Principles of instrumental analysis 5th edition pdf


    DESCRIPTION This text is written for a course that deals with the principles and applications of modern analytical instruments. Emphasis is placed upon the theoretical basis of each type of instrument, its optimal area of application, its sensitivity, Download or read Aqualeo's The. Access Principles of Instrumental Analysis 5th Edition solutions now. Our solutions are written by Chegg experts so you can be assured of the highest quality!. View: PDF | PDF w/ Links. Related Content Hi-Res PDF · Analytical Chemistry: An Introduction, Fifth Edition (Skoog, Douglas A.; West, Donald; Holler, F. James) Hi-Res PDF · Principles of Instrumental Analysis (Skoog, D. A.; West, D. M.).

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    Principles Of Instrumental Analysis 5th Edition Pdf

    Principles of Instrumental Analysis, 5th Edition free ebook. Principles of Instrumental Analysis, 5th Edition pdf download book online Principles of Instrumental. Principles of instrumental analysis / Douglas A. Skoog, James J. Leary Skoog, . analysis / Douglas A. Skoog, F. James Holler, Timothy A. Nieman. - 5th ed. Chemistry Spring semester, Text: "Principles of Instrumental Analysis", 5th Edition,. Douglas Skoog, F. James Holler and Timothy Nieman. Lecture.

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    Textbook Solutions. Looking for the textbook? We have solutions for your book! What is a transducer in an analytical instrument? This procedure, called dipolar decoupling, is similar to spin decoupling except that much higher power levels are used.

    A second cause of band broadening in solids is chemical shift anisotropy, which is discussed in Section 19B The broadening here results from changes in the chemical shift with orientation of the molecule or part of the molecule with respect to the external magnetic field. This type of broadening in solids is eliminated by magic angle spinning in which the sample is spun at greater than 2 kHz at an angle of With gaseious ionization sources, the sample is first volatilized by heating if necessary and then transmitted to the ionization area for ionization.

    In a desorption source, a probe is used and ionization takes place directly from the condensed phase. The advantage of desorption ionization is that it can be applied to high molecular weight and thermally unstable samples.

    The advantage of gaseous ionization sources are their simplicity and speed no need to use probe and wait for probed area to be pumped out. The most fragementation and thus the most complex spectra are encountered with electron impact ionization.

    Field ionization produces the simplest spectra. Chemical and electron impact ionization result in higher sensitivities than does field ionization. Both field ionization and field desoprtion ionization are performed at anodes containing numerous sharp tips so that very high electrical fields are realized. In field ionization, the sample is volatilized before ionization, whereas field desorption takes place an an anode that has been coated with the sample. The latter requires the use of a sample probe.

    Because SS is approximately half way between the filament and the target, the total difference in potential must be V, if the electron is to have 70 eV of energy at SS. After acceleration the velocity v can be calculated with the aid of Equation The presence of a negative dc voltage in the yz plane causes positive ions to move toward the rods where they are annihilated. In the presence of an added ac voltage, this movement is inhibited during the positive half of the cycle with the lighter ions being more affected than the heavier ions.

    Thus the yz plane acts as a low-pass filter removing heavier ions see Figure ???? The resolution of a single focusing mass spectrometer is limited by the initial kinetic energy spread of the sample molecules. This spread is minimized in a double focusing instrument by accelerating the sample through an electrostatic analyzer, which limits the range of kinetic energies of ions being introduced into the magnetic sector analyzer.

    Signicantly narrower peaks are the result. Chapter 20 A quadrupole ion trap is similar to a linear quadrupole filter except it as a spherical 3dimensional configuration. By a combination of fields, ions are temporarily stored within the trap.

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    They are then released sequentially by increasing the radio frequency voltage applied to the ring electrode. The ejected ions then strike a detector. A plot of detector signal vs. In an FT ICR instrument, ions are trapped in a cell by an electric trapping voltage and a magnetic field. Each ion assumes a circular motion in a plane perpendicular to the direction of the field. In modern instruments a radio frequency pulse that increases linearly in frequency is employed.

    A time domain image current is generated after termination of the pulse. Fourier transformation of the time decay signal yields the mass spectrum.

    Chapter 20 where the subscripts s and u designate standard and unknown respectively. Dividing one of these equations by the other gives the desired relationship.

    This conclusion is in agreement with the fact that the molecular mass of the unknown is The second conclusion is that the unknown must contain an odd number of nitrogen atoms. The difference in mass between 12C and 13C is 1. In tandem in space instruments, two independent mass analyzers are used in two different regions in space.

