GB2459580A

GB2459580A - Use of low molecular weight solvents in HiFAWS

Info

Publication number: GB2459580A
Application number: GB0908473A
Authority: GB
Inventors: Stuart Keith Ross; Sarah Marchant
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 2008-05-30
Filing date: 2009-05-15
Publication date: 2009-11-04
Also published as: WO2009144443A1; GB0908473D0

Abstract

Improved methods for identifying target molecules using High Field Asymmetric Waveform Spectrometry (HiFAWS) substantially eradicate the effect of water on the field dependency of target molecule ions. A carrier gas comprising a low molecular weight solvent with a proton affinity higher than that for water, such as ethanol or ammonia, is used to provide an electric field dependency of the target molecule ions substantially independent of any water vapour present in the carrier gas and/or the sample. HiFAWS is also known as FAIMS, rf-IMS, DMS, and FIS.

Description

Improved Methods for High Field Asymmetric Waveform Spectrometry (HiFAWS) The present invention is concerned with methods for performing HIFAWS, and in particular methods in which the electric field dependency of ions is rendered substantially independent of water concentration.

High Field Asymmetric Waveform Spectrometry (HiFAWS) is also known as Field Asymmetric Ion Mobility Spectrometry (FAIMS), Radio Frequency Ion Mobility Spectrometry (rf-IMS), Differential Mobility Spectrometry (DMS) and Field Ion Spectrometry (FIS). The technique has been described numerously including IA Buryakov etal, mt. J. Mass Spectrom. Ion processes, 1993, 128, 143-148, and RW Purves et al, Rev. Sci. Instrum., 1998, 69, 4094-4105. One of the major potential benefits of the technique is the ability to resolve ions that cannot be readily discriminated using ion mobility spectrometry (IMS).

HiFAWS is a technique capable of separating a wide variety of ions in the gas phase, from chemical warfare (CW) agents through to protein conformers. Ion identification in HiFAWS is related to changes in effective cross section of the ion and can be based on the propensity of ions to cluster/decluster, often with neutral molecules such as water. Separation of ions via this clustering/declustering mechanism relies on the ability of ions to form clusters with neutral molecules in the presence of low electric fields, resulting in an increased cross-sectional area of ion, and to decluster in the presence of high electric fields, resulting in a decreased cross-sectional area of ion. In HiFAWS ions are transported by a carrier gas between two electrodes, whereby an asymmetric waveform is applied to one of the electrodes. Ions are thereby displaced towards one and then the other electrode by the application of the waveform containing a short duration high field component and a slightly longer duration low field component of opposite polarity. The waveform is configured such that if the ion mobility is the same at high and low electric fields, the ion will experience zero net displacement towards an electrode and will be transmitted through the apparatus to the detector. However, at high electric field strengths the mobility of many ions change compared to their low field mobility resulting in a net displacement of the ions towards one or other of the electrodes. The mobility of atomic ions and small organic ions tend to increase at higher electric fields (and are displaced towards one electrode), whereas the mobility of proteins and larger molecules tend to decrease at higher electric fields (and are displaced towards the other electrode). To select which ions are transmitted into the detector, a direct current (DC) potential, referred to as a compensation voltage (CV), is applied to one electrode to eliminate the net displacement of the ion towards one electrode or the other. CV scans are generated by scanning through a range of positive and negative compensation voltages and measuring the ion abundance transmitted through the apparatus as a function of compensation voltage.

HiFAWS has found particular utility in the detection of CW agents, such as sarin. In HiFAWS, as also in the related technique of ion mobility spectrometry (MS), it is common that for each introduced agent, a monomer ion and possibly a dimer ion is formed and detected. In HiFAWS, discrimination of CW agents arises due to field dependence differences associated with the CW agent monomer ions. The dimer ions, if present, are not readily resolvable. The field dependency of the monomer ion can be greatly affected by the presence of water vapour, as shown by N Krylova et cii, J. Phys. Chem., 2003, 107, 3648 -3654. The presence of water vapour affects the field dependency by clustering around the monomer ion at low field strengths then declustering from the monomer ion at high field strengths where the number of molecules of water clustering around the monomer ion at low field strengths is dependent on the water concentration. Though water clustering/declustering can be regarded as beneficial in modifying the field dependency of monomer ions, it is also a problem in a detection system as variations in environmental moisture levels may lead to variations of water vapour concentration internally within the cell leading to

unpredictable monomer ion field dependence.

