2528
Anal. chem. 1909, 61, 2528-2534
trometry (14,15),and high-pressure negative ion mass spectrometry (16).It is expected, therefore, that C02 will most effectively thermalize the electrons formed in a HPECMS ion source and that kf will be greatest with this buffer gas. A second factor that may affect the relative formation rates of stable molecular anions in a HPECMS ion source is k,, the second-order rate constant for collisional stabilization of the excited intermediate in reaction 6. Ideally, the pseudofirst-order rate for this process, k,[B], should be fast with respect to the autodetachment rate constant, k,+ In order to gain a qualitative assessment of this factor for the various gases used here, a quantity called collision number, 2, is also listed for each buffer gas in Table 11. This quantity is the number of collisions, reported by Ahmed and Dunbar (20), required to quench a population of vibrationally photoexcited bromobenzene positive ions contained within an ion cycloton resonance cavity. Of the gases included in Table 11, COz was found to most effectively remove excess internal energy from the excited ion. It seems reasonable to suggest, therefore, that COz might also be relatively effective as a buffer gas in HPECMS for removing excess internal energy from excited polyatomic negative ions, as shown in the second step of sequence 6. SUMMARY With respect to use of C 0 2 as a buffer gas for HPECMS, the following conclusions can be drawn: (1)Simple EC spectra reflecting resonance and dissociative EC processes, alone, can be observed for compounds that are susceptible to secondary ion source reactions that involve gas-phase and surface-bound free radicals. (2) The high level of sensitivity normally expected of HPECMS is maintained. (3) The walls of the ion
source will be rapidly cleansed of hydrogen atoms that are deposited by prior use of hydrocarbon buffer gases. LITERATURE CITED Hunt, D. F.; Crow, F. W. Anal. Chem. 1978, 50, 1781. Martin, J. T.; Barchas, J. D.; Fad, K. F. Anal. Chem. 1982. 54. 1806. Dougherty, R. C. Anal. Chem. 1981, 53, 625A. Wentworth, W. E.; Chen, E. C. M. E k t r o n CBphrVe, Thecwy8nd%ctiCe in C h r o m e t m f l y ; ZlatklS, A., POOle, C. F., Eds.; Elsevier Scientific Publishing Company: New York, 1981; pp 27-88. Harrison, A. G. W”b1I o n h t k n Mass Spectrometry;CRC Press: Boca Raton, FL, 1983. McEwen, C. N.; Rudat, M. A. J . Am. Chem. Soc. 1981, 103, 4343. Stockl, D.; Budriklewicz, H. Org. Mess spectrom. 1982, 17, 376. Hllpert, L. R.; Byrd, 0.D.; Vogt, C. R. Anel. Chem. 1984, 56. 1842. Stemmler, E. A.; Hites, R. A. Anal. Chem. 1965, 57, 684. Sears, L. J.; Campbell, J. A,; Orlmsrud, E. P. Blamed. fnvkon. Mass Spectrom. 1987, 14, 401. Kassel, D. B.; Kayganlch, K. A.; Watson, J. T.; Aiilson, J. Anal. Chem. 1988, 60, 911. Warman, J. M.; Sauer, M. C. J . Chem. phvs. 1975, 62. 1971. Christophorou, L. G.; Grant, K. S.; Baird, J. K. Chem. phvs. Left. 1975, 30, 104. Woodin, R. L.; Foster, M. S.; Beauchamp, J. L. J . Chem. phvs. 1980, 72, 4223. George, P. M.; Beauchamp, J. L. J. Chem. phvs. 1982, 76. 2959. Gregor, I . K.; Guilhaus, M. Int. J . Mass Spectrom. Ion procesSes 1984, 56, 187-176. Stemmler. E. A.; Hitas, R. A. E k t m Capture Negetfve Ion Mess speck8 Of Envkonmentel Contemdnents 8nd Refeted Compounds; VCH Publisher, Inc.: New York, 1988. Hong. S. P.; Woo, S. B.; Helmy, E. M. mys. Rev. A 1977, 15, 1563. CMstophorou, L. 0.; Grant, M. W. A d v a n w In chemical m p b ; Priigoglne, I., Rice, S., E&.; John Wiley and Sons: New York, 1977; pp 413-519. Ahmed, M. S.; Dunbar, R. C. J . Am. Chem. Soc. 1987, 109, 3215.
RECEIVED for review June 6,1989.Accepted August 28,1989. This work was supported by the Chemical Analysis Division of the National Science Foundation under Grant No. CHE8711618.
Axial Magnetic Inhomogeneities and Low Energy Ion Injection in Fourier Transform Ion Cyclotron Resonance Spectrometry Eric L. Kerley, Curtiss D. Hanson, Mauro E. Castro, and David H. Russell*
Department of Chemistry, Texas A&M University, College Station, Texas 77843
Material choke in ceulens c“&W is crudal fur systems that must be used In strong magnetic Helds. Magnetk inhomogewltles generated by stray fteids present near para” lanee8 can dramatkally Mer the trajectories of low momentum ions. I n thls paper, low energy lon lnjectlon Into a Fourier trarufm kn cyclotron resonance spectrometry cell is studled by udng emplrlcal and theoretical data for a twosectlon cell constructed of paramagnetlc and diamagnetic materials.
