Direct infusion Mass Spectrometry

Perhaps the simplest form of mass spectrometry involves pumping the sample to be analysed directly into the mass spectrometer in a process termed direct infusion mass spectrometry. Typically, the liquid sample is contained in a syringe and a syringe pump is used to deliver a regular flow of liquid.

Direct infusion is suitable for use with samples that are pure or that are simple mixtures composed of only a small number of constituents. Crucially, the sample must also be free of contaminating factors that might interfere with mass spectrometric measurements, such as high levels of non-volatile salts/buffers and detergents.

Direct infusion is compatible with mass spectrometers equipped with electrospray ionisation, and we can acquire either intact molecular masses of analytes, or fragment them and measure the mass of the fragments to allow 'fingerprinting' of chemicals that aid identification.

Typical uses of direct infusion mass spectrometry are for confirmation of the identity of small chemicals and biomolecules, for measuring the mass of small proteins or for analysing macromolecular complexes, which can be preserved when analysed by the mass spectrometer in this way.

Direct infusion mass spectrometry involves use of the integrated syringe pumps attached to our mass spectrometers to deliver samples at flow rates of 200 microlitres per min or less, straight into the electrospray source of the instrument. We have instruments in which ionisation is by electrospray or by nanospray, allowing less sample to be used.

After ionisation, the ions are accelerated into the main body of the mass spectrometer where they are separated according to their mass-to-charge ratio. Separation occurs because ions having different mass-to-charge ratios respond differently to electric fields.

We can use mass spectrometers for direct infusion work that separate ions by use of quadrupole mass filter or by Orbitrap technology. Both instruments allow fragment ions to be generated, by collision with molecules of an inert gas, followed by measurement of the mass of the fragments.

Direct infusion mass spectrometry is useful for mass measurements of pure analytes, such as products of chemical reactions or purified pharmaceutical compounds.

Any samples that have previously been purified by chromatographic methods tend to be suitable and useful for analysis.

Some biomolecules, such as nucleic acids or carbohydrates, benefit from on-line LC-MS workflows are do not lend themselves to direct infusion MS.

Direct infusion tyically requires concentrations of analytes in the low micromolar range. Solutions should not contain non-volatile salts/buffers or detergents. Upwards of 50 microlitre volumes are preferred, although we can attempt sometimes measure mass spectra with lower volumes than this.