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Five Dimensions for Complex Sample Analysis: Enough Is Enough

Complex samples in the fields of metabolomics, lipidomics, proteomics, environmental and food analysis are often analyzed by direct injection into an ultra-high-resolution mass spectrometer (FT-ICR-MS or orbitrap-MS) or by LC coupled to a high-resolution mass spectrometer (e.g. LC-qTOF). But there are three major problems in the analysis of complex samples, which make an analysis very difficult:

At first, a fantastic mass resolution with high accurate mass is possible with an ultra-high-resolution mass spectrometer, which allows us to calculate a sum formula for each analyte. Unfortunately, it is not possible to construct the structure only with the sum formula; usually more than one (often ten or sometimes hundred) compound shows the same sum formula, which does not allow an identification of the analytes in the sample.

Secondly, the separation power of an one-dimensional LC coupled to a high-resolution mass spectrometer is often not sufficient for a baseline separation of complex samples. The calculation of a sum formula leads to several results due to the lower mass resolution in comparison with FT-ICR-MS or orbitrap-MS. In addition, the insufficient separation leads to coelutions, which means that several - often ten or more - compounds are in the ion source at the same time. Because of this coelution ion suppression takes place - especially with an electrospray ion source (ESI) -, which makes a quantitative analysis more difficult. Furthermore, the coelution leads to mixed MS spectra, increasing also the problems in spectra interpretation, especially with only mass resolution of e.g. 20,000.

The third problem is, that the method development for complex samples is very difficult and it takes time.

Therefore, we are working on a more generic analytical platform based on a two-dimensional chromatography method (LC+LC and GC+GC) coupled to an ion mobility-high resolution mass spectrometer. This approach works as a continuous multiheart-cutting system, using a long modulation time of four minutes (LC+LC) or 20 s (GC+GC), which allows the complete transfer of most of the first dimension peaks to the second dimension column without fractionation, in comparison to comprehensive two-dimensional chromatography (LCxLC or GCxGC).

Hence, it is possible to look at the data from a 2D analysis in a simple 1D chromatogram, which simplifies the data handling even when IMS as a third, MS as a fourth, and the fragmentation in the qTOF as a fifth dimension are introduced. The analysis of various complex samples, such as bio coal, waste water or Chinese herbs extracts with LC+LC or GC+GC-IM-qTOF-MS shows the separation power of this four dimensional separation method with an outstanding peak capacity of more than 8,000. The additional fifth dimension (fragmentation) together with the determined collision cross section (CCS) by the ion mobility unit (IM) is used as a helpful tool for identification of the analytes.

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