Why grades exist
Not every experiment needs 18.2 MΩ·cm water. Rinsing glassware with ultrapure water wastes it; preparing HPLC mobile phase with Grade 3 water invites trouble. Grading exists to match water quality to purpose — protecting results without overspending.
GB 6682-2008: three grades
China's national standard Water for analytical laboratory use — Specification and test methods (GB 6682-2008) defines three grades:
| Parameter | Grade 1 | Grade 2 | Grade 3 |
|---|---|---|---|
| pH (25 °C) | — | — | 5.0–7.5 |
| Conductivity (25 °C) | — | ≤1.0 µS/cm | ≤5.0 µS/cm |
| Oxidisable matter | — | ≤0.08 mg/L | ≤0.4 mg/L |
| Absorbance (254 nm) | ≤0.001 | ≤0.01 | — |
| Residue on evaporation | — | ≤1.0 mg/L | ≤2.0 mg/L |
| Reactive silica (SiO₂) | ≤0.01 mg/L | ≤0.02 mg/L | — |
Note that Grade 1 is judged by absorbance, not conductivity — organics and colloids do not show up in a conductivity reading. This is frequently overlooked.
ASTM / ISO 3696: Type I–III
The international equivalents (ASTM D1193, ISO 3696) map broadly onto the same tiers:
- Type I (ultrapure): resistivity ≥18.2 MΩ·cm at 25 °C, TOC typically < 10 ppb, bacteria < 1 CFU/ml. For HPLC, LC-MS, ICP-MS, cell culture, molecular biology.
- Type II (pure): resistivity ≥1 MΩ·cm, bacteria < 100 CFU/ml. For general reagent and buffer preparation, media, instrument feed.
- Type III (primary): resistivity ≥0.05 MΩ·cm (~20 µS/cm). For glassware pre-rinsing, water baths, autoclave feed.
Which grade for which work
- Trace inorganic analysis (ICP-MS / ICP-AES / AAS): Type I. Metal background must be minimal or detection limits are unreachable.
- Chromatography (HPLC / UPLC / LC-MS / GC-MS): Type I, and watch TOC — organic contamination causes ghost peaks and baseline drift.
- Cell culture / PCR / molecular biology: Type I, with attention to bacteria and endotoxin.
- Routine titration and reagent preparation: Type II is usually enough.
- Glassware washing, autoclave feed: Type III suffices; ultrapure water is wasted here.
A common waste
Many labs install one ultrapure system and route everything through it — including bottle rinsing and autoclave feed. Consumables then deplete far faster than expected, with the cost all carried by high-purity water.
The economical approach is tiered supply: one unit providing both an RO outlet (washing, autoclaving, water baths) and an ultrapure outlet (analysis, culture). Most lab systems support dual output — factor it into selection.
Do not treat RO water as pure water
RO permeate typically measures around 5 µS/cm (~0.2 MΩ·cm) — between Grade 3 and Grade 2. It suits rinsing and autoclave feed, but does not meet all Grade 2 criteria and should not be used directly for reagent preparation. Worth noting during method validation.
