Anal. Chem. 1994,66,92-98
Dynamic Range Extension in Glow Discharge Quadrupole Ion Trap Mass Spectrometry Douglas C. Duckworth,' Christopher M. Barshick, David H. Smith, and Scott A. McLuckey Analytical Chemistry Division, Oak RMge National Laboratory, Oak Ridge, Tennessee 3783 1-6375
Methods of extending the dynamic range of the quadrupoleion trap have been investigated by utilizing ions generated from a radio frequency powered glow dicharge. The success of the glow discharge quadrupole ion trap technique for inorganic analysisis largely dependentupon increasing the dynamic range. The dynamic range has been increased to lo5 by selective accumulation methods which isolate ions over a small mass region of interest. To accumulate selectively ions of interest, static and dynamic electric fields were applied so that, as all ions were gated into the trap, only the ions of interest were retained. Selective accumulation methods employed include mass-selectiveinstability, singlefrequency resonanceejection, combination rf-dc, and entrance end cap dc. These methods, capable of being used alone or in combmation, were evaluated with respect to ion-trapping efficiency and linearity of signal response with respect to the injection period. Linear injection curves were obtained, and the ability to couple sequential selective ion accumulation methods into a multiple ramp scan function for multielement quantification was demonstrated. Glow discharge mass spectrometry (GDMS) is a bulksolids elemental analytical technique most frequently employed for high-purity analysis of metals and semiconductor materials. The success of the technique is due to its multielement capability ( 70 elements), reasonably uniform relative elemental sensitivities, high absolute sensitivity,uncomplicated spectra relative to optical spectroscopy, and ease of sample preparation. Simple sample preparation results in fewer potential sources of contamination and represents a significant savings in sample preparation time in many instances. For nonconducting materials, samples need only be mixed with a conducting material and pressed into an appropriately shaped electrode, or they may be pressed directly and analyzed employing a radio frequency (rf) powered glow discharge.' Traditionally, GDMS has been performed using linear quadrupole and sector-based instruments, each having distinct advantages. Quadrupole mass filters are appealing mass analyzers for GDMS, given their relative low cost and simplicity. In the absence of isobaric interferences, majorto-high-ppb analyses are possible. Sector-based instruments offer the advantage of resolving many common isobaric interferences with mass resolution of several thousands and major-to-sub-ppb elemental determinations, all within a single analysis. The success of both mass analyzers is evidenced by the commercial availability of both sector and quadrupole mass analyzers. The utility of quadrupole ion traps has been extended through the development of methods for the injection of N
(1) Duckworth, D. C.; Marcus, R . K. Anal. Chem. 1989, 61, 1789.
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externallygenerated ions. For example, Louris and co-workers used injection lensesto sample ions from an electron ionization source? and Pedder et al. have utilized a dc quadrupole ion deflector to effect off-axis injection of chemical and electron ionization generated ions.3 Secondary ions resulting from cesium ion bombardment of solid and liquid matrices have been injected into, and analyzed by, a quadrupole ion trap for biomolecule analysis? Recently, matrix-assisted laser desorption has also been interfaced with the ion We are presently developing the quadrupole ion trap mass spectrometer as an analyzer for the analysis of glow discharge generated ions. The ability to inject externally generated ions into an ion trap has been demonstrated with an rf-powered glow discharge sourcelo and is based on prior success with other ion sources, including atmospheric sampling glow discharge ionization,' atmospheric pressure ionization,12 electrospray,13J4 and ion spray.l5 Several features of the quadrupole ion trap make it attractive as an analzer for GDMS. Interfacing glow discharge sources with the quadrupole ion trap is facilitated by the high operating pressure of the trap and the nearly monoenergetic, low-energy