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Characterization of a Second-Generation Focal-Plane Camera Coupled to an Inductively Coupled Plasma Mattauch-Herzog Geometry Mass Spectrograph Gregory D. Schilling,† Francisco J. Andrade,† James H. Barnes, IV,‡ Roger P. Sperline,§ M. Bonner Denton,§ Charles J. Barinaga,| David W. Koppenaal,| and Gary M. Hieftje*,†
Department of Chemistry, Indiana University, Bloomington, Indiana 47405, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, Department of Chemistry, University of Arizona, Tucson, Arizona 85721, and Pacific Northwest National Laboratory, Richland, Washington 99352
Mass spectrometry (MS) has become a workhorse of the analytical laboratory because of its ability to analyze a wide range of sample types and to produce both elemental and molecular information through use of alternative ionization sources.1 In the area of elemental analysis, inductively coupled plasma (ICP) ionization combined with mass spectrometry has set the standard for sensitivity and versatility due to the ability of the ICP to efficiently ionize most elements under a single set of operating conditions.2 Regrettably, most instruments do not take full advantage of this capability because they require the mass spectrum to be scanned in a sequential or peak-hopping manner. This mode of operation limits the effective duty cycle of the
measurement to 1/n, where n is the number of elements or isotopes that must be determined.3 Some methods, such as time-of-flight (TOF) MS, improve performance by acquiring a complete mass spectrum from a simultaneously extracted ion packet. The duty cycle in such instruments is limited, however, by the time it takes to fill the extraction region and acquire a complete mass spectrum. Although duty factors of greater than 50% have been reported with TOFMS,4,5 typical values are usually on the order of 10%.6 These limitations can be overcome by employing a mass spectrometer that measures all the mass-to-charge (m/z) values at once. In such an instrument, the duty cycle can theoretically reach 100%. With this increased duty cycle, limits of detection would be improved and required analysis times, sample sizes, or both reduced. Another benefit achieved by simultaneous detection is the ability to reduce the effects of correlated noise sources through the use of the ratio of multiple detector responses. When two analyte signals are acquired in synchrony, any multiplicative noise that affects the two can be eliminated by taking their ratio. Finally, spectral skew, i.e., artifacts that arise when a mass range is scanned during a time-dependent analyte peak, can be eliminated by using simultaneous detection. This benefit is most important when transient sample introduction techniques, such as chromatography, are used. These advantages can be realized to some extent by positioning several detectors along the focal plane of a spatially dispersive mass spectrometer. This approach has been commercialized by manufacturers in the form of multicollector instruments (typically thermal ionization and stable isotope ratio mass spectrometers). These instruments universally employ a limited number of
* To whom correspondence should be addressed. E-mail: hieftje@ indiana.edu. † Indiana University. ‡ Los Alamos National Laboratory. § University of Arizona. | Pacific Northwest National Laboratory. (1) Holland, J. F.; Enke, C. G.; Allison, J.; Stults, J. T.; Pinkston, J. D.; Newcome, B.; Watson, J. T. Anal. Chem. 1983, 55 (9), 997A-998A, 1000A, 1002A, 1004A, 1006A, 1008A, 1010A, 1012A. (2) Montaser, A. Inductively Coupled Plasma Mass Spectrometry; Wiley-VCH: New York, 1998.
(3) Barnes, J. H.; Schilling, G. D.; Sperline, R.; Denton, M. B.; Young, E. T.; Barinaga, C. J.; Koppenaal, D. W.; Hieftje, G. M. Anal. Chem. 2004, 76 (9), 2531-2536. (4) Yoon, O. K.; Zuleta, I. A.; Kimmel, J. R.; Robbins, M. D.; Zare, R. N. J. Am. Soc. Mass Spectrom. 2005, 16 (11), 1888-1901. (5) Tempez, A.; Schultz, J. A.; Della-Negra, S.; Depauw, J.; Jacquet, D.; Novikov, A.; Lebeyec, Y.; Pautrat, M.; Caroff, M.; Ugarov, M.; Bensaoula, H.; Gonin, M.; Fuhrer, K.; Woods, A. Rapid Commun. Mass Spectrom. 2004, 18 (4), 371-376. (6) McClenathan, D. M.; Ray, S. J.; Wetzel, W. C.; Hieftje, G. M. Anal. Chem. 2004, 76, 158A-166A.