Water concentration is thus a critical parameter in HiFAWS based detection. Water clustering occurs on application of a low electric field, and water declustering on application of a high electric field following the process A.Ht(H2O) (water clustering) -* A.H + (H20)1 (water declustering) as the applied electric field fluctuates between high field and low field. Note that depending on factors such as the properties of compound A and the strength of the electric field, complete declustering may not occur and the process may be better represented by A.H.(H2O) (water clustering) -* A.Ht(H2O)1 + (H2O), (water declustering), where m<n. This mechanism is the dominant process that influences the field dependency of CW agent monomer ions. Field dependence is primarily associated with differences in the cross sectional area of the monomer ion. The monomer ion becomes more field dependent as the water concentration increases, i.e. as the number of molecules clustering around the ion increases which thereby increases the cross sectional area of the ion.

It is clear that if monomer ions are to be confidently identified using HiFAWS the control of water concentration is critical. Failure to account for variations in water concentration is likely to result in a system with poor selectivity. There is therefore a need to overcome the water dependency effect by either providing modified and improved apparatus capable of allowing for changes in internal cell water concentration, or capable of removing water, or by providing new methods for alleviating or eradicating internal cell water dependency, or alternatively using a combination of both. One possibility to limit water vapour ingress into the detector is to use a rubber membrane. This approach is commonly used in IMS but can suffer from certain disadvantages associated with the transit time of analyte through the membrane. Such permeation effects can result in loss of system sensitivity. A further disadvantage associated with the fact that the rubber membrane does not prevent ingress of all water is that the water concentration entering the apparatus may still vary, thus effecting the field dependency. Alternative and improved approaches are therefore still required.

One means of modifying the field dependency of monomer ions has been to introduce a low molecular weight solvent to the carrier gas, which associates with the monomer ion to influence the field dependency. Low molecular weight solvents, such as methanol, 2-propanol, and 2-butanol, have been shown to improve analyte ion separation in HiFAWS analyses by DS Levin et al, Anal. Chem., 2006, 78, 96-106, and in particular in the analysis of peptides, whereby the low molecular weight solvent enables a reduction in the formation of multiply charged non-covalently bound peptide aggregate ions, as further reported by DS Levin et al, Anal. Chem. 2006, 78, 5443-5452.

The present application generally aims to provide HiFAWS based methods which alleviate or eradicate the effect of water on the field dependency of ions in HiFAWS.

Accordingly, in a first aspect, the present invention provides for use of a carrier gas comprising a low molecular weight solvent in HiFAWS detection of a target molecule in a sample, wherein the low molecular weight solvent has a proton affinity higher than that for water, to provide an electric field dependency of target molecule ions substantially independent of any water vapour present in the carrier gas and/or the sample.

As used herein, a low molecular weight solvent is a solvent which is capable of vaporisation at atmospheric pressure, and thus capable of being incorporated into, or modifying, a carrier gas for use in HiFAWS, and has a molecular weight of less than 200.

The Applicant has found that the presence of a low molecular weight solvent with a proton affinity higher than that for water in the carrier gas used to flow a target molecule between two electrodes in HiFAWS is capable of alleviating or even eradicating the effect of water on the field dependency of a target molecule ion, and in particular on the field dependency of a monomer ion of the target molecule. The effect of internal moisture in HiFAWS analysis originating from the target sample and/or the carrier gas can therefore be significantly reduced. A low molecular weight solvent of proton affinity higher than that of water also has the effect of reducing the level of ionisation of chemicals in a sample, leading to a more simplified background response during analysis.

