NANSLO REMOTE LAB ACTIVITY

SUBJECT SEMESTER: XXXXXXXXXXXXXXXXXXXXX

TYPE OF LAB:  XXXXXXXXXXXXXXXXXXXX

 

 

 

 

 

 

 

 

TITLE OF LAB:  Gas Chromatography

 

GC NANSLO Lab Activity in Word format last updated May 2, 2014.

 

Lab format: This lab is a remote lab activity.

 

Relationship to theory (if appropriate): In this lab you will learn the underlying principles behind the analytical technique of gas chromatography and learn some of the basic skills involved in the interpretation of a gas chromatograph.

 

Instructions for Instructors:  This protocol is written under an open source CC BY license. You may use the procedure as is or modify as necessary for your class.  Be sure to let your students know if they should complete optional exercises in this lab procedure as lab technicians will not know if you want your students to complete optional exercise.

 

Instructions for Students:  Read the complete laboratory procedure before coming to lab.  Under the experimental sections, complete all pre-lab materials before logging on to the remote lab.  Complete data collection sections during your online period, and answer questions in analysis sections after your online period.  Your instructor will let you know if you are required to complete any optional exercises in this lab.

 

Remote Resources:  Primary – Gas Chromatograph, Secondary – Pump and 5-component solution.

 

CONTENTS FOR THIS NANSLO LAB ACTIVITY:

 

Learning Objectives
Background Information
Equipment
Pre-lab Activities:  Pre-lab Question 1
Pre-lab Activities:  Pre-lab Question 2
Pre-Lab Activities:  Pre-lab Question 3
Pre-lab Assignment - Setting Up Your Data Table
Experimental Procedure
Exercise 1:  Iso-Thermal Trial
Exercise 2:  Ramping Trial
Exercise 3:  Ramping Trial
Post Lab Thoughts/Questions
Exercise 4:  (Optional) Pressure Changes
Preparing for the Gas Chromatography NANSLO Lab Activity

Equipment Used for the GC Lab

 

LEARNING OBJECTIVES:

 

After completing this laboratory experiment, you should be able to do the following things:

1. Compare and contrast different molecular structures for polarity and be able to rank molecules in terms of polarity.
2. Apply the concept of polarity and intermolecular forces of attraction in predicting retention times on a Gas Chromatograph (GC) column.
3. Improve upon given GC parameters and recommend new ones to obtain optimum resolution of peaks

 

BACKGROUND INFORMATION:

 

Polarity is the way molecules react with other molecules and interact with the world around them is determined by the composition and arrangement of atoms within the molecule.  One particular property that has a large influence on molecule-molecule interactions is polarity.  Polarity arises out of a difference between the atoms within the molecule based on electronegativity.  For example, a molecule such as H₂ does not have any polarity and is considered nonpolar because the atoms within the molecule are the same (both are H), and therefore there is no difference.  The same would be true for Br₂ or I₂ or any other diatomic molecule.  However, some molecules, such as H₂O, are polar, which we will explain in the following paragraphs.

 

Electronegativity is the strength of the attraction of a particular atom to the electrons in a covalent bond.  The larger the electronegativity the stronger the attraction and the more tightly the atom pulls on the electrons in a covalent bond.  The most electronegative element on the periodic table is Fluorine with a value of 4.0.  As you move across the table towards Fluorine, the electronegativity generally increases.  Likewise, electronegativity generally increases as you move up a group (column).  

 

 

 

The polarity of a bond within a molecule may be determined based on the electronegativity difference between the elements that are bonded together.  For example, in the bond between carbon and fluorine shown below, fluorine is the more electronegative atom and therefore the partial negative is on fluorine and the partial positive is on carbon.  The delta symbol (δ) indicates a partial charge.

