Vaporization sample

Fundamentally, introduction of a gaseous sample is the easiest option for ICP/MS because all of the sample can be passed efficiently along the inlet tube and into the center of the flame. Unfortunately, gases are mainly confined to low-molecular-mass compounds, and many of the samples that need to be examined cannot be vaporized easily. Nevertheless, there are some key analyses that are carried out in this fashion the major one i.s the generation of volatile hydrides. Other methods for volatiles are discussed below. An important method of analysis uses lasers to vaporize nonvolatile samples such as bone or ceramics. With a laser, ablated (vaporized) sample material is swept into the plasma flame before it can condense out again. Similarly, electrically heated filaments or ovens are also used to volatilize solids, the vapor of which is then swept by argon makeup gas into the plasma torch. However, for convenience, the methods of introducing solid samples are discussed fully in Part C (Chapter 17). 

Since detailed chemical structure information is not usually required from isotope ratio measurements, it is possible to vaporize samples by simply pyrolyzing them. For this purpose, the sample can be placed on a tungsten, rhenium, or platinum wire and heated strongly in vacuum by passing an electric current through the wire. This is thermal or surface ionization (TI). Alternatively, a small electric furnace can be used when removal of solvent from a dilute solution is desirable before vaporization of residual solute. Again, a wide variety of mass analyzers can be used to measure m/z values of atomic ions and their relative abundances. 

For troubleshooting suspected foam problems, vaporize samples of feed and bottoms to look for suspended solids. Also, one can look for the Tyndall effect as described in the section on condenser fogging. 

In Surface Analysis by Laser Ionization (SALI), a probe beam such as an ion beam, electron beam, or laser is directed onto a surfiice to remove a sample of material. An untuned, high-intensity laser beam passes parallel and close to but above the sur-fiice. The laser has sufficient intensity to induce a high degree of nonresonant, and hence nonselective, photoionization of the vaporized sample of material within the laser beam. The nonselectively ionized sample is then subjected to mass spectral analysis to determine the nature of the unknown species. SALI spectra accurately reflect the surface composition, and the use of time-of-flight mass spectrometers provides fast, efficient and extremely sensitive analysis. 

The specifications for CWA vapors are an agent concentration (mass of agent per volume of air) at the inlet to the CWA vapor sampling line and a response time. The specifications for the ACADA include two response times. A longer response time before an alarm requires a lower detection limit in order to offset the delay and greater potential exposure of the soldiers, versus the specification for the shorter response time. The specifications range from tens down to hundredths of a mg/m3, and depend on the toxicity of the CWA. A response time of 10 seconds or less has been requested for CWA in air. 

Because k varies from 0.1 to 0.5, the concentration is expected to vary from 18.9 ppm to 94.3 ppm. Actual vapor sampling is recommended to ensure that the TLV is not exceeded. 

Separation of mixtures in microgram quantities by passage of the vaporized sample in a gas stream through a column containing a stationary liquid or solid phase components migrate at different rates due to differences in boiling point, solubility or adsorption. 

Fig. 6.14 (a) OFRR vapor sensor responses to DNT vapor samples extracted with various sampling time at room temperature, (b) Calibration curve of DNT mass extracted by on SPME fiber under various extraction times at room temperature... 

In column chromatography the mobile (moving) phase is a liquid that carries your material through an adsorbant. I called this phase the eluent, remember Here a gas is used to push, or carry, your vaporized sample, and it s called the mobile phase. 

Fig. 21.2. Gas-solid chromatogram of vapor sample from bulging drum (column 5-ft X 1/8 in o.d. aluminum packed with molecular sieve 5A 60-80 mesh column temperature ambient 25°C to 28°C detector thermal conductivity at 80°C carrier gas Ar at 15cm3/min. (Reprinted/redrawn with permission from Analyt. Chem., 56, 603A (1984). Copyright 1984 American Chemical Society.)...

Vapor-flow sensor and vapor-sampling ports located near the vapor extraction well heads. 

The SVE system operated almost continuously (92% of the time) throughout the 72 days of operation. During this period, vapor flow remained within the range of 56 to 70 scfm (1615 to 1986 1/min) with an almost constant well head pressure (absolute) of 0.8 atm (vacuum equal to 81 in. H20 column or 6 in. Hg). Vapor samples were collected approximately every 5 days from each well head, GAC influent and effluent, and exhaust stack effluent. 

There are a variety of ways to do the calculations. Most of these, however, involve the calculation of the number of moles (n) from the ideal gas equation n = PV/RT. The mass of the vapor sample is calculated from the difference between measurements 1 and 2. The temperature (measurement 3) is converted to kelvin. The pressure (measurement 4) is converted to atmospheres. Measurement 5 is converted to liters. Inserting the various numbers into the ideal gas equation allows you to calculate the number of moles. The molar mass is calculated by dividing the mass of the sample by the moles. 

