Columns for HPLC - Western Analytical

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Columns for HPLCColumns for HPLCAnalytical columns packed with NUCLEOSIL ®Unmodified NUCLEOSIL ® , polar NUCLEOSIL ® phases and ion exchangersPackings with chemically bonded polar phasesThese materials are characterised by fast mass transfer, becausethe bonded organic layer of these stationary phases ismonomeric, thus minimising diffusion paths.Due to the availability of different functional groups it is possibleto vary the chemical characteristics of the surface andconsequently the adsorption characteristics of the stationaryphase. Silica packings with bonded polar phases thus combinethe advantages of partition chromatography (variation ofadsorption properties) and adsorption chromatography (rapidmass transfer in the stationary phase). Further advantagesof chemically bonded polar phases are:They do not “bleed”; equilibration with a liquid stationaryphase is not necessary as is the case in classical partitionchromatography.The influence of impurities in the eluent (e. g. water) onthe separation qualities of bonded polar phases is muchsmaller than is the case with unmodified silica gels andaluminium oxides for adsorption chromatography.Swelling and consequently variation in the permeabilityof the packed columns never occurs with chemicallybonded polar silica phases. Even the use of water orother aqueous eluents does not present any problems.Although the chemically bonded phases on the surface orthe silica gel form a monomeric organic layer, they have alarge surface concentration of functional groups, which isabout 3 to 4 functional groups per 100 Å 2 , i. e. about5 µeq/m 2 .Influence of functional groups on the separation characteristics of a stationary phasek’10,09,08,07,06,0Effect of modification on the separationof aromatic hydrocarbonsNO 2 phaseThe figure on the left shows the influence of the modificationof silica gel on the k’ values (capacity factors) of aromatic hydrocarbons,with n-heptane as eluent on unmodified NUCL-EOSIL ® as well as three chemically bonded polar phases,namely NUCLEOSIL ® CN, NH 2 and NO 2 . On the y-axis thek’ values of 9 aromatic hydrocarbons have been plotted. Theinfluence of the nitroaromatic groups on the separation is especiallyevident. A strong interaction between the electronicsystems results in the formation of charge transfer complexeswith considerably larger k’ values than for unmodifiedNUCLEOSIL ® .5,0NH 2 phase4,03,02,0NUCLEOSILunmodified1,00BenzeneNaphthaleneBiphenylAnthracenePhenanthrenePyreneChryseneBenzo[a]pyreneCN phaseBenzo[e]pyrene6.7.05MNwww.mn-net.com49
Columns for HPLCColumns for HPLCAnalytical columns packed with NUCLEOSIL ®NUCLEOSIL ® ion exchangersThese phases have been especially developed for HPLC.Contrary to adsorptions or reversed phase chromatographywith ion exchange chromatography there are three variableparameters for the mobile phase, which can influence theseparationthe type of buffer,the ionic strength andthe pH value.By changing the ionic strength of the eluent, the competitionbetween mobile phase and sample molecules for the functionalionic groups on the stationary phase is influenced.When the retention time of the sample molecules is too long,an increase in the ionic strength of the mobile phase will reduceretention times and vice versa.A change in the pH value of the mobile phase will alter thedissociation of analyte and buffer salt. These two effects influencethe equilibrium between the sample molecules andthe functional groups of the stationary phase. Thus the retentionof weak acids on an anion exchanger will increase whenthe pH value is raised, because a larger part of the acid ispresent in ionic form at higher pH values. If, on the otherhand, e.g. a phthalate buffer is used, increase of the pH valuecan increase the concentration of phthalate ions and thusthe ionic strength. This results in a decreasing retention time.Proper selection of buffer types and pH values can thus resultin enormous selectivity differences, which can be utilisedfor optimisation of a separation.Even for equal ionic strength of the mobile phase, differentions show different degrees of affinity to the ion exchangegroups. The following orders of increasing affinity can be givenfor anion exchangersF – < OH – < Cl – < CN – for cation exchangersH + < Na + < NH + 4 < K + < Cs + < Ag +Divalent ions are more strongly attracted to the ion exchangegroups than monovalent, and trivalent more than divalentions. Generally speaking, the greater the charge and effectiveradius of ions, the stronger is the attraction to the ion exchangegroups.+ + ++ +++ + +++ ++++ + + +++++ ++ ++++ +++www.mn-net.com6.7.0550www.mn-net.comMN
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Columns for HPLC - Western Analytical

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