Quantitative Response of IMS Detector for Mixtures Containing Two

Oct 16, 2012 - Faculty of Technology, Lappeenranta University of Technology, ... Department of Chemistry, University of Eastern Finland, P.O. Box 111,...
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Quantitative Response of IMS Detector for Mixtures Containing Two Active Components Jarosław Puton,*,† Sanna I. Holopainen,‡ Marko A. Mak̈ inen,§ and Mika E. T. Sillanpaä ‡̈ †

Faculty of Advanced Technology and Chemistry, Military University of Technology, Kaliskiego 2, 00-908 Warsaw, Poland Faculty of Technology, Lappeenranta University of Technology, Sammonkatu 12, 50130 Mikkeli, Finland § Department of Chemistry, University of Eastern Finland, P.O. Box 111, 80101 Joensuu, Finland ‡

S Supporting Information *

ABSTRACT: This study describes the relationship between the output signal of the ion mobility spectrometry (IMS) detector and the concentrations of two compounds being simultaneously introduced into the reaction section. Investigations were performed for three pairs of compounds, that is, dimethyl methylphosphonate (DMMP) and acetone, methyl tert-butyl ether (MTBE), and acetone, as well as trimethylamine (TMA) and n-nonylamine (NA). Vapors of the investigated compounds were produced in a two-channel generator with permeation sources and a dilution system based on mass-flow controllers. The generator design and the method of concentration determination are discussed in this paper. It was found that admixture can differently influence detection of an analyte. The presence of acetone does not effect the signal corresponding to dimer ions of DMMP. For pairs MTBE + acetone and TMA + NA characteristic peaks of analyte ions diminish with growing concentration of admixture, however, the detection based on the peak of the asymmetric dimer containing proton-bound molecules of both compounds is effective. For the detection of TMA in the presence of NA, the signal generated by the asymmetric dimer ions is meaningfully higher than the signals of monomer or dimer TMA ions measured without the NA admixture. The course of calibration dependencies was analyzed on the basis of a simple mathematical model of the reaction region. This model provided an estimation of the intensity of the signal for a given ionic species for definite concentration of analyte.

I

In practical applications of IMS, admixtures called dopants are often intentionally introduced to gases flowing through the detector. Their relatively high concentration causes that the dominating reaction ions contain one or two dopant molecules. Such ionic structures are known as alternative reactant ions. Dopants can increase the detection selectivity due to minimization of signals from interferents, separation of overlapping peaks, or improvement in the stability of the created ions.5 The detection of organophosphorous (OPC) warfare agents in the presence of ammonia6 or acetone,7 the measurement of biogenic amines in systems with nonylamine8,9 and the use of nicotinamide in narcotics detection10 are important examples of the application of dopants. The influence of dopants on the quantitative characteristics of detection has not received much attention. Eiceman et al. stated that the course of calibration dependencies for OPCs does not depend on the presence of dimethylsulfoxide admixture.11 Long et al. showed that the addition of dopants increases effectively the peak of ions created by organophosphorous pesticide (dimethoate).12 The results obtained by Sheibani et al. indicate increased detection sensitivity of aflatoxin when ammonia was used as a dopant.13

on mobility spectrometry (IMS) is an analytical method based on the ionization of an analyte, the separation of created ions of different mobilities, and the measurement of the current or charge carried by ions of a given kind.1,2 IMS provides differentiating of analytes and measurements of concentrations or masses. Qualitative analysis conducted with IMS requires a high value of resolving power. It can be achieved by a proper construction of IMS detectors and optimization of their operational parameters. Numerous experimental and theoretical works have been devoted to this problem.3,4 Much less attention has been paid to the quantitative aspects of detection with IMS. The intensity of the signal generated at a specific concentration of an analyte depends on the effectiveness of its ionization and ion collection efficacy. Therefore, the sensitivity of detection is determined by the kinetics and thermodynamics of ion−molecule reactions and also by the transport of ions to the transducer part of the detector. In general, the dependencies of a signal from IMS detectors on the analyte concentration are nonlinear. A drop of the signal generated by a given ionic species with an increase of the analyte concentration is sometimes observed. A particularly complicated situation appears when the sample contains, besides the analyte, some interfering substances. In such case, the intensities of the peaks corresponding to analyte ions usually depend on the interferent concentration. © 2012 American Chemical Society

