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Isotope separation using high–field asymmetric waveform ion mobility spectrometry
Nuclear Instruments and Methods in Physics Research A 450 (2000) 179}185
Isotope separation using high}"eld asymmetric waveform ion mobility spectrometry David A. Barnett , Randy W. Purves, Roger Guevremont * Institute for National Measurement Standards, National Research Council of Canada, Ottawa, Ont., K1A OR6, Canada PE SCIEX, 71 Four Valley Drive, Concord, Ont., L4 K 4V8, Canada Received 14 September 1999; accepted 15 December 1999
Abstract A new apparatus for gas-phase separation of stable elemental isotopes at atmospheric pressure is described. A gaseous mixture of chloride isotopes was generated using electrospray ionization and introduced into the analyzer region of a high-"eld asymmetric waveform ion mobility spectrometer (FAIMS). The ion current exiting the FAIMS was sampled into a quadrupole mass spectrometer for isotope identi"cation. 2000 Elsevier Science B.V. All rights reserved. PACS: 28.60.#s Keywords: Electrospray; Chlorine; Isotope separation; FAIMS
1. Introduction The separation and enrichment of stable isotopes generally requires expensive instrumentation and intensive method development, especially for large-scale production [1]. Enriched stable isotopes are gaining widespread use as internal standards in chemical analysis and tracers in biomedical studies [2], fuelled in part by the development of increasingly sensitive and versatile mass spectrometers. The demand for stable isotopes is in turn generating renewed interest in developing reliable and cost-e!ective sources of enriched isotopes. * Corresponding author. Tel.: #613-993-6339; fax: #613993-2451. E-mail address: [email protected] (R. Guevremont).
Recently, a new continuous #ow technique for the separation of gas-phase ions at atmospheric pressure and room temperature, referred to as high-"eld asymmetric waveform ion mobility spectrometry (FAIMS) was described [3}6]. Brie#y, FAIMS separates ions based on compound (element) dependent changes in mobility observed at high electric "elds. Cylindrical geometry FAIMS devices are ideal for use with dispersive ion sources, including electrospray ionization (ESI), because in addition to gas-phase separation, these devices also provide high transmission e$ciency due to an ion focusing mechanism [6]. The combination of ESIFAIMS-MS has been used for several applications including the separation of structural isomers [7]. In this study, the feasibility of using FAIMS for isotope separation was evaluated using the stable isotopes of chlorine.
0168-9002/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 0 ) 0 0 1 5 7 - 1
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D.A. Barnett et al. / Nuclear Instruments and Methods in Physics Research A 450 (2000) 179}185
2. Experimental An ESI-FAIMS interface was coupled in-house to a PE Sciex API 300 triple quadrupole mass spectrometer and has been described in detail previously [5]. A three-dimensional view of the instrument is shown in Fig. 1(a). The FAIMS ion "lter was composed of two inner cylinders, which were axially aligned and positioned about 5 mm apart, and an outer cylinder that surrounded the two inner cylinders. The inner cylinders (12 mm i.d., 14 mm o.d.), were about 30 and 90 mm long, respectively, while the outer cylinder (18 mm i.d., 20 mm o.d.) was about 125 mm long. The outer cylinder and the short inner cylinder of the FAIMS instrument were held at the same electrical potential (e.g., 0 V). The long inner cylinder had a high frequency
(210 kHz), high voltage (up to 4950 V p-p), asymmetric waveform (Fig. 1(b)) applied to it, thereby establishing an electric "eld between the inner and outer tubes. The dispersion voltage, indicated by DV in Fig. 1(b), is the maximum of the asymmetric waveform and can be varied between 0 and !3300 V. In addition to this waveform, a separate dc potential (compensation voltage, CV) was applied to the long inner cylinder. The electrospray needle was placed on the centre axis of the short inner cylinder, terminating about 5 mm short of the gap between the two inner cylinders. For the generation of chloride ions, the electrospray needle was held at !1950 V, giving a current of approximately !45 nA. Gas connections to the FAIMS device are also shown in Fig. 1(a). Compressed air was passed
Fig. 1. (a) Three-dimensional schematic view of the ESI-FAIMS-MS instrument (the FAIMS and MS interface plate (OR) were connected at a 453 angle). (b) Asymmetric waveform used in FAIMS. The maximum value of the waveform is called the dispersion voltage (DV).
