Thin layer chromatography/mass spectrometry: The advantages of tandem mass spectrometry

Abstract

The results of thin layer chromatography/mass spectrometry (TLCIMS) and thin layer chromatography/tandem mass spectrometry (TLC/MS/MS) experiments are described. Spectra from the analytes were obtained directly from the adsorbent without the need for extraction prior to analysis by mass spectrometry. These results illustrate the added specificity that the tandem mass spectrometer can afford in the detection and characterization of a range of compounds separated by TLC.Of the commonly used chromatographic methods, high resolution gas chromatography (HRGC) and high performance liquid chromatography (HPLC) are probably the two most powerful separative techniques currently available to the analytical chemist. Both have successfully been interfaced to mass spectrometers.The combination of gas chromatography with mass spectrometry (GUMS) is a well established and extremely versatile technique where interfacing presents few problems. The low pressure and volume of gas involved make it a relatively simple task to interface G C to a high-vacuum technique such as mass spectrometry. However, its use is generally limited to thermally stable, volatile molecules of molecular weight less than about 900 Da. Conversely, HPLC is capable of handling involatile and/or thermally labile molecules, but, because of the large volume of liquids involved, and the use of inorganic buffers, the problems of interfacing HPLC to a mass spectrometer are somewhat more complex than those encountered for GUMS. Also, there is no single universal interface, or ionization process, that can provide structural and molecular weight information for all the compounds amenable to HPLC.There are two other commonly used chromatographic methods that are used in conjunction with mass spectrometry, namely thin layer chromatography (TLC) and capillary direct fluid injection-supercritical fluid chromatography (DFI-SFC). Of the two, the latter has few of the advantages and many of the disadvantages encountered with HRGC and HPLC. Although the use of long columns in DFI-SFC enables a chromatographic resolution approaching that attained by HRGC to be achieved, at low temperatures and with relatively short analysis times,' the technique should be considered as a complementary technique to HRGC and HPLC, not an alternative.Where the resolution of the separation method is high, as in HRGC, HPLC and SFC, identification of * Author to whom correspondence should be addressed. components based on retention times is well established. TLC, however, as currently performed, cannot achieve the separation efficiency obtained with HRGC, HPLC and SFC. Consequently, the identification of components separated by TLC based solely on retardation factor (R,) values and UV or fluorescence properties will always leave a degree of uncertainty about the true identity of the compounds present.2 Positive identification has to be made by subjecting the analyte to spectroscopic analysis by infra red (IR), nuclear magnetic resonance (NMR) and mass spectrometry (MS). Of the three techniques, MS i...

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