The concentration of low molecular weight solvent in the carrier gas required to eradicate the effect of any water vapour on the field dependency is dependent on the concentration of water vapour in the sample and/or carrier gas. For example, a concentration of 200 ppm ammonia in the carrier gas is sufficient to eradicate the effect of concentrations of water of at least up to 1000 ppm. The concentration of a low molecular weight solvent required to eradicate the effect of a particular water concentration may be determined using routine methods. The concentration of the low molecular weight solvent in the carrier gas will usually be at least 10 ppm, and preferably at least 200 ppm.

The low molecular weight solvent is believed to bind/cluster to the target molecule ions in preference to water as a consequence of its higher proton affinity. Ammonia with a proton affinity of 204 kcal/mol binds to the target molecule ion in preference to that of water with a proton affinity of only 165 kcal/mol.

The concentration of low molecular weight solvent in the carrier gas can also dramatically affect the field dependence of the target molecule ion with changes in ion cross-sectional area now primarily being due to clustering/declustering of the solvent rather than water. The process A.H.(S) (solvent clustering) -� A.Ht(S)1 + S11 (solvent declustering), where m<n, thus dominates. This clustering/declustering can significantly change the effective ion cross sectional area of the target molecule ion resulting in larger high and low field mobility differences than that observed for water clustering/declustering. However, by using a fixed concentration of low molecular weight solvent in the calTier gas, for all analyses, the field dependency of a particular target molecule ion will remain substantially constant, and thus provide reliable detection. This is a major improvement to those analyses in which field dependency is dependent on an unknown water vapour concentration, in which any result is inherently unreliable.

Preferred low molecular weight solvents are ammonia and ethanol.

In a second aspect, the present invention also provides for a method for detecting a target molecule in a sample using HiFAWS comprising i. arranging for a HiFAWS apparatus having a region wherein to provide an

electric field, and having reactant ions therein,

ii. introducing the sample and a carrier gas comprising a low molecular weight solvent to the region, iii. colliding the sample with the reactant ions to produce a plurality of target molecule ions, iv. applying an asymmetric waveform comprising a high electric field portion and a low electric field portion to the region, and v. determining the electric field dependency of the target molecule ions, and thereby identifying the target molecule, wherein the low molecular weight solvent has a proton affinity higher than that for water, providing for an electric field dependency of target molecule ion substantially independent of any water vapour present in the carrier gas and/or the sample..

The method is in particular directed to detecting and identifying chemical warfare agents such as GA, GB and sarin, and chemical warfare simulant molecules such as dimethyl methyl phosphonate (DMMP), a simulant for sarin. The method may also be used for detecting explosives.

In an alternative embodiment of the second aspect the HIFAWS apparatus may be replaced with a combined HIFAWS-IMS apparatus. Such a combination may provide for further improved detection of a target molecule.

The present invention will now be described with reference to the following examples and figures in which Figures 1 a) to c) are graphical representations of HiFAWS results displaying compensation voltage against RF amplitude of the asymmetric waveform carried out with target molecule DMMP with a water concentration of 200 ppm and a) no low molecular weight solvent, b) 0.1 ppm ammonia, and c) 0.1 ppm acetone; Figures 2 a) to c) are graphical representations of HiFAWS results displaying compensation voltage against RF amplitude of the asymmetric waveform carried out with target molecule DMMP with an ammonia concentration of 1 ppm and a water concentration of a) 10 ppm, b) 200 ppm and c) 1000 ppm; Figures 3 a) to c) are graphical representations of HiFAWS results displaying compensation voltage against RF amplitude of the asymmetric waveform carried out with target molecule DMMP with an ammonia concentration of 200 ppm and a water concentration of a) 10 ppm, b) 200 ppm and c) 1000 ppm; Figures 4 a) to c) are graphical representations of HiFAWS results displaying compensation voltage against RF amplitude of the asymmetric waveform carried out with target molecule DMMP with a methanol concentration of 200 ppm and a water concentration of a) 10 ppm, b) 200 ppm and c) 1000 ppm; Figures 5 a) to c) are graphical representations of HiFAWS results displaying compensation voltage against RF amplitude of the asymmetric waveform carried out with target molecule DMMP with an ethanol concentration of 200 ppm and a water concentration of a) 10 ppm, b) 200 ppm and c) 1000 ppm; Figures 6 a) to c) are graphical representations of HiFAWS results displaying compensation voltage against RF amplitude of the asymmetric waveform carried out with target molecule DMMP with a propan-1 -ol concentration of 200 ppm and a water concentration of a) 10 ppm, b) 200 ppm and c) 1000 ppm; Figure 7 a) to c) are graphical representations of the compensation voltage for the DMMP monomer ion in the presence of a water concentration of 25 ppm (2), 200 ppm (3), and 1600 ppm (4). Figure 7a is in absence of ammonia, Figure 7b is in the presence of 0.1 ppm ammonia, and Figure 7c is in the presence of 200 ppm ammonia; Figure 8 is a schematic for the gas handling system delivering air, water vapour and modifier vapour to the HiFAWS apparatus.

