A typical mass spectrum is shown below. The horizontal, x-axis shows the m/z values of the ions (parent and daughter) that arrive at the detector. The vertical, y-axis shows the peak height which is proportional to the number of ions arriving in a given time.

The peak with the highest m / z value usually (but not always) corresponds to the parent ion and so can usually be used to measure the molar mass of the molecule. Sometimes, a fragment ion is the most abundant ion and so produces the tallest peak in the spectrum. Whether the tallest peak is due to the parent or a fragment, it is called the base peak and is arbitrarily assigned a relative height of 100%. Heights of all of the other peaks are then expressed as percentages of the base peak.

Click on the peaks, labelled with red markers, to display the fragmentation pattern.