analysis of a commercial oil by gas chromatography / mass...
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Analysis of a Commercial Oil by Gas Chromatography / Mass Spectrometry
1. Introduction
This experiment differs from most others in this course in three ways. First, its focus is
qualitative analysis and so only the identity of the unknown must be determined instead of
the concentration. Second, it is a union of two techniques, gas chromatography (GC) and
mass spectrometry (MS). This combination creates a 3-dimensional data analysis the gives
us additional useful information. Lastly, the unknown is a real-life sample, cooking oil. As
is the case with many “real-life” samples, the oil is not ready for analysis and must be
processed prior to injection into the instrument. The purpose of this laboratory is to prepare
the sample, analyze it with the GC/MS instrument, and identify the oil and its fatty acid
components.
GC/MS has numerous applications to chemical identification and analysis. It is the “gold
standard” in the field of forensics and has been used for drug detection, fire investigation,
environmental analysis, and explosives testing. GC/MS is also used by airport security to
detect substances in luggage or on human beings. Additionally, it can identify trace elements
in materials that were previously thought to have disintegrated beyond identification.
The GC/MS is composed of two major components: the gas chromatograph and the mass
spectrometer. The GC column separates analytes based on their relative boiling points and
affinity for the stationary phase. The molecules travel through the GC column at different
rates and therefore exit the column at different times. These “retention times” are the first
dimension of the GC/MS data on the x-axis. The analytes remain separated as they enter the
mass spectrometer, where they are ionized and the ions separated based on their mass to
charge (m/z) ratios. The mass spectrum of a species (ion abundance versus m/z) represents
the second and third dimensions of the analysis. See Figure 1 below for an example of the
three dimensional data.1
Figure 1: GC/MS 3-D Data Plot1
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Much research has been performed developing ionization methods for MS. The ionization
method determines the type of samples that can be analyzed by mass spectrometry. Electron
impact (EI) ionization and chemical ionization are used for gases and vapors. The instrument
in this laboratory uses electron impact (EI) ionization.
In an EI source, electrons are produced by heating a wire filament through which electric
current is run. The electrons are accelerated through the ionization space towards an anode;
in the ionization space, they bombard analyte molecules (M) in the gas phase and knock off a
valence electron. This creates a radical cation with an odd number of electrons:
M(g) + e- → M+● + 2e- (1)
The molecular ion (M+●) can also fragment into smaller ions which are present at smaller m/z
in the mass spectrum. Each molecule has a specific fragment spectrum known as its
“fingerprint”.
In this lab, your unknown is a cooking oil sample. Fats and oils belong to a class of
biomolecules called triglycerides. Triglycerides are derived from a glycerol core, with three
fatty acid chains attached via an ester linkage. A triglyceride has the shape of an “E” as
shown in Figure 2 below. The fatty acid chains can be identical, or, as is more common, are
different. Because fats and oils have such low volatilities, they cannot be analyzed directly by
gas chromatography. They must be broken down into more volatile components.
It is possible to decompose triglycerides into glycerol and the conjugate bases of their
constituent fatty acids by heating them with a strong base, such as NaOH. This hydrolysis
process which breaks the ester linkage is called saponification. The general reaction is
shown below.
C
C
C
H
H O
H O
H O
H
C
C
C
O
O
O
R
R'
R"
C
C
C
H
H O
H O
H O
H
H
H
H
+NaOH
CH3OH
C R
O
ONa
C R'
O
ONa
C R"
O
ONa
Triglyceride glycerol conjugate base
Figure 2. Saponification of a triglyceride.
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It is common to analyze fats and oils by performing this saponification reaction then
replacing the sodium with methanol to form three methyl esters of the constituent fatty acids
(Figure 3).
C R
O
ONa
BF3
CH3OHC R
O
OH3C
Conjugate base fatty acid methyl ester (FAME)
Figure 3. Esterification reaction.
These esters then can be extracted and injected onto the GC/MS instrument and the original
structure of the fat or oil determined. This type of analysis is called a fatty acid methyl ester
(FAME) determination.
