http://orgchem.colorado.edu/hndbksupport/GC/GCproc.html

Gas Chromatography: Procedure

(1) Add the sample to be injected to the syringe.

A 25µL glass Hamilton syringe is used to inject the GC samples. Only 2-4 µL of sample is

injected onto the column, which means that you fill only a small part of the barrel with

sample. Examine the syringe carefully before you fill it. The divisions are marked "5 - 10 -

15 - 20 - 25".

This is a 25 µL glass Hamilton syringe. You only inject 2.5 µL, so it will NOT be filled to the top.

Place the tip of the needle in the liquid. Slowly draw up a small amount of liquid by raising the

plunger, then press on the plunger to expel the liquid back into the liquid. This serves to “rinse”

the syringe with your sample, ensuring that what you will measure in the GC run is the

composition of your mixture. Repeat the rinse process one or two times. Then, draw up the

plunger slowly again while the needle is in the liquid and carefully fill the syringe with liquid about

halfway to the “5”.

(2) Inject the sample into the injector port.

You are need to do two things sequentially and quickly, so make sure you know where the

injection port is and where the start button on the recorder is.

Push the needle of the syringe through the injection port and immediately press the plunger

to inject the sample, then immediately press the start button on the recorder.

You will feel a bit of resistance from the rubber septum in the injection port; this is to be

expected and you should be prepared to apply some pressure to the syringe as you force the

needle into the instrument all the way to the base of the needle.

Push the needle of the filled syringe through the injector (as far as it will go) and quickly push the

plunger.

It is often hard to see the liquid in the syringe. If the syringe is clogged, the plunger will be

in the correct position but the barrel of the syringe will be filled with only air, as in the

bottom syringe in the photo to the left.

The best thing to do is to carefully examing the syringe after you think that you have filled

it. Hold it up to the light to get a better view.

Small air bubbles in the syringe will not affect the GC run (middle syringe in the photo to

the left). As long as there is enough liquid in the syringe, the GC run will work fine. If you

keep getting bubbles, just pull the plunger up a bit past the "halfway to the 5" mark to

compensate.

If you have a VERY large air bubble, you will not have enough liquid to show a reading on

the GC (e.g., the bottom syringe in the photo).

Remove the syringe immediately . . .

. . . and quickly press the start button on the integrating recorder or the start recording

button on the computer (ask your TA which device is connected to the GC that you are using)

Here's a close-up of the integrating recorder.

Here's a series of pictures showing how to run the computer program.

(3) Sit back and wait.

Observe the recorder. Within several minutes, it should record several peaks.

(4) End the GC run.

When you have seen all of the peaks which you suspect are in the mixture, or when the

recorder has shown a flat baseline for a few minutes or so, press stop on the recorder.

When you press stop, the recorder will print out the peaks, the retention times, and the areas

under the peaks. When it is done printing, you can press “enter” a couple times to advance

the paper.

Carefully tear the paper off the recorder. The paper is not perforated, so do not try to pull

up and expect it to pop out of the recorder. Instead, pull it down to start a tear from one

edge, and then continue the tear until the paper is cut and free.

This may seem trivial -- showing you how to

tear the paper. But too many times a student has tried to yank the paper out instead of

starting a tear and tearing it neatly. Yanking the paper can result in the paper being torn

below the plastic cutting surface on the recorder, and the paper gets jammed down inside

the recorder.

If this happens, the entire recorder has to be dis-assembled, a process which takes about 15

minutes, thus putting the entire GC out of service until it can be fixed.

http://orgchem.colorado.edu/hndbksupport/GC/GC.html

Gas Chromatography

Study Questions/Answers from the Handbook for Organic Chemistry Lab

In gas chromatography (GC), the stationary phase is a high-boiling liquid and the mobile

phase is an inert gas. In the organic chemistry teaching labs at CU Boulder, GC is used as

an analytical tool to find out how many components are in a mixture. It can also be used to

separate small amounts of material.

Movie on how to run a GC. On GoogleVideo - choose "smoothing" and "original size"

from the lower right pull-down menu for best video.

The GC Instrument

The process of gas chromatography is carried out in a specially designed instrument. A

very small amount of liquid mixture is injected into the instrument and is volatilized in a

hot injection chamber. Then, it is swept by a stream of inert carrier gas through a heated

column which contains the stationary, high-boiling liquid. As the mixture travels through

this column, its components go back and forth at different rates between the gas phase and

dissolution in the high-boiling liquid, and thus separate into pure components. Just before

each compound exits the instrument, it passes through a detector. When the detector “sees”

a compound, it sends an electronic message to the recorder, which responds by printing a

peak on a piece of paper.

