I’m sure you might have heard this phrase, but possibly not related to liquid chromatography. 😀
Anyway, if you read my last blog post on LC Ghost Peaks, you know that it is possible to experience ghost peaks and baseline disturbances from contamination in the aqueous part of your mobile phase. The question may arise as to where you should be obtaining your water to make mobile phase. This can either come from a purification system you may have at your facility, or your lab might be purchasing bottled water that is LC or LCMS grade. Whatever you choose, it should meet the requirements of Ultrapure Water, otherwise known as Type I water by the ASTM. The key requirements for Type I are listed in ASTM Method D1193-99E1 as follows:
Resistivity of >18 MegaOhms
Conductivity of <0.056 uL/CV
TOC (Total organic carbon) of <50 ppm
There are also specific requirements for maximum levels of sodium, chlorides and silica. However, excess levels of these would impact the resistivity and conductivity, so those are a good way to monitor those ions also.
Most in-lab purification systems such as MilliQ do monitor the above requirements to meet these qualifications. Usually the resistivity is monitored on a daily basis or each time water is withdrawn from the system.
You may be curious about the other types of water that are available and how they are obtained. Here is quick summary.
Type I- Distilled, deionized or purified by reverse osmosis, “polished” to remove additional ions by ion exchange, filtered to 0.2 µm with a membrane filter. This includes both HPLC grade water and LC-MS grade water. It depends on the brand, but LC-MS grade is often slightly lower in total carbon/organic content and it should be relatively free of metal ions that can create adducts when using LC-MS analyses.
Type II- Produced by distillation, but might be supplemented with ion exchange, reverse osmosis or additional ion exchange to achieve a conduction of less than 1.0 µS/cm. Type II is used as general reagent water, suitable for many common lab purposes, such as reagents for microbiology, spectrophotometry, electrochemistry or a final rinse for glassware.
Type III- Usually produced by reverse osmosis, but could be deionized or distilled. It is then filtered to 0.45 µm with a membrane filter. This type of water is often used as feed water to produce Type I. Sometimes it can also be used for cleaning glassware, media preparation or heated water baths.
Type IV- produced by reverse osmosis, deionization or ion exchange, with no additional polishing or filtration. Type IV is often used for industrial applications.
You might also wonder about what harmful materials could be in the water. I have narrowed it down to a few broad categories here:
Particulates- mostly sand, dust, and colloids. These can create clogs and result in high pressure. Colloidal materials can sometimes be irreversibly adsorbed, resulting in loss of efficiency.
Organics- These can coat packing material and impede mass transfer, result in loss of resolution and efficiency. In highly aqueous phases, these can also compete with analytes for active sites on the stationary phase. It is also possible that these can accumulate and elute later as ghost peaks. At high levels, these can also potentially change selectivity, cause RT shifts and tailing. It is also possible that at high levels, organics can increase pressure.
Bacteria or other microbes- These can grow and eventually product particulates and/or organic contamination. This could result in any of the above conditions.
Ions- These modify the ionic strength of the mobile phase and could affect retention, depending on the application. These are of greatest concern with LCMS because they can also create adducts, for example with Na+ (sodium) ions.
Clearly, it is important to make sure the above contaminants are not an issue with your water for mobile phases. If you need to ask questions specifically about the water that you use, please contact the vendor (for purchased water) or the manufacturer of your purification system.
Thank you for reading this blog post and good luck on your analysis.
If you are interested seeing more information on this topic from industry leaders, you might try performing an internet search for “purified water LCMS”. There are a considerable number of blog posts, articles and webinars available. Here are some suggestions for additional reading: