The novel analysis of uranyl compounds by electrospray-ion mobility-mass spectrometry

International Journal of Mass Spectrometry 333 (2013) 21–26

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The novel analysis of uranyl compounds by electrospray-ion mobility-mass spectrometry Christina L. Crawford a , Glenn A. Fugate b , Paula R. Cable-Dunlap c , Nathalie A. Wall a , William F. Siems a , Herbert H. Hill Jr. a,∗ a

Washington State University, Department of Chemistry, P.O. Box 644630, Pullman, WA 99164, United States Savannah River National Laboratory, Savannah River Site, Aiken, SC 29808, United States c Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831, United States b

a r t i c l e

i n f o

Article history: Received 2 August 2012 Accepted 2 August 2012 Available online 10 August 2012 Keywords: IMS Electrospray ionization Uranyl MS Forensics

a b s t r a c t This study relates the first mass identification of mobility peaks associated with uranyl species. These uranyl species were introduced into the gas phase by electrospray ionization and detected by ion mobility-mass spectrometry (IM-MS) to obtain rapid chemical information from uranyl compounds. Uranyl compound analysis in nuclear forensic science is typically performed using alpha, gamma, and mass spectrometry after extensive sample preparation and purification. Although providing highly sensitive isotopic and concentration information, these methods do not provide chemical information during the initial stages of analysis. Ion mobility spectrometry, when coupled with mass spectrometry, provides chemical information, including mass-identified mobility values, for analyte identification. In this study, uranyl compounds were detected in both the positive and negative ionization modes by electrosprayion mobility-time of flight mass spectrometry (ESI-IM-TOFMS). The results showed that the sample type influenced the analyte ions that formed in the negative mode and that ESI solvent composition was the main factor that influenced analyte ion formation in the positive mode analysis. These results indicate that ESI-IM-TOFMS can be used to obtain rapid, chemical information for the initial analysis of a sample containing uranyl compounds. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Nuclear forensic science provides information on the “intended use, history, and origin” of nuclear and radiological material [1]. In particular, the detection of uranium and its chemical speciation is critical to the area of nuclear safeguards. A wide variety of destructive and nondestructive instrumental techniques have been typically applied in nuclear forensic determinations to examine physical, chemical, forensic, and radioactive properties of the samples. Techniques often include microscopy (e.g., scanning electron microscopy, transmission electron microscopy) for physical characterization, crystal structures, and initial elemental analyses; mass spectrometry (e.g., thermal ionization mass spectrometry, secondary ion mass spectrometry, accelerator mass spectrometry, and inductively coupled plasma mass spectrometry) for isotopic analysis and elemental composition; and radiometric detection of alpha, beta, X- or gamma-ray radiation for isotopic identification and quantification [1–4].

∗ Corresponding author. Tel.: +1 509 335 5648; fax: +1 509 335 8867. E-mail address: [email protected] (H.H. Hill Jr.). 1387-3806/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ijms.2012.08.004

The most common nuclear forensic analysis techniques for uranium (U) have been alpha spectrometry, gamma spectrometry, and mass spectrometry (MS) [4]. Most MS techniques used in nuclear forensic studies require complete dissolution and significant purification of uranium materials from background matrices, especially in environmental samples; the MS techniques have also used ion sources that atomize the sample to enable extremely sensitive detection of isotopic ratios and total material content. However, these methods completely denature the uranium species from the original sample. Electrospray ionization (ESI), interfaced to a mass spectrometer, can preserve the chemical speciation of inorganic species as they transfer into the gas phase. Agnes and Horlick first demonstrated the use of electrospray ionization-mass spectrometry (ESI-MS) for the analysis of uranyl compounds (UO2 2+ ) in 1992 [5]. They identified several uranyl species in the gas phase including the UO2 + species. The detection of this species confirmed that ESI, which fundamentally is a redox process, was capable of transmitting uranyl species into the gas phase but that reduction of U(VI) to U(V) can occur during the transfer of the ion from the solution to the gas phase [6,7]. The U(V) uranyl species is rare in the solution phase because it will disproportionate into U(IV) and U(VI) species [8].

