HPLC-grade water and bacteriostatic water are both high-purity waters sold in sealed containers, and that is where the similarity ends. One is specified for what it does not contribute to a chromatogram; the other is specified for staying sterile and preserved in a vial that is entered repeatedly.
Side by side
| Property | HPLC-grade water | Bacteriostatic water |
|---|---|---|
| Purpose | Mobile-phase and sample preparation for chromatography | Preserved sterile diluent in multi-use vials |
| Filtration | Filtered through 0.2 µm (often 0.1 µm) to protect columns and pumps | Sterile-filtered or terminally sterilized as part of manufacture |
| UV absorbance | Specified to be very low across the UV range (for example below 0.01 AU at 254 nm) | Opaque between about 240 and 275 nm because of benzyl alcohol |
| Organic content | Very low TOC; no additives | 9 g/L benzyl alcohol, an organic compound by design |
| Sterility | Not claimed | Sterile, tested per USP <71> |
| Endotoxin | Not tested | Not more than 0.5 EU/mL |
| Preservative | None | 0.9% benzyl alcohol |
| Container | 1 L to 4 L bottles | 3 mL, 10 mL, 30 mL vials |
| Once opened | Use promptly; no microbial protection | 28-day in-use convention |
Why bacteriostatic water cannot be used as HPLC water
Benzyl alcohol is a UV-absorbing organic compound at 83 mmol/L. In a mobile phase it would produce a huge baseline at UV wavelengths, elute as an enormous peak on reversed-phase columns, and foul detectors. Even a small amount carried into a sample would appear as an interfering peak. See bacteriostatic water as a UV-Vis blank and when the preservative interferes.
Why HPLC water cannot be used as bacteriostatic water
HPLC-grade water is neither sterile nor preserved nor endotoxin-tested, and it is supplied in litre bottles that are open to the air every time they are poured from. It is excellent water for chromatography and an unsuitable diluent for anything that needs a sterile, preserved, multi-use product. See is bacteriostatic water sterile?
Grades in context
“HPLC grade” is a supplier specification for chromatography solvents; “USP” on bacteriostatic water is compliance with a pharmacopeial monograph. They are different systems of specification and are not ranked against each other. See chemical grades explained and USP water grades explained.
Related
Bacteriostatic water vs. ultrapure (Type I) water · Bacteriostatic water alternatives
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