How do the TNTplus methods incorporate the method blank or zero?

Document ID

Document ID TE3967

Version

Version 5.0

Status

Status Published

Published Date

Published Date 01/31/2019
Question
How do the TNTplus methods incorporate the method blank or zero?
Summary
TNTplus methods blank or zero
Answer
In several tests, the contribution of the reagent(s) to the final reading is of such a magnitude that it must be compensated for whenever the test is performed. Reagent blank refers to that portion of the test result contributed solely by the reagent. This produces a positive error in the test results.

Every effort is made to produce reagents with the lowest possible blank; for most reagents, it is less than 0.009 absorbance units. In the usual, non-color bleaching methods, this equates to greater than 97.95% transmission of light through a sample of pure water with the reagent(s). However, it is sometimes impossible or impractical to produce reagents with such a low blank. When using such reagents, it is best to determine the reagent blank by performing the procedure using high-quality water (deionized, distilled, organic-free, etc.) in place of sample to "zero" the instrument. The resulting value is then expressed in the concentration units of the test and is subtracted from each sample determination that uses the same reagent lot. Spectrophotometer software allows the reagent blank value to be stored and subtracted automatically from each sample value. The reagent blank needs to be determined only at first use, when a new lot of reagent has been opened, or if contamination is suspected.

In most tests, the reagent blank is so small the instrument may be zeroed on either an untreated portion of the original water sample or on deionized water. This will not result in a significant loss of accuracy unless the test is for very low levels of the analyte of interest. However, there are other tests where the reagent blank is significant and must be accounted for. This accounting can either be handled at the time of the test, which requires additional test reagents and equipment, or can be handled by using pre-determined reagent blank values. The TNTplus reagent sets use the pre-determined reagent blank approach and to illustrate their operation, the case of Chemical Oxygen Demand (COD) testing is used in this article.

The COD Digestion Vials, Low Range, pk/25 (Product # 2125825) as used in the standard COD method 8000, is specified to have a reagent blank absorbance that yields a result between 0 to 15 ppm COD versus a vial contained with only deionized water. In the method (Step 5) a second test vial is filled with deionized water and used as the reagent blank for the method. The significance of this is that one extra COD vial is used every day that this test is run and must be accounted for when ordering reagents.

The Chemical Oxygen Demand (COD) TNTplus Vial Test, LR (3-150 mg/L COD  (Product # TNT821), as used in the TNTplus COD method 8000, requests a reagent blank be analyzed only if very accurate results at low concentrations are needed, or when the reagents were in storage for long periods of time. For normal use, a Reagent Blank value that is built into the software of the DR-spectrophotometer is used for the analysis. The TNT821 reagent is specified to have a particular Extinction Blank Value (measurement against air).

 
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