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Evaluation of a 512-Channel Faraday-Strip Array Detector Coupled to an Inductively Coupled Plasma Mattauch-Herzog Mass Spectrograph Gregory D. Schilling,† Steven J. Ray,† Arnon A. Rubinshtein,†,‡ Jeremy A. Felton,† Roger P. Sperline,§ M. Bonner Denton,§ Charles J. Barinaga,| David W. Koppenaal,| and Gary M. Hieftje*,† Department of Chemistry, Indiana University, Bloomington, Indiana 47405, Department of Chemistry, University of Arizona, Tucson, Arizona 85721, and Pacific Northwest National Laboratory, Richland, Washington 99352 A 512-channel Faraday-strip array detector has been developed and fitted to a Mattauch-Herzog geometry mass spectrograph for the simultaneous acquisition of multiple mass-to-charge values. Several advantages are realized by using simultaneous detection methods, including higher duty cycles, removal of correlated noise, and multianalyte transient analyses independent of spectral skew. The new 512-channel version offers narrower, more closely spaced pixels, providing improved spectral peak sampling and resolution. In addition, the electronics in the amplification stage of the new detector array incorporate a sample-and-hold feature that enables truly simultaneous interrogation of all 512 channels. While sensitivity and linear dynamic range remain impressive for this Faraday-based detector system, limits of detection and isotope ratio data have suffered slightly from leaky p-n junctions produced during the manufacture of the semiconductor-based amplification and readout stages. This paper describes the new 512-channel detector array, the current dominant noise sources, and the figures of merit for the device as pertaining to inductively coupled plasma ionization. Inductively coupled plasmas (ICP) have revolutionized the field of elemental analysis.1,2 This impact is due in large part to the benefits that the ICP offers over previously used excitation/ ionization/atomization sources such as analytical flames or graphite furnaces. These characteristics include high excitation temperatures, low background, and a relatively inert excitation atmosphere.3 Furthermore, the high electron number densities and high excitation temperatures of the ICP result in efficient excitation and ionization of nearly all elements simultaneously. This aspect has been of benefit to ICP-optical emission spectrometry (OES) through the use of photodiode arrays (PDA) or charge* To whom correspondence should be addressed. † Indiana University. ‡ On leave from Nuclear Research Center, Israel Atomic Energy Commission, Beer-Sheva, Israel 84190. § University of Arizona. | Pacific Northwest National Laboratory. (1) Wendt, R. H.; Fassel, V. A. Anal. Chem. 1965, 37, 920–922. (2) Greenfield, S.; Jones, I. L.; Berry, C. T. Analyst 1964, 89, 713–720. (3) Olesik, J. W. Anal. Chem. 1991, 63, 12A–21A. 10.1021/ac900640m CCC: $40.75 2009 American Chemical Society Published on Web 05/22/2009
transfer devices (CTD) coupled to optical spectrographs, a combination that permits simultaneous multielemental analysis.4-9 Of course, ICPs have also been effectively used as ionization sources for mass spectrometry (MS).10 In ICPMS, improved limits of detection are achieved because lower backgrounds give rise to an overall higher signal-to-noise ratio, spectra are much simpler, and isotopic information can be obtained. Unfortunately, the inherently simultaneous nature of the ICP has not been fully realized in most MS applications, mainly because almost all commercial ICPMS systems are operated in a scanned or pulsed (in the case of time-of-flight mass spectrometry) manner. One type of mass analyzer that can be used as a mass spectrograph, i.e., in a continuous, simultaneously mass-dispersive manner, is the sector-field mass analyzer. Unfortunately, most sector-field mass spectrometers are used in a scanning mode due to the lack of a high-quality multichannel array ion detector. Earlier, photographic plates11,12 and electro-optic imaging detectors13,14 were used with sector-field instruments to achieve simultaneous mass analysis. However, such devices have fallen into disuse because of their inferior detection capabilities compared to single-channel ion detectors.15 However, there are commercially available instruments that utilize several singlechannel detectors simultaneously. These instruments, known as multicollector mass spectrometers, can acquire up to 12 m/z values simultaneously. Furthermore, moveable detectors or zoom optics are used to focus the desired ions onto the individual (4) Horlick, G. Appl. Spectrosc. 