Partial pressure of the analyte

In contrast to sample introduction via direct probe (Chap. 5.3.1), the components eluting from a GC capillary are quantitatively transferred into the ion source during a short time interval just sufficient to acquire about five mass spectra. Consequently, the partial pressure of the analyte is comparatively high during elution allowing sample amounts in the low nanogram range to be analyzed by capillary GC-MS. 

It is possible to obtain the value of fractional charge 5 from the dependence of A WF on partial pressure of the analyte gas (Pg) (6.96). This was done for the modulation of WF of poly(phosphazene)/benzoquinone layers containing iodine by tributyl phosphate (TBP) vapor (Li et al., 1996) and for polypyrrole by methanol (Blackwood and Josowicz, 1991 Topart and Josowicz, 1992a, b). The corresponding reaction, caused by charge transfer between iodine and TBP, is given by the following equilibrium reaction. 

Electrochemical sensors — Electrochemical sensors belong to the family of chemical sensors. Chemical sensors are devices that convert chemical information (concentration, - activity or partial pressure of the analyte) into a measurable signal. Chemical sensors contain two basic functional units a receptor and a transducer. The receptor interacts with the analyte and transforms chemical information into a form of energy that is converted further by the transducer into a useful analytical signal. 

Fig. 3.3-16 shows that the line width increases as the pressure of a gas increases (even by adding an inert gas at constant partial pressure of the analyte). As a result of condensation of the gas to a liquid, the entire band is reduced to one line. Solidification further reduces the band width. Since the integral intensity remains constant, the intensity of the band maximum increases, and so does the sensitivity (Schrader et al., 1981). In short Reducing the sample temperature increases the sensitivity. 

The rate constants for reaction of precursor ions H30", NO", and 02 " with different molecules were determined experimentally. To calculate the partial pressure of the analyte [M] it is necessary to know the value of constant k for each reaction and the concentration of [MEf. The method also allows simultaneous measurement of the concentration of several analytes. Studies have shown that ions such as HaO" and N react quickly enough with various organic compounds, but 02 " reacts quickly only with small molecules such as NO, NO2, or NH3. In the case of samples containing a large amount of moisture (e.g., breath samples or head-space of aqueous solution), cluster ions such as H30 (H20)i 2,3 may be formed in... 

The adsorption analysis can be performed using the experimental sensor effect data from gravimetric QCM measurements. The frequency shift, which is proportional to the partial pressure of the analyte, is correlated to the number of specific incorporation sites in the linear range of the isotherm slope. Additional evidence for specific interactions between the analyte and the polymer matrix can be demonstrated using infrared spectroscopic analysis [19]. NMR [4,7] can also provide information about imprinting effects. 

Given no other dominant source of electrons, the degree of ionization will increase as the number density (or partial pressure) of the analyte in the source decreases. The extent of ionization also increases as the ionization potential of the element decreases. The degree of ionization can be calculated using Saha s equation. 

For fast-flowing donor solutions with 1/ d 0 the mass transport is determined by the gas diffusion through the membrane, the partial pressure of the analyte in the donor solution, and the phase-transfer resistances. 

Using microporous membranes with a hydrophobic inner surface, the selectivity of gas dialysis against other volatiles results predominately from the ratio of the partial pressure of the analyte p, to the total pressure. In homogeneous membranes, the different solubilities of the gases in the membrane material are the main selecting factor. 

At the low pressures usually typical of quasi-ideal gas chromatography, the moving phase is practically unadsorbed by the adsorbent (stationary phase). Therefore the adsorbent surface is free, and there is no competition of carrier gas with analyte for adsorption. Usually the partial pressure of the analyte in the gas phase is small compared... 

At low partial pressure (more precisely low values of the ratio pIP of the partial pressure of the analyte to its vapor pressure) the amount of a substance adsorbed on a surface at equilibrium with the vapor is proportional to the partial pressure ... 

A small volume of blood is placed in a sealed container that has headspace above the surface of the sample. In a headspace analysis, Henry s law is employed. This law states that the partial pressure of the analyte above a liquid is proportional to its concentration in the liquid ... 