Received: June 29, 2012 Accepted: October 16, 2012 Published: October 16, 2012 9131

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kind of ions due to the recombination and reactions. The full mathematical description of the ion balance contains eq 3 for all ionic components as well as the Navier−Stokes and Poisson equations corresponding to the field of gas flow velocity and an electric field. Different simplifications in the modeling of the ion concentration distribution are possible. The further considered model of the reactant section of IMS detector is similar to the tubular reactor known from chemical engineering. It was assumed that only three kinds of ions, that is, reactant, monomer, and dimer ions like in the reactions described by eqs 1 and 2 will be taken into account. In a real situation, the reactant ions are created as a result of several reactions initiated by primary ionization. However, at water vapor concentration in the order of a few ppm, these processes are very fast in comparison with the time of ion’s movement through the space of the reaction section. It was assumed also that the influences of gas flow velocity, diffusion, and recombination on the concentration of ions are negligible. Further simplifications applied in the modeling were the uniform electric field and constant ionization density in the reaction section. The sketch of the reaction region is presented in Figure 1a. The set of

The results of systematic investigations of admixture influence on the detection sensitivity and cross-sensitivity effects in IMS detectors have not been published yet. The aim of this work is to study precisely the effect of admixture presence and its concentration on the detector signal for three pairs of analyte/admixture in the positive mode of IMS.

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THEORETICAL CONSIDERATIONS Theoretical analysis of the relationship between analyte concentration and the signal generated by IMS detectors is based on the consideration of the phenomena taking place in the reaction sections of detectors. The mathematical description of these phenomena is complicated because it is necessary to take into account the complexity of ion−molecule reactions and processes occurring during the transport of ions. The value of IMS detector signal is related to the space distributions of ion concentration in the reaction section. Precise calculations of these distributions are difficult because the ion balance is described by a set of nonlinear differential equations that should be solved for the physical system with a complicated geometry. Moreover, knowledge about the reaction rate constants, diffusion coefficients, mobilities, and other parameters is indispensable. The accurate values of these constants are often unknown. However, some of the experimentally obtained results can be explained on the basis of a simple, reactor model of the reaction section. Let us consider a typical situation of analyte ionization in the positive mode in the presence of water reactant ions. Analyte molecules (M) are usually ionized through association with water reactant ions,14 creating so-called monomer ions. The created ions can attach another analyte molecule producing proton-bound dimer ions.15 k rM

H3O+ + M ⎯→ ⎯ MH+ + H 2O k mM

MH+ + M ⎯⎯⎯→ M 2H+

(1) (2)

Analyte ions are generated in reaction section of IMS detector where analyte vapor is introduced. Concentration of given kind of ions in vicinity of shutter grid, that is, in front of the drift section, is determined by the time of ions residence in reaction section and concentration of analyte vapor. In general, reactions reverse to 1 and 2 are also possible. In some cases dissociation of dimer ions is observed.16 Further discussion will be limited to situations where monomer and dimer ions of analyte are stable and their decomposition in the reaction section, as well as during the movement in the drift section, does not occur. The ions participating in reactions 1 and 2 can be hydrated to a different degree. For this reason, the reaction rate constants for the formation of monomer (krM) and dimer (kmM) ions should be understood as overall values, describing the reaction rate for a given temperature and water vapor content.17 The concentration of a given kind of ions ni in the reaction region can be described with the following balance equation: ∂ni = div(ni vi) + DiΔni + Pi − Li = 0 ∂t

Figure 1. Modeling of reaction section in IMS detector. Sketch of reaction region (a) and results of modeling (solid lines) compared with experimental data (points) (b).