INTRODUCTION To achieve maximum performance in Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry, ion detection must occur in the absence of ion-neutral collisions. Because the duration of observation of the time domain ICR signal directly determines sensitivity and resolution, ICR detection is most effective at pressures of Torr or less. The pressure limitations of FT-ICR detection can be circumvented by separating sample introduction and mass analysis. 0003-2700/89/0361-2528$01.50/0
Separation of these steps can be achieved either temporally (e.g. by the use of pulsed valves (1,2))or spatially (e.g. by the use of external ion sources (3-6) and the two-section cell (7)). In each of these approaches, the desired result is coupling of a high-pressure (lo-’ Torr or greater) experimental step (e.g., ionization, ion-molecule reaction, or collision-induceddissociation) with ion detection at low pressure (106 Torr). Remote source techniques require that ions move from the source into the detection region; however, movement of ions from one region to another requires that the ions be accelerated toward the detection cell. That is, ions that have relatively low translational energy along the axis between source and cell (Z axis) are accelerated. Thus the process of Z-axisexcitation changes the conditions under which ion detection is performed. Ideally, ions trapped in the ICR cell have very low translational energies (thermal in the X-Y plane, a few electronvolts maximum along the 2 axis). The Z-axis energy is higher because ions must be trapped in an electrostatic trapping well such that their Z-axis motion is restricted. In order to trap sufficient numbers of ions for detection, trapping potentials that trap translationdy suprathermal ions are employed. Ions 0 1989 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 61, NO. 22, NOVEMBER 15, 1989
entering the FT-ICR cell from a remote source (either an external source or another cell) can have up to 10 eV of translational energy along the Z (magnetic) axis. Another factor to consider is the effects of Z-axis energy on motion of the ions in the X - Y plane. If during the ion injection process Z-axis energy is transferred into the X-Y plane, then the injected ion ensemble is trapped with significant spatial and energetic distribution resulting in loss of signal (8). Although collisional relaxation of injected ions is effective in reducing ion translational energy (9),collisional relaxation does not eliminate problems arising from spatial distribution. Ions encountering magnetically inhomogeneous regions during Z-axis ion injection can be deflected into the X - Y plane. In this paper, we attempt to describe the effects of axial magnetic inhomogeneities on ion partitioning and subsequent trapping of the ions in a two-section cell FT-ICR mass spectrometer. Axial inhomogeneities arise because of the weakly ferromagnetic or strongly paramagnetic properties of the materials used for construction of ICR cells. The effects of axial inhomogeneities on the FT-ICR signal have been studied previously and were found to have little effect on resolution or signal broadening (10). Factors influencing ion partitioning in two-section cells have also been considered (e.g. pressure or ion mass) but without considering the effects of magnetic inhomogeneities (11-13). Axial magnetic inhomogeneities localized in the conductance limit aperture of a two-section cell constructed of high magnetic susceptibility material cause significant redirection and spatial distribution of low-energy ions entering the ion cell. Therefore, a semiquantitative description of trapped ion distributions due to low-energy injection through an axially inhomogeneous magnetic region is useful for considering the design requirements for external ion source FT-ICR instruments. For example, some type of ion guide (either rf only quadrupoles or electrostatic lens elements) is required for external ion source instruments. The effect of a given means of injection on ion detection must be considered. Ions that are injected into the ion cell must have relatively low Z-axis kinetic energies. Thus, at some point along the ion path (source to ion cell) low-energy ions must pass through a potentially inhomogeneous magnetic field resulting in ill-defined ion trapping conditions and reduced instrument performance. EXPERIMENTAL SECTION Fourier Transform Ion Cyclotron Resonance Spectrometer. All experiments reported here were performed on a prototype Fourier transform ion cyclotron resonance mass spectrometer constructed at TAMU. The system is centered around a Nicolet 1280computer system and is equipped with an Oxford 3-Tlarge bore superconductingmagnet described previously (14). The vacuum system and electronics have been modified to accommodate a two-section cell. The two-section cell consists of two cubic cells (3.8 X 3.8 X 3.8 cm) mounted collinearly along the central axis of the magnetic field. The two cells share a common trap plate that also serves as a conductance limit for the differential pumping system. No modificationsto cell geometry were made to the cell with the exception of the thickness and aperture configuration in the conductance limit as specified below. The vacuum is maintained by two 200 L s-l oil diffusion pumps. Background pressures for both sections of the vacuum system were 1 x lo4 Torr or less. Ionization was performed by electron impact (50-eV electrons). The electron gun consisted of a resistively heated rhenium ribbon (1mm) mounted approximately 5 mm from the source region trap plate. An extractor having a 2-mm aperture was placed between the f h e n t and the trap plate and was maintained at ground potential. Trapping potentials were maintained between 0.5 and 10 V. Excitation of the ions in either the source or the analyzer region of the cell was performed by electronically switching the rf excite pulses between the cell regions. Ion Partitioning Studies. Ion partitioning was performed by pulsing the conductance limit trap plate from the normal ion
2520
