i think you might be down a rabbit hole here but i will give it a go.
whats the symptoms here in terms of a problem? if you a cramping regularly then using any electrolyte in training and increasing the dosage might show improvements.
i have been looking into the prevision hydration as i appear to be really suffering from the key issues and essentially bringing myself close to hospitalisation every ironman i do. its all going in but i am missing the key elements for processing so i end up bloated, passing out worrying everyone.
training wise in lock down i jave been upping the high5 zero tabs in 750 water bottle hence doubling / tripling the concentration and i have been doing some serious long rides in heat.
been getting the same salty bibs but none of the night restless legs or cramps.
i think your testing whilst commendable is open to flaws, collected sweat will evaporate quickly so be more concentrated.
Using cling film you might artificially increase output of other products as the skin wont breath the same way as in open air so contaminate more.
i know gatorade have testing strips which you send off as well.
just thinking there might be a more obvious answer