Application
Note: 10072
GC analyses of fats and oils, waxes and
surfactants: a demanding task for the injection
system
Introduction
Key Words
• Fats and Oils
• Waxes
• Surfactants
• Cold On-column
Injector
Analysis of fats, oils, waxes and surfactants is still one of
the most demanding application, which sets important
requirements to inlet design and represents a severe test
for modern, automated gas chromatograph. Some classes
of compounds, such as triglycerides, sterol esters,
poliethoxylated alcohols, heavy hydrocarbons and heavy
waxes, due to their boiling points, require an inlet system
capable to properly transfer a representative portion of the
sample to the analytical column thus avoiding
discrimination effects. Selection of the appropriate injector
design is therefore mandatory to produce quality and
reliable results. This short note will provide an overview
on some aspects of the injection requirements and will
demonstrate the applicability of this instrumentation to
the analysis of some important compounds in fats and
oils, waxes and surfactants.
What is the ideal injection system?
Cold on-column injector has been proposed since early
days as a method of choice for the injection of heavy
compounds into capillary columns. The reason was that
from the very beginning this injection system has been
recognised as the only one able to completely avoid
molecular weight discrimination and degradation in
comparison with other injection systems, especially the
classical split-splitless injector.
PTV injection systems were proposed as an
alternative, considering mainly that automation was easier.
Additionally PTV injector was considered to be less
sensitive to the dirt, in many cases present in the injected
sample. However, to cope with the requirements of this
application, PTV injectors must offer heating capability, in
terms of heating rate and maximum operating
temperature, not often found in instruments on the
market. The new BEST PTV injector, available with the
TRACE GC 2000, offers such level of performance. It is
therefore the ideal complement to the cold on-column
injector, for those high boiling compounds in dirty
matrices.
Figure 1.
Experimental
The standard instrument is a TRACE GC 2000 with AS
2000 autosampler and data system as shown in figure 1.
Results of analyses performed with a PTV injector
were compared to those obtained with a Cold on-column
equipped with a HOT (High Oven Temperature) device.
This is a slightly modified version of the standard cold oncolumn injector, having the advantage of allowing direct
introduction of the sample into the capillary column or
precolumn maintained at low temperature, even if the
oven temperature is kept very high.
When applicable, like in the case of the analysis of
compounds of relatively low volatility in clean matrices,
this configuration offers all the advantages of the classical
cold on-column injection, but drastically reduces the
analysis time because the oven temperature operates
almost isothermally or with limited temperature
excursion. Another important advantage is that the
residence time of compounds like triglycerides and sterol
esters, that can undergo partial degradation at high oven
temperatures, is dramatically reduced.
Additionally, the injection system eliminates
completely any sample loss due to discrimination and/or
thermal degradation, thus providing error free
quantitation.
Figure 2 shows in detail the Cold on-column injector
with the HOT device, and the injector zone Vs. oven
temperature correspondence.
The advantages delivered by this injection system
combined with the ability of the TRACE GC oven to be
heated and cooled very fast, allow this analysis to be
performed with high efficiency and speed.
Figure 3: PTV injector with Autosampler
Applications
Figure 2: Allowable HOT Cold On-Column injection temperatures Vs oven
temperatures using compressed air as cooling fluid
A TRACE GC 2000 equipped with the BEST (Brightly
Enhanced Sample Transfer) PTV injector and a FID
detector, is the alternative configuration used for this
application. The BEST PTV is a Programmable
Temperature Vaporising injector with some special
characteristics permitting an improved sample transfer
into the capillary column. A relatively long (12.5 cm)
vaporizing chamber, made of a “silcosteeled” liner (a
stainless steel liner with Restek deactivation procedure),
with an internal diameter of 1 mm improves the inertness
and the heat transfer to the injected sample. This allows
injection in splitless or split mode with no need of glass
wool. On top of this, the very low thermal mass
vaporizing chamber permits to achieve unprecedented
heating rates, 15°C/s, providing efficient sample
vaporization even for samples containing large quantities
of high boiling compounds.
Injections have been made manually or automatically
using an autosampler (AS 2000), suitable for both, on a
system shown in figure 3.
FATS and OILS, analysis by high temperature capillary GC
The most important analytes determined by this technique
are the triglycerides and the esters of some compounds
normally found in the unsaponified fraction of fats and
oils (sterols, triterpenic alcohols, tocopherol esters and
triterpenic dialcohols).
