Identification of Phosphatidylcholine Isomers in Imaging Mass Spectrometry Using Gas-Phase Charge Inversion Ion/Ion Reactions

Gas-phase ion/ion reactions have been enabled on a commercial dual source, hybrid QhFT-ICR mass spectrometer for use during imaging mass spectrometry experiments. These reactions allow for the transformation of the ion type most readily generated from the tissue surface to an ion type that gives improved chemical structural information upon tandem mass spectrometry (MS/MS) without manipulating the tissue sample. This process is demonstrated via the charge inversion reaction of phosphatidylcholine (PC) lipid cations generated from rat brain tissue via matrix-assisted laser desorption/ionization (MALDI) with 1,4-phenylenedipropionic acid (PDPA) reagent dianions generated via electrospray ionization (ESI). Collision-induced dissociation (CID) of the resulting demethylated PC product anions allows for the determination of the lipid fatty acyl tail identities and positions, which is not possible via CID of the precursor lipid cations. The abundance of lipid isomers revealed by this workflow is found to vary significantly in different regions of the brain. As each isoform may have a unique cellular function, these results underscore the importance of accurately separating and identifying the many isobaric and isomeric lipids and metabolites that can complicate image interpretation and spectral analysis.

Absorption mode Fourier transform mass spectrometry with no baseline correction using a novel asymmetric apodization function

RATIONALE

Absorption mode Fourier transform ion cyclotron resonance (FTICR) mass spectra offer significant benefits in terms of spectral resolution, signal-to-noise (S/N) ratio and measured mass accuracy. However, to date, methods for producing absorption mode spectra have created an undesirable baseline deviation as a consequence of FFT artifacts, resulting in interference of the frequency side-lobes of intense peaks. Methods for fitting and removing this deviation have been developed, but these are computationally intensive, slow and can be unreliable in practice.

METHODS

We have developed an approach for producing FTICR mass spectra which uses a new apodization approach to produce spectra which do not exhibit baseline deviation, whilst maintaining all the normal absorption mode benefits. This method involves the use of 'full' apodization function, replacing the more common Hann or half Hann functions, and where the user can control the position of the function maximum expressed as a fraction (F) of the transient length.

RESULTS

Absorption mode spectra produced using the new apodization function we propose provide all the normal benefits but do not exhibit baseline deviation that must be corrected prior to spectral interpretation. Additionally, varying the value of the F parameter allows users additional control over the compromise between the spectral resolving power and the S/N ratio. This is particularly beneficial in spectra with pronounced amplitude changes during the recording of the transient (detection).

CONCLUSIONS

The use of a 'full' apodization function, which may be asymmetric, prior to zero-padding and Fourier transformation, allows the production of absorption mode spectra which do not suffer from baseline deviation. Hence, it is no longer necessary to apply a baseline deviation correction in post processing, providing a significant performance advantage.

Producing Isotopic Distribution Models for Fully Apodized Absorption Mode FT-MS

Isotopic distributions are frequently used as part of the peak assignment process in the processing of mass spectra. The best methods for producing accurate peak assignments must account for the peak shape and resolving power. In other words, the full profile of the isotopic distribution is important. Conventional methods for modeling isotopic distributions generally assume a peak profile that is not applicable to fully apodized absorption mode spectra because the peak shapes in these spectra are distinctly different from those seen in normal (i.e., magnitude mode) spectra. We present results illustrating this problem and describe a method for producing more accurate isotopic distribution models for this class of spectra.