balance equations for individual kind of ions obtained by transformations of eq 3 has the following form: N ∂nr k n = − rM M nr + 0 ; ∂x K rE K rE

(3)

nr(0) = 0

∂nm k n k n = rM M nr − mM M nm ; nm(0) = 0 ∂x K mE K mE

where vi is the velocity of the ions resulting both from the gas flow and the drift in the electric field, Di is the diffusion coefficient, and t denotes the time. The Pi component corresponds to the generation of ions as a result of ionization and ion−molecule reactions and Li is the rate of losses of i-th

∂nm2 k n = mM M nm ; ∂x K m2E 9132

nm2(0) = 0

(4)

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The symbol E denotes the electric field, N0 is the rate of reactant ions production in a volume unit, and Ki is the mobility constant. The subscripts correspond to the reactant (r), monomer (m), and dimer (m2) ions and analyte molecules (M). The concentrations of ions calculated by solving the equation set 4 are given by formulas nr =

N0 (1 − exp(−arnMx)) k rMnM

nm =

N0 ⎛ a exp( −arxnM) − ar exp( −amxnM) ⎞ ⎟ ⎜1 − m k mMnM ⎝ am − ar ⎠

(5a)

(5b)

nm2 =

Figure 2. Diagram of possible ion−molecule reactions in reaction section of IMS detector.

N0am2 ⎛ a + am ⎜xnM − r k mMnM ⎝ ar am a 2 exp( −arxnM) − ar2 exp( −amxnM) ⎞ ⎟ + m a r a m (a m − a r ) ⎠

nonylamine (NA) were obtained from Sigma-Aldrich (Steinheim, Germany), and acetone, methyl tert-butyl ether (MTBE), and 40% trimethylamine (TMA) solution in water were purchased from Merck (Darmstadt, Germany). Proton affinity (PA) values for all chemicals used in studies are compared in Table 1. Air from compressor was used as carrier gas for the

(5c)

where ar =

k rM , K rE

am =

k mM , K mE

am2 =

k mM K m2E

Table 1. Proton Affinity Values for Chemicals Used in the Measurements

The peak area in the drift time spectrum corresponds to the electric charge carried by ions of a given kind. The quantity of the ions introduced into the drift region through the shutter grid is proportional to the concentration of the ions calculated for the outlet from the reaction section and the ion velocity vi = KiE. Therefore, the signal from the detector can be expressed by the formula

Si = γK ini(x RR )

(6)

compound

proton affinity, kJ/mol

acetone DMMP MTBE TMA NA

81219 902,18 91215 84219 94919 91720

IMS detector and gas generator. The air was purified with a rough filter and molecular sieves 13× (Alfa Aesar) contained in a cylinder of 1 L volume. The moisture level of the carrier gas introduced into detector can be estimated as less than 20 ppm. Pure nitrogen (99.999%) with water content less than 3 ppm was used as a drift gas. IMS Detector. The studies were performed with a spectrometer Ni-IMS (G.A.S. Dortmund, Germany). The detector of this instrument is a small drift tube with unidirectional gas flow. The length of the reaction region is equal to approximately 1 cm. The space in which the ionization of gas with β particles from 63Ni source occurs is approximately the same as the region where chemical ionization of analytes takes place. The drift section length in Ni-IMS detector is equal to 6.0 cm and the electric field in this region was about 320 V/ cm. Bradbury-Nielsen shutter grid was opened by voltage pulses with the duration of 0.15 ms. The detector temperature in all measurements was 70 °C. The carrier and drift gas flows were equal to 0.2 L/min. Drift time spectra were registered with firm software GASpector v. 3.99.035 DSP. The frequency of signal sampling was 50 kHz and final spectra were obtained with the signal averaging from 128 scans. As a measure of the detector signal, utilized in the studies of quantitative characteristics, a peak area was used, which was determined by the integration of the signal in the selected ranges of the drift time. Gas Generator and Calculation of Concentrations. The pneumatic system used for the introduction of two organic vapors with determined concentrations into the carrier gas was the basic part of the measurement set used in the investigations. The scheme of this system is shown in Figure 3. The vapors

