INSTRUMENTAL ANALYSIS AND SEPARATIONS

Detectors for Gas chromatography - LECTURE 24

There are a number of detectors used in GC. Generally the detector produces a signal in proportion to the amount of analyte present. Some types are based on concentration and others are mass dependent. The detectors produce an electrical signal that is sent to a recording and integrating device. The signal strength and retention time are recorded for further analysis by the chemist. For analysis, the analyte must be thermally stable and volatile. However, if the analytes meets this criteria, GC is preferable to HPLC because of its much higher resolution. GC detectors also typically have lower limits of detection and higher linear dynamic range.

Flame Ionization Detector

The most commonly used detector is the flame ionization detector (FID) it is a general carbon detector. It does not detect compounds that do not contain carbon such as nitrogen(N₂), oxygen(O₂), or water. The presence of N, O, or S in a carbon compound will tend to decrease the response of the FID.

The Carbon atoms (C-C bonds) are burned in a hydrogen flame. The hydrogen can be supplied ether from a cylinder or from an electrolytic hydrogen generator. The hydrogen must be pure to avoid background noise. A charcoal filter is often placed in the hydrogen supply line to remove any organic contaminants.

The response of the detector depends on the flow of the hydrogen, air and the makeup gas (if it is used). A certain amount of inert gas is needed for optimum response of the detector. Generally the flow from a capillary is too low so a makeup gas is used to provided the inert gas flow. The makeup gas has other beneficial effects such as stabilizing the detector, prolonging the lifetime of the jet, and purging any unswept areas of the detector. It is also very important to adjust the air and hydrogen gas flows for optimum response.

The FID must be heated. There are two main reasons for this. First, the burning of hydrogen in air produces water, which can reduce the detector response and even put out the flame. The second reason for heating the detector is to avoid condensation and deposition of compounds in the detector.

The detector response depends on the ionization of carbon atoms. Only a small portion are actually ionized (about 1 in 10,000), but since there is such a low background signal with the FID, this is enough. The ions carry a charge from the flame to the walls of the detector which surrounds the flame. The charge is electronically amplified and sent to a recording device. The FID is very sensitive down to 10^-12 g. It has a high linear dynamic range 10⁷ and is very robust and reliable.

Nitrogen Phosphorous Detector

Another common type of detector is the nitrogen/phosphorous detector (NPD). It is sometimes called the alkali flame detector or flame thermionic detector. It uses a bead of a compound such as rubidium silicate above a jet of H₂. The bead is heated by an electric current to 600-800^oC forming a plasma in the region of the bead. Nitrogen and phosphorous react in this boundry layer of plasma forming specific ions that carry a small current from the plasma to the charged collector (The mechanism of NPD response is still not well understood). The NPD is electronically similar to the FID, but since a flame is not sustained in the detector, hydrocarbon ionization does not occur resulting in a high selectivity. The response to N over C is about 10³ – 10⁵ greater and response to phosphorous 10⁴ – 10 ^5.5 higher than response to C. The linear dynamic range is 10⁴. However, the detector is only fairly reliable since the bead burns up over time causing drift in the signal.

Flame Photometric Detector

The flame photometric detector (FPD) is an element specific detector which can be used for analyzing many specific compounds However, commercially available instruments are generally limited to the detection of P and S containing molecules. The analytes are burned in a H₂ flame causing electrons to move to an excited unstable state. When the electrons return to the ground state, they emit a specific wavelength of light (526 nm for P and 394 nm for S). These wavelengths are monitored by a photomultiplier, amplified, and turned into an electrical signal. This detector is sensitive to 10^-9g and has a linear dynamic range for P of only 10³ – 10⁴. For S, the response is non-linear. This is because the response is due to the formation of the S₂ radical and therefore the response is proportional to the square of the sulfur content. Commercial instruments often have a square root adjustment function built in, but since the response can deviate from the theoretical square root relationship, the output can still be non-linear.

Electron Capture Detector

Another useful detector is the electron capture detector (ECD) It is an excellent detector for molecules containing an electronegative group such as Cl or F etc. (or derivitized molecules) It is probably the second most common detector after the FID. It is most often used for the trace measurement of halogen compounds in environmental applications for detecting insecticide and herbicide residues.

The ECD uses a radioactive source such as Ni⁶³ which produces Beta particles which react with the carrier gas producing free electrons. These electrons flow to the anode producing an electrical signal . When electrophillic molecules are present, they capture the free electrons, lowering the signal. The amount of lowering is proportional to the amount of analyte present. It is sensitive down to 10^-15 but the dynamic range is only about 10⁴.

Atomic Emission Detector

One of the newest gas chromatography detectors is the atomic emission detector (AED). The AED is quite expensive compared to other commercially available GC detectors, but can be a powerful alternative. The strength of the AED lies in the detector's ability to simultaneously determine many of the elements in analytes that elute from the column. It uses microwave energy to excite helium molecules (carrier gas) which emit radiation which breaks down molecules to atoms such as S, N, P, Hg, As, etc. These excited molecules emit distinctive wavelengths which can be separated by a grating and sent to the detector (typically a photodiode array) which produces the electrical signal. The atomic emission detector is very sensitive (10^-15) and has a dynamic range of 10⁴.

Photoionization Detector

The photoionization detector (PID) uses a UV lamp (xenon, krypton or argon lamp, depending on the ionization potential of the analytes) to ionize compounds. The ionization produces a current between the two electrodes in the detector. The detector is non-destructive and can be more sensitive than an FID for certain compounds (substituted aromatics and cyclic compounds for example).

Thermal Conductivity Detector

The thermal conductivity detector (TCD) consists of an electrically-heated wire or thermistor. The temperature of the sensing element depends on the thermal conductivity of the gas flowing around it. Changes in thermal conductivity, such as when organic molecules displace some of the carrier gas, cause a temperature rise in the element which is sensed as a change in resistance. Low molecular weight gases have high conductivities so hydrogen and helium are often used as carrier gases. Nitrogen and argon have similar conductivities to many organic volatiles and are often not used. However if nitrogen is used as a carrier gas, the detector can be used to measure hydrogen or helium. TCD's are often used to measure lightweight gases or water (compounds for which the FID does not respond). The TCD is not as sensitive as other detectors but it is a universal detector and is non-destructive. However, modern detectors called micro-TCD's have very small cell volumes, and new electronics that produce much higher sensitivities and wider linear ranges. Due to its increased sensitivity, and the fact that it is a universal non-destructive detector, it is again becoming more popular for certain applications.

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