ions formed in the glow discharge. Prior investigations have shown that the number of polyatomic interferences in the quadrupole ion trap is minimized due to spontaneous dissociationof weakly bound speciesand through ion-molecule reactions in the trap.g Metal argides, and to a lesser extent argon dimers, were found to dissociate in the trap under normal storage conditions in the presence of a helium bath gas. No metal argides have been observed to date, even with the shortest injection and trapping periods. Also, ions related to the argon support gas such as ArH+, Ar+, (2) Louris, J . N.; Amy, J. W.; Ridley, T. Y.; Cooks, R . G. Int. J. Mass Spectrom. Ion Processes 1989, 88, 97. (3) Pedder, R . E.; Yost, R. A.; Weber-Grabau, M. Proceedingsofthe 37th ASMS Conference on Mass Spectrometry and Allied Topics, Miami Beach, FL,May 1989; p 468. (4) Kaiser, R E., Jr.; Louris, J. N.;Amy, J. W.; Cooks, R . G. Rapid Commun. Mass Spectrom. 1989, 3, 225. (5) Doroshenko, V. M.; Cornish, T. J.; Cotter, R . J. Rapid Commun. Mass Spectrom. 1992, 6, 753. (6) Cox,K.A.;Williams,J . D.;Cooks,R.G.;Kaiiser,R. E.,Jr. Biol. MassSpectrom. 1992, 21, 226. (7) Schwartz, J. C.; Bier, M. E. Rapid Commun. Mass Spectrom. 1993, 7 , 27. (8) Jonscher,K.J.;Currie,G.;McMarmackA. L.;Yates, J.R.,IIIRapidCommun. Mass Spectrom. 1993, 7 , 20. (9) Chambers, D. M.; Goeringer, D. E.; McLuckey, S. A.; Glish, G. L. Anal. Chem. 1993,65, 14. (IO) McLuckey, S. A.; Glisb, G. L.; Duckworth, D.C.; Marcus, R. K. Anal. Chem. 1992. 64, 1606. (1 1) McLuckey, S. A.; Glish, G. L.; Asano, K. G. Anal. Chim. Acto 1989,225,ZS. (12) McLuckey, S. A.; Glish, G. L.; Asano, K. G., unpublished results. (13) Jardine,I.; Hail, M.; Lewis, S.; Zbou, J.; Schwartz, J . Proceedings ofrhr 38th ASMS Conference on Mass Spectrometry and Allied Topics, Tucson,AZ, June 1990; p 16. (14) Van Berkel, G. J.;Glish,G. L.; McLuckey, S. A. Anal. Chem. 1990,62,1284. (15) McLuckey, S. A.; Van Berkel, G. J.; Gliah, G. L.; Huang, E. C.; Henion, J. D. Anal. Chem. 1991, 63, 315. 00032700/94/036&0092$04.50/0
0 1993 Amerlcan Chemical Society
and Ar2+ were depleted through electron or proton transfer with residual gas-phase species in the trap. Residual gas species were primarily of low mass and did not contribute to the spectra because they fell outside the stability region. More strongly bound species, such as metal oxides and hydroxides, can be dissociated through collisional activation in the ion trap. Many such species pose problems as isobaric interferences in even sector-based, high-resolution mass spectrometry (MS). The quadrupole ion trap provides an attractive alternative, as even the strongest bound polyatomic species were dissociated. MS/MS dissociation efficiencies for such species, including BaOH+ (-5.5-eV dissociation energy), approached 100%. Because the absorption of energy by the primary ion is a resonance phenomenon, and only a selectedpolyatomic ion is excited, loss of nonresonantly excited ions via scattering is negligible in comparison to beam-type multiple quadrupole MS/MS experiments.I6J7 This is because nonexcited analyte ions are not subjected to energetic collisions with the He buffer gas; consequently the dissociation of polyatomic ions can be effected without losses of ions of different m/e values due to scattering. Despite the benefits of the quadrupole ion trap, limited dynamic range has been a fundamental limitation in its use for quantification. While absolute storage capacity of the ion trap depends on its dimensions and operating parameters, it is estimated to be approximately one million ions for commercial devices.’S In the absence of mass-selective ion accumulation, direct injection of glow discharge generated ions results in the trap being predominantly filled with discharge gas species. Argon- and residual gas-related species can comprise more than 75% of the total ion beam current, effectively decreasing the storage capacity for analyte ions. Likewise, major components can affect the accumulation of minor components. With an effective analyte ion storage capacity of lo5ions, mixture components with concentrations less than 10 ppm cannot contribute significantly to the spectrum without discrimination against more abundant species. Experimentally, limits of detection are much worse than theoretically predicted due to noise and low trapping efficiencies. Means of selective ion accumulation are thus required to improve the dynamic range of the quadrupole ion trap. Dynamic range improvement in an explosives detection application using single-frequency resonance ejection has already been demonstrated. l9 Barinaga and Koppenaal have used a linear quadrupole for discrimination and preselection of species formed in inductively coupled plasma sources.ZO Recently, sophisticated resonance excitation techniques based on broad-band waveforms have been employed for massselective ion accumulation.z1-z3 The methods applied in this (16) Duckworth, D. C.; Marcus, R. K. Appl. Spectrosc. 