D.A. Barnett et al. / Nuclear Instruments and Methods in Physics Research A 450 (2000) 179}185
through a gas puri"cation cylinder (charcoal/molecular sieve) and introduced into the FAIMS device. Gas entered through the Carrier In (C ) port, and exited via the Carrier Out (C ) and Sample Out (S ) ports. All three #ows could be adjusted (C "C #S ). In this study, the gas was intro duced through C at a #ow rate of 5 L/min. The gas exited through S at 1 L/min and through C at 4 L/min. A fraction of C , directed radially inward through the 5 mm gap between the inner cylinders, acted as a curtain gas. While the ions formed by ESI were driven radially outward through the gap by the electric "eld, the curtain gas prevented neutrals from entering the annular analyzer region. The curtain gas portion of C , along with the neutrals, exited the FAIMS device via the S port. The remainder of the gas #ow carried the electrospray ions along the length of the annular space between the outer cylinder and the long inner cylinder. If the combination of DV and CV was appropriate, ions were transferred to the vacuum chamber of the mass spectrometer (FAIMS-MS) through a `sampler conea placed at a 453 angle relative to the axis of the FAIMS cylinders. For clarity, the FAIMS-MS illustrated in Fig. 1(a) shows the ions exiting the FAIMS at a 903 angle. The diameter of the ori"ce in the sampler cone was approximately 260 lm. The ori"ce was electrically insulated from the FAIMS device and a separate voltage (OR) of !24 V was applied to it. An o!set voltage of !49 V was applied to the entire FAIMS unit (V ) to enhance the sensitivity of the FAIMS$ '+1 MS. The pressure inside the FAIMS analyzer was kept at approximately 770 Torr.
181
free chloride ions in the gas phase by preferentially binding most metal ions, ML>, thus preventing the formation of various metal}chloride species, i.e., MCl\, in solution [8]. L 4. Results and discussion The ability of FAIMS to transmit chloride ions generated from an ESI source is demonstrated in Fig. 2. Each trace represents an ion-selected CV spectrum (IS-CV spectrum), collected by scanning the CV from !1 to 69 V, while monitoring a mass to charge ratio (m/z) of !35. The dwell time and number of scans were kept constant for each spectrum. The "gure consists of a total of 16 IS-CV spectra. The "rst spectrum (top trace) was acquired with a dispersion voltage, DV, of 0 V (i.e., FAIMS was disabled). The intensity of this plot has been multiplied by a factor of 10 for presentation purposes. Each subsequent spectrum was acquired at increasing values of DV. The "nal spectrum in the series was acquired at DV"!3300 V. From this
3. Chemicals A stock solution containing 9.7 mg/ml of reagent grade ammonium chloride (Anachemia) was prepared in HPLC grade methanol (Anachemia). An aliquot of this stock solution was then diluted 1000-fold in an ESI bu!er containing 15 lM ethylenediaminetetraacetic acid (EDTA, Fisher Scienti"c) in methanol to give a sample solution containing 180 lM chloride. The EDTA-based electrospray bu!er was used to improve the yield of
Fig. 2. Series of 16 IS-CV spectra (m/z!35, CV"!1 to 69 V, 2.82 min duration) for a 180 lM solution of ammonium chloride. The dispersion voltage, DV, is varied from 0 V in the "rst spectrum (top trace) to !(500#200 n) for all subsequent traces up to a maximum of !3300 V (n"0}14).