    This is a rather straight-forward way to do tandem ms and some conventional mass spectrometers can be converted to tandem instruments. The advantages are that it is relatively easy to take all the different types of spectra product ion, precursor ion, neutral loss, multidimensional.

    The disadvantages are that the efficiency can be very low and thus the sensitivity can be low. Tandem in time instruments form the ions in a certain spatial region and then at a later time expel the unwanted ions and leave the selected ions to be dissociated and mass analyzed in the same spatial region.

    The efficiency can be fairly high and the process can be repeated many times. It is, however, only straight forward to take product ion spectra. Both approaches require quite expensive instrumentation.

    An M electron is ejected by X-radiation or by a beam of electrons. An N electron then descends to the M orbital while ejected a second N electron as an Auger electron.

    Auger and XPS peaks can be distinguished by comparing spectra obtained with two different sources such as an Al and Mg tube. Auger peaks are unaffected by the change in sources whereas XPS peaks are displaced by the change.

    The XPS binding energy is the minimum energy required to remove an inner electron from its orbital to a region where it no longer feels the nuclear charge. The absorption edge results from this same transition. Thus, in principle it is possible to observe chemical shifts by either type of measurement. Chapter 21 d The kinetic energy of an Auger electron is independent of source energy.

    Auger peaks do not change with the two sources, whereas XPS peaks do. With EELS, low energy electrons are incident on a surface, and the scattered electrons are detected and analyzed.

    Energy losses can occur due to vibrational excitation of molecules on the surface. With infrared absorption spectrometry, IR radiation is sent through a the sample and the transmitted IR radiation is measured. With Raman spectrometry, radiation in the visible region is sent through a sample, and the scattered 2 Principles of Instrumental Analysis, 6th ed.

    Chapter 21 radiation in the visible region is measured. The shifts away from the Rayleigh scattered line correspond to vibrational modes. EELS has the advantage that it is surface sensitive and can provide information on molecules adsorbed or bound to the surface.

    In both ion scattering spectroscopy and Rutherford backscattering spectroscopy, ions are scattered from a surface and the backscattered ions detected. In ISS, low energy ions 0. The experimental arrangements for both techniques are similar to that shown in Figure for RBS. With ISS, a low-energy electron impact source substitutes for the high-energy source shown.

    With ISS, information comes from the topmost layer of the sample, whereas with RBS, information can arise from as much as nm into the sample. It is difficult to obtain quantitative information with ISS, wherease with RBS, quantitative analysis is readily obtained.

    Principles of instrumental analysis / Douglas A. Skoog, James J. Leary - Details - Trove

    In static SIMS, conditions are arranged so that sputtering from the surface is slow compared to the time-scale of the experiment.

    Hence, the surface composition is little changed during the experiment. In dynamic SIMS, conditions are arranged so that the surface is sputtered away during the experiment providing information as a function of depth below the surface.

    A spatial image of surface composition is generated. Chapter 21 Surface photon techniques have the major advantage that the surface been examined does not have to be in an ultra high vacuum environment and can be in contact with liquids.

    Thus, the environment in the surface photon techniques can be more like that found in actual use catalyst, sensor, biological material, etc. The major disadvantage is that surface composition can be altered by adsorption of gases or by attraction of molecules to the surface. A buried interface is an interface found not at the surface, but below the surface in layered structures.

    Most surface characterization methods probe only the surface or a few nanometers into the surface. However, semiconductors, sensors, and many other materials contain interfaces that are buried below the surface and important to characterize.

    Another example is the interface between two immisicible liquids. Sumfrequency generation and second harmonic can provide access to buried interfaces. Sources of signals in SEM can be backscattered electrons, secondary electrons, and X-ray photons. In elastic scattering of electrons, the electrons interact with a solid in such a way that their direction is altered, but no energy is lost in the process.

    In inelastic scattering, part of the energy of the incoming electron is lost during the scattering process. The two types of scanning probe microscopes are the scanning tunneling microscope STM and the atomic force microscope AFM.

    In the AFM, the surface is scanned by a fine stylus mounted on a force-sensitive 4 Principles of Instrumental Analysis, 6th ed. Chapter 21 cantilever whose vertical position is sensed optically with a laser beam. The advantage of the STM is that the tip never makes contace with the sample surface and hence never disturbs or damages the surface.

    The primary advantage of the AFM is that it does not require that the sample be a conductor of electricity. The main disadvantage of the STM is that it requires that the surface be an electrical conductor.

