Minimum column temperature required for concurrent solvent evaporation in coupled HPLC-GC

Concurrent solvent evaporation using the loop-type HPLC-GC interface requires that the GC oven temperature be above the eluent boiling point at the given carrier gas inlet pressure in order to prevent eluent flowing into the GC capillary column. Corresponding oven temperatures representing minimum oven temperatures for eluent transfer were experimentally determined for solvents and solvent mixtures of interest for use as HPLC eluents. Evaluation of eluents for concurrent evaporation is discussed. Recommended lengths of uncoated column inlets (pre-columns) are derived from the mechanisms involved in solvent evaporation. Temperatures listed as minimum column temperatures for concurrently evaporating HPLC eluents are also useful for estimating maximum applicable column temperatures when working with the conventional retention gap or partially concurrent solvent evaporation techniques in coupled HPLC-GC.     

Inductive Charge Dispersal in Quinuclidinium Ions. Polar Effects,Part 13

Inductive charge dispersal to the α- β- and γ-positions of the solvated quinuclidinium ion has been examined by comparing the pK_a and the derived inductivities ρ_I of several 2- 3- and 4-substituted quinuclidinium perchlorates 4, 5 , and 6 , respectively. The same inductivity is observed at the practically equidistant β- and γ-positions. It, therefore, appears that polar substituent effects are transmitted directly through the molecule. As expected, inductivity is considerably higher at the α-positions where through-bond and direct induction coincide. The fact that the pK_a of all three series of salts correlate linearly with each other points to the common nature of these inductive electron displacements.     

Mechanisms and Stereochemistry of Heterolytic Fragmentation

Heterolytic fragmentation can proceed by several mechanisms. A study of the solvolysis of γ-aminoalcohol derivatives N? C? C? C? X, where X equals halogen or ? OSO₂R, shows that fragmentation may occur by a one-step mechanism or by several multi-step mechanisms depending on structural, electronic, and steric factors. The one-step synchronous mechanism has rigorous stereoelectronic requirements. It is associated with an increased reaction rate as evidenced by the “frangomeric” effect. Ketoxime derivatives RR′C?N? X may undergo Beckmann fragmentation by the synchronous mechanism or by prior rearrangement depending on their α-substituents. Fragmentation is frequently accompanied by conventional substitution, elimination, and rearrangement reactions. Under such circumstances knowledge of all possible mechanisms of a given substrate is essential to bring about a desired transformation.     

Epimeric 2-norbornyl cations in the solvolysis of 6-exo- and 6-endo-substituted 2-exo-norbornyl p-toluenesulfonates

Solvolysis of 6-exo- and 6-endo-substituted 2-exo-norbornyl tosylates $\text{1}$ and $\text{3}$, respectively, yields identical or different product mixtures depending on the inductive effect of the substituent, indicating that induction involves graded σ-participation.     

Bridging strain in the solvolysis of epimeric bicyclic toluenesulfonates

The large variations in the rate ratios for the epimeric bicyclic p-toluenesulfonates $\text{1}$ to $\text{5}$ are ascribed to differential bridging strain accompanying the formation of intermediate cations.     

Towards on-line SEC-GC of pesticide residues? The problem of tailing triglyceride peaks

SEC is used for clean-up of food extracts obtained for analysis of pesticide residues primarily to remove high molecular weight material, among which fat may be present at the largest concentrations. The triglyceride peak, which is usually very large, is eluted before the components of interest. The problem described in this paper deals with the tail of the triglyceride peak: this may intrude into the fraction containing the pesticides and contaminate the GC.     

Peak broadening in isothermal runs due to large retention gaps in capillary GC?

A simple formula is derived which allows estimation of the band broadening due to longitudinal diffusion in retention gaps for isothermal runs with the column held at the injection temperature. The broadening is strongly dependent on the linear gas velocity in the retention gap. If the internal diameters of the retention gap and the separation column are similar, the retention gap may have a length exceeding 100 m, even if the separation column is only 10 m long. A 0.5 mm I.D. retention gap attached to a 0.3 mm I.D. separation column (of interest for automatic on-column injection) may be several meters long. Retention gaps of 0.3 mm I.D. may be used for on-column injections into separation columns with I.D's. down to about 0.1 mm.     

Rapid thermal annealing and titanium silicide formation

Titanium silcides have been formed on monocrystalline (111) silicon substrates by rapid thermal annealing (RTA) of Ti layers deposited on Si at 700–800 °C for 1 to 240 s. The phase composition is dependent on the annealing temperature and time: at 700° and 750 °C for short annealing, TiSi and TiSi₂ are observed. At 800 °C and by increasing the exposure time at 700 ° and 750 °C, only TiSi₂ is detected. The growth of the total silicide thickness is found to be faster for RTA than for conventional furnace annealing and governed by two different mechanisms depending on the phases formed: in the range 700–750 °C, and 750–800 °C, activation-energy values of 2.6 ± 0.2 and 1.5 ±0.2 eV are found, respectively.For a thin deposited Ti layer (< 100 nm), the whole Ti is finally transformed into TiSi₂ with 20@ cm resistivity. For thicker Ti thicknesses, titanium oxide stops the reaction.     

Activated aluminum oxide selectively retaining long chain n-alkanes: Part II. Integration into an on-line high performance liquid chromatography-liquid chromatography-gas chromatography-flame ionization detection method to remove plant paraffins for the determination of mineral paraffins in foods and environmental samples

Aluminum oxide activated by heating to 300-400 °C retains n-alkanes with more than about 20 carbon atoms, whereas iso-alkanes largely pass non-retained (with characteristics described in more detail in Part I). This property is useful for the analysis of mineral oil contamination of foods and other matrices: it enables the removal of plant n-alkanes, typically ranging from C₂₃ to C₃₃, when they disturb the analysis of mineral paraffins (usually almost exclusively consisting of iso-alkanes). An on-line HPLC-LC-GC-FID method is proposed in which a first silica gel HPLC column isolates the paraffins from the bulk of edible oils or extracts and is backflushed with dichloromethane. In a second separation step, a 10 cm × 2 mm i.d. column packed with activated aluminum oxide separates the long chain n-alkanes from the fraction of the iso-alkanes which is transferred to GC-FID by the on-column interface and the retention gap technique. The retained n-alkanes are removed by flushing with iso-octane.