Solvent and reagent quality has long been a topic of interest to analytical chemists using liquid chromatography. Few references address the purity of solvents used to prepare mobile phases . Water delivered at the final purification stage is used to prepare the mobile phase. However, the initial pretreatment step is equally critical.
Water purified using ion exchange resins (DI)was fed to the polishing unit which combined UV photo-oxidation process with purification packs made of mixed -bed ion exchange resins and activated carbon. This type of polishing unit is recommended for HPLC and LC MS work .High backgrounds and unidentified peaks were observed both on PDA and MS plots . An alteration in the pretreatment step, that is a combination of reverse osmosis (RO) and electrodeionisation (EDI) was used. Analysis of these results clearly highlights the absence of contaminant peaks.
Service deionisation provides very good water quality in terms of ionic purity. A large part of charged organics are also retained well be the ion exchange resins. However, neutral organics and particulates have no affinity for the ion exchange sites and hence flow directly through the SDI bottles. Important variations of organic levels are expected because of the changes in the tap water composition. In addition some organic molecules will stick to the resin and lead to a loss of ion removal efficiency .
The advantage of a membrane based technique like RO in the pretreatment step is the rejection of a larger spectrum of contaminants: ions, organics and particulates. Ions typically are rejected at a 95-98% level , while organics are removed above 85%, depending on their size and polarity. Although none of the category of contaminants are fully removed, the RO significantly reduces the overall load of the contaminants.
To address this, RO is associated with EDI, which diminishes the levels of charged molecules (inorganic and organic ions ). The EDI technology combines selective cationic and anionic membranes, ion exchange resins, and a low continuous electrical current. The current allows for a continuous regeneration of resins. Performances of the RO-EDI should provide consistently pure water with a resistivity above 10 Meg Ohm.cm and a total organic level below 30 ppb.
Polishing units a re designed to handle TOC loads that are below 50ppb. Then TOC values in the final water are expected to be below 5ppb. Beyond a 50ppb TOC level in the pretreated water, the final water quality may reach values over 5ppb of TOC and certain organic molecules may be detected in high enough concentrations to interfere with LC - MS analyses. Other methods like distillation are not fully efficient for organic removal. Organic molecules with boiling points close to the boiling point of water, or with a very hydrophilic structure can be found in distilled water. (TOC levels typically range from 30-100ppb). When the levels are too high, the polishing unit cannot eliminate these organics.
To conclude, the performances of the polishing units are optimized with suitable pretreatment technologies. In particular the removal of both ions and organics is essential to work on HPLC and LC- MS in optimum conditions and to avoid interferences because of contaminants present in the water for mobile phase preparation. The combination of RO and EDI stands as a reliable pretreatment to offer consistency and quality of the delivered water.
(The author is Application Support Manager, Millipore India Ltd, Ban galore)