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The majority of ESI-MS studies with uranyl compounds have used various ESI solvent compositions to gain insight into the coordination chemistry of uranyl–ligand complexes in the solution phase [7–19]. Several reports have used a variety of solvent compositions including acetone [11], methanol [14,16], nitriles [20], water [21], and combinations of water, methanol, and acetic acid [22,23]. These reports showed that changes to the ESI solvent composition can influence the type of analyte ions formed [24,25]. A fundamental description of the use of water and organic solvents in the ESI process can be found in several reports [26,27]. Yet, in all of the above studies, the ionic species produced by these ESI methods were not consistent from one study to another. ESI-MS also produced gas phase ions with structures that were difficult to interpret without controlled fragmentation techniques and tandem mass spectrometry methods. Typically, electrospraygenerated ions were subjected to collision induced dissociation (CID) and then analyzed by tandem mass spectrometry (MS/MS) to determine the gas phase structure of the uranyl-containing ion. These additional analyses required MS/MS-capable instrumentation and increased analytical method time and complexity. Ion mobility spectrometry (IMS) is a rapid, gas phase separation and ion detection method that has been used extensively for detection of national security threats including chemical warfare agents [28], potential biological warfare agents [29], nuclear materials [23], and explosives [30]. Yet, little work has been conducted to rapidly detect uranyl ions with atmospheric pressure analytical techniques. Only one study has and detected uranyl (the most environmentally relevant form of uranium in the +6 oxidation state which exists as the +2 linear dioxo cation, UO2 2+ ) and other inorganic compounds by positive mode electrospray ionization-ion mobility spectrometry (ESI-IMS) [23]. Uranyl acetate and uranyl nitrate produced distinct mobility peaks for both compounds and averaged mobility spectra were collected for each sample in under 12 s. However, this work only detected positive ion species for the uranyl compounds; the utility of negative mode ion mobility spectrometry has been extensively demonstrated when detecting national security threats including explosives [31]. Also, no correlation of mobility peaks with mass identification has ever been performed for uranyl compounds. As a hybrid technique, electrospray ionization coupled to an ion mobility-time of flight mass spectrometer (ESI-IM-TOFMS) has provided rapid chemical information, including simultaneous mass identification of mobility values [32–34]. ESI-IM-TOFMS can supply several other types of chemical information beyond ESI-MS alone: mobility drift time data used for reduced mobility value calculations; mobility-mass correlation spectra for identification of ionic species including isobaric ions; mobility selection of ions for clear isotope pattern interpretation; and individual mobility and m/z spectra for both positive and negative ions. Fragmentation can also be induced in a hybrid IM-MS to elucidate structure without an additional CID fragmentation cell. Many complex samples contain analytes that ionize either as a positive or negative ion, depending on their gas phase proton and electron affinities. The collection of both positive and negative mode ion mobility-mass spectra (IM-MS) helps identify similarities and differences between the analyte ions formed from various compounds. Also, the two-dimensional (2D) IM-MS separation can greatly reduce the likelihood of a mass or mobility interferent masking the signal from a target analyte [35]. The goal of the present work is to demonstrate how ESIIM-TOFMS can obtain initial chemical information for uranyl compounds in both the positive and negative ionization modes. The secondary objective of this work was to determine if ESI-IM-TOFMS could differentiate several uranyl compounds from each other in the gas phase.

2. Materials and methods 2.1. Chemicals HPLC grade methanol, water, and glacial acetic acid were purchased from Sigma–Aldrich (St. Louis, MO) and were used without further purification. Uranyl chloride, uranyl nitrate hexahydrate, and uranyl acetate dihydrate were purchased from IBI Labs, Inc. (Boca Raton, FL). All mass and mobility calibrants were purchased from Sigma–Aldrich (St. Louis, MO) with the exception of 0.1 mg mL−1 standards of 2,4,6-trinitrotoluene (TNT), dissolved in 1:1 (v/v) solution of methanol:acetonitrile solvents, which were purchased from Accustandard (New Haven, CT).

