Article pubs.acs.org/ac
Tandem Differential Mobility Spectrometry in Purified Air for HighSpeed Selective Vapor Detection Marlen R. Menlyadiev and Gary A. Eiceman* Department of Chemistry and Biochemistry, New Mexico State University, Las Cruces, New Mexico, 88003, United States ABSTRACT: A tandem ion mobility instrument based on differential mobility spectrometry (DMS) was used to demonstrate selectivity in response through differences in field dependence of mobility for ions in purified air at ambient pressure. The concept of chemical selectivity solely from characteristic dispersion curves or from field dependence of ion mobility was experimentally demonstrated in three steps with mixtures of increasing complexity. In a mixture of four alcohols with carbon numbers four and below, distinct pairs of separation voltage and compensation voltage, applied to the first and second DMS stages, permitted isolation of ions from individual substances without detectable levels of other substances. In a three-component mixture of a ketone, alcohol, and organophosphorus compound, the same level of ion isolation was observed using specific and characteristic separation and compensation voltages on each DMS stage. In the last experiment, the isolation of product ions of individual substances from a mixture of 23 volatile organic compounds from four chemical groups was incomplete though the improvement in the ratio of analyte signal to chemical noise was calculated as 31 for DMMP and 106 for 1-hexanol. These findings demonstrate that chemical information available in dispersion curves can be accessed in response times below 100 ms through a tandem DMS measurement.
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swarm into a second drift region. Ions in this next drift region were photofragmented, and these ions were then characterized using a third drift region, preceded with another ion shutter. Although this instrument was not developed further, a similar tandem IMS instrument operated in air at ambient pressure has been used to explore the kinetics of ion decomposition.15 Tandem drift tubes with lengths of 1 m for each stage operated at 3 Torr in helium were shown to improve separation efficiency for mixtures of tryptic peptides eight times with an ion mobility-mass spectrometry instrument.16 Since ion mobility spectrometers for field applications at ambient pressure commonly are simple and inexpensive and provide low resolving power, tandem embodiments should be attractive for improved specificity in chemical measurements. One recent tandem instrument was comprised of differential mobility spectrometry (DMS) combined with IMS using a 15 mm long microfabricated DMS analyzer and two IMS drift tubes, each 10 mm long with a stacked ring design.17,18 Ions were passed from the ion source through the DMS at specific compensation voltages and drawn into IMS drift tubes providing characterization of the ions by mobility coefficients (K) and the field dependent differences in mobility coefficients (ΔK). Benefits of a tandem DMS/IMS were observed with a constant resolution over ion mass; however, characterization of an ion by K and ΔK did not provide chemical orthogonally and anticipated benefits of a tandem analyzer. In a larger scale embodiment of a DMS/IMS concept with the field asymmetric
n the past decade, ion mobility spectrometry (IMS) has emerged from applications in military preparedness and aviation security1 into use for clinical diagnostics,2 human metabolomics,3 analysis of foods,4,5 and air quality monitoring on the International Space Station.6,7 In these, the advantages of IMS include simplicity and small size of analyzers and convenience of measurements at ambient pressure. Complete ion mobility based instruments can be hand-held or pocket size. Additionally, configurations of IMS analyzers operated at reduced pressures and often in nonclustering atmospheres, when attached to mass spectrometers, are valuable in studies of structures and dynamics of gas phase ions of biomolecules.8−11 In the commercial ion mobility spectrometers intended for field applications, drift tubes have traditionally been single stage analyzers. These were and still are suitable for existing in-field applications; however, new and emerging chemical interferences or unconventional use of established instruments can compromise the confidence of chemical measurement with such analyzers. Despite the well-known advantages of tandem over single analyzers in mass spectrometry,12 similar shift in mobility analyzer design for field applications was slow and not widespread. Thus, this field is still largely undeveloped. Tandem configurations for IMS were first explored in the mid-1980s with a drift tube containing four stages of linear field gradients with line-of-sight drift regions. Ion shutters were used to separate a reaction region and three drift regions.13,14 Ions derived from a sample in the reaction region were injected through the first ion shutter into a drift region, and ion swarms were separated by drift time. At the end of this first drift region, another ion shutter synchronized to the first shutter was used to isolate a certain ion swarm and selectively introduce this © 2014 American Chemical Society
Received: September 27, 2013 Accepted: January 31, 2014 Published: January 31, 2014 2395