2. Safety
Protective eyewear must be worn at all times. Perform all steps of the extraction in the fumehood. Boron triluoride is toxic, so handle with caution; the boron triluoride/methanolsolution is lammable. Many internal parts of the GC/MS are very hot or carry dangerousvoltages. Do not open the cover of the GC. Notify the instructor if there is a problem withthe instrument. Discard all hazardous waste as directed in this procedure.
For more general safety in the laboratory, please refer the appendix.
3. Preparation of the unknown:
Obtain two large test tubes containing your unknown oil from your lab instructor. The test
tubes contain a few drops of identical unknown oil. Verify that the unknown ID numbers are
the same and document in your notebook. Retain the corks for each test tube, as you will use
them later.
Saponification and Esterification of the unknown oil. Add 3 mL of methanolic NaOH
solution to each test tube. Be careful not to use too much methanolic NaOH! Place the test
tubes in a beaker with hot water until all the oil dissolves into the solution. You may need to
remove the test tube from the water bath and gently mix the contents by shaking. The
methanol in the test tube should not be boiling vigorously. If it is, then, you may consider
removing the beaker off the hot plate, and possibly removing the test tube from the hot water
bath. Make sure to watch the test tube and not the beaker. Because the boiling point of
methanol is less than that of water; do not let the beaker of water come to a boil – this is too
hot. Do not let your sample dry out.
Obtain the boron trifluoride/methanol solution (which is stored in the refrigerator) from your
instructor. Add 5 mL of boron trifluoride solution to each test tube. Gently boil the test tubes
in the hot water bath for 3 minutes. The boron trifluoride/methanol solution must be kept
refrigerated when not in use. While you are esterifying the fatty acid, return the bottle to
your instructor immediately. After 3 minutes of gentle boiling, remove the test tubes from
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the hot water bath, and allow them to cool to room temperature. If your sample dries out, it is
okay to add a few mL’s of methanol to it. You may then have to briefly heat the test tube to
dissolve the sample back in to solution.
Extraction of fatty acid methyl ester (FAME). After the test tubes have cooled, add 5 mL
of saturated sodium chloride solution and 10 mL of cyclohexane to each test tube. Cork each
test tube and shake well. If the aqueous solution appears cloudy, add a few milliliters of
nanopure water and shake again. The organic and aqueous layers will separate upon
standing.
4. Operation of GC/MS
Note: The screenshots in Figures 4 – 6 are shown to give you a general idea of what the
instrument controller (aka “gameboy”) screen should look like. The specific parameters on
the controller should be verified versus what is written in the text of this procedure, not
versus the picture.
Load the proper method on the GC. Use the control module on the right side of the GC. All
operations on the controller module begin at the Status screen shown below. If the screen
does not appear like shown in Figure 4, press the ESC button until you see the Status screen.
Figure 4. Sample Status Screen on GC/MS
Note: this image is here to give you an idea of the screen.
It may look somewhat different from the real screen
At the Status screen, select the "Method Files" (F3 button), then select GC Methods (F5).
Several different methods will be listed. Press the up or down button to select the method
called "INSTRUMENTAL_LAB", then select Load (F8). If the correct method has already
been loaded, the control module will indicate this.
If the method parameters have been changed, the control module will ask if you want to save
the changes. If the control module prompts you to save the method before loading it, seek
immediate help from your instructor. Once the proper method has been loaded, press Esc
button until you are back to the Status screen.
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At this point, the GC is set up to run. While you should not have to change any parameters
for the GC, you should verify and document the settings of the GC. You can do so by
selecting Settings (F1) from the Status screen.
Figure 5. Sample Oven Settings
Note: this image is here to give you an idea of the screen.