2.2. Sample preparation For the negative mode analysis, each of the three uranyl compounds was dissolved in a 1:1 methanol:water ESI solvent. The positive mode ESI solvents used were: 9:1 (v/v) methanol:water and 1:1 (v/v) methanol:water. The positive mode ESI solvents were chosen as a representation of the typical spectrum of ESI solvents used in ESI-IMS and ESI-IM-MS [6]. Uranyl chloride was chosen for the positive mode analysis because it was readily dissolvable in the ESI solvents. The sample concentration ranged from 500 to 800 ␮M; this concentration range is typical of other ESI-MS analyses of uranyl compounds [14,18]. Sample preparation times ranged from 3 to 5 min per sample which was mainly spent performing the necessary serial dilutions to achieve the desired sample concentration. IM-MS data acquisition periods varied between 30 and 75 min to ensure adequate accumulation of signal from each ionic species produced by the analysis.

2.3. Instrumentation The electrospray ionization-ion mobility-time of flight mass spectrometer has previously been described [33,36,37]. Ions were generated by an electrospray ionization source constructed using a 60 ␮m I.D. × 145 ␮m O.D. fused silica capillary supplied by Polymicro Technologies, LLC (Phoeniz, AZ). The capillary was connected to a Hamilton 250 ␮L gas-tight sample syringe (Reno, NV) by stainless steel fittings from VICI Valco (Houston, TX). The electrospray solution was sprayed at a flow rate of 3 ␮L min−1 using a KD Scientific Inc. model 210 syringe pump (Holliston, MA). Ions were produced by applying a high voltage to the ESI setup biased 3.5 kV above the first electrode in the ion–molecule reaction region of the IMS. The ions were then pulsed into the mobility drift region by a Bradbury–Nielsen ion shutter with a 200 ␮s pulse width and a closure voltage of ±43.4 V. The ions drifted along a 521 V cm−1 uniform electric field against a countercurrent flow of heated nitrogen drift gas (200 ◦ C) at a flow rate of 1 L min−1 and ambient pressure (approximately 700 Torr). The ions entered a pressure interface region (1.6 Torr) via a 250 ␮m pinhole leak and then into an ion focusing region (2.5 × 10−2 Torr) of the TOFMS where a series of ion lenses guided the ions towards the TOF drift region. The ions entered the one meter V-shaped flight path of the TOFMS (1.9 × 10−6 Torr) with an acceleration potential of 800 V supplied by an orthogonal extraction pulser. The ions were detected by a Burle multi channel plate detector (Lancaster, PA). Spectra were viewed and processed by software developed at Ionwerks Inc. (Houston, TX) which was run on the ITT Visual Information Solutions IDL Virtual Machine platform (Boulder, CO). The values of the voltage settings for the ESI-IM-TOFMS were the same in both the positive and negative ionization mode. Predicted isotope patterns were generated by the open-source Isotope Distribution Calculator and Mass

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Spectral Plotting program from Scientific Instrument Services, Inc. (Ringoes, NJ) [38]. 2.4. Instrument calibration Mobility and mass calibrations were performed to ensure the accuracy and reproducibility of reduced mobility (K0 ) and m/z values for all the analyte ions found in this study. The use of K0 values is the standard way to compare data across various IMS systems. Since IMS systems are built in various configurations and used in diverse operating environments, K0 values (which are corrected for temperature (T, K) and pressure differences (P, Torr)) are used to identify species of interest in IMS [39]: K0 =