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D.A. Barnett et al. / Nuclear Instruments and Methods in Physics Research A 450 (2000) 179}185
several peaks; and (iv) the total ion current increases substantially. Three of the spectra in Fig. 2 have been #agged (i.e., DV"0, !2700, and !3300 V) by shading to zero, and are the focus of the discussion to follow. The IS-CV spectrum acquired at DV"0 V (i.e., with FAIMS disabled) is shown in Fig. 3(a). Since there is no applied electric "eld, there is no change in ion mobility, and the peak appears at CV&0 V. A mass spectrum acquired by setting CV"0 V is shown in Fig. 3(b). This ESI-FAIMS-MS spectrum is very similar to a conventional ESI-MS spectrum. The absolute signal in this spectrum is poor compared with a conventional ESI interface in which the ESI capillary would be located much closer (&1 cm) to the MS ori"ce plate. The majority of the ions in the mass spectrum are readily identi"able, and a detailed list is given in Table 1. Note that most of the ions identi"ed above m/z}120 are related to EDTA. The IS-CV spectrum acquired at DV"!2700 V is given in Fig. 4(a). The IS-CV spectrum shows that there are at least three distinct ions or chemical entities that are transmitted through FAIMS and yield an ion of m/z!35. Based on earlier reports [7,9], one of the peaks was expected to correspond to free chloride while the other two were expected to represent adducts that had at least partially dissociated in the FAIMS-MS interface to yield bare chloride ions. Assignment of these peaks was undertaken by sequentially tuning the CV to di!erent values and collecting mass spectra. Mass spectra collected at the same MS conditions used for the IS-CV spectrum in Fig. 4(a) are shown in Fig. 4(b), traces (i) through (iv). Given
Fig. 3. (a) Expanded IS-CV spectrum (m/z !35, CV"!1 to 9 V) from Fig. 2 of 180 lM solution of ammonium chloride acquired at DV"0 V. (b) Mass spectrum for the same solution collected at DV"0 V and CV"0 V.
series of spectra, there are four trends as the magnitude of DV is increased: (i) the compensation voltage at which the peak appears increases; (ii) the peak widths vary; (iii) the ion current separates into Table 1 List of prominent background ions observed in Fig. 3(b) m/z
Background ion
m/z
Background ion
!43 !45 !59 !62 !71, 73, 75 !75 !79, 81 !81, 83 !89
CH CO\ HCO\ CH COO\ NO\ HCl\ CH O(CO )\ Br\ (H CO )Cl\ HC O\
!97 !123 !125 !127 !145 !156 !164 !291
HSO\ [H (EDTA)-(CO )]\ H(NO )\, H C O Cl\ H C O Cl\ H (EDTA)\ NaH(EDTA)\ KH(EDTA)\ H (EDTA)\
D.A. Barnett et al. / Nuclear Instruments and Methods in Physics Research A 450 (2000) 179}185
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Fig. 5. Expanded IS-CV spectra (m/z!35 and !37, CV"50 to 69 V) from Fig. 2 acquired at DV"!3300 V.
Fig. 4. (a) IS-CV spectrum (m/z!35, CV"!1 to 69 V) from Fig. 2 of 180 lM solution of ammonium chloride acquired at DV"!2700 V. (b) Mass spectra for the same solution collected at DV"!2700 V and CV"(i) 6.7 V; (ii) 15.5 V; (iii) 31.8 V (iv) 34.5 V.
the low resolution of FAIMS, mass spectral overlap is inevitable, therefore, not all ions in the MS scans of Fig. 4(b) are related to chloride. The majority of the ions identi"ed from the spectrum in Fig. 3 and listed in Table 1 will appear at some value of CV between 0 and 40 V with DV set to !2700 V. All four mass spectra show the presence of some Cl\ ion. The "rst three spectra, Fig. 4(b) traces (i)}(iii), exhibit characteristic chloride isotope patterns at higher masses. In Fig. 4(b) trace (i), CV"6.7 V, a chloride adduct to oxalic acid (H C O ) is iden ti"ed at m/z!125 and !127. Oxalic acid is present as an impurity in the ESI bu!er. The oxalate ion (m/z!89) in this spectrum is formed from the
dissociation of the chloride/oxalic acid adduct species. In Fig. 4(b) trace (ii), CV " 15.5 V, an adduct of chloride with formic acid (H CO ) is identi"ed at m/z!81. Fig. 4(b) trace (iii), CV"31.8 V, was collected on the shoulder of the major peak at CV"34.5 V in Fig. 4(a). In this trace, a species containing two chloride ions is identi"ed as the hydrochloric acid adduct, HCl\ (m/z!71). Fi nally, a mass spectrum collected at the peak maximum, i.e., CV"34.5 V, yields a spectrum that shows the presence of free chloride ion (Fig. 4(b) trace (iv)). There are several other ions with similar mobility behaviour as chloride at DV"!2700 V, including nitrite (NO\) and nitrate (NO\), that also appear in the mass spectrum. In addition, there are peaks in this spectrum relating to solvated chloride