Chemical standards in ion mobility spectrometry

Analytica Chimica Acta 493 (2003) 185–194

Chemical standards in ion mobility spectrometry G.A. Eiceman a,∗ , E.G. Nazarov1 , J.A. Stone b a

Department of Chemistry and Biochemistry, New Mexico State University, Las Cruces, NM 88005, USA b Department of Chemistry, Queens University, Kingston, Ont., Canada Received 2 August 2002; received in revised form 21 May 2003; accepted 10 June 2003

Abstract Positive ion mobility spectra for three compounds (2,4-dimethylpyridine (2,4-DMP, commonly called 2,4-lutidine), dimethyl methylphosphonate (DMMP) and 2,6-di-t-butyl pyridine (2,6-DtBP)) have been studied in air at ambient pressure over the temperature range 37–250 ◦ C with (H2 O)n H+ as the reactant ion. All three compounds yield a protonated molecule but only 2,4dimethylpyridine and dimethyl methylphosphonate produced proton-bound dimers. The reduced mobilities (K0 ) of protonated molecules for 2,4-dimethylpyridine and DMMP increase significantly with increasing temperature over the whole temperature range indicating changes in ion composition or interactions; however, K0 for the protonated molecule of 2,6-di-t-butyl pyridine was almost invariant with temperature. The K0 values for the proton-bound dimers of 2,4-dimethylpyridine and DMMP also showed little dependence on temperature, but could be obtained only over an experimentally smaller and lower temperature range and at elevated concentrations. Chemical standards will be helpful as mobility spectra from laboratories worldwide are compared with increased precision and 2,6-di-t-butyl pyridine may be a suitable compound for use in standardizing reduced mobilities. The effect of thermal expansion of the drift tube length on the calculation of reduced mobilities is emphasized. © 2003 Elsevier B.V. All rights reserved. Keywords: Chemical standard; Ion mobility spectrometry

1. Introduction Ion mobility spectrometry (IMS) has become an analytical tool which is used in several demanding applications including the in-field or on-site detection of chemical weapons, explosives and illicit drug [1]. The principle of the method is the formation of ions representative of a sample and the characterization of these ions based upon their drift velocities, ϑd , in weak electric fields. The formation and the characterization ∗ Corresponding author. Tel.: +1-5056462146; fax: +1-5056466094. E-mail address: [email protected] (G.A. Eiceman). 1 Present address: SIONEX Corporation, Waltham, MA 02451, USA.

of ions both occur at ambient pressure in air or nitrogen. Under these conditions, the drift velocity is directly proportional to the electric field strength, E and a proportionality mobility coefficient, K(T ), as shown in Eq. (1) [2]: ϑd = K(T )E

(1)

This coefficient is a function of temperature and may be obtained, in principle, at a given temperature directly from physical measurements. For a potential difference V across a drift region of length l, an ion with an arrival time td has the mobility given by: K(T ) =

0003-2670/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0003-2670(03)00762-1

ϑd l/td l2 = = E V/ l td V

(2)

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G.A. Eiceman et al. / Analytica Chimica Acta 493 (2003) 185–194

The determination of accurate values of K(T ) for an ion by the direct use of Eq. (2) is complicated by several practical uncertainties. The time of arrival time of the maximum intensity of the ion at the detector is well defined but the reference zero for the time is uncertain. Spangler used the mid-point of the gating pulse as time zero and proposed this method for general adoption in IMS [3]. The value of V can be determined with high accuracy; however, the value of l cannot be assigned as the accurately measured distance between ion shutter and detector plate. This is because the drift field, determined by the potential drop V along the drift region, is never absolutely constant over that distance. Any inhomogeneity in the field leads to a larger drift time than for a strictly homogeneous field [4,5]. In addition, thermal expansion or contraction of the IMS drift tube will change the distance. Therefore, the length l should be replaced by an effective length leff , which is a function of temperature, and is unique for each spectrometer. K(T ) =

2 (T ) leff td V

(3)