L2 V · td

T  P  0 T

P0

(1)

where td is the ion’s drift time (s) as it travels a distance L (cm) along a drift tube with a potential difference of V (V). The positive mode mobility calibrants were 2,6-di-tertbutylpyridine (K0 = 1.40 cm2 V−1 s−1 for M+H+ and m/z 192.18) [40] and 2,4-lutidine (K0 = 1.95 cm2 V−1 s−1 for M+H+ and m/z 108.08) [41]; the negative mode mobility calibrant was TNT (K0 = 1.54 cm2 V−1 s−1 for M−H− and m/z 226.01) [31,42]. Mass-to-charge ratio spectra were internally calibrated using two known peaks that were assigned an exact mass for their respective ion species. A three-point quadratic fit was used to locate the center of each calibrant peak in time. Spectral interpretation was then based on the corresponding m/z values found for the additional peaks in the spectra. 3. Results and discussion 3.1. Negative mode analysis The majority of ESI-MS studies have been conducted in the positive ionization mode [5,8–10,12,13,16,17,19,43,44]. Only within the past five years have reports been published on negative mode ESI-MS studies of uranyl ions [14,15,18,22,45]. Negative mode IMS and IM-MS have been used since the 1970s to detect explosives and other compounds of interest to forensic science and national security; the use of negative mode ionization for uranyl compounds and compounds of interest to nuclear forensic science is a natural extension of this established technique. Fig. 1 shows negative ionization mobility-mass spectra of (a) uranyl nitrate, (b) uranyl acetate, and (c) uranyl chloride in a 1:1 (v:v) methanol:water solution; the corresponding peak list is presented in Table 1. These compounds showed distinct differences in their mobility and mass spectra, indicating that characteristic analyte ions formed depending on the type of uranyl salt. The U redox state for all the species reported in the tables and text was either U(V) or U(VI). Fig. 1(a) shows uranyl nitrate formed three primary analyte ions in the ESI-IM-TOFMS analysis. The first primary ion, represented on the 2D IM-MS plot in the bottom left quadrant of the figure, was the UO2 (NO3 )O2 − adduct ion due to the loss of nitric oxide at m/z 364.04 and a mobility drift time of 10.4 ms. The second primary ion was UO2 (NO3 )2 O2 − (in the bottom right quadrant) at m/z 425.99 and 10.9 ms. This species fragmented to form UO2 (NO3 )2 − (m/z 394.01, 10.9 ms) and, via a NO2 elimination reaction, formed UO2 (NO3 )O− (m/z 348.04, 10.9 ms). The third and final primary ion was the nitrate adduct UO2 (NO3 )3 − (shown on the right side of the figure at m/z 455.99 and 11.4 ms). This species fragmented to form UO2 (NO3 )2 O− (m/z 410.22 and 11.4 ms) and UO2 (NO3 )O− (m/z 348.04 and 11.4 ms). These clear fragmentation patterns (produced as the ions cross the pressure interface region between the IMS and MS) were obtained without the need for collision induced dissociation (CID) and subsequent MS/MS

Fig. 1. Negative ionization mode mobility-mass correlation spectra of (a) uranyl nitrate, (b) uranyl acetate, and (c) uranyl chloride. The mass spectra, with a m/z range of 300–500 Thomsons (Th), are shown at the top of the plots. The mobility spectra, with a drift time range of 8.0–14.0 ms are shown on the right side of the plots.

experiments. These MS techniques have commonly been used in ESI-MS studies of uranyl complexes to obtain structural information [8,10,12,13,16,19,43,44,46]. Fig. 1(b) shows the primary analyte ions for uranyl acetate. The first primary ion, m/z 419.99 and 11.7 ms, was the U(VI) species UO2 (CH3 COO)2 O2 − formed by the oxidative addition of O2 with the U(V) form of uranyl acetate. This ion formation mechanism was also seen by Groenewold et al. [47]. The U(VI) species fragmented to form the UO2 (CH3 COO)2 − species at m/z 388.08 and 11.7 ms and the UO2 (CH3 COO)2 O− species at m/z 345.18 and 11.7 ms. The uranylacetate cluster species, UO2 (CH3 COO)3 − , formed the second most intense mobility peak at m/z 447.13 and 12.9 ms (as shown in the top right of the 2D IM-MS spectra). The third most intense mobility peak in the spectrum was formed by the UO2 (CH3 COO)2 CH3 O− species (m/z 419.13 and 12.0 ms). A hydroxide substitution likely

