t Carrier gas

^-High voltage electrode t Carrier gas

FIGURE 4.5 A scheme of the thermionic detector (by courtesy of Shimadzu).

5. Thermionic Detector (Figure 4.5)

Principle of operation: adding an alkali metal salt to a flame enhances the response to compounds containing N2, P, S. The alkali source is an electrically heated ceramic bead of a sintered complex of an alkaline salt and silicate. The usual salt is rubidium silicate. The mechanism is not fully understood. Gas phase reactions involve free alkali metal atoms in the flame that are ionized by collision with carrier gas molecules.

Free radicals resulting from the pyrolysis of organic compounds containing P or N react with alkali metal atoms. Frequent replacement of the alkali source is still necessary. To overcome this drawback the alkali salt may be dissolved in water and introduced in the detector sensing volume as an aerosol or by means of a syringe pump.

A halogen specific detection method is based on halogen induced thermal electron emission. Detection limit 10"13 g of N/sec; 5.10"14 g of P/sec. The high sensitivity to nitrogen and phosphorous compounds makes this detector suited for pesticide residues and pharmaceuticals.

6. Surface Ionization Detector

The organic molecules from the GC are seeded in a hydrogen or helium supersonic beam and enter the vacuum chamber through a ceramic nozzle. The distance from the top of the nozzle to the surface is roughly 5 mm. In the vacuum chamber, the beam collides with Re02 or Pt surface for efficient positive ion production. The surface is always at a positive potential of 200 V against the collector electrode. The kinetic energy of the sample molecule, which is proportional to the nozzle

temperature, and the surface temperature are the most relevant parameters. ReO2 gives a 20 times higher sensitivity as positive ion-emitting surface.

The sensitivity is expressed as Coulomb per g of sample.

Limit of detection is in the nanogram range (e.g.,1013 g/sec for pyrene) (linear dynamic range 106).

7. Ion Mobility

The first successful use of ion mobility spectrometer (IMS) as detector in GC was in 1982.

Ion mobility spectrometry provides a rapid response to trace gases by converting sample molecules to ions at atmospheric pressure and by characterizing these ions with the help of their gas phase mobilities in weak electric fields.

Next to radioactive isotopes such as, 63Ni, 3H, and 241Am, which are still the ionization sources most commonly employed in IMS, other sources like photo-ionization, corona, or partial discharges, electrospray ionization, and flames have become increasingly popular. However, despite the rising number of regulatory requirements going along with the use of radioactive material, no nonradioactive ionization source unsurpassed the others because of their unique combination of simplicity, long-term stability, and robustness. Recently, manufacturers managed to phase out 63Ni sources. Radio frequency IMS analyzer can be used as a small detector in GC separations of volatile organic compounds since it provides a second dimension of chemical identity.

Ion mobility spectrometers consist of three parts, namely an ionization region, a drift region separated from the ionization region by an ion gate (shutter grid), and a detector. Gaseous samples are transported by a carrier gas into the ionization region where, in the case of a radioactive source, carrier gas molecules are ionized by radiation. So-called reactant ions are created, which undergo a series of reactions with molecules of the analyte to generate product ions that are directed by an electric field E.

D. Photometric Detection

Photometric detectors can be divided into three classifications: emission, absorption, and scattering. 1. GC/AED (Atomic Emission Detector)

An AED detector is a multielement detector capable of detecting elements with atomic emission lines in the vacuum UV, UV-VIS, and near IR portions of the electromagnetic spectrum.

AED allows multielement measurement.

Plasma sources are capable of producing intense emission from the elements. Types of plasma used in chromatographic detection are microwave induced plasmas (MIP) and inductively coupled plasma (ICP). An argon plasma is sustained in a microwave cavity which focuses into a capillary discharge cell. The most widely used cavities are cylindrical resonance cavities and "surfatron" that operates by surface microwave propagation along a plasma column. Atmospheric pressure cavities are very simple to interface with capillary GC columns.

Other plasmas are glow discharge plasmas, and direct current plasmas with a continuous Direct Current arc. A typical AED uses a 50 W microwave generator and a reentrant cavity to focus the energy into a 1 mm i.d. fused silica tube in which a plasma is sustained by a steady flow of helium makeup gas. A spectrometer employing a diffraction grating and a movable photodiode array (PDA) views the plasma axially and can detect the emitted radiation in the 160 to 800 nm region with a 0.1 nm resolution at 400 nm. All major hetero atoms, the halogens, and most metals (e.g., Pb, As, Sn, Hg) can be detected with high sensitivity (LOD 0.1 to 30 pg/sec). In the Pulsed Discharge Emission Detector (PDED) the GC effluent is passed directly into the discharge and the resulting emission spectra are observed. Coupling with a vacuum UV monochromator allows observations of atomic emissions, e.g., Cl, Br, I, and S.

2. Flame Photometric Detector (FPD) (Figure 4.6)

Principle of operation: a flame breaks down large molecules. The high temperature of the flame stimulates atoms and species that are brought to an excited state (S$ or PO*) and relax with emission of a light of characteristic wavelength. In a common burner design, the flame burns on a set of concentric tubes that deliver the reagent gases.

This detector is well adapted for sulfur, phosphorus, or tin determination. Two flames are often used to separate the region of sample decomposition to sample emission. Response is dependent on the environment of the sulfur atoms (thiols, sulfides, disulfides, thiophenes). The FPD can also detect iron.

Limit of detection is around 10"12 g P/sec or 10"10 g S/sec.

3. GC/FTIR (Fourier Transform Infra Red)

MS cannot distinguish closely related structural isomers because they exhibit very similar mass spectra. Infrared (IR) spectroscopy provides information on the intact molecule. There are three basic types of GC-FTIR instruments: (a) light pipe, (b) matrix isolation, and (c) subambient trapping. A light pipe is a narrow bore (100 to 200 fim i.d.) borosilicate capillary with a smooth thin layer of gold coated on the inside surface. Reflection occurs with gold coating thus increasing path length of the cell by a factor of ten or more according to Beer's law. A schematic of GC/FTIR instrumentation is displayed in Figure 4.7.

In the Michelson interferometer a collimated light beam is divided at a beam splitter into two coherent beams of equal amplitude that are incident normally on two plane mirrors. The reflected beams recombine coherently at the beam splitter to give circular interference fringes at infinity focused by a lens at the plane of the detector (see figure on GC-FTIR).

For monochromatic light of wavelength A0 and intensity B(A0) the intensity at the center of the fringe pattern as a function of the optical path difference x between the two beams is given by

Carrier gas

FIGURE 4.6 A scheme of the flame photometric detector (by courtesy of Shimadzu).

Carrier gas

FIGURE 4.6 A scheme of the flame photometric detector (by courtesy of Shimadzu).


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