species, i.e. Cl(H O)\ (m/z!53, !55) and Cl(CH OH)\ (m/z!67, !69). These sol vated species are likely a consequence of the recombination of chloride and solvent vapour within the jet expansion region of the mass spectrometer. Fig. 5 shows the "nal IS-CV spectrum #agged from Fig. 2 (DV"!3300 V), plotted over the CV range of 50 to 69 V. The Cl isotope is included in this "gure to demonstrate the separation of isotopes using FAIMS. There are "ve points to be emphasized pertaining to the results in Fig. 5. (i) The heavier chlorine isotope, Cl, appears at lower compensation voltage. This observation is consistent with previous reports for small ions (low mass, increasing mobility with increased electric "eld
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D.A. Barnett et al. / Nuclear Instruments and Methods in Physics Research A 450 (2000) 179}185
strength) in which CV is inversely dependent on ion mass within a series of ions having homologous structures [9]. (ii) The accuracy of the isotope ratio determined from this "gure is poor. The poor isotope ratio re#ects the mass bias within the quadrupole mass analyzer. (iii) The two CV peaks are not symmetric, as there is some `tailinga for both ions on the low CV side. This peak asymmetry appears to be an artifact of the 453 ion extraction from FAIMS into the mass spectrometer used in this study. (iv) Comparison of the measured ion current between this spectrum (DV"!3300 V) and the one acquired at DV"0 V shows a 75-fold increase in sensitivity as a result of a two-dimensional ion focusing mechanism that occurs within the FAIMS analyzer [6]. (v) The chloride}formic acid adduct observed earlier disappears at higher DV. The apparent loss of this species with increasing magni-
tude of DV suggests that the increased ion motion within the FAIMS analyzer may result in dissociation of the adduct species. A similar observation was made for the adduct ions of hydrochloric acid and oxalic acid. Mass spectra collected at CV values of 59.3, 60.0, and 61.8 V with DV held constant at }3300 V are shown in Fig. 6. These spectra show that FAIMS is able to separate the two isotopes on a continuous basis as illustrated by the changing proportions of the two isotopes with increases in CV. In addition to the bare chloride ions at m/z values of !35 and !37, the spectra also show nitrite (m/z!46) and two solvated chloride species, Cl(H O)\ and Cl(CH OH)\, at m/z values of !53, !55, !67 and !69. The exact origin of the solvated chloride species is uncertain but because the Cl/Cl isotope ratio in the solvated ions matches that of unsolvated Cl\, it is likely that the chloride ions are transmitted through FAIMS without solvation, followed by recombination with solvent vapour during the cold jet expansion into the reduced pressure region of the mass spectrometer.
5. Conclusions In this study, a new technique for the gasphase separation of stable isotopes of chlorine at atmospheric pressure and room temperature is reported. With appropriate development, this method may o!er signi"cant advantages for the production of enriched isotope standards for chlorine and several other elements. While mass spectrometry is used in the initial method development to con"rm the separation of isotopes, the production of enriched isotopes could be done at atmospheric pressure and room temperature using FAIMS.
Acknowledgements Fig. 6. Mass spectra of a 180 lM solution of ammonium chloride collected at DV"!3300 V and CV values of (a) 59.3 V, (b) 60.0 V and (c) 61.8 V.
The authors thank Mine Safety Appliances Company, Pittsburgh, PA for their support in this project.
D.A. Barnett et al. / Nuclear Instruments and Methods in Physics Research A 450 (2000) 179}185
References [1] S. Villani, Isotope Separation, American Nuclear Society, USA, 1976. [2] R.R. Wolfe, Radioactive and Stable Isotope Tracers in Biomedicine, Wiley, New York, 1992. [3] I.A. Buryakov, E.V. Krylov, E.G. Nazarov, U.K. Rasulev, Int. J. Mass Spectrom. Ion Proc. 128 (1993) 143. [4] R.W. Purves, R. Guevremont, S. Day, C.W. Pipich, M.S. Matyjaszczyk, Rev. Sci. Instrum. 69 (1998) 4094.
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[5] R.W. Purves, R. Guevremont, Anal. Chem. 71 (1999) 2346. [6] R. Guevremont, R.W. Purves, Rev. Sci. Instrum. 70 (1999) 1370. [7] D.A. Barnett, B. Ells, R.W. Purves, R. Guevremont, J. Am. Soc. Mass Spectrom. 10 (1999) 1279. [8] D.A. Barnett, G. Horlick, J. Anal. At. Spectrom. 12 (1997) 497. [9] D.A. Barnett, R.W. Purves, R. Guevremont, Appl. Spectrosc. 53 (1999) 1367.
