Josh P. Roberts has an M.A. in the history and philosophy of science, and he also went through the Ph.D. program in molecular, cellular, developmental biology, and genetics at the University of Minnesota, with dissertation research in ocular immunology.
Lab water is a lot purer than what comes from the city tap, yet it may still not be pure enough to meet the requirements of today’s molecular biology, protein, and cell culture laboratories. For these applications, you’ll minimally need a purifier that can deliver what’s called ASTM Type 1 water.
“All companies that I’m familiar with follow the ASTM [formerly the American Society for Testing and Materials] standard,” said Julie Akana, product manager for water purification at Thermo Fisher Scientific. “It shouldn’t matter what company you choose, you’ll get the same quality.”
That said, there are a wide variety of point-of-use water purifiers to choose from, from small coffee maker-like units that sit on a bench top to larger, plumbed-in, under-sink or wall-mounted systems that can deliver a continuous supply of 2 liters per minute or more. Some are better for HPLC and mass spectrometry applications while others are better for PCR. Most require semi-pure lab water from a centralized system, or an in-lab pre-treatment system, as input. But what they all have in common is the ability to deliver nearly ion-free water with low organic, pyrogen, particulate, and colloid counts.
What's your application?
With a Type 1 system, “all the hard work is happening in the ion exchange cartridge,” says Akana. Accessories may be added to further purify the water for specific applications.
Thus the first question Sartorius arium® national sales manager Anthony Chiarello asks customers is, “What are you using the water for?”
For instrumental analysis you want to get rid of as much total oxidizable carbon (TOC) as possible, and Chiarello recommends adding a UV bulb to the system. A low-pressure UV lamp offers not only the obvious bactericidal effect, but also create free hydroxyl radicals that cleave uncharged organic molecules into mildly charged products. These can then be picked up and removed by the scavenger resins in the filtration cartridges.
For biological work, where macromolecules such as endotoxins are an issue, Chiarello recommends ultrafiltration (UF), passing the water through something akin to a membrane or hollow fiber resin with a low molecular weight cutoff. “It’s going to give them RNase-free water, DNase-free water, pyrogen-free water.”
But be advised: the ultrafilter itself is composed of organics, says Bob Applequist, product manager for the Labconco water purification line, “so it imparts organics back into the water. For HPLC you do not want to have that water filtered through an ultrafilter.” If your lab is doing both analytical and molecular biological work, Applequist recommends a unit that lets users choose whether to draw water through the ultrafilter or bypass it.
Feed water?
The vast majority of Type 1 lab water purification units sold in North America are fed by a building-wide system that removes most of the impurities, says Chiarello. But because of construction, maintenance, electrical and other costs the trend is away from such systems; newly built laboratories tend to create Type 1 water without the benefits of a central system, he says.
The conventional way to do this is using a pre-treatment system, says Akana. Because pre-treatment is typically a much slower process than “polishing” pre-treated water, the pre-treated water is stored for use by the on-demand Type 1 system. It can also be used for feeding dishwasher rinse cycles, water baths, autoclaves, and humidifiers, as well as for things like making up non-critical reagents.
More recently vendors have introduced “tap to Type 1” systems, integrating the pre-treatment (typically reverse osmosis (RO)) and polishing systems. While larger units require external water storage, many smaller units utilize an internal tank. Many systems allow for both pre-treated and Type 1 water to be drawn.
Even in cases where there is a central lab water system it may not meet the polishing system’s requirements for feed water. Many companies offer free water test kits “to ensure that pre-treated water is really pre-treated water, and that their in-house system is being properly maintained,” points out Akana.
How much do you need?
Sartorius sells a specialized cartridge for feeding a polishing unit with tap water. But “the economics are such that if you’re using more than 5 liters/day you’ll be changing that cartridge too often,” says Chiarello. “There’s a finite capacity in even the best resin in the world. It’s simply a matter of how much contamination you put into the system, and how much water.”
There is a wide spectrum of systems offering daily capacities to meet the needs of any lab. These, as well as flow rate, are generally published by the manufacturer and can be found on their web sites. For example, EMD Millipore’s Super-Q® Plus System produces up to 12 liters/min and 4,000 liters/day. At the other end of the spectrum, the company’s non-plumbed Simplicity unit – with a 2 liter refillable reservoir – is designed for labs using 1 to 5 liters per day and boasts a flow rate of > 0.5 liters per/min. Perhaps more typical are Thermo Scientific Barnstead’s 15–200 liters/day capacities.
If you’re looking at pre-treatment as well as polishing, pay attention to at least two other numbers: The rate at which the RO system can fill a tank, and the size of the tank itself. Even though the RO can be rate limiting, tanks can be filled overnight to allow for a substantial reservoir – Labconco offers a range of tanks with up to 70 liters capacity – from which to draw, Applequist points out.
Accessories, bells and whistles
Many manufacturers offer what’s called “remote dispensing” options, in which the Type 1 water is delivered from the end of a flexible tube (in some cases up to 10 feet) rather than directly from a spigot on the unit, even for some smaller coffee maker-like benchtop units. For some the remote dispenser doubles as a display unit containing all the control functions so the main unit can be placed in a cabinet or otherwise inaccessible location. Some Thermo Fisher systems, for example, allow for up to three remote dispensers that can be used simultaneously.
TOC monitors, in many cases required by the FDA and other regulatory bodies for specific applications, are available on many units. Similarly, many units have RS232 ports that allow printing of reports, and Sartorius’ units feature an SD card slot for data collections.
Some units will allow the flow to be automatically turned off after a specified time or volume has been reached, for set-it-and-forget-it operation. And for those who forget to use this function, or if other disasters occur, leak detectors capable of shutting off water flow are also available.
The bottom line is to choose a system that meets your lab’s current (or better yet, anticipated) needs in terms of feed water usage, output quality and quantity, reporting, space- and plumbing requirements, convenience and even styling. And remember to estimate high, Chiarello says. “Once you have the purified water system up on the wall, people will use it for everything.”