The precursor model of FAB applies well to ionic analytes and samples that are easily converted to ionic species within the liquid matrix, e.g., by protonation or deprotonation or due to cationization. Those preformed ions would simply have to be desorbed into the gas phase (Fig. 9.6). The promoting effect of decreasing pH (added acid) on [M+H] ion yield of porphyrins and other analytes supports the precursor ion model. [55,56] The relative intensities of [Mh-H] ions in FAB spectra of aliphatic amine mixtures also do not depend on the partial pressure of the amines in the gas phase, but are sensitive on the acidity of the matrix. [57] Furthermore, incomplete desolvation of preformed ions nicely explains the observation of matrix (Ma) adducts such as [M+Ma+H] ions. The precursor model bears some similarities to ion evaporation in field desorption (Chap. 8.5.1). 

Combining (6.98), (C.19b), and (C.26) yields dependence of Vq on partial pressure of the donor-acceptor gas. In direct analogy with ISFET operation in the presence of interfering ions (6.71) we can write the following for the transistor in some arbitrary gas phase and the analyte. 

Upon exposure to a given partial pressure, p, of a gas-phase analyte, equilibrium is established when a certain fraction, 0, of adsorption sites are filled. The activity of adsorbate in the ambient phase can typically be approximated by the partial pressure of the adsorbing gas, so that Equation S.3 can be written as ... 

Then, the oxygen partial pressure of the surrounding gas atmosphere is the driver for a change in the electron concentration.These examples clearly show that DTEGs are based on the same physical principles as conductometric gas sensors, since in both cases the analyte concentration modulates the electron density. However, the measurand is different. In conductometric devices, the material property conductivity changes and, hence, the resistance of a sensor varies with the analyte concentration. In contrast, the determination of thermopower is more complicated, since not only the thermovoltage has to be measured, but also a known temperature difference has to be apphed or, at least, measured. 

Characterization of Porous Samples. Adsorption isotherms are a useful analytical method for characterizing porous samples. This measurement consists of monitoring the amount of a molecular species taken up by the sample as a function of the relative saturation of the species in the gas phase over the sample. This relative saturation is defined as p/p, where p is the partial pressure of the test species and p is its saturation vapor pressure at the temperature of the run. In a typical measurement, p/p is increased from zero to a value near 1 (onset of bulk condensation) and then decreased back down to zero. For many porous samples, hysteresis is observed between the adsorption and desorption branches of the isotherm obtained in this manner (14). 

In order to use Equation A2.32 in the program it is necessary to use an analytical expression of the intemal energy, u, of the steam-water mixture as a function of the total volume, F (m ), its weight and the partial pressure of the steam or temperature as given in Section A2-2. 

As a rule, a modifying agent is both a component of the carrier gas and a constituent of the stationary phase (surface layer of adsorbent). The situation is somewhat reminiscent of the retention in liquid chromatography, where the surface properties depend on the concentration of the modifier in the mobile phase. Analyte retention volume depends on the nature and partial pressure of the carrier gas additive as well as on the nature of adsorbent surface. Therefore the use of a phase system composed of a (modifier-carrier gas) mixture as mobile phase offers the possibility of fine tuning of selectivity and retention by adjusting the modifier content of both phases [10]. 

Cl differs from what we have encountered in mass spectrometry so far because bimolecular processes are used to generate analyte ions. The occurrence of bimol-ecular reactions requires a sufficiently large number of ion-molecule collisions during the dwell time of the reactants in the ion source. This is achieved by significantly increasing the partial pressure of the reagent gas. Assuming reasonable... 

Cl is a gas-phase ion-molecule reaction in which the analyte (molecule) is ionized via a proton transfer process. (For more detailed description of the Cl processes and analytical applications, a classic book by Harrison [17] is recommended.) The formation of the reactive ions in this ion-molecule reaction process is triggered by El ionization of a reagent gas that is, most commonly, methane, isobutene, or ammonia. The partial pressure of the reagent gas (1-0.1 Torr) is much higher than that of the analyte (ca. lO" to 10 Torr), so the gas molecules can be considered as a protective shield for the analyte molecules to avoid direct El ionization. El ionization of methane results in the fragmentation of methane molecular ion and some of these ions react with neutral methane. The ionization of the analyte molecule occurs by proton transfer between reagent gas ions and the analyte, or to a less extent, by adduct formation. Some characteristic mechanistic steps for methane Cl can be summarized as follows ... 

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