Isotope separation using high}"eld asymmetric waveform ion mobility spectrometry David A. Barnett , Randy W. Purves, Roger Guevremont * Institute for National Measurement Standards, National Research Council of Canada, Ottawa, Ont., K1A OR6, Canada PE SCIEX, 71 Four Valley Drive, Concord, Ont., L4 K 4V8, Canada Received 14 September 1999; accepted 15 December 1999
Abstract A new apparatus for gas-phase separation of stable elemental isotopes at atmospheric pressure is described. A gaseous mixture of chloride isotopes was generated using electrospray ionization and introduced into the analyzer region of a high-"eld asymmetric waveform ion mobility spectrometer (FAIMS). The ion current exiting the FAIMS was sampled into a quadrupole mass spectrometer for isotope identi"cation. 2000 Elsevier Science B.V. All rights reserved. PACS: 28.60.#s Keywords: Electrospray; Chlorine; Isotope separation; FAIMS
1. Introduction The separation and enrichment of stable isotopes generally requires expensive instrumentation and intensive method development, especially for large-scale production [1]. Enriched stable isotopes are gaining widespread use as internal standards in chemical analysis and tracers in biomedical studies [2], fuelled in part by the development of increasingly sensitive and versatile mass spectrometers. The demand for stable isotopes is in turn generating renewed interest in developing reliable and cost-e!ective sources of enriched isotopes. * Corresponding author. Tel.: #613-993-6339; fax: #613993-2451. E-mail address: [email protected] (R. Guevremont).
Recently, a new continuous #ow technique for the separation of gas-phase ions at atmospheric pressure and room temperature, referred to as high-"eld asymmetric waveform ion mobility spectrometry (FAIMS) was described [3}6]. Brie#y, FAIMS separates ions based on compound (element) dependent changes in mobility observed at high electric "elds. Cylindrical geometry FAIMS devices are ideal for use with dispersive ion sources, including electrospray ionization (ESI), because in addition to gas-phase separation, these devices also provide high transmission e$ciency due to an ion focusing mechanism [6]. The combination of ESIFAIMS-MS has been used for several applications including the separation of structural isomers [7]. In this study, the feasibility of using FAIMS for isotope separation was evaluated using the stable isotopes of chlorine.
0168-9002/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 0 ) 0 0 1 5 7 - 1
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2. Experimental An ESI-FAIMS interface was coupled in-house to a PE Sciex API 300 triple quadrupole mass spectrometer and has been described in detail previously [5]. A three-dimensional view of the instrument is shown in Fig. 1(a). The FAIMS ion "lter was composed of two inner cylinders, which were axially aligned and positioned about 5 mm apart, and an outer cylinder that surrounded the two inner cylinders. The inner cylinders (12 mm i.d., 14 mm o.d.), were about 30 and 90 mm long, respectively, while the outer cylinder (18 mm i.d., 20 mm o.d.) was about 125 mm long. The outer cylinder and the short inner cylinder of the FAIMS instrument were held at the same electrical potential (e.g., 0 V). The long inner cylinder had a high frequency
(210 kHz), high voltage (up to 4950 V p-p), asymmetric waveform (Fig. 1(b)) applied to it, thereby establishing an electric "eld between the inner and outer tubes. The dispersion voltage, indicated by DV in Fig. 1(b), is the maximum of the asymmetric waveform and can be varied between 0 and !3300 V. In addition to this waveform, a separate dc potential (compensation voltage, CV) was applied to the long inner cylinder. The electrospray needle was placed on the centre axis of the short inner cylinder, terminating about 5 mm short of the gap between the two inner cylinders. For the generation of chloride ions, the electrospray needle was held at !1950 V, giving a current of approximately !45 nA. Gas connections to the FAIMS device are also shown in Fig. 1(a). Compressed air was passed
Fig. 1. (a) Three-dimensional schematic view of the ESI-FAIMS-MS instrument (the FAIMS and MS interface plate (OR) were connected at a 453 angle). (b) Asymmetric waveform used in FAIMS. The maximum value of the waveform is called the dispersion voltage (DV).
