Gas chromatography is an established analytical separation technique. It applies to the analysis of organic compounds. The GC is the instrument that makes it possible. It consist of an oven, a column, an injector, eventually an autosampler, a detector and a pneumatic system. The compounds are injected onto a column, vaporized, and separated according to their polarity and/or boiling point depending on the column chosen.

Olfactometry is the measurement of odors, good or bad. Olfactometers are commonly used in food, flavor and fragrance companies. These instruments are used mostly for consumer evaluation. GC-Olafctometry adds a dimension to the technique.

When they are separated prior to the olfactometry, the compounds responsible for an odor can be smelled one by one, and the odor broken down into multiple odors.
The instrument used for the olfactometry part is usually called a sniffer port.

Identification of odor, or compound responsible for an odor is one of the main applications of GC-O. This applies, for example, to the identification of the compound(s) responsible for the odor emitted by a flower. It can also be used for deformulation of products or, in a less pleasant application, the identification of the compound(s) responsible for an odor contamination in a product.

Identification of compounds is fairly easy using the spectroscopy technique available today. Knowing which one is responsible for the odor is a different story. Moreover, the human nose is often more sensitive than the detector available (MS, FID, PID, etc...). Therefore, an analysis of a product by GC/MS might not be enough to identify the odorous compounds.
This is where the human nose comes into play. By comparing the response from the detector (FID, MS, ...) to the response of the panelist (person who performs the GC-O) , it is easy to find out which compounds smell, which ones don't and what they smell like.

Quantitation of odor is a bit more challenging and requires preparation and training. It is not as exact as the quantitation done by straight GC or GC/MS, but gives a good idea of the quantity of compound used. It can, however, be combined with the results (if any) from the detector. As with a GC or GC/MS the panelist must be trained to recognize the odor intensity at different concentrations.

Important: The most important issue with GC-O is safety. Although the quantity of compound is small, smelling dangerous compounds can be hazardous. GC-O should not be conducted on unknown sample. A profile of the main components must be acquired and studied before running a sample by GC-O. If any harmful compounds are found, it is still possible to conduct GC-O, but alarms must be set to warn the panelist of the elution of such compounds.
Injection techniques such as headspace can be used to avoid injecting too many harmful compounds. Headspace injection is usually a valuable tool for GC-O and odor studies, as most of the compounds work well with this technique.

The instrumentation for GC-O

The sniffer port is of course the main apparatus for conducting GC-O. Special attention must be paid to the comfort offered to the panelist. For optimal results, the panelist must focus on the smell and nothing else. Not all sniffers are created equal and amazingly, the results will vary with the quality of the instrument.

A sniffer must have the following options:

Heated interface
Make-up gas
Supply of humid air to prevent nasal dryness and improve the sensitivity of the panelist
Adjustable sniffing cone for optimal comfort
Device for generating an analog signal for the odor intensity

Inside the GC-O system

The column output is split between the GC-detector and the sniffing port. The split ratio is adjusted with the diameter and length of the output branches.
The humid air is added before the heated interface to prevent condensation. The flow is adjustable for optimum comfort.

Make-up gas can be added to speed up the elution of the odor.

Fraction collector

Often during a GC-O run, some compounds can be smelled by the panelist but are not detected by the physical detector (FID, MS, etc...). In this case, fraction collection is a useful tool. The collection of the areas where the compounds are smelled is done by repetitive injection/ collection. The fractions collected can then be analyzed by GC/MS or any other technique (such as UV-VIS, IR, NMR).

The Prep 9000

The Prep 9000 is designed to fit on top of the sniffer 9000. By using the same configuration as for the GC-O run, the integrity of the retention time is preserved and the transition time from sniffing to collection is minimized. The system is programmed either from the internal timer or from the external event from the GC. The Prep 9000 collects the compounds on adsorbent-filled tubes. Up to 10 fractions can be collected in a single run. The fractions collected are then extracted by solvent or each tube thermally desorbed. With the proper solvent, the efficiency of the system is close to 90%.

Additional features to improve sniffing ports

Additional tools can help the panelists conducting efficient GC-O. These tools include devices to help generate accurate signals, prevent the thermal degradation of the compounds, and software to record the comments made during the GC-O run.