1990.14, 649. (17) King, F. L.; Harrison, W. W. Int. Muss Spectrom. Ion Processes 1989, 89, 171. (18) March, R. E.; Hughes, R. J. Quadrupole Storage Mass Spectrometry; John Wiley and Sons: New York, 1989. ( 19) McLuckey, S. A.; Goeringer, D. E.; Glish, G. L. J . Am. Soc. Mass Spectrom. 1991, 2, 11. (20) Barinaga, C. J.; Koppenaal, D. W. Proceedings of the l l s r ASMS Conference on Muss Spectrometry and Allied Topics, San Francisco, CA, May 30-June 4, 1993. (21) Julian, R. K., Jr.; Cooks, R. G. A d . Chem. 1993, 65, 1827. (22) Kelly, P. E. US.Patent No. 5,134,286, 1992. (23) Shaffer, E. A.; Karnicky, J.: Buttrill, S. E., Jr. Proceedings of the 4lst ASMS Conference on Mass Spectrometry and Allied Topics, San Francisco, CA, May 30-June 4, 1993.
work can all be effected with standard capabilities of the commercially available ion trap mass spectrometer. The selectiveaccumulation techniques employed here arise, in part, from ion isolation methods employed in quadrupole ion trap based MS/MS experiments, where all ions are accumulated and the mleof interest is isolated. These techniques, including mass-selectiveinstability,18combination rf-dc,l8 and resonance ejection,4J5J9J4 have been described in detail elsewhere. In addition, the use of entrance end cap dc potentials was evaluated as a novel means of high mass-to-charge discrimination. These techniques were characterized in terms of linearity and relative trapping efficiencies with respect to injection time. Linearity is required for the use of relative injection periods as a means of quantification. Finally, linked scan functions were used to generate a multiple-ramp scan function for multielement analyses.
EXPERIMENTAL SECTION The rf glow discharge ion trap mass spectrometer system was described in detail previously.1° An rf flow discharge source, constructed in-house, and a Finnigan ion trap mass spectrometer (ITMS) (Finnigan Corp., San Jose, Ca) were used. Ions extracted through the exit aperture of the glow discharge source were focused through a three-element lens system. The second element consisted of two half-plates which served as a beam deflector to gate ions into the trap. A He buffer gas pressure of 1 mTorr was found to be optimum for trapping injected ions. The rf glow discharge source, which has also been described in detail previou~ly,~~ consisted of a direct insertion probe through which a 13.56-MHz excitation potential is directly coupled. The probe was inserted through a l/2-in. Cajon fitting into a 2 3/4-in. Conflat flange-mounted four-way cross, which served as the counter electrode. The discharge was sustained in the cross region with an argon support gas (-700 mTorr) and was operated at a power of 50 W, neglecting small power losses in cables and couplings. NIST SRM 1103 Free Cutting Brass served as the sample throughout this work. RESULTS AND DISCUSSION The description of the mass-selective ion accumulation methods used here is facilitated by referring to the stability diagramassociated with the ion trap (Figure 1). The stability diagram is a plot of the dimensionless parameters a, versus qz, where a, = -8eU/mr,2Q2
qz = -4eV/mrtQ2
and where e is the charge on the ion, U is the applied dc voltage, m is the mass of the ion, ro is the radial trap dimension, fl is the angular frequency of the rf, and Vis the applied rf voltage. The stability diagram relates the operating parameters listed above, of which ro and Q remained constant in the present work. For an ion of a given mle, therefore, the working point (24) Kaiscr,R. E., Jr.;Cooks,R. G.;Stafford,G.C.,Jr.;Syka, J. E. P.;Hemberger, P. H. Int. J. Muss Spectrom. Ion Processes 1991, 106, 79. (25) Duckworth, D. C.; Marcus, R. K. J. AMI. At. Spectrosc. 1992, 7, 71 1.
