Any method for testing Nitrate and Nitrite in biological fluids like aqueous humour?
There is 1982 publication describe nitrate, nitrite, and $\ce{[^{15}N]}$-nitrate in biological fluids. It's abstract describe the procedure briefly:


  Abstract: A new automated system for the analysis of nitrate via reduction with a high-pressure cadmium column is described. Samples of urine, saliva, deproteinized plasma, gastric juice, and milk can be analyzed for nitrate, nitrite, or both with a lower limit of detection of $\pu{1.0 nmol} \; \ce{NO3−}$ or $\ce{NO2−/ml}$. The system allows quantitative reduction of nitrate and automatically eliminates interference from other compounds normally present in urine and other biological fluids. Analysis rate is 30 samples per hour, with preparation for most samples limited to simple dilution with distilled water. The application of gas chromatography/mass spectrometry for the analysis of $\ce{^{15}NO3−}$ in urine after derivatization to $\ce{^{15}NO2}$-benzene is also described.


Thousands of samples of human and rat urine, human saliva, and plasma have been analyzed for nitrate and nitrite using this system. Typical ranges of nitrate and nitrite levels in biological samples are shown in the Table.

$$
\begin{array}{ccc}
\hline
\text{Biological fluid} &amp; \text{$\ce{NO3-}$ Concentration, $\pu{\mu M}$} &amp; \text{$\ce{NO2-}$ Concentration, $\pu{\mu M}$}  \\ \hline
\text{Urine} &amp; 250-2000 &amp; \text{ND}^a \\
\text{Saliva} &amp; 200-600 &amp; 30-210 \\
\text{Plasma} &amp; 15-60 &amp; \text{ND}^a \\
\text{Gastric juice} &amp; 50-85 &amp; 0.4-60^b \\
\text{Milk} &amp; 20-30 &amp; \text{ND}^a \\\hline
\end{array}
$$
$^a$Not determined. $\; \; ^b$Nitrite concentrations at the high end of this range are from people with gastric pathologies.

In general, for methods of the detection and determination of nitrite and nitrate, read Ref.2  (a review) and 3. However, Ref.1 claimed that the method in Ref.2 did not work for biological fluids.



References:


Laura C. Green, David A. Wagner, Joseph Glogowski, Paul L. Skipper, John S. Wishnok, Steven R. Tannenbaum, “Analysis of nitrate, nitrite, and $\ce{[^{15}N]}$nitrate in biological fluids,” Analytical Biochemistry 1982, 126(1), 131–138 (https://doi.org/10.1016/0003-2697(82)90118-X).
Qiu-Hua Wang, Li-Ju Yu, Yang Liu, Lan Lin, Ri-gang Lu, Jian-ping Zhu, Lan He, Zhong-Lin Lu, “Methods for the detection and determination of nitrite and nitrate: A review,” Talanta 2017, 165, 709–720 (https://doi.org/10.1016/j.talanta.2016.12.044).
M. P. Stainton, “Simple, efficient reduction column for use in the automated determination of nitrate in water,” Analytical Chemistry 1974, 46(11), 1616–1616 (https://doi.org/10.1021/ac60347a051).