Anal. Chem. 2001, 73, 2605-2612
Accurate Mass Liquid Chromatography/Mass Spectrometry on Quadrupole Orthogonal Acceleration Time-of-Flight Mass Analyzers Using Switching between Separate Sample and Reference Sprays. 2. Applications Using the Dual-Electrospray Ion Source Jean-Claude Wolff,*,† Christine Eckers,† Ashley B. Sage,‡ Kevin Giles,‡ and Robert Bateman‡
GlaxoSmithKline, New Frontiers Science Park North, Third Avenue, Harlow, Essex CM19 5AW, U.K., and Micromass UK Ltd., Floats Road, Wythenshawe, Manchester, M23 9LZ, U.K.
A new electrospray dual sprayer, LockSpray, was developed for accurate mass measurements on a quadrupole orthogonal acceleration time-of-flight mass spectrometer (oa-Q-ToF). With the dual-sprayer ion source, both sprays are orthogonal to each other. A mechanism similar to the one employed on the multiplexed electrospray source (MUX) allows switching between reference and sample sprayer. The reference sprayer is optimized for low flow rates, whereas the sample sprayer is a conventional Z-spray type sprayer. Earlier work using a modified MUX ion source on an orthogonal acceleration time-of-flight instrument showed promising results. In this paper, examples obtained with the LockSpray, specifically designed for accurate mass measurements on an oa-Q-ToF, are presented. The examples include results obtained for the identification of impurities in drug substances such as cimetidine and rosiglitazone, using accurate mass tandem mass spectrometry in both positive and negative ion electrospray modes. Good mass accuracies, i.e., within 2 mDa of the theoretical value, were obtained in MS and MS/MS operation. The use of liquid chromatography/electrospray mass spectrometry (LC/ESI-MS) combined with accurate mass measurement on a (quadrupole) orthogonal acceleration time-of-flight mass spectrometer (oa-Q-ToF) is becoming an established method to help in the structural identification of unknown compounds in the pharmaceutical industry.1-10 To routinely get good accurate mass measurement using these techniques, it is necessary to also have * Corresponding author: (fax) +44-(0)1279-627655; (e-mail)
[email protected]. † GlaxoSmithKline. ‡ Micromass UK Ltd.. (1) Cottee, F.; Haskins, N.; Bryant, D.; Eckers, C.; Monte´, S. Eur. Mass Spectrom. 2000, 6, 219-224. (2) Eckers, C.; Haskins, N.; Langridge, J. Rapid Commun. Mass Spectrom. 1997, 11, 1916-1922. (3) Lee, M. J.; Monte´, S.; Sanderson, J.; Haskins, N. J. Rapid Commun. Mass Spectrom. 1999, 13, 216-221. 10.1021/ac001419a CCC: $20.00 Published on Web 05/05/2001
© 2001 American Chemical Society
reference material introduced into the mass spectrometer. Methodologies by which this can be achieved with minimal disruption to the chromatographic separation, chromatographic integrity, and sensitivity of the determinations are being investigated. The most appropriate method seems to use multiple sprayer ion sources. Several such systems have been described.11-15 The multiple sprayer technique is not only applied in conjunction with timeof-flight but also with magnetic sector mass spectrometers.16,17 In previous work, the use of a modified multiplexed electrospray (MUX)18 ion source on an orthogonal acceleration time-offlight instrument (LCT) showed promising results.19 In this paper, (4) Jonsson, A. P.; Carlquist, M.; Husman, B.; Ljunggren, J.; Jornvall, H.; Bergman, T.; Griffiths, W. J. Rapid Commun. Mass Spectrom. 1999, 13, 1782-1791. (5) Hopfgartner, G.; Chernushevich, I. V.; Covey, T.; Plomley, J. B.; Bonner, R. J. Am. Soc. Mass Spectrom. 1999, 10, 1305-1314. (6) Zhang, H.; Henion, J.; Yang, Y.; Spooner, N. Anal. Chem. 2000, 72, 33423348. (7) Michelsen, P.; Karlsson A. A. Rapid Commun. Mass Spectrom. 1999, 13, 2146-2150. (8) Zhang, H.; Heinig, K.; Henion, J. J. Mass Spectrom. 2000, 35, 423-431. (9) Zhang, N.; Fountain, S. T.; Bi, H.; Rossi, D. T. Anal. Chem. 2000, 72, 800806. (10) Hoogenboom, A. C.; Niessen, W. M. A.; Little, D.; Brinkman, U. A. Th. Rapid Commun. Mass Spectrom. 1999, 13, 125-133. (11) Eckers, C.; Wolff, J.