Industries Information

Residual gas analyzers (Rags) are spectrometers that are used to identify the gases present in vacuum environments.  Residual gas analyzers perform this function by producing ionizing molecules of the gas sample, separating the resulting mixture of ions according to their charge-to-mass ratios, providing an output signal, which is a measure of the relative species present, and counting the rate at which the ions hit the detector. The spectrometers within residual gas analyzers analyze the process by which a mixture of ionic species is separated according to the mass-to-charge ratio (m/z). The analysis may be qualitative and/or quantitative.  There are two common styles of spectrometric analysis used, quadrupole and magnetic sector analysis, although there are specialized and custom designs available.

Quadrupole mass spectrometers consist of an ion source, ion optics to accelerate and focus the ions through an aperture into the quadrupole filter, the quadrupole filter itself with control voltage supplies, an exit aperture, an ion detector, detection electronics, and a high-vacuum system.  The mass spectrometer operates only in a high vacuum whereas the gas it is being used to analyze is often at atmospheric pressure. To reduce the pressure the gas is sampled twice using a "double sniffer" configuration.

Magnetic sector residual gas analyzers use one of two methods to read and identify samples.  Single focusing analyzers focus a circular beam in a path of 180, 90, or 60 degree. A variety of forces influence the particles and separate the ions due to their different mass-to-charge ratios. In double focusing analyzers, an electrostatic analyzer is added to separate particles with difference in kinetic energies.

Residual gas analyzers ionize their samples using a variety of technologies and methods including electron impact ionization (EI), field ionization (FI), and chemical ionization (CI).  EI uses an electron beam, usually generated from a tungsten filament, to ionize gas-phase atoms or molecules. An electron from the beam knocks an electron off of analyte atoms or molecules to create ions.  FI removes the electrons from any species by interaction with an intense electrical field. CI forms new ionized species when gaseous molecules interact with ions. The process may involve transfer of an electron, a proton or other charged species between the reactants.

Finally, there are two distinct detector types that can be found within residual gas analyzers. Electron multiplier tubes are similar in design to photomultiplier-tubes. They consist of a series of biased dynodes that eject secondary electrons when they are struck by an ion. They therefore multiply the ion current and can be used in analog or digital mode.

Faraday cup residual gas analyzers use a metal cup that is placed in the path of the ion beam. It is attached to an electrometer, which measures the ion-beam current. Since a Faraday cup can only be used in an analog mode it is less sensitive than other detectors that are capable of operating in pulse-counting mode.

 Refractometers are widely used to measure the amount of dissolved substances in solutions containing mainly sucrose. The relationship between refractive index and the amount of dry substance content is well known for sucrose and is the basis of the BRIX scale, which is the measure of the number of grams of sucrose present per 100 grams of aqueous sugar solution. That is, 50 BRIX means 50 grams of solid per 100 grams of solution. This relationship holds for a large number of similar substances and so the BRIX scale is widely used in the food industry. Unfortunately the refractive index of a sugar solution changes with temperature while its solid content remains the same.

Refractometers measure the critical angle of refraction through a sample. The critical angle of refraction will change with concentration. It will normally increase with an increase in concentration. Color, gas and dissolved particles do not affect measurement results. Refractometers can be designed for either continuous or sample monitoring.

Important specifications for refractometers are the refractive index range or BRIX range, depending on which is being used, and the accuracy. The pressure range of the refractometers is important, as are the maximum flow velocity and the sample size.

Sample temperature control is a popular feature available for some refractometers. Another special feature is an automatic prism wash system, which will clean the prism without having to remove it from the refractometer. Other features include temperature compensation, temperature sensing, sanitary ratings, programmability, self-calibration and alarms.

Common display types for refractometers include analog meters or gauges, digital displays and video terminal displays. They can be handheld, as a field instrument might be, portable with wheels or handles for ease in transporting, or more permanently fixtured instruments. Analog outputs such as current, voltage and frequencies are often available. Computer signals and other digital outputs can be found in these devices as well.

 Raman spectrometers measure the wavelength and intensity of inelastically scattered light from molecules. They are used to determine the chemical composition of a sample based on the wavelength and intensity of the light passing through the sample. Raman spectroscopy is based on the theory of Raman scattering, which states that light is scattered due to the vibrations of the molecules in the substance or substrate through which light passes. This vibration scatters the light and changes its energy from that of the incident light. In this way, Raman spectrometers use the Raman effect by comparing the different energies of the incident light and the scattered photons.

Raman spectrometers are similar to infrared (IR) spectrometers in that both an IR spectrometer and Raman spectometer measure the vibrational energies of the molecules in a sample. Infrared spectroscopy or IR spectroscopy measures the absorption of light of certain energies that correspond to the vibrational energy of the molecule. Since Raman scattering is different from infrared absorption, the two methods of spectroscopy are often used to provide complementary data.