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ion mobility spectrometry FAIMS/IMS/MSn instrument, ions at reduced pressure in nonclustering gases were found at combinations of CVs and drift times suggestive of some orthogonality though this was not quantitatively determined.19,20 This design was inverted with an IMS/DMS/MS measurement where one peak for tyrosine-tryptophan-glycine in the IMS was resolved in the DMS into two conformer peaks.21 Differential mobility spectrometer is especially suitable through simplicity and function for tandem configurations of mobility instruments for field applications. A planar DMS provides a capability where the selection of ions, for passage into a second mobility stage, is facile and continuous; thus, this mobility technique was chosen as the mobility method for development as a tandem instrument at ambient pressure. In DMS, ions are passed in a gas flow continuously between metal plates where differences in the dependence of ion mobility under strong electric fields between the plates (E/N > 2 Td) form the basis for ion separations. The electric fields in the small DMS analyzers used here typically created by applying 1.18 MHz asymmetric waveforms providing up to 30 kV/cm electric fields between metal plates through application of up to 1500 V to one of the plates (the separation voltage (SV)). Although ion residence times in such analyzers, with lengths of 15 mm, are few milliseconds, a sweep of the superimposed dc field, via application of the compensation voltage (CV) with ranges from −40 to 15 V, provides a measure of all ions in the analyzer with times of 1 to 3 s. In order to obtain a comprehensive measure for a single substance, one approach is to scan a range of separation voltages while continuously obtaining spectra from the scan of compensation voltage: producing plots of ion intensity, separation voltage, and compensation voltage. Such data sets, or dispersion plots, provide a rich measure of characteristics of ions and high analytical value; the time for such scans can be more than 3 min. A more serious weakness in a single DMS with such parameters is that ions above 200 amu tend to cluster at compensation voltage near 0 V leaving little analytical resolution. There have been two approaches to overcoming this last limitation. The first is to combine single DMS analyzers with other instruments, for example, as detectors with gas chromatographs and as prefilters with mass spectrometers. In the second approach, sometimes included with DMS in hyphenated instruments, reagents have been added into the DMS analyzer. This created changes in the ion or ion’s environment with effects on ΔK of ions and sometimes shifts in compensation voltages allowing resolution of ion peaks. In a tandem DMS/DMS measurement where an ion can be selected in a first stage with a specific combination of SV and CV and then measured subsequently under another combination of SV and CV, the whole of mobility dependence seen in an entire dispersion plot is accessible with a small instrument and response times which can be ∼10 ms, the combined residence time in two sequential DMS analyzers. A central interest in this work is to examine what selectivity can be achieved by sampling positions within a dispersion plot without the need for gas modifiers in the supporting atmosphere. In the absence of field dependences of mobility on electric field strength, a tandem DMS instrument would provide little advantage over a sequential measurement; however, ions in a DMS can provide access to a subtle “orthogonality”, even in a purified gas atmosphere. Ions exhibit changes in mobility when
exposed to strong E/N arising from hydration and dehydration of the core ion22 and from reversible temperature-dependent shifts of ion geometry .23 Substances over a range of moieties have been characterized using DMS, and there is a suggestion that functionality contributes significantly to the patterns seen in dispersion plots. Thus, solvation-desolvation dynamics alone may be sufficient to isolate an ion in a first stage of DMS and induce changes in the ion structure, or solvation condition in a second stage, providing a tandem measurement with a chemical, not only physical, basis of characterization. In addition to what is understood as an intrinsic “orthogonality”, DMS has several other attractions for tandem mobility measurements including (i) a simplicity and capability to alter easily the alpha function of ions by adding vapors at ∼1% (v/v) into the supporting gas atmosphere,24−27 usually purified air or nitrogen, (ii) small drift tubes,28 and (iii) possible operation as an ion filter rather than a scanning instrument thus improving the signal-to-noise ratio.12 In prior investigations with DMS/DMS, interfaced to a mass spectrometer,29−31 the structure of a DMS/DMS instrument and electrical control of each stage independently by two computers were demonstrated though results were complicated by poor control of vapor concentrations, ion transformations in the mass spectrometer interface region, and lack of synchronization between two DMS stages. The main objective of this investigation was to determine if alpha functions alone provide sufficient differences in ion behavior to introduce some element of chemical “orthogonality” without the use of ion transformations between DMS stages through ion fragmentation or the addition of modifying reagents in the supporting atmosphere. This was complimented by an integrated electronics-software package which brought both DMS stages under a single control, synchronizing changes in compensation voltage and separation voltage. Another point of interest was to determine if the speed of response in a DMS/ DMS instrument approaches the predicted response time of ∼10 ms, the lower limit of response governed by ion residence time in the two DMS stages. In these studies, Faraday plates were used in place of a mass spectrometer to avoid ion transformations in air-vacuum interfaces and to improve the reliability of mobility spectra interpretation. Finally, vapor concentrations of substances were controlled using a gas chromatograph as an inlet to the DMS/DMS instrument, allowing delivery of various substances in a single measurement with their amounts in the range of 1−10 ng. At these levels, collisions will not occur between sample neutrals and ions derived from a substance, providing reliable measures of ion mobility in a purified gas atmosphere where water alone is the reagent gas.
