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Ag Chemicals |
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Introduction Many agricultural chemicals (pesticides and herbicides) have chiral centers. And in most cases, only one of the enantiomers has the desired potency. Obviously, this is important information to the analytical chemist performing environmental analyses. In other cases, the active ingredient is achiral but may have a chiral analog normally occurring as an impurity. Scanning environmental samples for the optically active analog can be used to indicate their presence or absence. Two notorious pesticides, p,p’-DDT and p,p’-DDD are of significant concern to environmental chemists due to the fact that they are biologically active and non-degradable. The biological activity goes beyond their toxicity as an insecticide and unfortunately includes toxicological activity towards vertebrates. Additionally, these molecules are of a class of compounds that has been implicated as endocrine disruptors. Therefore, it is of considerable value to be able to monitor levels of DDT and DDD in soil, surface water and ground water. Although both insecticides are achiral (i.e. exhibit no optical activity), commercial formulations typically contain o,p’- isomeric analogs (~10 to 20%) which are optically active. By monitoring environmental samples for the presence of these analogs, the highly probable presence of the achiral, toxic components may be indicated.
o,p’- DDT In the following figure we see two major components in a sample suspected to contain p,p’-DDT. We have included the the chiral separation conditions along with the chromatograms. UV/Vis absorbance detector produce a good response due to the compound's aromatic rings. However, there is little qualitative value in the results. The PDR-Chiral Advanced Laser Polarimeter not only offers important quantitative information but clearly indicates that the two components are both optically active and that the first peak to elute is the positive (+) enantiomer.
From these results, one can reasonably suspect that the sample does indeed contain p,p’-DDT. o,p’- DDD In the next example we show a similar sample, substituting DDD for the DDT. Again, we have a nice response with the UV detector but only the laser polarimeter positively indentifies the two enantiomers. The presence of o,p'-DDD indicated by this data certainly justifies sending the sample out for further analysis to confirm the presence of p,p’-DDD. In the two environmental examples presented here, the PDR-Chiral Laser Polarimeter was used to indicate the presence enantiomers based upon on optical rotation. From their presence, the presence of their toxic cousins in the samples can be inferred. For the purpose of methods development, the detector also confirmed the elution order and complete separation of the optical isomers. For qualitative capability, the PDR-Chiral Laser Polarimeter clearly shows an additional dimension that it offers over conventional UV absorbance detection. |
