Adsorption with minute samples

Another problem, that of adsorption at any of the several interfaces (such as gas-Uquid and gas-solid), may arise with minute samples. Martin found gas-chromatographic retention volumes to be markedly affected by such adsorptions and in some cases to be the cause of a larger contribution to retention volume than solution in the bulk stationary phase. He both measured and calculated the extent of such adsorptions for a variety of solutes. Adsorption at a gas-liquid interface is serious for polar stationary phases when surface areas are relatively high, when the ratio of stationary phase to solid support is low, and when the temperature is low. The effects of adsorption are minimized by the use of relatively highly loaded columns and nonpolar stationary phases and by avoiding solute-stationary phase pairs in which the infinite-dilution activity coefficients deviate markedly from unity. Ottenstein indicated that adsorption on a liquid surface can be considered negligible in packed columns when liquid loadings exceed 5% and activity coefficients are less than 10. 

The adsorption isotherm up to 200 mm. Hg was completed and afterwards the sample was evacuated. Evacuation was started at —183° and continued for 40 minutes at that temperature. Thereafter the temperature was increased to —78° and evacuation continued at that temperature for thirty minutes. The sample was cooled down in He and evacuated. A second CO isotherm was taken at —183°. The difference between the two isotherms was determined to be 2.7 % of a monolayer. To insure that this sample had not, in some way or other, been oxidized, the temperature was raised to —78° with the sample still kept in the CO atmosphere. The sample was kept at — 78° for a few minutes only and the dry-ice bath again substituted by the liquid oxygen bath. During the temperature cycle more CO had been adsorbed so that the amount of chemisorption was now equal to 10.6% of a monolayer, 7.9% having been taken up during the temperature cycle. 

In all of the experiments so far described in Section VI,B, hydrogen was employed throughout the experiment and at flow rates of about 12 ml/minute. We ran several experiments in which the hydrogen was replaced by helium at the highest temperature of activation, the sample cooled in helium, and the injections carried out in helium with the sample at room temperature. Adsorptions were 20 pmoles/gm at 295° activation, 110 at 345°, and 270 at 395°. These samples were then heated to 400° in about 8 minutes. About one-half of the adsorbed carbon monoxide was evolved by 95°, usually in two peaks the last of which was followed by a long tail. In an experiment with activation at 395°, nitrogen was used instead of helium. Nearly the same results were obtained. 

IR spectra of the samples were obtained in the range of 4000 cm to 900 cm by a Nicolet 710 (Nicolet Analytical Instmments) FT-IR spectrometer with a MCT detector. All spectra were recorded after 100 scans. A self-supporting disk of catalyst sample was mounted in a transmission IR cell. Prior to the methanol adsorption experiment the sample was dried under vacuum at different temperatures, then exposed to the methanol vapour for 60 minutes and dried under vacuum again. 

The batch recycle differential reactor is used to obtain adsorption rate data to evaluate various kinetic models for cadmium ion adsorption on Sol-AD-lV. Approximately 0.25 g of the solid extractant is placed between glass beads inserted in a glass tube of 1.0 cm internal diameter. Solutions of various cadmium ion concentrations (25 to 300 mg/1) are contacted in a batch mode over 720 minutes, with intermediate sample taken for atomic absorption spectroscopic measurements of the cadmium ions. The external mass transfer resistances are minimized by operating at 16 ml/min. Results of the experiments are shown in Figure 7.23. 

Charcoal Tubes Reference has been made earlier to adsorption, which is the property of some solid materials, such as activated charcoal, to physically retain solvent vapors on their surfaces. In environmental health testing, the adsorbed vapors are removed, generally with a solvent, in a laboratory. The solvent is then analyzed by physical methods (gas chromatography, etc.) to determine the individual compounds whose vapors, such as benzene, were present in the sampled air. Industrial atmospheric samples can be collected in small glass tubes (4 mm ID) packed with two sections of activated charcoal, separated and retained with fiberglass plugs. To obtain an air sample, the sealed ends of the tube are broken off, and air is drawn through the charcoal at the rate of 1 liter per minute by means... 

The samples were submitted to the sulfidation procedure described above, followed by 2 h of heating at 673 K, under vacuum (about 2x10 3 Pa). After cooling under vacuum, pyridine was adsorbed at room temperature for 30 minutes. The samples were then outgassed in three steps of 1 h the first one at room temperature and the others at 423 K and 523 K. Spectra were taken before pyridine adsorption and after each outgassing step, with a FTIR spectrometer Bruker IFS-88 (spectral resolution set at 1 cm ). Each spectrum represented the average of at least 50 scans. 

Specific surface areas of the catalysts used were determined by nitrogen adsorption (77.4 K) employing BET method via Sorptomatic 1900 (Carlo-Erba). X-ray difiraction (XRD) patterns of powdered catalysts were carried out on a Siemens D500 (0 / 20) dififactometer with Cu K monochromatic radiation. For the temperature-programmed desorption (TPD) experiments the catalyst (0.3 g) was pre-treated at diflferent temperatures (100-700 °C) under helium flow (5-20 Nml min ) in a micro-catalytic tubular reactor for 3 hours. The treated sample was exposed to methanol vapor (0.01-0.10 kPa) for 2 hours at 260 °C. The system was cooled at room temperature under helium for 30 minutes and then heated at the rate of 4 °C min . Effluents were continuously analyzed using a quadruple mass spectrometer (type QMG420, Balzers AG). 

Platinum was determined in seawater by adsorptive cathodic stripping voltammetry in a method described by Van den Berg and Jacinto [531]. The formazone complex is formed with formaldehyde, hydrazine, and sulfuric acid in the seawater sample. The complex is adsorbed for 20 minutes at -0.925 V on the hanging mercury drop electrode. The detection limit is 0.04 pM platinum. 

Distinguishing between adsorption on to the cell surface and the actual transfer across the cell membrane into the cell may be difficult, since both processes are very fast (a few seconds or less). For fish gills, this is further complicated by the need to confirm transcellular solute transport (or its absence) by measuring the appearance of solutes in the blood over seconds or a few minutes. At such short time intervals, apparent blood solute concentrations are not at equilibrium with those in the entire extracellular space, and will need correcting for plasma volume and circulation time in relation to the time taken to collect the blood sample [30]. Nonetheless, Handy and Eddy [30] developed a series of rapid solution dipping experiments to estimate radiolabelled Na+... 

After 1 h on an orbital shaker, pH is recorded and 5 mL aliquots of each thin slurry are filtered for ICP analysis. Over powders, Pt adsorption is complete within minutes [19,23], and 1 h contact is sufficient to ensure equilibration. Samples of the parent solution (before contact) are also filtered and analyzed with ICP, which can be used to determine not only metal uptake, but also dissolution of the support. The metal uptake, in terms of surface density (mol/m2), is calculated by dividing the concentration difference by the SL ... 

Specific Antibody Determination. Serum samples were prepared from each bleed by centrifugation to remove clotted material. 100 ul of the sera was incubated for 30 minutes with sufficient H-STXOL to provide a ca. 20 fold excess of hapten to the anticipated quantity of specific binding sites. The radioactivity of the protein pellet was determined after ammonium sulfate precipitation. After correction for a small amount of non-specific adsorption of label by control sera proteins the mg/ml of specific antibody in the sample was calculated. 

---