 

 

The polarity of a molecule not only depends on the electronegativity differences (if any) between the atoms in the molecule, but also on the 3-dimensional shape of the molecule.  For example, carbon dioxide (shown below on the left) has two polar bonds but the overall molecule is nonpolar because the two bond dipoles cancel each other out. However, the water molecule (shown below on the right) which also has two polar bonds is in fact polar because the bond dipoles do not cancel out due to the tetrahedral electron geometry around the oxygen atom.  (Recall that there are two lone pairs of electrons on the oxygen atom).

 

 

How polar a molecule is will also depend on the strength of the molecular dipole.  For example, the net dipole for dichloromethane is smaller than the net dipole in chloroform.

 

Intermolecular Forces of Attraction: The polarity of a compound influences how strongly the molecules are attracted to each other.  Polar compounds are  capable of having dipole-dipole interactions which are very strong.  Nonpolar compounds can only have London dispersion forces which are very weak.

 

Gas Chromatography:  Chromatography separates compounds or particles based on a specific physical property.  There are several types of chromatography that are commonly used such as thin layer chromatography, column chromatography, and gas chromatography.  The device that is used in this lab activity is called a “gas chromatograph” or simply a “GC”.  In the gas chromatography technique that will be used in this lab, molecules will be separated based on their polarity.  In every form of chromatography, there are two phases, a stationary phase and a mobile phase.  For the gas chromatography, the stationary phase is a column through which the molecules will pass as they are carried through by the mobile phase which, in this case, is just air.  Often, light gases such as helium or hydrogen are used as the mobile phase.

 

The interaction with the column determines how long the molecule will take to get through the column before being detected by the instrument.  This causes different molecules to have different retention times (the time it takes for the molecule to get through the column). The stronger the interaction between the molecule and the column, the longer the molecule will stay on the column and therefore come out at a later time.  The weaker the interaction, the faster the molecule will come out of the column.  Molecules that are similar in polarity to the column will have stronger interactions.  In other words, a nonpolar compound will interact more strongly with a nonpolar column and a polar compound will interact more strongly with a polar column.

 

In addition to polarity, the molecule’s volatility (i.e. – boiling point) will impact the retention time.  There is a direct relationship between the strength of the intermolecular forces of attraction and the volatility of a substance.  The stronger the attraction between the molecules, the less volatile a substance will be.  The compound’s molar mass also contributes to the volatility.  Molecules that are larger are capable of having stronger intermolecular forces of attraction due to greater magnitude of the temporary dispersion forces and therefore will be less volatile.  Substances with lower boiling points (more volatile) will vaporize more quickly than substances with higher boiling points.  Therefore, the more volatile molecules might start moving down the column sooner than the less volatile molecules, essentially getting a head-start, depending on what the starting temperature of the column is.  For example, if you start the column out at a temperature that is at or above the boiling points of the chemicals, they will all “flash boil” at the same time and start down the column simultaneously. Of course, even if the column never reaches the boiling point of the chemical, it will still evaporate and move down the column (just like water will evaporate at room temperature).  It is the combination of volatility and polarity which determines how quickly a molecule will move through the gas chromatography column.

 

The result of all this is that different types of molecules can be separated from each other as they move through the gas chromatography column if the parameters such as column temperature and gas flow (pressure) are set correctly.  In this activity, you will be attempting to completely separate four different chemicals and determine which peaks in the gas chromatogram correspond to which chemicals.

 

As an example, here are the chromatograms produced by three of the four individual chemicals that you will be using in this lab activity as well as the chromatogram produced by running a mixture of all three chemical compounds through the GC. 

 

Chromatogram 1: Methanol	Chromatogram 2: Butyl Acetate
Chromatogram 3:  2-Butanone	Chromatogram 4:  Mixture of All Three

 

Note that it is the order in which the compounds come off of the column that we are focusing on in this introductory lab activity.  There is a lot of other information that can be gained from detailed analysis of a chromatogram.  For example, the area under the peak for each compound indicates the relative amount of that compound in the overall mixture.  Also, the shape of the peak can tell us some interesting things about the chemical compound that produced it.  However, these more advanced topics will be left for a future lab activity.

