For most of the past 40 or so years that pressure-driven liquid chromatography (LC) has been used, problems related to air bubbles in the pump and detector have been at the top of the list of LC-related problems. The regular use of helium sparging or vacuum degassing reduced these problems to an acceptable level in many laboratories. More recently, the introduction of in-line vacuum degassing systems has become the norm in most laboratories. My unscientific, finger-in-the-wind survey through e-mail questions, the classes I teach and Chromatography Forum ( http://www.chromforum.com/) suggests that bubble-related problems have given up their historic top position on the LC problem list. This is good news! However, something else must take its place and I believe that check-valve problems have taken that highest (or lowest, depending upon your perspective) position.

Over the last 25 years of writing this column, check valves have been a significant portion of the discussion at least 10 times. The last column dedicated to check valves was published in early 2006.¹ This triggered additional questions and gradually, through the haze, it began to appear that most problems related to sticky check valves were encountered when acetonitrile was used as the organic component of the mobile phase. One of the troubles with trying to get a better handle on this problem is that data are very diffuse and tend to be confounded by changes that make it difficult to pin down a single reason why check-valve problems are so common. This prompted me to issue a request to you,² the everyday users of LC, for information that might help clarify this situation. Your responses form the foundation of this month's "LC Troubleshooting" discussion.

Over a year's time, I received inputs from 14 readers, scattered throughout the world in several different industries. Five different brands of equipment and seven models were mentioned, and the comments dispelled my original thoughts that sticky check valves were specific to one or two manufacturers — I believe the problem is nearly universal for ball-type check valves. Also, the problem is associated with acetonitrile — in only one instance was another solvent (tetrahydrofuran) mentioned as the organic present with sticking check valves. Finally, it is not one brand of acetonitrile that is the problem — often, users claimed to improve things by switching from brand X to brand Y, but just as often a change from Y to X was recommended by another party. But before we look at what you had to say, let's review how check valves work.