There is probably no common consensus when a sample should
be considered complex. For many analysts, the combined presence
of compounds belonging to entirely different substance groups
(e.g., proteins, carbohydrates, plant secondary compounds, and exogenous
compounds) may present one dimension of complexity.
The other dimension may be that some analytes are only present
at trace levels, while the concentration of others may be in the high
g/kg range. Things become even more complex when one analyte
represents a trace compound in one sample, but features as a major
ingredient in the next sample.