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Internal Calibration on Adjacent Samples (Incas) with Fourier Transform Mass Spectrometry

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Internal Calibration on Adjacent Samples (Incas) with Fourier Transform Mass Spectrometry Anal. Chem. 2000, 72, 5881-5885 Internal Calibration on Adjacent Samples (InCAS) with Fourier Transform Mass Spectrometry Peter B. O'Connor* and Catherine E. Costello Mass Spectrometry Resource, Department of Biochemistry, Boston University School of Medicine, 715 Albany Street, R806 Boston, Massachusetts 02118 Using matrix-assisted laser desorption/ionization (MAL- ability to isolate and fragment selected ions for MSn, and high DI) on a trapped ion mass spectrometer such as a Fourier mass accuracy and resolution. This last property, high mass transform mass spectrometer (FTMS) allows accumula- resolution/accuracy, is limited primarily by the ion abundance- tion of ions in the cell from multiple laser shots prior to dependent space charge frequency shifts that are commonly in detection. If ions from separate MALDI samples are the ∼10-100 ppm range depending on severity, although space accumulated simultaneously in the cell, ions from one charge errors can usually be minimized by using a calibration sample can be used to calibrate ions from the other spectrum with total ion abundance similar to the analyte spectrum. sample. Since the ions are detected simultaneously in the Several interesting new methods have been proposed to correct cell, this is, in effect, internal calibration, but there are these frequency shift parameters including Easterling's linear no selective desorption effects in the MALDI source. This frequency adjustment13 and Bruce's DeCal method.14 These method of internal calibration with adjacent samples is methods propose to correct the frequencies of the ions by a demonstrated here on cesium iodide clusters, peptides, known, constant space charge factor applied before mass calibra- oligosaccharides, poly(propylene glycol), and fullerenes tion and show significant promise in this regard. The alternative and provides typical FTMS internal calibration mass is internal calibration. accuracy of <1 ppm. Internal calibration is common throughout mass spectrometry and consists of mixing several compounds of known molecular Matrix-assisted laser desorption/ionization (MALDI)1-4 has not weight with the analyte prior to mass analysis and then using the only become a routine ionization mode in mass spectrometry known masses to calculate the unknown ones. The improvement ∼ laboratories but it also enables use of a crucial method for in mass accuracy is significant (usually 10-fold) and very reliable; biochemical research. The ability to accurately measure molecular however, generating a mass spectrum of the mixture is sometimes weights of a wide variety of large biomolecules including pro- problematic as ionization efficiencies vary significantly among teins,2,5,6 oligosaccharides,7,8 and oligonucleotides4 up to several standards and maintaining signal parity between the calibrant and the analyte can be difficult. million daltons in molecular weight has revolutionized the field. When MALDI mass spectrometry is performed on a time-of- Also, the continuing evolution of methods to perform tandem mass flight instrument, it is common to place a spot containing a spectrometry9 allows biochemists to obtain more information than calibrant sample on the target near to the spot having the analyte ever before from their samples. of interest. Then, one calibrates the mass spectrometer im- Fourier transform mass spectrometry (FTMS) incorporating mediately prior to acquiring the spectrum of the analyte, thus a MALDI ion source allows some unique advantages such as the minimizing the calibration shifts due to slow temperature changes ability to store and accumulate ions10,11 in the Penning trap,12 the in the length of the flight tube, surface aberrations in the MALDI target such as pits or scratches, or field effects that vary with * Corresponding author: (phone) (617) 638-6705; (fax) (617) 638-6761; (e-mail) [email protected]. sample position on the target that can cause mass errors in the (1) Karas, M. I.; Bachmann, D.; Bahr, U.; Hillenkamp, F. Int. J. Mass Spectrom. 0.1% range. This method is, however, an external mass calibration - Ion Processes 1987, 78,53 68. and is subject to the attendant shot-to-shot mass accuracy shifts. (2) Hillenkamp, F.; Karas, M.; Beavis, R. C.; Chait, B. T. Anal. Chem. 1991, 63, 1193A-1203A. However, the same method can be used in an FTMS, wherein (3) Karas, M.; Bahr, U.; Giessmann, U. Mass Spectrom. Rev. 1991, 10, 335- the ionization and detection processes are almost completely 357. decoupled, and the ions from the calibrant and the analyte spots (4) Berkenkamp, S.; Kirpekar, F.; Hillenkamp, F. Science 1998, 281, 260- 262. can be accumulated together in the Penning trap prior to analysis. (5) Castoro, J. A.; Wilkins, C. L. Anal. Chem. 1993, 65, 2621-2627. As the trapped ions are simultaneously analyzed, any space charge - (6) Fenselau, C. Anal. Chem. 1997, 69, 9, A661 A665. shifts or other drifts in the mass calibration are canceled by doing (7) Okamoto, M.; Takahashi, K.; Doi, T.; Takimoto, Y. Anal. Chem. 1997, 69, 2919-2926. internal calibration. This technique is shown here and in a related (8) Harvey, D. J. Mass Spectrom. Rev. 1999, 18, 349-450. (9) McLafferty, F. W. Tandem Mass Spectrometry; Wiley: New York, 1983. (13) Easterling, M. L.; Mize, T. H.; Amster, I. J. Anal. Chem. 1999, 71, 624- (10) O'Connor, P. B.; Costello, C. E. Anal. Chem. 2000, 72, 5125-5130. 