Examples

HiFAWS apparatus (Sionex Value Added Component; SVAC) was supplied by Sionex Corporation, Bedford, USA. Topographic dispersion plots were obtained using software developed at Sionex Corporation.

The HiFAWS apparatus was modified to interface, via 50 tm orifice plates, with two separate quadrupole mass spectrometers, one tuned for external positive ions (SXP 600. VG Quadrupoles, UK) and one tuned for negative ions (SXP Elite, VG Quadrupoles, UK). The HiFAWS apparatus was interfaced with mass spectrometry to allow confirmatory identification of ions detected. All trends observed on the Sionex SVAC could be reproduced on the modified apparatus. The modified apparatus excludes detection electrodes so that on application of the appropriate compensation voltage the ions pass directly through the apparatus and into the mass spectrometer, via the pinhole orifice. Driving electronics for the modified apparatus were supplied by S ionex Corporation.

Now having regard to Figure 8 the gas handling for the instrument is as shown.

Purified air is flowed through sieve packs 5 containing a mixture of 5A/13X molecular sieve. The bulk of the air stream (0 -11 min') passes through a mass flow controller 6. A residual air stream (0 -50 ml miii') is flowed through a second mass flow controller 7 which is connected in line to a water bubbler 9 allowing water vapour to be delivered to the bulk air stream in a controlled and constant manner.

Purified air is also flowed through a CW vapour generator 11 and a third mass flow controller 8 (0 -50 ml miii') to deliver CW agent vapour to the conditioned bulk air stream. Excess vapour is filtered and removed to exhaust 12. At the modifier injection point 10, a syringe drive (Harvard Apparatus) is used to inject modifier headspace vapour into the air stream. The bulk air flow is delivered to the HiFAWS apparatus 13.

All mass flow controllers are manufactured by MKS Instruments and are connected to a MKS four channel readout. The water concentration in the bulk air stream is measured using a MCM Dewmatic (Moisture Control and Measurement, UK) and can be controlled and maintained at any water concentration between 2ppm and l000ppm.

The CW vapour generator is a GIlO vapour generator (Graseby lonics, now Smiths Detection, UK). The vapour concentration in the bulk air stream has not been accurately quantified but is estimated to be 0.04 mg m3. For experiments using the Sionex SVAC instrument the total flow rate is maintained at 400 ml miii' and a cell temperature of 35°C. For experiments using the mass spectrometers, the total flow rate is maintained at 600 ml miii' at a cell temp of 35°C. ii

Now having regard to Figure 1, the difference in the behaviour of ions, in particular DMMP monomer ion 1, as a function of applied peak rf field strength between (a) 200 ppm water, (b) 200 ppm water and 0.lppm ammonia, and (c) 200 ppm water and 0.1 ppm acetone is quite pronounced. These figures in fact show the effect of small amounts of low molecular weight solvent (a dopant) on the field dependency of the DMMP monomer ion 1. The magnitude of the compensation voltage is proportional to the magnitude of the field dependency of the monomer ion. In the presence of low concentrations of either ammonia or acetone (0.1 ppm) the field dependency of the monomer ion is significantly reduced, and in the presence of acetone substantially eradicated. Use of dopants in HiFAWS, as opposed to use in IMS, would thus appear to have a negative effect on field dependency, and a negative effect on the ability of HiFAWS to resolve ions, and thus target chemicals.

