Four Ways to Better Water Quality in LC-MS

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High-purity water is key to the success of analyses performed using (ultra) high-performance liquid-chromatography-mass spectrometry, (U)HPLC-MS—also known as LC-MS

This article will discuss several aspects of working with high-purity water in LC-MS analyses, including quality, handling, complementary purification technologies and the importance of maintenance.

Start with the best-quality water
Natural water contains several major classes of contaminants, including inorganic ions, organic molecules, particulates and colloids and bacteria and their by-products. As these contaminants are also present in tap water, water must be carefully purified before it can be used in highly sensitive analytical techniques, such as LC-MS, in order to avoid impacting analyses.

Laboratory water purification system manufacturers have defined laboratory water types as Type 1, 2 or 3 according to the level of contaminants contained in the water. Type 1 is the purest, and is referred to as "ultrapure," "high purity" or "Milli-Q water." Type 2 water refers to pure water, or "Elix water." Type 3 water is the lowest grade of laboratory water.

To avoid interferences with measured analytes in (U)HPLC and LC-MS applications, and to maintain the (U)HPLC system in proper working condition, Type 1 ultrapure water is extensively used in the preparation of standards, blanks and samples, and as a component of mobile phase. Water is also needed to wash and rinse containers: For LC-MS applications, this water should also be ultrapure, in order to avoid introducing a contaminant to an otherwise uncontaminated sample.

Advanced water quality monitoring enables information to be obtained about both ionic and organic contaminants, so the user can retain control over water quality at all times. High-precision resistivity monitors provide data on the ionic purity of the water dispensed from the water purification system. A resistivity value of 18.2 MΩ·cm means the number of ions contained in the ultrapure water isn’t significant (total ionic concentration is

Total Oxidizable Carbon   (TOC) monitors measure organic purity in general, and show levels of organic contamination in terms of parts per billion (ppb). Traditionally, a low TOC level is achieved through UV photooxidation technology that reduces TOC in ultrapure water to less than 5 ppb.

High TOC levels mean there’s a greater probability interferences with analytes will occur, or experiments will fail, leading to lost time to understand the reason for failure, and/or to repeat the experiment.

Handle with care
Ultrapure water is an extremely good solvent, and once dispensed from the water purification system, it tends to be contaminated quickly from a variety of sources, including the laboratory environment and any storage containers used. Water quality can also degrade if the water purification system doesn’t work properly.

Although, ideally, ultrapure water should be dispensed from the water purification system just before use in LC-MS analyses, in real-life situations, everyone may not have this option, and it may be necessary to store ultrapure water for a short time.

If this is the case, then there are several factors to take into consideration. First and foremost, it’s important to choose an appropriate container for the application—and to select the best quality possible.

In addition to absorbing contaminants from the laboratory atmosphere, ultrapure water that’s in contact with a storage container will also absorb ions and organics. Glass containers will release ions, and over time, ultrapure water stored in a glass bottle will pick up these ions. For LC-MS users, ionic contamination can be an issue when ions form adducts, creating additional peaks in mass spectra, complicating data analysis and creating interferences with measured analytes.

Glass containers can also release organics into water, but in smaller quantities than plastic containers, such as polyethylene carboys, which can leach plasticizers. If users must store ultrapure water for LC-MS, it’s best to do so in glass—preferably borosilicate bottles—rather than plastic containers, and for the shortest time possible.

LC-MS users of ultrapure water should also be aware that any containers used for water collection—including caps—should always be cleaned thoroughly, and then rinsed several times with ultrapure water to remove any traces of contaminants or detergents before water collection.

Combine purification technologies 
No single water purification technology is capable of fully removing all classes of water contaminants. Therefore, in addition to selecting a system that meets the laboratory's criteria with regard to sensitivity analyses, monitoring needs and system size, LC-MS scientists should also choose an ultrapure water purification system that uses a combination of several purification technologies.

A typical water purification chain starts with a pre-treatment pack followed by a reversed osmosis (RO) membrane. Together these technologies will decrease the level of organics from 1,000 ppb to 50 to 100 ppb, and provide Type 3 water in which more than 95% of all ions have been removed. RO treatment is followed by electrodeionization in the Elix module to further remove ions, as well as treatment with a germicidal UV lamp to prevent bacterial growth. This Type 2 water has a low TOC level and resistivity at or above 5 MΩ.cm. However, the water must be further treated to obtain water of a quality suitable for (U)HPLC analyses. This last stage consists of final purification (or polishing) steps, using ion-exchange resins, synthetic activated carbon and UV photooxidation, in order to produce 18.2 MΩ·cm ultrapure water with a TOC level at or below 5 ppb.

Finally, a point-of-use filter can be placed at the end of the chain, with the dual purpose of adding a purification step immediately before water is delivered and used, and also preventing retro-contamination of the purification chain from airborne sources. For LC-MS, the focus of this final purification step can be either to prevent particles and bacteria in ultrapure water, and/or to remove traces of organics that are rarely still present in the water.

 

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