Any uncertainty in leff will lead to a larger uncertainty in K as suggested by Eq. (3). For example, an uncertainty of 2% in leff produces a 4% uncertainty in K(T ). An alternative to Eqs. (1)–(3) for determining K(T ) is by comparison of an ion of unknown mobility to that of a standard (an ion of known mobility) as shown: K (unknown) td (standard) = K (standard) td (unknown)

(4)

In this approach, the uncertainty in leff is removed [6]. For comparison and tabulation purposes, K(T ) is usually converted to a reduced mobility or K0 as defined by Eq. (5), in which p is the drift gas pressure in Torr and T is the temperature in K [2,7]: K0 = K(T )

p 273.16 760 T

(5)

At any given temperature and pressure, K(T ) is proportional to K0 and hence the reduced mobility of an ion may be obtained from the ratio of its td value to that of an agreed upon standard ion obtained under identical experimental conditions: td (standard) K0 (unknown) = K0 (standard) td (unknown)

(6)

An early compilation of extant ambient pressure reduced mobilities was published with the caveat that many of the ions listed had not been mass-identified and also with the suggestion that drift times should be obtained with reference to an agreed standard [8]. Several standards have been proposed for use in mobility spectrometers operating in the positive ion mode with proton transfer chemistry. Karpas [8], following the earlier work of Lubman and Kronick [9], suggested the use of 2,4-dimethylpyridine (2,4-DMP, commonly called 2,4-lutidine) citing a relatively high proton affinity for 2,4-DMP and a propensity to produce a single peak, the protonated molecular ion. Karpas assumed a constant K0 = 1.95 cm2 V−1 s−1 in experiments conducted at 150, 200 and 250 ◦ C. Eiceman and Karpas provided tables of reduced mobilities measured relative to 2,4-DMP for a variety of compound types [1]. The use of protonated dimethyl methylphosphonate (DMMP)·H+ and the proton-bound dimer (DMMP)2 H+ as standard ions was investigated by Shoff and Harden [6]. They found a slight increase in K0 for DMMP·H+ (0.00018 cm2 V−1 s−1 K−1 ) and a slight decrease (0.00043 cm2 V−1 s−1 K−1 ) for (DMMP)2 H+ over the temperature range −13 to 207 ◦ C. They noted that the hydration of protonated 2,4-DMP was less than that of DMMP·H+ and stressed that ion hydration at low temperatures can be a problem, changing ion mass and hence K0 values. There is no recognized standard for the calibration of mobility spectrometers in modern analytical measurement science though several IMS manufacturers have placed internal standards in commercially available drift tubes for specific applications. Any compound to be used as a universal standard in proton transfer chemistry would have ideally the following properties: a relatively high proton affinity so ionization is competitive with analytes in proton transfer reactions; readily obtainable in sufficient purity so that extraneous ions are not generated; the protonated form should be thermally stable up to the maximum operating temperatures of most analytical instruments (∼250 ◦ C); the ion should exhibit a minimal tendency to cluster with vapor neutrals of the standard; the mobility should be sufficient for resolution at all temperatures from the reagent ion (usually (H2 O)n H+ ); and finally, the protonated form should have a minimal tendency to hydrate.


In this paper, a comparative study is given for three compounds that are possible standard compounds for use with mobility spectrometry in the positive mode. The compounds are, 2,4-DMP, DMMP and 2,6-di-t-butyl pyridine (2,6-DtBP). The latter was chosen because DtBP·H+ has a much lower affinity for water than do other protonated pyridines such as 2,4-DMP owing to steric hindrance [11]. Under carefully controlled conditions, the effects on mobilities of changes in drift gas temperature and water content and of electric field were studied to assess the qualities of these compounds for use as reference standards in existing mobility spectrometers and in future instruments where improved precision is sought.

2. Experimental

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a commercial drier (R&D Separations GC-2 Disposable Moisture Getter, Alltech Associates, IL). The moisture content of the air was 0.1 ppm as measured by the Panametric MIS-2 meter. When required, the moisture level was adjusted upwards by introduction of water vapor from a temperature controlled reservoir. The amplifier and ion shutter electronics were created in-house. An Intel 80286-based computer was interfaced with an Advanced Signal Processing (ASP) board (Graseby Ionics Ltd., Watford, UK) to obtain spectra with digital signal averaging after analog-to-digital conversion of spectra; spectra are collected and processed using WASP software v 3.0 (Graseby Ionics Ltd.). Software parameters for data collection were: number of spectra averaged, 64; frequency of averaging, 40 Hz; number of data points, 640; and frequency of signal conversion, 40 kHz.