It was also shown in these experiments that as the water concentration in a sample increased the compensation voltage required to detect the DMMP monomer ion, at a particular applied rf potential, increased. Thus, at an applied rf potential of 1000 V the compensation voltage for DMMP monomer ion in the presence of 0.1 ppm ammonia increases from -3.75 V (25 ppm water) to -8.43 V (1600 ppm water). Now having regard to Figure 2, the effect of water concentration on the compensation voltage and thus the field dependency for the DMMP monomer ion I can also be observed in the presence of 1 ppm ammonia, wherein the field dependency and the compensation voltage are dependent on the concentration of water present.

Now having regard to Figure 3, the effect of water concentration on the field dependency (compensation voltage) of the DMMP monomer ion 1 is substantially eradicated when 200 ppm of ammonia is used in the analysis, with a substantially identical result being produced in the presence of a water concentration of (a) 10 ppm, (b) 200 ppm and (c) 1000 ppm.. A marked improvement in the field depdency of the monomer ion with 200 ppm ammonia is also seen, when compared to the use of 0.1 ppm ammonia. Such a concentration of ammonia would therefore appear to improve to resolution of ions, and also to eradicate the effect of water vapour on filed dependency.

Now having regard to Figure 7, the effect of ammonia on the compensation voltage of the DMMP monomer ion (at an applied rf potential of 1200V), in the presence of a water concentration of 25 ppm 2, 200 ppm 3 and 1600 ppm 4 can be seen. In (a) the absence of ammonia the DMMP monomer ion has a highly variable compensation voltage, between -6 V and -16 V, depending on the concentration of water, thus accurate determination of DMMP would prove difficult. In (b) the presence of 0.1 ppm ammonia, thus with ammonia used as a dopant, the compensation voltage remains highly variable, between -5 V and -14 V, with the effect of the dopant predominantly to slightly reduce the compensation voltage, at a particular water concentration. In (c) the presence of 200 ppm ammonia the variability in compensation voltage is substantially eradicated, with a good field dependency (compensation voltage) of about -10 V being observed for the DMMP monomer ion, with clustering of ammonia ions. Furthermore, 200 ppm ammonia has the effect of increasing the field dependency of DMMP monomer ion compared to unmodified water-based chemistry when the water concentration present is lower than about 200 ppm. Ammonia not only produces the desired effect of eradicating the effect of water,

but also provides a good field dependency.

Thus when ammonia is used at low concentrations in HiFAWS, i.e. at concentrations associated with dopants (0.lppm -lppm), water remains the principal factor in the field dependence of the DMMP monomer ion. At 200ppm ammonia, however, the field dependence of the DMMP monomer is effectively substantially independent of water concentration. It is hypothesised that due to the higher proton affinity of NH3 (204 kcal/mol) compared to H20 (165 kcal/mol) that NH3 binds/clusters preferentially to the monomer ion.

Now having regard to Figures 4, 5 and 6, similar experiments were conducted with ppm of methanol, 200 ppm ethanol and 200 ppm propan-1-ol at different water concentrations. The field dependency of monomer 1 was observed to be substantially unaffected by water concentrations varying between 10 ppm and 1000 ppm.

These low molecular weight solvents also affect the field dependency of the DMMP monomer ion, as can be seen by comparing Figures 3, 4, 5 and 6. Each low molecular weight solvent, which is referred to as a modifier when used at high concentration, as opposed to a dopant when used at low concentration (0.1 to 1 ppm), has a unique effect on the field dependency. Ethanol has the most pronounced effect. The magnitude of the effect is also modifier concentration dependent. However, if the same concentration of modifier is routinely used for HiFAWS analysis then the reliability of the analysis, over that of an analysis that is dependent on an unknown concentration of water, is vastly improved.

