Mass spectrometry (MS) has become a fundamental tool for compound identification or confirmation by virtue of its ability to perform elemental composition determination (formula ID) by accurate mass measurements. The speed, sensitivity, and ease of interfacing the technique with gas and liquid chromatographs make MS the technique of choice for many applications.
In addition to accurate mass measurements, the isotope abundance distribution for an ion also provides information unique to a given chemical formula. The mass spectral accuracy required for accurate isotope modeling, however, has previously not been easily obtainable.
Mass accuracy is the measure of an instruments error in determining the theoretical (exact) mass of an ion. It is usually expressed in parts per million relative to the measured mass values or in absolute units of milli-Daltons (mDa) (Figure 1). Every unique formula has a unique mass value, which is the basis for formula ID using accurate mass measurements.
Unfortunately, due to measurement error, mass accuracy alone can rarely provide a unique formula, particularly at higher mass values (above 400 Da). For example, the formula search on an instrument capable of obtaining a mass accuracy of 1 ppm results in a list of 34 formula candidates for an unknown compound at 500 Da containing the elements C, H, N, O, S, and Cl.
The formula candidates can be pared down by imposing chemical constraints such as: limiting the possible elements in the formula search, restricting the minimum and maximum number of atoms for each element, electron state, and utilizing any other complementary knowledge of the unknown sample. For true unknowns, even on the highest resolution instruments such as FT-ICR systems, mass accuracy alone is not enough to identify a unique formula.
An ion’s isotope pattern is also unique for each formula and significantly richer in information than the measurement of a single peak position, as with accurate mass measurements. It is composed of many peaks, each with unique relative intensities based on their isotopic abundances and unique relative mass positions based upon the mass of each isotope.