2.1. Instrumentation 2.2. Chemicals and solutions A gas chromatograph/ion mobility spectrometry (GC/IMS) instrument was used in these experiments. A 30 m XTI-50 capillary column (Restex Corporation, Bellefonte, PA) was used in the Hewlett-Packard model 5880A gas chromatograph with automated split injection. The drift tube for the mobility spectrometer, constructed with alternating conducting and insulating rings has been described and characterized previously [5]. The insulating rings were high-temperature stable Teflon (PTFE) and the conducting rings, which were embedded in the Teflon spacers, were stainless steel. Ions from the 10 mCi 63 Ni source were injected into the drift region using an ion shutter composed of two grids. In almost all experiments, the ion shutter was open for 208 ␮s and each scan was initiated on the rising edge of the rectangular shutter pulse. The time scale was set to zero at the center of the rectangular waveform. The temperature of the whole instrument was fixed using a Model 6100 Controller (Omega Engineering Inc., Stamford, CT) and the drift gas temperature was measured using an Omega Series CCT-23 0/400C transducer with model MIS-2 meter (Panametric Inc., Waltham, MA). The drift gas was in-house air, treated using an Addco Model 737 Pure Air Generator (Miami, FL) followed by several scrubbing towers (1 m long × 30 cm i.d.) containing 5 Å molecular sieve and

The chemicals used in this work, methyl methylphosphonate, 2,4-dimethylpyridine (2,4-DMP) and 2,6-di-t-butyl pyridine, were obtained from various manufacturers in the highest available purity. Two stock solutions, one containing 5.9 × 10−3 mol l−1 of DMMP and 2,6-DtBP, and the other the same concentrations of 2,4-DMP and 2,6-DtBP, were made up in methylene chloride. 2.3. Procedure A 1 ␮l sample of a stock solution was injected into the gas chromatograph which was programmed from 60 to 200 ◦ C at 20 ◦ C min−1 and mobility spectra were taken at 1 s intervals throughout the analysis. The widths of individual chromatographic peaks were between 8 and 15 s at base line and consequently it was possible to record approximately 10 mobility spectra during the elution of each compound. The compounds exhibited retention times of: methylene chloride, 3.50 min; 2,4-DMP, 6.45 min; DMMP, 6.45 min; and 2,6-DtBP, 8.15 min. Stored ion mobility spectra (about 540 spectra for each chromatographic injection) were converted to ASCII format and imported into an EXCEL spreadsheet.

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3. Results and discussion The major peak in the spectrum in the absence of any sample, the reactant ion peak or RIP, is due to the hydrated proton, (H2 O)n H+ [11,12]. When a substance is eluted from the chromatograph and is placed in the ion source of the IMS drift tube, the RIP is decreased in intensity and one or two new peaks appeared. The total ion current is constant consistent with conservation of charge. The profile of the chromatographic peak was simulated both by the loss of peak height of the RIP and by the sum of the intensities of all product ions as functions of time. Since about 10 mobility spectra were recorded over the elution time of one chromatographic peak, the effect of a change in analyte concentration by at least one order of magnitude was observable in that time. When the temperature of the spectrometer was raised, the drift times of all the ions decreased and this is evident in Figs. 1–3

which show the results obtained over the temperature range 37–250 ◦ C for 2,4-DMP, DMMP and 2,6-DtBP, respectively. The same amount of analyte was injected into the chromatograph at each temperature and each spectrum shown was obtained at the time of maximum analyte concentration in the spectrometer. A measure of the reproducibility of this method and of all aspects of the mobility spectra are shown for each temperature in Fig. 3 where two spectra are shown, one superimposed on the other. The two spectra in each instance, obtained from the two different DtBP-containing mixtures, show the excellent reproducibility of drift time and peak profile. 3.1. Mobility spectra for prospective chemical standards The spectra obtained for 2,4-DMP are shown in Fig. 1 and contain three peaks at low temperature but

Fig. 1. The ion mobility spectra obtained with 2,4-DMP at 2.9 mg/m3 moisture and field of 222 V cm−1 . The temperatures (◦ C) at which the spectra were obtained are, from the bottom up: 37, 54, 78, 95, 119, 150, 184, 215, and 250. The spectrum shown with the broken line that is superimposed on the top spectrum is that obtained in the absence of 2,4-DMP.


