Trace Analysis via LEMFS Determination of Polycyclic Aromatic Hydrocarbons (PAHs) • PAH’s: large ε , Φ ≈ 0.2 – 1, 10-100 ns fluor. lifetime • Example (using N 2-pumped dye laser, time-resolved): Detection Limits (PAHs in water): 9 Benzene – 19 ppb (10-7 M) 9 Napthalene – 1.3 ppt (10-11 M) 9 Anthracene – 8.9 ppt (10-11 M) 9 Pyrene – 0.5 ppt (10-12 M)-Limited by background fluorescence • 6 decades LDR 4 Emission Versus Absorbance? ¾ Let’s assume a measurement uncertainty of 0.001 For an emission measurement: Noise = 0.001 Blank = 0.000 Signal at detection limit = 3N = 0.003 Is this signal detectable via absorbance measurement?Is this signal detectable via absorbance measurement?...
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This note was uploaded on 05/15/2010 for the course CHEM 221 taught by Professor Joelgoldberg during the Spring '10 term at UVM.

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April04_2005_2pp - 1 April 4, 2005 ¾ Todays Office Hour:...

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