1976, 30, 113–123. (5) Barnard, T. W.; Crockett, M. I.; Ivaldi, J. C.; Lundberg, P. L.; Yates, D. A.; Levine, P. A.; Sauer, D. J. Anal. Chem. 1993, 65, 1231–1239. (6) Ivaldi, J. C.; Barnard, T. W. Spectrochim. Acta, Part B 1993, 48B, 1265– 1273. (7) Bilhorn, R. B.; Epperson, P. M.; Sweedler, J. V.; Denton, M. B. Appl. Spectrosc. 1987, 41, 1125–1136. (8) Pilon, M. J.; Denton, M. B.; Schleicher, R. G.; Moran, P. M.; Smith, S. B., Jr. Appl. Spectrosc. 1990, 44, 1613–1620. (9) Sims, G. R.; Denton, M. B. Talanta 1990, 37, 1–13. (10) Houk, R. S.; Fassel, V. A.; Flesch, G. D.; Svec, H. J.; Gray, A. L.; Taylor, C. E. Anal. Chem. 1980, 52, 2283–2289. (11) McCrea, J. M. Appl. Spectrosc. 1971, 25, 246–252. (12) Vouros, P.; Desiderio, D. M.; Leferink, J. V. M.; McCloskey, J. A. Anal. Chem. 1970, 42, 1275–1277. (13) Tuithof, H. H.; Boerboom, A. J. H.; Meuzelaar, H. L. C. Int. J. Mass Spectrom. Ion Phys. 1975, 17, 299–307. (14) Beynon, J. H.; Jones, D. O.; Cooks, R. G. Anal. Chem. 1975, 47, 1734– 1738. (15) Barnes, J. H., IV; Hieftje, G. M. Int. J. Mass Spectrom. 2004, 238, 33–46.
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detectors.16 Clearly, the development of a multichannel array ion detector capable of achieving results comparable to those of singlechannel devices for a broad range of m/z values is desired. Ideally, the array detector should be as sensitive as a single-channel detector, have a uniform response across its length, exhibit low noise, and not be the limiting factor in resolution. The advantages that can be realized through simultaneous mass analysis have previously been discussed in detail on both a theoretical17 and experimental18 basis. In short, these advantages include a much higher duty cycle, reduced correlated noise through ratioing, and more accurate transient analyses. Higher duty cycles result in improved limits of detection, shorter analysis times, and reduced sample consumption. One such array detector capable of achieving the above advantages is the focal plane camera (FPC).18-25 The FPC, so named because of its ability to capture a simultaneous snapshot of the ion distribution along the focal plane of a mass spectrograph, uses Faraday strips connected in a 1:1 fashion to high-gain integrating operational amplifiers. This arrangement is able to record signals from a range of mass-to-charge (m/z) values simultaneously. The newest version, described and evaluated in this report, utilizes 512 collector/amplifier pairs, each of which offers two individually selectable gain levels. It also employs a sample and hold feature on each channel, so truly simultaneous integration of selected channels is possible. The new FPC has been coupled to a Mattauch-Herzog mass spectrograph (MHMS), and its performance has been characterized with ICP ionization. EXPERIMENTAL SECTION Sample Introduction System. For all experimental work presented here, a Cetac (Omaha, NE) U-6000AT+ ultrasonic nebulizer (USN) was used to introduce samples into the ICP. The evaporation and condensation portions of the USN were operated at 160 °C and 5-10 °C, respectively. Sample and waste flows to and from the USN were regulated by means of a Rainin (Emeryville, CA) RP-1 four-head peristaltic pump operated at a sample uptake rate of 1 mL/min. Additionally, a Cetac membrane (16) Belshaw, N. S.; Freedman, P. A.; O’Nions, R. K.; Frank, M.; Guo, Y. Int. J. Mass Spectrom. 1998, 181, 51–58. (17) Solyom, D. A.; Hieftje, G. M. J. Am. Soc. Mass Spectrom. 2003, 14, 227– 235. (18) Schilling, G. D.; Andrade, F. J.; Barnes, J. H., IV; Sperline, R. P.; Denton, M. B.; Barinaga, C. J.; Koppenaal, D. W.; Hieftje, G. M. Anal. Chem. 2007, 79, 7662–7668. (19) Barnes, J. H., IV; Schilling, G. D.; Sperline, R.; Denton, M. B.; Young, E. T.; Barinaga, C. J.; Koppenaal, D. W.; Hieftje, G. M. Anal. Chem. 2004, 76, 2531–2536. (20) Barnes, J. H., IV; Schilling, G. D.; Hieftje, G. M.; Sperline, R. P.; Denton, M. B.; Barinaga, C. J.; Koppenaal, D. W. J. Am. Soc. Mass Spectrom. 