189

Fig. 2. The ion mobility spectra obtained with DMMP at 2.9 mg/m3 moisture and field of 222 V cm−1 . The temperatures (◦ C) at which the spectra were obtained are, from the bottom up: 94, 119, 150, 184, 215, and 250. The spectrum shown with the broken line that is superimposed on the top spectrum is that obtained in the absence of DMMP.

only two at the higher temperatures. The peak with smallest drift time is the RIP while the other two have been identified by Schoff and Harden [6] as the protonated molecule, 2,4-DMP·H+ , and the proton-bound dimer (2,4-DMP)2 H+ , the latter having the larger drift time. The intensity of the peak for the proton-bound dimer relative to that of the protonated molecule decreases with increasing temperature and is not observable in the spectra for temperatures above 78 ◦ C. The intensity of the proton-bound dimer peak also decreased at a fixed temperature as analyte concentration in the ion source decreased. The enthalpy of association of 2,4-DMP with 2,4-DMP·H+ , as shown in Eq. (7), has not been measured; but, the values for 2-methylpyridine (23.0) and for 2,6-dimethylpyridine

(23.3 kcal mol−1 ) [10] 2, 4-DMP · H+ +2, 4-DMP → (2, 4-DMP)2 H+

(7)

would suggest a value of 23 kcal mol−1 . If any (2,4-DMP)2 H+ is formed in the ion source at temperatures above 78 ◦ C, the lifetime is obviously too short for survival of the ion during passage through the drift region. Mobility spectra obtained for DMMP are shown in Fig. 2 and are very similar to those for 2,4-DMP (see Fig. 1). Both the protonated molecule (DMMP·H+ ) and the proton-bound dimer ((DMMP)2 H+ ) are present in the spectra at low temperatures; however, (DMMP)2 H+ is not present at the higher temperatures. It is notable that the drift times of these two ions

190


Fig. 3. The ion mobility spectra obtained with 2,6-DtBP at 2.9 mg/m3 moisture and field of 222 V cm−1 . The temperatures (◦ C) at which the spectra were obtained are, from the bottom up: 37, 54, 78, 95, 119, 150, 184, 215, and 250. There are two spectra for each temperature obtained from different chromatographic injections. The spectrum shown with the broken line that is superimposed on the top spectrum is that obtained in the absence of 2,6-DtBP.

are almost identical with those of the respective ions of 2,4-DMP. Coincidentally, the retention times of the two compounds were the same on the GC column used in this study. The most obvious difference between the spectra in Figs. 1 and 2 is the temperature above which the proton-bound dimer is no longer present. This occurs at 215 ◦ C for DMMP, which is 130 ◦ C greater than for 2,4-DMP for the same gas phase concentration of neutral molecules. Protonation of DMMP will be on oxygen and, by analogy with aliphatic acid esters, probably on P=O. [13] The enthalpy of formation of symmetrical proton-bound dimers by oxygen bases is constant at 32 ± 2 kcal mol, independent of the nature of the molecule [14,15]. Since the entropies of formation of proton-bound dimers are, for nitrogen and oxygen bases, approximately the same, symmetrical oxygen dimers are more stable than symmetrical nitrogen dimers all other things being equal. The DMMP dimer should therefore exist at a higher drift

tube temperature than can the 2,4-DMP dimer, as was observed. Amongst the three compounds, 2,6-DtBP was unique in that no proton-bound dimer was visible in any spectrum, even the spectrum from 37 ◦ C as seen in Fig. 3. This is consistent with the extraordinarily large decrease in standard entropy of >60 cal K−1 mol−1 for the association of protonated 2,6-DtBP with 2,6-DtBP·H+ [10]. The value is large because of the highly restricted motion of the t-butyl and constituent methyl groups in the proton-bound dimer compared with those in the neutral molecule and protonated molecule. Using this value, together with the standard enthalpy of formation of (2,6-DtBP)2 H+ of 23 kcal mol−1 (determined by the same authors), the ratio at equilibrium of the concentrations of protonated molecule and proton-bound dimer may be calculated. At a concentration of 77 mg/m3 for 2,6-DtBP in the ion source and a temperature of


