Accurate and interference-free multiplexed quantitative proteomics using mass spectrometry
Embodiments are directed to a method, a computer readable medium encoded with instructions that, when executed, perform a method, and a system for performing mass spectrometry analysis. Molecules of different samples may be labeled with a chemical tag, allowing a multiplexed analysis of multiple samples. The labeled molecules may be fragmented, each fragmented molecule creating at least two separate ions. The relative abundance of each of the heavier ions, which may comprise the original molecule from the sample, may be measured. A relative abundance of the labeled molecules in each of the samples may be determined from the measured relative abundances of the heavier ions.
1. A method of performing a mass spectrometry analysis on a mixture of a plurality of samples, wherein each of the plurality of samples comprises at least a first type of precursor ion labeled with at least one type of chemical tag selected from a plurality of chemical tags, wherein each of the plurality of samples comprises a plurality of precursor ions of the first type, the method comprising:
fragmenting the labeled precursor ions of the mixture to form a plurality of ions comprising a first subset of ions and a second subset of ions, wherein:
each ion of the first subset of ions comprises at least a portion of the respective chemical tag but not the respective molecule; and
each ion of the second subset of ions comprises at least a portion of the respective chemical tag and the respective molecule;
measuring abundances of ions at a plurality of different mass-to-charge ratios, the mass-to-charge ratios being expected mass-to-charge ratios of the second subset of ions; and
determining a relative mixing ratio of the plurality of samples through an iterative process, said iterative process comprising repeating steps of:
determining an expected isotopic envelope of the labeled precursor ions of the mixture for an initial mixing ratio of the plurality of samples based at least in part on an isotopic envelope of the precursor ions of the first type;
determining expected relative abundances of ions of the second subset of ions based at least in part on the determined expected isotopic envelope of the labeled precursor ions of the mixture for the initial mixture ratio of the plurality of samples;
calculating a difference between the determined expected relative abundances of ions of the second subset of ions and the measured abundances of ions at the plurality of different mass-to-charge ratios; and
selecting a subsequent mixing ratio of the plurality of samples based on the calculated difference.
2. The method of claim 1 , further comprising creating the mixture of the plurality of samples by:
labeling at least one type of molecule of each of a plurality of samples with a respective chemical tag, wherein each of the plurality of samples comprises a plurality of molecules of the at least one type of molecule;
mixing together the labeled molecules from each of the plurality of samples;
ionizing the labeled molecules to form the labeled precursor ions;
injecting the labeled precursor ions into a mass spectrometer; and
selecting the labeled precursor ions for analysis.
3. The method of claim 2 , wherein the at least one type of molecule is selected from the group consisting of a protein, a peptide, a polysaccharide, a lipid, RNA, DNA, and a metabolite.
4. The method of claim 1 , wherein selecting the labeled precursor ions for analysis uses a plurality of discrete windows.
5. The method of claim 2 , wherein ionizing the labeled molecules comprises an act selected from the group consisting of electrospray ionization (ESI), nano-electrospray ionization (nESI), matrix assisted laser desorption ionization (MALDI), atmospheric chemical ionization (APCI), atmospheric photo ionization (APPI), and sonic spray ionization (SSI).
6. The method of claim 1 , wherein fragmenting the labeled precursor ions results in at least one respective tag to break with or without loss of at least one charge.
7. The method of claim 6 wherein manipulating the labeled precursor ions comprises a proton transfer reaction.
8. The method of claim 1 , wherein the isotopic envelope of the precursor ions of the first type indicates expected relative abundances of the precursor ions of the first type due to natural isotopic variation of the precursor ions of the first type.
9. The method of claim 1 , wherein determining the expected isotopic envelope of the labeled precursor ions of the mixture is further based on an isotopic envelope of the at least one type of chemical tag.
10. The method of claim 2 , wherein a duration of time during which ions are injected into the mass spectrometer is determined based upon at least one characteristic of the precursor ions.
11. The method of claim 10 , wherein the at least one characteristic is selected from the group consisting of a charge state of the precursor ions, a mass-to-charge ratio of the precursor ions, an intensity of the precursor ions, or a type of molecule of the precursor ions.
12. The method of claim 1 , wherein determining the mixing ratio of the plurality of samples through the iterative process comprises:
determining relative expected abundances of ions of the second subset of ions for the subsequent mixing ratio of the plurality of samples; and
selecting the subsequent mixing ratio of the plurality of samples as the determined mixing ratio of the plurality of samples based at least in part on the determined expected relative abundances of ions of the second subset of ions for the subsequent mixing ratio of the plurality of samples.
13. The method of claim 1 , wherein determining the expected relative abundances of ions of the second subset of ions is further based on expected mass variations of the first type of precursor ion within the respective sample resulting from multiple fragmentation patterns of the labeled precursor ions.