where xRR is the length of reaction section and γ is the proportionality coefficient. The concentration dependencies of the signal, that is, the calibration curves, calculated with eqs 5 and 6 are presented in Figure 1b. Calculations were performed for following values of parameters: Kr = 2.61 cm2/(V s), Km = 2.30 cm2/(V s), Krm2 = 1.80 cm2/(V s), krM = 2.0 cm3/s, kmM = 1.5 cm3/s, xRR = 1.0 cm, and E = 140 V/cm. Presented model of reaction section of IMS detector is very simplified but it makes it possible to estimate influence of particular parameters (for example, mobility coefficients and reaction rate constants) on the intensity of the signal. If two active substances, that is, analyte (M) and admixture (D), are simultaneously introduced into IMS detector, their ionization can occur according to reactions described by eqs 1 and 2. Additionally, numerous reactions in which the exchange of the molecule in monomer or dimer ions take place are possible. In the case of monomer ions, the exchange of molecule can be forced by the difference in the proton affinity.18 The interaction of dimer ion with a neutral molecule can lead to a displacement reaction, which takes effect in the creation of symmetric or asymmetric proton bound dimer.15 The scheme of the set of possible reactions which can take place when two active components are introduced into the reaction section of IMS detector is presented in Figure 2.

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EXPERIMENTAL SECTION Reagents and Gases. All chemicals used in this study were analytical grade and they were used without additional purification. Dimethyl methylphosphonate (DMMP) and n9133

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Figure 3. Scheme of gas generator; Designations: pmp, pump (compressor); flt, filter; mfc, mass-flow controller; tc, thermostatic container; ps, permeation source.

by the stream of drift gas introduced into the reaction section of the detector. The accuracy of gas generator system can be derived by analyzing of eq 7. There are two main factors characterizing the precision of component concentration quantifying. The first one is an uncertainty of determination of the emission intensity. It is defined by accuracy of the measurement of permeation source mass decrement. Relative error of the value of emission from used sources was less than 6.5%. The second factor influencing the gas generator accuracy is precision of dilution systems containing set of mass-flow controllers. The uncertainty of dilution was estimated by total differential method using technical data of controllers. Obtained values of relative errors of dilution were not higher than 8.0% for concentration of component emitted from permeation source ps1 and 9.2% for component from the source ps2. Measurement Methodology. DMMP, MTBE, and TMA were the compounds for which the calibration dependencies were determined in the presence of acetone and NA as admixtures. The sources of analyte vapors were placed in container tc1 being a part of a double-stage dilution system. Such a procedure guaranteed the precise adjustment of the analytes concentration in a wide range. The concentrations of admixtures were controlled in the system with a single-stage dilution. For the selected concentration of the admixture, a series of drift time spectra measurements for several analyte concentrations were performed. Peak areas were taken as a measure of signal intensity corresponding to quantity of given kind of ions. Certain ionic structures were not verified using IMS-MS studies, however, drift time spectra for analytes and admixtures have been reported earlier.9,21−24 All compounds used in experiments demonstrate drift time spectra containing two characteristic peaks produced by monomer and dimer ions of analyte. When analyte and admixture are simultaneously introduced into detector, asymmetric dimers are observed with drift times between values for symmetric dimers of both

were generated from permeation sources ps1 and ps2 placed in the thermostatic containers tc1 and tc2. The sources used for the introduction of acetone were made from Teflon tubes closed with Swagelok caps. In the case of other chemicals, glass vials closed with silicon or Viton membrane were applied. The quantity of the substance introduced into gases from permeation sources was determined by the weighing method. The gas flows in the pneumatic system were adjusted with 6 mass-flow controllers F-201 CV operating with control blocks E-7000 (Bronkhorst High-Tech, B.V., Netherlands). The gas stream passing through the container with source ps1 was diluted in a double-stage system in which the controllers mfc1a, mfc1b, mfc1c, and mfc1d were used. The dilution of the gas stream passing over the source ps2 took place in a single-stage system consisting of mfc2a and mfc2b controllers. The concentrations of vapors in the gas introduced into IMS detector can be calculated on the basis of the balance of gas flows in a pneumatic system: C1 =