2004, 15, 769–776. (21) Barnes, J. H., IV; Schilling, G. D.; Sperline, R. P.; Denton, M. B.; Young, E. T.; Barinaga, C. J.; Koppenaal, D. W.; Hieftje, G. M. J. Anal. At. Spectrom. 2004, 19, 751–756. (22) Barnes, J. H., IV; Schilling, G. D.; Stone, S. F.; Sperline, R. P.; Denton, M. B.; Young, E. T.; Barinaga, C. J.; Koppenaal, D. W.; Hieftje, G. M. Anal. Bioanal. Chem. 2004, 380, 227–234. (23) Barnes, J. H., IV; Sperline, R.; Denton, M. B.; Barinaga, C. J.; Koppenaal, D.; Young, E. T.; Hieftje, G. M. Anal. Chem. 2002, 74, 5327–5332. (24) Koppenaal, D. W.; Barinaga, C. J.; Denton, M. B.; Sperline, R. P.; Hieftje, G. M.; Schilling, G. D.; Andrade, F. J.; Barnes, J. H., IV Anal. Chem. 2005, 77, 418A–427A. (25) Schilling, G. D.; Andrade, F. J.; Barnes, J. H., IV; Sperline, R. P.; Denton, M. B.; Barinaga, C. J.; Koppenaal, D. W.; Hieftje, G. M. Anal. Chem. 2006, 78, 4319–4325.
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desolvation unit was employed to remove excess solvent from the USN aerosol. With the desolvation system, the membrane coil was held at a temperature of 160 °C; a 3.5-4.0 L/min counterflow of argon (99.998%, Airgas, Radnor, PA) was used to carry away the solvent vapor. A separate argon flow of 0.7-1.2 L/min was delivered by a mass flow controller (Tylan General, FC-280 SAV, Carson, CA) operated with a MKS 647A multigas controller unit and was used to carry the aerosol through the USN/membrane desolvator combination and into the central channel of an ICP torch. The ICP was maintained by a PlasmaTherm (St. Petersburg, FL) model 2500D radio frequency power supply and a model 2500E impedance-matching network. Additional argon for the outer (16-17 L/min) and intermediate (1.1-1.5 L/min) channels of the ICP was delivered by two MKS (Andover, MD) 1179A mass flow controllers. Mattauch-Herzog Mass Spectrograph. The MHMS has been described in detail previously,19,26,27 so only a brief account and information pertinent to the current study will be presented here. Ions are extracted from the ICP via a three-stage differentially pumped interface that employs a 0.75 mm sampling-cone orifice, a 0.5 mm skimmer-cone orifice, and a 1.0 mm third-stage aperture. An ion accelerating potential of 1.0-1.2 kV is applied to the sampler and skimmer cones. A copper torch shield was used to limit a secondary discharge between the plasma and the highvoltage interface of the MHMS. An Einzel lens and a pair of dcquadrupoles follow the third-stage aperture and are used to focus and shape the ion beam onto a 50 or 100 µm entrance slit. A 31.8° electrostatic analyzer (ESA) of radius 160.3 mm sits at a distance of 113.3 mm behind the entrance slit. The theoretical resolving power (R) of the MHMS is dictated by the entrance slit and the radius of the ESA by eq 1 R)
rESA 2S
(1)
where rESA is the radius of the ESA and S is the slit width. Here, the ESA radius of 160.3 mm dictates that the theoretically attainable resolving power is ∼800 when a 100 µm slit is used and ∼1600 with a 50 µm slit. A 90° magnetic sector with a 76 mm long focal plane follows the ESA. The field strength of the magnetic sector can be changed by use of an electromagnet and is varied to focus the desired range of m/z values onto the detector array. Generally, the MHMS is operated in a continuous ion beam mode in order to achieve the highest duty cycle possible. However, for the limit of detection studies, the MHMS was operated with a pulsed ion beam. This method was used so the dark current and signal could both be determined without resetting the detector. To pulse the ion beam, the (-) ESA was switched in coordination with the global reset of the FPC by means of two pulse generators. The first pulse generator (PDG-2510, Directed Energy, Inc., Fort Collins, CO) was set to deliver a 5 V, 500 µs square pulse that was triggered from the global reset pulse of the FPC. The second pulse generator (100B, Systron Donner, Concord, CA), triggered off the first function generator, was (26) Burgoyne, T. W.; Hieftje, G. M.; Hites, R. A. J. Am. Soc. Mass Spectrom. 1997, 8, 307–318. (27) Barnes, J. H., IV; Schilling, G. D.; Denton, M. B.; Koppenaal, D. W.; Hieftje, G. M. J. Anal. At. Spectrom. 2003, 18, 1015–1018.