The lead content of biological samples is usually very small, rendering gravimetric methods impracticable, and methods have often relied upon the formation of coloured complexes with a variety of dyes. More recently, the development of absorption spectroscopy using vaporized samples has provided a sensitive quantitative method. Oxygen measurements using specific electrodes offer a level of sensitivity which is unobtainable using volumetric gas analysis. 

Mass spectrometry is traditionally a gas phase technique for the analysis of relatively volatile samples. Effluents from gas chromatographs are already in a suitable form and other readily vaporized samples could be fairly easily accommodated. However the coupling of mass spectrometry to liquid streams, e.g. HPLC and capillary electrophoresis, posed a new problem and several different methods are now in use. These include the spray methods mentioned below and bombarding with atoms (fast atom bombardment, FAB) or ions (secondary-ion mass spectrometry, SIMS). The part of the instrument in which ionization of the neutral molecules occurs is called the ion source. The commonest method of... 

This agreement is considered to be quite good when allowance is made for the fact that these are independently measured runs at concentrations varying by a ratio of 5000 to 1 The 10 ppm run at 120°C is probably in error due to a trace contaminant in the vapor samples observed in the potentiometric titration curve. This problem was not observed at 80°C. 

The vapor sample under investigation may not eontain only one kind of speeies. It is desirable to learn as mueh as possible about the vapor composition from independent sources, but here the different experimental conditions need to be taken into account. For this reason, the vapor composition is yet another unknown to be determined in the electron diffraction analysis. Impurities may hinder the analysis in varying degrees depending on their own ability to scatter electrons and on the distribution of their own intemuclear distances. In case of a conformational equilibrium of, say, two conformers of the same molecule may make the analysis more difficult but the results more rewarding at the same time. The analysis of ethane-1,2-dithiol data collected at the temperature of 343 kelvin revealed the presence of 62% of the anti form and 38% of the gauche form as far as the S-C-C-S framework was concerned. The radial distributions calculated for a set of models and the experimental distribution in Figure 6 serve as illustration. 

The determination of explosives in soils has been mostly commonly associated with the detection of unexploded ordnance such as land mines (both anti-personnel and anti-tank). Chambers et al. [70] designed sampling subsystems for soil/vapor sampling. A probe was used to extract and concentrate vapors of explosives in the pore volume of soil in the vicinity of land mines with sub-part-per-biUion detection limits for TNT and related explosive munitions compounds [70]. As an... 

The unique simplicity of TNT detection by this method renders the fabrication of an operable device built around amplified fluorescent polymer technology a relatively simple proposition in comparison to other technological platforms. The method relies purely on changes in fluorescence intensity. AH that is theoretically required is a means of introducing vapor samples to a conjugated polymer film, a light source to excite the polymer film, and a photodetector to measure the emission intensity as a function of time. 

Volume 1 consists of chapters covering the development. Instrumentation, and results of a wide range of materials, including background correction lasers, inductively coupled-mass sp>ectroscopy plasmas, electrothermal vaporizers, sample introduction, and Fourier transform atomic spectrocopy. 

Aboveground tank testing is performed by inserting vapor sampling probes under the tank bottom. To ensure detection of leakage from any point on the tank floor, evacuation probes are placed under the perimeter of the tank and one or more air injection probes are placed beneath the center of the tank. A program of air injection and/or evacuation is initiated to collect samples from under the tank. These samples are analyzed for the presence of tracer. 

Bulk sensors certainly have a role in chemical sensing of explosives, but the subject of this book is the other basic type sensor, one that seeks molecules released from the bulk of the explosive material in an object. We will refer to these as trace chemical sensors. They are sometimes called vapor sensors, but that seems a less accurate description when they are applied to explosive molecules, which may not always be found in a vapor state. As we shall see in Chapter 5, that requires us to understand where and how to look for these molecules. It will become apparent upon a little reflection that the two types of sensors are complementary and are best used in different situations. Furthermore, even when trace sensors are used, in some situations sampling of particles of soil or vegetation or sampling from surfaces may prove to be more productive that vapor sampling. For underwater sources the term vapor sensing is also inappropriate. 

Some of the molecules that do make their way into the free air above the boundary layer are likely to sorb onto the surface of any object that is in their flow path. Once this happens, that molecule is effectively lost for collection by vapor sampling techniques, reducing the available concentration in a sample. In many search areas plants form the most available surfaces for molecules to fall upon. Hence, it is possible that plant surfaces near a source might form a reservoir for molecules that could be profitably exploited by innovative sampling techniques. Certainly, it is well recognized that when plants take in water through their root system that they may be also taking in the molecules released from a nearby source [17]. 

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