Mass spectrometry for structural elucidation
Mass spectrometry is a key method for structural elucidation. When combined with orthogonal methods such as NMR, it can be used to identify unknown structures. The basis for this lies in its ability to precisely determine the mass of the molecule or fragment being examined. This method is also highly sensitive, meaning a great deal of structural information can be gleaned from even very small amounts. We primarily use high-resolution mass spectrometers for structural elucidation. These are Q-ToF (time of flight) mass spectrometers that work with ESI (electrospray ionization).
Important mass spectrometry parameters for structural elucidation
Mass accuracy: The masses can be measured accurately to 4 decimal places. As a result, sum formulas can be determined for a mass signal.
Resolution: Due to its high resolution, the isotope pattern is visible for the mass signal. This pattern is specific to an elemental composition and also backs up the sum formula. Distinctive isotope ratios in particular, such as those of the halogens chlorine and bromine, are very clearly visible.
Fragment analysis: Both low-energy and high-energy measurements are conducted in parallel as standard. The low-energy measurement is used to detect the molecular peak of the substance. In the high-energy measurement, the substance is fragmented. It is possible to generate sum formulas for the fragments, which can then provide valuable information about the structure of the substance. In order to increase the selectivity, it is also possible to choose targeted ions and fragment them as part of an MS/MS test. This is especially useful when there are overlapping signals.
Pairing with UPLC: The combination of a high-resolution mass spectrometer and a UPLC can also be used to test complex substance mixtures via mass spectrometry. This makes it possible to obtain structural information about secondary components that are only present in very small amounts. Once again, fragmentation experiments can be carried out for each signal in the chromatography.
In addition to combined LC-MS mass spectrometry, which is particularly suitable for non-volatile and semi-volatile substances, we also offer GC-MS for determining the structure of volatile substances using mass spectrometry.
GC-MS and GC-MS/MS can be used to address a variety of analytical questions. Depending on the composition of the sample, different ionization methods such as EI (electron ionization) and CI (chemical ionization) can be used. Whereas CI is essentially used to detect molecular peaks, EI results in the heavy fragmentation of the substances, which provide valuable information for structural elucidation. There are also extensive databases available for this purpose.
MALDI (matrix-assisted laser desorption/ionization) is used to test polymers, as it is also suitable for testing substances with greater molar masses. This is a mild ionization method that can detect a substance’s molecular peak. When it comes to testing polymers, a variety of signals are observed over a large mass range. The signal distances and properties can be used to draw conclusions about the structure, composition, end groups and average molar mass of the polymer.