A mass spectrometer can help you locate unknown ions in the mass spectrum so that you can identify them. When charged particles move through the magnetic field of a mass spectrometer, they're deflected (pulled off course) by the magnetic field; they then hit the detector, as the first figure shows.
![The inside of a mass spectrometer.](https://www.dummies.com/wp-content/uploads/458265.image0.jpg)
The number of ions that bend the right amount at a particular magnetic field strength to hit the detector is measured. On a mass spectrum, the molecular weight of the fragment (m/z) is shown on the x-axis versus the relative number of fragments that hit the detector on the y-axis (the intensity). The most intense peak on the mass spectrum is arbitrarily assigned an intensity value of 100. The next figure shows the mass spectrum for pentane.
![The mass spectrum for pentane (C<sub>5</sub>H<sub>12</sub>).](https://www.dummies.com/wp-content/uploads/458266.image1.jpg)
It's actually not the weight of the fragment that's measured on the x-axis of the mass spectrum, but the mass (m) to charge (z) ratio (called the m/z value). A fragment that has a +2 charge will require a smaller magnetic field to deflect it toward the detector than a fragment that has a +1 charge, and will therefore show up on the mass spectrum at half the m/z value. However, since most fragments simply have charges of +1 (and since the vast majority of undergraduate classes will not discuss fragments with more than one charge), the m/z axis is often simply said to represent the molecular weight of the fragment. That is, given that the mass-to-charge ratio is m/z, and the charge (z) equals +1, in most cases, m/z = m/1 = m.
Referring again to the mass spectrum for the molecule pentane (C5H12), this spectrum contains several peaks that are worth pointing out. The molecular ion peak, or M+ peak, is the peak at m/z 72, which represents the molecular weight of the molecule (72 amu). The tallest peak in the spectrum, which is always given a value on the y-axis of 100, is called the base peak. The base peak represents the most abundant fragment that hits the detector. In some spectra, the base peak is also the M+ peak, although in this case it's not.
The mass spectrum for pentane is fairly simple, but it still contains a lot of peaks, each of which represents a different fragment that was formed by electron ionization. Usually, you can't expect to be able to identify all the peaks in a mass spectrum (or even most of them).