YSI 9600 - Manual (Page 73)

The blank is typically purified water which has a salinity which is roughly that of the water at the site for
brackish and oceanographic applications and simply deionized or distilled water for freshwater
applications. The blank and standard solutions are analyzed in a manner identical to the samples described
above. The blank photodiode readings are defined as P
o
for determining absorbance values. The standard
and sample photodiode readings are defined as P. To correct for temperature effects, the 9600 uses the
following ratios (see details in section 8.8):
P
o
= blank signal (Hz)/reference signal (Hz)
P = sample signal (Hz)/reference signal (Hz)
As shown in the equation below, Beer’s law defines absorbance as directly proportional to concentration
where a is absorptivity, b is path length, and c is concentration.
Absorbance = A = log (P
o
/P) = abc Beer’s Law
Periodic calibration of the instrument determines an absorbance value for the standard solution and linear
interpolation between two standard readings results in a corrected standard absorbance value (A
std
) used in
the calculation of each sample nitrate-N concentration as shown in the equation below:
Sample conc. (mg/L nitrate-N) = (A
sample
/A
std
) x std conc. (mg/L nitrate-N)
8.8 SYSTEM LINEARITY
For the 2 mm detector, typical absorbance values range from 0-1 absorbance units for the range 1-10 mg/L
nitrate-N. Deviation from linearity begins in the vicinity of 6 mg/L where the accuracy falls off to +/- 10%.
Shown below are typical calibration curves before and after a 30 days field deployment.
Linearity of 9600 Absorbance Readings Before and After Deployment
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0 2 4 6 8 10 12 14 16
Nitrate-N, mg/L
Absor
b
ance
pre
post
YSI Environmental 73 Model 9600 Nitrate Monitor