Ten parts per billion is one microgram of lead in a litre of water. In more human terms: a single grain of salt, split into roughly a thousand pieces, one of those pieces, dissolved in a litre.
That is the limit the WHO guidance and the BIS standard for drinking water settle on, and it is low enough that the measurement is as much a process problem as an electronics problem.
Where the lead comes from
Rarely from the source water. Lead enters at the plumbing — old service lines, lead solder on copper pipe, brass fittings with the wrong alloy. It is a corrosion problem, which means the concentration depends on how long the water sat in the pipe, its pH, and its temperature. A sample taken after the tap has run for two minutes and one taken first thing in the morning are different samples.
Why the container matters
At this concentration, the walls of the container are a participant. Lead adsorbs onto glass. A sample collected in an unacidified bottle and carried around for a day can read low because the analyte left the water and joined the glass.
The standard practice exists for that reason: acidify to keep the metal in solution, chill, and measure sooner rather than later. Any field instrument that skips this step is measuring a different sample than the lab does, which is exactly how portable devices acquire a reputation for being unreliable.
What portable measurement has to prove
The instrument question — can a handheld potentiostat resolve a few micrograms per litre — is answerable with an analog front end and a good electrode. The harder question is whether the whole chain, from sampling to reading, produces a number that means the same thing as the laboratory method.
That means paired studies. Same sample, split in the field, one half run on the handheld and one half sent to an accredited lab, across the range of matrices that matter: tap water, borewell water, treated effluent, a soil extract. Not a spike recovery in deionised water.
That is the work ahead of a device like AnalyteX, and it is the least photogenic part of building an instrument. It is also the only part that turns a demonstration into a measurement.