How to verify or adjust the reagent blank on the 5500 sc or Series 5000 Silica Analyzer in the field?
Adjusting a reagent blank on a 5500sc or Series 5000 Silica Analyzer using the standard additions method in the field.
The reagent 1 (R1) blank value can be found on the reagent bottle. If the sample has very low silica concentrations and has issues with high readings (one to two ppb high), that are not caused by blueing of the sample cell or instrument issues, then a field blank determination can be performed to adjust the blank value.
The method used to determine the blank on the 5500sc silica analyzer is the standard additions method. This method requires a stable sample concentration feed to the analyzer for the duration of the testing, which will be a couple hours. Begin by performing a grab sample test with the sample water, and record the analyzer reading. Collect 500 ml of sample, and to this add 2 ml of 500 ppm standard, then run this spiked sample as a grab sample and record the reading. Collect 500 ml of sample, and to this add 4 ml of 500 ppm standard, then run this spiked sample as a grab sample and record the reading. Repeat running grab samples spiked with 6, 8, and 10 ml, recording each reading.
When the spiked grab sample readings have been collected, then chart these on graph paper (or in Excel). Spike value - 0, 2, 4 etc., ppb is the x-axis; sample reading in the y-axis. Draw a best-fit straight line through the charted points. Where the line crosses the x-axis is the value of the reagent blank. (The intercept will be a negative value, but it is entered as a positive blank value.)
Determination of reagent blank is performed at concentrations near the detection limit of the analyzer, and the spiked sample readings noted above are subject to noise and interference. The lab ware and mixing vessels must be scrupulously clean for good results. It is best to repeat each standard addition at least three times to assure results are repeatable. Final accuracy of the blank determination will probably be around +/- 1 ppb, and very likely in this range of agreement with the value on the Molybdate reagent bottle.
A disagreement of 1-2 ppb between two analyzers running sample that is itself only 1-2 ppb (or even less) can be VERY DIFFICULT to isolate. If the process system is stable and runs a known baseline response following regeneration, and no other fault can be found in the analyzer, the most practical solution may be to adjust the blank value on this analyzer so that readings match the known baseline.