Lipid analysis by thin-layer chromatography-A review of the current state

The number and type of atmospheric pressure techniques suitable for sampling analytes from surfaces, forming ions from these analytes, and subsequently transporting these ions into vacuum for interrogation by MS have rapidly expanded over the last several years. Moreover, the literature in this area is complicated by an explosion in acronyms for these techniques, many of which provide no information relating to the chemical or physical processes involved. In this tutorial article, we sort this vast array of techniques into relatively few categories on the basis of the approaches used for surface sampling and ionization. For each technique, we explain, as best known, many of the underlying principles of operation, describe representative applications, and in some cases, discuss needed research or advancements and attempt to forecast their future analytical utility.

Established and emerging atmospheric pressure surface sampling/ionization techniques for mass spectrometry

Graphite-assisted laser desorption/ionization (GALDI) mass spectrometry (MS) was investigated for analysis of cerebrosides in a complex total brain lipid extract. Conventional MALDI MS and GALDI MS were compared regarding lipid analysis by using high-vacuum (HV, <10-6 Torr) LDI time-of-flight mass spectrometry and intermediate-pressure (IP, 0.17 Torr) linear ion trap mass spectrometry. Cerebrosides were not detected or detected with low sensitivity in MALDI MS because of other dominant phospholipids. By using GALDI, cerebrosides were detected as intense mass peaks without prior separation from other lipid species while mass peaks corresponding to phosphatidylcholines (PCs) were weak. The signal increase for cerebrosides and the signal decrease for PCs in GALDI MS were more significant in HV than in IP. MSn experiments of precursor ions corresponding to cerebrosides and PCs in brain lipid extract were performed to identify the detected species and distinguish isobaric ions. Twenty-two cerebroside species w...

Colloidal graphite-assisted laser desorption/ionization mass spectrometry and MSn of small molecules. 1. Imaging of cerebrosides directly from rat brain tissue.

Due to a high background in the low-mass region, conventional MALDI is not as useful for detecting small molecules (molecular masses <500 Da) as it is for large ones. Also, spatial inhomogeneity that is inherent to crystalline matrixes can degrade resolution in imaging mass spectrometry (IMS). In this study, colloidal graphite was investigated as an alternative matrix for laser desorption/ionization (GALDI) in IMS. We demonstrate its advantages over conventional MALDI in the detection of small molecules such as organic acids, flavonoids, and oligosaccharides. GALDI provides good sensitivity for such small molecules. The detection limit of fatty acids and flavonoids in the negative-ion mode are in the low-femtomole range. Molecules were detected directly and identified by comparing the MS and MS/MS spectra with those of standards. Various fruits were chosen to evaluate the practical utility of GALDI since many types of small molecules are present in them. Distribution of these small molecules in the fruit was investigated by using IMS and IMS/MS.

Colloidal Graphite-Assisted Laser Desorption/Ionization MS and MSn of Small Molecules. 2. Direct Profiling and MS Imaging of Small Metabolites from Fruits

Coupling of planar chromatography to mass spectrometry (MS) and especially ambient MS is a relatively new field of great interest. The direct sample access at ambient conditions and the feasibility to obtain mass spectra free of contamination within a minute or even within seconds greatly contributes to the progress of planar chromatography. Targeted recording of mass spectra on zones of interest is performed after evaluation of the chromatogram, thus providing high efficiency. Reported approaches for coupling are divided into elution-based and desorption-based techniques. Devices of both categories are commercially available. As a consequence of increasing importance, a rethink of the terminology of liquid chromatography with MS has to be considered.

Coupling of planar chromatography to mass spectrometry

An interface to allow on-line qualitative and quantitative full-plate detection and analysis of compounds separated by thin-layer chromatography (TLC) is presented. A continuous wave diode laser is employed as a desorption source. Atmospheric pressure chemical ionization mass spectrometry ionizes and subsequently identifies the desorbed sample molecules. Besides direct laser desorption on untreated TLC plates, graphite particles were used as a matrix to couple in the laser power and improve the efficiency of desorption.

A new interface to couple thin-layer chromatography with laser desorption/atmospheric pressure chemical ionization mass spectrometry for plate scanning

The desorption of an analyte by a continuous wave diode laser from a porous surface of a thin-layer plate covered with a graphite suspension is presented. The thermally desorbed analyte molecules are ionized in the gas phase by a corona discharge at atmospheric pressure. Therefore, both essential processes—the desorption and the ionization of analyte molecules, which are often performed in one step—are separated. The target preparation is easy and fast since no additional extraction process is required. The mass spectrometric background signal was mostly limited to the low mass range showing no interference with typical compounds of interest. In this study, the calmative and antihypertensive drug reserpine was chosen as model analyte, which is often used for specification of mass spectrometers. No fragmentation was observed because of efficient collisional cooling under atmospheric pressure. The influence of diode laser power and the composition of the graphite suspension were investigated, and a primary optimization was performed. Copyright © 2004 John Wiley & Sons, Ltd.

Thin-layer chromatography combined with diode laser desorption/atmospheric pressure chemical ionization mass spectrometry.

Continuous wave diode laser is applied for desorption of an analyte from a porous surface of a thin-layer plate covered with a graphite suspension. The thermally desorbed analyte molecules are ionized in the gas phase by a corona discharge at atmospheric pressure. Therefore, both essential processes – the desorption and the ionization of analyte molecules, which are often performed in one step – are separated. Reserpine was chosen as model analyte, which is often used for specification of mass spectrometers. No fragmentation was observed because of efficient collisional cooling under atmospheric pressure. The influence of diode laser power and the composition of the graphite suspension were investigated, and a primary optimization was performed. An interface to allow online qualitative and quantitative full plate detection and analysis of compounds separated by thin-layer chromatography is presented.

Scanning Diode Laser Desorption Thin-Layer Chromatography Coupled with Atmospheric Pressure Chemical Ionization Mass Spectrometry

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A new interface to couple thin-layer chromatography with laser desorption/atmospheric pressure chemical ionization mass spectrometry for plate scanning

Thin-layer chromatography combined with diode laser desorption/atmospheric pressure chemical ionization mass spectrometry.

Scanning Diode Laser Desorption Thin-Layer Chromatography Coupled with Atmospheric Pressure Chemical Ionization Mass Spectrometry