Table 2.

Considerations for the analysis of skin volatile organic compounds.

Workflow	Analytical Step	Considerations
Experimental design	Define intended population	Patient selection
		Separate into training and independent validation cohorts
		Define the anatomical location of skin sampling
Sample preparation	Pre-sampling	Avoid personal care products at the location of sampling for a minimum of 24 h
		Consider patient medication and use of xenobiotics
	Skin VOC collection and storage	Direct contact vs. non-contact methods
		Choice of sampling material (cotton/PDMS/SPME)
		Sampling time
		Sources of contamination
		Sample storage (direct analysis or storage at -80 freezer)
	Sampling technique	Consider the use of pre-concentration headspace step or direct injection
		Use of internal standards in patch
		QCs (spiking with internal standards to monitor and correct analytical variability)
MS analysis	Analytical platform	Online or offline platforms
		Optimisation of MS parameters
	MS data collection	Run order (randomisation) and batch effects
Data Analysis	Data pre-processing	Data deconvolution, scaling, peak alignment
		Removal of irreproducible, contaminant compounds
	Statistical analysis	Descriptive statistics
		Univariate analysis
		Multivariate analysis (e.g., PCA, PLS-DA, OPLS-DA)
		Prediction model (e.g., ROC analysis)
	Biological interpretation	Metabolic pathways (KEGG, HMDB)