Technology

Ion Mobility Spectrometry (IMS) is an analytical technology to separately detect gaseous compounds in a mixture of analytes. The separation is based on the specific drift times, that ionized compounds need to pass a fixed distance (drift tube) in a defined electric field

Compared to other techniques e.g. TOF-MS, ions travel at atmospheric pressure versus a flow of inert drift gas. The drift time of each substance is determined by its ion‘s mass and geometric structure, as slowing collisions with the drift gas molecules are more frequent for sterically demanding structures. Therefore IMS can even differentiate isobaric molecules. 

For detection, the resulting ion current is measured by an electrometer as a function of time. Atmospheric Ionization of molecules can be obtained by several techniques. G.A.S. uses photoionization with a 10.6eV UV-lamp or soft chemical-ionization initiated by a low-radiation tritium (H3) source (below excemption limits of EURATOM). While the first directly produces positive ions, the latter generates rectand ions with the gas atmosphere by a cascade of reactions following the collision of a fast electron emitted from the β-radiator H3[1]. The so-called Reaction Ion Peak (RIP) representing the total of all ions available is formed. In nitrogen and air, resp., the reactand ions can be descibed as H+(H2O)n and O2-(H2O)n. Chemical ionization of analytes by reactand ions then result in the formation of specific analyte ions, when the affinity of the analyte towards the reactand ion is higher when compared to water. The proton affinity of water is 691kJ/mol, so all molecules with a higher proton affinity will be ionized by proton transfer, which is typically given for all heteroatom-organic compounds. 

 

G.A.S. Gesellschaft für analytische Sensorsysteme mbH
BioMedizinZentrumDortmund
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E-Mail : info@GAS-Dortmund.de

IMS Working Principle

Ion Mobility Spectrometry (IMS) is an analytical technology to separately detect gaseous compounds in a mixture of analytes. The separation is based on the specific drift times, that ionized compounds need to pass a fixed distance (drift tube) in a defined electric field.

Compared to other techniques e.g. TOF-MS, ions travel at atmospheric pressure versus a flow of inert drift gas. The drift time of each substance is determined by its ion‘s mass and geometric structure, as slowing collisions with the drift gas molecules are more frequent for sterically demanding structures. Therefore IMS can even differentiate isobaric molecules. For detection, the resulting ion current is measured by an electrometer as a function of time. Atmospheric Ionization of molecules can be obtained by several techniques. G.A.S. uses soft chemical-ionization initiated by a low-radiation tritium (H3) source (below excemption limits of IAEA resp. EURATOM).


In a first step rectand ions are generated by a cascade of reactions following the collision of a fast electron emitted from the β-radiator with the drift gas atmosphere[1]. As a consequence the so-called Reaction Ion Peak (RIP) representing the total of all ions available is formed. In nitrogen and air, resp., the reactand ions can be descibed as H+(H2O)n and O2-(H2O)n. Chemical ionization of analytes by reactand ions then result in the formation of specific analyte ions, when the affinity of the analyte towards the reactand ion is higher when compared to water. The proton affinity of water is 691kJ/mol, so all molecules with a higher proton affinity will be ionized by proton transfer, which is typically given for all heteroatom-organic compounds.

[1] Eiceman, G. and Karpas, Z., Ion Mobility Spectrometry, ISBN 0-88493-2247-2

 

working_principle

Gas Chromatography (GC) coupled to Ion Mobility Spectrometry (IMS)

Complex analyte mixtures like for example food, flavours, natural gas and environmental applications often demand a second and independent separation step in order to seperately analyse the multiplicity of compounds at lowest (ppb) concentration levels.

To overcome the limitation of IMS with regard to separation efficiency G.A.S. equips almost all of its IMS systems with gas chromatographic (GC) columns. The volatile compounds of samples under testing are pre-separated in time by a GC column. The discrete compounds elute directly into the IMS ionization chamber, so that analyte and/or ion interactions and a competition of analytes on the reactand ions can be avoided. This set-up enhances the sensitivity of the detector towards individual compounds.

The GC-IMS setup enables a twofold separation of analyte mixtures and the detection by the IMS electrometer. Since the IMS measurements are extremely fast (30ms / spectrum) a continuous and high-resolution recording of analyte signals is provided. Shown figure sketches the GC-IMS sample flow and detection leading to a 3D-dataset of GC- and IMS separation, same as the corresponding intensity which are processed using G.A.S. Laboratory Analytical Viewer (LAV) and additional software tools.


 

GC_IMS_Scheme

Laboratory Viewer and GC-IMS Library Search

G.A.S. offers a comprehensive line-up of software tools supporting automated data acquisition as well as data visualization, evaluation, calibration, quantification and characterization.

GC-IMS technology implies a 2-dimensional separation of chemical compounds coupled to a highly sensitive detector. The Laboratory Analytical Viewer (LAV) software enables a very intuitive data handling and 2- or 3-dimensional representation of measurements. If required, several measurements are easily comparable in an overlay mode or as a difference topographic plot. Furthermore each dimension of separation is easily accessible by using the GC- or IMS-monitor, which gives an one-dimensional view on the data or rather a chromatogram or ion mobility spectrum.
In many applications it is important to determine the concentration of certain substances routinely. By using an user-friendly quantification tool calibration curves and models can be established and are subsequently available for automatic substance quantification. If necessary, the established quantification model can be transferred onto the G.A.S. measurement system, resulting in a computer-independent analysis (on-site measurements, e.g.).
Yet another important function, especially in the field of quality control or flavour analysis, e.g., is the analysis of substance-specific fingerprints. For this purpose the LAV tool was customized and extended with special plugins, which are precisely suitable for the user’s desired scope. These plugins support analysts in the domains of evaluating signal patterns (identifying specific marker compounds, different amount of substances, e.g.) of different samples and also facilitate a comprehensible graphical representation of the results obtained by fingerprint analysis.

A further in-house software tool named GCxIMS Library Search allows the  identification and characterization of unknown volatile organic compounds (VOC’s) easily and fast. Compound determination is realized by matching both (GC & IMS) compound-specific separation properties.The software comes with a full non-restricted version of the NIST2014 Retention Index Database including ~400.000 annotated Kovats/Lee retention indices and ~83.000 compounds as well as a IMS drift time database developd by G.A.S. containing several hundred substance specific drift times. Both libraries can be customized and expanded by the user.





 

Chromatogram

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