trapping potential to some potential less than the trapping potential for 60 ps to 18. In most cases, the conductance limit was pulsed to ground. Ions formed in the analyzer region were removed by rf sweeps. The condition of an ion-free analyzer region was verified by detecting in the analyzer region with a partitioning pulse duration of zero. Benzene and argon were the neutral reagents in all studies with the molecular ion being the ion of interest for benzene. The emission current was maintained at less than 100 nA for all studies in order to avoid ion partitioning as a result of space-charge explosion along the Z axis. In addition, at sample pressures of 1 X la-' Torr and emission currents of 100 nA, the number of ions formed (ca 106)is small and ion trapping and detection are not complicated by space charge effects. The quantitative signal-to-noise study was performed by estimating the noise level as the arithmetic mean of the magnitude of the signal at 10 points of the base line near the peak of interest. The mean noise level was then compared to the peak height of interest. It was assumed that for a constant number of scans that the signal is directly related to the number of ions (15). Magnetic Field Calculations. Magnetic field topologies for various two-section cell materials were calculated by using the Poisson/Superfwh codes. (POISSON/SUPERFISHwas developed by R. F. Holsinger and K. Halbach. The codes are distributed R. K. Cooper, Accelerator Theory and Simulation Group, AT-6, L a Alamos National Laboratory, Los Alamos, NM 87545.) The Poisson representation of the magnetic field was solved numerically by implementing a nonuniform triangular mesh (IS). Input for the program consists of defining the parent magnetic field strength (H), the geometry of the two-section cell, and the magnetic permeability (1 + x,) of the cell material. Magnetic susceptibilities (x,) were measured by the Gouy method (17). Calculations were performed on a VAX 11/780 computer. Ion Trajectories. Ion trajectories were calculated and displayed by using the trajectory calculation program SMON (V4.0) (SIMON PC/PSZ V4.0 was developed by D. C. McGilvery and modified by D. A. Dahl. The program is distributed by D. A. Dahl, MS 2208, EG&G Idaho, Inc., Idaho National Engineering Laboratory, P.O. Box 1625, Idaho Falls,ID 83415.) on a math coprocessor equipped PC/AT type IBM compatible computer. SIMION allows placement of electrodes in a user-defined array as well as allowing magneticfields to be defined along any axis within the array. The array used for all calculationswas 90 X 25 points (X,Yin SIMON; Z,Y in FI?-ICRconventions). The spacing between grid points correspondsto 1mm in the actual system. The array was defined for positive values of the ordinate; the remainder of the cell was generated by rotation. Maximum deviation in the refinement of the array was set to O.OOO1 V. Magnetic fields were vectorially added by coordinate as estimated from the Poisson plots. RESULTS AND DISCUSSION The original two-section ICR cell configuration used by this group for the study of ion partitioning was constructed entirely of 2.54 mm thick 304 stainless steel plate. The conductance limit aperture was 4 mm in diameter and tapered from both sides at approximately 30'. No benzene positive ion partitioning into an ion-free analyzer region was observed for benzene ions formed by electron impact at an emission current of 100 nA, ionization periods of up to 10 ms, and trap potentials ranging from 0.5 to 10 V. On the basis of signal-tonoise (S/N) measurements, most of the ions initially located in the source region were lost when the conductance limit was pulsed to ground. The original cell configuration was modified for the following experiment by enlarging the conductance limit and placing a 70% transmittance gold grid across the enlarged aperture (grid diameter = 23 mm). This cell yielded inefficient ion partitioning a t trapping potentials of 7-8 V. No partitioning was observed for lower or higher trapping potentials. The two-section cell was tilted approximately 5' in all directions with respect to the Z axis with no significant effect on ion partitioning efficiency. By comparison of signal-bnoise ratio measurements for ions detected in the source region (prior to partitioning) and in the analyzer region (after par-
2530
ANALYTICAL CHEMISTRY, VOL. 61. NO. 22, NOVEMBER 15, 1989
titioning), it appeared that approximately 90% of the ions initially located in the source region were lost when the conductance limit was pulsed, but the remaining ions (ca. 10%) were partitioned almost evenly between the source and analyzer regions. Significant ion losses can be attributed to the transmkive properties of the gold grid because approximately 30% of the ions will be lost on each pass through the grid. In the subsequent attempt to efficientlypartition ions, the stainless steel analyzer and source region trap plates were replaced with oxygen-free high conductivity copper plates. This cell has a sauare-edee conductance limit auerture of 6 mm diameter. 0; the basis of comparative S / N ratio measurements, we estimate that at least 80% of the Ar+ ions formed in source region partition to the analyzer region. Ion partitioning was observed for this cell at trap potentials of 1 4 V. The effects of higher trapping potential on ion partitioning were not studied. We performed an experiment to qualitatively investigate the effects of space charge on AI+ partitioning a t 2 X lW' Torr s~urcepressure. The trap voltage was set to 4 V, the ionization event lasted 5 ms, and the partitioning pulse duration was set to zero. The emission current was varied in Subsequent experiments between 35,60,and 110 nA. No AI+ partitioned to the analyzer region a t an emission current of 35 nA, hut at 110 nA of emission current, the signal for ion partitioning of AI* was intense. The analyzer cell was quenched during the ionization event; hence this signal was not due to AI+ formation in the analyzer cell. This result implies that partitioning experimentsperformed with emission currents in the high nanoamperes to the microamperes range must be performed auefdly to avoid spacecharge ion partitioning. While partitioning by Coulombic repulsion could be of some utility in analytical studies, it is undesirable for controlled probes of ion chemistry. Overall, space charge partitioning would result in poor control ion location and is avoided in all experiments reported here. Magnetic Materials. All materials poaseas magnetic properties that alter the shape and I d intensity of a magnetic field into which the material is placed. The induced magnetic flux density (magnetic induction, B (kg A-' s2,the tesla)) inside a material placed within a homogeneous magnetic field is given by eq 1where H (A m-') is the magnetic force within the material, I (A m-I) is the intensity of magnetization within the material, and (4sX N A-3 is the magnetic permeability of a vacuum. H is due to the prevailing field into which the object has been placed. The material composing the object interacts with the magnetic force in order to increase or decrease the magnetic induction within the M y . This effect can be quantifed as the magnetic susceptibility, xm,defined in eq 2. xm is, of course, unitless as formulated here.
