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Drip, Drip, Drip . . .
Oct 1, 2008
By: John W. Dolan
LCGC North America

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John W. Dolan
This "LC Troubleshooting" installment marks the beginning of the 26th year that I have been writing this column. Over that time, changes in instrument design, and especially column technology, have occurred that have made liquid chromatography (LC) an easy-to-use and reliable process. Some of the major problems with routine operation now are minimal. For example, bubbles resulting from poorly degassed mobile phase no longer hold first place on the top 10 LC problems list — automatic in-line degassers are largely responsible for this change. Some problems, however, have not gone away and probably never will. Leaks fall in this category. Leaks are so ubiquitous that it is easy to assume that everyone knows how to handle them, so we don't even train new workers on how to deal with them. This is highlighted by a quick search through my database of past "LC Troubleshooting" columns — "leak" or "leaks" only showed up as a keyword in 18 of approximately 280 columns. So, I guess it is time to go back to the basics and spend a little time on leaks for this month's topic.

How Do I Know There's a Leak?

Leaks rarely occur without giving some other warning at the same time. Most commonly, leaks are accompanied by a low or fluctuating pressure readout. Many LC systems are equipped with leak detectors. These are simply a pair of electrical contacts located in a low spot in the column oven, autosampler, pump, or other system component. When a leak occurs, the mobile phase runs to the low spot and completes the circuit between the contacts and triggers an alarm. With the help of a leak detector, you usually will know which module has the problem, but you might need to look further to identify the specific source of the leak.

If the leak causes a steady drip, drip, drip of mobile phase, it is fairly easy to track to its source. Smaller leaks can be a bit more of a challenge. Some leaks are so slow that the mobile phase evaporates before a visible drop is formed — these can leave buffer residues behind. One thing to look for is a white deposit or crystalline fuzz of buffer salts at one of the connections — this is a sign of a past or present leak. Some very small leaks can be frustrating to identify — probing them with a paper towel or laboratory wiper does not seem to show any moisture present. One of my favorite leak-detection tools for such cases is a small piece of thermal printer paper. Thermal paper is becoming a rare commodity in the laboratory, but you might find it in a balance printer, a fax machine, or in a drawer where you keep printer supplies. Another source is a charge-card receipt, most of which come printed on thermal paper. Cut a thin pointer from the paper, for example, 1 cm wide at one end by 5–10 cm long, tapering to a point. Thermal paper is very sensitive to small amounts of organic solvent (you can test this by placing a drop of alcohol or acetonitrile on the paper), so you can use this probe to poke at the fittings or other location where you suspect a leak. If a leak is present, the tip of the paper will turn black. (Note to self: do not throw away that last "useless" roll of thermal fax paper.)

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Source: LCGC North America, 10/1/2008
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