Anal. Chem. 2005, 77, 3461-3465
Archiving and Absolute Quantitation of Solutes Separated by Single Charged Droplet Coulomb Explosion Samuel F. W. Bakhoum,† Michael J. Bogan,† and George R. Agnes*
Department of Chemistry, Simon Fraser University, 8888 University Drive, Burnaby, BC, V5A1S6, Canada
The electrospray (ES) ion source relies on the transfer of low-volatility solutes to the gas phase as an outcome of coulomb explosion events of charged droplets generated by electrical atomization. Introduced here are two methods for archiving compounds separated by coulomb explosion of single droplets having net charge that had been levitated in an electrodynamic balance. We categorized compounds separated by the explosion as either material ejected, including progeny droplets, or the material retained in the main residue. The potential for this methodology is illustrated by (i) qualitative characterization of solute states, aqueous versus precipitated in the separated material, and (ii) absolute quantitation of solutes separated by such an event. For a droplet containing 5 × 107 20-nm-diameter fluospheres, its first encountered coulomb explosion event resulted in the ejection of 1.70% of them. The capability to acquire such detailed information regarding the individual steps in the process of transferring low-volatility solutes to the gas phase in an ES ion source is essential to develop strategies for absolute quantitation in applications of ES mass spectrometry. In the process of generating gas-phase ions in an electrospray, the first step is the electrical atomization of the solution to form an aerosol consisting of unipolar charged droplets and it is known to vary with changes in the physical properties of the liquid solution.1 The fact that the ion signal intensity of low-volatility solutes as measured by a mass spectrometer varies as a function of the solution’s physical parameters as well, indicates that there is a cascade effect on the subsequent steps leading to gas-phase ion production.2 It has also been documented that variation of a solute’s concentration in the micromolar range affects the ion signal intensity of other solutes due to concentration and surface activity driven solute partitioning within droplets with net charge in the period of time leading up to3,4 droplet coulomb explosion.5-8 * To whom correspondence should be addressed. E-mail:
[email protected]. Ph: 604 291-4387. Fax: 604 291-3765. † Authors contributed equally to this work. (1) Zeleny, J. Phys. Rev. 1914, 3, 69-91. (2) Tang, K.; Page, J. S.; Smith, R. D. J. Am. Soc. Mass Spectrom. 2004, 15, 1416-1423. (3) Enke, C. G. Anal. Chem. 1997, 69, 4885-4893. (4) Haddrell, A. E.; Agnes, G. R. Anal. Chem. 2004, 76, 53-61. (5) Rayleigh, L. Philos. Mag. 1882, 14, 184-186. (6) Doyle, A.; Moffett, D. R.; Vonnegut, B. J. Colloid Sci. 1964, 19, 136-143. (7) Taflin, D. C.; Ward, T. L.; Davis, E. J. Langmuir 1989, 5, 376-384. 10.1021/ac048113e CCC: $30.25 Published on Web 05/04/2005
© 2005 American Chemical Society
The dynamic nature of gas-phase ion production in response to changes in solution composition has led to widespread application of the method of internal standards to achieve relative quantitation by electrospray (ES)-MS.9,10 To enable absolute quantitation of all solutes in demanding applications such as speciation,11 determination of formation constants, metabolic profiling, and protein expression,12-14 quantitative understanding of the dynamic behavior of each step in the path leading to gas-phase ion production in the ES process is needed. To that end, droplet coulomb explosion, a key step in the ES process that leads to the production of gas-phase ions, has been characterized by other investigators with respect to the physical outcome of such an event. For instance, the explosion has been measured to partition mass and net charge unequally; 15-25% of the droplet’s net charge is released along with
