How are readings validated below 0.1 NTU on laboratory Turbidimeters?

Document ID

Document ID TE6145

Published Date

Published Date 27/05/2017
Question
How are readings validated below 0.1 NTU on laboratory Turbidimeters?
Summary
Validating readings below 0.1 NTU.
Answer
According to USEPA 180.1 there is a well-defined linear response from 0.012 to 40 NTU's and Hach uses the 20 NTU calibration point to establish this response.  When calibrating any Hach turbs the <0.1 NTU is not a calibration point and is only used to develop the stray light value.  The lamp is not on when the S0 point is engaged and the actual <0.1 NTU Stablcal® vial is placed in the instrument only because it itself gives off stray light.  This also duplicates the interface or matrix of a typical sample cell used during analysis.  After the S0 calibration point, the stray light value is developed and subtracted from the rest of the calibration points (S1, S2, S3, etc).  Pure water has a 0.012 NTU value itself, so the calibration curve actually covers the 0.012-20 NTU range with a perfectly linear response.  The S1 calibration point (20 NTU std) includes this linearity for the ultra-low measurement range (0.012-1.00 NTU).  This also explains why a "dirty" instrument can hold an accurate calibration curve.

If the customer wants to validate the calibration curve at ultra-low level values suggest the 0.1, 0.3, 0.5 and 1.0 NTU standards Stablcal® ULR Verification Kit, 100mL (Catalog Number/Order Code 2714600). Keep in mind that for ultra-low measurements there are two main errors: stray light and contamination of the sample.  Hach has continuously lowered stray light sources with the turb line so that leaves sample contamination.  The following will minimize sample contamination and user errors:

1)  Clean sample cells- clean with soap and DI water followed by acid rinsing (1:1 HCl), then ultra-filtered DI water (15x), then cap sample cells immediately to avoid air contamination.

2)  Index sample cells- after cleaning and wiping with silicon oil, rotate sample cells filled with ultra-filtered DI water and find the lowest NTU value, then mark.

3)  Remove entrapped air- always a positive interference, let sample stand for several minutes or apply vacuum without contaminating sample.

4)  Polish sample cells- Use 1-2 drops of silicon oil with oiling cloth.

5)  Use one sample cell- after indexing, one sample cell can be used to eliminate differences in optical qualities of sample cells.