Influence of alkyl chain length on the stability of n-alkyl-modified reversed phases. 1. Chromatographic and physical analysis

The Influence of the llgand alkyl chain length, chemically bonded at the surface of a silica substrate, on statlonary phase stabilty In llquld chromatography practlce is reviewed. Several factors affecting long-term stability of modern reversed- phase high-performance liquid chromatography (RPHPLC) phases are consldered and their lndlvldual contrlbutlons are evaluated In this paper and the following paper In this issue. The stationary phases under study were identically modifled on the same batch of sillca substrate to ellmlnate differences In substrate properties and synthesis conditions. Modiflcatlons with llgand alkyl chain length between C1 and C18 were performed such that an approximately equal ligand density was obtained for the the seven RP-HPLC phases studied. These n-alkyldimethylsiloxysilane bonded phases were exposed to sknulated aglng experlments. A subsequent chromatographic characterizatlon regarding changes In capacity, lipophilic and polar selectlvlty, and sillca degradatlon related separation performance was carried out. Comparison with results determined by other characterization methods, llke bulk analysis, elemental analysls, and solid-date 29Si crosspolarizatlon magic angle spinning NMR revealed that with longer n-alkyl llgands gradually better substrate shieldlng properies were obtained. The ligand alkyl chain length affects the stability of reversed phases to a large extent. Short ligand modified phases changed drastically uslng relatively aggresslve eluents. Organlc modifier rlch eluents combined with longer ligand modifled phases are preferable for a longer llfetlme of RP-HPLC columns, especially when aggressive eluents are used

Soil microbes contribute to the classic plant diversity-productivity pattern

Ecosystem productivity commonly increases asymptotically with plant species diversity, and determining the mechanisms responsible for this well-known pattern is essential to predict potential changes in ecosystem productivity with ongoing species loss. Previous studies attributed the asymptotic diversity–productivity pattern to plant competition and differential resource use (e.g., niche complementarity). Using an analytical model and a series of experiments, we demonstrate theoretically and empirically that host-specific soil microbes can be major determinants of the diversity–productivity relationship in grasslands. In the presence of soil microbes, plant disease decreased with increasing diversity, and productivity increased nearly 500%, primarily because of the strong effect of density-dependent disease on productivity at low diversity. Correspondingly, disease was higher in plants grown in conspecific-trained soils than heterospecific-trained soils (demonstrating host-specificity), and productivity increased and host-specific disease decreased with increasing community diversity, suggesting that disease was the primary cause of reduced productivity in species-poor treatments. In sterilized, microbe-free soils, the increase in productivity with increasing plant species number was markedly lower than the increase measured in the presence of soil microbes, suggesting that niche complementarity was a weaker determinant of the diversity–productivity relationship. Our results demonstrate that soil microbes play an integral role as determinants of the diversity–productivity relationshi

Influence of alkyl chain length on the stability of n-alkyl-modified reversed phases. 2. Model dissolution study

A dlrect, In situ study of the hydrolysis and dissolution of various n-alkyl modHied phases was performed with =Si maglc angle spinnlng NMR spectrometry as a model for statlonary phase aging In laboratory practice. WRh thls study, the substantlal effect of the ligand length on the dlssolutlon reactlon of the alkyl "ed phases under aggresslve eluent conditions is conflrmed. As expected, longer n-alkyl ligands gradually show better substrate shleldlng properties. Above a certaln critical ligand length, shielding by longer ligands inhibit the dissolution rate drastically by affectlng the hydrolysis and dissolution processes. The type of solvent system used, In tiquki chromatography practice or for dissolution studies as reported here, mainly affects the proportions of dissolved silicates in the eluent. On the basis of thls in sltu dlssolutlon study a model for ligand and substrate hydrolysis and dissolutlon with the involved reactions is presented

Influence of alkyl chain length on the stability of n-alkyl-modified reversed phases. 2. Model dissolution study

A dlrect, In situ study of the hydrolysis and dissolution of various n-alkyl modHied phases was performed with =Si maglc angle spinnlng NMR spectrometry as a model for statlonary phase aging In laboratory practice. WRh thls study, the substantlal effect of the ligand length on the dlssolutlon reactlon of the alkyl "ed phases under aggresslve eluent conditions is conflrmed. As expected, longer n-alkyl ligands gradually show better substrate shleldlng properties. Above a certaln critical ligand length, shielding by longer ligands inhibit the dissolution rate drastically by affectlng the hydrolysis and dissolution processes. The type of solvent system used, In tiquki chromatography practice or for dissolution studies as reported here, mainly affects the proportions of dissolved silicates in the eluent. On the basis of thls in sltu dlssolutlon study a model for ligand and substrate hydrolysis and dissolutlon with the involved reactions is presented