Laboratory water is classified into grades, most often Type I (ultrapure), Type II (pure) and Type III (reverse-osmosis or general-purpose water). Type I is reserved for the most sensitive work, such as HPLC, LC-MS, PCR and cell culture. Type II suits buffers, reagents and general chemistry. Type III covers glassware rinsing, autoclave feed and the feed stage of higher-grade systems. The grade is chosen by the most demanding application the water must serve, and the sections below set out what each grade means, how it is produced and how to match it to a method.
The standards behind the grades
No single document defines "lab water". Three frameworks are in common use, and they are related but not identical.
- ASTM D1193 defines four types of reagent water (Type I to IV). The 2024 edition also describes additional grades addressing microbiological contaminants, and since the 2018 revision the types may be produced by alternative technologies, provided the constituent specifications are met, according to ASTM International.
- ISO 3696 specifies three grades for analytical laboratory use. Grade 1 has a maximum conductivity of 0.1 µS/cm, Grade 2 has 1.0 µS/cm and Grade 3 has 5.0 µS/cm with a pH range of 5.0 to 7.0, as summarised by Xylem Australia.
- CLSI guidance for clinical laboratories moved away from Type I, II and III designations in 2006 in favour of water that is fit for purpose, with Clinical Reagent Laboratory Water (CLRW) described in most detail (Xylem).
The practical consequence is that "Type I", "Grade 1" and "ultrapure" are used loosely by vendors and in methods, yet the numerical limits differ. When a method, an accreditation scope or a purchase order names a standard, the water should be specified against that standard by name.
Type I, II and III compared
The figures below follow the ASTM D1193 values as reported by Across International. Published summaries differ slightly between editions, so the current edition of the standard is the reference for any accredited work.
| Grade | Resistivity at 25°C | Conductivity | TOC | Typical production |
|---|---|---|---|---|
| Type I | ≥ 18.2 MΩ·cm | ≤ 0.055 µS/cm | < 10 ppb | Distillation or equivalent, then mixed-bed ion exchange and 0.2 µm filtration |
| Type II | ≥ 1 MΩ·cm | ≤ 1 µS/cm | < 50 ppb | Distillation with a still producing distillate below 1.0 µS/cm |
| Type III | > 4 MΩ·cm | < 0.25 µS/cm | < 200 ppb | Distillation, ion exchange, electrodeionisation or reverse osmosis, then 0.45 µm filtration |
| Type IV | ~0.2 MΩ·cm | ≤ 5 µS/cm | No specific limit | General non-critical use |
The production routes in the final column are those listed in the ASTM D1193-06 (reapproved 2018) specification. Modern laboratory systems usually reach Type I by combining reverse osmosis, ion exchange, UV treatment and ultrafiltration rather than by distillation.
Resistivity, TOC and what they leave out
Resistivity (or its inverse, conductivity) is the parameter monitored continuously by laboratory water systems. The theoretical maximum for pure water is 18.2 to 18.3 MΩ·cm at 25°C, and the NIH laboratory water white paper notes that even 10 to 15 ppb of carbon dioxide absorbed from clean air can pull a reading from 18.2 down to about 10 MΩ·cm. Ultrapure water therefore loses resistivity quickly once exposed to air, and it is best used shortly after it is dispensed.
The same document makes a point that is often missed: resistivity measures ionic contamination only. Non-ionic organic material such as sucrose barely changes the reading, and plate counts tend to underestimate viable microorganisms. Four parameters are therefore reviewed together:
- Resistivity or conductivity for dissolved ions.
- Total organic carbon (TOC), which the NIH paper identifies as critical in the 1 to 5 ppb range for applications such as HPLC.
- Microbial counts and endotoxin, which require separate periodic testing.
- Particulates, controlled by filtration at 0.2 µm or finer.
The broader classes of impurity are inorganic ions, organics, particles and colloids, microorganisms and their by-products, and dissolved gases. Each grade is, in effect, a statement of how many of these classes have been reduced and by how much.
How the water types differ in preparation
The common names describe a process, whereas the Type designations describe a result. The distinction matters when buying or specifying water.
Reverse-osmosis (RO) water. RO removes the majority of dissolved solids, particles, bacteria and large organics by forcing water through a semipermeable membrane. It is the usual basis of Type III water. Quality depends on the feed, so Type III water can vary between systems, regions and seasons, which is why it is used for less critical tasks and as feed to higher-grade systems (Sartorius buyers guide).
Deionised (DI) or demineralised water. Resin beds exchange dissolved ions for hydrogen and hydroxide ions, producing water with low conductivity on demand. It does not remove uncharged organics or microorganisms on its own.
Distilled water. Boiling and condensation remove most minerals and non-volatile matter. Volatile organic compounds can co-distil, and for that reason distilled water is not recommended for HPLC, LC-MS, ICP-MS, PCR or cell culture; those methods call for Type I ultrapure water at 18.2 MΩ·cm, according to John Morris Group Australia.
Ultrapure water. This is Type I water, typically produced from pre-treated feed (Type II or III) by ion exchange, UV and ultrafiltration stages. It is the grade that meets the tightest resistivity, TOC, endotoxin and particle limits.
Matching the grade to the application
The safest rule is to set the grade by the most sensitive method the water supports, then confirm it against the method's own requirements.
Type I applications
Type I is the choice wherever trace contaminants interfere with detection or with biological activity. This includes:
- HPLC, UHPLC and LC-MS, where organic contamination raises the UV baseline or produces ghost peaks in mass chromatograms
- ICP-MS, ICP-OES and atomic absorption, where nanogram-per-litre levels of calcium, sodium or iron can shift baselines
- PCR, qPCR and sequencing, where metal ions and nucleases inhibit amplification
- Mammalian cell culture, where endotoxins and organic contaminants can alter outcomes
These uses are described in Lab Manager's guide to purification systems and standards, which also notes that media prepared with Type II or III water can introduce variability in osmolality and endotoxin load.
