Sample introduction

Samples submitted for analysis can be introduced into the mass spectrometer in many forms. Probably the most common means are (1) direct introduction, (2) gas chromatography (GC), and (3) liquid chromatography (LC).

In direct introduction the sample can be introduced via a sample probe or plate through a vacuum lock, and can subsequently be ionized via EI, CI or matrix-assisted laser desorption ionization (MALDI; see Section 2.4). Alternatively, the sample can be introduced as a liquid stream into an ion source at atmospheric pressure, after which it is subjected to electrospray ionization (ESI; see Section 2.3). Direct injection does not offer any form of sample separation.

In contrast, both gas and liquid chromatography enable the samples of interest to be separated into individual components prior to introduction into the mass spectrometer ion source. Gas chromatography involves sample introduction with the requisite that the sample components must be volatilized prior to separation, and results in a gas sample being introduced to the mass spectrometer (i.e. EI, CI). Figure 5-2 shows the chromatogram obtained after a mixture of three simple phenolic compounds - phenol,

a a 70 so 90 ioo 110 120 wo 140 150

Figure 5-1. Mass spectrum of 2-methoxy 4-vinylphenol.

o-cresol and 2,5-dimethylphenol - was injected in the gas chromatograph. This analysis utilized a 15-m long DB5 capillary column with an initial column temperature of 50°C heated to 320°C at 15°C per minute. The retention time is the time that elapses between injection of the mixture and the elution (detection) of an individual compound and depends on the volatility of the compound combined with the affinity of the compound for the stationary phase inside the capillary column. Liquid chromatography involves a liquid sample being separated into individual components and these components being introduced into a mass spectrometer ion source at atmospheric pressure (i.e. ESI).

It is important to remember the few restrictions imposed by electrospray when considering an LC-MS analysis. Common solvents like methanol, water, acetonitrile and volatile salts (below 25 mM) like ammonium acetate and ammonium bicarbonate are acceptable in the mobile phase, whereas phosphate salts/buffers, mineral acids or other nonvolatile components cannot be used. Unfortunately, this conflicts with many of the routine mobile phases used for the analysis of phenolic compounds and anthocyanins, necessitating changes in methods when going from LC to LC-MS analyses.

retention time

Figure 5-2. Gas chromatogram obtained after injecting a mixture of three phenolic compounds into a gas chromatograph equipped with a 15-m long DB5 column. The temperature was increased from 50 to 320°C at 15°C per minute.

retention time

Figure 5-2. Gas chromatogram obtained after injecting a mixture of three phenolic compounds into a gas chromatograph equipped with a 15-m long DB5 column. The temperature was increased from 50 to 320°C at 15°C per minute.

Many other analytical techniques can be coupled to mass spectrometers. These so-called hyphenated techniques, like GC-MS and LC-MS, include but are not limited to ICP-MS (inductively coupled argon plasma), SCF-MS (supercritical fluid), NMR-MS (nuclear magnetic resonance) and IR-MS (infrared).

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