Hach DR/850 Procedures Manual page 48

CHEMICAL ANALYSIS INFORMATION
In some situations, using the pre-programmed curve may not
be convenient:
a) Running tests where frequent calibration curve checks are required.
b) Testing samples which give a consistent test interference.
Consider the following before adjusting the calibration curve:
1. Will future test results be improved by adjusting the curve?
2. Are interfering substances consistent in all the samples that you
will test?
Any precision and test range information provided with the procedure
may not apply to an adjusted curve calibration.
You can adjust many of the calibration curves by following the steps
found in the test procedures. Working carefully is important. After the
adjustment, it is wise to run standard solutions of several concentrations
to make sure the adjusted curve is satisfactory. Perform standard additions
on typical samples to help determine if the adjusted curve is acceptable.
Think of the standard adjust measurement as a two-step process. First,
the instrument measures the sample using the pre-programmed
calibration. Second, it multiplies this measurement by an adjustment
factor. The factor is the same for all concentrations. The instrument will
remember the factor indefinitely and will display the standard adjustment
icon when it is used.
Adjust the calibration curve using the reading obtained with a Hach
Standard Solution or carefully prepared standard made from a
concentrated Hach Standard Solution. It is important to adjust the curve in
the correct concentration range. For most purposes, Hach recommends
adjusting the curve using a standard concentration that is 70 to 85% of the
maximum concentration range of the test.
For example, the Hach pre-programmed method for fluoride has a range
of 0-2.0 mg/L F. To adjust the calibration curve, use a standard with a
concentration between 1.4-1.6 mg/L. Hach provides a 1.60 mg/L Fluoride
Standard Solution (80% of the full range). This is a convenient standard to
use for adjusting the calibration curve.
If the range of all your samples is known to be below a concentration that
is less than 50% of the full range (50% of 2.0 is 1.0 mg/L), then adjust the
standard curve with a standard that is within that range. For example, if all
the samples contain 0.6-0.9 mg/L F, you may use a 1.00 mg/L fluoride
standard to adjust the curve. You may use the 1.00 mg/L standard because
it is closer to the sample range you are working with.
, continued
48