Figure 1. (a) Photograph and (b) circuit diagram of the FPC-512 camera. The circuit diagram shows the integrating amplifier circuit (capacitive transimpedance amplifier, CTIA) that incorporates two levels of gain and the sample and hold amplifier (SaHA) that enables truly simultaneous data acquisition.
adjusted to deliver a pulse of amplitude equal to the optimized (-) ESA voltage and with a pulse duration of 500 ms. The output of the second function generator was connected to the (-) ESA. Focal Plane Camera. The FPC used in these studies consisted of an array of 512, 8.5 µm wide, titanium-nitride-coated aluminum strips set on 12.5 µm centers. The titanium-nitride plating was used as an oxidation protectant for the aluminum fingers. In addition, each Faraday strip is connected to its own capacitive transimpedance amplifier (CTIA) by means of directly depositing the collectors onto an amplifier/multiplexer integrated circuit (IC). This design avoids individual wire-bond connections to each Faraday collector, as in previous devices, and offers simplified manufacturing, a more robust device, and the ability to utilize narrower, more closely spaced collectors. However, even with this design, the collectors were interleaved so every alternate collector was connected to the amplifiers stationed at opposite ends of the IC. As a result, connections to the collectors were made on 25 µm centers. A photograph and circuit diagram of the FPC-512 are shown in Figure 1. A comparison of the physical characteristics of the FPC-512 with the previous FPC versions is given in Table 1. The gain of the CTIA is related to the inverse of the capacitance in the feedback loop. The current FPC device uses two separate capacitors in each CTIA channel. The two capacitors differ in capacitance by a factor of 1000, and either the smaller one or both can be employed to adjust the gain between two different levels. The gain level can be set individually for each of the 512 collectors. For the high-gain setting, only one capacitor (on the order of 8 fF) is used and
Table 1. Physical Comparison of Three Generations of Focal Plane Camerasa
number of pixels pixel width (µm) pixel spacing (µm) fill factor pixel height (mm) gain levels minimum readout time (all channels, ms) a
FPC-32
FPC-128
FPC-512
32 145 30 83 1.6 1 1
128 45 5 90 6.35 2 (1:100) 0.4
512 8.5 4 68 6.35 2 (1:1000) 14 (LabVIEW-limited)
The FPC-512 camera was used for all work presented here.
generates an output from the CTIA of about 20 µV for each collected singly charged ion. The low-gain setting for each channel then provides 20 nV for each collected singly charged ion. In addition to the CTIAs, each Faraday strip also has its own sample-and-hold amplifier (SaHA). The charge from the CTIAs can be simultaneously gated into the SaHAs and then sequentially interrogated while new ions are being collected on the Faraday strips. This design provides truly simultaneous data acquisition across all 512 channels and allows the charge on each SaHA to be averaged over multiple nondestructive reads to reduce read noise associated with the circuitry. The 512-channel FPC also incorporates a test device to characterize any differences between the CTIAs and SaHAs of the various channels. The device operates by injecting charge into every channel simultaneously. The injected charge on each channel is the same; therefore, the output of each channel should Analytical Chemistry, Vol. 81, No. 13, July 1, 2009
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reflect any differences among the channel circuitry. Variations across all 512 channels of the array were on the order of 2% RSD. Furthermore, the differences can be used to create a flat-field correction to be applied to all subsequent data. When the flatfield correction is applied, the precision across the channels improves to 0.1% RSD. The array detector was cooled by means of a Peltier chiller attached to the back of the circuit board at a point directly beneath the mounted amplifier IC. Heat from the “hot” side of the Peltier cooler was carried out of the vacuum chamber by a 381 mm long heat pipe. The heat pipe was cooled by a brass jacket that carried a flow of a 50:50 mixture of ethylene glycol/water that was maintained at a temperature of 0 °C by a recirculating chiller (Neslab Instruments, Inc., Endocal, Newington, NH). This setup maintained a temperature of -38 °C at the face of the silicon chip, which was monitored by means of an onboard AD590 temperature sensor, and reduced the effects of Johnson noise in the detector. Data Analysis. The FPC integrates charge continuously. This charge is read though nondestructive readouts (NDRO) at regular intervals throughout the total integration period. Therefore, a plot of total charge accumulated as a function of NDRO or time can