Analytical Chemlshy, Vol. 66,No. 1, January 1, 1994
95
0.4
,
'
.
.
I
z Stability 0.2 0 .o
mu -0.2 -0.4 -0.6 -0.8
0.0
0.5
1 .o
1.5
mure 1. Stability diagram for the quadrupole ion trap. Lerger filled circles, posltioned along the a2 = 0 line, represent ions of larger mle values (smaller q2 values).
on the diagram is determined by V,the amplitude of the ring electrode rf signal, and U,the dc field. The latter is usually imposed by adding a dc signal to the ring electrode. The fl, and & lines on the diagrams indicate constant-frequency conditions in the relevant dimension (radial, r, or axial, z). In the most common mode of operation, the end caps of the trap are both grounded, and the dc potential applied to the ring electrode is 0 V, thereby yielding a value of 0 for a,. When the amplitude of V is increased, ions of sequentially higher mass, as indicated in Figure 1 by the filled circles on the (I, = 0 line, are axially ejected from the trap at the edge of the stability diagram, whereb, = 1 (4, = 0.908). The boundaries of the stability diagram can be further utilized to discriminate against certain ions through the manipulation of voltages, both rf and dc, on the ring and end cap electrodes. To illustrate the need for selective ion accumulation, an ion trap mass spectrum of NIST SRM 1103 Free Cutting Brass is shown in Figure 2a, where no selective ion accumulation was employed. Operating along the uz = 0 line of the ion stability diagram (Figure 1) and with an rf trapping potential corresponding to qcutoffof m/e 15, this spectrum represents the worst case, where all ions are accumulated simultaneously over the 0.5-ms injection period and without preferential accumulation of the analyte ions of interest. The presence of argon and residual gas ions limits the detectability of analyte species as the trap fills. As seen in Figure 2a, argon and numerous residual gas contaminants dominate the spectrum, while the contribution from matrix elements is just visible above noise. Longer injection periods were found to result in severe space charge, which limited resolution and further accumulationof analyte species. The following sections describe and illustrate the use of common isolated techniques-mass-selective instability,resonance ejection,and combination rf-dc-employed during injection to reduce space-charge effects, allowing longer accumulations of less abundant ions. Additionally, a technique utilizing a dc potential on an end cap electrode was developedto discriminate against high-mass species. Mass-SelectiveInstability. Mass-Selective instabilityz6is one of the more common techniques employed in ion trap (26) Stafford, G. C.. Jr.; Kelly, P. E.; Syka, J. E. P.; Reynolds, W. E.; Todd,J. F. Int. J . Mass Spcctrom. Ion Processes 1W,60, 85.
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l Pb+
(3.73%) 25
50
75
100
126
150
175
200
mlz Flgwe 2. Radio frequency powered glow discharge mess spectra of NIST SRM 1103 Free CuWng Brass demonstratlng sebcttve ion accumulation uslng mass-selecthre Instablflty: (a) lowmas8 cutoff, 15 amu; (b) lowmess cutoff, 45 amu.
mass spectrometry and is used to eject ions from the trap sequentially. Mass-selective instability is accomplished by increasing the rf amplitude on the ring electrode, thereby forcing ions through the,!?z= 1 (qz = 0.908) stability boundary in the order of increasing mass-to-charge ratio. The use of this technique for selectiveion accumulation is shown in Figure 2b. An rf trapping potential corresponding to a low-mass cutoff of 45 amu was used to prevent the accumulation of many unwanted species such as HzO+, HsO+,HsO', Ar+, and ArH+ ions. Copper, zinc, and lead ions (59.27, 35.72, and 3.73% elemental, respectively) are now observed, employing the same 0.5-ms injectionperiod used for the spectrum in Figure 2a. Polyatomic interferences,arising from reactions between zinc and water vapor in the ion source, are clearly visible. In comparison to Figure 2a, the trapping efficiency, defined as the relative number of analyte ions accumulated and detected for a constant injection period and ion population, has improved. For the 0.5-ms injection period employed, the signal-to-noise ratio has been enhanced by a factor of 25, allowing the use of longer injection times. Resonance Ejection. For the remaining methodsof selective ion accumulation to be described, we use as our test case the accumulation of tin ions, present in NIST 1103brass at 0.88% elemental. To improve trapping efficiency for Sn+ it is necessary to discriminate against discharge gas related ions, matrix-related ions (Cu+, Zn+), and, at a higher mass than Sn+, lead ions (3.73% elemental).
