Identify compounds
1. Inject a blank sample into the GC, equipped with an FID and sniffing port, under the
analytical conditions chosen for the aroma sample.
Most odorants will elute between 700 and 1600 n-paraffin retention indices from non-polar
columns like the DB5.
For aroma extracts, the blank sample is a mixture of the solvents used in the extraction,
and are concentrated in the same way as the aroma isolate. Some volatiles in aroma extracts
may derive from trace impurities of the solvents. For headspace techniques, a blank run is
also recommended to check impurities coming from the tubings and/or adsorbents used.
2. Inject the aroma sample (0.5
µl per solvent extract) in the on-column or splitless mode
and simultaneously start the GC sniffing.
For both on-column and splitless injections of solvent extracts the initial temperature of
the GC oven is held slightly above the boiling point of the solvent for 1 to 2 min. After very
rapid heating to 50
°C to 60°C the heating rate usually is 4°C or 6°C per minute up to the
final temperature of 220
°C to 240°C for DBWAX columns and up to 250° to 280°C for
DB5 capillary columns.
3. Inject the mixture of 10 to 100
µg/ml n-alkanes.
The injection mode (e.g., headspace or liquid injection) should be the same as for the aroma
sample.
4. Determine the retention time of the alkanes and of the peaks to be identified in the
flavor sample and calculate their retention indices according to the following equation
(Van den Dool and Kratz, 1963):
RI = 100
× n(C) + 100 × [T
(a)
- Tn
(CS)
/Tn
(CL)
- T
(a)
]
where n(C) is the number of carbons; T
(a)
is the retention time of the compound of
interest; Tn
(CS)
is the retention time of smaller hydrocarbons, i.e., eluting before the
compound of interest; and Tn
(CL)
is the retention time of larger hydrocarbon, i.e.,
eluting after the compound of interest.
This is shown in Figure G1.3.1.
5. Compare RI indices and aroma quality as assessed during GC sniffing (Acree et al.,
1984; Ullrich and Grosch, 1987) with those in the literature (Kondjoyan and Berda-
gué, 1996), and the Flavornet (http://www.nysaes.cornell.edu/flavornet).
This will give a list of possible candidates matching the RI and sniffing criteria.
6. Repeat steps 1 to 5 on a column of different polarity.
A DBWAX and DB5 column are recommended because most literature data is available
on these phases and they represent a polar column (DBWAX) and an apolar column (DB5),
which separate compounds differently.
7. Inject the aroma sample on the same types of columns (i.e., DBWAX and DB5) using
an MS detector.
Focus identification on the odor-active regions as determined by GC-sniffing and RI values
as published in the Flavornet (http://www.nysaes.cornell.edu/flavornet). The fragmenta-
tion pattern obtained for the compounds can be compared with those in data banks like the
Wiley/NBS Registry of Mass Spectral data (McLafferty and Stauffer, 2000) or the NIST 98
library (National Institute of Science and Technology).
8. Inject reference compound.
An aroma compound is unambiguously identified only when aroma quality, the GC
retention indices, and the MS spectra are the same as for the reference compound.
Current Protocols in Food Analytical Chemistry
G1.3.2
Identification and
Quantification of
Aroma
Compounds