Sometimes Peak Defects Are Not the Sign of a Problem

Last month, I demonstrated the feasibility of a 250 µL injection in a split/splitless injection port ("Is there a limit to the volume you can inject into an Agilent split/splitless inlet?", using the EPA Method 521 (nitrosamines in drinking water) list. As you can see in the chromatogram below, seven of the eight peaks are nice and symmetric. Peak number 2, however, shows exaggerated fronting.

N-nitrosomethylethylamine is the only compound on this list that exists as a set of enantiomers. I suspected that the front defect on peak 2 was due to the partial resolution of a minor isomer. A suspicion which I recently confirmed while working on a rapid 8270 screening run using a short, narrow bore column (results shown below).

I was able to achieve this partial separation by taking advantage of the higher efficiency of a narrow bore, thin film column and minimizing my injection band by combining split injection with a narrow inlet liner. You may have noticed that the n-nitrosomethylethylamine peak in the first chromatogram is approximately 20 seconds wide while the partially resolved peaks in the second (8270) chromatogram are about 1.2 seconds wide each (or 2.4 seconds wide total). In order to properly define peaks which are this narrow, you need a fast acquisition speed. Not being able to resolve the enantiomers is not an issue, but if you look at the aniline - bis(2-chloroethyl)ether separation, you'll see that a slow acquisition rate could easily result in the loss of resolution for this isobaric pair. I used a 7890-5975C for this work, but a 5973 with the fast electronics package should be sufficient.