 

EQUIPMENT:

 

• Paper
• Pencil/pen
• Computer with Internet access (for the remote laboratory and for data analysis
• Remote:  Gas chromatograph and pumping system to deliver chemicals

 

PRE-LAB ACTIVITIES

 

Pre-lab Question 1:

 

Rank the following in order of increasing polarity: 

 

Most polar:  _______________________________________________________ Middle:  ____________________________________________________________ Least polar:  _______________________________________________________

 

 

 

 

 

 

 

 

 

 

 

Pre-lab Question 2:

 

Assuming two compounds have similar boiling points, will a nonpolar compound have a longer or shorter retention time than a polar compound on a polar column?

 

 

Pre-lab Question 3:

 

Which of the following molecules should have a shorter retention time on a nonpolar column?  Explain why.

 

 

 

 

 

 

Pre-lab Assignment - Setting Up Your Data Table: 

The compounds you will test in this experiment are methanol, butyl acetate, isopropanol, and 2-butanone.  Set up a data table with the Lewis structure, boiling point, and molar mass of each.

Based on the information presented in the Background section, predict the order in which these chemicals will come through the GC column under optimal conditions for separation.

 

 

EXPERIMENTAL PROCEDURE

 

Once you have logged on to the remote lab system, you will perform the following laboratory procedures.  See Preparing for the Gas Chromatography NANSLO Lab Activity below.

 

Exercise 1:  Iso-Thermal Trial

 

4 compounds: Methanol, butyl acetate, isopropanol, and 2-butanone.

Trial #1:  Run the mixture of the 4 compounds isothermally at the following profile:

• Start temp = 90°C
• Hold time = 1 min  (how long to stay at the Start Temp)
• Ramp rate = 0 °C/min
• Final temp = 90°C
• Hold time = 5 min  (how long to stay at the Final Temp)
• Total Time = 6 min
• Pressure = 7 kPa

 

1. What do you notice about the chromatogram that is produced?  Are all the compounds separated from each other?
   
   
2. Export a copy of the chromatogram (graph) and paste it into a document on your computer.  Insert this graph into your report.
   
   

Exercise 2:  Ramping Trial

Trial #2:  Run the mixture of the 4 compounds while ramping the temperature up.  

• Start temp = 70°C 
• Hold time = 1 min 
• Ramp rate = 10°C/min 
• Final temp = 90°C 
• Hold time = 3 min 
• Total Time = 6 min 
• Pressure = 7 kPa

 

1. What do you think will be different about  this run?
   
              Prediction:
   
   
2. What do you notice about the chromatogram that is produced?  Are all the compounds separated from each other?
   
   
   
3. How does the result compare with your prediction?
   
   
   
4. Export a copy of the chromatogram and paste it into a document on your computer.  Insert this graph into your report.
   
   

Exercise 3:  Ramping Trial

 

Trial #3 and beyond:   Improve the separation of the peaks from the last two runs by varying only the ramp rate and starting and ending temperatures for your next 2 runs.  Keep the pressure at 7 kPa and keep the total length of time 10 minutes or under.  Make a data table for your parameters and observations.  Your goal is to completely separate all four compounds from each other.  

Here are the limits for the various settings in the GC profile for the specific GC we are using:

 

Minimum Temperature:  30°C
Maximum Temperature:  160°C
Minimum Ramp Rate:  0°C/min
Maximum Ramp Rate: 10°C /min
Minimum Pressure:  1 KPa (above room pressure)
Maximum Pressure:  19 KPa (above room pressure)

Choose some profile settings that you think will give you even better separation of the peaks:

Trial #3:

 

• Start temp = ___ °C
• Hold time = ___ min
• Ramp rate = ___ °C/min
• Final temp = ___ °C
• Hold time = ___ min
• Total Time = ___ min
• Pressure = 7 kPa

 

1. Export a copy of the chromatogram and paste it into a document on your computer.  Insert this graph into your report.
   
   
2. Which of the four peaks represents the isopropyl alcohol?
   
   
3. How does the boiling point of isopropyl alcohol compare with the boiling point of the other three compounds?
   
   
4. Explain why the isopropyl alcohol peak appears where it does in the gas chromatogram.
   
   

POST LAB THOUGHTS/QUESTIONS:

 

1. If you ran the same mixture of compounds through a polar GC column, assuming optimal conditions, in what order would they appear at the end of the separation?
   
