Why do some peaks front and others don't in the same method?
I often get asked how I come up with the topics that are covered in my "LC Troubleshooting" column every month. Most of the
time, the source is either questions I receive via e-mail (see instructions at the end of this column if you have a question
to ask me) or in live classes that I teach around the world. This month, the topic originates in the latter category. One
course attendee had observed a problem with her liquid chromatography (LC) method in which a peak fronted. Most of us are
familiar with tailing peaks, in which the peak rises from the baseline rapidly, but returns to the baseline more slowly, resulting
in a distorted peak, where the second half of the peak is broader than the first half, forming a "tail". In the present case,
injections of the reference standard gave normal, nearly symmetrical peaks, but when the samples were injected, the peaks
rose more slowly from the baseline than normal, resulting in a fronting peak, where the first half of the peak is broader
than the second half. The question, of course, was what was going on and how to correct the problem.
Divide and Conquer
One of the troubleshooting principles that I frequently mention is what I call the "divide-and-conquer" technique. This is
simply the process of sequentially dividing the problem into (usually) two major parts by changing some variable and observing
the result. The goal is to find changes that either do or do not make a difference in the observed problem. This helps to
eliminate possible causes of the problem. As we go through a series of such steps, we progressively eliminate potential problem
sources until we are left with the root cause of the problem. For example, in the first step, we may eliminate half of the
possible causes, in the second, half of the remaining causes, and so on. It doesn't take long to be left with just one or
two things that may have to be sorted out to correct the problem.
A second principle of troubleshooting is to use the scientific method and change just one variable at a time. In this manner,
we will know which variables influence the problem and which do not. Often it is tempting to change several items together,
just to get the problem solved and get back to work, but in the long run, this approach doesn't help us identify the real
problem source so that we can avoid or minimize future problems. For example, for a problem observed in the chromatogram,
we might change the column, guard column, in-line filter, and make a fresh batch of mobile phase. This may fix the problem,
but it does not get to the root cause of the problem — for example, use of the wrong pH for the mobile phase — and therefore
may waste money (we may have discarded a perfectly good column) and invite future problems (perhaps we should add a step to
double-check the mobilephase pH).
Usually, we apply the divideandconquer technique along with changing just one thing at a time during troubleshooting. This
may be a conscious or subconscious process. Let's try this approach with the present problem.