The Flame Ionization Detector (FID) is the most widely and successfully used gas chromatographic (GC) detector for analyzing volatile hydrocarbons and many carbon containing compounds. It is highly reliable, provides great sensitivity, and has a wide linear range of detection. Here we will give an overview of its theory of operation and its place in gas chromatography.
The FID was developed in 1957 by scientists working for the Commonwealth Scientific and Industrial Research Organization in Melbourne, Australia. With a linear range for 6 or 7 orders of magnitude and limits of detection in the low picogram, the sensitivity of the FID is without parallel in the realm of GC detectors.
Family Tree – Ionization detectors
A number of very sensitive gas chromatographic detectors are based on the principle that a gas mixture behaves as an insulator at ordinary temperatures – unless electrons, or electrically charged atoms or molecules, are present. Some common examples would be the Flame Ionization Detector (FID), the Nitrogen Phosphorous Detector (NPD), the Electron Capture Detector (ECD), and the Photo-Ionization Detector (PID).
FID operation depends on the creation of charged particles produced from compounds by temperature in a flame. Simply put, the number of charged particles present will in some way be proportional to the concentration of the material of interest. In the absence of organic molecules in the carrier gas, this flame is very poor in charged particles, because the combustion of hydrogen with oxygen delivers only a very small number of ions or electrons.
The FID is constructed of a small volume chamber into which the gas chromatograph’s capillary column is directly plumbed. Usually the small diameter capillary is used to feed column effluent mixed with a hydrogen and oxygen through a hollow stainless steel needle (called a jet) where it is burned at the tip. The flame is lit using an electronic igniter, which is actually an electrically heated filament. It is within this flame polarized in an electric field that a complex process takes place. Free electrons and positively charged carbon species are created which then enter a gap between two electrodes. The jet itself serves as an electrode, where an appropriate potential is applied between it and a collector. Usually the collector is a cylindrical tube surrounding the jet, but in some instruments it can be a piece of platinum wire.
The applied potential imposed between the electrodes serves a dual purpose – to lower the electrical resistance across the gap, and to cause an electrical current to flow in the presence of ions. The resultant current is measured by an electrometer.
Detection for the Masses
The FID is a mass flow sensitive detector. That is to say, that for organic compounds the intensity of the signal is proportional to the mass flow of carbon. This is one aspect of the differences between the FID and the Thermal Conductivity Detector (TCD), the latter’s response being concentration dependent.
In any GC detector system interruptions in flow rate might affect peak shape. However, in mass flow sensitive detectors like the FID, this will not affect the total area count. Such interruptions with concentration dependent detectors will cause area counts to no longer represent the mass of the compound flowing through the detector.
Maintaining the flow rate of carrier and makeup gases, applied voltage, flame temperature and detector temperature can go a long way to optimize and stabilize the FID. Note that there is an important distinction between detector housing temperature and flame temperature, the latter being a function of the hydrogen / air ratio, and a key to optimize ionization efficiency. Its operating range, 100-420 deg C, gives it an obvious advantage in programmed temperature applications. Precise temperature control is not a requirement for the FID, however heating the detector housing ensures that water vapor produced from combustion does not condense in the detector. In hazardous work areas or applications it might be a good idea to fit the exhaust with a flame trap or properly vent the FID chimney.
Carbon is Key
The FID responds only to substances that produce charged ions when burned in a hydrogen/air flame. The response from an organic compound is proportional to the number of carbon atoms which can be oxidized under the conditions within the FID. For example butane has twice the number of carbon atoms as an equivalent volume of ethane, so it will, within limits, produce a response twice that of ethane.
Just as instrumental in the FID’s success for determining organic compounds are those things which the detector will not respond to. The FID does not respond to water, or to permanent gases such as N, O, CO, etc. As such it is ideally suited for trace analysis in moist samples and air pollution studies. If desired, CO & CO2 can be quantitated by converting to CH4 by reduction with hydrogen in a Ni catalyst tube and subsequently measured by the detector.
There is no response from fully oxidized carbons such as carbonyl or carboxyl group, and response diminishes with increasing substitution of halogens, amines, and hydroxyl groups. The detector does not respond to inorganic compounds apart from those that are ionized in FID conditions, e.g. approximately 2,000 degrees C.
Making Up is Best to Do
In practice, without makeup gas, the total flow of gases into the detector is too small to get the most sensitive and widest linear response from the FID. In other words, the sum total of the column, fuel, and oxidant flows are insufficient to optimize flame conditions for detection.
In order to maintain the best analytical conditions, additional gas must be constantly supplied to the detector. This gas makes up the additional needed gas flow and so is termed makeup gas. The makeup gas needs to be inert so that its addition doesn’t upset the fuel and oxidant balance, and it needs to be added in relatively large amounts (~30+ ml/min in some detector designs). Therefore, nitrogen is a good gas of choice. Helium would work also but is a nonrenewable resource and more expensive. All gas flows are controlled by adjustable gas regulators.
End of the Line
The FID is a destructive detector. This means that unlike non-destructive detectors, such as a TCD or a PID, compounds which have passed through the FID are no longer available in their original form for subsequent detection by other devices. This may not be important for certain detectors, designed to measure total sulfur or nitrogen, but it is an important consideration when plumbing in line with other detectors.
authored by: Edward Zachowski is President, and Stan Paleologos Laboratory Director, at Alpha Omega Technologies, Inc., 1025 Rte 70, Brielle, NJ 08730. Phone (800) 842-5742, email [email protected], websites www.aoti.net , www.6890.com , & www.5890.com .