Biomedical Engineering Reference
In-Depth Information
1.4.2 Calibration based on decomposition of SNAP
For this method S-nitroso-N-acetyl-D, L-penicillamine (SNAP, FW
220.3) is
decomposed to NO in solution in the presence of a Cu (I) catalyst [77]. The result-
ant NO generated can then be used to calibrate the sensor. The reaction proceeds in
accordance to the following reaction:
2 RSNO
→
2 NO
RS-SR
The stoichiometry of the reaction dictates that the fi nal generated NO concentration
will be equal to the concentration of SNAP in the solution. The method can be summa-
rized as follows. Saturated cuprous chloride solution is fi rst prepared by adding 150 mg
CuCl to 500 mL distilled water. This solution is then deoxygenated by purging with
pure nitrogen or argon gas for 15 min. The fi nal, saturated CuCl solution will have a
concentration of approximately 2.4 mM at room temperature. The solution is light sen-
sitive and must therefore be kept in the dark prior to use.
The SNAP solution is then prepared separately as follows. EDTA (5 mg) is dis-
solved in 250 mL of pure water (HPLC grade, Sigma) and then adjusted to pH 9.0
using 0.1 M NaOH. The solution is then deoxygenated using the method described
above. A quantity of 5.6 mg of SNAP is then added to the solution to result in a SNAP
concentration of
0.1 mM. SNAP solution is also extremely sensitive to light and tem-
perature and must therefore be stored refrigerated and in the dark until required. Under
these conditions, and in the presence of the cheating reagent (EDTA), the decomposi-
tion of SNAP occurs extremely slowly. This allows the solution to be used to calibrate
NO probes throughout the day. In practice, calibration is performed by placing an NO
sensor into a vial containing a measured amount of the CuCl solution and known vol-
umes of the SNAP stock solution are then injected into the vial and the fi nal concentra-
tion of NO can be calculated using dilution factors.
The concentration of SNAP in the stock solution is calculated as follows:
[
C
]
[
A
W
/(
M
V
)]
1000
where
C
concentration of SNAP (
µ
M);
A
purity of SNAP;
W
weight of SNAP
(mg);
M
volume of the solution in liters (l).
If SNAP purity, for example, is 98.5% then the concentration of SNAP is calculated as:
MW of SNAP (mg/mmole); and
V
[
C
]
[98.5%
5.6/(220.3
0.25)]
1000
100.1
µ
M
Figure 1.3 shows a typical calibration curve generated using an NO microsensor
and the SNAP method described.
1.4.3 Calibration based on chemical generation of NO
This method of calibration generates known concentrations of NO based on the reac-
tion of nitrite with iodide in acid according to the following equation:
2 KNO
2
2 KI
2 H
2
SO
4
→
2 NO
I
2
2 H
2
O
2 K
2
SO
4
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