Radio frequency analyzers

Quadrupole analyzer. A mass filter that, creates a quadrupole field with a DC component and an RF (radio frequency) component in such a manner as to allow transmission only of ions having a selected mass-to-charge (m/z) ratio. 

If the potential applied across the cathode and the anode is constant (dc), conductive materials can be analyzed if the potential is varying at radio frequency (rf), both conductive and non-conductive materials can be analyzed. 

Theory. If two or more fluorophores with different emission lifetimes contribute to the same broad, unresolved emission spectrum, their separate emission spectra often can be resolved by the technique of phase-resolved fluorometry. In this method the excitation light is modulated sinusoidally, usually in the radio-frequency range, and the emission is analyzed with a phase sensitive detector. The emission appears as a sinusoidally modulated signal, shifted in phase from the excitation modulation and partially demodulated by an amount dependent on the lifetime of the fluorophore excited state (5, Chapter 4). The detector phase can be adjusted to be exactly out-of-phase with the emission from any one fluorophore, so that the contribution to the total spectrum from that fluorophore is suppressed. For a sample with two fluorophores, suppressing the emission from one fluorophore leaves a spectrum caused only by the other, which then can be directly recorded. With more than two flurophores the problem is more complicated but a number of techniques for deconvoluting the complex emission curve have been developed making use of several modulation frequencies and measurement phase angles (79). 

The quadrupole analyzer uses oscillating electrical fields to separate ions based on their stable trajectories. The analyzer consists of four parallel poles placed between the ion source and the detector in such a manner that the path of the ion beam travels through the middle. The rods have DC voltages applied to opposite rods to carry the same charge, yielding one set of positive rods and one set of negative rods. All four rods have an oscillating radio frequency (RP) applied to them (Honour, 2003). If the ion mass is too... 

Solid state NMR spectroscopy is used to determine molecular structures by analyzing the static and dynamic features of the material. In NMR experiments, both a magnetic field and a radio frequency field are applied to a solid sample or a solution resulting in an absorption of energy, which is detected as an NMR. Spectrometers are also available for high resolution solid state NMR. Nuclei in... 

Quadrupole or radio-frequency mass analyzers have only recently become available with the mass range and resolution 42) to be of general use in inorganic and organometallie mass spectrometry, although they have been popular as small mass spectrometers built into specific systems as reaction monitors. They do have the advantage of essentially linear... 

Emission spectroscopy utilizes the characteristic line emission from atoms as their electrons drop from the excited to the ground state. The earliest version of emission spectroscopy as applied to chemistry was the flame test, where samples of elements placed in a Bunsen burner will change the flame to different colors (sodium turns the flame yellow calcium turns it red, copper turns it green). The modem version of emission spectroscopy for the chemistry laboratory is ICP-AES. In this technique rocks are dissolved in acid or vaporized with a laser, and the sample liquid or gas is mixed with argon gas and turned into a plasma (ionized gas) by a radio frequency generator. The excited atoms in the plasma emit characteristic energies that are measured either sequentially with a monochromator and photomultiplier tube, or simultaneously with a polychrometer. The technique can analyze 60 elements in minutes. 

Quadrupole mass analyzers consist of four rods that produce a radio frequency quadrupole field. Only a single mass-to-charge ratio has a stable trajectory through the mass analyzer, but the electric potentials can be swept rapidly, either continuously or in discrete steps to measure a variety of masses. Quadrupole mass spectrometers are used in a wide variety of applications, but are not widely used in cosmochemistry. 

In the quadrupole mass analyzer, focusing electrodes direct and accelerate the ionized fragments into a mass filter consisting of four cylindrical electrodes in a vacuum. Tire cylindrical electrodes establish a combination radio-frequency and direct-current electrical field that permits only those ions with a specific, selected mass-to-charge ratio to pass all the way through the filter. The rest of the ions impact die electrodes and do not travel to the exit. Varying the electrical field allows ions with other masses to pass through the filter. 

In a quadrupole device, not as accurate and precise as double-focusing instruments but fast, a quadrupolar electrical field comprising radio-frequency (RF) and direct-current components is used to separate ions. Quadrupole instruments as mass analyzers are used together with ESI as the ion source the configuration employing a three-dimensional quadrupolar RF electric field (Wolfgang Paul, University of Bonn, 1989 Nobel prize for physics) is referred to as an ion trap analyzer (see below). 

There is also a standard test method for determination of major and minor elements in coal ash by inductively coupled plasma (ICP)-atomic emission spectrometry (ASTM D-6349). In the test method, the sample to be analyzed is ashed under standard conditions and ignited to constant weight. The ash is fused with a fluxing agent followed by dissolution of the melt in dilute acid solution. Alternatively, the ash is digested in a mixture of hydrofluoric, nitric, and hydrochloric acids. The solution is analyzed by (ICP)-atomic emission spectrometry for the elements. The basis of the method is the measurement of atomic emissions. Aqueous solutions of the samples are nebulized, and a portion of the aerosol that is produced is transported to the plasma torch, where excitation and emission occurs. Characteristic line emission spectra are produced by a radio-frequency inductively coupled plasma. A grating monochromator system is used to separate the emission lines, and the intensities of the lines are monitored by photomultiplier tube or photodiode array detection. The photocurrents from the detector... 

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