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Abstract No.: TuP-022
Session: Clinical Chemistry Applications
Presentation date: Tue, Aug 29, 2006
Presentation time: 14:30 – 16:00

Simultaneous Steroid Profiling in Biological Samples Using High Field Fourier Transform Mass Spectrometry

Michael L. Easterling1, Pat R. Langridge-Smith2, C. Logan Mackay2, Ruth Andrew3, Jonathan R. Seckl3, Christian B. Berg1, Christopher J. Thompson1, J. Paul Speir1

1 Bruker Daltonics, Inc., Billerica, United States
2 SIRCAMS, School of Chemistry, University of Edinburgh, Edinburgh, United Kingdom
3 Centre for Cardiovascular Science, Queen's Medical Research Institute , Edinburgh , United Kingdom

Correspondence address: Michael L. Easterling, Bruker Daltonics, Inc., FTMS, 40 Manning Road, Billerica, 01821 United States.

Web site: http://www.bdal.com

Keywords: APPI (see Ionization, Atmospheric Pressure Photo-); Fourier Transform ICR; Plasma; Steroids.

Novel aspect: Improved limits of detection for steroid profiling using a high-field hybrid Q-FT-ICR mass spectrometer.

 

Introduction
Glucocorticoid hormones are key regulators of fuel homeostasis, stress and inflammation. They are produced within the adrenal gland but also in tissues from inert hormones by the enzyme 11β-hydroxysteroid dehydrogenase 1. Within a cell, concentrations of inert and active steroid determine tissue-specific responses, and changes in these parameters have been implicated in diseases associated with ageing (cardiovascular, cognitive decline). In the rodent the active hormone is corticosterone, formed in tissues by reduction of the 11-oxo group in 11-dehydrocorticosterone. We report here the use of a 9.4 Tesla hybrid Q-FTMS instrument for the simultaneous detection of glucocorticoids in rat plasma extracts as well as adrenal and liver cytosol.

Methods
Steroids were recovered from rat plasma and cytosol by organic extraction. Steroids were extracted from tissues in acidified ethanol followed by solid-phase extraction. Extracted steroids were reconstituted in methanol and the extracts analysed on a 9.4 Tesla Qq FT-ICR mass spectrometer (Bruker Daltonics, Billerica, MA), using a variety of ionisation techniques (nanoESI, APCI, APPI, MALDI) to establish limits of detection.

Preliminary results
A major advantage of the hybrid Q-FT-ICR mass spectrometer is its ability to selectively isolate and accumulate ions over a prescribed m/z range. This enables the dynamic range of the instrument to be increased by selectively accumulating or enriching ions over a given m/z window. This is particular beneficial for the analysis of low abundant species such as glucocorticoids, in which structural fragmentation data combined with accurate mass measurement is essential for characterisation of modified steroids from direct cell extracts. Because of the high mass resolution coupled with high mass accuracy, it was possible to unequivocally assign spectral peaks to glucocorticoids by elemental mass alone, obviating the need for any prior chromatography. The limits of detection achieved are superior to those obtained in our previous work at lower field using 3.0 T APEX II and 4.7 T APEX III instruments (87 fg for corticosterone, 385 fg for 11-DHC) and enable unambiguous identification of glucocorticoids in plasma extracts and cellular cytosol.

In ESI experiments on mixtures of these two steroids, the ionisation efficiency of corticosterone was observed to be 2.5-3 times higher than that of 11-DHC. Cationisation has been found to be 100 times more efficient than protonation in ESI. For APCI the ionisation efficiency increases slightly with addition of acid (up to 0.2% by volume). However, ionisation efficiency of corticosterone by ESI (cationisation) is at least one order of magnitude greater than the ionisation efficiency observed by APCI. In the presentation, we will compare LODs obtained using ESI-MS, and those from other ionisation techniques (APCI, APPI, MALDI).