An internal standard compound, 2-methyl-3-methanone, n-alkane ([C.sub.8]-[C.sub.20]) and authentic compounds including acetaldehyde, trans-2-decenal, 2-methylpropanal, 2-methylbutanal, 3-methylbutanal, pentanal, hexanal, heptanal, (E), 2-heptenal, 2-methylthiazole, benzaldehyde, octanal, benzeneacetaldehyde, nonanal, decanal, (E,E), 2,4-decadienal, ethanol, 1-propanol, 2-furanmethanol, 1-hexanol, 1-octen-3-ol, fufural, tetradecanal, 1-octanol, acetone,
2-propanone, 2,3-butanedione, 2-butanone, 2-heptanone, 3-octanone, dimethyldisulfide, pyrazine, methylpyrazine, 2,3-dimethylpyrazine etc.