Vapors, calibration gases

EPA Method 25A is the instrumental analyzer method for determination of total gaseous organic concentration using a flame ionization analyzer. The method apphes to the measurement of total gaseous organic concentration of vapors consisting primarily of alkanes, alkenes, and/or arenes (aromatic hydrocarbons). The concentration is expressed in terms of propane (or other appropriate organic calibration gas) or in terms or carrion. 

While this permits more sensitive and accurate reading of concentrations in the 0-to-10% range, this type of instrument is not sufficiently sensitive to give precise indications of concentrations at the TLV of many toxic gases and vapors. In addition, they lack specificity, do not read directly in TLV units (ppm), and are subject to interferences. All combustible gas and vapor indicators are calibrated by the manufacturer using one specific gas or vapor such as methane, and a calibration curve is provided, in percent LEL, for the calibration gas only. 

A PID can accurately measure gases and vapors at low ppmv or even ppb levels. However, the PID is not a selective monitor. It has very little ability to differentiate between chemicals. The PID can tell us how much of a gas or vapor is present, but we have to deduce the exact gas or vapor present. When approaching an unknown chemical release, the PID is set to its calibration gas of isobutylene. Once... 

Correction factors are scaling factors used to adjust the sensitivity of the PID to directly measure a particular gas compared to the calibration gas. The lower the CF, the more sensitive the PID is to a gas or vapor. 

Consider a calibrator gas certified to contain 15% O2 (L/L or mol/mol) and 5% CO2, the remainder being N. The mole fractions (or F) of the gases in the dry mixture are 0.15,0.05, and 0.80, respectively. This mixture, after saturation with water vapor at 37 °C (to mimic a patient s blood or alveolar air), is introduced into a blood gas instrument s measuring chamber (held at 37 °C to mimic a patient s body temperature) for the purpose of calibrating the instrument for subsequent measurements of gases in patients samples. If the local barometric pressure, P(Amb), on this occasion is 747 mmHg, then the humidified calibrator gas is present in the chamber at ambient, barometric pressure, such that... 

The mode of calibration is determined by the design of the instrument. Most instruments contain a barometer or a transducer responsive to P(Amb) so that barometric pressure is always known to the microprocessor. With such instruments, only a keyboard entry of the fractional composition of O2 and CO2 in low and high calibrator gas mixtures needs to be made. Today, most analyzers auto-calibrate without the necessity for user input. The microprocessor will calculate the values for PO2 and PCO2 (according to Dalton s law) for gases saturated with water vapor at 37 °C. 

However, if highly flexible calibration gas mixtures consisting of liquid components with short response times need to be generated, vaporizers or saturators are suitable solutions. 

I6J2 Calibration Gas Generators Using the Vaporization Tichnigue... 

A quasi-continual output can be assumed, when the concentration pulses in the calibration gas can no longer be ascertained. To achieve this, the time lag between two pulses must be short in relation to the residence time of the carrier gas in the vaporizer. 

Because saturators and vaporizers for laboratory use are usually made of glass equipment, higher pressures are unpopular. That is why the commercially available jet probe has become popular (see Figure 16-8). A carrier gas stream flows through an injector pump and creates an underpresure of about 0.4 bar (absolute) on the sucking side. The pressure difference of 0.6 bar (or a pressure ratio of 1 0.4) in relation to the precursory calibration gas is sufHcient to guarantee the supercritical flow and to suck up a defined part of the flow (in the range of 1 SCCM, for instance). 

Hill and Powell (1968) have recently written a comprehensive text on non-dispersive infrared gas analysis. They have discussed applications and sampling techniques in science, medicine, and industry instrumentation and detecting systems and methods for producing calibration gas and vapor mixtures. 

To determine the concentrations of vapor phase alkaloids detected in ETS by APCI mass spectrometry, it is necessary to have vapor phase standards which can be used for instrument calibration. Gas dilution is perhaps the best way to calibrate for compounds in the gas phase. Gas dilution requires that a standard of known concentration and a method for accurately and reproducibly diluting the standard are available. Permeation tubes and diffusion tubes, housed in a constant temperature oven, are well suited for generating gas standards with known analyte concentrations. Table 1 includes the analyte, source, and typical source effusion rates used for investigating ETS along with the ion monitored for quantitative analysis of each analyte. 

The reactor outlet was directly connected to a quadrupole mass spectrometer (Balzers QMS 200) and to a UV-analyzer (ABB LIMAS 1IHW) in parallel. NH3, NO, NO2, N2O, O2, and He were dosed from bottled calibrated gas mixtures by mass flow controllers, while water vapor was added by means of a saturator. The catalyst temperature was measured by a K-type thermocouple directly immersed in the catalytic bed. 

Component Gaseous or vaporized component of a calibration gas mixture, known in quantity and quality, and directly used for examination of the calibration . 

The fixed points in the lTS-90 are given in Tabie 11.39. Platinum resistance thermometers are recommended for use between 14 K and 1235 K (the freezing point of silver), calibrated against the fixed points. Below 14 K either the vapor pressure of helium or a constant-volume gas thermometer is to be used. Above 1235 K radiometry is to be used in conjunction with the Planck radiation law,... 

It is also desirable to spot test the instrument s response between calibrations. For this purpose, several suppliers of compressed gas prepare cylinders containing almost any desired concentration of the gas or vapor of interest. If it is not practical... 

Combustible gas detection systems are frequently used in areas of poor ventilation. By the early detection of combustible gas releases before ignitible concentration levels occur, corrective procedures such as shutting down equipment, deactivating electrical circuits and activating ventilation fans can be implemented prior to fire or explosion. Combustible gas detectors are also used to substantiate adequate ventilation. Most combustible gas detection systems, although responsive to a wide range of combustible gases and vapors, are normally calibrated specifically to indicate concentrations of methane since most natural gas is comprised primarily of methane. 

Temperature Tgo in the range between 3.0 and 24.5561 K is defined in terms of 3He or 4He constant volume gas thermometers (CVGT), calibrated at the triple points of Ne and H2, and at a temperature between 3.0 and 5.0 K that has been obtained from vapor pressure versus temperature relations for He. 

Figure 15 gives a diagrammatic representation of a volumetric line which is used in connection with a high-temperature Calvet microcalorimeter 67). Other volumetric lines which have been described present the same general features (15, 68). In the case of corrosive gases or vapors, metallic systems may be used 69). In all cases, a sampling system (A in Fig. 15) permits the introduction of a small quantity of gas (or vapor) in a calibrated part of the volumetric line (between stopcocks Ri and Ro in Fig. 15) where its pressure Pi is measured (by means of the McLeod gage B in Fig. 15). The gas is then allowed to contact the adsorbent placed in the calorimeter cell C (by opening stopcock Ro in Fig. 15). The heat evolution is recorded and when it has come to completion, the final equi-...

Kolb, B., Welter, C., Bichler, C. (1992) Determination of partition coefficients by automatic equilibrium headspace gas chromatography by vapor phase calibration. Chromatographia 34, 235-240. 

Of the three general methods, the last seems to be the most practical. Theoretically, with high enough concentrations of hydrocarbons, the first method, the headspace analysis, should be both the most accurate and the easiest to calibrate. Operationally, it leaves much to be desired both because of the problems of sensitivity and those of the accommodation of the larger molecules in water. The second method, vacuum degassing, requires much more equipment than the third method and requires that large amounts of water vapor be removed before the sample is injected into the gas chromatograph. The last method is so much less complicated that even with its calibration problems it has been adopted almost universally. 

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