be generated. If a constant ion flux is impingent on the FPC, the resulting data are linear with respect to NDRO or time. Linear regression is then used to determine the slope of this line, which is equal to the ion current on the detector. This slope can be used to determine the total number of ions that were collected during the entire integration period; since the linear regression is performed on many data points, the read noise of the measurement is greatly reduced. This method was used for all data analysis except for the determination of limits of detection (LOD). For LOD data, ions were pulsed through the MHMS, and the total charge accumulation was taken as the difference between the average of several NDROs after the pulse and the average of several NDROs before the pulse. This method was used to directly determine the dark current and subtract it from the signal at fast repetition rates. Several repetitions of this method were summed to achieve a 10 s total integration period for the limits of detection determinations. Reagents. A 1% HNO3 solution was used to prepare all analytical standards through serial dilution. Prior to making the 1% HNO3 solution, reagent-grade nitric acid (70%, Mallinckrodt Baker Inc., Phillipsburg, NJ) was purified in a Teflon subboiling distillation apparatus (SBS-108, Savillex, Minnetonka, MN). For the measurement of limits of detection, resolution, and isotope-ratio accuracy, serial dilution of 1 × 105 ng mL-1 multielementalstandardsprovidedbySPEXCertiprep(Metuchen, NJ) were used. For isotope-ratio precision studies, a 1 × 106 ng mL-1 atomic absorption silver standard (Johnson Matthey Electronics, Ward Hill, MA) was diluted to 0.5 × 103 ng mL-1. For the determination of dynamic range, a 1000 µg mL-1 holmium atomic absorption standard was used to make solutions varying in concentration from 0.1 ng mL-1 to 1 × 105 ng mL-1. All data were background-subtracted, using the 1% HNO3 solution as a blank. RESULTS AND DISCUSSION Detector Noise. The detector noise was characterized in terms of the read noise, the dark current, and the read-to-read dark-current fluctuations both with the ion source or ion optics on and with them off. The read noise was taken as the standard 5470
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Figure 2. Spectrum of the lanthanide series, acquired in three separate mass windows delineated by color. A 10 ppb multielement solution and 10 s integration time were used.
deviation of 10 successive NDROs of the detector. Prior to determination of this standard deviation, any slope, or dark current was subtracted from the 10 reads. The dark current was taken from the slope through 250 NDROs over a 1.92 s integration period. The fluctuations in the dark current were taken as the standard deviation of the slope across 10 repetitions. Values for the average read noise, dark current, and dark current noise were determined to be 60 charges, -254 charges/s, and 57 charges/s, respectively, and were found to be independent of the ion source and ion optics being on or off. These values were compared to the total noise observed in a measurement, which was taken as the standard deviation of the total number of charges collected throughout a 1.92 s integration. The total number of collected charges was determined using the slope method described above. On average, the total noise was 116 charges. This value compares well with the expected dark current noise for a 1.92 s integration (1.92 s × 57 charges/s ) 109 charges) and suggests that dark current noise is the dominant factor in governing the total noise. Typical values for the FPC-128 for the read noise, dark current, and dark current noise were 100 charges, -70 charges/s, and 12 charges/s, respectively. When several measurements were averaged, both the read noise and the dark current noise improved with the square root of the number of measurements that were averaged. In addition, use of the sample and hold option reduced the read noise in the same manner as averaging multiple reads but did not improve the dark current or the dark current noise. Since the dark current noise seems to be the major source of background noise in the system, use of the sample and hold method is not expected to improve the overall noise. Although no experimental work was performed to determine the source of this dominant noise, the chip supplier suggested that instability in the dark current and thus in the detector is due to imperfect manufacturing of the semiconductor ICs used in the detector. The flaws, however, are thought to be manufacturing-related and not inherent to the design, as additional chips, manufactured with the same design, appear to behave in a much more stable manner. Unfortunately, the new chips were not immediately available for full characterization.