q1 − q2 + q3 − q4 ε1 q1 − q2 q1 q1 − q2 + q3 q1 − q2 + q3 − q4 + q5 − q6

C2 =

q5 − q6 ε2 q5 q1 − q2 + q3 − q4 + q5 − q6

(7)

where ε1 and ε2 are the emissions from permeation sources ps1 and ps2 determined as a decrement of mass in a unit of time and q1−q6 are the volume flows of gas, as shown in Figure 3. The concentrations calculated using eq 7 refer to the inlet of IMS detector. Most of the calibration dependencies presented in this work were calculated for concentrations determined in such way. The exceptions from this rule are the experimental dependencies presented in Figure 1b in which the analyte concentration was determined for the reaction section of the detector. In this case, the concentration is two times lower because of the dilution of the carrier gas containing the analyte 9134

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components.15 The reproducibility of measurement results was checked during the series of measurements. A wider description of measurement methodology with the algorithm of measurements and an example of repeatability checking results is contained in the Supporting Information, section S1.

this graph. The relationship between the signal generated by reactant ions and analyte concentration is presented in Figure 4b. When the acetone admixture was not present in the carrier gas, the hydrated protons constituted the reactant ions. At high (0.9 or 3.6 ppm) concentrations of acetone, the generation of acetone ions is very fast. For this reason, it can be assumed that, at high acetone concentration, the reactants are dimer acetone ions. The calibration dependencies determined for the peaks corresponding to dimer analyte ions are presented in Figure 4c. The set of calibration dependencies for the situation when only DMMP was introduced into the detector was presented in Figure 1b. It makes possible the comparison of the theoretical results based on the simple mathematical model with experimental data. From the calibration dependencies presented in Figure 4b one can conclude that the signal from IMS detector in the nonselective mode, that is, based on the measurement of the electric charge carried by reactant ions, does not practically depend on the presence of the admixture in the carrier gas. Similarly, the results presented in Figure 4c give evidence that the signal generated by analyte dimer ions poorly depends on the acetone concentration. Practical deficiency of the dependence of calibration curves on the admixture concentration is not obvious because the chemical ionization of the analyte at the absence of acetone and at its high concentration proceeds in different ways (see Figure 2). If only the analyte is introduced into the detector, the production of dimer analyte ions is a result of two successive association reactions. The production of dimer analyte ions from acetone reactants proceeds through two displacement reactions. The displacement reactions in which DMMP molecule is replaced by the molecule of acetone (r.8b and r.9b in Figure 2) are not very probable. It is indicated by the lack of influence of acetone concentration on the dimer peak intensity, which is visible in Figure 4a,c. The mathematical description of the production of asymmetric dimers (r.8b) and analyte dimers (r.9b) for acetone reactant ions can be the same as for the case when the gas introduced into the detector contains only the analyte. Therefore, the concentrations of acetone reactant ions, asymmetric dimers, and dimers containing two analyte molecules can be described by eq 5 with appropriate modified values of mobility coefficients and reaction rate constants. The approximate equality of signals for reactant ions Sr and dimer analyte ions Sm2 for zero and high concentration of acetone admixture could be expressed with equations

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RESULTS AND DISCUSSION The registered drift time spectra which were utilized for the calculation of calibration dependencies demonstrated below are presented in the graphs placed in the Supporting Information, section S2. An elevated baseline for drift times between successive peaks was not observed in all analyzed spectra. It means that monomer and dimer analyte ions produced in reaction region were stable and did not dissociate during movement in drift region.16 DMMP/Acetone: A Classic Example of the Use of a Dopant. The drift time spectra measured at DMMP concentration equal to 100 ppb and different concentrations of acetone are shown in Figure 4a. The ranges of detector current integration and calculation of the signal for the determination of calibration dependencies are also marked in

