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By Dr. Estelle Riche, Brand Insights Contributor, Global Application Specialist, Milli-Q® Lab Water Solutions, from the Life Science business of Merck KGaA, Darmstadt, Germany
Purified water has many uses in a laboratory, from reagent and buffer preparation to glassware rinsing, and it plays a key role in the success of many experiments. Water purity is essential for the success of many analyses. Understanding the potential impact of water contaminants on liquid chromatography-mass spectrometry (LC-MS) and inductively coupled plasma-mass spectrometry (ICP-MS) analyses can help you select the optimal water purification solution for your laboratory.
Water plays a major role in an analytical laboratory. It can be used as an experimental blank, in sample preparation, to prepare standards, and even for washing and rinsing sample and reagent containers. In laboratories performing LC-MS, it may also be used to prepare mobile phases, while in ICP-MS, it may be used, in combination with acids, for rinsing instrument lines or cleaning the cones or nebulizers. In all cases, it is important to use high-quality water to ensure accurate and reliable results because any contaminant present in the water will potentially affect the results.
Potential effects of water contamination
There are many potential effects of water contamination on experimental outcomes. It can cause erroneous results, which can lead to a need for troubleshooting. Ghost peaks or noisy baselines in LC-MS, for example, can make it difficult to interpret your data. In addition, it may reduce the lifetime of your column, or, if you are performing ICP-MS, force you to clean your nebulizer or cones more often than normal. Another potential effect of water contamination is the need to repeat experiments, causing you to lose some sample as well as reagents. This can be an issue if you don’t have access to a large amount of sample.
Using water that is free of your analytes of interest and of contaminants that could have an impact on the analytical instrument itself will help you ensure you obtain reliable and accurate results.
What’s in your water?
When you turn on the faucet in your lab, a lot more than H2O comes out—many trace contaminants are present in tap water. These contaminants are broken up into five families. The first is organics—either molecules from nature, i.e., the degradation of leaves and trees, or contaminants from human activity such as pesticide or medication residues. The second family is ions, e.g., calcium or magnesium, that come from the rocks and the ground the water has been in contact with. Next is particles such as pieces of sand or dust, as well as colloids. Bacteria may also be present, although tap water is treated to ensure that no harmful bacteria are present. The last family of contaminants is gasses, e.g., dissolved gases such as oxygen or carbon dioxide.
Even though many of these families of contaminants appear in low levels that are safe for drinking, they could have an impact on results or/and instruments, especially when using LC-MS or ICP-MS to perform trace or ultra-trace analyses.
Inorganic salts and dissolved organics are known to affect most laboratory experiments and are monitored online in advanced laboratory water systems via resistivity and total oxidizable carbon (TOC) monitoring. In most cases, when using ultrapure water, the TOC should be at 5 ppb (micrograms per liter) or below and the resistivity should read 18.2 MOhm.cm at 25°C, which means there are no ions in the water. But neither of these figures alone reflect the total purity of your water, so it is very important to look at both values.
Several norms define the quality required for specific laboratory water applications, and water purification system manufacturers have defined three different types of purified water. The type of water you select for specific laboratory techniques will depend on their sensitivity to water contaminants. Type 3 water is the least purified, often by reverse osmosis, and is used for regular rinsing, heating baths, and any other basic activity. Type 2 is called pure water and is purer than Type 3. It can be used for all general laboratory applications such as to prepare buffers or reagents and for the final rinse in glassware washing machines. Type 1 water is ultrapure or reagent-grade water. This is the purest water grade and is what we recommend for critical applications such as LC-MS, ICP-MS, and molecular or cellular biology.
How is water purified?
Each water purification technology has its benefits and limitations. Water purification systems therefore combine several technologies in order to remove the different contaminants and provide the appropriate quality of water for critical applications.
Water is typically purified in two steps. The first step is called pre-treatment, which turns tap water into pure water. This step removes over 95% of all contaminants from water. Tap water usually has ppm (mg/L) levels of ions and organics. After pre-treatment, the water contains only ppb (µg/L) levels of ions and around 50 ppb of organics.
The second step, called polishing, results in ultrapure water. After the polishing step, the levels of ions are in the ppt range and organics are below 5 ppb.
You may choose to combine a system delivering pure water (such as a Milli-Q® IX system) with a polishing system that delivers ultrapure water (such as the Milli-Q® IQ 7000 system) or select an all-in-one solution that delivers both pure and ultrapure water (such as the Milli-Q® IQ 7003/05/10/15 system).
