An easily overlooked fact
ICP-MS detection limits are usually limited not by instrument sensitivity but by blank values. And a principal contributor to those blanks is the water used for standards, dilutions and system rinsing.
If your water carries 1 ppb of iron, your iron detection limit cannot be below 1 ppb — regardless of what the instrument cost.
Which elements are most affected
- Ubiquitous contaminants: Na, K, Ca, Mg, Fe, Zn, Al, Si — present in water-treatment materials, glassware and airborne dust.
- Trace toxic elements: Pb, Cd, As, Hg, Cr — low environmental background, but any container or reagent contribution raises the baseline immediately.
- Silicon: worth special attention. Reactive silica is not fully removed by conventional ion exchange, and glass vessels leach it.
Where contamination comes from
- Water source and treatment system. Tubing, fittings and reservoir materials leach metals; aged resin loses exchange capacity.
- Containers. Ordinary glass continuously leaches Na, Si and B. Use PFA / FEP / quartz, acid-leached and conditioned.
- Air. Lab dust carries Fe, Al and Zn. Open vessels can be contaminated in minutes.
- Reagents and standards. Acids — especially grades below trace-metal purity — carry their own metal background.
- Operator. Powdered gloves, skin contact, pipette tip material all contribute.
Quantifying the water's contribution
Run a simple experiment: aspirate your ultrapure water directly (undiluted, unacidified) and measure the elements you care about.
- Below instrument detection limit → water is not the bottleneck; look at containers and reagents.
- Consistently measurable → that value is your blank floor, and no detection limit below it is achievable.
Then compare water drawn directly from the outlet against water stored in a carboy for 24 hours. The difference is often striking, and it separates "the system is inadequate" from "the storage practice is inadequate".
Bringing blanks down
- Draw fresh, use immediately. Do not store ultrapure water; it picks up contamination from container and air.
- Use the right containers. PFA or FEP, pre-soaked in dilute nitric acid and rinsed with ultrapure water. No glass for trace metals.
- Shorten the plumbing. The shorter the run from unit to point of use, the less contact area.
- Watch silica. Conventional ion exchange removes silica only partially; confirm the reactive-silica spec at selection (good systems reach <0.1 ppb).
- Replace cartridges on schedule. Saturated resin stops adsorbing and can release what it already captured.
Which system specs matter
- Resistivity: ≥18.2 MΩ·cm, sustained (reflects total ionic load)
- Reactive silica: <0.1 ppb
- Heavy metals: in the <0.01 ppb range
- TOC: <5 ppb (organics complex metals and interfere with nebulisation and ionisation)
A system quoting only "18.2 MΩ·cm" without silica and metal background data cannot be assessed for trace work. Ask for those two figures directly.
A practical conclusion
For trace metals, bring the blank down before discussing detection limits. And the first suspect is always water — it is used in the largest volumes and is most easily masked by a reading that merely looks compliant.