    The chief disadvantage of the AFM is that stylus tip comes in physical contact with the surface which may alter its characteristics. Proceeding as in Solution , we obtain 6 Principles of Instrumental Analysis, 6th ed.

    This probably accounts for the differences 8 Principles of Instrumental Analysis, 6th ed. Chapter 22 seen. The mean of MacInnes values is 0. These results are very close. If an indicator exhibits nernstian behavior, it follows the Nernst equation with its potential changing by 0. The alkaline errors in pH measurements with a glass electrode arise from the exchange of singly charged ions, such as sodium or potassium ions, in the surface of the glass membrane with the protons from the water.

    The potential then responds to the alkali metal activity as well as to the hydrogen ion activity. Hydration of the surface of the glass takes place in which singly charged metal ions in the glass are exchanged with protons of the water. An electrode of the first kind is a metal that responds directly to the activity of its metal ion.

    An electrode of the second kind is a metal electrode whose potential depends on the activity of an anion that forms a precipitate with the electrode metal ion. A gas-sensing probe functions by permitting the gas to penetrate a hydrophobic membrane and altering the composition of liquid on the inner side of the membrane. Thus, there is no direct contact between the electrodes and the test solution as there is with membrane electrodes.

    In a membrane electrode the observed potential is a type of boundary potential that develops across the membrane that separates the analyte solution from a reference solution. Chapter 23 It is caused by charge separation created by the differences in the rates at which ions migrate across the interface.

    Uncertainties include 1 the acid error in highly acidic solutions, 2 the alkaline error in strongly basic solutions, 3 the error that arises when the ionic strength of the calibration standards differs from that of the analyte solution, 4 uncertainties in the pH of the standard buffers, 5 nonreproducible junction potentials with solutions of low ionic strength and 6 dehydration of the working surface. An ionophores is a neutral, lipophilic compound that form complexes with analyte ions.

    When incorporated into a membrane, the target ions are carried across the solution2 Principles of Instrumental Analysis, 6th ed. Chapter 23 membrane boundary by the formation of the complex. The separation of charge across the solution-membrane barrier produces a nernstian response toward the analyte. Potentiometric titrations are widely applicable and do not require an indicator. They are inherently more accurate than titrations with an indicator. They can also be used for titrations of colored or turbid solutions.

    A disadvantage is that they often require more time than a visual titration. The equipment needed is more expensive than that for a visual titration. The operational definition of pH is based on the direct calibration of the pH meter with carefully prescribed standard buffers followed by potentiometric determination of the pH of unknown solutions.

    This definition ensures that pH measurements can be easily duplicated at various times and in various laboratories. Microfabricated ion-selective electrodes are of small size and require only small volumes of solution.

    They are small enough and rugged enough to be used in portable instruments for field use. If —0. Ag-AgCl needed to reduce the concentrations to 10—6 M. Sample 1 at — 1. Chapter 24 The spreadsheet is attached. In amperometry, we measure the current in a cell at a fixed value of electrode potential. In pulse voltammetry, an excitation signal is used that consists of a series of pulses that increase in size as a function of time.

    The RDE usually constains a single disk. A ring-disk electrode contains a second ringshaped electrode that is electrically isolated from the center disk. After an electroactive species is generated at the disk, it is then swept passed the ring where it undergoes a second electrochemical reaction.

    The entire array of charged species and dipoles at the electrode-solution interface is the electrical double layer. The interface behaves like a capacitor when the potential across it is changed.

    Its magnitude is limited by the rate at which a reactant is brought to the electrode surface by diffusion, migration , or convection. A diffusion current is the 1 Principles of Instrumental Analysis, 6th ed. Chapter 25 limiting current when mass transport is by diffusion and migration and convection have been eliminated. Laminar flow is a type of liquid flow in which layers of liquid slide by one another in a direction that is parallel to a solid surface.

    In adsorptive stripping methods, the analyte is deposited by physical adsorption. The deposited analyte is determined by stripping in the same way by the two methods. A high supporting electrolyte concentration is used to minimize the effects of migration to the electrode surface and to reduce the cell resistance which decreases the IR drop. The reference electrode is placed near the working electrode to minimize the IR drop that can distort voltammograms.

    Most organic electrode processes involve hydrogen ions. Unless buffered solutions are used, marked pH changes can occur at the electrode surface as the reaction proceeds. Stripping methods are more sensitive than other voltammetric procedures because the analyte can be removed from a relatively large volume of solution and concentrated in a small volume. After concentration, the potential is reversed and all the analyte that has been deposited can be rapidly oxidized or reduced, producing a large current.