loo
1 Pb+ Pb+
PbOH+
Sn+
120
140
180
180
do
200
do Id0
12b
140
mh
m/Z
'OQ
1
la
J T
lia
2w
220
I
Sn+
,v+ SnOH+
,
, 120
140
160
180
200
60
mlZ
Flguro 5. Radlo frequency powered glow discharge mass spectra demonstrating the selective accumulation of Sn+ In NIST SRM 1103 FreeCuttlngBrassusingresonanceejedon: (a)noresonancee]ectlon; (b) resonance ejectron of W+.
80
1w
120
140
mh
180
180
200
do
Figtwo 4. Radlo frequency powered glow dkcharge mass spectra demonstrating the selec#ve accumulation of Sn+ In NIST SRM 1103 Free Cutting Brass uslng a combination rf-dc notch flker: (a) no V, on ring electrode; (b) -21 V, on rlng electrode.
major Pb isotopes, instead of multiple discrete or broad-band Resonance ejection is a common technique which has been employed for mass range extension4 and ion i s o l a t i ~ n . ' ~ * ~ ~excitation potentials. With high voltages on the excitation waveform, masses adjacent to resonant ions can be ejected Resonance ejection is effected when the frequency of an ac with some success. voltage applied between the end cap electrodes becomes Tin hydroxide, generated by the reaction of the metal oxide resonant with the natural, or secular, frequency of an ion's with water vapor in the ion trap, was also observed (Figure motion in the trap. When the amplitude of the ac potential 3b). It was noted previously that metal oxides react exois great enough, the resonant ion is axially ejected from the thermically with water to form the hydroxide.lO Given the trap. In this experiment, we have utilized mass-selective long residence time in the trap (-0.5 s), the oxide is efficiently instabilitytoreject all ions present below m / e 67,and resonance converted to the hydroxide. It was noted previously that the ejection of lead ions at m / e 206-208, while injecting ions pressure of water vapor in the ion source resulted in the extracted from the glow discharge. Argon dimers, normally formation of metal oxides, which were observed to be of greater abundant in glow discharges, were not a concern, as none abundance relative to the hydroxidewith the linear quadrupole were observed even with the shortest ion accumulation and mass analyzer.1° Conversely, the metal hydroxides were storage periods. Argon dimers do not contribute appreciably preferentially observed in the ion trap as a result of gas-phase to the total ion signal as they are readily lost through ion-molecule reactions. dissociation and charge transfer. Combination rf-dc. Combination rf-dc is the operating Parts a and b of Figure 3 illustrate the use of resonance mode most akin to that of a linear quadrupole in that only ions ejection to effect the ejection of lead ions. In Figure 3a, no with combinations of a, and q, at the apex of the stability resonance ejection was applied. Ions were injected for 430 diagram are trapped. This is accomplished by applying a dc ms, resulting in space-charge broadening of the peaks. When potential to the ring electrode that results in an a,value greater resonance ejection was employed (Figure 3b), the spectral than 0 (see Figure 1). By operating at constant u, > 0, this contribution from lead was almost completely removed, and technique serves as a notch filter, where the width is inversely the signal-to-noise ratio was improved by a factor of 8. Space related to the value of uz, and the centroid of the mass range charge was significantly reduced, resulting in unit resolution accumulated is directly related to qz. and the ability to accumulate ions for longer periods. The Application of combination rf-dc is illustrated for our test Pb+ ion signal remaining results fromusing a single-frequency, case in Figure 4. Several species can be observed in Figure high-voltage (6 VFp) excitation potential, centered on the Analytical Chemistty, Vol. 66,No. 1, Jnuary 1, 1994