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Table 1 Peak assignments for the ions found in the negative ionization mode analysis of three uranyl salts by ESI-IM-TOFMS. Each spectra was collected for 30 min in a 1:1 (v/v) methanol:water solvent at a concentration of 500 ␮M. References are given for the species found in the ESI-MS literature unless there is no literature value available (N/A). Uranyl salt

Measured m/z

Exact mass

Ion drift time (ms)

K0 values (cm2 V−1 s−1 )

Analyte ion identification

Ref.

Uranyl nitrate

348.04 364.04 394.01 410.22 425.99 455.99

348.0234 364.0183 394.0163 410.0112 426.0061 456.0041

10.9; 11.4 10.4 10.9 11.4 10.9 11.4

1.63; 1.55 1.71 1.63 1.55 1.63 1.55

UO2 (NO3 )O− UO2 (NO3 )O2 − UO2 (NO3 )2 − UO2 (NO3 )2 O−‡ UO2 (NO3 )2 O2 − UO2 (NO3 )3 −

[14] [14]

345.18 360.00 374.02 388.08 405.09 419.13 419.99 433.06 447.13

345.0488 361.0438 374.0516 388.0672 405.0700 419.0856 420.0571 433.0649 447.0805

11.8 10.7 10.9 11.7 12.0 12.0 11.7 12.2 12.9

1.49 1.64 1.61 1.49 1.46 1.46 1.49 1.44 1.37

UO2 (CH3 COO)O− UO2 (CH3 COO)O2 − UO2 (CH3 COO)(CHO2 )− UO2 (CH3 COO)2 − UO2 (CH3 COO)2 HO− UO2 (CH3 COO)2 CH3 O− UO2 (CH3 COO)2 O2 − UO2 (CH3 COO)2 (CHO2 )− UO2 (CH3 COO)3 −

Uranyl acetate

Uranyl chloride

339.95 374.94

339.9783 374.9472

9.91 9.91

1.78 1.78

−

UO2 Cl2 UO2 Cl3 −

[14] [14]

[18,22]

[47] [18,22] N/A N/A

A double dagger symbol (‡) indicates that the proposed peak assignment matches only the nominal mass of the measured ion.

occurred, replacing the methoxide ion in the previous species to form UO2 (CH3 COO)2 HO− (m/z 405.09, 12.0 ms). The CH3 O− species most likely formed from the ESI solvent containing methanol, a species previously seen by Stipdonk et al. [48]. Other species of interest in the mobility-mass analysis were UO2 (CH3 COO)2 CHO2 − (m/z 433.06, 12.2 ms) and UO2 (CH3 COO)CHO2 − (m/z 374.02, 10.9 ms). The mobility analysis discerned that these two species, which only differ by an acetate molecule in their molecular formulas, were not related by fragmentation but, instead, were independently formed as shown by their distinct mobilities. In this case, the mobility separation clearly provided information about ion species formation that would otherwise have been difficult to obtain without a tandem ESI-MS technique. The UO2 (CH3 COO)O2 − species (m/z 360.00, 1.64 cm2 V−1 s−1 ) was another example of a species that independently formed from the UO2 (CH3 COO)2 O2 − species since they each have a distinct mobility and thus are not related via fragmentation. In a negative mode extractive electrospray ionization (EESI)-MS analysis of uranyl acetate, only the UO2 (CH3 COO)3 − species at m/z 447 was formed. Subsequent CID and MS/MS experiments were used to generate fragment ions and interpret the MS spectra [22]. The UO2 (CH3 COO)3 − species was again the only ion species produced in an electrospray Fourier transform ion cyclotron resonance mass spectrometry (ESI-FT-ICR-MS) study of uranyl acetate. Other species may have been present but were eliminated in the ICR cell to perform an infrared multiphoton dissociation (IRMPD) analysis of the UO2 (CH3 COO)3 − species [18]. By contrast, this ESI-IM-TOFMS method, shown in Fig. 1(b), produced several primary ion species for uranyl acetate. The production of multiple ions is especially useful in analyses where a complex matrix can obscure a single analyte ion. Fig. 1(c) shows the only uranyl chloride ion species formed was a chloride adduct at m/z 374.94 and 9.91 ms; fragmentation of this adduct formed the molecular ion species at m/z 339.95 and 9.91 ms. Chloride is a common negative mode reactant ion in IMS and chloride adducts are often seen in negative mode IMS and MS analyses because of chloride’s high electron affinity [49–52]. The major source of the chloride ion in the sample spectra was presumably from the sample itself. A chloride reactant ion peak was present in the background IM-MS spectra but was most likely a memory effect. Fig. 2 demonstrates the ability of ESI-IM-TOFMS to provide another level of chemical information: clear and easy-to-interpret isotope patterns. ESI-IM-TOFMS, by separating various ion species

