Create CollectionIdentifiers.md

CollectionIdentifiers.md

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+# USI Collection Identifiers
+
+Universal Spectrum Identifier (USI) for Mass Spectrometry has as its second field a collection identifier.
+The specification itself does not prescribe the complete list of allowable collection identifiers. Rather,
+the list is maintained here at GitHub and is extensible in an open manner without changing the specification.
+
+# Permitted Collection Identifiers:
+
+1. PXDnnnnnn - The preferred collection identifier is of the form PXDnnnnnn, such as PXD017269
+(http://proteomecentral.proteomexchange.org/dataset/PXD017269). The PXDnnnnnn SHOULD be used for all proteomics
+mass spectrometry datasets that have been registered at ProteomeXchange.
+
+2. MSVnnnnnnnnn - A collection identifier for a datasets contained at the MassIVE repsositorym such as MSV000078556. This type of 
+identifier SHOULD only be used when there is not a corresponding PXDnnnnnn identifier for the dataset, as is often the case
+for metabolomics datasets.
+
+3. RPXDnnnnnn - a ProteomeXchange identifier for a reprocessed dataset of the form RPXDnnnnnn, such as RPXD006668
+(http://proteomecentral.proteomexchange.org/dataset/RPXD006668). The RPXDnnnnnn SHOULD ONLY be used when there is no
+corresponding PXDnnnnnn identifier available (as in the case where an RPXDnnnnnn dataset derives from a non-ProteomeXchange
+dataset.) The PXDnnnnnn identifier should always be used when possible.
+
+4. PXLnnnnnn - A ProteomeXchange spectral library identifier of the form PXLnnnnnn. All spectral libraries registered
+through ProteomeCentral are given a PXLnnnnnn identifier. These collections contain representative spectra often aggregated
+from several replicates, rather than original scan events in MS datasets as denoted by PXDnnnnnn identifiers.
+
+There are no other approved collection identifiers at this time.
+