John W. Dolan

Divide and Conquer

This is perhaps the most powerful strategy for problem isolation. I borrowed the term years ago from John Hinshaw, who writes the "GC Connections" column for LCGC. My partner, Tom Jupille, calls it the Binary Search. Whatever term you like to use, the divide-and-conquer process helps you to efficiently get to the root cause of a problem. It is very simple — just figure out a test that divides all possible problem sources into two large parts. Make the test and the results should allow you to eliminate half the potential sources. Repeat this process with the remaining possibilities until you have found the root cause of problem. This technique comes so naturally after a while that it seems trivial, yet I am regularly surprised to find it ignored in the troubleshooting process.

At this point, we could shift to the most obvious cause (see below) — pump problems — but we will continue with the divide-and-conquer strategy. You might wonder if we have a problem with the instrument or with the method itself. My next step in divide-and-conquer might be to determine which of these major areas is at fault. So I put a new C18 column (or one I've dedicated to this process) in the system and run the column manufacturer's column test. I'll inject the test sample six times so that I can calculate retention-time precision. If I can get results that are close to what the column manufacturer obtained in terms of column plate number, peak tailing, and retention time, this suggests that the instrumentation is working correctly. Don't expect exactly the same values the manufacturer quotes, but you should be within 5–10%. Remember that the column manufacturer uses an LC system that has been optimized for column testing, so it will show off their columns in the best light. If the retention time precision of the test mixture meets your method-suitability requirements, the results point to a problem related to the method itself — the column and mobile phase would be the next items to examine. If retention precision is poor, look next for problems associated with flow rate or on-line mixing. Three major influences on flow rate are check-valve performance, pump-seal wear, and bubbles. On-line mixing problems can be isolated by comparing results with hand-mixed mobile phases.