Geology Reference
In-Depth Information
S
OCH
2
CH
3
OCH
2
CH
3
N
Cl
O
P
Cl
Cl
Chlorpyrifos
λ
max
=288nm
UV-region
of natural sunlight
Wavelength(nm)
Figure 6 Spectral absorbance of the insecticide chlorpyrifos (in methanol) over UV
wavelengths. For the environmentally relevant range of the UV spectrum (UV-
A/B), l
max
occurs at 288 nm, with significant absorbance at higher wavelengths
(to 310 nm), suggesting natural sunlight is capable of inducing phototransfor-
mation (thanks to J. Weber, Lancaster University, for generating the absorb-
ance spectrum)
From Figure 6, A
288 nm
¼
0.49 and was generated with C as
B
0.1 mM
(or 1
10
4
M) and l
¼
1 cm, the value of e
288 nm
is therefore:
0
:
49
10
4
1
¼
4900M
1
cm
1
e
288 nm
¼
ð
6
:
25
Þ
Values of l
max
and e
max
are useful to assess whether a compound might
absorb ambient light and therefore be susceptible to environmental pho-
todegradation. Values of e howeverneedtobecalculatedforeachwave-
length if full quantification of photochemical processes is required. Indeed
for chlorpyrifos, significant absorbance occurs at wavelengths 4290 nm
suggesting that photodegradation is worth considering with respect to its
environmental fate. It is worth noting that the solvent in which the
chemical resides may have a strong influence on the absorption spectrum,
and is important when trying to replicate environmental conditions for a
chemical with a low aqueous solubility. It is therefore relatively common
practice to replicate photochemical experiments using a number of solvent-
water mixtures to assess the solvent effect on the spectral absorbance.
Indeed, OECD guidelines with respect to aqueous photodegradation
studies on compounds with low solubility, recommend that co-solvents
