Geology Reference
In-Depth Information
a good example are the porphyrins (of which chlorophyll is a member).
These provide useful 'light sensitive' systems that can harvest light
energy and in the case of chlorophyll perform the vital role of photo-
synthesis.
Absorption of light at a given wavelength (A
l
) by a solution of a
chromophoric molecule can be treated quantitatively according to the
Beer-Lambert law. The ratio of the light intensity transmitted through
the chemical solution (I
l
) to the incident light intensity (I
o,l
) can be
related to the chemical concentration (C) according to,
A
l
¼
log
I
l
I
o
;
l
¼
e
C
l
ð
6
:
23
Þ
where C is expressed as molar concentration (M), e the decadic (log)
molar absorption coefficient (M
1
cm
1
) and l the light path length in
the chemical solution (cm). I is often expressed in units of Einstein cm
2
s
1
, (where an Einstein is 6.02
10
23
photons or a 'mole' of photons)
with absolute light irradiance (expressed as Joules s
1
m
2
or Watts
m
2
) measured using a chemical actinometer for a specified wavelength
range.
6.6.2 Photolysis in Aqueous Systems
Light absorbance studies can be conducted using a UV/vis spectropho-
tometer programmed to irradiate a solution at individual wavelengths
over part, or all, of the solar spectrum. Quartz-glass cuvettes (l
¼
1 cm)
are usually employed which hold the chemical solution and a reference
solution (i.e. typically the solvent but without the chemical to assess the
absorbance artefacts of the holding solvent). Water is the solvent used
for environmental fate studies, but apolar or monopolar organic com-
pounds (including many pesticides) are only sparingly soluble in water
so an organic solvent - water solution is often used, where the organic
solvent is strongly polar and water-miscible such as methanol or aceto-
nitrile. Figure 6 shows the spectral absorbance of chlorpyrifos (a cur-
rently used organophosphorus insecticide) in pure methanol between the
wavelengths 200-400 nm. Chlorpyrifos displays several absorbance
maxima, but within the UV-B/UV-A range of the solar spectrum the
maximum absorbance occurs at 288 nm and e288 nm can be calculated
by rearranging Equation 6.23 to:
e
288 nm
¼
A
288 nm
ð
6
:
24
Þ
C
l
