Sample Introduction after Planar Separation

Although analysis by planar chromatographic techniques and mass spectrometry has been practised for many years, there remain three principal methods of sample introduction.

The first method involves the analyte of interest being recovered from the adsorbent by elution with an appropriate solvent and introduced into the mass spectrometer's ion source as a discrete sample. After the area of interest of the chromatogram has been identified, usually by visual means, the perimeter of the area of interest, the 'spot', is marked with a soft-leaded pencil. The adsorbent is loosened from the backing with a flat-bladed tool such as a spindle screwdriver or microspatula. The loosened material is removed from the plate and mixed with a small quantity of solvent. Ultrasonification can assist in the extraction process. A small plug of quartz wool packed into a drawn-out Pasteur pipette (Figure 1) acts as a filter medium. The filtered adsorbent is washed with one aliquot of solvent.

Depending upon the method of ionization to be used, the extracted material may be introduced into the ion source by using the direct insertion probe for the ionization of volatile anlaytes by EI/CI; or by a desorption probe or cassette, such as for analysis by

Figure 1 Analyte recovery by solvent elution.

FAB, LSIMS or MALDI (matrix-assisted laser desorption ionization), for the ionization of polar, nonvolatile and/or thermally labile analytes. This method of sample introduction is particularly useful when the sample application methods of multispotting or streaking methods are used to obtain sufficient analyte for multi-MS-MS experiments.

In the second method the analyte and adsorbent are not separated. Both are introduced into the ion source simultaneously. After visual location, the spot is scraped off the plate and introduced into the ion source by one of the methods described above.

Both of the above methods destroy the integrity of the chromatogram. A third method of introduction uses a dedicated probe that enables the whole chromatogram to be introduced into the FAB/LSIMS ion source, so that a spatially resolved total ion current chromatogram of the organic material on the adsorbent may be obtained. With this type of probe it is necessary to use aluminium-backed plates. A strip approximately 10 mm x 65 mm, which is essentially one track of a developed plate, is cut out and mounted on the end of the probe. An appropriate matrix is painted down the length of the strip and the chromatogram is driven at a constant rate, with a maximum of 50 mm min"1, by a stepping motor. The pulses to the stepping motor are controlled in conjunction with the scan control of the mass spectrometer, with the result that the whole chromato-gram is scanned by the FAB/LSIMS beam. Mass spectral information is acquired for the whole chrom-

atogram. The analysis is analogous to the column chromatography experiment in that the analytes are ionized sequentially throughout the chromatogram.

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