Flame ionization and thermal conductivity have been the most common detectors in gas chromatography (GC) (link to GC page) for decades. Both are compatible with capillary and standard-bore GC. But what makes them tick, and when should you choose one over the other? Let’s put them face-to-face to compare strengths, limitations and best applications of flame ionization detection (FID) and thermal conductivity detection (TCD) (links to product pages).
Flame Ionization Detection
First developed in the 1950s1, FID is the most common method used with all types of GC (link to previous blog post).
In a typical FID design, the mobile phase passes from the GC column into an oven, keeping the sample in gas phase. As it moves into the detector, the eluent is mixed with combustion gas, then oxidant. The mixture travels up to a jet nozzle and into the flame to burn.
Voltage applied between the nozzle (positive) and the collector plates (negative) accelerates any reduced carbon ions toward the plates, for detection by a sensitive ammeter. The signal is converted to voltage, amplified, filtered, and recorded by the GC data system. The effluent—mostly carbon dioxide and water—vents through an exhaust port.
Reasons to Choose FID
FID detector electronics filter out high-frequency background noise, for very robust signal-to-noise performance.
- Good response to most analytes
- Outstanding sensitivity and resolution: down to a few picograms of carbon per second
- Wide linear range; generally better linearity than TCD
- Low cost
- Simple to use
- Robust, reliable
When Not to Use FID
- For analytes that lack carbon-hydrogen bonds (not detectable by FID)
- When you want your sample back – destructive method
- Situations where volatile combustion gas and flame jet are problematic
- For stationary phases that may bleed contaminants at high temperature, or samples high in chlorinated or other compounds that burn inefficiently
- Determination of any ionizable hydrocarbons
- Petrochemical analyses
- Biodiesel testing
- Complementary detection in parallel with GC-MS
- Technical Note 10426: Porting the Agilent Blood Alcohol Application Note 5990-9021EN (PDF)
- Application Note 10316: Analyzing Residual Solvents in Pharmaceutical Products Using GC Headspace with Valve-and-Loop Sampling (PDF)
- Application Note 14105: Determination of Free and Total Glycerin in Pure Biodiesel (B100) by GC in Compliance with EN 14105 (PDF)
- Webinar: Automated In-Line Sample Preparation and GC Analysis of Biodiesel According to EN14105 & ASTM D658
Thermal Conductivity Detection
Based on technology that predates even FID1, TCD relies on differences in thermal conductivity between target analytes and a carrier gas. Many compounds have relatively low thermal conductivity in the gas phase, so they can produce negative peaks against the background from a highly thermally-conductive carrier such as helium or hydrogen. Other carriers are used as well—in theory, anything that isn’t an analyte of interest.
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A typical TCD design directs carrier gas flow to two temperature-controlled chambers: one that receives the column eluent (column flow) and another that receives only carrier gas (reference flow). Each chamber surrounds one of four filaments arranged in a bridge circuit.
Electric current applied across the filaments heats them. Carrier gas flowing through each chamber removes that heat at a constant rate, creating a stable background temperature. When sample compounds with relatively lower thermal conductivity pass through the column flow chamber, they remove less heat. The resulting transient increase in temperature changes the filament’s resistance, and is detected as a difference from the reference chamber baseline.
Reasons to Choose TCD
TCD detects any species that differs from the carrier gas in thermal conductivity – essentially any gas, given the right carrier. It responds similarly to set analyte concentrations within a class, for example, across organic compounds, so can be used to estimate relative concentration of each peak (ratio of peaks).
- Universal; responds to any species that isn’t the carrier gas
- Improving sensitivity: down to single-digit parts-per-million (link to application note) in newer models
- Non-destructive to samples
- Low cost
- Simple to use
- Robust, reliable alternative for analytes that respond poorly to other detectors, or where FID combustion gas is impractical
When Not to Use TCD
- When sensitivity and resolution are key; lower with TCD than FID and other methods
- When cross-talk between similar hydrocarbons could be troublesome
- Initial determination of unknowns
- Fixed gas ( O2, N2, CO, CO2, H2S, NO, NO2, etc.), water and solvents analysis
- Aggressive acids and amines
- Water in industrial solvents
- Trace ammonia in gas samples
- Poster: Enhanced Sensitivity Miniaturized Thermal Conductivity Detector for the Determination of Water, Ammonia, and Trace Impurities in Industrial Solvents (PDF)
- Application Note 10351: Analysis of Flue Gases with GC and TCD Detection (PDF)
- Easy swapping between FID, TCD, and other methods: Analytical Repeatability, Accuracy, and Robustness of Instant Connect GC Modules (PDF)
Keep an eye out for parts II and III in the Detector Detective series, where we’ll explore GC-MS and specialty detection methods.
- Hinshaw, JV. Flame Ionization: GC’s Workhorse Detector. LCGC North America. Jul 01, 2015; 33(7):470-477. http://www.chromatographyonline.com/flame-ionization-gcs-workhorse-detector. Accessed online 14 Oct, 2015.