A second-generation Faraday-strip array detector has been coupled to an inductively coupled plasma MattauchHerzog geometry mass spectrograph, thereby offering simultaneous acquisition of a range of mass-to-charge ratios. The second-generation device incorporates narrower, more closely spaced collectors than the earlier system. Furthermore, the new camera can acquire signal on all collectors at a frequency greater than 2 kHz and has the ability to independently adjust the gain level of each collector. Each collector can also be reset independently. With these improvements, limits of detection in the hundreds of picograms per liter for metals in solution have been obtained. Some additional features, such as a broader linear dynamic range (over 7 orders of magnitude), greater resolving power (up to 600), and improved isotope ratio accuracy were attained. In addition, isotope ratio precision as low as 0.018% RSD was achieved.
10.1021/ac052026k CCC: $33.50 Published on Web 06/03/2006
© 2006 American Chemical Society
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detectors (n e 9) that are set in fixed positions to acquire signals for specific isotope ratio measurements. Such multicollector instruments are almost never employed to acquire a full elemental mass spectrum; for such measurements, an auxiliary electronmultiplying detector is used that requires scanning a magnetic field.7,8 An improvement to these instruments would be to utilize an array of independent detectors, with minimal dead space between pairs, to span the entire mass range. This arrangement would avoid scanning altogether, but for such an array to be accepted, its figures of merit must be comparable to those of a scanning instrument with a single multiplying detector. Detector arrays have been used in the past. Two such devices are the photographic plate and the electrooptic imaging detector. Although such devices offer the advantages of simultaneous detection, they suffer from several limitations when compared to single-channel detectors. These disadvantages include limited dynamic range, low sensitivity, lengthy analysis procedures, multiple conversion steps, and nonuniform response across the arrays.9,10 These limitations restricted the use of early array detectors in everyday mass spectrometry, leaving the door open for the development of an array detector that is capable of higher performance. Two generations of such an array detector have been developed and characterized by coupling them to an ICP MattauchHerzog mass spectrograph (MHMS). This mass spectrograph geometry is ideal for use with an array detector because it continuously and simultaneously focuses all m/z values onto a flat focal plane. The array detector devices, based on Faradaystrip detection, have been termed focal plane cameras (FPC) because of their ability to collect an image of the ion distribution along the focal plane of the double-focusing mass spectrograph. The first-generation detector has been characterized and discussed extensively in previous publications.3,11-15 It was composed of 32 individual ion collectors and will therefore be referred to as FPC32 in the following discussion. The present paper will focus on the figures of merit of the second-generation detector, which has 128 channels and will be denoted FPC-128. With respect to limits of detection, these array detectors have been shown to be comparable to a single-channel secondary electron multiplier (7) Barshick, C. M., Duckworth, D. C., Smith, D. H., Eds. Inorganic Mass Spectrometry: Fundamentals and Applications; Marcel Dekker: New York, 2000; Vol. 23, p 512. (8) Wieser, M. E.; Schwieters, J. B. Int. J. Mass Spectrom. 2005, 242 (2-3), 97-115. (9) Koppenaal, D. W.; Barinaga, C. J.; Denton, M. B.; Sperline, R. P.; Hieftje, G. M.; Schilling, G. D.; Andrade, F. J.; Barnes, J. H. I. V. Anal. Chem. 2005, 77 (21), 418A-427A. (10) Barnes, J. H.; Hieftje, G. M. Int. J. Mass Spectrom. 2004, 238 (1), 33-46. (11) Barnes, J. H. I. V.; Sperline, R.; Denton, M. B.; Barinaga, C. J.; Koppenaal, D.; Young, E. T.; Hieftje, G. M. Anal. Chem. 2002, 74 (20), 5327-5332. (12) Knight, A. K.; Sperline, R. P.; Hieftje, G. M.; Young, E.; Barinaga, C. J.; Koppenaal, D. W.; Denton, M. B. Int. J. Mass Spectrom. 2002, 215 (1-3), 131-139. (13) Barnes, J. H. I. V.; 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 (6), 769-776. (14) Barnes, J. H. I. V.; 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 (6), 751-756. (15) Barnes, J. H. I. V.; 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 (2), 227-234.