The latest Milli-Q® systems generate ultrapure water from tap through the following combined sequence of technologies (see FIGURE 1). The pretreatment step combines reverse osmosis, which removes between 95 – 99% of all contaminants, electrodeionization (EDI), which further removes ions, and a mercury-free bactericidal lamp. The storage tank provides multi-targeted prevention of bacterial, particulate, and CO2 contamination, and water is regularly recirculated so that high-quality pure water is always on hand. Once Type 2 pure water is produced, a mercury-free photo-oxidation lamp oxidizes remaining traces of organic molecules, then the water goes through the polishing cartridge containing activated carbon and ion-exchange media to produce ultrapure water.
The impact of water quality for LC-MS
Water plays an essential role in LC-MS, and organic contamination will directly affect the quality of the data obtained by affecting baselines or causing extraneous peaks (see FIGURE 2). Milli-Q® Lab Water Solutions offers different point-of-use purifiers for sensitive analyses. Users have a choice between a 0.22 µm screen filter that removes particles and microorganisms larger than 0.22 microns, and a final filter that contains C18 silica to remove traces of organics lingering in the water and is optimal for ultra-trace analyses. Other point-of-use polishers are available, such as one designed specifically to remove endocrine disrupters such as phthalates of bisphenol A or one specific for volatile organic contaminant removal.
Also be aware of particles and bacteria—they can accumulate on column frits and cause back pressure. Bacteria can also multiply and release contaminants into your column. Lastly, water with even a small number of ions can cause the mass spectra to become more complex, as the sodium or potassium ion combine with the fragments and cause adduct peaks, and/or cause ion suppression.1
You must, therefore, use water that contains as little organics, bacteria, particles, and ions as possible to prepare your mobile phase, samples, standards, and experimental blanks.
What happens to stored ultrapure water?
Unfortunately, stored ultrapure water does not stay ultrapure. As soon as the water comes out of the purification system, it is susceptible to contamination. It can be contaminated by the container it is placed in, the environment or air in the lab, or even organic solvents used in your lab, which can absorb into the ultrapure water and generate contamination.
As a rule of thumb, freshly purified water should be used whenever possible for optimal results. And, if you are conducting sensitive organic analysis such as LC-MS, avoid storing water in a carboy or another plastic container because they can release plasticizers into the water; borosilicate bottles are preferred.
The impact of water quality for ICP-MS
ICP-MS is the most sensitive type of elemental analysis technique, and the most interesting because it can measure a wide range of elements and levels. However, the more sensitive the technique, the more important laboratory conditions become. Similarly, the more sensitive the analysis, the more important the water quality becomes.
The laboratory environment presents a significant risk to water quality, as ions present in the air can easily be absorbed by ultrapure water. Ionic contamination of water can cause inaccurate measurements of the analyte, noisy background and decreased sensitivity, and memory effects. It is therefore necessary to control contamination in the lab and preferably work under clean-room conditions.
If you are conducting trace or ultra-trace elemental analysis, an additional purification step is recommended. The Milli-Q® IQ Element water purification and dispensing unit is specifically designed to answer the most stringent requirements of trace elemental analyses. It offers an extra step in purification comprised of two features. The first is an ion exchange cartridge that combines a mixture of high-purity ion exchange media to remove ions down to trace levels. The second is an ultra-clean 0.1-micron final filter with a charged membrane that ensures particles and colloidal traces are removed from the water. The dispensing unit fits in a hood and features a footswitch for hands-free rinsing (see FIGURE 3).
The bottom line
Water is an important solvent that needs to be handled with care. As analytical techniques are becoming increasingly sensitive, the potential impact of water quality on experimental results cannot be overlooked. Being knowledgeable about which contaminants can have an impact on your experiments will help you to wisely choose the most appropriate water-purification solution for your experiments.
Selecting a water system goes beyond the need for contaminant-free water that meets the lab’s output and regulatory demands. The latest Milli-Q® water purification systems offer additional benefits, such as ergonomy, ease of use, and simple maintenance; they have been designed to facilitate data traceability and management, and they support environmental sustainability through the reduction of water usage, the use of mercury-free lamps, and an overall reduction in size.
For additional information about this topic, we invite you to:
Watch the webinar Good Water Practices for Reliable LC-MS and ICP-MS Analyses.
Get more information on the Milli-Q® IX System
Get more information about Milli-Q® Lab Water Solutions
1Khvataeva-Domanov A, Mabic S. Why and How to Avoid Ionic Contamination in Water Used for LC–MS Analyses. Spectroscopy Online 2015;13(3):28–32. http://www.spectroscopyonline.com/why-and-how-avoid-ionic-contamination-water-used-lc-ms-analyses