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4a, including Sn+,Pb+, and PbOH+. Some contribution from a8Zn+is observed because the low-mass selective instability cutoff was set at m / e 60. Application of -21 v d c to the ring electrode resulted in the selective accumulation of ions in the mass range of -75-180 amu. Ejection efficiency for lead ions was nearly 100%. Mass resolution and signal-to-noise ratios were both improved; the latter was improved by a factor of 7 for the same accumulation period (450 ms). In the absence of space charge effects arising from the presence of Pb+, longer accumulation periods could be employed for increased sensitivity. A limitation in this approach is the loss of trapping efficiency when one is operating close to the apex of the stability diagram. As a, increases, the trapping well depth decreases, resulting in a reduced effective trapping volume. Over a mass filtering range of 80-1 00 amu, trapping efficiencies for the ions of interest were not found to be adversely affected relative to rf-only operation. End Cap Vae. The application of a dc potential to one of the end caps of the ion trap results in a low-passmass filter,27*28 effectively repositioning the & = 0 line (see Figure 1) to PZ > 0. As heavier ions have lower qzvalues, they no longer have stable trajectories in the trap after injection and are ejected. Parts a-c of Figure 5 illustrate the effect of applying 0.0, -3.2, and - 4 . 3 Vdc, respectively, to the entrance end cap electrode. Mass-selective instability (rf level, m / e 65) was employed in each experiment. With no dc voltage applied (Figure 5a), space charge effects (note the poor resolution) and a reduction in trapping efficiency were observed. With -3.2 v d c applied to the end cap (Figure 5b), lead ions were not trapped; no Pb+ was observed in the mass spectrum, while SnOH+ was observed. The ability to reduce the contribution of this polyatomic ion from the trapped ion population is illustrated in Figure 5c. The isotope of lowest mass for SnOH+ is separated from that of greatest mass for Sn+ by only 9 amu, yet SnOH+ was completely ejected from the trap during accumulation. This was accomplished without discrimination against the high-mass isotopes of tin and indicated a sharp transition from stability to instability. The low m / e limit, at which this low-pass filtering technique is applicable without compromising the trapping efficiency, has not been fully evaluated, but the technique is thought to be applicable to masses of at least 100 amu and greater. At lower m/e values, perturbation of the trapping fields becomes extreme and trapping efficiency suffers. Application of the dc potential to the entrance end cap, as opposed to the exit end cap, was chosen due to a focusing effect that was observed. This effect and the improvement in trapping efficiency are illustrated in Figure 6, where 208Pb+and 120Sn+are plotted against entrance end capvoltage. As end cap voltage was increased, Pb+ signal intensity was observed to increase by a factor of 2 before Pb+ ions became destabilized. The decrease in space-charge effects, resulting from discrimination against Pb+, is evident as Sn+ intensity rises sharply as Pb+ signal decreases at v d c = -2.5. Analytical Characterization. The choice of which selective ion accumulation techniques, alone or in combination, depends ~~~~~
~
~
(27) Todd, J. F. J.; Penman, A. D.;Smith, R. D. In(.J . MassSpecrrom. Ion Processes 1991, 106, 117. (28) Asano, K. G.;Goeringer, D. E.; McLuckcy, S. A.; Hockman, D.; Stiller, S. W. Proceedings of the 41st ASMS Conference on Mass Spectrometry and Allied Topics, San Francisco, CA, May 30-June 4, 1993.
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lool
Pb+
1
u n a
-d* . I
II
Sn+
Figure 5. Radio frequency powered glow discharge mass spectra demonstrating the selectlve accumuletlon of Sn+ In NIST SRM 1103 Free Cutting Brass using an entrance end cap dc voltage to effect a low-pass filter: (a) grounded end caps; (b) -3.2 Vdoon entrance end cap: (c) -4.3 Va on entrance end cap. L
I
18 16
-0.5
0.0
0.5
1.0
1.6
2.0
2.5
;;
3.0
Entrance Endcap Voltage (Vdc)
Figure 6. Effect of entrance end cap voltage on trapping efflclencies of Pb+ and Sn+.
