At CLIO (Centre Laser Infrarouge d’Orsay), two new end stations based on tandem mass spectrometry (MS/MS) are proposed to the users. Our motivations for developing these new instruments range from enhancing the resolving power of MS/MS for separating and characterizing isomeric and isobaric molecules to a fundamental understanding of the correlation of the shape of a molecule with intra-molecular interactions. For this purpose, a differential mobility spectrometry (DMS) device has been integrated to our two MS/MS spectrometers which were already coupled with the CLIO IR beam. As a result, our two end stations allow for the infrared multiple photon dissociation (IRMPD) spectroscopic characterization of molecular ions which are not only mass-selected but also shape-selected through DMS. These DMS-MS/MS-IRMPD end-stations are particularly valuable for the separation and identification of isomeric ions encountered upon MS/MS analysis of complex mixtures as plasma or urine samples, and targeted metabolomics is the research direction of our group.
Tandem mass spectrometry (MS/MS) analysis has become invaluable across a broad range of fields and applications such as, for example, genomics, proteomics, metabolomics, forensic, drug discovery, environment, geology, … The analysis of complex mixtures, however, requires the coupling of MS/MS with gas or liquid chromatographic methods. A faster alternative for the separation of the multiple components of these mixture is ion mobility spectrometry (IMS) which can be hyphenated to MS/MS instruments. Ion mobility is the proportionality factor between an ion’s drift velocity in a gas and an electric field of strength E. Two types of IMS instruments can be distinguished according to whether separation is based on time- or space-dispersion of the ions with different mobility.
Gas phase IR spectroscopy of DMS- and mass-selected isomers can thus be performed. This is illustrated in Figure 2 in the case of two complexes saccharide isomers (methyl‑α‑glucose and methyl‑α‑mannose). The DMS spectrum has two overlapping peaks. Once the CV value is set for the maximum transmission of one isomer, its IRMPD spectrum can be recorded. Based on the IR absorption spectra of molecular structures derived from quantum chemical calculations, a clear correlation can be made between DMS separation an allow for the structural characterization of the two isomeric forms of the saccharide.
We made the choice of building a DMS device which allows for space-dispersion of the ions, and thus allows for a continuous transmission of the ions in the ion trap where they can be accumulated and then interrogated upon IR-FEL activation. The working principle is illustrated in Figure 1 where three ions formed upon electrospray are entrained by a gas flow between two parallel electrodes from atmospheric pressure to the low pressure region of the MS/MS instrument. Blue ions are transmitted to the mass spectrometer because the transverse displacements at high and low electric field compensate one another. Selective transmission of green or red ions can be obtained by adjusting a so-called (DC) compensation voltage (CV) applied to one DMS electrodes.
Integration of such DMS device on end stations of synchrotron and FEL should be of interest for selection of molecular ions prior to their spectroscopic interrogation. A first implementation will be done at the UE52_PGM Nanocluster trap end-station at the BESSY II synchrotron (HZB, Berlin). This DMS-MS/MS approach should be particularly profitable for studies of the structurally informative NEXAMS spectroscopic method (Near Edge X-ray Absorption Mass Spectrometry).
F. Berthias et al., Identification and quantification of amino-acids and related compounds based on Differential Mobility Spectrometry, Analyst 2020. DOI .
Y. L. Wanget al., Infrared isomer-specific fragmentation for the identification of aminobutyric acid isomers separated by differential mobility spectrometry, Int. J. Mass Spectrom. 443 (2019), 16-21. DOI
F. Berthias et al., Resolution and Assignment of Differential Ion Mobility Spectra of Sarcosine and Isomers, J. Am. Soc. Mass Spectrom. 29 (2018), 752. DOI