How can sample turbidity interfere with colorimetric methods?

Document ID

Document ID TE10454

Version

Version 2.0

Status

Status Published

Published Date

Published Date 27/03/2019
Question
How can sample turbidity interfere with colorimetric methods?
Summary
Discussion of checking for and adjusting for potential interference from sample turbidity on methods followed using a colorimeter or spectrophotometer.
Answer
Suspended solids in a sample can lead to turbidity or cloudiness of the sample. During a colorimetric measurement using a colorimeter or spectrophotometer some of the light going through the sample would be scattered by the solids in the sample. The light that is scattered is not transmitted. The light that is not transmitted is assumed to be absorbed by the instrument. Because of this sample turbidity can be a positive interference in colorimetric methods.

Many colorimetric methods will utilize a sample blank which will help remove interference from small amounts of sample color and turbidity (See also: What is the difference between a reagent blank and a sample blank?). For TNT+ reagent sets, many of these methods can use TNT919 to add a sample blank to the method (See also: Which TNTplus® chemistries utilize the Sample Blank Vials for TNTplus Vial Test?). The threshold of maximum turbidity that can be accounted for by only using a sample blank varies depending on the method and more importantly the sample matrix.

If there is a concern about if sample turbidity is causing an interference in measurement, either if the method is not using a sample blank or if there are concerns that the turbidity is too high to be accounted for with a sample blank alone, what would be recommended is to perform standard additions and calculate percent recovery. If percent recovery on standard additions is within what would be considered an acceptable tolerance (this would be a user set/determined tolerance) then no other action would need to be taken. If however, the percent recovery is outside of this tolerance, then sample pretreatment to adjust or account for turbidity would be recommended. This could include sample dilution, centrifugation, or sample filtration (NOTE: filtration may not be recommended in some methods). Repeat standard additions on pretreated samples to verify that recoveries on pretreated samples are within that acceptable tolerance.