Hach DR2400 - Adjusting the Standard Curve

Chemical Analysis
Chemical Analysis
Page 40
5940022ChemAnal.fm
Example:
A sample measures 10 mg/L. A separate 100-mL portion of the sample was
spiked with 5 mL of a 100-mg/L standard solution. The spiked solution was
measured by the same method as the original sample. The result was 13.7 mg/L.
Acceptable percent recovery values are 80-120%.
3.2.4 Adjusting the Standard Curve
The DR/2400 Spectrophotometer has over 120 Hach programs permanently
installed in memory. A program usually includes a pre-programmed calibration
curve. Each curve is the result of an extensive calibration performed under ideal
conditions and is normally adequate for most testing. Deviations from the curve
can occur from using compromised testing reagents, defective sample cells,
incorrect test procedure, incorrect technique, or other correctable causes.
Interfering substances or other causes may be beyond the analyst’s control.
In some situations, using the pre-programmed curve may not be convenient:
• Running tests where the reagents are highly variable from lot to lot.
• Running tests where frequent calibration curve checks are required.
• Testing samples which give a consistent test interference.
Consider the following before adjusting the calibration curve:
• Will future test results be improved by adjusting the curve?
• Are interfering substances consistent in all the samples that you will test?
• Any estimated detection limit, sensitivity, precision, and test range
information provided with the procedure may not apply to an adjusted
curve calibration.
The calibration curves can be adjusted by following the steps found in the test
procedure. Generally, you add test reagents to a blank and standard solution.
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. Performing standard additions on typical samples may also help
determine if the adjusted curve is acceptable.
Think of adjusting a 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 until the program is
exited and will display the standard adjustment icon when it is used. You can
return to the pre-programmed curve any time by selecting the Hach Program
from the main menu.
X
s
13.7 mg/L=
X
u
10 mg/L 100 mL×
105 mL
------------------------------------------------ 9.5 mg/L==
K
5 mL 100 mg/L×
105 mL
-------------------------------------------- 4 . 8 m g / L==
%R
100 13.7 mg/L 9.5 mg/L–()×
4.8 mg/L
----------------------------------------------------------------------------- 88%==