Because the ISUS is an active optical instrument with an internal UV light source and spectrometer, gradual changes in the baseline spectra can occur. To check for instrument drift, we recommend users check the baseline prior to deployments by sampling an aliquot of ultra pure de-ionized water (DIW). To do this, first clean the probe tip and warm up the instrument for 8-10 minutes. Then log 1 minute of data with the probe tip submersed in DIW, making sure that there are no bubbles present on the probe tip. If the nitrate reading for the DIW sample is >-2 or <+2 uM, the instrument has not drifted significantly. If the nitrate reading is outside this range, the instrument has drifted and should be corrected by updating the reference spectrum in the cal file. For V3 instruments, the ISUSCom software has a function to update the cal file. For V1 & V2 instruments, the ISUSCal program is required. After uploading the new cal file to the instrument, it is a good idea to run another DIW check to make sure the procedure has worked.