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(SEM). In addition, isotope ratio precision has been greatly improved due to simultaneous multimass detection. EXPERIMENTAL SECTION Focal Plane Camera. The FPC devices operate by accumulating charges that are collected on gold Faraday strips. Each Faraday collector uses a capacitive trans-impedance amplifier (CTIA) to integrate the accumulated charges. The value from each CTIA is then channeled through an Altera Cyclone (Altera Corp., San Jose, CA) field-programmable gate array (FPGA) device to a LabVIEW PCI-6534 digital I/O acquisition card (National Instruments Corp., Austin, TX) installed in a Dell Optiplex GX260 personal computer (Dell, Round Rock, TX). The individual channels are selected by a low-noise multiplexer. A photograph of the circuit board and Faraday collectors is shown in Figure 1a and a diagram of the CTIA-multiplexer circuit in Figure 1b. The Altera FPGA is used to address each CTIA to set its gain, to command both the independent and global resetting of the CTIAs, and to command the sampling and transmission of voltage values from the CTIA. Digital-to-analog conversion is performed on the FPGA circuit board, examined by the programmed logic in the FPGA, and transmitted to the digital interface card by the FPGA. The FPGA is programmed to detect incipient overflows of individual CTIAs, reset each prior to overflow, and record the event, all at speeds exceeding those possible by means of the personal computer. The gain of a CTIA is inversely proportional to the capacitance in the feedback loop of the operational amplifier. Unlike the FPC32 unit, the FPC-128 camera has two user-selectable gain settings that are chosen by connecting or disconnecting a second capacitor (see Figure 1b). The capacitances of the two capacitors differ by a factor of ∼100, so gains of × (low gain) and 100× (high gain) are achievable for each collector channel independently and under computer control. In addition to the second gain level, the FPC128 device also has a faster acquisition rate than the FPC-32 camera. Specifically, the FPC-128 model can acquire data from all collectors in less than 500 µs. Future FPC detectors will feature multiple and automatically selectable gain levels for maximum analytical utility. The FPC-128 device was also improved over the FPC-32 array in many other respects, as is shown in Table 1. The FPC-128 unit utilizes 45-µm-wide collectors set on 50-µm centers, thereby providing higher spatial resolution, less intercollector dead space, and correspondingly better spectral definition. Unlike the FPC32 camera, the length of each collector in the FPC-128 unit was increased to span the height of the magnetic sector and the entrance slit of the MHMS, enabling more of the ion beam to be collected. However, because the width of each collector was reduced, the effective ion collection area of a single Faraday strip and the total array length are only slightly larger for the FPC-128 than for the FPC-32 system. Operation of the FPC-128 camera relies on two user-defined parameters to set the integration period. These parameters are set by means of a custom-written LabVIEW (National Instruments) program interfaced with the detector through the aforementioned LabVIEW data acquisition card and FPGA. The first parameter, the integration period, is defined by the number of nondestructive readouts (NDRO) and the second parameter is the time interval between each pair of readouts, where a
Figure 1. (a) Photograph and (b) circuit diagram of the FPC-128 array detector system. The photograph shows the exposed electronics on the surface of the detector board. The circuit diagram shows how two levels of gain in the FPC-128 camera are achieved.
Table 1. Comparison of FPC-32 and FPC-128 Cameras
number of collectors collector width collector active height collector active area intercollector spacing length of combined collector array
FPC-32
FPC-128
32 145 µm 1.6 mm 0.232 mm2 30 µm 5.4 mm
128 45 µm 6.35 mm 0.286 mm2 5 µm 6.4 mm
NDRO is the reading of the accumulated charge without clearing it from the capacitor. The output file contains the value of each NDRO; therefore, the ion flux on each collector can be individually and independently accessed and registered. Upon completion of all NDROs, any desired collector can be reset, by shorting the capacitors to ground, thereby removing all
charge. The LabVIEW software enables several repetitions of this cycle to be automatically made. The shortest integration period using all channels of the FPC-128 is roughly 500 µs, but shorter integrations can be taken if fewer than all 128 channels are used. To reduce the noise associated with the detector, a Peltier cooler was placed on the backside of the ceramic carrier containing the collector chip. The heat from the “hot” side of the Peltier cooler was carried out of the vacuum chamber by means of a heat pipe. The heat pipe was cooled either by a heat sink and fan or by water cooling via copper tubing coiled 13 times around the pipe. A temperature sensor was not employed to directly read the temperature of the collector chip, but experiments were completed to ensure the effectiveness of the cooling system. Analytical Chemistry, Vol. 78, No. 13, July 1, 2006
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Table 2. Operating Conditions of the ICP-MHMS-FPC-128 Sample Introduction System sample uptake rate 1 mL/min USNa chamber temperature 160 °C USN condenser temperature 5-10 °C Inductively Coupled Plasma Source forward power 1.25 kW reflected power