B =& +II) Xm
= I/H
(1) (2)
Materials fall into one of several categories. If the magnetic susceptibility of a substance is positive, the substance is termed paramagnetic. If xmis negative, the material is diamagnetic. Ferromagnetic materials have very large positive susceptibilitiesresulting from cooperativecoupling among the electron spins of such materials. The origin and dependence of para- and ferromagnetism are different; consequently, ferromagnetismis not simply a special case of paramagnetism. Ferromagnetic materials become paramagnetic above the Curie tempertaure. The materials considered in this paper are diamagnetic (copper) or paramagnetic (304stsinless steel) only. It must be noted that 304 (or 18-8) stainless steel has a Curie point that is below 0 "C if the amount of cold work performed in machining the parts has not been excessive (28).
nrrue 1.
POisar plot Of lwoe8cmn cell msbllcted Of p a r a m a m The magnetic
material (x, = 0.16) bamed in a 3-T magnetic %!d. Z axis is the abscissa; the Y axis is the ordinate.
Both diamagnetic and paramagnetic susceptibilities are roughly constant for increasing H, whereas ferromagnetic susceptibilities decrease significantly as H increases. At the saturation point, B - Hand I level off and a break in B occum. As H is increased past the saturation point, B increases slowly and indefinitely (18). Values for the magnetic Susceptibilityof 304 stainleas steel vary widely depending upon the exact composition and history of the sample. To obtain a reasonable estimate of the magnitude of xmfor the stainless steel used for the construction of our ICR cells, the magnetic susceptibility of a sample of raw cell material was measured by using the Gouy method (see Experimental Section); a magnetic susceptibility of 0.16 was obtained. This compares favorably with the lower end of literature values which range from 119 (indicating a ferromagnetic component) to 0.4 (18). Consequently, the conclusions drawn in this study could represent a lower limit of the effects observed. As a control, the susceptibility of the oxygen-free high conductivity copper stmk used in cell construction was also measured; a value of 0.00 was obtained. Mapping Inhomogeneities. Equations 1and 2 describe the magnetic induction on the interior of a substance placed in a magnetic field. These fields are directly related to the fringing fields of interest generated outside of the ICR cell plates. Calculations of magnetic fields outside of materials submersed in parent fields are not simple because the geometry of an object placed in the field plays a significant role in the perturbed form of the field. In most araes,the equations describing these stray fields cannot be solved explicitly. The Poiason/Superiiih codes allow the calculation of the topology of a magnetic system given values for the susceptibility of the material of choice, the geometry of the system of interest, and the parent field magnitude and direction. Figure 1contains the result of Poisson computations for a 304 stainless steel cell of the geometry employed for these partitioning studies. The value of xm used for the calculation was 0.16; the value measured by the Gouy method. The higheat intensity of magnetization is found wherever the length of the material along the parent field (Z)axis is larger than the width (Xor Y dimension) of the object. For example, the excitereceive plates in Figure 1show significantlyincreased magnetic induction, whereas the trap plates have increased induction only near the edges in the X-Y plane. An explanation for this increased induction (and, hence, axial inhomogeneity) near the trap plate apertures can be rationalized by considering the region of the plates immediately surrounding the apertures as thin cylinders with lengths greater than their widths. The inhomogeneity for the center aperture is somewhat more intense than those for the outer trap plates; a result of two effects. The field topology calculation takes into account magnetic homogeneity of the field due to the superconducting magnet. This field is somewhat weaker a t the outer trap plates than a t the center of the cell resulting in reduced aberration. Secondly, the field interacting with
ANALYTICAL CHEMISTRY, VOL. 61, NO. 22, NOVEMBER 15, 1989
the outer trap plates is less affected by the excite-receive plates than is the center trap plate. It is imperative to realize that the cell geometry synergistically contributes to local magnetic topologies. Local, axial inhomogeneities as those depicted in Figure 1 would have little effect on the single cell ET-ICR experiment because ion interaction with these regions would be minimal in this case. Such inhomogeneities are destructive, however, to experiments requiring ions to pass through the inhomogeneous regions near the trap plate apertures (e.g., an ion partition experiment). A Poisson plot of the two-section cell system with oxygen-free high conductivity copper (not shown) used for cell construction (x, = 0.00) demonstrates that the cell is effectively transparent with respect to the parent magnetic field giving rise to no significant magnetic inhomogeneities throughout the cell. Trajectory Calculations. SIMION was used to model ion trajectories during partitioning in a two-section cell with a 304 stainless steel conductance limit trap plate. This version of SIMION can be used to "paint" magnetic fields along any axis and with any reasonable magnitude. For the trajectory calculations, a homogeneous parent magnetic flux density of 3 T was defined throughout the entire cell along the Z axis. The magnetic field inhomogeneities in the region of the conductance limit aperture were estimated from Figure 1and added to the parent field. It is important to understand that there is little benefit in trying to rigorously mimic the axial magnetic inhomogeneities near the conductance limit aperture of the actual two-section cell used here for two reasons. First, the magnetic susceptibility of the stainless steel composing a given part can be known only approximately. This is due to the fact that machining of p& may lead to precipitation of magnetic phases within the material. Also, slight variances in alloy composition lead to large changes in x,. It is probably not pcasible to know the magnetic susceptibility of a given part (assuming that it is homogeneous in composition) to better than a factor of 3. Thus,two identical items machined from the same stock could have significantly different magnetic properties. Secondly, as mentioned above, the magnitude of fields generated outside of materials bathed in magnetic fields are strongly geometry dependent such that in describing one real case explicitly, the principles of interest would be obscured. Furthermore, varying operating parameters (trapping potential, for example) for a given system significantly changes the effect of magnetic inhomogeneities. In this paper, the thrust will be to represent the form of the inhomogeneity near the aperture with intensities that are considered to be reasonable. With SIMION, several specific examples of the interaction of ions with the magnetic inhomogeneity of the aperture region depicted in Figure 1 are considered below. These examples are based on ion partitioning in a two-section cell. In all cases considered, the cell consisted of two 3.8-cm3cells joined by a conductance limit. The conductance limit was 3 mm thick and had a 4 mm diameter squared-edge aperture. The symmetry of the system was considered to be cylindrical along the Z axis. The outer trap plates were set to a potential of 4 V while the conductance limit was maintained at ground. Four volts was chosen as the trapping potential because this is the maximum potential used for ion partitioning for twosection cell studies performed on this instrument. Ions placed near the Z axis of one cell travel toward the aperture where they encounter the magnetically inhomogeneous region defined above. SIMION provides views of the resulting trajectories in all three planes. The perturbation of the trajectories of the ionizing electrons will not be considered here. The momentum of the electrons piercing the defined magnetic inhomogeneities is about 1% of the lowest momentum ion considered below. Intuitively,
2531
Y
L
Figwe 2. Y-Zplane projection of the trajecbks of ten mlz 100 bns with E, varying from 0.19 to 3.48 eV as they partltlon through an axial inhomogeneity as described in the text.