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EXPERIMENTAL SECTION Instrumentation. The tandem differential mobility spectrometer shown in Figure 1 was built using the designs and components previously employed for single stage DMS analyzers32 and slightly modified for two stages. One ceramic DMS plate with bonded metal was cut to isolate the analyzer region and was set in front of another unmodified DMS plate containing an analyzer and a Faraday plate. A second set of plates was prepared and assembled into the tandem DMS analyzer using a 0.5 mm thick Teflon gasket to separate the plates and establish a flow through the analyzer. Plates were retained in a Teflon-aluminum frame adapted and enlarged 2396
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isobutyl acetate, methyl valerate, isopropenyl acetate, tert-butyl acetate, n-butyl acetate, isopropyl acetate, isopropanol, 1butanol, 1-hexanol, 1-heptanol, pinacolone, 2,4-dimethyl-3pentanone, cyclohexanone, cycloheptanone, acetone, hexanone-2,2-methyl-3-pentanone, 2,6-dimethyl-4-heptanone, 3methylcycylohexanone, heptanal, octanal, and dimethyl methylphosphonate (DMMP). Hexadecane (Sigma-Aldrich, Inc.) was used as a solvent with mixtures. An alcohol mixture contained methanol, ethanol, isopropanol, and 1-butanol at 500 ng/μL; a second mixture contained 1-heptanol, cyclohexanone, and DMMP at 1000 ng/μL; and a third mixture contained the 23 organic compounds (listed above) at 400 ng/μL. Procedures. Generation of Dispersion Plots for Individual Substances. Dispersion plots of methanol, ethanol, isopropanol, 1-butanol, 1-hexanol, 1-heptanol, cyclohexanone, and DMMP were generated individually using a syringe filled with neat sample and placed into the injection port. When vapor flux into the tandem DMS analyzer became constant, the separation voltage was stepped from 600 to 1500 V in 9 V increments and a compensation voltage was swept from −16 to +5 V in 0.2 V steps. Isolation of Ion Peaks in Tandem DMS. In the general procedure to establish elution order, DMS1 was set to all ion pass mode (power off for SV and CV), DMS2 was set to a particular SV, and the complete range of compensation voltages was scanned. In the ion peak selection mode, DMS1 was set to a specific combination of SV and CV, DMS2 was set to a given SV value, and CV was scanned over a relatively narrow window of ∼2 V at 10 Hz. In order to extract a particular ion peak, the ion intensity at a single CV value was plotted versus retention time and constituted 1 column of intensity from 10 generated in the narrow sweep. In another instance (DMMP ion selection plot), the CV for DMS2 was not swept and instead was locked to a single CV value.
