6
listed above. For example, if the activity of Cl
-
is ten times higher than nitrate in the sample, Cl
-
may cause 7% error for the nitrate
measurement, while CH
3
COO
-
and SO
4
2-
in the same ratio may only cause 0.7% and 0.05% error, respectively.
When testing samples with high interference levels, such as wastewater, chlorinated water, and soil water, the interference ions
will be counted as nitrate with the coefficient factor and cause measurement errors. Some commonly existing interference ions,
such as Cl
-
, Br
-
, I
-
, S
2-
, HCO
3
-
and NO
2
-
can be reduced by using Nitrate Interference Suppression Buffer (NISB). Add NISB at 1:1
volume ratio to all sample/standard solutions (e.g. add 5 mL NISB to 5 mL solution) in place of ISA throughout the sensor
conditioning, calibration and measurement.
NOTE
: Nitrate Interference Suppression Buffer (NISB) is purchased separately, see ACCESSORIES.
NOTE
: When using NISB, do NOT add ISA. White precipitate may form in sample solution after mixing with NISB, it will not
affect the measurement.
NOTE
: Strong interference ions such as ClO
4
-
and SCN
-
require their concentrations to be up to 2000x lower than nitrate in the
sample to avoid high errors.
IV. MEASUREMENT VALIDATION METHODS
:
1.
Calibration Check:
a.
Choose one standard solution within the testing range, and prepare the solution according to the measurement procedure.
b.
Test readings in the standard solution and the value should be within 10% of the expected value. If not, the measurement
result may not be reliable. The source of error should be identified and the measurement should be redone after
correction.
2.
Known Addition Recovery Check:
This validation method works well for validating low concentration measurement (1-60 ppm-N) by using concentrated
standard solutions to increase the nitrate concentration in the sample by a known value, optimally double the measured value,
and then compare the value with the increase in Pen readings. For about 10 mL sample, one spike of standard solution with
the 100
µ
L pipette will increase the sample nitrate concentration by roughly 1% of the standard solution concentration,
example ~1 ppm increase by one spike of 100 ppm standard solution, ~10 ppm increase by one spike of 1000 ppm standard
solution.
Spike reference for different sample concentration for 10 mL sample volume (
V
/mL)
Measured Sample Concentration (
ppm NO
3
-
-N
)
×
100
µ
L pipette
Standard Solution (
ppm NO
3
-
-N
)
1-3
1
100
3-9
3
100
9-20
1
1000
20-60
3
1000
a.
Add
n
spikes (volume increase:
𝑽
𝒂𝒅𝒅
=0.1
×
n/mL) of standard solution (
Cs
/ppm) to the sample solution after measurement
(
C1
/ppm) and swirl for mixing.
b.
Take the 2
nd
measurement (
C2
/ppm) and calculate the recovery
using the following equation.
𝑅𝑒𝑐𝑜𝑣𝑒𝑟𝑦
%
=
𝐶
!
×
100
𝑉
×
𝐶
!
+
𝑉
!""
×
𝐶
!
×
(
1
/
(
1
−
𝐷
!
)
)
(
𝑉
!""
+
𝑉
)
Where Df is the dilution factor when spiking unprepared standard solutions, 1/51 for using ISA and
½
for using NISB. If
adding prepared standard solutions, Df is 0.
The recovery should be within 10% of the expected value. If not, the measurement result may not be reliable. The source of
error should be identified and the measurement should be redone after correction.
MAINTENANCE
I. ROUTINE MAINTENANCE
1.
After each use
ALWAYS
rinse the Nitrate sensor with clean water (preferably DI, RO, or distilled) and then carefully pat the
sensor with a clean, soft, lint-free cloth to remove any water drops.
2.
ALWAYS
replace the protective cap on the Nitrate sensor after each use.
3.
Do not touch the ISE membrane as oil from your finger may contaminate the sensor.
4.
Do not drop, throw, or otherwise strike the PT6 as this may void the warranty.
5.
Do not store the PT6 in a location where the ambient temperatures exceed its operating/storage temperature limits.