Type II applications
Type II is appropriate for buffer and reagent preparation, microbiological media, ELISA and immunochemistry workflows, histology reagents and general spectrophotometry. It is also a common feed for clinical analyzers, autoclaves and washers. For most buffers Type II is sufficient, whereas standards for trace-level analysis are prepared with Type I (John Morris Group Australia).
Type III applications
Type III serves glassware rinsing, heating baths, autoclave feed and the first stage of a Type I or II system. Glassware used for trace analysis should have a final rinse in Type I or II.
A quick reference
| Application | Minimum practical grade |
|---|---|
| HPLC, LC-MS, ICP-MS | Type I |
| PCR, sequencing, cell culture | Type I (nuclease-free and endotoxin-controlled where relevant) |
| Buffers, reagents, ELISA | Type II |
| Microbiological media | Type II |
| Glassware washing, autoclave feed | Type III |
| Clinical analyzers | CLRW-grade, per the analyzer manufacturer |
Using a higher grade than required is technically acceptable, since Type I water can serve applications that call for Type II, but the extra polishing carries a cost without benefit for non-critical tasks.
Healthcare and clinical settings
Clinical analyzers are specified against CLRW rather than the ASTM tiers, and the analyzer manufacturer's water requirements take precedence. Beyond the purity grade, facilities also monitor the microbial condition of their distribution systems; an overview of water hygiene testing in healthcare sets out the main categories of testing used in regulated care environments.
Choosing a grade for a new workflow
A structured approach limits both over-specification and failed experiments.
- List every method the water will serve and note the most sensitive one. That method sets the grade.
- Check the method or standard by name. If it cites ASTM D1193, ISO 3696 or CLRW, use that definition rather than a vendor's label.
- Characterise the feed. Hardness, conductivity, chlorine and TOC in the incoming water determine how quickly consumables are exhausted and whether a Type I system can meet specification.
- Define verification. Record which parameters are checked, how often and against which standard, particularly under ISO/IEC 17025 accreditation.
- Plan for point-of-use quality. Ultrapure water degrades in storage, so dispense it close to use and monitor resistivity and TOC trends rather than waiting for an alarm.
Our perspective
Water is the largest single component of most laboratory preparations, and it is the reagent least often specified. In peptide research, reconstitution, dilution and buffer preparation all depend on a solvent whose composition is known. A grade chosen for convenience can introduce ions, organics or microbial load that confound analytical readouts, particularly in chromatographic purity verification.
At Au Peptide Labs, we supply bacteriostatic water and related laboratory supplies alongside our research peptides, stocked in Australia and dispatched from Melbourne. Our position is that supplies and compounds should be handled as a coherent system: peptides with batch-specific certificates of analysis, diluents suited to the protocol, and consumables that do not undermine the result. Selecting the correct diluent, whether sterile or bacteriostatic, is part of protocol design rather than an afterthought.
We recommend that every laboratory document the water grade used at each step of a workflow, in the same way it records lot numbers and storage conditions.
Dr. Authur
Diluents and supplies for peptide research workflows
For peptide reconstitution, the choice between sterile water and a bacteriostatic diluent affects how long a prepared solution can be held, and the client's guide to bacteriostatic versus sterile water covers the distinction. Buffer composition is a separate question, addressed in the guide to peptide buffer selection, which covers pH, salts and additives.
Au Peptide Labs stocks bacteriostatic water together with insulin syringes and alcohol wipes under Lab Supplies, for laboratory research use only. Orders ship from Australia, and Express Post is available for a one-off fee of $10. All products are strictly for laboratory research and are not for human or veterinary use.
Frequently asked questions
What is the difference between Type I and Type II lab water?
Type I is ultrapure, with resistivity of 18.2 MΩ·cm and very low TOC, endotoxin and particle levels. Type II has a minimum resistivity of 1 MΩ·cm and a higher TOC allowance. Type I is used for HPLC, PCR and cell culture, while Type II is used for buffers, reagents and general chemistry.
Can distilled water replace ultrapure water?
Not for sensitive methods. Volatile organic compounds can carry over during distillation and create baseline noise in chromatography or inhibit enzymatic reactions. Distilled water suits general tasks, whereas HPLC, LC-MS, ICP-MS, PCR and cell culture call for Type I.
Is Type III water the same as reverse-osmosis water?
In most laboratories, yes. Type III is generally produced by reverse osmosis, and its quality depends on the feed water, so it varies between systems. It is used for rinsing, autoclaves and as feed to higher-grade systems.
Why does ultrapure water lose resistivity after dispensing?
Atmospheric carbon dioxide dissolves into it rapidly. Only a few parts per billion are needed to lower the reading well below 18.2 MΩ·cm, so ultrapure water should be used promptly after it is collected.
Sources
- ASTM D1193-24 Standard Specification for Reagent Water
- ASTM D1193-06 (2018) Standard Specification for Reagent Water
- Laboratory Water, NIH Office of Research Facilities
- Different types of pure water for the lab, Xylem Australia
- Exploring Different Types of Pure Water for Labs, Across International
- Lab Water System Buyers Guide, Sartorius
- Distilled Water: How It's Made, Lab Uses and Purity Limits, John Morris Group
- Distilled vs Demineralised vs Deionised Water, John Morris Group
- A Comprehensive Guide to Lab Water Purification Systems and Standards, Lab Manager
- Types of water hygiene testing in healthcare