This first edition helps you to get the best from the Flame Ionisation Detector (FID) for capillary GC, along with troubleshooting tips and maintenance recommendations.
FID Detector Use and Optimisation The most widely used of all detectors in capillary GC is both sensitive and widely applicable. It’s necessary, however, to understand the key parameters and settings in order to get the most for the detector and avoid issues with sensitivity and separation. These key facts for
FID operation act as a Quick Guide for FID optimization.
Three gases are required for FID detection with capillary columns :
A FUEL gas, typically Hydrogen
An OXIDISER gas, typically Air
A MAKEUP gas, typically Nitrogen or Helium
The Fuel gas and Oxidiser combine to form the flame in which the analyte will combust and ionise. The makeup gas assists the analyte up into the flame and through the detector and minimizes dead volume effects.
Detector Gas Flow Rates
The ratio’s of detector gas flow rates are crucial for optimum sensitivity and good peak shape. Most FID detectors operate at maximum sensitivity with a volumetric flow rate of between 20 and 40 ml/min. This is a combination of carrier and makeup gas flows. As most capillary columns operate between 0.1 and 10 ml/min, well below the optimum required for maximum sensitivity, we can set the makeup gas flow rate accordingly.
Typical FID default flow rates:
Carrier Gas:
Makeup Gas:
Hydrogen:
Air:
1ml/min.
30ml/min.
30ml/min.
300ml/min
If the makeup gas flow rate is too low, the analyte band is not swept effectively through the dead volume of the detector and peak tailing / broadening will occur. If the makeup gas flow is too high, then peak shape will be good but sensitivity may be reduced as the analyte is swept through too quickly for the detector to respond fully.
Figure 1: Typical FID detector schematic
FID Sensitivity
The sensitivity of the FID detector depends upon several factors that include:
Combustion gas flow rates (and their relative ratio sometimes called the Gas Stoichiometry)
Makeup gas flow rate (see above)
Flame jet exit diameter
Carrier Gas Flow Rate
Relative positions of jet (anode) and collector (cathode)
Position of the column relative to the jet exit
Detector temperature (to a lesser extent)
The optimum stoichiometry of the combustion gases is typically 10:1, air to hydrogen.
(a)The sensitivity of the detector will typically deviate with hydrogen flow as shown in Figure 2.
The FID detector burns a Hydrogen / Air flame which is hot enough to pyrolise most organic compounds which pass through it producing cations and electrons which move between the anode and cathode to form a minute current. The exact mechanisms of ion production are not well understood, however the ‘composition’ of the flame, that is the ratio of air to hydrogen, appears to have a significant effect on ion production and therefore sensitivity. The response of the detector is directly proportional to the number of reduced carbon atoms moving between the two electrodes per unit time.
If hydrogen flow rate is too high this tends to reduce the dynamic linear range of the detector and one should use the manufacturers recommended hydrogen flow rate wherever possible as a starting point.
Figure 2:Effect of Hydrogen on
relative sensitivity of FID detector
(b) Makeup gas flow rate (see above)
(c) Flame jet exit diameter
The jet diameter will dictate the rate at which the carrier gar flows into the flame – i.e. the volumetric flow and linear velocity of the gas eluting in the flame. With packed GC columns the carrier gas flow rate (and hence the volumetric flow into the flame) is usually high enough to produce a reasonable detector response. Standard FID jets have exit diameters of approximately 0.5–0.7 mm, which is suitable for most applications.
A smaller jet of about 0.3 mm i.d. is often used with capillary columns to gain sensitivity (about 1.5x) through increasing linear velocity of the carrier into the flame. A narrow FID jet is not recommended for packed-column use because stray column packing support easily can clog the jet passage. Conversely, narrower jets prevent the tip of a capillary from accidentally protruding into the flame.
Capillary GC carrier flow can be operated in two modes – ‘Constant Pressure’ or ‘Constant Flow’ In constant pressure mode, the flow of the carrier gas can reduce as the oven temperature increases. Whilst the FID detector is not ‘Mass Flow Sensitive’ some baseline rise may be observed during the temperature gradient.