based on their mobility, can generate isotope patterns that are free of overlapping peak patterns which may obscure the recognition of a particular isotope pattern in a complex sample. For more detailed information on isotope pattern identification and interpretation, refer to McLafferty and Turecek’s Interpretation of Mass Spectra [53]. This figure compares the experimental isotope pattern and the

Fig. 2. Comparison of (a) experimental and (b) predicted MS spectra showing the isotope pattern for the chloride adduct of uranyl chloride (UO2 Cl3 − at m/z 375). The spectra show arbitrary intensity units versus mass-to-charge (m/z) ratios from 370 to 382 Th.

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Table 2 Peak list for the positive ionization mode background solvent spectra and analysis of uranyl chloride by ESI-IM-TOFMS in two solvent systems: (a) 9:1 (v/v) methanol:water and (b) 1:1(v/v) methanol:water. References are given if the species was found in the ESI-MS literature, regardless of uranyl salt analyzed. Measured m/z value

Exact mass

K0 value (cm2 V−1 s−1 )

Peak ID

Ref.

(a) 9:1 (v/v) methanol:water 287.37 329.13 347.05 361.01 365.06 379.13* 405.03

287.0433 329.0539 347.0645 361.0801 365.0751 379.0907 N/A

1.53 1.54 1.53 1.52 1.53 1.52 1.40

UO2 OH+‡ UO2 CH3 COO+ UO2 CH3 COO(H2 O)+ UO2 CH3 COO(CH3 OH)+ UO2 CH3 COO(H2 O)2 + UO2 CH3 COO(CH3 OH)(H2 O)+ Unknown

[10,11]

(b) 1:1 (v/v) methanol:water 270.23* 287.14 329.34 346.98 361.10 364.99 378.98*

270.0406 287.0433 329.0539 347.0644 361.0801 365.0750 379.0907

1.40 1.52 1.52 1.52 1.53 1.52 1.52

UO2 +‡ UO2 OH+‡ UO2 CH3 COO+ UO2 CH3 COO(H2 O)+ UO2 CH3 COO(CH3 OH)+ UO2 CH3 COO(H2 O)2 + UO2 CH3 COO(CH3 OH)(H2 O)+

[10] [10,11]

(1) A double dagger symbol (‡) indicates that the proposed peak assignment matches only the nominal mass of the measured ion or that an assignment could not be made based on the measured mass of the ion. (2) The starred species indicate a parent ion which was formed in the ESI source but fragmented before the mass analysis.

