Agriculture Reference
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via robotics (if a fully automated option is available), and (iv) the elimination of
maintenance of the liner and column because contamination by nonvolatiles does not
occur as much as it does with liquid injections. However, several factors in
uence the
quality of generated data. Speci
cally, the pro
le of volatiles obtained is dependent on
the type, thickness, and length of the SPME
fiber used, as well as on the incubation and
extraction time, temperature, and addition of salts [27]. In addition, the relative
concentration of analytes in the headspace does not re
ect the relative concentrations
in the sample because of the differences in volatility of compounds. Taking into account
all these factors, it is essential to analyze the samples under well-de
ned and constant
conditions [84].
After desorption in a hot GC injection port and separation by a GC column,
isolated volatiles are introduced in the ion source of a mass spectrometer, where they
are fragmented, typically using EI at 70 eV. The fragments of all the volatile
compounds are recorded as the abundance of each ion of different mass-to-charge
ratios ( m / z ) (Figure 2.5).
Up to now, HS (SPME)-MS e-nose has been applied mainly in food authenticity
studies. In most cases, the intensities of particular fragments ( m / z ) are subsequently
submitted to multivariate data analysis for statistical evaluation. In general, equili-
bration times (HS-MS) and incubation and extraction times (HS-SPME-MS) ranged
between 10 and 60 min, followed by desorption and acquisition (3
-
10min) of MS
fingerprints of isolated volatile compounds.
Vera et al. conducted a study to classify and characterize a series of beers according
to their production site and chemical composition [85]. The analyzed beer samples
were of the same brand but obtained from four different factories. The results obtained
in this study enable consideration of the HS-MS (e-nose) as a potential aroma sensor
because it is capable of discriminating and characterizing the samples according to
their predominant aromas with the help of multivariate analysis.
Mildner-Szkudlarz and Jelen demonstrated the potential of HS-MS (e-nose) as a
rapid tool for volatile compounds analysis with subsequent multivariate data analysis
(PCA) for differentiation between EVOO samples adulterated with hazelnut oil [86].
This method allowed detection of olive oil adulteration with different contents of
hazelnut oil ranging from 5 to 50% (v/v). Figure 2.6 shows average spectrum of
hazelnut oil, pure EVOO, and EVOO with 5 and 50% (v/v) of adulteration obtained
using HS-SPME-MS technique. Several groups of ions can be observed that grouped
around ion m / z 43, 55, 70, and 83 for EVOO and m / z 43, 60, 74, and 96 for hazelnut
oil. Changes in speci
c ion intensities in the HS-SPME-MS spectrum could be very
cautiously correlated with the changes of particular components in pure and adulter-
ated oils detected using HS-SPME-GC-MS.
HS (SPME)-MS e-nose has also been used to characterize and identify cheeses [87]
and the country of origin of tempranillo wines [88], to study off-
avors in milk [89],
and to detect unwanted fungal growth in bakery products [90].
2.3.3 Ambient Desorption/Ionization-Mass Spectrometry
The introduction of ambient desorption/ionization methods enabled a great simplifi-
-
cation and an increase in speed of MS-based measurements. Unlike the conventional
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