This phenomenon will be eliminated when operating in ‘Constant Flow’ mode as the instrument adjusts the pressure to maintain constant carrier flow during the temperature gradient. Some modern FID detectors offer the ability to ramp the make up gas flow so that overall ‘carrier + make up’ flow does not alter even when operating the carrier gas in constant pressure mode.
(e) Relative positions of jet (anode) and collector (cathode)
The relative positions of jet (anode) and collector (cathode) are fixed by the instrument manufacturer to give the optimum sensitivity.
(f) Position of the column relative to the jet exit
If the tip of the capillary column is placed too low in the FID jet, there will be a large unswept volume and peak broadening will occur.
If the end of the column is touching the jet tip, the polyimide column coating will ‘bake out’ and may chip, releasing small particles of polyimide into the flame, causing ‘spikes’ on the baseline.
Spikes on the GC baseline - potentially caused by
incorrect positioning of the capillary column in the FID detector.
The ideal position for the column end is 1-2mm below the jet orifice.
(g) Detector temperature (to a lesser extent)
The response (sensitivity) of the detector is not closely linked with detector temperature providing two basic conditions are met. First, the detector must be at a minimum of 150oC for stable operation and to avoid water condensation in the detector housing; and second that the detector temperature is between 20 and 50oC above the maximum column oven temperature for that particular application. Any higher than this and stationary phase may bleed from the portion of the column inside the detector causing baseline disturbances, noise and potentially poor peak shape due to analyte adsorption on exposed silica surfaces.
The FID detector is highly robust and reliable with a wide linear range. However, as with most GC detectors, there is a reliance on high purity bases and routine maintenance to ensure optimal performance. We have highlighted here some of the key maintenance and troubleshooting operations, which will be further highlighted during the next Essential Guide Webcast on April 26th.
(a) Gas supplies:
All detector gases must meet minimum purity and flow-control specifications. Table I summarizes carrier, combustion, and makeup gas purity, purification, and regulation requirements for some common detector systems.
In addition to procuring sufficiently pure gases, chromatographers must ensure that appropriate gas pressure regulators and purifiers are in place. An inexpensive regulator can add impurities to the gas passing through and fail to regulate the gas pressure accurately. Without proper purification, minute leaks between a tank and an instrument can compromise gas purity and eliminate the advantage of more-expensive, high-purity gases.
Reduction in analyte or increase in background noise is usually due to improperly set gas flow rates (poor gas quality) or a contaminated detector. Gas flows and purity have been discussed above.
Detectors become contaminated with combustion products that form on the jet and collector – eventually resulting in occlusion and loss of electrical insulation and a high background signal. Cleaning is usually fairly straightforward but care should be taken not to deform the orifice in the jet tip otherwise on-going reproducibility issues will occur.
(c) Difficulty Lighting the Detector:
This is usually due to the glow plug being inoperable or improperly set gas ratios.
Do not use piezo-electronic (static) lighters to ignite the detector flame as these may cause irreparable damage to the detetctor electronics.
Often it is necessary to reduce the air flow and increase gradually whilst lighting the glow plug. The flame will light (with a ‘pop’) at the correct air / hydrogen stiochiometric ratio after which the air flow can be gradually increased to the desired value.
The makeup gas flow is important to correct detector operation and the makeup gas should be flowing before attempting to light the detector. A blocked FID jet may also cause issues lighting the jet – follow your manufacturers guidelines for jet cleaning.
These are usually attributable to poor positioning of the capillary column relative to the detector jet tip. Peak broadening or tailing occurs when the column is positioned too low into the detector.
(e) Baseline issues:
Rising baselines can be attributed to changes in carrier gas flow during temperature programmed operation. This issue is discussed elsewhere in this newsletter.
Spikes in the baseline are caused when particulate matter passes through the flame region of the detector. This is usually due to baked polyimide from the column external surface if the detector is too hot or the column is too high in the detector and the column end is ‘crushed’ as the column nut is tightened.
Spikes in the baseline may also be due to electrical surges (rapid voltage changes). One should ensure a ‘smoothed’ electrical supply to the GC instrument.
