d, v, w and ammonium ions and additional sequence-specific fragment ions associated with particular amino acid residues. Oligosaccharides Oligosaccharides may be sequenced using tandem mass spectrometry in a similar manner to peptide sequencing. Fragmentation generally occurs on either side of the glycosidic bond (b, c, y and z ions) but also under more energetic conditions through the sugar ring structure in a cross-ring cleavage (x ions). Again trailing subscripts are used to indicate position of the cleavage along the chain. For cross ring cleavage ions the nature of the cross ring cleavage is indicated by preceding superscripts. Oligonucleotides Tandem mass spectrometry has been applied to DNA and RNA sequencing. A notation for gas-phase fragmentation of oligonucleotide ions has been proposed. Newborn screening Newborn screening is the process of testing newborn babies for treatable genetic, endocrinologic, metabolic and hematologic diseases. The development of tandem mass spectrometry screening in the early 1990s led to a large expansion of potentially detectable congenital metabolic diseases that affect blood levels of organic acids. Limitation Tandem mass spectrometry cannot be applied for single-cell analyses as it is insensitive to analyze such small amounts of a cell. Theses limitations are primarily due to a combination of inefficient ion production and ion losses within the instruments due to chemical noise sources of solvents. Future outlook Tandem mass spectrometry will be a useful tool for protein characterization, nucleoprotein complexes, and other biological structures. However, some challenges left such as analyzing the characterization of the proteome quantitatively and qualitatively. See also Accelerator mass spectrometry Cross section (physics) Mass-analyzed ion-kinetic-energy spectrometry Unimolecular ion decomposition References Bibliography External links An Introduction to Mass Spectrometry by Dr Alison E. Ashcroft