What causes refractive indexes and how can they be eliminated?
Refractive index cause and correction
Refractive indexes are caused by a matrix difference between the carrier and the standards and/or samples. The difference may be related to pH, salt concentration, or some other differing characteristic. They are commonly seen when running chemistries requiring digestion, brackish water samples, or chemistries involving acid preservation, such as ammonia. Refractive indexes can be eliminated by matching the carrier to the matrices of the samples and standards. For this reason, if sulfuric acid is being used to preserve the samples, it is recommended that acid of the same concentration be used for the carrier. Review the Lachat QuikChem method carefully. Some methods recommend a slightly different buffer formula for preserved versus non-preserved samples. This problem will be more evident in manifolds that use larger sample loops. If preserved samples with a deionized water carrier can be run without seeing a refractive index, there is no need to make any changes to the procedure. In the case of brackish water samples, it is often impossible to match the carrier to the sample matrices exactly because the salinity levels vary from one sample to another. In this situation, the software can use brackish integration to compensate for the refractive indexes.