Sr(H2O) ≈ Sr(Ac) (H 2O) (Ac) Sm2 ≈ Sm2

(8)

Superscripts in eq 8 correspond to the kind of reactant ions. Calculating the signals with eqs 6 and 5a, we obtain K r(H2O) (H 2O) k rM

≈ Figure 4. Detection of DMMP in the presence of acetone. Drift time spectra (a) and calibration dependencies (b, c) for different concentrations of admixture.

(1 − exp(−ar(H2O)x RR nM))

K r(Ac) (Ac) k rM

(1 − exp(−ar(Ac)x RR nM))

(9)

The dependency (eq 9) has to be fulfilled for any value of analyte concentration nM. It is possible only when 9135

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≈

Article

(H 2O) k rM (Ac) k rM

(10)

The equality of signals for dimer analyte ions can be analyzed in a similar way (see Supporting Information, section S3). Taking advantage of eqs 6, 5c, and 10, it can be shown that K m(H2O) (Ac) KdM

≈

(H 2O) k mM (Ac) kdmM

(11)

In this formula, K(Ac) dM dimer ions and k(Ac) dmM is

denotes the mobility of asymmetric the rate constant for reaction r.9a. Equations 10 and 11 exhibit that, in this case, the reaction rate constants are approximately proportional to the mobility of ions participating in these reactions. The mobility coefficients and reaction rate constants presented in eqs 10 and 11 are compiled in Table 2. The Table 2. Measured Mobility Coefficients and Calculated Reaction Rate Constants for the Ionization of DMMP for Hydronium and Acetone Reactant Ions reaction r.1 r.8a

r.2 r.9a

ion

mobility, cm2/(V s)

rate constant, cm3/s

production of monomer analyte ions 2.76 3.1 × 10−9 H+(H2O)3 + Ac2H 2.38 2.4 × 10−9 (H2O) (Ac) K r(H2O)/K r(Ac) = 1.16 k mM /kdmM = 1.25 production of dimer analyte ions (DMMP) 2.37 2.3 × 10−9 H+(H2O) (DMMP)H+Ac 2.04 2.2 × 10−9 (Ac) K m(H2O)/KdM = 1.15

(H2O) (Ac) k mM /kdmM = 1.05

values of mobility were determined on the basis of the drift time spectra and parameters of IMS detector used in the measurements. Reaction rate constants were calculated with equations resulting from the parametric model by Su and Chesnavich.25,26 These values should be seen as an estimation because they could differ significantly from the experimental data.27 In accordance with the expectation, the mobilities and rate constants drop with the growing ion mass. However, the changes of values of these parameters are small. The results in Table 2 demonstrate that the equality of proportions in eqs 10 and 11 is fulfilled with accuracy of 10%. Small and synchronous changes of mobility coefficients and the rate constants when hydronium reactant ions are replaced with acetone dimer ions cause that the calibration dependencies for both kinds of reactant ions are almost the same (see Figure 4). MTBE/Acetone: Compounds with Similar Proton Affinity. Drift time spectra measured for mixtures with different contents of MTBE at constant concentration of acetone are presented in Figure 5a. The relationship between the signal (characteristic peaks areas) and the concentration of both components was analyzed. In contrast to the detection of DMMP, the course of the calibration curves for dimer MTBE ions depends strongly on the concentration of acetone admixture (Figure 5b). The intensity of the dimer MTBE peak diminishes with the increase of the acetone concentration. The calibration curves for the asymmetric dimer consisting of proton-bound acetone and MTBE molecules are compared in Figure 5c. The initial parts of the calibration curves are similar at the concentrations of acetone lower than 300 ppb. This means that the efficiency of the generation of asymmetric dimer

