It would be easy to answer if you can elaborate your doubts. Several things are being considered during the analysis for quality control and ensure the accuracy of the obtained data. The results can be easily cross checked following conventional methods i.e. azide method for BOD and Calorimetic/titrimetic method for COD following APHA, using some of the known samples. Similarly, TOC can be done for some of the quality control samples from TOC analyzer.

Unfortunately cross-checking results is not an option as this is a small lab that does not have other instruments at it's disposal.

May main concern is the degree of confidence in the results, and i'm also concerned that I have not seen many academic papers using these methods

It depends on the quality of materials, reagents you use and also your accuracy and experience. But, based on my experience, Hach analysis devices are accurate and reliable.

You can check its precision and accuracy using standard samples.

Web of Science gives 12 entries with keywords Hach + COD. Measurement of various chemical parameters with this instrument is very common in WWTP and environmental analysis labs. You can compare your instruments's paraweters with other models on Hach's webpage https://www.hach.com/quick.search-quick.search.jsa?keywords=spectrometer or ask the company on reliability and reproducibility of measurements with their instrument or to send application notes on a specific technique. My experience with Hach instruments has been very positive.

Jessica,

how you perform calibration? with phthalate standard solutions?

If you use original HACH tests - where do you see the issue?

The question is ok when you make the reagent by your own.

regards,

GB