LaMotte SMART Spectro Spectrophotometer - Manual (Page 12)

12 SMART Spectro Operator’s Manual 2.11
The choice of the correct wavelength for testing is important. It is interesting to
note that the wavelength that gives the most sensitivity (lower detection limit)
for a test factor is the complementary color of the test sample. For example
the Nitrate-Nitrogen test produces a pink color proportional to the nitrate
concentration in the sample (the greater the nitrate concentration, the darker the
pink color). A wavelength in the green region should be selected to analyze this
sample since a pinkish-red solution absorbs mostly green light.
 REAGENT BLANK
Some tests will provide greater accuracy if a reagent blank is determined to
compensate for any color or turbidity resulting from the reagents themselves.
A reagent blank is performed by running the test procedure on 10 mL of
demineralized or deionized water. Use sample water to SCAN BLANK. Insert
the reacted reagent blank in the colorimeter chamber and select SCAN
SAMPLE. Note result of reagent blank. Perform the tests on the sample water as
described. Subtract results of reagent blank from all subsequent test results.
NOTE: Some tests require a reagent blank to be used to SCAN BLANK.
 SPECTROPHOTOMETER TUBES
Spectrophotometer tubes which have been scratched through excessive use
should be discarded and replaced with new ones. Dirty tubes should be cleaned
on both the inside and outside. Fingerprints on the exterior of the tubes can
cause excessive light scattering and result in errors. Handle the tubes carefully,
making sure the bottom half of the tube is not handled.
LaMotte Company makes every effort to provide high quality spectrophoto-
meter tubes. However, wall thicknesses and diameter of tubes may still vary
slightly. This may lead to slight variations in results (e.g. if a tube is turned while
in the sample chamber, the reading will likely change slightly). To eliminate this
error put the tubes into the sample chamber with the same orientation every
time.
The tubes that are included with the spectrophotometer have an index mark
to facilitate this. If possible, use the same tube to SCAN BLANK and SCAN
SAMPLE.
 SELECTING AN APPROPRIATE WAVELENGTH
The most appropriate wavelength to use when creating a calibration curve
is usually the one which gives the greatest change from the lowest reacted
standard concentration to the highest reacted standard concentration. However,
the absorbance of the highest reacted standard concentration should never
be greater than 2.0 absorbance units. Scan the lowest and highest reacted
standards at different wavelengths using the %T/ABS mode to fi nd the
wavelength which gives the greatest change in absorbance without exceeding
2.0 absorbance units. Use this wavelength to create a calibration curve.