Erica Tennenhouse, PhD
Bumping and foaming are problems that commonly arise during evaporation and can lead to sample loss and contamination. As Jim Dawson, president of Heidolph North America (Elk Grove Village, IL), explains, the primary culprits are applying “too much heat, too much rotational speed, or too much vacuum” to a particular application.
Bumping occurs when the sample boils and bubbles rapidly, causing the product that is being concentrated to splash out of the vessel. Roland Anderson, laboratory products manager at KNF Neuberger (Trenton, NJ), gives the following analogy: “If you had sauce in a pan that you were looking to concentrate, and you were stirring it at a very low temperature, you would be able to concentrate it without splashing the sauce everywhere. If you were simply to turn the heat up on that saucepan and let it start bubbling, you would splash that sauce around the outside of the pot.”
Foaming, on the other hand, results from “surfactants, which can be present in natural extracts as well as artificially made products,” explains Bernie Hertz, laboratory evaporation product manager at BUCHI Labortechnik AG (Switzerland). Foaming is a slower process than bumping, he says, and foam tends to stay inside the evaporating flask for a longer period of time.
Tricks of the trade
The traditional way to avoid bumping during evaporation is through visual inspection, says Anderson—simply watching the samples and adjusting the vacuum pressure so that bumping doesn’t occur.
Related Video: Choosing the Best Rotary Evaporator for Your Application
Foaming cannot be avoided per se, Hertz notes; it can only be reduced. “The most common method [to minimize foaming] is to open and close the stop cock whenever foam appears,” he says, adding, “A great way to diminish the issue is to slowly apply vacuum.”
Along with these methods, several products are available that can reduce sample loss and contamination when bumping and foaming occur, help eliminate bumping altogether, and minimize foaming.
Bump traps, foam brakes, and more
To specifically address the issue of foaming, Hertz suggests using an anti-foam agent, a large flask, or a dedicated glass condenser, which provides much more space for foam to expand. Dawson adds that a simple glass foam brake can help keep foam from entering the receiving flask.
A bump trap, which is a secondary flask that is smaller than the main flask, is also often used during evaporation. This additional flask is placed right above the main flask so that if bumping occurs, the sample will get caught in the bump trap before getting drawn into the rest of the rotary evaporator or condenser.
The solution is not elegant, Anderson notes. “Putting a bump trap in line doesn’t reduce the bumping, but it does prevent the sample loss.”
Vacuum controllers get closer to the root of the problem. By controlling the speed of the pump, these devices can drastically reduce or eliminate the likelihood of bumping.
KNF Neuberger’s new vacuum controller has an automatic feature that senses the vapor pressure of the solvent and uses that as a set point. This feature allows the user to optimize the speed of evaporation without bumping. “When using that controller, we don’t ever recommend people put a bump trap in line,” says Anderson.
Then there are foam sensors, which “detect the foam inside the evaporating flask and aerate the system slightly to make sure that the foam collapses,” Hertz explains. “The foam sensor allows automatic, unattended distillation of foaming samples.”
In Dawson’s opinion, automation is the ideal solution to bumping and foaming. “We want to manage the conditions by automatically adjusting the vacuum,” he says, pointing out that Heidolph’s AUTOaccurate program supports precise automatic pressure control for evaporation.
“But what’s interesting,” says Dawson, “is that despite all these advances in automation, chemists tend to prefer to adjust manually.” Budgetary constraints may be part of the reason, but then there are those seasoned and patient chemists who simply feel more comfortable with the manual methods they have always used, he explains.