Figure 5. Detection of MTBE in presence of acetone. Drift time spectra for defined concentration of admixture (a) and calibration dependencies for symmetric (b) and asymmetric dimer ions (c).

ions through reactions r.1 and r.5 as well as r.3 and r.7 are similar. When the acetone concentration is increased from 54 to 270 ppb, the maximum value of the signal corresponding to the asymmetric dimer ions rises. The stability of these ions indicates that they interact weakly with MTBE molecules. It can be concluded that MTBE dimers are created mainly in reaction r.2 but not in reaction r.9a. A further increase in the acetone concentration (610 ppb and 2.4 ppm) causes the dominating process of the asymmetric dimer production to be the displacement reaction r.8a. This mechanism is less effective than the reactions r.1 and r.5 or r.3 and r.7. This induces the decrease of the slope of the initial parts of the characteristics in Figure 5c. At high concentrations of the dopant, the dependence of the area of the peak corresponding to the asymmetric dimer on the concentration of MTBE is monotonic in a wide range of concentrations. This makes possible the quantitative measurements of MTBE in the presence of acetone dopant. TMA/NA: Detection of Amines Using a Dopant of Low Mobility Coefficient. The results of the study of TMA detection in the presence NA admixture are shown in Figure 6. The calculation of exact TMA concentration was not possible 9136

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performed without the dopant and for the asymmetric dimer at NA concentration equal to 770 ppb are compared in Figure 6c. It can be seen that, for a single peak, the detection with a dopant gives a more intense signal than for the measurements made without the NA admixture.

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CONCLUSIONS Results of research featured here have shown that the presence of an admixture can differently influence the detection of an analyte. A peak of the dimer OPC ions does not change its position and intensity when an acetone admixture is introduced. The constancy of the intensity of this peak is not obvious because the dimer analyte ions are created in various ion−molecule reactions for different reactant ions present in the system. Simple theoretical consideration based on the balance of ion concentrations in the reaction section leads to the conclusion that the constancy of the intensity of the dimer ions peak occurs when the reaction rate constants are proportional to the mobilities of ions participating in reactions. This proportionality is conserved with the accuracy better than 10% in the case of detection of DMMP without a dopant and in the presence of acetone. For systems in which proton affinity of analyte and admixture are similar, that is, for pairs MTBE/ acetone and TMA/NA, the intensity of the symmetric analyte dimer depends on the admixture concentration. Effective detection of MTBE and TMA can be performed on the basis of the peak corresponding to the asymmetric dimer built from proton-bound analyte and dopant molecules. If the dopant content is appropriately high, the intensity of the signal generated by the asymmetric dimer does not depend strongly on the dopant concentration.

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ASSOCIATED CONTENT

S Supporting Information *

Description of measurement methodology, drift time spectra utilized for calculation of calibration dependencies, and derivation of eq 11. This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 6. Detection of TMA in presence of n-nonylamine. Drift time spectra for defined concentration of admixture (a), calibration dependencies for asymmetric dimer ions (b), and setting-up of calibration dependencies for different kinds of ions (c).

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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because the sources of TMA vapors were permeation standards containing a water solution of this compound. The drop of source mass resulted from the simultaneous emission of TMA and water. It made impossible accurate estimation of the analyte concentration in the gas flowing over permeation standard. For this reason, the relative values of TMA concentration, calculated on the basis of the dilution in the vapor generator system, were applied in the graphs. The drift time spectra (Figure 6a) contain clearly separated peaks corresponding to the hydronium reactant ions, monomer and dimer ions of both amines, and also the asymmetric dimers containing TMA and NA molecules. The concentration dependence of the signal generated by asymmetric dimer (Figure 6b) is similar, as in the case of MTBE detection in the presence of acetone. An increase in the NA admixture concentration causes a drop in the TMA detection sensitivity at low concentrations and a simultaneous broadening of the dynamic measurement range. Calibration dependencies measured for monomer and dimer ions when TMA detection is

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dx.doi.org/10.1021/ac3018108 | Anal. Chem. 2012, 84, 9131−9138