A solute’s chromatographic peak may be characterized in many ways, two of which are shown in this figure. Retention time, tr, is the time between when we inject the sample injection and when we see the maximum response for the solute’s peak. Another important parameter is the baseline width, w, which, we measure by extending tangent lines from the inflection points on either side of the chromatographic peak through the baseline. Although usually we report tr and w using units of time, we can report them using units of volume by multiplying each by the mobile phase’s velocity, or in linear units by measuring distances with a ruler.
In addition to the peak for the solute, the chromatogram shown above also includes a small peak that elutes shortly after we inject the sample into the mobile phase. This peak is for nonretained solutes. Because these solutes do not interact with the stationary phase, they move through the column at the same rate as the mobile phase. The time required to elute nonretained solutes is called the column’s void time, tm.