25 ◦ C, only 5% of the ions are in the form of the (2,6-DtBP)2 H+ and at 45 ◦ C less than 1% of ions is present as (2,6-DtBP)2 H+ . The concentration of the neutral 2,6-DtBP in the experiments is far less than 77 mg/m3 so the proton-bound dimer could not form. As seen, the peak area of 2,6-DtBP·H+ relative to that of the RIP remains constant from 37 to 250 ◦ C. As expected, because of its higher mass the drift time of 2,6-DtBP·H+ molecule is significantly greater than those of 2,4-DMP·H+ and DMMP·H+ . 3.2. Mobilities as function of temperature The calculation of ion mobilities using Eq. (3) requires a knowledge of the value of leff . Although it is not possible to determine the effective length of the spectrometer, for purposes of comparison of the ions generated by the three compounds, the measured physical length of the drift region was used in calculating

191

a set of reduced mobilities. At 37 ◦ C, the length was 5.10 cm. The length lT at any other temperature T was obtained using the equation lT = 5.1(1+0.00014(T − 37)), where 0.00014 C−1 is the coefficient of thermal expansion of the Teflon. At 250 ◦ C, l becomes 5.25 cm and the change in l2 from 37 to 250 ◦ C to be used in Eq. (3) is 6%. In Fig. 4, the reduced mobilities are shown as functions of temperature and were calculated with corrections for thermal expansion of the drift region. Mobilities are shown in Fig. 4 for protonated 2,4-DMP, DMMP and 2,6-DtBP, for proton-bound dimers of 2,4-DMP and DMMP and for the RIP. The temperature ranges for over which the dimers were studied, as noted earlier, are much reduced compared with those of the other ions. The moisture content was 2.9 mg/m3 for all the points shown in the figure. A cursory glance at Fig. 4 shows that the reduced mobility of the RIP exhibits the greatest temperature-dependence, increasing linearly by almost

COEFFICIENT OF REDUCED MOBILITY

2.7

2.5

2.3

2.1

1.9

1.7

1.5

1.3 0

50

100

150

200

250

300

o

TEMPERATURE ( C) Fig. 4. Calculated reduced mobilities as functions of temperature at a moisture content of 2.9 mg/m3 moisture (䉫) (2,4-DMP)2 H+ , (䊊) (DMMP)2 H+ , (䉱) 2,6-DtBP·H+ , (䊉) 2,4-DMP·H+ , (䉬) DMMP·H+ , (×) RIP (0.1 ppm moisture), (+) RIP.

192


50% from 37 to 250 ◦ C. Protonated 2,4-DMP and DMMP showed significant but smaller temperature dependences but both graphs are non-linear at the lower temperatures. The non-linearity for DMMP·H+ commences at a higher temperature than does that for 2,4-DMP·H+ , consistent with the generally higher enthalpies of hydration of protonated oxygen bases compared with protonated pyridines [16]. The graphs for the two proton-bound dimers and for protonated 2,6-DtBP show little temperature dependence. The slopes of the linear portions of the graphs are presented in Table 1. Also shown in the table are the results obtained by Schoff and Harden [6] in their study of DMMP over the temperature range 90–220 ◦ C and 2,4-DMP over the range 40–240 ◦ C with a controlled moisture content of 5.2 ± 0.7 mg/m3 . It is to be noted that they observed a negative temperature

Table 1 Calculated reduced mobilities Ion

Temperature range (◦ C)

dK0 /dT (cm2 V−1 s−1 T−1 )