Figure 1. Block diagram of tandem differential mobility spectrometer (a) and enlarged view of analyzer (b).
from the prior single DMS analyzers. The ion source was a 2 mCi foil of 63Ni located inside a SAF 2507 Super Duplex Stainless compression fitting union (Swagelok, El Paso, TX). Electronics for the tandem DMS including the waveform generator and amplifiers were provided by ChemRing Detection Systems (Charlotte, NC) and were adapted from two control boards for JUNO, a hand-held DMS analyzer. Software was also provided by ChemRing Detection Systems originating with their JACS software and modified for data acquisition and control of parameters with a tandem DMS. These electronics and software permitted synchronous operation of two identical waveform generators for each DMS and collection of detector signals from the single pair of Faraday plates (Figure 1). The makeup gas was air at 1.5 L/min and was purified through a 13× molecular sieve to a moisture level of 1 ± 0.5 ppmv, as measured by a Moisture Image Series 2 (Panametrics, Inc. Waltham, MA) moisture meter. The residence times of ions in both DMS stages were 1 ms and the total time spent in analyzer including that in the intermediate regions was 2.8 ms. At an ambient pressure of 660 Torr and gas temperature of 45 °C, 1 V of separation or compensation voltage (SV or CV) corresponded to a 0.1 Td field. A model 5890 series II gas chromatograph (Hewlett-Packard Corp, Avondale, PA) was equipped with split/splitless injector and a Rtx-50 30 m × 0.25 mm × 0.25 μm capillary column (Restek, State College, PA). Chromatographic parameters were carrier gas, nitrogen at 3.3 cm/s; split ratio, 100:1; injector temperature, 150 °C; initial oven temperature, 30 °C; final temperature, 150 °C; program rate, 10 °C/min. The column, between the gas chromatography (GC) oven and DMS/DMS analyzer was held at 130 °C in a heated transfer line, 25 cm long × 6 mm o.d. In studies with four alcohols, GC oven conditions were isothermal at 50 °C. Reagents and Samples. A total of 23 volatile organic compounds were obtained from Sigma-Aldrich, Inc. (St Louis, MO) in the highest purity available and included ethyl acetate,
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RESULTS AND DISCUSSION One feature of measurements by DMS is the dispersion plot which is a plot of ion intensity, separation voltage, and compensation voltage as shown in Figure 2, a composite of four alcohols showing product ions and the reactant ions. Curves for
Figure 2. Composite plot of individual dispersion curves for methanol, ethanol, isopropanol, and 1-butanol in purified air. 2397
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differences. The pattern for methanol arises from a proton bound dimer which undergoes dissociation with increased E/N to the protonated monomer of methanol after SV of ∼1000 V. Separation of ion peaks for alcohols in general is improved with increased SV, though peak intensities may be simultaneously decreased. The dispersion plot demonstrates a major advantage of DMS over other IMS methods, namely, knowledge of content of an ion mixture which becomes evident not in a single spectrum but rather in the patterns of field dependence of mobility. The content encoded in Figure 2 discloses dependences of ions on hydration, through the characteristic cluster-decluster mechanism of relatively small ions which should be a moiety and mass specific. Since the level of moisture here is too low for a single collision between an ion and a water molecule during the period for cluster formation, the solvation should be predominantly associated with interactions in supporting the gas atmosphere. In a DMS/DMS measurement, a pair of voltages for CV and SV is chosen to pass a certain ion peak or ion swarms through the first DMS or DMS1. The second pair of voltages, chosen from specific locations in the dispersion curve, was applied to the second DMS or DMS2. Only ions matching both criteria are passed to the detectors under these conditions. A chromatogram from GC/DMS/DMS analysis of the alcohol mixture is shown (Figure 3a) as inverted intensity of the reactant ion peak versus time with the electronically inactivated
each ion disclose the dependence of the mobility coefficient on electric field strength as shown in eq 1: K (E /N ) = K (0)[1 + α2(E/N )2 + α4(E/N )4 + ....]