   
2. Draw a picture of what you think the chromatogram would look like.

 

 

Exercise 4: (Optional) Pressure Changes

 

*Optional Trials:  What impact do you think changing the pressure in the GC column will have?  Make a prediction and repeat one of your trials from above using a different pressure. Export a copy of the chromatogram and paste it into a document on your computer. Insert this graph into your report.

 

Prediction: 

 

Evidence:

 

Conclusion:

 

PREPARING FOR THE GAS CHROMATOGRAPHY NANSLO LAB ACTIVITY:

 

Read and understand the information below before you proceed with the lab!

  

Scheduling an Appointment Using the NANSLO Scheduling System

 

Your instructor has reserved a block of time through the NANSLO Scheduling System for you to complete this activity. For more information on how to set up a time to access this NANSLO lab activity, see www.wiche.edu/nanslo/scheduling-software.  

 

Students Accessing a NANSLO Lab Activity for the First Time

 

You must install software on your computer before accessing a NANSLO lab activity for the first time. Use this link to access instructions on how to install this software based on the NANSLO lab listed below that you will use to access your lab activity – www.wiche.edu/nanslo/lab-tutorials.

 

1. NANSLO Colorado Node -- all Colorado colleges.
2. NANSLO Montana Node -- Great Falls College Montana State University, Flathead Valley Community College, Lake Area Technical Institute, and Laramie County Community College.
3. NANSLO British Columbia Node -- Kodiak College.

 

Reviewing these Instructions

 

We've provided you with two ways to learn how to use the gas chromatograph for this NANSLO lab activity:

 

1. Read these instructions or print this lab activity.
2. Watch Video 2 below. 

NOTE:   The conference number in this video tutorial is an example.  See “Communicating with Your Lab Partners” below to determine the toll free number and pin to use for your NANSLO lab activity.

 

In addition, watch Videos 1, 3, and 4 to help you understand how gas chromatography works.

 

VIDEO 1:  An animation of the dynamic process of how polar and nonpolar molecules
are physically separated on a gas chromatography column (2:29 minutes)

 

 

 

VIDEO 2:  Gas Chromatography Remote Web-based Science Lab (RWSL) Lab Interface Tutorial

 

 

 

VIDEO 3:  In-depth look at the principles of gas chromatography (2:21 minutes)

 

 

 

VIDEO 4:  Animation of how the peaks are produced
on the chromatogram (9 seconds): 

 

 

 

GAS CHROMATOGRAPH RWSL NANSLO LAB INTERFACE INSTRUCTIONS

 

NANSLO LAB ACTIVITY INTERFACE

 

The equipment control software shown below is written using the LabView software from National Instruments.  The user interface is presented as a LabView control panel which will be referred to as the lab interface for the remainder of this tutorial.

 

Figure 1:  Remote Web-based Science Lab (RWSL) GC Lab Interface.

COMMUNICATING WITH YOUR LAB PARTNERS

 

As soon as you have accessed this lab interface, call into the toll free conference number shown on the lab interface to communicate with your lab partners and with the Lab Technicians.  Use the PIN code noted to join your lab partners.  Only one person can be in control of the equipment at any one time so talking together on a conference line helps to coordinate control of the equipment and creates a more collaborative environment for you and your lab partners.

 

GAINING CONTROL OF THE GC

 

Right click anywhere in the gray area of the lab interface and choose “Request Control of VI” from the dialogue box that appears when multiple students are using the GC at the same time.  After you request control, you may have to wait a short time before you actually receive control and are able to use the features on this lab interface.