Table 2. Isotope-Abundance-Corrected Limits of Detection for the FPC-512, FPC-128, and FPC-32 Cameras, Given in Parts-Per-Trillion isotope 133
Cs Ba 139 La 140 Ce 141 Pr 145 Nd 149 Sm 151 Eu 161 Dy 165 Ho 167 Er 172 Yb 203 Tl 206 Pb 138
FPC-512a
FPC-128a,b
11 11 12 13 10 10 10 9 4 4 5 5 10 9
FPC-32c 0.7 2 2
0.4
0.6
0.3
1
a Noise was determined from off-mass collectors. b Data taken from ref 25. c Data taken from ref 19.
Figure 3. Linear dynamic range of the FPC-512 camera obtained from holmium solutions. Both gain levels were used to collect data from samples spanning 8 orders of magnitude in concentration. Error bars are included but are indistinguishable from the data points. The roll-off at the high concentration end of the curve is due to ion transmission losses in the mass spectrograph and not to detector saturation.
Simultaneous Mass Detection Range. As was mentioned previously, only a narrow range of m/z values can be detected simultaneously with the current FPC because of the limited length of the array. The number of simultaneous masses can be calculated from eq 2
( ) r512 r1
2
)
mH mL
(2)
where r1 and r512 are the flight radii of ions in the magnetic sector that are collected by pixels 1 and 512, respectively. Here, mL and mH are the m/z values that follow trajectories of r1 and r512, respectively. The r1 and r512 values were determined by calculating the exact locations of pixels 1 and 512 on the focal plane. These locations were in turn used to determine the
Figure 4. Isotope ratio precision as a function of integration time for the FPC-32, FPC-128, and FPC-512 cameras, including the counting statistics limit (red dashed line) based on the count rate observed with FPC-128. The precision trends with the counting statistics limit for the FPC-32 and FPC-128 systems; however, a rolloff is demonstrated at longer integration times with FPC-512. Notice the differences in concentrations used among the three detectors.
circular paths that ions would take through the magnetic field to be collected by pixels 1 and 512. The radii of the circular paths were used as r1 and r512. On the basis of the calculated r1 and r512 values for the FPC-512 array, the left side of eq 2 is equal to 1.15. Therefore, given any limiting m/z, the corresponding lower or higher m/z that can be simultaneously detected can be calculated. If the low mass is 6 amu, the highest mass that can be acquired at the same time is 6.9 amu. As a result, the two isotopes of lithium cannot be determined at once with the current array detector. In contrast, if the low m/z is 227 (227Ac), a mass range of 34 amu can be simultaneously acquired, which would include isotopes from all elements in the actinide series. Because of the radioactive nature of the actinides, the lanthanide series was chosen to demonstrate the mass range of the FPC-512. An acquired spectrum of the lanthanides is shown in Figure 2. In this plot, three mass windows were combined to cover the entire lanthanide series. Limits of Detection. Limits of detection were determined at the 3σ level by using 10 repetitions of 10 s integrations for both the analyte and the 1% nitric acid blank. Each 10 s integration consisted of the summation of 20 individual 500 ms integrations using the pulsed ion beam method described earlier. In all cases, the blank was subtracted from the analyte signal and the noise was determined at a featureless region of the mass spectrum. These off-mass background levels were used because contamination-free detection limits were desired. A comparison of detection limits for the FPC-512, FPC-128, and FPC-32 array detector generations is shown in Table 2. In general, the detection limits are slightly worse for the FPC-512 system than for the FPC-128 or FPC-32 cameras. Several reasons contribute to this finding. First, the fill factor, or the fraction of the ion collection area to the total detector area, is reduced from 90% in the FPC-128 camera to 68% in the FPC-512 unit. Given a peak that spans several pixels in both cases, more ions will be lost in the dead area of the FPC512 than with the FPC-128. Yet, the holmium signals used to determine detection limits were about 1.4 times greater with FPCAnalytical Chemistry, Vol. 81, No. 13, July 1, 2009
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Figure 5. Spectra of thallium and lead obtained using (a) FPC-32 with a 100 µm MHMS entrance slit width, (b) FPC-128 with a 100 µm MHMS entrance slit width, and (c) FPC-512 with a 100 µm MHMS entrance slit width. The points in each spectrum indicate the response of the individual collectors of each array detector. The resolution is shown to improve with FPC generation. With the FPC-512 camera, the detector is no longer the limiting factor in resolution.