(1)
where terms are: K(0), the mobility coefficient under low field conditions; α2, α4..., specific coefficients of even powers of the electric field. E/N is in units of Td. Equation 1 can be simplified where an α function is used to describe dependence of the E on E/N: K (E /N ) = K (0)[1 + α(E /N )]
(2)
where α(E/N) = α2(E/N) + α4(E/N) +... The α function for small ions in a purified air atmosphere is strongly associated with ion clustering with moisture because hydration enthalpies are characteristic of ions. Ion mass also affects α functions as seen with ions from the same chemical family but with different mass. There is, however, no quantitative assignment of the role of moiety and mass in forming the α function. Practically, differences in dispersion curves provide opportunity for separation of ion peaks in measurements with a single DMS analyzer; for example, a mixture of alcohols would be resolved best with voltages with this DMS greater than 900 or E/N of 90 Td. The dispersion plot provides more than dependence alone; it conveniently reveals chemical transformations of ions including fragmentation and dissociation reactions, which can occur in a DMS and are dependent on field strength and gas temperature. Access to this chemical information for a measurement is possible with a single DMS at a large expense of time. A scan of CV values requires 1−5 s although signal intensity can be monitored at a single combination of SV and CV providing response largely governed by residence time for an ion in the analyzer, perhaps 4−5 ms. Sampling of ion behavior at two or more SV and CV combinations can be achieved by “ion jumping”, where electronics is set to new values repeatedly. A disadvantage of this approach is the loss in dwell time for any particular ion and the corresponding loss in sensitivity. In the DMS/DMS concept described here, specificity of response is improved over a single DMS by passing ions through two combinations of SV and CV where there is no compromise on selection of dwell time. In both instruments, the dwell time is at maximum for each combination of SV and CV. Additionally, ion residence time governs speed of response and with ion flux at 15 m/s, the time of response should be below 5 ms. A composite of dispersion curves for four alcohols is shown in Figure 2, with electric fields normalized as E/N and scaled in units of Townsends, and contains ions for protonated monomers of methanol, ethanol, isopropanol, and 1-butanol.33 The plot also includes dispersion curves derived from ion peaks for proton bound dimers in the DMS spectra and also the reactant ions which were H+(H2O)n in positive polarity and O2−(H2O)n in negative polarity. The dependences of mobility on E/N are seen in dispersion curves of individual ions which were characteristic in both absolute positions and in slopes and shapes, even in purified air at 1 ppm moisture and 45 °C. For example, ion peaks for all protonated monomer ions appear at a CV of ∼−2 V for SV of 600 V and the dispersion curve for isopropanol separates from others at ∼900 V SV and ∼−8 V CV. In contrast, the dispersion plot for the protonated monomer of 1-butanol has a relatively flat dependence of mobility on E/N and becomes distinctive at ∼800 V SV and −3 V CV. The curves for proton bound dimers are characteristically near 0 to −2 V in CV over a range of SV with minor 2
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Figure 3. Plots of ion intensity and retention time for four alcohol mixture with (a) inverted peak intensity of reactant ion peak with DMS1 in all pass mode and DMS2 set at SV of 850 V and CV of −5.8 V and peak intensity with voltage pairs (SV, CV) for DMS1 and DMS2 of (b) 850 V, −5.4 V, and 900 V, −5.8 V; (c) 850 V, −2.5 V, and 900 V, −2.8 V; and (d) 850 V, −1.0 V, and 900 V, −1.2 V; and (e) 850 V, −0.4 V, and 900 V, 0.4 V. Conversion to E/N is 1 V = 0.1 Td. 2398
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analyzer would exhibit overlap of ion peaks at any SV below 1000 V and above 1300 V. A plot of inverted intensity for the reactant ion peak for the GC/DMS/DMS measurement is shown in Figure 5a where substances are nearly baseline
DMS1 and DMS2 operated with SV of 850 V and a narrow CV scan over the reactant ion peak. There is partial separation of the four components with peaks from methanol, ethanol, and isopropanol significantly convolved. When the mixture is analyzed with a tandem DMS set to each of two pairs of CV1 with SV1 and CV2 with SV2, the extraction of ions based on characteristic dispersion curves is shown by extracted ion chromatograms (XIC) of the alcohols in Figure 3b−e. While these plots are a type of ion specific chromatogram, the purpose of the gas chromatograph is understood principally as a inlet to rapidly deliver substances of precise amounts to the tandem DMS where complications from ionization chemistry are minimized. Consequently, the patterns seen in Figure 3a can be regarded as the analytical response to a substance (or signal, S) mixed with chemical noise. In these instances, the signal intensity is decreased 10-fold, yet the chemical noise (intensity for other alcohols in a single plot from any part of Figure 3b−e, is effectively reduced to zero. Thus, the signal-to-noise ratio is improved, as described for tandem mass spectrometry,12 although the increase in these measurements is too large to be calculated since the noise is effectively below the detection levels. Since differences between α plots in DMS are largest for small polar molecules of all substances, the concept of tandem DMS with selectivity from sampling two positions in a dispersion curve was extended to molecules with differing moieties including an organophosphorus compound, a cyclic ketone, and an alcohol with a carbon number of seven. Consequently, the dispersion curves for these substances (seen in Figure 4 as a composite) appear above −7 V CV for all SV
Figure 5. Extracted ion chromatograms (XIC) for mixture of DMMP, 1-heptanol, and cyclohexanone: (a) XIC with DMS1 in all pass mode and SVDMS2= 500 V and CVDMS2= −0.4 V; (b) XIC with SVDMS1 = 500 V, CVDMS1= −0.4 V and SVDMS2= 1000 V, CVDMS2= −0.8 V; (c) XIC with SVDMS1= 600 V, CVDMS1= 0 V and SVDMS2 = 1400 V, CVDMS2 = 2.2 V; (d) XIC with SVDMS1= 700 V, CVDMS1 = 0 V and SVDMS2= 1500 V, CVDMS2= 4.0 V.