D.A. Barnett et al. / Nuclear Instruments and Methods in Physics Research A 450 (2000) 179}185
through a gas puri"cation cylinder (charcoal/molecular sieve) and introduced into the FAIMS device. Gas entered through the Carrier In (C ) port, and exited via the Carrier Out (C ) and Sample Out (S ) ports. All three #ows could be adjusted (C "C #S ). In this study, the gas was intro duced through C at a #ow rate of 5 L/min. The gas exited through S at 1 L/min and through C at 4 L/min. A fraction of C , directed radially inward through the 5 mm gap between the inner cylinders, acted as a curtain gas. While the ions formed by ESI were driven radially outward through the gap by the electric "eld, the curtain gas prevented neutrals from entering the annular analyzer region. The curtain gas portion of C , along with the neutrals, exited the FAIMS device via the S port. The remainder of the gas #ow carried the electrospray ions along the length of the annular space between the outer cylinder and the long inner cylinder. If the combination of DV and CV was appropriate, ions were transferred to the vacuum chamber of the mass spectrometer (FAIMS-MS) through a `sampler conea placed at a 453 angle relative to the axis of the FAIMS cylinders. For clarity, the FAIMS-MS illustrated in Fig. 1(a) shows the ions exiting the FAIMS at a 903 angle. The diameter of the ori"ce in the sampler cone was approximately 260 lm. The ori"ce was electrically insulated from the FAIMS device and a separate voltage (OR) of !24 V was applied to it. An o!set voltage of !49 V was applied to the entire FAIMS unit (V ) to enhance the sensitivity of the FAIMS$ '+1 MS. The pressure inside the FAIMS analyzer was kept at approximately 770 Torr.
181
free chloride ions in the gas phase by preferentially binding most metal ions, ML>, thus preventing the formation of various metal}chloride species, i.e., MCl\, in solution [8]. L 4. Results and discussion The ability of FAIMS to transmit chloride ions generated from an ESI source is demonstrated in Fig. 2. Each trace represents an ion-selected CV spectrum (IS-CV spectrum), collected by scanning the CV from !1 to 69 V, while monitoring a mass to charge ratio (m/z) of !35. The dwell time and number of scans were kept constant for each spectrum. The "gure consists of a total of 16 IS-CV spectra. The "rst spectrum (top trace) was acquired with a dispersion voltage, DV, of 0 V (i.e., FAIMS was disabled). The intensity of this plot has been multiplied by a factor of 10 for presentation purposes. Each subsequent spectrum was acquired at increasing values of DV. The "nal spectrum in the series was acquired at DV"!3300 V. From this
3. Chemicals A stock solution containing 9.7 mg/ml of reagent grade ammonium chloride (Anachemia) was prepared in HPLC grade methanol (Anachemia). An aliquot of this stock solution was then diluted 1000-fold in an ESI bu!er containing 15 lM ethylenediaminetetraacetic acid (EDTA, Fisher Scienti"c) in methanol to give a sample solution containing 180 lM chloride. The EDTA-based electrospray bu!er was used to improve the yield of
Fig. 2. Series of 16 IS-CV spectra (m/z!35, CV"!1 to 69 V, 2.82 min duration) for a 180 lM solution of ammonium chloride. The dispersion voltage, DV, is varied from 0 V in the "rst spectrum (top trace) to !(500#200 n) for all subsequent traces up to a maximum of !3300 V (n"0}14).