It may look somewhat different from the real screen
The oven (Figure 5) settings should be:
Initial temperature 190°C
Initial time 5 min
Rate 2°C/min
Final temperature 200°C
Final temp time 5 min
Once the oven settings are verified, select the Inlet (Figure 6) settings (F1). They should be
set to the following values:
Inlet temperature 250°C
Split ratio 1:50
Flow 1.20 mL/min
Figure 6. Sample Inlet Settings screen
Note: this image is here to give you an idea of the screen.
It may look somewhat different from the real screen
Finally, check the Detector settings (F4). There is only one value for the Detector setting:
Detector temperature = 280°C
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At the computer start the ChemStation software by double-clicking the "Instrument #1" icon
on the desktop (Figure 7). Do not click the one it says “Instrument #1 Data Analysis”, as it
will not help you collecting data. Once ChemStation starts up, two windows (Instrument
Control and Top) are visible on the computer monitor. The Instrument Control window will
primarily be used.
Figure 7. ChemStation Software
Load the proper method file. Go to "Method" and select "Load". Select
"C:\MSDChem\1\Method\Instrumental_Analysis.M" as the method file.
Before injecting samples on the GC/MS system, you must evaluate the tune used by the mass
spectrometer for that analysis. In the Instrument Control window, go to the "View" menu,
select "Tune and Vacuum Control…" and wait a few seconds.
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Once the Instrument Control window has entered the Tune and Vacuum control mode, go to
the "Tune" menu, and select "Tune Evaluation". Evaluation of the mass spectrometer tune
will start. The instrument will make a clicking sound, and the computer monitor will display
a series of plots. When the instrument is finished performing the Tune Evaluation, the results
will print automatically. Next, check the results. All the checks should read "OK". If any do
not read "OK", talk to your instructor immediately. Retain the tune evaluation along with
your results.
Once tuning is finished, go to the "View" menu and select "Instrument Control". At this
point, a dialog box will read "Be sure tune file is saved. Switch view now?" Click "yes" to
continue.
Once the method has been loaded, go to "Method" and select "Run". A new window will pop
up (Figure 8). Fill the following in each box:
Data Path: D:\Instrumental_analysis\
Data File Name: Enter your name
Operator Name: Enter your name
Figure 8. Start Run Window
When all the information is entered, click the "Run Method" button (not "OK" button). If
you click "OK", you will have to go to "Method" and select "Run" again.
Next, go to the GC, press the "Prep Run" button and wait until the "Not Ready" LED is off
(Figure 9).
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Figure 9. GC Front Panel
Fill the syringe (Figure 10) with the top organic layer which contains the analyte, and expel
excess liquid, bringing the volume to 1 μL. If you need a demonstration of proper injection
into a GC, ask your instructor.
Figure 10. GC Syringe for Injection of Sample
Carefully insert the needle into the center of the injector (Figure 11) so that the hub of the needle
butts up against the injector. The needle will penetrate into a rubber septum (inside the injector).
Figure 11. GC Injection Port
Inject the sample in one swift, quick motion, then quickly withdraw the needle from the
injection port. Immediately press the "Start" button located on the front of the instrument
(Figure 8). It is important not to keep the needle in the injector for a long time. Once it has
penetrated the septum, the sample must be injected as soon as possible and as smooth as
possible.
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Once the run has started, you should see a message (Figure 12) on the computer saying
"Acquisition Override Solvent Delay (3.00 minutes)?" DO NOT CLICK ON ANYTHING!
SIMPLY IGNORE! This is normal.
Figure 12. Message Window upon Starting Run - ignore
Each run lasts 15 minutes. While waiting for the run to complete, clean the syringe with
methanol and work on any cleaning or laboratory tasks. Repeat the previous procedure for
your second sample. Two pages will print out per injection: one page with the peak area
percent report and the other with the chromatogram.
Repeat the above procedure for each replicate sample. When the analysis is complete, DO
NOT TURN ANYTHING OFF! THE GC/MS MUST BE KEPT RUNNING AT ALL
TIMES!
Dispose all of your sample in the test into the large “Recovered cyclohexane” flask in the
middle fume hood. Discard the empty test tubes and corks.