The type of GC used in the organic chemistry teaching labs is shown below: Gow-Mac

series 350/400. Click on the photo for a detailed enlargement.

The GC consists of an injection block, a column, and a detector. An inert gas flows through

the system. The injection chamber is a heated cavity which serves to volatilize the

compounds. The sample is injected by syringe into this chamber through a port which is

covered by a rubber septum. Once inside, the sample becomes vaporized and is carried out

of the chamber and onto the column by the carrier gas.

The large photo below is of the inside of one of the older GC models, but it still shows

useful information. It shows the column in the oven and the insulated chamber that houses

the detector. Click on the thumbnails to see larger photos of the column and detector, as

well as the inside of the injector port (showing the septum).

inside of the injector port

the septum

the column

the detector inside the housing

On the Varian 920 and Gow Mac 350 chromatographs, detection of the compounds is achieved

with a thermal conductivity (TC or hot wire) detector.

The column (see the photo above) is an integral part of the GC system. On the outside, all

you see is a long stainless steel tube, 1/8 to 1/4 inch in diameter and 4-5 feet long, which is

coiled to fit inside the instrument. Inside the column is the important component: the

stationary phase composed of the high-boiling liquid. The liquid is usually impregnated on

a high surface area solid support like diatomaceous earth, crushed firebrick, or alumina.

The liquid can be applied in various concentrations: the more liquid, the more sites it has to

interact with the compounds.

All of our GCs have columns which are five feet long and 1/8" or 1/4" in diameter and

contain a methyl silicone polymer liquid phase (OV-101, 1.5%) on a diatomaceous earth

support (chromosorb G). Methyl silicone is a liquid phase of intermediate polarity, and

non-polar compounds such will separate according to their respective boiling points.

The carrier gas is an inert gas, helium. The flow rate of the gas influences how fast a

compound will travel through the column; the faster the flowrate, the lower the retention

time. Generally, the flow rate is held constant throughout a run. (The GCs at CU Boulder

are set at a flow rate of 55 mL/min.)

This is where the carrier gas enters the Varian

GCs and where the gas flow rate can be adjusted. Click on the photo above for details.

In a professional laboratory, the GC conditions would be critical for another experimenter

trying to duplicate your observations. All of our GCs have the same columns (1.5% OV-

101 on Chromasorb G) and the same flow rate (55 mL/minute) and detector bridge current

(150 mAmps). Each instrument will have a different setting for:

column temperature

injection port temperature

detector temperature

It is a good practice to write down some of the settings on the instrument. The values for

these temperaturs are viewed by turning the knob on the instrument below the gauge --

click on the thumbnail below to see detailed photos of how to do this.

reading temperatures on the Gow-Mac

Recorders

Two devices are used to record the GC traces/areas under peaks:

integrating recorders

computer program

Each type of device records the messages sent to them by the detector as peaks, calculates

the retention time, and calculates the area under each peak; all of this information is

included in the printout. For similar compounds, the area under a GC peak is roughly

proportional to the amount of compound injected. If a two-component mixture gives

relative areas of 75:25, you may conclude that the mixture contains approximately 75% of

one component and 25% of the other.

An integrating recorder is pictured below. Click on the photo for a detailed picture and

the location of the start button (press when you inject), the stop button (press when you

have seen your peaks, it tells the recorder to do the calculations and to print), and the enter

button (paper feed).

The screen of one of the computers is pictured below. A "Shortcut to GasChrom" is on the desktop

- double click to launch the program. Once inside the program, press Start, Stop, and Print as

appropriate.

Retention Time (RT)

The retention time, RT, is the time it takes for a compound to travel from the injection port

to the detector; it is reported in minutes on our GCs. The retention time is measured by the

recorder as the time between the moment you press start and the time the detector sees a

peak. If you do not press start at the same time you inject your sample, the RT values will

not be consistent from run to run.

Factors that affect GC separations

Efficient separation of compounds in GC is dependent on the compounds traveling through

the column at different rates. The rate at which a compound travels through a particular GC

system depends on the factors listed below:

Volatility of compound: Low boiling (volatile) components will travel faster

through the column than will high boiling components

Polarity of compounds: Polar compounds will move more slowly, especially if the

column is polar.

Column temperature: Raising the column temperature speeds up all the

compounds in a mixture.

Column packing polarity: Usually, all compounds will move slower on polar

columns, but polar compounds will show a larger effect.

Flow rate of the gas through the column: Speeding up the carrier gas flow

increases the speed with which all compounds move through the column.