Many of these compounds can be analysed using
different techniques like SFC, HPLC or HT-GC. The latter
is the best considering the resolution power that it can
provide in the shortest time.
Triglycerides
Fig. 4 shows the analysis of a mixture of Hydrogenated
palm Kernel oil 75%+ Hydrogenated soybeans oil 25% in
iso-Octane performed with the HOT OC and PTV in
splitless mode with an empty liner.
Column:
Carrier gas:
Detector:
H.O.T COC %
BEST PTV %
0.73
4.21
5.89
16.02
12.18
7.33
7.16
5.29
4.28
5.01
3.22
9.14
18.80
0.5
0.19
0.70
4.14
5.76
16.07
12.29
7.22
7.05
5.23
4.23
5.00
3.00
9.35
19.03
0.57
0.32
Sterol esters
Fig. 5 shows the analysis of an extract of olive oil, injected
using the HOT OC. This sample includes some
compounds, such as heavy esters of sterols that in
presence of catalytic sites and at relatively high
temperature can be degraded in steroidal hydrocarbons.
The complete absence of steroidal hydrocarbons in the
chromatogram is a further confirmation of the high
inertness of this technique.
The chromatogram in figure 6 shows that results
obtained with the PTV injector in splitless mode with
empty liner are comparable with the HOT on-column
injector. In particular no products from thermal catalytic
reactions appear to be present.
B)
Oven progr:
C30
C32
C34
C36
C38
C40
C42
C44
C46
C48
C50
C52
C54
C56
C58
Table 1.
A)
Sample:
COMPOSITION
Hydrogenated palm Kernel oil 75% +
Hydrogenated soya beans oil 25%
in iso-Octane
260ºC (1 min) up to 310ºC at 10ºC/min; up to
360ºC (5 min) at 5ºC/min
10m x 0.32 mm ID SE52. 0.15 µm film thinkness
4ml/min (constant flow)
FID
Figure 4: Comparison between HOT On-column and PTV injections for fast
analysis of Triglycerides:
A) HOT On-column
B) BEST PTV
The two chromatograms are very similar and no
relevant discrimination and degradation is noticed in the
PTV (see Table 1). Even in splitless mode, the compounds
are transferred correctly into the column, thanks to the
absence of glass wool, the good deactivation of the liner
and the fast heating ramp. Moreover the very fast ovenheating rate provided by the TRACE GC 2000 permits to
complete the analysis in just thirteen minutes.
Figure 5: Analysis of extract of olive oil using a HOT on-column injection
1. Alcohol C 21 IS
2. Alcohol C 22
3. Alcohol C 24
4. Squalene
5. Alcohol C 26
6. a Colestanol IS
7. a Tocoferol
8. Alcohol C 28
9. Campesterol
10. Stigmasterol
11. b Sitosterol
12. D 5 Avenasterol
21. Wax ester C 44
22. Wax ester C 46
13. Cycloartenol
14. 24-Methyl cycloartenol
15. Citrostadienol
16. Wax ester C 35 IS
17. Wax ester C 36
18. Wax ester C 38
19. Wax ester C 40
20. Wax ester C 42
23. Colesteril epta decanoate IS
24. Campesterol oleate
25. b Sitosterol oleate
26. Cyclo artenol oleate
27. 24-Methylen cycloartenol oleate
28. Citrostadienol oleate
In addition to these
offices, Thermo Electron
Corporation maintains
a network of representative organizations
throughout the world.
Figure 6: Analysis of extract of olive oil using PTV
Conclusions
The analysis of all the classes of compounds examined in
this paper can be efficiently performed using both HOT
on-column injector and PTV.
Cold on-column injector with the HOT device is
suggested in case of clean matrices. It offers exceptionally
good results avoiding problems due to discrimination and
degradation of the high molecular weight and
thermolabiles compounds. Additionally the analysis time
is shorter.
The PTV injector is the most suitable solution in case
of dirty matrices. Its capability to preserve sample
integrity and to provide complete sample transfer has
produce comparable results with those obtained with the
Cold on-column injector with the HOT device. Finally,
the complete range on injection systems available with
Trace GC 2000 can provide the correct solution in any
case, covering both clean and dirty samples, with a wide
range of molecular weight compounds.
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AN10072_E 12/04C