“Oh, and what do you study in grad school?”

Answering the question, “What are you studying in grad school?” is never an easy task. Especially if the person really cares what the answer is. If they don’t care, “Chemistry” is usually a sufficient answer, and we move on. I’ve come up with several stock answers that vaguely describe synthetic organic chemistry in somewhat easy-to-understand English for non-chemistry majors (First, I have to explain that ‘synthetic organic chemistry’ is not an oxymoron…)

But if the conversation really gets scintillating (that’s the term I’m going to use, at least), I get to try to explain concepts like flash column chromatography to non-chemists. Over time, I think I’ve formulated a fairly good explanation-by-analogy. I’ll lay it out below the jump. Let me know how I did – especially if you yourself are not completely familiar with column chromatography. What concepts related to your field have you had to explain to non-scientists?

The final operation in chemical synthesis is purifying the product. Often, synthetic chemists use a procedure called column chromatography. There are several webpages about column chromagography (here, here, here, here), but they can be a bit technical. So let’s try a thought experiment:
Imagine a hallway (a big hallway). Imagine this hallway filled with trees (I told you it was a big hallway). Imagine lots of bags at one end of the hallway – ziplock ’snack bags’, grocery bags, trash bags, those huge lawn clipping bags – and everything in between. Imagine a huge fan at the same end of the hallway with the bags (think the size of fan in that ventilation tower Will Smith walks through to get to the Men in Black headquarters).

When that huge fan is turned on, the bags start to fly down the hallway. They’d all exit the hallway at the same time – except for the trees. The trees catch the bags and prohibit their movement – but not to the same extent. The small bags might get caught for a short time, but get unstuck quickly and flow relatively easily down the hallway. The largest bags you might say have a higher ‘affinity’ for the trees. They get caught significantly and have a much harder time freeing themselves – and the intermediate sizes get stuck proportional to their ‘affinity’ for the trees. Eventually the smallest bags exit in roughly one large clump of bags. A while later the medium bags, and eventually (if ever) the largest bags will exit the hallway.

This is exactly analogous to column chromatography. It involves a glass column (the hallway, although ours are vertical), an adsorbent or stationary phase (the trees), an eluent or mobile phase (the wind), and the crude product mixture (the bags of various sizes). There are a number of variables (column size, adsorbent height, eluent composition, and flow rate) to tune in chromatography.

Once all the variables are optimized (a topic for another day, this is why they call chromatography more of an art than a science), I load the sample on the top of the bed of adsorbent (put the bags in the hallway), cover it with a volume of eluent, and push the eluent through the column (turn the fan on). The column does its thing – the different compounds in my crude product mixtures have different affinities for the adsorbent compared to the eluent (different bag sizes) and the various compounds exit the column at (hopefully) different times. FINALLY, I have my purified product.

I’ve attached a picture of a column I ran a few months ago (above). This is a fairly decent representation of a column chromatography setup. Fortunately, my starting material and product were bright yellow, and I could watch the different compounds elute off my column. Click through to see a larger version.


  1. I’ve been involved in methylation reactions in mammalian biochemical reactions since my Ph.D. (now my postdoc, in embyros). Explaining ‘methylation’ to people can be tricky, but I usually just explain that I’m looking at how little building blocks get moved in the liver – then talk about how bad things get when it goes wrong (heart disease, diabetes, kidney failure etc).
    In terms of embryo methylation, I usually ask them if they know that DNA is the code of life, and that every cell in their body is running off those instructions (they usually do)… then point out that the same instructions give different cells different outcomes – so there must be something else written on top of the code to let cells know what to do.
    Then I go into methylation being a black marker which draws on the blueprints of life, to hide bits of messages… and then go on about how embryos need to regulate this marking or things go wrong.

  2. Pretty good. I like the black marker part.

    I think my favorite example of explaining a complex concept was when I tried to explain entropy to my family. I managed to distill it down to one sentence:

    Shaking a bag of M&Ms isn’t going to separate the colors.

  3. Two thoughts come to mind. (1) Einstein claimed that an expert is someone who can explain quantum physics to his or her grandmother (or something to that effect).

    (2) I find that using hand gestures to show my research (ring-formaiton followed by ring expansion) works well when explaining my research to my in-laws. However, my father-in-law usually then asks me, “what proof do you have that you created what you wanted and not something else.” I then have to explain NMR (again through hand gestures) and talk about “interpreting the molecular language.”

  4. Whew. NMR is always a fun one to explain, isn’t it?

    I’ve found that people seem to accept that hydrogen atoms can be thought of as miniature magnets, each with slightly different strengths based on where it is in the molecule. When we sweep through the magnetic field strength (an ok first approximation for how we acquire NMR – at least at the non-scientist level), at some point the external magnetic field will equal that of every hydrogen atom. The hydrogen atom will return a signal. And fortunately for us, this is very characteristic and predictable. We can view the various response signals and extrapolate what our compound looks like.

    Not quite as fun or colorful as arboretums and M&Ms, but they at least understand that we can do it, even if they don’t fully grasp how it’s done.

  5. Nice analogy. I use the mall as an analogy for chromatography. If the entire class got dropped off at one end of a mall and were picked up at the other end, I would (since I hate shopping and malls in general) be an unretained analyte. Someone else who loves shopping would stop at a lot of stores and “interact” there. They would take longer to get to the exit.

  6. Dear collegaues

    Can you explain how determine experimentally the detection limit? (please, no theorical information (noise/signal, 3DS etc), just what kind of experiment I need to do). Thank you in advance. Best regards

  7. Dear Judy

    Thank you in advance for your help.
    I have 10 days to make this experiment.
    I need to determine the LD/LQ in an analytical methodology for a specific toxin in food using HPLC.

  8. Dear Mitch

    Yes, I can make a calibration curve and residual plots for determination the linearity of FLD detector. I work with the external standart method using calibrant solutions (disolve in mobile phase).

    Can I know the LD and LQ from calibration curve ? or these are Instrumental Detection or Quantitation Limits, respectively ?

    Thank you so much for your help !
    Best regards

  9. Pingback: Chemistry Blog » Blog Archive » Chemistry Lab Demonstrations: Candy Chromatography

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