 

Figure 2:  Selecting "Request Control of VI"

 

RELEASING CONTROL OF THE GC

 

 To release control of the GC so that another student can use it, right click anywhere in the gray area of the lab interface and choose "Release Control of VI" from the dialogue box that appears.

 

Figure 3:  Selecting "Release Control of VI"

 

GC VIEW WINDOW

 

The Image view Window displays the real-time video feed from a digital camera focused on the GC, the syringe, and the robotic pumping system.

 

Figure 4:  Image View Window

 

CAMERA PRESETS AND PAN-TILT-ZOOM CONTROLS

 

Several camera preset positions have been programmed for use with this lab interface.  Use these to look more closely at the equipment or to view what is happening in the lab itself.  Pre-set position 2 for example allows you to view the screen on the GC that tells you the current temperature of the column.

 

Figure 5:  Camera Preset Positions

 

The four arrows used to pan and tilt allow you to move the camera right to left and up and down.  The two zoom buttons allow you to zoom in to see a closer look at the equipment such as shown in Figure 6 or zoom out to view more of the room.

 

 

Figure 6:  Pan, Tilt, & Zoom controls

 

Using the Cameras to View the GC Apparatus

There are two digital cameras set up for the GC  lab.  First, we'll describe the options when "Camera Selection 1" is available.  

Camera Selection = 1

This camera allows you to see what is going on in the lab and on the GC apparatus itself.  To determine what Camera Presets are available when Camera 1 is being used hover over the preset buttons and a pop-up menu will appear showing the views, e.g. 1 – Full view.  Use Camera 1, Camera Preset 2 to see the screen on the GC itself.

For each camera preset view, additional camera options are available when Camera 1 is being used (see Figure 7.)

 

1. Use the up and down arrows to tilt the camera up or down.
2. Use the right and left arrows to pan right or left.
3. Use the left "Zoom OUT" arrow and right "Zoom IN" arrow to zoom out and in.

 

Figure 7:  Pan-Tilt-Zoom Camera Control

 

Camera Selection = 2

This camera allows you to see a closeup of the needle that injects  the sample compound into the GC.  To determine what Camera Presets are available when Camera 2 is being used, hover over the preset buttons and a pop-up menu will appear showing the views.  

 

Make sure Camera 2, Camera Preset 2 is selected to view the needle itself. 

 

SETTING UP THE GC RUN PROFILE

 

Using the parameters set out in your lab procedure, change the profile settings using the GC lab interface (highlighted in the red box).  Once the profile has been defined, select the “Submit Profile” button.

WARNING:  Once you click Submit Profile, you cannot stop the process!  If possible, have your lab partners double check your settings before you submit them.

 

Figure 8:  GC Run Profile Settings

 

RUNNING THE GC PROFILE

 

Camera 1, Camera Preset 2 shows the screen on the GC itself which tells you the current temperature of the column.

 

Figure 9:  Camera 1, Camera Preset 2 shows GC temperature.

 

The “GC Status” field will indicate when the GC has reached the proper pressure and temperature settings.  At that point, the “Inject Needle” message appears in the GC Status window.  When the "Insert Needle" button is clicked, it causes the robot to move the pump forward and insert the needle into the GC column.  Select Camera 2, Camera Preset 2 to view this needle.

 

 

Figure 10: Select the "Insert Needle" button.

 

  After the needle has been inserted, the "Input and Collect" button is available.  When this button is selected, the robotic pumping system will deliver the correct amount of sample compound.  The needle will be withdrawn from the column automatically.

 

Figure 11:  Select the "Inject and Collect" button.

 

Data is now being generated.

 

Figure 12:  GC is running data.

 

 

VIEWING THE DATA GENERATED BY THE GC IN A GRAPHIC FORMAT (CHROMATOGRAM)

 

Select the “Graph” tab on the lab interface to view the data being generated.  At first, the data on the chromatogram will be noisy as shown in Figure 13. This will smooth out as soon as the chemicals begin to work their way out of the end of the GC column and impact on the detector.