-C.; Haskins, N.; Bateman, R.; Hoyes, J.; Preece, S. Presented at the 47th ASMS Conference on Mass Spectrometry and Allied Topics, Dallas, TX, June 13-18, 1999. (12) Dresch, T.; Keefe, T.; Park, M. Proceedings of the 47th ASMS Conference on Mass Spectrometry and Allied Topics, Dallas, TX, June 13-18, 1999; pp 18651866. (13) Andrien, B. A.; Whitehouse, C.; Sansone, M. A. Proceedings of the 46th ASMS Conference on Mass Spectrometry and Allied Topics, Orlando, FL, May 31June 4, 1998; p 889. (14) Whitehouse, C.; Gulcicek, E.; Andrien, B.; Shen, S. Proceedings of the 47th ASMS Conference on Mass Spectrometry and Allied Topics, Dallas, TX, June 13-18, 1999; pp 454-455. (15) Jiang, L.; Moini, M. Anal. Chem. 2000, 72, 20-24. (16) Takahashi, Y.; Fujimaki, S.; Kobayashi, T.; Morita, T.; Higuchi, T. Rapid Commun. Mass Spectrom. 2000, 14, 947-949. (17) Bordoli, R. S.; Woolfitt, A. R. and Bateman, R. H. Proceedings of the 43rd ASMS Conference on Mass Spectrometry and Allied Topics, Atlanta, GA, May 21-26, 1995; p 98. (18) De Biasi, V.; Haskins, N.; Organ, A.; Bateman, R.; Giles, K.; Jarvis, S. Rapid Commun. Mass Spectrom. 1999, 13, 1165-1168.
Analytical Chemistry, Vol. 73, No. 11, June 1, 2001 2605
Figure 1. Picture of the LockSpray ion source, showing sample and reference sprayer orthogonal to each other and to the sampling cone.
we present the results obtained using a new electrospray dual sprayer, LockSpray, which was developed specifically for obtaining accurate mass measurements on either a LCT or an oa-Q-ToF. Both sprays are orthogonal to each other on the dual sprayer. A mechanism similar to the one employed on the MUX source allows switching between reference and sample sprayers. The sample sprayer is a conventional Z-spray type, whereas the reference sprayer does not have desolvation gas and is suitable for providing a low flow rate of the reference material. As opposed to the proof-of-concept work,19 tandem mass spectrometry (MS/MS) measurements were carried out on the oa-Q-ToF in this study. The reference sprayer acts as the source of the reference lock mass for both MS and MS/MS experiments. The LockSpray allows exact mass measurement of MS/MS spectra using an external reference material, as opposed to previously reported work (e.g., refs 2 and 5), which has used either the precursor ion or fragment ions of known structure, within the spectrum itself, to provide the reference for mass measurement. The examples shown here include results obtained for the identification of impurities in drug substance such as cimetidine (I) and structure elucidation work on rosiglitazone (II). These results are obtained using accurate mass tandem mass spectrometry in both positive and negative ion electrospray modes. EXPERIMENTAL SECTION Reagents, Calibrants, and Compounds Investigated. All reagents and calibrants were obtained from Sigma-Aldrich (SigmaAldrich Co. Ltd., Poole, Dorset, U.K.), unless otherwise stated. Solvents were from BDH (Poole, Dorset, U.K.). The drug substances cimetidine (N-cyano-N′-methyl-N′′-[2-[[(5-methyl-1H(19) Eckers, C.; Wolff, J.-C.; Haskins, N. J.; Sage, A. B.; Giles, K.; Bateman, R. Anal. Chem. 2000, 72, 3683-3688.
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imidazol-4-yl)methyl]thio]ethyl]guanidine, C10H16N6S (I)) and rosi-
glitazone [5-[4-[2-(N-methyl-N-(2-pyridyl)amino]ethoxy]benzyl]thiazolidine-2,4-dione, C18H19N3O3S (II)) were dissolved in 0.05 M ammonium acetate, adjusted to pH 5 with acetic acid, and acetonitrile in a proportion of 70:30 volume fraction. Solutions of 5 mg of cimetidine /mL and 0.2 mg of rosiglitazone/mL were prepared. Dual Electrospray Design and Operation. The ion source has two separate electrosprays, one for the sample stream and one for the reference stream, that are positioned around the sampling cone at 90° to each other (Figure 1). Sample eluent is introduced to the ion source via the standard electrospray probe of the Z-spray assembly, while the reference solution is introduced via the low flow rate secondary sprayer. Both sprayers are held at the same ionization voltage and have an equal flow of nitrogen nebulizer gas. The reference sprayer, as this is designed for low flow rates (