RIP Lutidine·H+ DMMP·H+ 2,6-DtBP·H+ (Lutidine)2 H+ (DMMP)2 H+

37–250 78–250 150–250 37–250 37–78 95–150

3.2E−3 1.07E−3 1.4E−3 1.9E−5 4.6E−4 1.5E−4

± ± ± ± ± ±

1E−4(3.2E−3)a 1.0E−4(1.9E−4)a 1E−4(1.8E−4)a 1.2E−5 1.9E−5 1E−4(−4.3E−4)a

a Values from [5], not corrected for thermal expansion of the drift cell.

coefficient for the mobility of (DMMP)2 H+ the cause of which was not immediately evident. However, thermal expansion of the drift cell was not taken into consideration and when it is the negative coefficient is eliminated [17].

2.3

1.9

2

-1 -1

Ko (cm V s )

2.1

1.7

1.5

1.3 150

200

250

300

350

400

450

E (V/cm) Fig. 5. The effect of electric field and moisture content on reduced mobilities at 150 ◦ C and moisture of (a) 0.08 mg/m3 : (䉫) RIP, 2,4-DMP·H+ , (䊊) DMMP·H+ , () (DMMP)2 H+ , (+) 2,6-DtBP·H+ ; and of (b) 2.9 mg/m3 : (䉬) RIP, (䊏) 2,4-DMP·H+ , (䊉) DMMP·H+ , (䉱) (DMMP)2 H+ , (×) 2,6-DtBP·H+ .


3.3. Field and moisture effects The effects of change of electric field in the drift region on the reduced mobilities are shown in Fig. 5 for the fixed temperature of 150 ◦ C. Experiments were completed at two moisture levels, 0.08 and 2.9 mg/m3 . There is no apparent influence of field on the reduced mobilities at a fixed moisture level but all ions show slight increases in mobility at 0.1 ppm over that at 4 ppm. This is most noticeable for the RIP, less so for 2,4-DMP·H+ and DMMP·H+ and is very small for the dimers and for 2,6-DtBP·H+ . The effect of increased water concentration would be to increase the average hydration levels of all the ions, with those ions that have the greatest tendency to hydrate showing the greatest change. With the exceptions of 2,6-DtBP·H+ and (H2 O)n H+ there is no available data on the hydration enthalpies and entropies of the ions. However, a proton-bound dimer, especially one with no acidic hydrogens such as those of 2,4-DMP and DMMP, will certainly be less prone to hydration than the protonated molecule, consistent with observation. As expected, 2,6-DtBP·H+ shows a minimal change of reduced mobility with change in moisture content while (H2 O)n H+ shows the largest change. For the latter ion it is calculated from the thermodynamic data obtained by Grimsrud and Kebarle [18] that under equilibrium conditions, (H2 O)2 H+ is the only ion of significance that is present at 150 ◦ C and 0.08 mg/m3 moisture while at 150 ◦ C and 2.9 mg/m3 , (H2 O)3 H+ and (H2 O)2 H+ are present in equal equilibrium concentrations. The higher average ion mass should result in a lower reduced mobility, as is observed. The enthalpy and entropy of hydration of 2,6-DtBP·H+ are 12.5 and 41 cal K−1 mol−1 , respectively, the former being an extremely small negative value for any hydration reaction so that even at 37 ◦ C and 2.9 mg/m3 moisture the calculated equilibrium concentration of the hydrate is 105 times less than that of 2,6-DtBP·H+ . The very small increase in reduced mobility of this ion with increased moisture level must have an explanation other than being due to hydration. Few other reports on the use of chemical standards as calibrants for calculations of mobilities have occurred in the past several years. An exception is that of Tabrizchi and the proposal to use the reactant ion peak, (H2 O)2 H+ as a reference in mobility spectra [19]. His studies on the mobilities of ions generated from eight

193

compounds, including DMMP, found that peak separations remained fairly constant over the temperature range 30–250 ◦ C. This is contrary to theory and certainly contrary to the results presented in Fig. 4. This proposal to use the reactant ion as an internal standard to which all other ions are referenced should not be acceptable for a range of instruments under a range of temperatures or moistures.