    The purpose of the electrodeposition step in stripping analysis is to concentrate the analyte in the thin film of mercury on the surface of the working electrode. The advantages of a mercury film electrode compared with platinum or carbon electrodes include 1 the high hydrogen overvoltage on mercury, 2 the ability to detect metal ions that are reduced at cathodic potentials, 3 the ability to be used in stripping methods, and 4 the ease of preparation by electrodeposition.

    The disadvantages include 1 the poor 3 Principles of Instrumental Analysis, 6th ed. Chapter 25 anodic potential range, 2 the relatively large residual currents, and 3 the toxicity of mercury. Chapter 25 Equation gives the relationship between peak current, diffusion coefficients and scan rate. See the attached spreadsheet 6 Principles of Instrumental Analysis, 6th ed. See attached spreadsheet c 3. Problems can also arise if the electrode dimensions become comparable to the double-layer thickness or to molecular dimensions.

    In some cases for nanoelectrodes, new theories and experimental approaches may be necessary. The mobile phase then passes over or through the stationary phase.

    The general elution problem arises whenever chromatograms are obtained on samples that contain species with widely different distribution constants. When conditions are such that good separations of the more stongly retained species are realized, lack of resolution among the weakly held species is observed. Conversely when conditions are chosend that give satisfactory separations of the weakly retained compounds, severe band broadening and long retention times are encountered for the strongly bound species.

    The general elution problem is often solved in liquid chromatography by gradient elution and in gas chromatography by temperature programming. In gas-liquid chromatography, the mobile phases is a gas, whereas in liquid-liquid chromatography, it is a liquid. In liquid-liquid chromatography, the stationary phase is a liquid which is immobilized by adsorption or chemical bonding to a solid surface. The equilibria that cause separation are distribution equilibria between two immiscible liquid phases.

    In liquid-solid chromatography, the stationary phase is a solid surface and the equilibria involved are adsorption equilibria. In GC, the retention factor is varied by changing the column temperature as is done in temperature programming. In LC, variations is achieved by altering the composition of the solvent as in gradient elution. The number of plates in a column can be determined by measuring the retention time tR and the width of a peak at its base W. Decreasing the peak widths will increase resolution as will increasing the peak separation.

    Increasing separation can be done by lengthening the column to increase the number of plates or increasing the selectivity. Chapter 26 Longitudinal diffusion is much more important in GC that in LC.

    Longitudinal diffusion is a large contribution to H at low flow rates. The initial decreases in H in plots of plate height vs. Because gaseous diffusion coefficients are orders of magnitude larger than liquid values, the phenomenon becomes noticeable at higher flow rates in GC than in LC. The minimum is sometimes not observed at all in LC. Gradient elution is a method of performing liquid chromatography in which the composition of the mobile phase is change continuously or in steps in order to optimize separations.

    Zone separation is influenced by 1 packing that produce distribution coefficients that differ significantly; 2 increases in the column length; 3 variations in mobile phase composition LC ; 4 optimizing column temperature GC ; 5 changes in the pH of the mobile phase LC ; 6 incorporation of a species in the stationary phase that selectively complexes certain analytes LC.

    Slow sample introduction leads to band broadening. Chapter 26 through In gas-liquid chromatography, the stationary phase is a liquid that is immobilized on a solid. Retention of sample constituents involves equilibria between a gaseous and a liquid phase.

    In gas-solid chromatography, the stationary phase is a solid surface that retains analytes by physical adsorption.

    In a soap bubble meter, a soap film is formed in a gas buret through which the effluent from a GC column is flowing. The flow rate is then determined from the time required for the film to travel between two of the graduations in the buret. Temperature programming involves increasing the temperature of a GC column as a function of time. This technique is particularly useful for samples that contain constituents whose boiling points differ significantly.

    Principles of Instrumental Analysis

    Low boilin point constituents are separated initially at temperatures that provide good resolution. As the separation proceeds, the column temperature is increased so that the higher boilin constituents come off the column with good resolution and at reasonable lengths of time. Chapter 27 where j is the pressure correction factor given by Equation A concentration-sensitive detector responds to the concentration of the analyte in the mobile phase, whereas a mass-sensitive detector responds to the number of analyte molecules or ions that come in contact with the detector.

    Peak areas for a concentrationsensitive detector increase as the flow rate decreases because the analyte is in contact with the detector for a longer period.