predicted isotope pattern for the UO2 Cl3 − species. The peaks at m/z 375, 377, and 379 correspond to UO2 Cl2 + 35 Cl− , UO2 Cl2 + 37 Cl− and UO2 35 Cl37 Cl + 37 Cl− , respectively. 3.2. Positive mode analysis A (+)ESI-IM-TOFMS analysis was conducted to determine if initial mobility and mobility-mass information could also be found in the positive mode for a single uranyl compound. Previous work using (+)ESI-MS to study single uranyl compounds (most often uranyl nitrate, uranyl acetate, or uranyl citrate[16,19]) identified a wide variety of positive mode species [9,13,14,44,48,54]. Yet, no such analyses have provided mobility and mobility-mass information for uranyl compounds. As shown in the above section, mobility analyses can provide an independent identification of a uranyl ion using reduced mobility values found for each ion species, as well as complementary information about a uranyl ion species’ size (related to the measured reduced mobility value) relative to its m/z value. Table 2 shows the reproducible analyte ion species formed for uranyl chloride in two ESI solvent systems: 9:1 (v/v) methanol:water and 1:1 (v/v) methanol:water. These solvents were chosen because they represent typical solvents used in ESIIMS and ESI-IM-MS analyses [6]. The table lists the nominal m/z values, average reduced mobility values (n = 3), and proposed peak identifications for the species detected in each solvent. Only the reproducible ion species found across multiple spectra for each solvent have been listed. These peak assignments were directed by previous ESI-MS literature reports and typical product ion formation mechanisms in IMS. The solvent systems produced comparable uranyl species. In general, the analyte ion species were formed by adduction of the uranyl species with the reactant ions present in the system. A common reactant ion, found in each ESI solvent, was the Na+ ion at m/z 22.99; the sodium ion was most likely a contaminant from the ESI solvents. Water clusters ((H2 O)n H+ or (H2 O)n Na+ where n = the number of water molecules, typically 1–3) with reactant ions were also common species in IMS and ESI-IMS [42]. The ammonium ion, NH4 + at m/z 18.04, was also a common positive mode reactant ion and was present in trace amounts in laboratory air and the IMS drift gas introduction system [42]. A variety of gas phase adduct ions formed with uranyl, including adducts with acetate, water, methanol, and combinations of these

species. Acetate was a persistent reactant ion in the IMS and, even after daily thermal and monthly solvent cleaning cycles, could not be eliminated. The starred species in Table 2 were the primary ions which formed in the ESI source. These primary ions were present only during the mobility experiment but fragmented upon entering the pressure interface region of the IM-TOFMS. Fragmentation caused species to form that had the same reduced mobility values but different m/z values. Thus, in the 9:1 (v/v) methanol:water solvent, the species at m/z 379.13 was the parent ion for the fragment ions at m/z 365.06, 361.01, etc. since they each have the same reduced mobility value of 1.52 cm2 V−1 s−1 . 4. Conclusions This study mass-identified mobility peaks for uranyl compounds analyzed by ESI-IM-TOFMS. The results of the negative ionization mode analysis showed that the anions (Cl− , NO3 − , and CH3 COO− ) present during the negative mode analysis of the uranyl compounds produced distinct analyte ions that enabled clear identification of each uranyl compound from the other. Subsequent fragment ions, created in the pressure interface region of the MS, provided a clear picture of the uranyl compound ions created in the system. Thus, the negative ionization mode can be used to distinguish uranyl acetate, uranyl nitrate, and uranyl chloride from each other in an ESI-IM-TOFMS. The results of this positive mode ESI-IM-TOFMS analysis of uranyl chloride provided unique mobility and mobility-mass values for uranyl species in these typical ESI solvents. The positive mode work also demonstrated that, even with different ratios of methanol and water in an ESI solvent, the same ion species could be produced and the mobility values for these species agreed to within ±0.02 cm2 V−1 s−1 . The choice of ESI solvent should be carefully considered when analyzing uranyl compounds by this technique due to the influence of the solvent on the analyte ions formed. Positive mode ESI-IM-TOFMS analysis was capable of detecting uranyl species but may not be able to distinguish the chemical type of the uranyl compound as distinctly as the negative ionization mode. This work also established the first mass identified K0 values needed for future ESI-IMS and IMMS work. These K0 values establish baseline mobility values for uranyl species and will aid future work using ESI-IMS and IMMS to rapidly (<1 min data acquisition periods) detect uranyl compounds.

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