Table I. Effects on Ion Partitioning Through a Stainless Steel Conductance Limit Trap Plate'
s,
ion
E,, eV
fate
trapped?
mm
1
3.485 2.763 2.125 1.593 1.171 0.848 0.604 0.478 0.290 0.190
XP b
yes no no no yes yes no yes no no
2 3 4 5 6 7 8 9 10
QP' QP QP XP
XP XPd XP RP QP
(E,)-'/z, 4,4, eV-'/Z
eV
ma
0.9
0.56
0.70
0.8
2.9 2.8 4.1 3.7
1.1 1.3 1.8 1.4
0.63 0.47 0.56
2.4 1.6 1.9 1.2
0
See text for a description of the parameters: S,, AEu, and ion center-of-motion shift upon successful partitioning. 'QP = ion quench during partitioning through aperture. dIon 7 successfully partitioned with a center-of-orbit shift, but collided with the conductance limit in < 60 MS. 'RQ = reflect then quench.
4. bXP =
it would seem that lower momentum would result in increased charged particle deflection and preliminary SIMION trajectories performed in this lab indicate this to be the case. As one referee has pointed out, such deflection of the electron beam could result in the formation of ions that are spatially disallowed from partitioning to the analyzer region of the two-section cell. Energy, Partitioning, and Inhomogeneity. In order to study the effect of Z-axis energy on low-energy ion partitioning, SIMON calculations were performed on ten m l z 100 ions initially placed uniformly across the 2 axis of the cell. The initial placement of ions in the gradient electric trapping field determines the ion's Z-axis energy on reaching the conductance limit (0.01eV in the X-Y plane). Ions beginning closer to the outer trap plate have up to 4 eV of Z-axis translational energy when they reach the conductance limit. Ions placed near the conductance limit have near zero translational energy in the Z axis when they reach the aperture. The ions were placed such that their cyclotron orbit guiding centers coincided with the Z axis. Figure 2 shows a trajectory plot of 10 ions ranging from 0.19 to 3.48 eV as they are allowed to undergo partitioning through the axial inhomogeneity defined above. Figure 2 qualitatively portrays the spatial and energetic distribution of ions arising from lowenergy interaction with a magnetic inhomogeneity. Table I quantitatively summarizes the conditions and fate of the 10 ions in this study. The trajectories were terminated at 60 ps if collision with an electrode had not occurred. This time limit represents the minimum period for an event on the Nicolet FTMS-1000 and, therefore, the shortest partitioning pulse. The plot corresponds to a projection of the trajectories at X = 0 (2 is the horizontal coordinate; Y is the vertical coordinate). It is apparent that the aperture inhomogeneity dra-
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ANALYTICAL CHEMISTRY, VOL. 61, NO. 22, NOVEMBER 15, 1989
D
C
--
Figure 3.
Enlarged view of the conductance limit trap plate aperture
during ion partitioning: (A) ion collision on partitioning (for example, ion 2 in Table I), (B) ion gukling center shift and gain of energy in the X - Y coordlnate on partitioning (for example, ion 1 in Table I), (C) the ion reflects off the axial inhomogeneity and returns to the first cell region (ion 9 in Table I).