5. Data analysis
In order to analyze the results, switch to the Data Analysis module of ChemStation. Go to
"View" in menu, select "Data Analysis (offline)" and wait. The Data Analysis module should
start at this point.
Once Data Analysis has started, load your data file to analyze the results. Go to "File" and
select "Load Data File…" from the menu. A new dialog box will ask for the data to be
selected. Select the data you have just collected. If you don't see your data, ensure that the
directory path is set correctly. It should be D:\Instrumental_Analysis\. If not, select the
correct path by clicking the "Path" button shown on the window. Once you have located
your data, click "OK" to load.
Next, print out the mass spectrum of each primary component of the chromatogram; there
should be three. Move the cursor to the apex of the chromatogram peak and RIGHT
DOUBLE-CLICK. The mass spectrum for that particular peak should be displayed on the
lower half of the screen in a new window. Print the mass spectrum. To do so, go to “File”,
select “Print”. When asked what to print, select “Selected Window”, and select “1”. This
will print your mass spectrum shown on the lower half of the screen.
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Determination of the cooking oil identity using GC data. You can determine the identity
of your unknown cooking oil using the GC chromatograms. Compare the retention times and
relative peak area percentages of your sample to standard oil data as discussed below.
For the identification of unknown oil, download the Excel spreadsheet for GC/MS analysis.
I t i s a t t h e s a m e s i t e w h e r e t h e l a b o r a t o r y p r o c e d u r e s a r e p o s t e d
(http://chemistry.creighton.edu/~jiro/CHM466/ ) You must use this spreadsheet. You should
compare the retention time and the area percent of your sample numerically to the standard
oil values listed on the spreadsheet. Your sample data should be inserted and lined up below
the oil you think your unknown is. You are expected to turn in the print out of the
spreadsheet as well as two copies of report sheet.
In addition, record your brief reasoning and discussion in your notebook, focusing on your
analysis process. Write this neatly and skip every other line.
Determination of fatty acid identities using MS data. Next, you will use the mass
spectrum of each peak in the chromatogram to elucidate the identity of the fatty acid
components of the cooking oil.
Remember, each mass spectrum you collected is of a methyl ester derivative of the fatty
acid. For example, the structure of methyl myristate, a derivative of myristic acid
(tetradecanoic acid), is shown below in Figure 13.
CH3 CH3
O
O
Figure 13. Chemical Structure of Methyl Myristate
You can think of your unknown as having the general structure of H 3COOC-(CH2)n-CH3. For
example, methyl myristate, a 14 carbon fatty acid derivative, would have n = 12 by this
notation. Also, note that the 14 carbon chain possesses no double bonds. The shorthand
notation for this fatty acid is C14:0.
You can see from this structure that each fatty acid methyl ester (FAME) has the same
structural pattern of: H3COOC-(CH2)n-CH3 (the end groups are shown in bold here) You
must determine two things: the value of n and the number of double bonds in the chain
(if any). Then you will report the formula of each original fatty acid in the shorthand
notation.
The highest m/z peak in a mass spectrum (Figure 14) is called the molecular ion (M+) peak.
The molecular ion is the unfragmented FAME; it has not lost any atoms, only an electron.
Remember, mass spectrometry discriminates between isotopes, so we can use the nominal
masses of carbon (12 amu), hydrogen (1 amu) and oxygen (16 amu) for determining mass.
For example, for methyl myristate2, the molecular ion is the peak at 242 amu in Figure 14.
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This peak is composed of the end groups (H3COOC- & -CH3, which contribute to 74 of
the 242. The remaining 168 is attributed to the 12 CH 2 groups (12 x 14 = 168). When the
carbon chain contains no double bonds, as in this example, the mass of the chain (i.e.
168) is a multiple of 14 (the mass of CH2). If the chain contained any double bonds we would
expect the mass to be smaller by 2 amu per double bond (i.e. 166 if it contained 1 double
bond).