on the m / e value of the analyte and on the presence or absence of abundant neighboring ions. In addition, the analyte ion will be of either greater or lesser mass than the Yinterferencen, meaning that the selective ionization technique(s) used will
I
0
1
1
55
110
Inlmollon Tlma
I 185
220
(ma)
Flgurr 7. Comparison of Sn+ trapping efficiency as a function of inJectlon time and selective ion accumulation technique employed.
be case-specific. AS a result, elemental responses will not be uniform because of different injection and trapping characteristics. To improve dynamic range, injection periods must be extended for minor and trace analytes; therefore, quantification schemescommon to beam instruments (Le., utilizing relative beam intensities) will not suffice even for semiquantitative analyses. To perform analytical measurements, it is necessary to select an independent variable to relate to concentration. Ion injection time was chosen here, as it is easily measured and manipulated. It is important that the response be linear as a function of injection period. The signal of Sn+ (0.88%) in NIST SRM 1103 brass is illustrated in Figure 7 as a function of injection period. This plot shows the variation of signal versus injection time for the various selective accumulation techniques employed. Even with mass-selective instability, employing low-mass cutoffs of m / e 42 and 65, the response C U N ~ S become nonlinear as space-charge effects develop at an injection period of 100 ms. This nonlinearity is due to a decrease in trapping efficiencyand a loss of signal amplitude related to peak broadening. The broadening of the peaks associated with space-charging results in an overall decrease in ions detected, since the shoulders of the peak fall outside the assigned analog-to-digitalchannels of the detection system. A slight increase in trapping efficiency is noted when the matrix ions, Cu+ and Zn+, are discriminated against using a mass cutoff of m / e 65. Simultaneous accumulation of Pb+ with Sn+, however, is still problematic. This indicates that, for optimum performance, the analyte of interest should be isolated from both lower and higher mass species during injection. Best results were observed for the remaining techniques-combination rf-dc, end cap Vdc, and resonance ejection-where the ion was isolated. In our test case, combination rf-dc provided the highest trapping efficiency. This is due to part of the fact that ion trapping is very efficient at the moderate qz value employed (qz = OS), where the trapping potential is greater than at the boundaries of the stability diagram. As stated earlier, trapping efficiency is case-specific; discrimination against ions of close m / e values with combination rd-dc (i.e., operation at the apex of the stability diagram, where well depth is more shallow) resulted
-
60 d
100
120
140
mlz
IW
I80
200
=?+
Po
Flgurr 8. Illustrationof multiple-ramp scan function (a) and resultant mass spectrum (b) of selected elements in NIST SRM 1103 Free Cuttlng Brass.
in reduced trapping efficiency. Application of dc voltage to the end cap was the next-most efficient accumulation technique. Resonance ejection performed well also and might be improved with broad-band resonance ejection. To apply these isolation techniques to multielement analyses, various scan functions must be linked together, as illustrated in Figure 8a; Figure 8b shows the resulting mass spectrum. Four different scan functions (numbered 1 4 ) are sequentiallyconnected in Figure 8a, which include minimum rf injection levels of 30,40,65, and 125 amu, respectively. In each case, some degree of mass-selective instability was used. Relative rf trapping (low m / e cutoff) potentials are plotted on the ordinate. Relative injection times are indicated along the abscissa and are proportional to concentrations. Injection periods are indicated by the constant-potential region on the left of each scan function. Scan function 1 is unusual and represents the most difficult situation for mass-selective ion accumulation. The major isotopes of nickel, a minor constituent (0.15% elemental) observed in Figure 8b, are situated between intense argon and matrix (Cu+ and Zn+) ion signals. Mass-selective instability and combination rf-dc were employed to eliminate low m / e residual gas ions and lead ions, respectively. Trapping of Ni+ was experimentally determined to be optimum when set at an rf trapping potential corresponding to an mass cutoff of -30 amu, instead of 40 amu. While mass-selective instability at the higher trapping potential reduced the space-chargeeffects of argon ions, this resulted in a qz value too high for efficient trapping of nickel ions (Le., well depths are more shallow at A n a w l Chemistry, Vol. 66, No. 1, Jenusry 1, 1994