matically affects ion partitioning trajectories. It is noteworthy that no deflection of the ion paths is observed for the same calculations performed on an oxygen free high conductivity copper cell (not shown). Three basic trajectories (or combination of these) are observed for the stainless steel cell: (i) (Figure 3A) the ion collides with the conductance limit during the first pass, (ii) (Figure 3B) the ion partitions but gains energy in the X-Y plane usually followed by collision with the conductance limit, or (iii) (Figure 3C)the ion undergoes reflection and subsequent collision with the conductancelimit. In the first case, ions can normally be trapped in the second cell region by proper timing of the conductance limit grounding pulse. Guiding center shifts occur for all ions entering the inhomogeneity within the aperture. Figure 4 shows the projection of the trajectories for the ions that partition in Figure 2 onto the X-Y plane. It should be clear from the figure that the axial inhomogeneity due to the stainlw steel conductance limit induces significant complexity on ion partitioning trajectories a t low translational energy. In addition, one observes that ions formed with little spatial distribution and equal energy in the cyclotron orbit (X-Y plane energy) are distributed spatially and can gain significant energy in the X-Y plane upon partitioning through the axial inhomogeneity. The mechanism for cyclotron orbit energy gain is deflection into the X-Y plane due to the X-Y component of the magnetic field in the conductancelimit aperture. These latter effects are summarized in Table I (A, is the center-of-orbit shift upon partitioning, AE, is the energy gained in the X-Y plane). From the f i i e s and Table I, one sees that only four of the ten ions partition such that the ions are trapped in the second cell region. Ion no. 7 partitions but collides with an electrode before 60 ws. These five ions have an appreciable energy spread (0.7 eV) with respect to the defined energy spread of zero before partitioning. On extrapolation of this result to the observable experiment, it appears that much of the ion signal would be lost on partitioning ions in the cell for this example. In addition, ions that are partitioned to the second cell region are not as detectable as before partitioning due to the large resultant energy and spatial distributions of the ions (8). The dependence of ion trajectories on Z-axis energy can be determined upon examination of the trajectories of the five ions that partition to the second cell region in the 10-ion study above (Figures 3 and 5 and Table I). Figure 5 shows pro-
i-tE,*
5eV
Figure 4. X - Y plane projection of the ions that partitlon In the Ion energy study. Ions a-e correspond to ions 1, and 5-8 in Table I, respectively. _ = --J AIc--'1
--q?&3ggEE45k---L
-
--JZz?-k
- _ I
f -
-
Figure 5. SIMIONplot
of the Y - 2 plane pro/ectlon of an ion injected into an ICR cell. The ion beglns with 8 e V in the Zdirection, the left trap plate is maintained at 5.0 V, and the right trap plate Is at 5.4 V. The ion reflects off magnetic inhomogeneities in the trap plate apertures. jections of the ion trajectories onto the X-Y plane for these five ions as well as the physical definitions of some of the parameters in Table I (S,, AEu, and AJ. Physically, S, is a measure of how well the ion cyclotron guiding center follows a given flux line in the axially inhomogeneous region. S, is defined as the path length of the X-Y plane projection of the trajectory from the cessation of stable cyclotron oscillation in the source region to the resumption of a stable orbit in the analyzer region. Upon examination, one observes that the ion orbit guiding center generally follows a magnetic flux line on entering and leaving the inhomogeneity but tends to cross field lines in the central region of the inhomogeneitywhere the field lines are parallel to the parent field. Therefore, S, is a measure of the perpendicular component of ion motion with respect to the portion of the magnetic inhomogeneity that is parallel to the parent field (see Figure 1). In two cases discussed below, the ion guiding center crosses field lines in the entry region of the inhomogeneity (where the flux lines are approximately oriented 45O off the Z axis) and the projection of the ion trajectory into the X - 2 plane must be considered. AE, is the energy transferred from the Z-axis component to the X-Y plane during partitioning through the aperture
ANALYTICAL CHEMISTRY, VOL. 61, NO. 22, NOVEMBER 15, 1989
Table 11. Effects of Mass on Ion Partitioning through a Stainless Steel Conductance Limit Trap Plate” mass
fate
trapped?
S, mm
M,, 4, eV mm
100 200 300 400 500 600 700 800 900 1000
XP QP QP QP XP XP XP XP XP QP
yes
3.3*
0.63
1.6
0.15
1.6 2.3 1.7 1.7 1.2 1.6b
0.32 0.46 0.21 0.34 0.05 0.05
1.8 2.1 1.9 1.8 1.4 0.2
0.058 0.056 0.041 0.046 0.039 0.037
(ME,)-’/* (amu.eV)-’l2
no no no
yes yes yes yes yes yes Legend is as for Table I. see text.
Adjusted for X - 2 plane component,
axial inhomogeneity. In addition, the arithmetic mean of the z-axis energy on entering and leaving the axial inhomogeneity, E,, is defined by eq 3. A, is the shift of ion cyclotron guiding (3) centers in the x-y plane on partitioning from the source region to the analyzer region. Two trends are observed for the partitioning of five m/z 100 ions through the inhomogeneity: (i) the x-y plane projection of the path through the inhomogeneous region becomes less linear with increasing energy, and (ii) S, correlates well with (E,)-l/z. If the magnetically inhomogeneous region is considered to be a crude magnetic analyzer and E, to be a reasonable estimate of the actual mean z-axis energy within the inhomogeneity, then S, can be expressed as a function of the reciprocal of ion momentum. Because mass is constant in this study, S, can be expressed as a function of (,??,)-l/z. A plot of S, versus (&)-l/z is reasonably linear ( r = 0.980), and the first-order regression of the plot very nearly passes through 0,O ( b = -0.12 mm) as would be expected. A higher order of correlation between S, and Z-axis energy could only be approached through more rigorous treatment of the form of the inhomogeneous magnetic field within the aperture and the magnitude of E, with respect to position in the aperture region. Mass, Partitioning, and Inhomogeneity. In a study precisely analogous to the energy study above, the mass dependence on ion partitioning was examined. Ions of m/z 100-1000 were “partitioned”through the paramagnetic conductance limit using SIMON calculations with the same array definitions as before. All ions were placed 21 mm from the outer trap so that the energy upon reaching the magnetically inhomogeneous aperture region would be 0.758 eV. Ions were given 0.01 eV of energy in the X-Y plane and placed such that the guiding center of each ion’s orbit before partitioning coincided with the Z axis (X = Y = 0) of the cell. The results of this study are summarized in Table 11. As mentioned earlier, S, represents the component of the ion’s path (while within the inhomogeneous region) that is perpendicular to the local magnetic field. Analysis of the trajectories of the m / z 100 and the m/z 1000 ions in this study indicate that a large fraction of this perpendicular component is in the X-Z plane. In this case, the X-Y plane projection of an ion’s trajectory gives poor S, values. Table I1 reflects adjusted values for these ions which consider the X-Z plane components. Again, two trends emerge: (i) trajectories are more linear for higher momentum ions, and (ii) S, correlates with momentum. For this study, S, varies linearly with respect to (ME,)-1/2,where M is the ion’s mass in amu. E, must be included in this correlation because it is related to both the ion’s initial and final energy. A plot of S, versus (ME,)-1/2is roughly linear ( r = 0.915) with a y intercept of 0.60 mm.
