Q2 2009 / Flame Ionization Detectors

First developed in 1957 by scientists working for the Commonwealth Scientific and Industrial Research Organization in Melbourne, Australia, the Flame Ionization Detector (FID) is today considered a highly reliable detector. It provides a high level of sensitivity combined with a wide linear range of 6 or 7 orders of magnitude (10⁶ to 10⁷) and limits of detection in the low pictogram. Many in the industry believe its sensitivity is so powerful it is without parallel among Gas Chromatographic (GC) detectors.

FIDs – from the family tree of ionization detectors

A number of 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).

Typical FID setup showing connections for ignitor, electrometer and the applied electrical potential.

An FID’s operation depends on the creation of charged particles produced from compounds by temperature in a flame. Simply put, the number of charged particles present is proportional to the concentration of the material examined. In the absence of organic molecules in the carrier gas, the flame is relatively poor in the number of charged particles present because the combustion of hydrogen with oxygen delivers only a small number of ions or electrons.

An FID is constructed with 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.

Within this flame, polarized in an electric field, a complex process takes place. Free electrons and positively charged carbon species are created. The electrons and charged carbon species 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.

A cross-section of a Flame Ionization Detector.

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

An 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. In fact, this is one of the chief differences between FIDs and Thermal Conductivity Detectors (TCDs) – the latter’s response is concentration dependent.

In any GC detector system, interruptions in the flow rate might affect peak shape. However, in mass flow sensitive detectors like the FID, this will not affect the total area count. Interruptions with concentration dependent detectors will typically cause the area counts to inaccurately represent the amount 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 °C, gives it an obvious advantage in programmed temperature applications.

Overall GC performance has improved in recent years as with this PerkinElmer Clarus 600 GC, which can deliver faster injection-to-injection time than conventional GCs.

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 that 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.

Diplomatic immunity

Just as instrumental in an FID’s success for determining organic compounds are the substances that the detector ignores. An FID does not respond to water, or permanent gases such as N₂, O₂, and CO₂. This makes it ideal for trace analysis of moist samples and for air pollution studies. If desired, CO and CO₂ can be quantified by converting them to CH₄ by reduction with hydrogen in a Ni catalyst tube and then measured by the detector. This is particularly handy when these compounds are in concentrations so low as to escape detection by a TCD.

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 of approximately 2,000°C.

Making up is great to do

In practice, without a makeup gas, the total flow of gases into the detector is too small to gain the most sensitive and widest linear response from an FID. In other words, the sum total of the column, fuel, and oxidant flows are insufficient to optimize flame conditions for detection. So in order to maintain the best analytical conditions, additional gas must be constantly supplied to the detector. Since this gas makes up the additional needed gas flow it is termed “makeup gas.”

The makeup gas needs to be inert so that its addition doesn’t upset the fuel and oxidant balance. It also needs to be added in relatively large amounts (~30+ ml/min in some detector designs). Nitrogen is a good choice. Helium would also work, but is a non-renewable resource and more expensive. All gas flows are controlled by adjustable gas regulators.

At the end of the line

Remember, an FID is a destructive detector. This means that unlike non-destructive detectors, such as TCDs or PIDs, the compounds that pass through an FID are no longer available in their original form for subsequent detection by other devices. This may not matter for some detectors, such as those designed to measure total sulfur or nitrogen, but it may be an important consideration when an application requires plumbing in-line with other detectors. SGR

Photos courtesy of PerkinElmer Corporation.

For more information contact Ed and Stan at:

Alpha Omega Technologies, Inc 1025 Rte 70, Brielle,

NJ 08730.

Phone: (800) 842-5742; Email: info@aoti.net;