Preparation for CDOM absorption lab (spectrophotometry) Mary Jane Perry

Major absorbers in seawater are: * water * CDOM – chromophoric or color-absorbing dissolved organic matter * absorbing particles CDOM has 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

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 filtered scattering (with integrating sphere and ac9 “c” vs. “a” tube) role of blanks contribution of salts at UV wavelengths magnitude and spectral slope (S) as way to characterize – S ( – REF ) a CDOM ( ) = a CDOM ( REF ) e please correct handout, p. 6

blanks

Beer ’ s Law: within linear range, quantitative relationship with concentration: I n = I 0 exp-(  c  L) I n / I 0 = exp-(  c  L) ln (I n / I 0 ) = -(  c  L)  is the molar absorption coefficient (m 2 mole -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 (I n / I 0 ) = -(  c  L) = -( a  L) plot: I n / I 0 vs. c ln I n / I 0 vs. c

I n = I 0 exp-(  c  L) I n / I 0 = exp-(  c  L) ln (I n / I 0 ) = -(  c  L) = -( a  L) Chemists and spectrophotometers ln (I 0 / I n ) = (  c  L) transform to log 10 : A =  ln(I 0 /I n ) = 0.434( a  L) conversion between spectrophotometer and ac9: a = A  (0.434  L) -1 =  A  L -1 (caution: L for the spec is in cm; convert to m)

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

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  m) and 0.2  m filters * pathlength (1 and 10 cm pathlength, vs. 25 cm with ac9) * integrating sphere for scattering Assignments – coordinate data * with your group * among spec groups * between spec and ac9 labs