A mass spectrometer measures the mass-to-charge ratio (m/z) of ions. The mass spectrometer itself does not directly measure the mass of atoms in kilograms (kg). Instead, it determines the ratio of the mass of an ion to its charge.
The mass spectrometer compares the m/z ratio of ions to a reference standard. Historically, the reference standard was based on the mass of a carbon-12 (C-12) isotope, which was assigned a mass of exactly 12 unified atomic mass units (u). However, in modern practice, the reference standard is based on a more precise and accurate value derived from the International System of Units (SI).
To convert the measured m/z ratio to the mass in kilograms, you need to use the experimental value of 1 unified atomic mass unit (u), also known as 1 Dalton (Da), which is approximately equal to 1.66054 x 10^-27 kg. This value is obtained by comparing the mass of C-12 to the SI unit of mass, the kilogram.
So, the conversion from m/z ratio to mass in kilograms can be done using the experimental value of 1 Da:
Mass (kg) = m/z ratio * 1 Da * (1.66054 x 10^-27 kg/Da)
By multiplying the m/z ratio by the conversion factor, you can obtain the mass of the ion in kilograms.
It's important to note that the mass spectrometer provides relative masses or mass-to-charge ratios, not absolute masses in kg. The conversion to kg is performed using the experimental value of 1 Da, which is an approximation based on the currently accepted value for the atomic mass unit.