The fingerspan is a device used to generate accurate odor intensity. For a full span of the panelist's finger, the signal output is set at 100%. During the run, the panelist spreads his fingers. The more the fingers are spread, the more intense the signal.
This technique yields reproducible results for semi-quantitation and evaluation of odor thresholds.

For recording comments, several techniques are used. Software has been developed to assist the characterization of the odor. The problem with this type of software is that it usually distracts the panelist from the sniffing.

Other techniques include video taping the GC-O experiment; unfortunately, this is time consuming. Indeed, the video needs to be transcribed. Using an assistant to write down the comments and retention time requires finding someone willing to spare the time to listen to the panelist. Voice recognition software is probably the most efficient and least distracting technique to record comments (for more details, see the software section).

Voice recognition software

To the best of our knowledge, the only voice recognition software for GC-O is Nose to Text. Today's computers can easily handle voice recognition. The main computer requirements are a large memory (~256Mb) and a good sound card.

Voice recognition presents several advantages when applied to GC-O. The main advantage is that it lets the panelist focus on the experiment. The other is that it records both retention time and comments. In addition to the recognition (acquisition) function, it features several reporting functions as well as library search capabilities.

Each panelist must "train" the voice recognition software. This process takes 10 to 15 minutes and allows the voice recognition program to adapt its algorithm to the pitch and intonation of the speaker. The panelist is equipped with a headset microphone connected to the sound card. All start and stop commands are vocal commands.

During the run, as the panelist smells a compound, he describes the odor and impressions (such as intensity). The comments are recognized and added to the result box along with the retention time. At the end of the run, reports can be created.

Nose to Text is designed to accommodate several panelists (one at a time).

Reporting functions

One of the interesting features of Nose to Text is the reporting function. Several types of reports can be generated depending on the recipient.

The GC report function merges the data recognized to a GC report. The GC report is first generated using the odor intensity trace and then the recognized data are merged to this report. This type of report is designed for GC users.
The odor report lists all the odors found and associated comments in the order in which they are listed. Following compounds (data files) are added the same way. The odor report function is a tool designed for non-GC personnel. It gives a quick view of the odor found in the samples. This type of report does not include the retention time.
The target odor report lists only the odor targeted. A list of targeted odors is created. Once the run is finished, the software compares the results to this list and reports the targeted odor(s) found with the associated comments. Again, file comparison is possible

Library Search capabilities

The library feature has been added to facilitate the identification of compounds. As no commercial odor library is available, the Nose to Text library has been designed to be very flexible. Any company with an in-house library can import the data into Nose to Text.

Since the library is designed for GC, it includes retention indices for up to 10 columns. Other fields includes compound name, odor description, and up to 6 custom fields (for parameters such as MW, Formula, Synonym, CAS, structure files...)

The search can be done by odor, compound name, or any of the custom fields. The retention indices are used to restrict the search to a limited range of indices.

Conducting a GC-O run

On this page, we will look at how to conduct GC-O. After describing the setup, we will review how to conduct a run and what you should expect from it.

Conducting a GC-O run requires complete focus from the panelist. A few steps can be taken in order to minimize distraction and improve results.

Instrument setup

GC-O experiments need to be conducted in a quiet environment. While the panelist smells, he must not be disturbed. Someone walking in the lab, a ringing telephone or any other noise is a distraction. Unfortunately, labs are often crowded and space is limited. It is therefore difficult to place the GC-O system in a lab by itself. Steps can be taken to improve the setup. Keep the GC-O system away from the main traffic area. Install the system near the end of the lab where there is as little traffic as possible.

Situate the sniffing port such that the panelist is facing the wall and not the entrance. If possible, install a divider between the lab and the sniffing port. The surrounding area must also be cleared of odors. An overpressurized lab is recommended. The quality of the instrumentation and the comfort of the panelist must be optimized for best results.

The sniffing port must have a humid air supply. This improves user comfort by preventing nasal dryness, and it increases the olfactory sensitivity of the panelist. The sniffing port cannot be placed in a hot area (such as on top of the GC). As the oven heats up, the top of the GC becomes hot, making it very uncomfortable for the panelist. It is best to have the sniffing port installed to the side of the GC (about 20 inches away).

Staying focused

The panelist must be seated in a natural and comfortable position. Standing up during the GC-O run is exhausting and the panelist is liable to lose his concentration.

Place your elbows around the sniffing port and hold your head such that your nose is placed near the sniffing cone.