4. Conclusions The ions, (H2 O)2 H+ , 2,4-DMP·H+ and DMMP·H+ show significant increases in reduced mobilities over the temperature range from near ambient to 250 ◦ C. As a chemical standard where ions of other reduced mobilities can be referenced, none of these ions is eminently suitable. In particular, the usual RIP, (H2 O)2 H+ is not a suitable standard, showing the largest temperature coefficient and probably the ion most susceptible to moisture content. The reduced mobilities of both 2,4-DMP·H+ and DMMP·H+ are linearly proportional to temperature at the upper end of the temperature range but both deviate from linearity at the low temperatures. Since this departure is probably due to hydration changing the mass of the ion, the non-linearity will be susceptible to the moisture content of the drift gas which is often different from instrument to instrument. The reduced mobilities of the proton-bound dimers (2,4-DMP)2 H+ and (DMMP)2 H+ show an almost negligible change over the available experimental temperature. However, because of its low enthalpy for dissociation, the range of temperature over which (2,4-DMP)2 H+ can be used is limited at the high temperature end (DMMP)2 H+ is useable to a higher temperature than is (2,4-DMP)2 H+ but both ions would require a large increase in the concentration of the neutral molecule over that used in this study in order to increase the temperature range. Such a large concentration would be detrimental to the observation of other analytes competing concurrently for protons. Like the dimers, 2,6-DtBP·H+ shows negligible effect of temperature on its reduced mobility and the fact that it does not produce a proton-bound dimer means that all protons accepted by 2,6-DtBP are concentrated in this one ion. The high proton affinity of 2,6-DtBP should allow for favorable competition for protons with most

194


analytes and therefore might be used concurrently with them. We suggest that 2,6-DtBP is worthy of consideration for use as a standard compound in ion mobility spectrometry carried out at ambient pressure. Pending confirmation of these findings and agreement within the community of IMS investigators, the proposed standard will become the preferred reference compounds in this laboratory.

Acknowledgements Financial support of NASA (grant no. 00-HEDS01-110) and US Army (grant no. DAAH04-95-1-054) is gratefully acknowledged. References [1] G.A. Eiceman, Z. Karpas, Ion Mobility Spectrometry, CRC Press, Baton Rouge, 1994. [2] E.A. Mason, E.W. McDaniel, Transport Properties of Ions in Gases, Wiley, New York, 1987.

[3] G.E. Spangler, Anal. Chem. 65 (1993) 3010. [4] T.M. Miller, J.T. Moseley, D.W. Martin, E.W. McDaniel, Phys. Rev. 173 (1968) 115. [5] G.A. Eiceman, E.G. Nazarov, J.A. Stone, J.E. Rodriguez, Rev. Sci. Instrum. 72 (2001) 3610–3621. [6] D.B. Shoff, C.S. Harden, in: Proceedings of the Fourth International Conference on Ion Mobility Spectrom, Cambridge, 1995, pp. 6–9. [7] H.H. Hill, J. Chromatogr. 373 (1986) 141. [8] Z. Karpas, Anal. Chem. 61 (1989) 684. [9] D.M. Lubman, M.N. Kronick, Anal. Chem. 55 (1983) 867. [10] M. Meot-Ner, L.W. Sieck, J. Am. Chem. Soc. 105 (1983) 2956. [11] S.H. Kim, K.R. Betty, F.W. Karasek, Anal. Chem. 50 (1978) 2006. [12] G.E. Spangler, J.P. Carrico, J. Mass Spectrom. Ion Process. 52 (1983) 267. [13] A. Bagno, G. Scorrano, J. Phys. Chem. 100 (1996) 1536. [14] T.J. larsen, T.B. McMahon, J. Am. Chem. Soc. 104 (1982) 6255. [15] M. Meot-Ner, J. Am. Chem. Soc. 106 (1984) 1257. [16] M. Meot-Ner, J. Am. Chem. Soc. 106 (1984) 1265. [17] S. Harden, private communication, 2001. [18] E.P. Grimsrud, P. Kebarle, J. Am. Chem. Soc. 95 (1973) 7939. [19] M. Tabrizchi, Appl. Spectrom. 55 (2001) 1653.