Choosing instruments for drinking-water testing

Drinking-water testing is one of the few laboratory workflows where the parameter list is decided for you. The regulator defines what must be reported, the method defines how, and the laboratory’s job is to build a bench that produces those results repeatably. That makes instrument selection a sequencing exercise rather than a shopping exercise.

What the reporting list decides

Before comparing instruments, write two columns: the parameters you must report, and the concentration range you must report them at. Almost every instrument decision follows from those two columns. Two laboratories with the same water source can end up with very different benches because one reports a single chloride value and the other separates six anions.

A useful test: if the report distinguishes between nitrate and sulphate, you need separation. If it only reports a sum such as hardness or alkalinity, titration is enough.

Group 1: the parameters measured every day

pH, conductivity, dissolved oxygen and turbidity are measured on almost every sample and need no preparation. They are also the measurements most likely to be silently wrong, because a dirty electrode or a scratched turbidity cell produces a plausible number. This group deserves the most attention to routine care, not the most money.

A multi-parameter benchtop meter plus one turbidity meter covers the group. If sampling happens in the field, a portable set is a separate decision: the field value and the laboratory value are different measurements and should be reported as such.

Group 2: digestion and colorimetry

COD, ammonia, total phosphorus and total nitrogen share a workflow: digest, cool, dilute, read. The reading instrument can be a dedicated COD analyser or a spectrophotometer, but the digestion block is what constrains throughput. A laboratory that owns one block and one reader for four parameters will queue, so the batch size matters as much as the wavelength range.

Group 3: separation for individual anions

Ion chromatography answers the question titration cannot: how much of each anion. It also brings infrastructure — eluent preparation, high-purity water, column care and a longer start-up routine. Laboratories usually add it when the anion workload is steady enough that outsourcing has become the bottleneck.

Group 4: elemental analysis for trace metals

Trace metals at µg/L levels need atomic absorption. Flame covers the mg/L to sub-mg/L range and is the simpler routine; the graphite furnace reaches lower and demands more operator skill and more method work. Decide on the reporting limit, not on the ambition of the laboratory.

A buying order that survives audits

1. Multi-parameter meter and turbidity meter — the daily parameters.
2. Digestion block, then the reader that fits the parameter list.
3. Analytical balance for standard preparation; the whole traceability chain starts here.
4. Ion chromatograph, when individual anions are reported routinely.
5. Atomic absorption, when trace metals are on the list.

Where laboratories lose accuracy

Three failures account for most out-of-control results: samples held longer than the method allows, digestion recovery treated as an assumption rather than a check, and a calibration range that does not bracket the sample. None of them is fixed by a better detector, and all of them are fixed by writing the workflow down.