YSI 9600 - Manual (Page 72)

and NED through line B. The output of all four pumps meet at a T-connection and continue through a
mixing coil and detector cell. After a steady state concentration is achieved in the detector flow cell, buffer
is pumped into the cadmium column and then all pumps stop to allow the color forming reaction to reach
completion. After this waiting period, two readings are taken in the detector: (1) the SAMPLE photodiode
signal which measures the amount of light passing through the 2mm sample path and (2) the REFERENCE
photodiode signal which is a measurement representative of the LED output. After the photodiode readings
are logged, a wash solution is pumped through the mixing coil and detector to prevent staining in the
system. YSI has found that the solution exiting the detector typically contains 60 ppm Cd
+2 which is a
product of the reduction of dissolved O
2
in the sample and reagents. For this reason, the effluent must be
collected in waste bags for disposal in compliance with local and federal regulatory agencies.
8.4 PUMPING SEQUENCES
The 9600 software has two preset schedule routines which define the parameters described above. The
routines are for normal (0-5 mg/L) or higher (5 mg/L and up) nitrate-N concentrations expected in the
environment water samples. The two routines differ in only two ways – both a shorter color forming
reaction time and longer wash cycle for higher nitrate concentrations – to minimize precipitation of the
colored reaction product and prevent staining of the detector.
8.5 FLOW RESTRICTION PROBLEMS
The pumps used in the 9600 cannot function properly with (and can be damaged by) excess backpressure in
the system. The cadmium column is the most likely source of this problem – and proper operation of the
instrument requires use of the Model 9612 Cadmium Column Assembly which MUST be purchased from
YSI. Also, excess air introduced into the system will cause cadmium column degradation and unacceptable
increases in back pressure. ANY additional restriction to flow in the system can cause the above schedule
routines to no longer be optimal with serious degradation of instrument performance.
8.6 CALIBRATION OF SAMPLE AND STANDARD PUMPS
The 9600 measures nitrate by comparing the absorbance reading from a standard to that from the
environmental water after each has been processed by the system. These two solutions of nitrate (standard
and sample) are introduced into the system by separate pumps which almost certainly pump slightly
different volumes of fluid. Thus, without some correction for this effect, an error in the comparison of
standard to sample would be introduced with a consequent compromise of system accuracy.
This potential problem is overcome in the 9600 during the Predeployment Check described in Section 4.3
above. During this test, the same solution (usually a 1 mg/L nitrate standard) is introduced into the system
by both sample and standard pumps and the relative absorbances are determined after the chemical reaction
sequence shown above in Section 8.1 has taken place. These relative values are then applied to the
calibration sequences for all points in the study immediately following the Predeployment Check. For
example, if the absorbance of the “standard” in the predeployment check is 10 % greater than that for the
“sample”, then the fact that the standard pump is introducing more volume into the system is automatically
accounted for in all subsequent calculations.
8.7 CALIBRATION DETAILS
The 9600 contains standard and blank solutions for periodic calibration. The standard solution
concentration is typically 1 mg/L nitrate-N, but can be chosen by the user as described in Section 2.3.3.
YSI Environmental 72 Model 9600 Nitrate Monitor