    Peak areas for a mass-sensitive detector are not greatly affected by flow rate. Using CS for concentration sensitive and MS for mass sensitive, we find for each of the detectors listed: Atomization of the analyte and excitation of atomic emission is brought about by passing the eluent through an energetic microwave field.

    Chapter 27 c The thermionic detector is based on the ion currents produced when the mobile phase is combusted in a hydrogen flame and then passed overa heated rubidium silicate bead. It is used primarily for analytes that contain phosphorus or nitrogen. Electronegative functional groups are particularly effective at capturing electrons and reducing the ion current.

    The ions and electrons produced are collected by a pair of biased electrodes giving rise to a current. Chapter 27 e Advantages, flame photometric: A total ion chromatogram is obtained by summing the ion abundances in each mass spectrum and plotting versus time. The combination of GC with MS allows the identification of species eluting from the chromatographic column.

    The total ion chromatogram gives information similar to a conventional GC chromatogram.

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    By monitoring selected ions, information about specific species can be obtained. By scanning the mass spectrum during the chromatography experiment, species eluting at various times can be identified. Gas chromatography coupled with tandem mass spectrometry allows even more specific identifications to be made. The effluent from the GC column is either continuously monitored by the second technique or collected and measured. Chapter 27 The inner surface of a PLOT column is lined with a thin film of a support material, such as a diatomaceous earth.

    This type of column holds several times a much stationary phase as does a wall-coated column. Its inner walls are coated with a thin layer of the mobile phase.

    Fused silica columns have greater physical strength and flexibility than glass open tubular columns and are less reactive toward analytes than either glass or metal columns. Desirable properties of a stationary phase for GC include: Film thickness influences the rate at which analytes are carried through the column, with the rate increasing as the thickness is decreased. Less band broadening is encountered with thin films. Bonding involves attaching a monomolecular layer of the stationary phase to the packing surface by means of chemical bonds.

    Cross linking involves treating the 5 Principles of Instrumental Analysis, 6th ed. Chapter 27 stationary phase while it is in the column with a chemical reagent that creates cross links between the molecules making up the stationary phase. The retention index for an analyte is a measure of the rate at which it is carried through a column compared with the rate of movement of two normal alkanes, one that moves faster than the analyte and the other thatmoves more slowly.

    To obtain the retention index of an analyte on a given column, the log of the adjusted retention times for the two alkanes and the analyte are determined. The retention index for butane is always and for pentane The retention index for the analyte is then derived by interpolation between the two logarithmic retention indexes of the alkane see solution to Problem The distribution coefficient for a polar compound will be larger on the carbowax 20M column than on the nonpolar SE column.

    Since the three compounds are relatively polar, they would be less compatible with a nonpolar solvent such as a silicone oil than with didecylphthalate. Since less time would be spent in the stationary phase for the silicone oil, tR would be smaller. This increases causes a marked increase in CSu Table and thus an increase in H. Reduced efficiency and an increase in H results.

    Thus, the sample will be put on the column in a narrow band with less initial zone spreading. Chapter 27 d Increasing the flow rate may cause either increases or decreases in H depending on the flow rate as can be seen in Figure b. A decrease in particle size also makes the CMu term smaller.

    Both effects lead to a smaller plate height Table In most cases, H increases as the temperature decreases. Gas-solid chromatography is used primarily for separating low molecular mass gaseous species such as carbon dioxide, carbon monoxide and oxides of nitrogen. Gas-solid chromatography has limited application because active or polar compounds are retained more or less permanently on the stationary phase.

    In addition, severe tailing is often observed due to the nonlinear characteristics of the physical adsorption process.

    Three methods for improving resolution include: In partition chromatography, k is conveniently varied by using a two or more component solvent system and varying the ratio of the solvents. In adsorption chromatography on an alumina packing, it is generally best to increase the polarity of the mobile phase as the elution proceeds.

    Thus the rato of acetone to hexane should be increased as the elution proceeds. The linear response range of a detector is the range of analyte concentration or mass over which the detector responds linearly. It is the same as the dynamic range defined in Section 1E The fitting is then replaced and pumping is resumed.

    Separation is achieved either through partitioning of the neutral ion-pair or as a result of electrostatic interactions between the ions in solution and charges on the stationary phase resulting from adsorption of the organic counter-ion.

    Chapter 28 g In ion chromatography, the stationary phase is an ion-exchange resin, and detection is ordinarily accomplished by a conductivity detector.

