Question

a. If you have two chromatograms of the same mixture, but they are from two different GC instruments, then how do you determine which one gives you better separation. For example, if the first one gives short, broad peaks and the 2nd one gives tall, sharp peaks then which instrument gives you better separation? Explain your answer.

b. What is the retention time of peaks in the Gas Chromatograph tell you about the compound? For example, if you have a chromatograph with two peaks corresponding to C1 and C2, then what does it mean for RT1 to be less than RT2?

Answer 1

Gas chromatography (GC) is a common separation technique used in analytical chemistry. The analyte is vaporized and injected onto the system. The mobile phase used in GC is an inert gas which helps the transportation of the analyte through the column whereas the stationary phase used can be a solid (gas-solid chromatography) or a liquid (gas-liquid chromatography).

a) Chromatographic efficiency, N is used to quantify the width of a chromatographic peak. The efficiency of a GC instrument depends upon the the peak maxima and its width. And N is inversely related to the rate of band broadening per unit time. So in the given case the second one gives a better separation as as the peaks are sharp and tall.

b) Retention time (RT) is the time taken for the analyte to pass through the column.

Stronger the interaction of the compound with the stationary phase, longer it takes to migrate through the column (more RT). In the given example, C2 interacts stronger with the stationary phase (high RT), and therefore lacks behind C1 in its movement through the column. As a result, compound C1 has a shorter retention time than C2.