Classic kit: Craig's rotary evaporator
Years ago, a non-chemist friend of mine visiting my lab asked me what a rotavap was for. After my explanation, he asked, 'do you guys ever make hooch with that thing?' I was reminded of this when I visited my cocktail mixologist friends Tony Conigliaro and Spike Marchant. Tucked under the eaves of the roof of their bar in north London, UK, is the cramped laboratory where they prepare the exotic concentrates, syrups, and decoctions that make their cocktails so special. In addition to a pair of glass-fronted refrigerators crammed with fruits and leaves, steel shelves groaned with equipment - blenders, mixers, filters, shakers, and much more. By the window stood their pride and joy: a gleaming rotary evaporator. At its lower end spun a large pear-shaped flask filled with green leaves and a clear liquid, with droplets of clear condensate dripping gently into the receiver. The sight made me grin.
One of Walther Büchi's first commercial rotary evaporators, dating from 1959
© BÜCHI GMBH
At some point in most synthetic chemistry the solvent has to be removed from a mixture. For solids this might be done in open containers using very gentle heat. But otherwise one had to set up a distillation, methodically assembling glassware joined by means of carefully bored corks, applying a Bunsen flame ever so gently to the flask. To speed up the process, a vacuum could be applied to lower the boiling point. The problem, as every chemist knows, was the ever-present danger of bumping that arises from local superheating. Since boiling chips lose their usefulness under vacuum, a stream of bubbles introduced with a capillary (Chemistry World, October 2008, p79) can provide nucleation sites. But this raises the pressure and limits what can be distilled.
Some dogged chemists tried to find ways to induce nucleation by mechanical means, such as violently shaking the entire distillation apparatus to induce bubble formation - an operation not for the faint of heart. In his magisterial book on laboratory technique, Avery Morton rather incredulously reported that 'a way has even been found of rotating the flask during distillation', as if the chemist in question had lost his mind.
Indeed, in 1924, the Irish chemist H G Becker had reported that by using a well-lubricated, extra long ground glass joint, it was possible to spin the flask using a loop of rope and a pulley driven by an electric motor. The rotation - not too fast, Becker said - mixed the solvent enough to prevent hotspots, but also provided a thin film of liquid that would favour evaporation. But although already used in industry, the principle was too radical for laboratory chemists.
Lyman C Craig
© NATIONAL LINRARY OF MEDICINE
But his extraction methods meant that removing solvent was always an issue. In 1950, in a paper amounting to less than a page, he described his invention - a flask, spun by Becker's pulley, from which solvent could be distilled under reduced pressure into a second flask. It sparked a host of imitators and developments. In 1957, an ambitious Swiss glassblower, Walther Büchi, began selling the first commercial rotavap. They went like hot cakes, becoming known simply as 'Büchis' - leaving Craig almost forgotten - and the pace of synthetic chemistry accelerated by an order of magnitude.
© ANALYTICAL CHEMISTRY
50 years later, the device began to invade kitchens led by pioneers like Ferran Adria and Heston Blumenthal, and now the laboratories of mixologists. So next time you drink a mint julep or a horseradish Bloody Mary, just think: those flavours may have come through, of all things, a rotavap.
Andrea Sella teaches chemistry at University College London, UK
L C Craig, J D Gregory and W Hausmann, Anal. Chem. 1950, 22, 1462