Raman spectrometers often use lasers, the most typical being an argon ion laser. A laser spectrometer offers many benefits, including focused, high-powered excitation of the tested substance which results in a high incidence of light scattering. Because the Raman effect measures the difference between scattered and incident light, Raman spectrometers are particularly useful in gathering data from a small section of a sample. Consequently, Raman spectrometers are being used to develop confocal microscopy techniques. The incident laser radiation of the Raman spectrometer is focused with the microscope objective on a point in the sample. The resulting Raman spectrum contains data almost exclusively from a point within the sample.

Suppliers of Raman spectrometers are located across the United States and across the world. They meet a variety of regulatory requirements and quality standards.

Polarimeters are instruments used to measure the rotation of the plane of vibration in polarized light. Light polarization can be measured using a Stokes vector polarimeter. The polarization properties of a particular material can be measured with a Mueller matrix polarimeter. Typically, Mueller matrix polarimeters are used to determine the purity of materials such as sugar, vitamins, steroids, and antibiotics. Polarimetry of thin films and surfaces is called ellipsometry, which is used to provide information about layers that are thinner than the wavelength of the light itself, down to a single atomic layer or less. An interferometer uses the interference pattern of two beams of light to make very precise measurements. Interferometers can measure minute surface irregularities in optical devices such as mirrors. In astronomy, interferometers can measure the distances between celestial objects and their diameters. The most sensitive polarimeters are based on interferometers, while more conventional polarimeters use polarizing filters, wave plates or other devices.

Choices for polarimeters include: complete vs. incomplete, sequential vs. simultaneous, monochromatic vs. polychromatic, non-imaging polarimeter vs. imaging polarimeter, and manual (visual) vs. electronic (automatic). For pharmaceutical testing, most work is done using 589 nm light (sodium D line). For electronic (automatic) polarimeters, total accuracies of 0.01° are possible. The light source in a polarimeter can be a spectral arc lamp (sodium or mercury), or an incandescent lamp (tungsten). Spectral arc lamps can be designed for visible, ultraviolet (UV) and infrared (IR) wavelengths, and are suitable for stability of wavelength and long-term calibration. Incandescent lamps are less expensive than spectral arc lamps, but limited to visible and IR radiation.

ASTM International (formerly called the American Society for Testing and Materials (ASTM), maintains ASTM C1426 regarding the standard practices for verification and calibration of polarimeters. Other standards describe how to use polarimeters in standard tests (i.e. ASTM F218 for analyzing stress in glass). The Society of Manufacturing Engineers (SME) maintains SME EM930116 for the rapid contour measurement of composite structures utilizing a three-dimensional (3D) tracking interferometer.

Water quality photometers and colorimeters are ion-specific computer-interfaced probes designed to determine the concentration of a solution from its color intensity. The color of a solution may be inherent or derived by adding another reagent to it. Monochromatic light from an LED light source passes through a cuvette containing a solution sample. Some of the incoming light is absorbed by the solution. As a result, light of a lower intensity strikes a photodiode. The amount of light that penetrates a solution is known as transmittance. Transmittance can be expressed as the ratio of the intensity of the transmitted light, It, and the initial intensity of the light beam, Io, as expressed by the formula:

T = It / Io

Photometers and colorimeters produce an output voltage, which varies in a linear way with transmittance, allowing a computer to monitor transmittance data of a solution thus producing an output that can be converted to concentration.

Mounting options for water quality photometers and colorimeters are quite varied. They can be handheld, portable meters or benchtop or fixtured instruments. Handheld meters are popular for laboratory use, where a scientist may be taking readings from many different samples. Portable meters typically have wheels, handles or another device for transportation. Benchtop and fixtured instruments are permanently mounted for long-running use.

Displays for the instruments can be analog meters, a numeric or alphanumeric digital display or video, CRT or LCD. Another option is to have no local display at all and have the data gathered by another instrument. Likewise, the user controls can also be analog or digital or can be operated through a host computer. To simplify the instruments, preprogrammed devices without user controls are available.

Electrical output options for water quality photometers and colorimeters are the standard voltage and current outputs as well as an analog pulse or frequency or a change in state of switches or an alarm. Serial and parallel interfaces can help connect these instruments to a host computer.

Some features for water quality photometers and colorimeters are battery power for greater mobility, auto wavelength selection and automatic calibration. Temperature compensation is available in some models, as are self-test diagnostic routines to verify sensitivity, zero point, etc. Water resistant housings and data storage and logging capabilities enhance the versatility of these instruments.