John H. Knox

The performance of packings in high-speed liquid chromatography : III. Chemically bonded pellicular materials

The reduced plate height, h, has been determined as a function of the reduced velocity, v, for Permaphases ETH and ODS** for several solutes at temperatures from 20° to 60° (0 < k′ < 10). The data are interpreted in terms of the equation n n n nFor Permaphase ETH A lies between 1.6 and 1.9 while C lies between 1.9 × 10−2 and 5.6 × 10−2. These relatively high values of C for a pellicular material probably indicate slow mass transfer in the polymeric stationary phase. The relatively indicate slow mass transfer in the polymeric stationary phase. The relatively low value of A indicates that reasonably good packing has been achieved. For Permaphase ODS anomalous values are obtained. A increasing with degree of retention while C is near zero. This is an artefact arising from the increasing asymmetry of peaks as k′ increases. n nHeats of transfer have been determined with both materials. For Permaphase ETH ΔH for transfer from the stationary to mobile phase (hexane + 1% ethanol) lies between 1.8 and 3.7 kcal mol−1 but is not the only factor determining k′. For Permaphase ODS values of ΔH cover a wider range (0.9 to 5.9 kcal mol−1) and closely follow the values of k′; thus retention on Permaphase ODS is largely determined by the heat of transfer from the stationary phase to the mobile phase (methanol—water, 40:60).

Band dispersion in chromatography--a universal expression for the contribution from the mobile zone.

It is generally assumed that the dispersion which is covered by the C term of Van Deemter type equations arises from processes occurring in the static zone, while the dispersion covered by the A term arises from processes occurring in the mobile zone. It is also now widely accepted that the contribution to h, the reduced plate height, from mobile zone processes increases with a modest power of v, the reduced flow velocity. A reassessment of data acquired since the 1960s suggests that this power falls with increasing velocity, but may be relatively high at reduced velocities, v, in the range 1-30. Data for a wide variety of materials over a wide range of v have been re-examined and are well fitted by an equation of the form: h = B/v + [1/A + 1/(Dv(n))](-1) + Cv. With C < or = 0.02 in accordance with the theoretical value for slow equilibration in the static zone, n is found to be in the range 0.5-1.0 with the lower values applying to glass bead packings, and the higher values applying to porous spherical packing materials. The equation provides a decreasing power of velocity in the A term in agreement with experimental data. It is now clear that nearly all of the dispersion previously assigned to processes in the static zone actually occurs in the mobile zone. Accordingly, substantial improvements in column performance in LC may well be achieved by better packing of columns, or by designing structures such as monolithic beds and two dimensional designs on chips, which can provide more uniform structures than the beds of spherical particles widely used in current

Capillary electrochromatography - the LC analogue of capillary GC

Capillary electrochromatography is a form of high performance liquid chromatography which uses an electric field to drive the eluent through the column. It can readily be coupled to mass spectrometry without loss of chromatographic efficiency, and rivals capillary gas chromatography in overall performance.

Homogeneous and heterogeneous processes in the gas-phase oxidation of isobutane and isobutene

According to most theories of the slow gas-phase oxidation of hydrocarbons, the major product producing reactions are homogeneous and it is supposed that the main effect of surface is on kinetically important reactions which produce relatively little final product. According to one theory of hydrocarbon oxidation all primary products arise from the homogeneous unimolecular decomposition of peroxy radicals or from reactions of radicals produced from such decompositions. If this is so, the distribution of the products formed directly from the peroxy radicals should be unaffected by reactant pressures (in particular, oxygen pressure) and should change gradually with temperature provided that conditions are chosen to avoid secondary oxidation. By analyzing the reaction products from the oxidation of isobutane and isobutene between 250° and 350°C when the extent of oxidation is sufficiently small that secondary oxidation is negligible we have found that, while the percentage yields of the major products (about 80% isobutene from isobutane, and about 75% acetone+formaldehyde from isobutene) are little affected, reactant pressure and temperature have such profound effects on the distribution of the minor products that the simple hypothesis outlined above is untenable. Furthermore, investigations of the effect of change of surface have revealed that the initial distribution of minor products is highly sensitive to the nature of the reaction vessel wall: the surfaces investigated were clean Pyrex; HF washed Pyrex; KCl-, B 2 O 3− , and NaOH-coated Pyrex; and silverplated Pyrex. It is concluded that, contrary to general belief, a considerable part, if not the whole, of the minor products arise in heterogeneous processes which by consequence play a very important part in the reactions between 250° and 350°C. However, general considerations demand that the heterogeneous processes are not independent but are consequent on primary homogeneous processes involving free radicals. It is proposed that three primary homogeneous reactions occur in the early stages of the oxidation of isobutane and three in the oxidation of isobutene: Isobutane C 4 H 10 +X=C 4 H 9 +HX C 4 H 9 +O 2 =C 4 H 8 +HO 2 C 4 H 9 +O 2 =C 4 H 9 OO Isobutene C 4 H 8 +HO 2 =Me 2 CO+CH 2 OH C 4 H 8 +HO 2 =Me 2 COH+CH 2 O C 4 H 8 +HO 2 =C 4 H 8 OOH and that these homogeneous reactions are followed by the heterogeneous decompositions of the peroxy radicals into complex surface sensitive mixtures of products, C 4 H 9 OO+wall=complex mixture A C 4 H 8 OOH+wall=complex mixture B With isobutene there will necessarily be secondary homogeneous processes producing formaldehyde or CO by oxidation of CH 2 OH and acetone by oxidation of Me 2 COH. This mechanism explains why the ratio of major to minor products is little affected by composition, total pressure, temperature, or surface while the distribution of minor products is highly sensitive to these parameters.

The mechanism of combustion of pentane in the gas phase between 250° and 400°C

The initial stages of the slow reaction between gaseous n-pentane and oxygen have been studied under static conditions between 250° and 400°C by analysis of the reaction products, using gas chromatography. The dependence of initial fractional yields on surface, mixture composition, and temperature has been established. The reaction has been proved to be propagated homogeneously by carrying out competitive experiments, from which the rate of radical attack on pentane relative to 2-methyl propane is 1.35±0.05. Nevertheless, the initial fractional yields of products depend strongly upon the nature of the reaction vessel surface, indicating that an important part of the reaction is heterogeneous. Trends in product yields with composition at 290°C indicate that RO2 radicals can: (a) abstract H from RH to give ROOH, which subsequently decomposes heterogeneously; (b) decompose homogeneously to pentenes and O-heterocycles; and (c) diffuse to the walls to give predominantly acetone. The rate constants which can be deduced for (a) and (b) on the basis of a calculated rate for (c) are in good agreement with those proposed elsewhere. At about 250°C, the entire product appears to arise from heterogeneous reactions of radicals, while above 350°C the major products arise from homogeneous pyrolysis of ROO radicals.