Length of the column: The longer the column, the longer it will take all

compounds to elute. Longer columns are employed to obtain better separation.

Generally the number one factor to consider in separation of compounds on the GCs in the

teaching labs is the boiling points of the different components. Differences in polarity of

the compounds is only important if you are separating a mixture of compounds which have

widely different polarities. Column temperature, the polarity of the column, flow rate, and

length of a column are constant in GC runs in the Organic Chemistry Teaching Labs. For

each planned GC experiment, these factors have been optimized to separate your

compounds and the instrument set up by the staff.

Process gas chromatograph GOW MAC Instrument Co

The Series 400 and 400-P Isothermal, Thermal Conductivity Detector (TCD) Gas

Chromatographs are rugged, compact, affordable, service-free GCs designed for high

capability performance while withstanding rough, student or industrial use. They meet the

demands for teaching the principles of gas chromatography, chemistry and biochemistry.

As of January of 2005, the Series 400 GCs has replaced our very popular Series 350 GCs.

- Isothermal Operation

- Detector: Thermal Conductivity Detector (TCD)

- Direct On-Column Injection or Gas Sample Valve (manual or pneumatic)

- Single or Dual Column

- Microscale Compatible: heated, threaded exit ports (optional)

- Accommodates 1/8” or ¼” o.d. metal or 6 mm glass columns

- Signal Amplifier, x10 (optional)

- Independent, Direct Set, Temperature Controls for Column Oven, Detector, Injection

Ports

- Operating temperature: ambient to 300 °C

- Power: 110-220 V, 50-60 Hz

https://wiki.brandeis.edu/twiki/bin/view/Chem/GowMacGCs

GOW-MAC 350 series and 400 series Thermal Conductivity

Gas Chromatograph

How It Works and What It Does

The GOW-MAC measures the changes in thermal conductivity of helium caused by the presence of

a sample's components. A small amount of sample is injected into a long tube called a column. The

helium carrier gas is forced into the column under pressure. The helium acts as a mobile phase,

which sweeps the sample over the stationary phase inside the column. The sample components

migrate through the column at rates depending on their attraction to the stationary phase. Since

these attractions are probably different, the sample components will be separated. As the

separated components exit the column, a detector measures the changes in the thermal

conductivity. This information is used to generate a chromatogram, which plots the emergence of

the components on a time basis along the X-axis. The area of the peak drawn represents the

quantity of the component. The center of the peak marks the time of emergence of that

component. The time of emergence of the next component is represented by the distance to the

next peak. All of this information can be used to identify the components of the sample.

Directions for Use

https://wiki.brandeis.edu/twiki/bin/view/Chem/GOWMAC_SOP

Tips for Use

1. DO NOT TOUCH THE GC DIALS! Contact Gary Koltov for any adjustments. 2. There are 2 manuals for the GCs, one for the DACS Strip Chart Software that runs on the

PC and one for the Gas Chromatograph itself. These manuals are located in Gary Koltov's office. The manuals have a troubleshooting section.

3. Email Gary Koltov with any questions. 4. The Strip Chart manual notes that the software needs to be recalibrated if the PC or GC is

moved and/or changed. 5. There are 2 GC 400's and 4 GC 350's in the organic chem lab. The 350's will be upgraded

eventually. 6. What is the difference between the 350 vs the 400? The answer is that different

temperature regimes are used for the columns. These regimes are adjusted manually. 7. The 400 is more reliable in that the column temperature stays where it is set. The 350

needs more fiddling and attention. 8. A special piece of hardware called a DACS module connects the GC to the PC. Configuring

this setup is described in the DACS software manual. 9. Special note: The GC's usually generate reliable data, except in the following scenario:

There is a known bug in the GC software where the software sometimes doesn't calculate the area under the graph peaks correctly. In certain circumstances, the bottom of the curve between 2 peaks does not hit the baseline. When that happens, the software groups the areas from those 2 peaks together and calculates 1 large area instead of 2 smaller ones, leading to an incorrect concentration calculation. The only workaround is to calculate those 2 areas by hand. GOW-MAC is working on a fix. As an example, this bug has been seen in the Dehydration Experiment, which analyzes 4 low molecular weight alkenes, 25% by volume of each. Notice in the table below how close the boiling points are

to one another. As such, this solution is very difficult to separate so the GC software doesn't recognize the double peak.

Substance Boiling Point

4-Methyl-1-Pentene 54℃

4-Methyl-2-Pentene 54℃

2-Methyl-1-Pentene 58℃

2-Methyl-2-Pentene 67℃

Web Resources

GOW-MAC 400 Series FAQ and info