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D.A. Barnett et al. / Nuclear Instruments and Methods in Physics Research A 450 (2000) 179}185
several peaks; and (iv) the total ion current increases substantially. Three of the spectra in Fig. 2 have been #agged (i.e., DV"0, !2700, and !3300 V) by shading to zero, and are the focus of the discussion to follow. The IS-CV spectrum acquired at DV"0 V (i.e., with FAIMS disabled) is shown in Fig. 3(a). Since there is no applied electric "eld, there is no change in ion mobility, and the peak appears at CV&0 V. A mass spectrum acquired by setting CV"0 V is shown in Fig. 3(b). This ESI-FAIMS-MS spectrum is very similar to a conventional ESI-MS spectrum. The absolute signal in this spectrum is poor compared with a conventional ESI interface in which the ESI capillary would be located much closer (&1 cm) to the MS ori"ce plate. The majority of the ions in the mass spectrum are readily identi"able, and a detailed list is given in Table 1. Note that most of the ions identi"ed above m/z}120 are related to EDTA. The IS-CV spectrum acquired at DV"!2700 V is given in Fig. 4(a). The IS-CV spectrum shows that there are at least three distinct ions or chemical entities that are transmitted through FAIMS and yield an ion of m/z!35. Based on earlier reports [7,9], one of the peaks was expected to correspond to free chloride while the other two were expected to represent adducts that had at least partially dissociated in the FAIMS-MS interface to yield bare chloride ions. Assignment of these peaks was undertaken by sequentially tuning the CV to di!erent values and collecting mass spectra. Mass spectra collected at the same MS conditions used for the IS-CV spectrum in Fig. 4(a) are shown in Fig. 4(b), traces (i) through (iv). Given
Fig. 3. (a) Expanded IS-CV spectrum (m/z !35, CV"!1 to 9 V) from Fig. 2 of 180 lM solution of ammonium chloride acquired at DV"0 V. (b) Mass spectrum for the same solution collected at DV"0 V and CV"0 V.
series of spectra, there are four trends as the magnitude of DV is increased: (i) the compensation voltage at which the peak appears increases; (ii) the peak widths vary; (iii) the ion current separates into Table 1 List of prominent background ions observed in Fig. 3(b) m/z
Background ion
m/z
Background ion
!43 !45 !59 !62 !71, 73, 75 !75 !79, 81 !81, 83 !89
CH CO\ HCO\ CH COO\ NO\ HCl\ CH O(CO )\ Br\ (H CO )Cl\ HC O\
!97 !123 !125 !127 !145 !156 !164 !291
HSO\ [H (EDTA)-(CO )]\ H(NO )\, H C O Cl\ H C O Cl\ H (EDTA)\ NaH(EDTA)\ KH(EDTA)\ H (EDTA)\
D.A. Barnett et al. / Nuclear Instruments and Methods in Physics Research A 450 (2000) 179}185
183
Fig. 5. Expanded IS-CV spectra (m/z!35 and !37, CV"50 to 69 V) from Fig. 2 acquired at DV"!3300 V.
Fig. 4. (a) IS-CV spectrum (m/z!35, CV"!1 to 69 V) from Fig. 2 of 180 lM solution of ammonium chloride acquired at DV"!2700 V. (b) Mass spectra for the same solution collected at DV"!2700 V and CV"(i) 6.7 V; (ii) 15.5 V; (iii) 31.8 V (iv) 34.5 V.
the low resolution of FAIMS, mass spectral overlap is inevitable, therefore, not all ions in the MS scans of Fig. 4(b) are related to chloride. The majority of the ions identi"ed from the spectrum in Fig. 3 and listed in Table 1 will appear at some value of CV between 0 and 40 V with DV set to !2700 V. All four mass spectra show the presence of some Cl\ ion. The "rst three spectra, Fig. 4(b) traces (i)}(iii), exhibit characteristic chloride isotope patterns at higher masses. In Fig. 4(b) trace (i), CV"6.7 V, a chloride adduct to oxalic acid (H C O ) is iden ti"ed at m/z!125 and !127. Oxalic acid is present as an impurity in the ESI bu!er. The oxalate ion (m/z!89) in this spectrum is formed from the
dissociation of the chloride/oxalic acid adduct species. In Fig. 4(b) trace (ii), CV " 15.5 V, an adduct of chloride with formic acid (H CO ) is identi"ed at m/z!81. Fig. 4(b) trace (iii), CV"31.8 V, was collected on the shoulder of the major peak at CV"34.5 V in Fig. 4(a). In this trace, a species containing two chloride ions is identi"ed as the hydrochloric acid adduct, HCl\ (m/z!71). Fi nally, a mass spectrum collected at the peak maximum, i.e., CV"34.5 V, yields a spectrum that shows the presence of free chloride ion (Fig. 4(b) trace (iv)). There are several other ions with similar mobility behaviour as chloride at DV"!2700 V, including nitrite (NO\) and nitrate (NO\), that also appear in the mass spectrum. In addition, there are peaks in this spectrum relating