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high qz values). After the ions were injected for 209 ms, the ion-trapping potential was increased to an rf level corresponding to a m / e cutoff of 50 amu, to isolate the nickel ions, for 8 ms. Simultaneously, a 6-V resonance ejection signal, with a frequency corresponding to m / e 63.1, was applied to reduce space-charge effects of the matrix ions. All ions were then ejected by ramping the rf trapping potential from 30 through 62. It was determined that isolatingthe Ni+ in this manner resulted in a better signal response, as opposed to injecting at a high qz value while applying the resonance ejection. The shortcomings noted for scan function 1 illustrate the need for broad-band resonance ejection. Broad-band resonance ejection for argon, matrix ions, and lead ions would be beneficial in this application as the minor constituents could be trapped at an optimum qz value and isolated in the same event. Tailored broad-band waveforms21J2 have been developed and applied to organic applications.21,22,28It appears that these techniques hold promise for elemental applications as well. Scan functions 2-4 are more straightforward than scan function 1. In scan function 2, copper and zinc ions are accumulated for 4 ms with a low-mass cutoff of m / e 40, to eliminate Ar+. Given the abundance of the matrix relative to the minor species at higher masses, no low-pass (or notch) filtering was employed. Scan 3 .was used for accumulation of Sn+(0.88%) and employed mass-selectiveinstability (with a low-mass cutoff of 67 amu) and combination rf-dc to prevent trapping of Pb+ ions. An extended ion injection period (21 1 ms) was employed to generate more ion signal from this minor constituent. Lead ions were accumulated for a shorter period than Sn+(88 ms; scan 4) due to its higher concentration. Only mass-selective instability was used during this accumulation (low-mass cutoff, 125 amu). As illustrated in Figure 8b, these scan functions can be linked together to generate a mass spectrum with major and minor constituents generating an approximately equal signal intensity for constituents other than Ni+, as previously noted. Given the demonstrated linear injection curves and the similar intensities observed in Figure 8b, it should be possible to quantify elemental concentration based on injection times. For example, the time required to generate a Sn+ signal of intensity equal to a matrix element, Cu+, can be used as a quantitative measure of relative concentration. Of course, this would have to be adjusted for differences in elemental responses, which may include differences in ion source phenomena (Le., atomization, ionization) and differences in
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AnalVtlcel chemlsby, Vol. 66, No. 1. Jenuery 1, 1994
trapping efficiencies. The limiting aspect of such quantification is the potential need for the developmentof new response factors when scan functions are altered. Having developed scan functions for an application, analysis times should be comparable to current GDMS methods. A detection limit for Iz2Sn+in Figure 8b yields a limit of detectability of 51 ppm. Not all parameters have been optimized, especially injection period, which, based on the resolution, could extend detection limits further. A limit of detection of 10 ppm has been demonstrated in earlier work employing the mass-selective accumulation techniques described here.I0 Reduction in detector noise, known to originate from glow discharge generated electrons, should significantly improve signal-to-noise characteristics.
CONCLUSIONS We have found selective ion accumulation techniques to be an effective means of extending dynamic range in quadrupole ion trap mass analysis. These techniques, including mass-selective instability, resonance ejection, combination rfdc, and entrance end cap dc, result in the elimination of abundant ions from the trap. These would otherwise fill the trap volume, resulting in detrimental space charge effects on both mass resolving power and trapping efficiency. The last injection technique is a novel means of low-pass filtering and provides a very sharp stability cutoff at high masses. Sophisticated broad-band resonance ejection techniques clearly hold promise for inorganic ion trap mass spectrometry applications. However, the results described here indicate that there are a variety of tools available for mass-selective ion accumulation without recourse to tailored wave forms. Utilization of these techniques for GD-ITMS allows the detection of major and minor (- 10 ppm) constituents in solid matrices. The ability to obtain linear signal response with respect to injection period, and the ability to coupleindependent scan functionsfor multielementmonitoring,provide a potential means of quantification. ACKNOWLEDGMENT The authors acknowledge the helpful contributions of Dr. Gary L. Glish in the early phases of this work. Research was sponsored by the US.Department of Energy, Office of Basic Energy Sciences,under Contract DE-AC05-840R21400 with Martin Marietta Energy Systems, Inc. Received for review August 9, 1993. Accepted October 4, 1903.O Abstract published in Aduancc ACS Abstracts, November 15, 1993.