One of the challenges of GC-O is to constantly smell during the run. Respiration is one of the most difficult things to master.

If you breathe too fast, you will hyperventilate and eventually pass out; too slowly and you might miss some effluents. You must find a rhythm that is comfortable and does not induce dizziness. Everybody is different and must find his own rhythm.

A main factor in getting optimal results is to stay focused during the run. Panelists have their own tricks for avoiding disturbance. Closing your eyes and using ear plugs are two ways to isolate yourself from any disturbing elements. Again, GC-O is very demanding. Take a break between runs; walk away from the instrument and catch your breath! After four or five runs, you will be probably be done for the day.

External influence

External influence refers to that which makes the panelist think he should smell something. An obvious example is the FID trace, which is displayed in real time on the monitor. A peak on the display prompts the panelist to, have the feeling he should smell something, and in fact he probably will.

Depending of the purpose of the run, it is sometimes best to conduct what is called a blind run. This is when the monitor is turned away from the panelist so the FID trace is not displayed.

Multi-panelist configurations: Few systems offer the multi-post option. This configuration is a great system as several panelists can smell the same thing at he same time. This is also a situation that prompts the most external influences. If one panelist suspects another panelist smells something, he himself will smell something.

Careful measures must be taken to avoid interaction between panelists. Place a screen between each sniffing port. Avoid any noisy signal generation.
Isolate the panelists from noise either with ear plugs or with headsets playing music.

Before you start

Safety is a factor to be considered. Although the quantity of compound is small, GC-O should not be conducted on unknown compounds. Before conducting, identify the compounds present in the sample and check MSDS for any harmful compounds.

Odor identification

Identifying all the odors in a sample is challenging. Training is an important factor in identifying odors. The more you conduct GC-O, the better you will be at it. Training should consist of smelling and familiarizing yourself with different types of odors. A known mixture should be prepared and run several times. The results should be compared to the literature or compound definitions. If a compound poses a problem to a panelist, it is best to run it by itself. Note that panelists react differently to some compounds. It is possible that panelist A will smell a compound and panelist B will not. Define a list of terms (odor descriptors) to be used and associate it with real compounds from the training mixture.

It usually requires several runs to identify all odors in a sample. The first run should be considered a familiarization run to get a handle on the types of odors. The following runs help refine the descriptions of the odors.

Take a break between each run, step away from the instrument and relax. It has been shown that after a few runs (5 or so) the "sensitivity" of the panelist decreases, so you should not expect to run too many samples per day.

As you smell the compounds, try to identify the type of odor detected, then narrow it down to the precise odor. If you cannot identify it precisely, try to narrow it down to a type of odor and try to define it as much as possible (nice fruity odor, foul odor). This is where a fraction collector can be useful.

If, despite all your efforts, an odor cannot be identified, collect it on a trap and run it by itself on a GC/MS or some other technique available for identification. Search the literature for a description of the compound. Run the collected compound by itself and familiarize yourself with the smell.

Odor threshold

The odor threshold is the minimum quantity that can be detected by panelists. This threshold will vary depending on the panelist.

It is best to conduct the experiment on a single compound. Prepare several solutions by consecutive dilution. Optimize the run for time. It should be as short as possible. Inject each solution in a random order without the knowledge of the panelist.

Recording odor intensity

The odor intensity is a trace just like the chromatogram from the FID. It is generated by the panelist as he smells the compounds. The signal generated by the panelist is sent to the external detector of the GC data system. There are two types of odor intensities

Square signals, indicative of the presence of an odor
Odor peaks, indicative of the intensity of the smell.

The odorous compounds are easily identified by overlaying the FID trace and odorogram.

Overlay of the FID trace and Odorogram

The square odor intensity

This type of signal is generated with an On-Off switch. It indicates the presence of an odor with no indication of the intensity. As the panelist smells a peak, he depresses a switch that sends a fixed signal to the data system. The switch is kept on for as long as the odor elutes.

The odorogram intensity

The intensity of the signal is a function of the odor intensity. It is generated with a progressive signal generator such as the Fingerspan. Evaluating the odor intensity requires training. This is equivalent to calibrating the detector by injecting a compound at different concentrations (high, low and average). As the panelist starts smelling a peak, he slides the cursor (if using a Fingerspan device). When the odor intensity is at its highest level,, the cursor is held in position. As the odor decreases, the cursor is bought back to zero.