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Mass Spectrometry & Spectroscopy
Enhancing Mass Spectrometric Performance with Field Asymmetric Waveform Ion Mobility Spectrometry
Robert W. Smith1* , Lauren Brown1 , Danielle Toutoungi2 , James C. Reynolds1 , Billy Boyle2 , Colin S. Creaser1
1. Centre for Analytical Science, Loughborough University, Loughborough, Leicestershire. LE11 3TU 2. Owlstone, 127 Cambridge Science Park, Milton Road, Cambridge. CB4 0GD
*Corresponding author: Robert W. Smith, Owlstone Ltd, 127 Cambridge Science Park, Milton Road, Cambridge CB4 0GD, Email:
rob.smith@
owlstone.co.uk
There is growing interest among mass spectrometrists in fi eld-asymmetric waveform ion mobility spectrometry (FAIMS) as a technique that can potentially provide fast orthogonal separation in line with a mass spectrometer. To explore the capabilities of this approach, a miniaturised FAIMS device combined with mass spectrometry (MS) has been used for the separation and determination of a wide range of analytes, including active pharmaceutical ingredients and excipients, drug metabolites, potentially genotoxic impurities, peptides and proteins. The system proved to be compatible with sample introduction via direct infusion, HPLC and thermal desorption. On a single mass analyser, FAIMS-selection of precursor ions before in-source dissociation improved product ion spectra, aiding identifi cation of a drug metabolite in urine and of plasma proteins.
This article reviews the advantages of incorporating miniaturised, chip-based fi eld asymmetric waveform ion mobility spectrometry with mass spectrometry (FAIMS-MS). FAIMS, also known as differential mobility spectrometry, exploits the structural and ion mobility differences between ions under high and low electric fi elds, allowing a rapid separation that is highly orthogonal to mass spectrometry and liquid chromatography. FAIMS acts as an on-line fi lter for ions entering the MS and is used to select ions and remove interferences prior to mass analysis.
Ions are carried between a pair of electrode plates positioned between the ion source and mass spectrometer inlet by a fl ow of neutral gas (typically nitrogen at atmospheric pressure). Alternating high and low electric fi elds cause ions to oscillate between the electrodes. The magnitude of the asymmetric waveform (dispersion fi eld or DF) can be optimised for separation, as a result of differences in ion mobility at high and low fi eld strengths. Differential mobility causes a drift towards one of the electrodes, resulting in a neutralising collision. This sideways drift can be cancelled out, guiding ions through the device by using an opposing compensation fi eld (CF) which can be held static or scanned to produce a spectrum. Peak location in the CF spectrum is characteristic of ion structure and can be used as a dimension of separation or to preferentially transmit analyte ions whilst other interfering ions are neutralised and fi ltered out.
The ability to separate isobaric ions is a feature of FAIMS that makes hyphenation with mass spectrometry ideal [1-4]. FAIMS can also be used to rapidly remove interferences from sample matrices that would otherwise require extra steps of sample preparation and separation which increase the analysis time. A further advantage of using FAIMS to remove interferences is that lower limits of quantitation can be achieved [1-3]. FAIMS selection of ions prior to in-source collision-induced dissociation (FISCID-MS) enables tandem experiments to be performed on a single mass analyser platform, signifi cant enhancements in fragmentation spectra were achieved by the addition of a FAIMS separation. The ability to select precursor ions based on differential mobility reduces the complexity of product ion spectra by removing interferences from multiple precursors, similar to mass-selection for selected reaction monitoring performed with a tandem mass spectrometer [2,3]. The enhancement in selectivity and improved quantitative performance offered by combining these techniques is demonstrated for a variety of analytes and matrices.
Experimental Materials and Methods
The chip-based FAIMS spectrometer (Owlstone Ltd., UK) features multiple planar electrode channels (100 µm gap) to enable high dispersion fi elds (<300 Td) with a 700 µm path length to give short ion residence times (50 – 250 µs) for rapid compensation fi eld (CF) scanning (Figure 1). FAIMS was combined with an Agilent 6230 TOFMS (Agilent technologies, UK). Samples have been introduced by direct infusion, thermal desorption (Unity1, Markes International, UK) and high performance liquid chromatography (HPLC 1200 series, Agilent Technologies, UK). The materials and methods vary for each application, the details of which can be found in corresponding references.
Figure 1. Photographs showing chip-based FAIMS (a) on a fi nger, showing the scale of the miniaturised device; (b) close-up of FAIMS chip showing the multiple parallel gaps between the electrode plates (ions travel through the device into the plane of the page); (c) incorporated into ESI source of TOF MS, with FAIMS control system positioned below the source.
Results and Discussion Separation of Isobaric Compounds
2,4,6-trimethylaniline (2,4,6-TMA) and N,N-dimethyl-m-toluidine (N,N-DMT) are potentially genotoxic impurities (PGIs), which are also isobaric. The level of PGIs needs to be monitored in active pharmaceutical ingredients (APIs) because they possess structural characteristics that may exhibit carcinogenicity. Conventional chromatographic techniques (GC-MS and LC-MS) are widely used for monitoring levels of PGI compounds in APIs; however lengthy sample preparation and chromatographic separation reduce sample throughput. There is a need for new analytical strategies to meet the needs of the fast- paced pharmaceutical research and discovery environment [5,6]. Thermal desorption- mass spectrometry (TD-MS) offers an alternative to conventional methods but lacks the selectivity of separation prior to detection. A TOF MS provides qualitative information with moderate quantitative performance, although isobaric analytes cannot be distinguished. The addition of FAIMS separation gives an extra stage of selectivity without increasing analysis time.
INTERNATIONAL LABMATE - JANUARY/FEBRUARY 2014
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