resolved chromatographically. Identical measurements with ions selected using pairs of CV and SV with DMS/DMS measurements, as noted in crosshairs of the Figure 4, are shown in Figure 5b−d and demonstrate a high capability of DMS/ DMS to remove chemical noise, that is, other substances in the mixture. The low SV in DMS1 provided some ion “selection” with high transmission coefficients and separation from the reactant ion peaks and protonated monomers. This alone is simplifying and useful. The real key to DMS/DMS over DMS alone is that there are two layers of selectivity, not one. The idea of tandem instrumentation is based on the benefits of two “gates”12 in improving signal over chemical noise. It should be noted that there may be more powerful and selective combinations of SV1/CV1 and SV2/CV2 than those employed here, which can provide even better selectivity of detection for individual compounds in this particular mixture. Some small levels of chemical noise in Figure 5b−d are apparent though difficult to reliably calculate due to electronic noise. Therefore, an even more complex mixture with a large range of substances is needed to test the concept of selectivity through accessing two regions of a dispersion plot for ions in purified gas. The composite dispersion plot for 23 volatile organic compounds is too complex to display and instead the ion peaks at two SV values of 700 and 1000 V are shown (Figure 6) from GC/DMS/DMS where the tandem DMS was operated
Figure 4. Composite plot of individual dispersion curves for DMMP, 1-heptanol, and cyclohexanone in purified air.
values, with the farthest position for the protonated monomers seen for cyclohexanone. The proton bound dimers of dimethyl methylphosphonate (DMMP) and cyclohexanone on the other hand, appear at the other extreme of the CV scale with their ion peak positions at ∼3−5 V. The protonated monomer for the third substances, 1-heptanol is no greater than −3 V CV for 950 V SV and the analytical space is contracted compared to the prior dispersion plot in Figure 2. Specifically, overlap of dispersion curves is greater and more complex than with that from alcohols. A consequence is that spectra from a single DMS 2399
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the voltage pairs for DMS/DMS measurements of each substance are marked as cross hairs.
Figure 6. Topographical plots of ion intensity, retention time and compensation voltage for mixture of 23 compounds: (a) SVDMS2 = 700 V; (b) SVDMS2 = 1000 V. DMS1 was in all pass mode in both cases. Peak identities are as follows: (1) isopropanol, (2) acetone, (3) ethyl acetate, (4) isopropyl acetate, (5) 1-butanol, (6) tert-butyl acetate, (7) isopropenyl acetate, (8) pinacolone, (9) 2-mehyl-3-pentanone, (10) isobutyl acetate, (11) 2,4-dimethyl-3-pentanone, (12) 2-hexanone, (13) n-butyl acetate, (14) methyl valerate, (15) 1-hexanol, (16) heptanal, (17) 2, 6-dimethyl-4-heptanone, (18) 1-heptanol, (19) cyclohexanone, (20) dimethyl methylphosphonate, (21) octanal, (22) 3-methylcyclohexanone, (23) cycloheptanone.
Figure 7. Dispersion plots of 1-hexanol (a) and DMMP (b) in purified air with crosshairs indicating SV/CV combinations used for selective DMS/DMS detection of 1-hexanol and DMMP in Figure 8.
