CHM 3430L Cal Poly Pomona Chemistry Non volatile GC Sample Treatment Lab

Butter Flavored Popcorn on Rt-2560
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18
20
Time (min)
GC_FF1272
1.
2.
3.
4.
5.
6.
7.
Peaks
Methyl myristate
Methyl palmitate
Methyl stearate
Methyl oleate
Methyl vaccenate
Methyl linoleate
Methyl linolenate
Column
Sample
Diluent:
Injection
Inj. Vol.:
Liner:
Inj. Temp.:
Oven
Oven Temp.:
Carrier Gas
Flow Rate:
Detector
Instrument
Notes
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tR (min)
9.256
12.178
15.755
17.001
17.116
18.852
21.193
Structural
Nomenclature
C14:0
C16:0
C18:0
C18:1 cis-9
C18:1 cis-11
C18:2 cis-9,12
C18:3 cis-9,12,15
Rt-2560, 100 m, 0.25 mm ID, 0.20 µm (cat.# 13198)
Butter flavored popcorn
Hexane
1 µL split (split ratio 100:1)
Topaz 4 mm ID straight inlet liner w/wool (cat.# 23300)
250 °C
160 °C to 250 °C at 2 °C/min (hold 5 min)
H2, constant flow
2 mL/min
FID @ 250 °C
Agilent 7890A GC
The sample was prepared using sodium methoxide.
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CHM 3430L Unit #3
Non-volatile GC Sample Treatment
One of the most important requirements for GC analyses is that samples for GC analyses
must be volatile. Not all samples, such as fats and organometallic compounds, however, are
volatile. Derivatization, which is the process of chemically modifying a species to produce a new
species which has properties more suitable for analysis, is needed for these non-volatile species.
Section A. Transesterification of Fatty Acids
Fats, known as “lipids” in technical term, constitute a major class of biological substances
that include fatty acids, their naturally occurring compounds, and other substances related to them
chemically. The simple lipids are esters of various long-chain fatty acids with glycerol. Lipids
are the fundamental constituents of all biological cell membranes and provide a major portion of
our caloric intake. Consequently, their accurate characterization is of considerable importance to
several branches of science and industry.
The most widely accepted method for the GC analysis of non-volatile lipids involves their
conversion to the volatile methyl esters of the fatty acids, followed by gas chromatographic
separation and identification on the basis of their relative retentions under specific
chromatographic conditions.
Question 1. The most widely accepted method for converting non-volatile lipids is through
conversion reaction which produces volatile methyl esters, shown below. This reaction is called
____________.
Question 2. The reaction vessel contains excess lipids, methanol, sodium methoxide, produced
methyl esters, and glycerol. An aqueous extraction must be performed, following the above
reaction. Why?
Question 3. The aqueous extraction will produce two immiscible layers. Why is this happening?
Question 4. Which layer, organic or aqueous, should be injected into the GC?
Question 5. In this experiment, the content of myristic (C14:0), palmitic (C16:0), stearic (C18:0),
oleic (C18:1), linoleic (C18:2), and linolenic (C18:3) in a sample of a fat (unknown) will be
determined. What do the numbers in each parenthesis stand for?
Wet Experiment Procedure
1. Check out the prepared set of standard solutions of each ester (~10 mg/mL; from the
stockroom). Carefully read the labels on the bottle and do NOT mix the standards.
2. Inject 0.5 µL of each fatty acid methyl ester (FAME) standard (including methyl myristate,
methyl palmitate, methyl stearate, methyl oleate, methyl linoleate, and methyl linolenate;
each ~10 mg/mL) by the air sandwich technique to establish its retention time for
identification. To save time, run two standards simultaneously. Draw 1 µL of air, 0.5 µL
of FAME standard #1, 1 µL of air and 0.5 µL of FAME standard #2 into the syringe.
3. Obtain your unknown sample and weigh out ̴ 25 mg (1 drop) of it into a screw-capped vial.
4. Perform the transesterification: pipet in 1 mL of methyl tertiary-butyl ether (MTBE) and
add 100 µL of a 25% sodium methoxide solution in methanol. Cap the vial and shake it
occasionally for one or two minutes, then allow it to set for five minutes. Perform the
extraction: add about 1 ml of water to the vial and shake, then allow the top ether layer to
separate. Centrifuge if layer separation is slow.
5. Carefully withdraw enough of the upper layer with a plastic eyedropper and transfer it to
a 1 mL screw-capped vial. Be careful to withdraw only the upper layer.
6. Inject exactly 1.0 µL of the extract by the air sandwich technique and start the GC run.
Unit 3 Assignment 1
Please see Canvas Module 3 L3P1 Assignment for details.
Section 2. Organometallic Compounds
With the rapid growth in industry, more and more organometallic compounds are found
in the environment we all live. The toxicity and mobility that these compounds have raise a great
concern to environmental chemists. The most important and abundant organometallic
compounds that are present in the environment are organomercury, organolead and organotin
species. Although, classically, those compounds are analyzed via inductively coupled plasma
(ICP) MS or atomic absorption spectroscopy, GC–MS is a powerful tool for organometallic
compound analysis. Similar to fatty acid analysis, organometallics must be derivatized to form
volatile or semi-volatile compounds via ethylation or phenylation. Solid phase microextraction
(SPME) is often used to facilitate the extraction and injection of organometallic derivatives.
Question 1. A small volume of volatile compound (in the liquid form) is pipetted in a glass vial
and capped. What is going to happen next?
Question 2. Sometimes it is necessary to sample the vapor to avoid the impurities in the sample.
A SPME needle is thus developed. As indicated on the diagram of the needle, which part is
responsible for sample extraction?
Question 3. What determines the amount of analyte being absorbed on the fiber?
Question 4. Should a thin coating or thick coating be used on the fused-silica fiber?
Question 5. How can you enhance the extraction efficiency of analyte from the liquid?
Question 6. The figure below indicates the extraction and injection of sample through SPME.
Following the extraction of analyte onto the fiber in the headspace, the fiber inside the SPME needle
is introduced into the injector port. The high temperature at the injection port quickly remove the
analytes from the coating. GC separation of the analytes is hence started. If a trace amount of
organometallic (