Figure 14. Mass Spectrum of Methyl Myristate2
Use this information about interpretation of mass spectra to determine the identities of the
original fatty acid chains in your unknown. Remember that although we know that a double
bond is present, we do not know the location of this bond. Don’t forget: to accurately
determine the identity (original chain length) of the fatty acid, you must also account for the
two carbons that are present on each end of the chain.
Reporting Requirements
Once your data analysis is complete, report the following on the reporting sheet:
The identity of your unknown oil (determined by retention time and area percent
comparison with standard oils using the spreadsheet from Blueline).
The identity of each fatty acid in your oil (determined from FAME mass spectra
identification). (If double bonds are present, report the chain length and number of
double bonds). Report in the format of C16:0, as was described for myristic acid.
You can either print out the sheet on the following page or pick up a copy in the lab. Report
your data on this sheet rather than the cards.
Make sure that your reasoning and mass spectrum interpretation is clearly documented in the
results and discussion section of your notebook for each FAME. Describe the steps in your
math as if you are explaining to someone unfamiliar with the method.Your laboratory grade
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depends on both your descriptions and the correctness of your results. It is important to
clearly describe your reasoning.
Your grade will be determined out of 100% as follows:
70% Oil identification (20%) and explanation (50%)
30% fatty acid formula and explanation (10% for each)
6. References
1. http://www.spectroscopynow.com/FCKeditor/UserFiles/Image/specNOW_advertorials/1105_MS_Thermo/Thermo_msadvertorial_nov05_ig1a_large.jpg
2. http://www.lipidlibrary.co.uk/ms/arch_me/me_sat/M0014.gif
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GC/MS Fatty Acid Reporting
Name: _________________ Lab Section: _____ Unknown Number: ____________
Unknown Oil Identity: ___________________________
Grading:
Possible score Your score
Oil identification 20
Oil ID explanation 50
Fatty acid 1 ID and explanation 10
Fatty acid 2 ID and explanation 10
Fatty acid 3 ID and explanation 10
TOTAL 100
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Peak
#RT (min)
M+●
m/z
Fatty acid formula
(in the format of C24:2)
1
2
3
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Instrumental Analysis Laboratory Safety Rules
A. Instructions: Carry out all manipulations in accordance with instructions and the
safety rules and procedures given herein.
B. Eye Protection: All students and staff working in the laboratory must wear safety
glasses at all times.
If a student needs to be reminded more than three times to wear goggles, she/he will
be dismissed from lab for the remainder of the day, and will not be given an
opportunity to make up the work.
C. Apparel: The clothes you wear in lab are an important part of your “safety
equipment,” and should offer protection from splashes/spills. Closed toed shoes
(sneakers are fine), Full-length pants or a full-length skirt, and A shirt that completely
covers your torso (i.e. at minimum, a t-shirt).
In other words, you must NOT wear shorts to lab. You must NOT wear flip-flops,
sandals, or crocs. You must NOT wear tank tops, halter tops, spaghetti-strap tops, or
low cut jeans to lab. Exposed abdomens, hips, and backs are not safe in the lab.
D. Gloves: Gloves are an important part of personal protection. Gloves will be available
at all times in the laboratory. Your instructor will require their use when appropriate.
E. Food: Food, drinks, and gum are not allowed in lab. None at all, not even water
bottles.
F. Sanitation Issues: Be sure to wash your hands before leaving lab, before you eat
anything outside of lab, and before you answer your cell phone.
G. Music: Individual headphones are not allowed. Your may choose to play music for
the entire class.
H. Cell Phones and Other Electronic Devices: Cellular phones and other electronic
devices that you do not need to perform your laboratory work should be put away.
I. Other: All students are explicitly prohibited from:
1. conducting any unauthorized experiments.
2. removing chemicals or apparatus from the laboratory for any reason.
3. working in the lab alone, or at other than regularly scheduled lab periods.
4. smoking in the laboratory or within 20 feet of any doorway.
5. impeding movement in aisles or through doorways with bags, skateboards, etc.