2533
Magnetic Trapping. Magnetic inhomogeneities in the entry aperture of a cell can give rise to ion trapping effects for ions injected from external sources. This trapping arises from the ability of magnetic inhomogeneities to redirect the energy component distribution along other axes. For example, Figure 5 is the Y-Z plane projection of an ion being injected into a rectangular ICR cell at 8.0 eV along the Z axis and 0.01 eV in the X-Y plane. The left trap plate is biased to 5.0 V while the right plate is held at a potential of 5.4 V. A magnetic field of 3.0 T was placed over the entire potential array oriented along the Z axis (horizontal). The inhomogeneityused for the two-section cell studies above was doubled in magnitude and placed in both apertures. The form of the inhomogeneity was not altered. It is important to note that doubling the inhomogeneity intensity still results in a very weak aberration with respect to that possible for 304 stainless steel (see Magnetic Materials, above). The ion enters the cell at about 3.0 eV along the Z axis and approximately 2.6 eV of this energy is transferred into the X-Y plane resulting in a significant gain in cyclotron orbit radius. The ion guiding center is also shifted as the ion is deflected into the X-Y plane. The ion falls into the cell trapping well accelerating to roughly the cell’s center (as evidenced by the projection of an expanding helix). As the ion approaches the right trap plate, it decelerates along the Z axis, encounters the inhomogeneity in the aperture region, and is reflected back along the Z direction. Upon reflection, the ion guiding center shifts in the X-Y plane and the ion loses approximately 1eV of energy from the X-Y plane into Z-axis motion. It is obvious in this example that the ion is not actually trapped in the cell and would most likely collide with the left trap plate, but it remains equally obvious that the inhomogeneities of this cell would assist in ion trapping. Stronger inhomogeneities or geometrically optimized inhomogeneities would result in efficient trapping. CONCLUSIONS Several conclusions emerge on considering the partitioning of ions in a two-section FT-ICR cell with an axial magnetic inhomogeneity in the region of the conductancelimit aperture. First, ion deflection in the X-Y plane (measured as S, or A,) increases with increasing ion momentum. A, only roughly correlates with momentum and is highly path sensitive. AE, roughly decreases with increasing ion momentum and is also highly path sensitive. Whether energy was varied with a set mlz ratio or mlz was varied for a given injection energy, it is apparent that significant energy gain and ion deflection results under the low-momentum conditions defined above. The resulting ion packets trapped after such ill-defined injection will have poor spatial (5-10 mm diameter) and significant energy (0.7eV) distributions. The implications of this work are far-reaching throughout FT-ICR. In the two-section cell experiment, one must choose to optimize geometry or use low susceptibility materials in order to build cells that perform well under the conditions of the experiment. Optimizing geometry largely centers around the use of plates that are thin in the 2 direction. This increases the demagnetizing effect which causes the induced magnetization, I, to approach the field due to the parent magnetic force, h H . This group has opted to use oxygen-free high conductivity copper for cell construction. The twusection cell in use here has worked well with good partitioning efficiency for approximately 2 years. The implications of ion dispersion due to axial magnetic inhomogeneities are important for external ion source FT-ICR. For the same number of ions, the detectable signal is degraded as the spatial and energetic definition of an ion packet becomes less defined. This effect could be particularly dramatic for cells using fine grids of high magnetic permeability materials
Anal. Chem. lWQ,61, 2534-2540
2534
where the wire thickness approaches the mesh size. In this case, ions that partition must be intimate with axial inhomogeneities. This effect is offset somewhat by the fact that the grids are normally thin along the 2 axis. However, if the wire's thickness along the 2 and X (or Y) axes is comparable, local inhomogeneities could be quite strong. Finally, the performance degradation of stainless steel single cell instruments (19) could largely be due to the effects discussed above. The signal would degrade due to the cell's decreasing ability to trap ions properly (small energetic and spatial distribution), as the permeability of the material increases with time. LITERATURE CITED Sack, T. M.; Gross, M. L. Roc. Annu. Conf. Mass Specfrom. A M Top., 31st 1983,396. Carlln, T. J.; Frelser, B. S. Anal. Chem. 1983,55, 571-574. Kofel, P.; Alleman, M.; Kdkma4, Hp.; Wanczek, K . P . Int. J . Mass Sp@Ct?Om.IOn @ 1985, 65,97-103. McIver, R. T.; Hunter, R. L.; Bowers, W. D. Int. J . Mass specbwn. IOn RVCe1985,64,67-77. Atford, J. M.; Wllliems, P. E.; Trevor, D. J.; Smalley, R. E. Znt. J . Mess Spectrom. Ion &xe.?ses 1986, 72,33-51. HaWn, C.D.; Castro, M. E.; RusseH, D. H.; Hunt, D. F.; Shabanowltz, J. ACS Symp. Ser 1987,No. 359, 100-115.