512 than with FPC-128. This result is most likely due to the number of pixels that were used in each case. For the FPC-128 camera, only a single pixel was used to determine the signal for LOD determinations, whereas several pixels were used with the FPC-512. The difference in performance between the two detectors lies clearly in the noise level. With FPC-512, the blank noise during the holmium measurement was about 14 times higher than with the FPC-128. This increased noise is most likely caused by the degraded p-n junctions in the semiconductor chip. Even with this elevated noise, LODs are still in the single part-per-trillion range for most isotopes. Linear Dynamic Range. Linear dynamic range is very important when simultaneous acquisition of signals from multiple analytes is desired. If the detector is limited in dynamic range, it might be necessary to dilute or concentrate a sample and run it multiple times to determine analytes that span a broad concentration range. The linear dynamic range of the FPC-512 device was determined from holmium solutions of concentrations ranging from 1 × 10-3 to 1 × 106 ng/mL. Both integration time and amplifier gain levels were changed to accommodate the wide range of ion currents resulting from the various solution concentrations. The resulting dynamic range, shown in Figure 3, spans at least 8 orders of magnitude. The selectable gain levels were found to differ by a factor of 1227. This value was obtained by taking the quotient of the signals for the 1 × 101 ng/mL solution, using each gain level, and was employed to correct for the difference in gain for the other solutions. The plot in Figure 3 shows a roll-off at 1 × 106 ng/mL, due to ion transmission losses in the mass spectrograph and not to detector saturation. Isotope Ratio Accuracy and Precision. Isotope ratio precision is one of the major driving forces behind simultaneous mass analysis. As was stated previously, any correlated noise between analyte channels can be reduced by taking the ratio of two or more simultaneously collected signals. Although previous FPC versions have demonstrated precision levels on the order of 60 ppm and accuracies generally in the 1% error range, the FPC-512 performance was disappointing, most likely because of imperfect manufacturing of the semiconductor chips used in the detector. These bad semiconductors cause the detector to become unstable over long periods that are required 5472
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for ultraprecise isotope ratio data. This effect can be seen in Figure 4, which compares the isotope ratio precision as a function of integration time among the three detector generations. The precision is shown to roll off at longer integration times with the FPC-512 detector. Even with this recognized problem, precision levels of 300-500 ppm and accuracies of 2-10% error were routinely obtained during 1-10 s long integrations. Resolving Power. In order to resolve two mass-spectral peaks with an array detector, there should be at least one collector between the two peaks that receives only a small amount or no signal. Ideally, the array detector will have a high enough density of narrow collectors not to be the limiting factor in dictating resolving power. In the Mattauch-Herzog geometry, the resolving power is defined by eq 1. As indicated before, a slit width of 100 µm in the MHMS instrument should yield a theoretical resolving power of 800. The experimental data, shown in Figure 5, provide a resolving power of 640 for the 205 Tl peak in the plot in part c of Figure 5. The discrepancy from the expected value could be due to an actual slit width that was slightly wider than 100 µm, fluctuations in ion energies, electrostatic analyzer voltages, or the magnetic field, all of which would degrade resolving power. Fluctuations in ion energies are suspected; with a glow discharge ionization source, in work to be published elsewhere, the theoretical resolving power has been attained experimentally. These findings show that the dimensions of the collectors in the FPC-512 are not the limiting factor in governing resolution for the system as a whole. With previous detector versions, the resolving power of the system was limited by the size of the collectors in the array detector unless an unusually wide entrance slit was used. CONCLUSIONS The FPC-512 device has shown improvements over previous FPC versions despite problems in the chip-manufacturing process. While noise, limits of detection, and isotope ratio data have all suffered slightly, the resolving power of the system has been improved and is now dictated by the MHMS instrument and not by the width of the array detector fingers when typical MHMS slit widths are used. Future generations of the FPC with properly manufactured semiconductors should restore or improve upon
the figures of merit while maintaining the enhanced resolving power. In addition, an even greater number of collectors will be used in order to simultaneously detect a much broader region of the mass spectrum. A FPC version containing 1696 collectors is already planned that will be capable of the simultaneous detection of 96 m/z values on the high mass end, which would span from molybdenum isotopes to lawrencium isotopes.
Pacific Northwest National Laboratory is operated by Battelle Memorial Institute for the Department of Energy under Contract DE-AC06-76RLO-1830. The authors thank SPEX Certiprep for providing multielement standards for this work.
ACKNOWLEDGMENT Support for this work was provided by the U.S. Department of Energy, Office of Nonproliferation Research and Engineering.
Received for review March 27, 2009. Accepted May 7, 2009. AC900640M
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