The analysis of the 23 component mixture with DMS1 in all pass mode and DMS2 fixed on a narrow range encompassing the reactant ion peak is shown in Figure 8a. The separation of substances in chromatographic time removes nonquantitative effects from the ionization step where the response can be strongly matrix dependent. Nonetheless, all substances regardless of absolute or relative retention can be regarded as matrix for calculation of improvements in signal-to-chemical noise for tandem DMS measurements.34,35 These are shown for 1-hexanol in Figure 8b and DMMP in Figure 8c. As with prior results, pairs of SV and CV were successful in isolating an ion for the substance from the other substances or chemical noise. Yet in each instance, an additional peak in the chromatograms of Figure 8b,c demonstrated incomplete isolation of ions. A few observations can be made from these results: (i) the improvement in the signal-to-noise ratio from each substance, derived from peak areas for the individual substance and the other 22 substances was 106-fold for 1-hexanol (change in ratio from 0.0425 to 4.50) and 31-fold for DMMP (change in ratio from 0.0426 to 1.34). (ii) In any direct extension of these concept to a stand-alone vapor monitor without a chromatographic inlet, a main concern in response to a single substance would arise in ion source chemistry rather than in ion filtering by tandem DMS. (iii) The increase in signal-to-noise ratio was
with all ion pass for DMS1 and sweeping of CV for DMS2. The results are consistent with prior studies with DMS where the reactant ion peak at 700 V SV (Figure 6b) appears at ∼−5 V CV and is displaced to CV more negative than −10 V with SV 1000 V and not seen in Figure 6b. Protonated monomers of substances, which are seen from −5 to −2 V CV at SV of 700 V, are dispersed from −8 to −1 V CV with SV of 1000 V; this is consistent with ions exhibiting positive α functions in most instances. Finally, the peaks for proton bound dimers of these substances are all displaced in a direction opposite the protonated monomers, that is, these ions have negative α functions. Although only two points on the dispersion curves are illustrated in these plots, the complexity and variability suggest that characteristic dependences are general as expected and should allow high selectivity by DMS/DMS, depending on suitable selection of pairs of SV and CV. There are no rules developed yet to guide or maximize selectivity from neither these complex mixtures nor even simple mixture; therefore, the choice of voltage pairs was from manual inspection and somewhat arbitrary pending advances in computational analysis of complex dispersion plots. The dispersion curves for two substances, 1-hexanol and DMMP are shown in Figure 7, and 2400
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Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The financial support from the National Science Foundation, Award No. CHE-1306388, is gratefully acknowledged as is material support from ChemRing Detection Systems with specialized electronics and software. Machine shop contributions from John Tobin and technical discussions with John Petinarides, Todd Griffin, and Paul Rauch were received gladly in this project.
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Figure 8. Plots of ion intensity and retention time for a mixture of 23 volatile organic compounds with (a) inverted peak intensity of reactant ion peak with DMS1 in all pass mode and DMS2 set at SV of 500 V and CV of −1.8 V and peak intensity with voltage pairs (SV, CV) for DMS1 and DMS2 of (b) 600 V, −0.5 V, and 1200 V, −1.4 V; and (c) 1000 V, 1.2 V, and 1500 V, 4.4 V. Chromatographic peak identities are the same as in Figure 6
achieved here without the addition of modifying gases which is now being developed to aid selectivity with a single stage DMS analyzer as a filter before mass spectrometers. Measurements here were made in a purified air atmosphere only. Additional selectivity in tandem DMS may be possible with modifiers or reagents introduced between DMS1 and DMS2.
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CONCLUSIONS A capability to operate a tandem DMS analyzer with characteristic combinations of separation voltage and compensation voltage on each of the DMS stages demonstrates that selection of ions from particular substances in mixtures is possible with improvements in signal-to-noise ratios (analyte peak area/sum of all peaks) of 31 or higher. In some instances the ratio was not measurable at current detection limits for “chemical noise”. This was attained with times of response below 100 ms while conventional dispersion curves require two to four minutes. These findings demonstrate that selectivity based on mobility differences, including ion solvation, at two positions in dispersions plots was suitable to isolate the response from one substance from others of comparable molar mass and chemical function. The tandem DMS analyzer presented in this work can potentially be useful not only as a selective standalone detector for field applications, as described in this work, but also as a selective mobility prefilter for various types of mass spectrometers in applications requiring elimination of chemical noise prior to mass spectrometry measurement.
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