to solvated chloride species, i.e. Cl(H O)\ (m/z!53, !55) and Cl(CH OH)\ (m/z!67, !69). These sol vated species are likely a consequence of the recombination of chloride and solvent vapour within the jet expansion region of the mass spectrometer. Fig. 5 shows the "nal IS-CV spectrum #agged from Fig. 2 (DV"!3300 V), plotted over the CV range of 50 to 69 V. The Cl isotope is included in this "gure to demonstrate the separation of isotopes using FAIMS. There are "ve points to be emphasized pertaining to the results in Fig. 5. (i) The heavier chlorine isotope, Cl, appears at lower compensation voltage. This observation is consistent with previous reports for small ions (low mass, increasing mobility with increased electric "eld
184
D.A. Barnett et al. / Nuclear Instruments and Methods in Physics Research A 450 (2000) 179}185
strength) in which CV is inversely dependent on ion mass within a series of ions having homologous structures [9]. (ii) The accuracy of the isotope ratio determined from this "gure is poor. The poor isotope ratio re#ects the mass bias within the quadrupole mass analyzer. (iii) The two CV peaks are not symmetric, as there is some `tailinga for both ions on the low CV side. This peak asymmetry appears to be an artifact of the 453 ion extraction from FAIMS into the mass spectrometer used in this study. (iv) Comparison of the measured ion current between this spectrum (DV"!3300 V) and the one acquired at DV"0 V shows a 75-fold increase in sensitivity as a result of a two-dimensional ion focusing mechanism that occurs within the FAIMS analyzer [6]. (v) The chloride}formic acid adduct observed earlier disappears at higher DV. The apparent loss of this species with increasing magni-
tude of DV suggests that the increased ion motion within the FAIMS analyzer may result in dissociation of the adduct species. A similar observation was made for the adduct ions of hydrochloric acid and oxalic acid. Mass spectra collected at CV values of 59.3, 60.0, and 61.8 V with DV held constant at }3300 V are shown in Fig. 6. These spectra show that FAIMS is able to separate the two isotopes on a continuous basis as illustrated by the changing proportions of the two isotopes with increases in CV. In addition to the bare chloride ions at m/z values of !35 and !37, the spectra also show nitrite (m/z!46) and two solvated chloride species, Cl(H O)\ and Cl(CH OH)\, at m/z values of !53, !55, !67 and !69. The exact origin of the solvated chloride species is uncertain but because the Cl/Cl isotope ratio in the solvated ions matches that of unsolvated Cl\, it is likely that the chloride ions are transmitted through FAIMS without solvation, followed by recombination with solvent vapour during the cold jet expansion into the reduced pressure region of the mass spectrometer.
5. Conclusions In this study, a new technique for the gasphase separation of stable isotopes of chlorine at atmospheric pressure and room temperature is reported. With appropriate development, this method may o!er signi"cant advantages for the production of enriched isotope standards for chlorine and several other elements. While mass spectrometry is used in the initial method development to con"rm the separation of isotopes, the production of enriched isotopes could be done at atmospheric pressure and room temperature using FAIMS.
Acknowledgements Fig. 6. Mass spectra of a 180 lM solution of ammonium chloride collected at DV"!3300 V and CV values of (a) 59.3 V, (b) 60.0 V and (c) 61.8 V.
The authors thank Mine Safety Appliances Company, Pittsburgh, PA for their support in this project.
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References [1] S. Villani, Isotope Separation, American Nuclear Society, USA, 1976. [2] R.R. Wolfe, Radioactive and Stable Isotope Tracers in Biomedicine, Wiley, New York, 1992. [3] I.A. Buryakov, E.V. Krylov, E.G. Nazarov, U.K. Rasulev, Int. J. Mass Spectrom. Ion Proc. 128 (1993) 143. [4] R.W. Purves, R. Guevremont, S. Day, C.W. Pipich, M.S. Matyjaszczyk, Rev. Sci. Instrum. 69 (1998) 4094.
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[5] R.W. Purves, R. Guevremont, Anal. Chem. 71 (1999) 2346. [6] R. Guevremont, R.W. Purves, Rev. Sci. Instrum. 70 (1999) 1370. [7] D.A. Barnett, B. Ells, R.W. Purves, R. Guevremont, J. Am. Soc. Mass Spectrom. 10 (1999) 1279. [8] D.A. Barnett, G. Horlick, J. Anal. At. Spectrom. 12 (1997) 497. [9] D.A. Barnett, R.W. Purves, R. Guevremont, Appl. Spectrosc. 53 (1999) 1367.