The new technique, described in an ASAP article in Analytical Chemistry, uses a silicon wafer plate covered with germanium nanodots (GeNDs) as the ionisation platform for laser desorption/ionisation (LDI) mass spectrometry (MS).

This removes the background matrix fragment signals that can mask low molecular weight analyte peaks during protein sequence or identification experiments.

MALDI-MS (matrix assisted laser desorption/ionisation) is widely used in the field of biological analysis, with companies providing mass imaging instruments for biomarker discovery and for use in pharmaceutical drug discovery applications.

The technique is ideal for such applications as it is a soft ionisation method that allows fragile biomolecules such as proteins, peptides and sugars to be analysed without the fragmentation that can occur using standard ionisation techniques.

However, the use of laser desorption/ionisation (LDI) has been somewhat hampered by the need to use a matrix that can provide protons to the analyte to encourage ionisation.

The use of an organic matrix has several disadvantages such as matrix deposition and the fact that strong background peaks often originate from the matrix which interfere with low mass sample ions.

While techniques using non-volatile inorganic matrixes such as buckyballs, carbon nanotubes and various metal oxide particles have been described, their use has not been widely adopted due to their low sensitivity and reduced versatility as well as the risk of laser irradiation scattering the particles inside the ionisation chamber.

The researchers, led by Dr Hiroaki Sato, of the Research Institute for Environmental Technology, AIST, Japan, used molecular beam epitaxy (MBE) to create the germanium nanodots with diameters between 150nm and 200nm and a height of about 50nm on silicon wafers that could then be mounted on a standard MALDI plate using electroconductive adhesive tape.

If silicon plates without the nanodots were spotted with the peptide angiotensin-I, only background peaks were observed.

However, when wafers with the nanodots were used, the molecular ion peak was observed with almost no fragmentation and a detection limit of 800 amols.

Only three digests were observed in the peptide mass fingerprint (PMF) analysis of bovine serum albumen (BSA) digests using a CHCA (alpha-cyano-4-hydroxycinnamic acid) matrix; a considerable amount of matrix cluster peaks sequence coverage to 34 per cent.

The use of the new GeND-MS technique enabled sequence coverage to be increased to 47 per cent with the observed signals free from interfering peaks - allowing the distinct observation of many digests.

The researchers have so far achieved an upper detectable mass limit of 16952 Daltons which equates to the molecular ion peak for myoglobin, as larger molecules are hard to ionize using matrix-free LDI-MS (laser desorption ionisation-MS).

The ionisation technique was also of use in MS/MS protein identification studies, with the researchers concluding that: "the GeND chip has considerable potential for use as a basal platform for soft desorption/ionisation techniques".