Editing Molecular mass (section)

== Determination ==
=== Mass spectrometry ===
{{main|Mass spectrometry}}
In mass spectrometry, the molecular mass of a small molecule is usually reported as the [[monoisotopic mass]]: that is, the mass of the molecule containing only the most common isotope of each element. This also differs subtly from the molecular mass in that the choice of isotopes is defined and thus is a single specific molecular mass out of the (perhaps many) possibilities. The masses used to compute the monoisotopic molecular mass are found in a table of isotopic masses and are not found in a typical periodic table. The average molecular mass is often used for larger molecules, since molecules with many atoms are often unlikely to be composed exclusively of the most abundant isotope of each element. A theoretical average molecular mass can be calculated using the [[standard atomic weight]]s found in a typical periodic table. The average molecular mass of a very small sample, however, might differ substantially from this since a single sample average is not the same as the average of many geographically distributed samples.

=== Mass photometry ===
Mass photometry (MP) is a rapid, in-solution, label-free method of obtaining the molecular mass of proteins, lipids, sugars and nucleic acids at the single-molecule level. The technique is based on interferometric scattered light microscopy.<ref>Young et al. (2018). Quantitative imaging of single biological macromolecules. Science 360, 423-427. DOI: https://doi.org/10.1126/science.aar5839</ref> Contrast from scattered light by a single binding event at the interface between the protein solution and glass slide is detected and is linearly proportional to the mass of the molecule. This technique can also be used to measure sample homogeneity,<ref>Sonn-Segev, A., Belacic, K., Bodrug, T. et al. Quantifying the heterogeneity of macromolecular machines by mass photometry. Nat Commun 11, 1772 (2020). https://doi.org/10.1038/s41467-020-15642-w </ref> to detect protein [[oligomerisation]] states, and to identify complex macromolecular assemblies ([[ribosomes]], [[GroEL]], [[Adeno-associated virus|AAV]]) and protein interactions such as protein-protein interactions.<ref>Soltermman et al. Quantifying protein-protein interactions by molecular counting using mass photometry.  Angew. Chem Int Ed, 2020, 59(27), 10774-10779</ref> Mass photometry can accurately measure molecular mass over a wide range of molecular masses (40 kDa – 5 MDa).

=== Hydrodynamic methods ===
To a first approximation, the basis for determination of molecular mass according to [[Mark–Houwink equation|Mark–Houwink relations]]<ref>Paul, Hiemenz C., and Lodge P. Timothy. Polymer Chemistry. Second ed. Boca Raton: CRC P, 2007. 336, 338–339.</ref> is the fact that the [[intrinsic viscosity]] of [[Solution (chemistry)|solution]]s (or [[suspension (chemistry)|suspensions]]) of macromolecules depends on volumetric proportion of the dispersed particles in a particular solvent. Specifically, the hydrodynamic size as related to molecular mass depends on a conversion factor, describing the shape of a particular molecule. This allows the apparent molecular mass to be described from a range of techniques sensitive to hydrodynamic effects, including [[dynamic light scattering|DLS]], [[size exclusion chromatography|SEC]] (also known as [[Gel permeation chromatography|GPC]] when the eluent is an organic solvent), [[viscometry]], and diffusion ordered [[nuclear magnetic resonance spectroscopy]] (DOSY).<ref>{{cite journal|author1=Johnson Jr., C. S. |journal=[[Progress in Nuclear Magnetic Resonance Spectroscopy]]|year=1999| volume=34|pages=203–256|title=Diffusion ordered nuclear magnetic resonance spectroscopy: principles and applications |issue=3–4|doi=10.1016/S0079-6565(99)00003-5|bibcode=1999PNMRS..34..203J }}</ref> The apparent [[Hydrodynamic radius|hydrodynamic size]] can then be used to approximate molecular mass using a series of macromolecule-specific standards.<ref>{{cite journal|author1=Neufeld, R. |author2=Stalke, D. |journal=[[Chem. Sci.]]|year=2015| volume=6|pages=3354–3364|title=Accurate Molecular Weight Determination of Small Molecules via DOSY-NMR by Using External Calibration Curves with Normalized Diffusion Coefficients |issue=6 |doi=10.1039/C5SC00670H|doi-access=free|url=http://goedoc.uni-goettingen.de/goescholar/bitstream/handle/1/11975/c5sc00670h.pdf?sequence=1 |pmid=29142693 |pmc=5656982}}</ref> As this requires calibration, it's frequently described as a "relative" molecular mass determination method.

=== Static light scattering ===
It is also possible to determine absolute molecular mass directly from light scattering, traditionally using the [[Zimm plot|Zimm method]]. This can be accomplished either via classical [[static light scattering]] or via [[multi-angle light scattering]] detectors. Molecular masses determined by this method do not require calibration, hence the term "absolute". The only external measurement required is [[Static light scattering#Theory|refractive index increment]], which describes the change in refractive index with concentration.