632. (11) Mize, T.; Amster, J. Anal. Chem., submitted. (14) Bruce, J. E.; Anderson, G. A.; Brands, M. D.; Pasa-Tolic, L.; Smith, R. D. J. (12) Penning, F. M. Physica 1936, 3, 873-894. Am. Soc. Mass Spectrom. 2000, 11, 416-421. 10.1021/ac000770t CCC: $19.00 © 2000 American Chemical Society Analytical Chemistry, Vol. 72, No. 24, December 15, 2000 5881 Published on Web 11/08/2000 paper11 with a wide range of samples selected to be extreme cases ground potential to the trapping voltage of 20 V after a delay of of differences in ionization efficiencies. The samples range from approximately 500-1000 µs from the laser pulse varied according cesium iodide clusters, fullerenes, and poly(propylene glycol) to to the mass range expected. A gas pulse is then injected into the oligosaccharides and peptides. Mass accuracy of 0.1-1 ppm is mass spectrometer through a pulse valve system (pressure in the typical. analyzer stage of the vacuum system rises to ∼1 × 10-6 Torr for 2 s) for cooling the ions to the center of the cell. The ions are EXPERIMENTAL SECTION then held in the cell for 5-10 s while the pressure pumps back ∼ × -10 These experiments were performed using an IonSpec HiRes- down to a base pressure of 1 10 Torr prior to excitation MALDI Fourier transform mass spectrometer with a 7-T active (frequency sweep from 100 to 2000 Da in 4 ms at 130 V amplitude) shielded superconducting electromagnet. This instrument has a and detection (acquisition rate of 2 MHz into 512k 12-bit data differentially pumped vacuum system with three turbomolecular points). The analyzer tube pressures are measured with an ion ∼ pumps (Pfeiffer TMU 260), analog electronics (24 DAC voltages, gauge positioned 50 cm from the cell and above the turbopump s 16 TTL triggers, rf quadrupole circuit, high-power excitation flange so that these pressures are only approximate to the actual amplifier, mixer/preamplifier for detection), dual pulsed valve inlet pressures in the cell. The signal is then apodized (quarter-sine), system, two ionization guages (Granville Phillips model 274 zerofilled, and magnitude mode Fast-Fourier transformed. Peak Bayard-Alpert type), quadrupole ion guide, and capacitively picking is done by a standard method, and the peaks are coupled closed cylindrical cell (similar to the design by Beu and centroided by fitting the upper half of the peak to a second-order Laude,15 but with additional external trapping plates). The ion polynomial and finding the point of zero gradient (note that this requires a minimum of three points on the top half of the peak). source is described below. The system shares the magnet, data The sample spots are deposited on the facets of the probe by system, and computer with a modified IonSpec HiResESI mass carefully depositing ∼1 µL of saturated 6-azo-2-thiothymine (ATT) spectrometer. The magnet is a Cryomagnetics 7-T active shielded matrix in methanol in a thick layer. The layer of matrix is created superconducting magnet with a 5-in. warm bore; the 5-G line is by using an Eppendorf gel-loader tip (Brinkmann Instruments, ∼10 cm from the face of the magnet. The data system consists of Inc. Westbury, NY) dipped into the matrix solution so that a small a 40-MHz frequency synthesizer, an Arbitrary waveform generator quantity of the matrix enters the tip by capillary action and then (40 MHz, 1 megapoint), voltage sequence programmer (for spotting this droplet onto the probe. This is repeated 20-50 times controlling DACs as a function of time through the pulse until a completely white layer of matrix is created. Then, 1 µLof sequence), pulse sequence generator (1 µs timing resolution), and analyte solution, typically dissolved at 10 µM in water with 0.1% transient digitizer (2 megapoints in acquisition and accumulation trifluoroacetic acid, is dropped onto the surface and allowed to buffers). The computer is a Pentium III 600 computer with 128- dry. The cesium iodide spots were created using a similar MB RAM, 8-GB hard drive. The instrument control software is technique; however, no matrix was used and only a saturated IonSpec's Explorer version 6.72 running in MS-DOS. solution of CsI in water was pipetted onto the probe. The cesium The MALDI mass spectrometer uses an IonSpec 10-faceted iodide laser desorption/ionization (LDI) spectra required ∼30% sample probe, a 377-nm N laser (Laser Sciences, Franklin, MA) 2 more laser power than the MALDI spectra of peptides or with a motor-driven beam steering mirror (Newport Optics, Irvine, oligosaccharides.
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