    A guard column is a short column through which the mobile phase flows before it reaces the injection region and the analytical column in HPLC instruments. The composition of the guard column is similar to that of the analytical column except that the particles are generally larger to minimize pressure drop.

    The purpose of the guard column is to remove particulate matter and contaminants from the mobile phase and to saturate the mobile phase with the stationary phase so that losses of that phase from the analytical column are minimized. Normal-phase partition chromatography and adsorption chromatographyare similar in the respect that the stationary phases in both are polar, whereas the mobile phases are relatively nonpolar.

    Chapter 28 In adsorption chromatography, separations are based on adsorption equilibria between the components of the sample and a solid surface. In partition chromatography, separations are based on distribution equilibria between two immiscible liquids.

    In size-exclusion chromatography separations are based upon the size, and to some extent the shape, of molecules with little interactions between the stationary phase and the sample components occurring. In ion-exchange chromatography, in contrast, separations are based upon ion-exchange reactions between the stationary phase and the components of the sample in the mobile phase. Pneumatic pumps are simple, inexpensive and pulse free. They consist of a collapsible solvent container housed in a vessel that can be pressurized by a compressed gas.

    This pump has limited capacity and pressure output and is not adaptable to gradient elution. The pumping rate depends on solvent viscosity.

    Screw-driven syringe pumps consist of a large syringe in which the piston is moved by a motor-driven screw. They are pulse free and the rate of delivery is easily varied. They suffer from lack of capacity and are inconvenient when solvents must be changed. Reciprocating pumps are versatile and widely used.

    They consist of a small cylindrical chamber that is filled and then emptied by the back-and-forth motion of a piston. Advantages include small internal volume, high output pressures, adaptability to gradient elution, and constant flow rates that are independent of viscosity and back pressure. The pulsed output must be damped.

    In suppressor-column ion chromatography the chromatographic column is followed by a suppressor column whose purpose is to convert the ions used for elution to molecular species that are largely nonionic and thus do not interfere with conductometric detection 4 Principles of Instrumental Analysis, 6th ed.

    Chapter 28 of the analyte species. In single-column ion chromatography, low capacity ion exchangers are used so that the concentrations of ions in the eluting solution can be kept low.

    Detection then is based on the small differences in conductivity caused by the presence of eluted sample components. A gas-phase sample is needed for mass spectrometry. The output of the LC column is a solute dissolved in a solvent, whereas the output of the GC column is a gas and thus directly compatible. When vaporized, however, the LC solvent produces a gas volume that is times greater than the carrier gas in GC.

    Hence, most of the solvent must also be removed. Many of the GC detectors are unsuitable for HPLC because they require the eluting analyte components to be in the gas-phase. A number of factors that influence separation are clearly temperature dependent including distribution constants and diffusion rates. In addition, temperature changes can influence selectivity if components A and B are influenced differently by changes in temperature.

    Because resolution depends on all these factors, resolution will also be temperature dependent a For a reversed phase chromatographic separation of a steroid mixture, selectivity and, as a consequence, separation could be influenced by temperature dependent changes in distribution coefficients. Chapter 28 as a consequence, separation could be influenced by temperature dependent changes in distribution coefficients. If the second peak were twice as broad as the first, Rs and N would be smaller.

    From the spreadsheet the largest percentage loss occurs between 1. For a normal-phase packing, Equation applies. The critical pressure of a substance is the vapor pressure of that substance when it is at its critical temperature. The properties of a supercritical fluid that are of particular importance in chromatography are its density, its viscosity, and the rates at which solutes diffuse in it.

    The magnitude of each of these properties lies intermediate between a typical gas and a typical liquid. Pressure increases cause the density of a supercritical fluid to increase which causes the k value for analytes to change. Generally increases in pressure reduce the elution times of solutes.

    Chapter 29 In contrast to GC, SFC separations can be carried out at relatively low temperatures making the technique applicable to thermally unstable and nonvolatile species. The analytes are collected by dipping the restrictor into a few mL of solvent and allowing the gaseous fluid to escape or by using an adsorbent like silica. On-line extractions involve transferring the effluent from the restrictor directly to a chromatographic system such as a GC. Sample handling between the extraction and measurement is eliminated in an on-line extraction.

    In a static extraction, the extraction cell is pressurized under static conditions and the cell contents are later transferred by flow of fluid from the pump. SFE is generally faster than liquid-liquid extractions.

    The solvent strength can be varied by changes in pressure and temperature. The recovery of analytes in simpler with SFE because the many supercritical fluids are gases at ambient temperature.

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