 

Figure 13:  Data is "Noisy"

 

When this happens, you will see a dramatic change in the scale on the left side of the chromatogram as shown in Figure 14.  It often takes 5 to 6 minutes for the run to occur from the beginning to the end of the column. You will see separate peaks for separate chemicals as they exit the chromatograph column.

 

Figure 14:  Real data is being captured.

 

ANALYZING THE DATA GENERATED BY THE GC (CHROMATOGRAM)

 

Once the chromatogram is completed, you want to analyze it to determine where the peaks are located and perhaps their intensity.   You can do this by clicking on the “Cursor” button.  The cursor will appear in the center of the chromatogram.  Click and drag the cursor to a peak on the chromatogram.  The location of the peak in time (x-axis) and the amplitude of the peak (y-axis) is displayed in the Cursor box.  See Figure 15.

 

Figure 15:  Peak in time and amplitude based on area of chromatogram selected.

 

 

MANIPULATING THE VIEW ON THE CHROMATOGRAM

 

There are several tools available to you when viewing the chromatogram.  The tools most often used are:

 

1. The “Cursor” button (see Figure 15 above) displays a cursor in the center of the chromatogram that can be moved around.   
2. When selecting the center button on the small toolbar at the bottom right corner of the chromatogram, additional tools are visible.  See Figure 16.
   
   
   Figure 16:  Tool options for Chromatogram.   
   
   The two tools most frequently used are:
   
   
1. The "zoom in" button allows you to select an area of the chromatogram that you want to examine more closely.  Move the magnifying glass displayed within the chromatogram when the "zoom in" button is selected to the location where you want a closer look.  See Figures 17-18.
   
   
   Figure 17:  Zoom in to get a closer look at the chromatogram.
   
   
   Figure 18:  Magnifying glass available.
   
   When you use the zoom in tool, the cursor sometimes disappears.  If this happens, click on the "Cursor" button to turn the cursor off and click it again to turn it back on.  The magnifying glass will reappear in the middle of the screen.
   
   
   Figure 19:  Click the "Cursor" button twice to see the magnifying glass.
   
   
2. The chromatogram will always zoom out to its fullest display when the "zoom out" button is selected.  See Figure 20.
   
   
   Figure 20:  Zoom out to get a view farther away of the chromatogram.
   
   
3. After using the "Zoom In" and "Zoom Out" tools, you may may have to select the Cursor Control tool - the left button on the small toolbar at the bottom right corner of the chromatogram.  The cursor will again appear in the center of the chromatogram.  See Figure 21.
   
   
   Figure 21:  Cursor Control Button.

 

EXPORTING THE CHROMATOGRAM DATA

 

You can export to your clipboard a graph image of the chromatograph or the data and paste it into a document on your computer.  Using the drop-down menu, select what you want to capture –graph image or graph data.  Next, open up the document that you will paste the captured information onto.  For example, if you selected “Graph Image,” you could open Word and select “Paste.”  The image would be placed where your cursor is on the document.  If you selected “Graph Data,” you could open a spreadsheet such as Excel and paste the data into the spreadsheet. 

 

Figure 22:  Exporting image or data to clipboard.

 

BEGINNING A NEW EXERCISE WITH A DIFFERENT GC PROFILE

 

When you have completed your data collection for one exercise, you can click on the "Profile" tab and change the profile settings for your next run.

 

EQUIPMENT USED FOR THE GC LAB

 

NANSLO uses the Vernier Mini Gas Chromatograph for this lab.  To see how the Vernier Mini Gas Chromatograph is used to collect data, see http://vernier-videos.s3.amazonaws.com/training_html5/mp4/Intro_to_Gas_Chromatography.mp4.   Note, that the description of how the data is collected in this video differs from this lab only with respect to location of collection.  In the Vernier video, a computer attached to the GC collects the data.  In this NANSLO lab activity, students use the NANSLO lab activity control panel to capture that data remotely.

 

For more information about NANSLO, visit www.wiche.edu/nanslo.

 

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Creative Commons Attribution 3.0 United States License 3  

 

 

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