Mass spectrometry is an analytical technique used to identify unknown compounds, quantify known substances, and determine molecular properties. A key measurement in this technique is the mass-to-charge ratio (m/z).
What is Mass-to-Charge Ratio (m/z)?
The mass-to-charge ratio (m/z) is a fundamental property measured in mass spectrometry. It represents the ratio of an ion’s mass to its elementary charge, allowing instruments to separate and detect ions.
The m/z value is dimensionless, typically expressed in units of Daltons per charge number. It is a ratio that accounts for the number of charges carried by the detected ion, which significantly influences the observed value.
Understanding the Mass Component (m)
The ‘m’ in the m/z ratio refers to the mass of the ion. This mass is determined by the sum of the atomic masses of all atoms within the ion. It involves recognizing the difference between average molecular mass and monoisotopic mass.
Average molecular mass considers the weighted average of all naturally occurring isotopes for each element. In contrast, monoisotopic mass uses the exact mass of the most abundant stable isotope for each atom. For smaller molecules, the monoisotopic mass often corresponds to the most intense peak in a mass spectrum. However, for larger molecules like proteins, the monoisotopic peak may not be the most abundant or even observable due to the complex distribution of isotopes.
Understanding the Charge Component (z)
The ‘z’ in the m/z ratio represents the charge of the ion, which is an integer indicating the number of elementary charges. This process, called ionization, typically involves the gain or loss of electrons.
Ions can carry a single positive or negative charge, such as when a molecule gains a proton (protonation, resulting in a +1 charge) or loses a proton (deprotonation, resulting in a -1 charge). Larger molecules, particularly biomolecules like proteins, frequently acquire multiple charges (e.g., +2, +3, or more) during the ionization process. The charge state significantly impacts the observed m/z value, as a higher charge results in a lower m/z for the same mass.
Calculating m/z: Step-by-Step Examples
The m/z value is calculated using the formula: m/z = (mass of ion) / (charge of ion). The mass of the ion is typically expressed in Daltons (Da). The charge (z) is an integer representing the number of positive or negative charges on the ion.
Consider a molecule with a molecular mass (M) of 100 Da. If this molecule gains a single proton (H+), its mass becomes 100 + 1.0078 (mass of a proton) = 101.0078 Da. Since it carries a +1 charge, the m/z value would be 101.0078 / 1 = 101.0078. This is often denoted as [M+H]+.
Alternatively, if the same molecule (M = 100 Da) loses a proton, its mass becomes 100 – 1.0078 = 98.9922 Da. With a -1 charge, the m/z value is 98.9922 / 1 = 98.9922 (absolute value is typically reported). This is referred to as [M-H]-.
For a larger molecule with a molecular mass (M) of 1000 Da that acquires two protons, its mass would be 1000 + (2 1.0078) = 1002.0156 Da. Since it carries a +2 charge, the m/z value would be 1002.0156 / 2 = 501.0078. This demonstrates how multiple charges reduce the observed m/z value, bringing larger molecules within the detection range of mass spectrometers.