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Preparation for CDOM absorption lab (spectrophotometry) Mary Jane Perry

Preparation for CDOM absorption lab (spectrophotometry) Mary Jane Perry.

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Preparation for CDOM absorption lab (spectrophotometry) Mary Jane Perry

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  1. Preparation for CDOM absorption lab(spectrophotometry)Mary Jane Perry

  2. Major absorbers in seawater are: * water * CDOM – chromophoric or color-absorbing dissolved organic matter * absorbing particlesCDOMhas an operational definition:* absorption of water that has been passed through a filter (0.2 m plastic filter or G/FF filter, nominally 0.7 m)– collodial gels– viruses– small prokaryotes– small eukaryotes that “wiggle” through filter* minus a blank and sometimes forced = zero at 715 nm

  3. Today’s lab: 2 ways to look at CDOM absorption* bench-top spectrophotometer, from 200–800 nm* in-water ac9, from 412–676 nm (8 wavebands)Examine:filter size (0.2 and ~ 0.7 microns); note freshly filteredscattering (with integrating sphere and ac9 “c” vs. “a” tube)role of blankscontribution of salts at UV wavelengthsmagnitude and spectral slope (S) as way to characterize– S (l–lREF)aCDOM() = aCDOM (REF) eplease correct handout, p. 6

  4. blanks

  5. Beer’s Law: within linear range, quantitative relationship with concentration: In = I0 exp-( c  L) In / I0 = exp-( c  L) ln (In / I0 ) = -( c  L) is the molar absorption coefficient (m2mole-1)c is the concentration of the dye (mole m-3) L is the path length that the light must travel (m) we combine  and c into a, absorption coefficient (m-1 )ln (In / I0 ) = -( c  L) = -(a L)plot:In / I0 vs. c ln In / I0 vs. c

  6. In = I0 exp-( c  L) In / I0 = exp-( c  L) ln (In / I0 ) = -( c  L) = -(a L)Chemists and spectrophotometersln (I0 / In ) = ( c  L)transform to log10: A = 0.434 ln(I0/In) = 0.434(aL)conversion between spectrophotometer and ac9:a = A (0.434  L)-1 = 2.304 A L-1(caution: L for the spec is in cm; convert to m)

  7. Spectrophotometer – example with single monochromatorIn lab, also spec with two monochromators; integrating sphere; and 1- and 10-cm pathlenths

  8. Variants* water 1. water collected beyond mouth of DRE 2. Damariscotta River Estuary water (dock) 3. Biscaye Pond freshwater* blanks – tap, RO, Milli-Q* filtered through G/FF (~0.7 mm) and 0.2 mm filters * pathlength (1 and 10 cm pathlength, vs. 25 cm with ac9)* integrating sphere for scatteringAssignments– coordinate data * with your group* among spec groups* between spec and ac9 labs

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