The effect of a modifier on the field dependency of CW agents GA and GB monomer ions in the presence of water was also investigated. Both GA and GB could be discriminated from each other, and a known interferent chemical, in the presence of ppm ethanol and 800 ppm water at an applied RF potential of 1000 V. Discrimination was further improved as the RF potential was increased through 1200 V to 1400 V. Moreover, there is good discrimination between the reactant ion peak, the monomer cluster ion and the dimer cluster ion. Compensation voltages for the reactant ion, GB monomer and GB dimer at an applied RF potential of 1000 V in the presence of 200 ppm ethanol and water concentrations of 5, 200 and 800 ppm are

shown in Table 1.

Table 1. Change in compensation voltage with water concentration for the reactant ion, GB monomer and GB dimer at an applied RF potential of 1000 V in the presence of 200 ppm ethanol.

Water Ethanol GB Monomer GB Dimer Concentration Reactant ion _______________ _______________ 5ppm -8.434 -3.747 -0.232 200ppm -9.606 -4.333 -0.232 800ppm -10.788 -4.919 -0.232 There is a shift in compensation voltage of approximately 1 V for the GB monomer ion in increasing water concentration from 5 ppm to 800 ppm, and thus a higher ethanol concentration would be required to completely eradicate the effect of water on the compensation voltage for GB monomer ion.

Analysis of mass spectra for the GA and GB monomer cluster ions at an applied RF potential of 1000 V showed clustering with up to four ethanol molecules.

GA and GB could also be discriminated from each other, and a known interferent chemical, in the presence of 200 ppm propan-2-ol and 800 ppm water at an applied RF potential of 1200 V. There is good discrimination between the reactant ion, monomer cluster ion and dimer cluster ion. Discrimination was increased at an RF potential of 1400 V. Mass spectra for the GA and GB monomer peaks showed evidence of GA clustering with up to three propan-2-ol molecules and GB with up to four propan-2-ol molecules. The spectra showed little evidence of water clustering even at a water concentration of 800 ppm.

Claims

CLAIMS1. Use of a carrier gas comprising a low molecular weight solvent in HiFAWS detection of a target molecule in a sample, wherein the low molecular weight solvent has a proton affinity higher than that for water, to provide an electric field dependency of target molecule ions substantially independent of any water vapour present in the carrier gas and/or the sample.
2. A use according to Claim 1, wherein the concentration of the low molecular weight solvent in the calTier gas is at least 10 ppm.
3. A use according to Claim 2, wherein the concentration of the low molecular weight solvent in the carrier gas is at least 200 ppm.
4. A use according to Claims 1 to 3, wherein the low molecular weight solvent is ammonia or ethanol.
5. A method for detecting a target molecule in a sample using HiFAWS comprising i. arranging for a HiFAWS apparatus having a region wherein to provide anelectric field, and having reactant ions therein,ii. introducing the sample and a carrier gas comprising a low molecular weight solvent to the region, iii. colliding the sample with the reactant ions to produce a plurality of target molecule ions, iv. applying an asymmetric waveform comprising a high electric field portion and a low electric field portion to the region, and v. determining the electric field dependency of the target molecule ions, and thereby identifying the target molecule, wherein the low molecular weight solvent has a proton affinity higher than that for water, providing for an electric field dependency of target molecule ion substantially independent of any water vapour present in the carrier gas andlor the sample.
6. A method according to Claim 5, wherein the concentration of the low molecular weight sovent in the carrier gas is at least 200 ppm.
7. A method according to Claim 5 or Claim 6, wherein the low molecular weight solvent is ammonia or ethanol.
8. A method according to Claims 5 to 7, wherein the method is for detecting a target molecule in an extreme environment, such as in a dessert or a rainforest, wherein water vapour concentration may be highly variable.