Table 1. Summary of common spectroscopic assays that may be of use to cancer microbiome research.

Technique	Specific form	Notes
NMR		Most of use for broad profiling of a range of metabolites Higher reproducibility than MS.
MS	Gas chromatography	This is particularly useful for the identification and quantification of volatile metabolites such as short-chain fatty acids
	Liquid chromatography	Can be used in ‘profiling’ (i.e. detection, identification and relative quantification of a particular group of metabolites, e.g. bile acids, lipids, etc), or as a ‘targeted’ technique (i.e. specific identification and accurate quantification of each member of a group of metabolites, e.g. individual amino acids) The instrument can be used in positive or negative ionisation mode, depending upon the specific metabolites of interest May be performed with chromatography step performed in a number of different ways, including Hydrophilic interaction chromatography: Useful for identifying small polar molecules Reverse phase chromatography: Detects a broad range of metabolites, ranging from moderately polar to moderately apolar molecules
	Desorption electrospray ionisation (DESI)	A form of ambient MS, where intact samples, in their native environment, are directly ionised. The instrument used to perform ionisation is directly linked to a mass spectrometer
	Rapid evaporative ionisation MS (REIMS)	A further form of ambulatory MS. A radiofrequency electrical current is applied to a sample via a stainless steel monopolar probe. The vapour generated contains gas phase ions of metabolites and structural lipids. The vapour is channelled to a mass spectrometer attached to the instrument using an incorporated vacuum system