Bacteriostatic water is sterile and endotoxin-controlled, which sounds ideal for analytical work, but it carries 9 mg/mL of benzyl alcohol, and an analytical method sees that as a 0.9% impurity. Whether it matters depends entirely on the detector. This guide goes method by method.
UV-visible spectrophotometry
Interferes. Benzyl alcohol has an aromatic ring with absorbance maxima near 257 nm and a much stronger band below 220 nm. At 9 mg/mL the absorbance is large. A blank of bacteriostatic water against a sample diluted in the same water cancels the baseline in principle, but any mismatch in preservative concentration between blank and sample, and any analyte band in the 200 to 270 nm region, is compromised. Use preservative-free water.
HPLC with UV detection
Interferes. Benzyl alcohol elutes as a substantial peak on reversed-phase columns and absorbs at the wavelengths most methods monitor (210, 220, 254, 257 nm). It can co-elute with early-eluting analytes and it contaminates the column with each injection. Samples, standards and diluents for HPLC-UV should be prepared in preservative-free water or the mobile phase. The same property is what makes HPLC-UV a good method for assaying the preservative itself; see the benzyl alcohol assay.
Gas chromatography
Interferes. Benzyl alcohol is volatile enough to chromatograph and appears as a peak with flame ionization or mass spectrometric detection. Its oxidation products, benzaldehyde and benzoic acid, may appear as additional peaks in older vials. Unsuitable as a diluent for GC unless the method has been validated with it.
LC-MS
Usually tolerable, sometimes not. Benzyl alcohol ionizes weakly under electrospray and is generally not a major background contributor, but at 0.9% it can cause ion suppression of co-eluting analytes and contribute a benzyl (tropylium) fragment at m/z 91 in the source. Methods should be checked for suppression with and without the preservative; when in doubt, use preservative-free water.
Total organic carbon (TOC)
Interferes completely. Benzyl alcohol is organic carbon, roughly 7 g of carbon per litre at 0.9%, which is orders of magnitude above any TOC limit. Bacteriostatic water is never a blank or diluent for TOC.
Conductivity and pH
Minimal effect. Benzyl alcohol is non-ionic and does not change conductivity or, at 0.9%, pH. Bacteriostatic water behaves like pure water in electrochemical measurements, apart from any dissolved CO2. See the pH of bacteriostatic water.
Titrimetric and gravimetric methods
Method-dependent. Acid-base titrations are unaffected; oxidative titrations (permanganate, dichromate) will consume oxidant on benzyl alcohol and give high results. Gravimetric methods that evaporate the solvent will leave a benzyl alcohol residue unless it is driven off at temperature.
Fluorescence
Usually tolerable. Benzyl alcohol is weakly fluorescent in the deep UV and rarely interferes with visible-range fluorophores, but it can quench or scatter in some systems. Validate.
Summary table
| Method | Bacteriostatic water as diluent or blank |
|---|---|
| UV-Vis, HPLC-UV, GC, TOC | No |
| LC-MS, fluorescence, oxidative titration | Only if validated |
| Conductivity, pH, acid-base titration | Generally acceptable |
The rule
If the detector responds to organic carbon or to UV absorbance, or the method is quantitative to the percent level, use preservative-free water for everything the instrument sees. Bacteriostatic water belongs upstream of analysis, for example as a multi-use sterile solvent for preparing a research material that will later be analyzed after dilution in the proper diluent. The broader picture is in laboratory uses of bacteriostatic water.
All BAC Water Supply products are supplied for laboratory research use only and are not for human or veterinary use.