(7) OhederI, S.; LiUejoh, D. Roc. Annu. Conf. Mass Spectrom. A M ~ o .Ip33rd tew. 727-728. (8) . . Henson. C . D.: Kerlev, E. L.: Russell, D. H. TmW Anal. Ghem. (2nd Ed.) 1989. 1 1 , 1171187. (9) Elklnd, J. L.; AJford, J. M.; Welss, F. D.; Laaksonen. R. T.; Smalley, R. E. J . Chem. P h 9 . 1987, 15,2397-2399. (IO)Lauklen, F. H. Znt. J . Mass Spectrom. Ion RVCesses 1986, 73, 81-107. (11)Glancaspro, C.;Verdun, F. R.; Muller, J.-F. Int. J . Mass Spectfom. IOn procesSeS 1988,72,63-71. (12)Verdun, F. R.; QlenWprO, C. A M I . Chem. 1986, 56, 2097-2099. (13)Honovich, J. P.; Markey, S. P.; Wang, T.C. L.; Shih, M.C. PTOC. ASMS Conf. h4aSS spS&O" A M Top., 35ih 1987; 1112-1113. (14)K d y , E. L.; Russell, D. H. Anal. m.1989,67. 53-57. (15)comlsarow, M. 8. J . C b m . Phys. 1978,60, 4097-4104. Php. 1987,2,149-172. (16) Wbrsbw, A. M. J . (17)Selwood, P. W. Magnetochmistry;Interscience: New York, 1943 p 2. (18)Bozorth, R. M. Femwnegnetlpm; D. Van Nostrand, Inc.;New York, 1951,Chapters 1, 2,and 5. (19)Dunbar, R. C.,personal communication.
m.
RECEIVED for review May 9,1989. Accepted August 4,1989. This work was supported by the National Science Foundation (CHE-8821780). We gratefully acknowledge the Texas Agricultural Experiment Station for providing a portion of the funds for purchase of the Nicolet FTMS-1000 mass spectrometer.
Measurement of Linear Alkylbenzenesulfonates in Aqueous Environmental Matrices by Liquid Chromatography with Fluorescence Detection Mark A. Castles,' Billy L. Moore,* and Susan R. Ward
The Procter and Gamble Company, Ivorydale Technical Center, Cincinnati, Ohio 4521 7
A hbh-performance IlquM chromatography method for the determlrurtlon of Unear atkytbenzenerulfonate (LAS) In complex envlrmental matrkm ha8 been developed. The m a thod takes advantage of the Inherent sendtlvlty of fluorescence detectbn, as c0mpru.d to that OMahred with conventlonal UV detecth or the popular derulfonatbn-gas chro" p h y method for cktamwng LAS in compkx matrlcc#r. The chromatography has boen &&!ped to comblne a l d the isomers of each homokgue into dngle peaks repreebn#ngthe respec#veC,,-C,, LAS condltu8nts. This serves to enhance the detection levels, as well as the overall accuracy and prechlon of the measurement. Sdld-phase extraction procedures have been employed to isdate the LAS from such and matrices as rewage treatment plant hrlluent, final e"t, the "qwnt recelvlng river water. The avofago recovery of LAS from in#utmband ef"b and rlver waters was found to be 95 %, 85 %, and 81 % , respectively. The precision of the method, In terms of the relative standard deviation for influent, effk.nt, and rhrer water was determined to be 1.4%, 4.6%, and 13.5%, respecthrely. The lknn d detecth ( S I N 2 3) was round to be 1.5 ng per compornnt injected onto the column or 2 ppb of total LAS concentrated from a 200mL sample volume. Therefore, the limit of qwntitation ( S I N 1 found to be 7-10 ppb. A dkcurdon of the aMIlyHCa1 IO) method and the rerun0 from the anatyois of varkw samples is presented.
* Author to whom correspondence should be addressed. Current address: Degussa Corp., Mobile, AL.
0003-2700/89/0361-2534$01.50/0
Linear alkylbenzenesulfonates (LAS), shown in Figure 1, are water-soluble materials used extensively in detergent formulations as surface-activeagents. Ionic surfactants are unique in the sense they possess both hydrophobic and hydrophilic characteristics, due to the ionic and long chain alkyl portions of the molecule. This property promotes considerable concentrations of the surfactant at interfaces, thus dissolving hydrophobic materials into the aqueous phase. These properties account for the widespread use of LAS as an active cleaning agent in detergent formulations. Commercial LAS materials consist of a mixture of homologues and isomers. Individual LAS homologues and isomers are identified by the length of the hydrophobic alkyl substituent and by the position of attachment of the alkyl chain to the benzene ring. The predominant formulation consists of alkyl chain lengths from Clo to Cia, with the average distribution being approximately Clz. Isomer configurations range from the 2-phenyl position to the 7-phenyl position, depending on alkyl chain length. Due to the nature of anionic surfactant product usage, the majority of LAS is disposed to sewage treatment facilities. LAS is highly removed (e.g. 1 9 5 % for activated sludge processes) during the sewage treatment process as result of biodegradation processes (I) and adsorption to solids (2). Due to the high removability of LAS, only very small quantities (Le. low parts per billion levels) are discharged from wastewater treatment plants into the receiving waters. Analytical methodology to quantitate low parts per billion levels in environmental matrices is required to validate mathematical models designed to predict environmental concentrations of @ 1989 American Chsmlcal Society
