Stationary Phase In Gas Chromatography Engineering Essay

unmoving Phase In Gas Chromatography Engineering EssayStationary kind in Gas Chromatography (GC) is the part of the chromatographic system where the mobile course will proceed and distribute the solutes betwixt the phases. Stationary phase plays a vital role in determining the selectivity and guardianship of solutes in a mixture. There ar two major types of GC which be gas- substantiality chromatography and gas- naiant chromatography.In gas-solid chromatography, identical framework is apply as both the nonmoving phase and upkeep material. The common adsorbents used let in alumina, molecular sieve such as zeolite and clay, silicon oxide and active carbon. In gas-liquid chromatography, the nonmoving phase is a liquid which is immobilized or adsorbed on a solid foul material such as silica particles. The material of nonmoving phase ranges from polymers such as polysiloxanes, polyesters, polyethylene glycols to fluorocarbons, and liquid crystals. In addition, the stati onary phase may consist of either porous particles, solid particles or a fibrous material such as paper.There are various types of stationary phases available because the choice of stationary phase world the most suitable one depends on the house of components. The primary rule of separation is like change state like where non- pivotal analytes will divider strongly into non-polar stationary phases and polar analytes partition into polar phases.Polysiloxanes, for instance are the most common stationary phases. They possess the greatest variety and are stable, robust and versatile. Besides that, they spate resist oxidation and offer noble solute diffusivitites into the polymer coupled with excellent chemical and thermal stability. degree Celsius% methyl group substituted is the most basic polysiloxane being used and is non polar. The diagram at a dispiriteder place shows the basic structure of 100% dimethyl substituted polysiloxane.Because a variety of groups backside be incorporated into the structure, polysiloxane exhibit a wide range of polarities ranging from non polar to polar. This burn down be done by replacing the methyl groups with other functional groups in the polymer structure. The structure be scurvy-scale is a general representation of substituted polysiloxane.The R groups rout out be methyl(-CH3), phenyl(-C6H5), trifluoropropyl(-CH2CH2CF3) or cyanopropyl(-CH2CH2CH2CN). X and Y show up the ploughshare of an aggregate in the overall polymeric stationary phase composition. The increase in the percentage of substitution of these polar groups increases the polarity of the liquid phase to various degree.For instance, 5% diphenyl-95% dimethyl polysiloxane.In this structure, R1 and R2 are phenyl groups and R3 and R4 are methyl groups. M and N have the value of 5% and 95% respectively.Table below shows virtually of the common stationary phases used in gas-liquid chromatography.Stationary PhaseCommon Trade NameTemperatureCApplicationsPol ydimethyl siloxaneOV-1, SE-30350hydrocarbons, drugs, steroidsPoly(phenylmethyldimethyl)siloxane (10% phenyl)OV-3, SE-52350Fatty acid methyl esters, alkaloids, drugsPoly(phenylmethyl) siloxane (50% phenyl)OV-17250Drugs, steroids, pesticides, glycolsPoly(trifluoropropyldimethyl)siloxaneOV-210200Chlorinated aromatics, nitroaromatics,alkyl substituted benzeneMethyl-5% phenyl polysiloxaneSE-54, OV-23, DB-5, SPB-5, BP-5, HP-5, ULTRA 2, RTx-5, CPSil-850-325Similar to methyl polysiloxane. Slightly more than selective due to phenyl content. Excellent thermal stability.Methyl 50% Phenyl PolysiloxaneOV-17, DB-17, SPB-7, BP-10, HP-17, RTx-17, AT-50,40-325Added selectivity-higher phenyl content.Retains similar compounds longer than methyl silicone.Efficient separations of drugs, sugars and steroids. nice thermal stability.6% Cyanopropylphenyl 94% MethylpolysiloxaneDB-1301, RTx-1301, HP-130130-320Selectivity for polarizable and polar compounds. Exhibits little retention of polyaromatic compoun ds. serious thermal stability.Methyl 7% Cyanopropyl 7% Phenyl PolysiloxaneDB-1701, CPSil-19, RTx-1701, AT-1701280Unique selectivity of cyanopropyl and phenyl groups.Not truly a polar phase.Good thermal stabilityMethyl 25% Cyanopropyl 25% Phenyl PolyciloxaneDB-255, HP-255, CPSil-43, RTx-225, AT-25540-240 paired phase.Efficient separations of fatty acids and alditol acetate derivatives of sugars.Fair thermal stabilitySilicone OilDC-550180-200Moderately polar substrate,used for alkylbenzenes and naphthalene homologsSilicone maunder RubberSE-30400Non polar,for highest temperature work. Used for steroids and polycyclical aromaticsFor polydimethyl siloxane, the -R groups are all hydrophobic giving liquid the least polarity and has the following general structure.Poly(cyanopropylphenyldimethyl) siloxanes are another polar stationary phases. They are used in separating compounds which obligate several hydroxyl groups such as steroids.Another type of stationary phase is polyethylene glycol s (PEGs) which is shown below.This stationary phase is non-silicon-containing stationary phase and is most widely used after siloxanes in the analysis of polar solutes. They are moderately polar and was considered the most polar stationary phase available due to the problem in coating and crabby-linking of polar siloxane on the stationary phase. Besides, they are well known for their unique selectivity and high polarity as a liquid phase. The polyethylene backbone of these pillars is contrary than polysiloxane phases. Strong polar dispersive interaction in the phase is imparted by the oxygen group in the polymer backbone. It alike provides a precise strong dipole interaction as the phase itself is capable of hydrogen bonding which is the bonding amidst a strong polar group (OH, NH) and a compound with strong electronegativity (F, O, N). Stationary phases with jump or FFAP in their names to a fault belong to polyethylene glycol. Polyethylene glycols stationary phases have 10 0% of the stated material because they are not substituted. They have several disadvantages such as slight stable, less(prenominal) robust and limited maximum temperature compared to most siloxanes. In addition, they exhibit shorter lifetimes and have high susceptibility to wrongfulness upon over-heating or exposure to oxygen. However, the unique separation properties of polyethylene glycol have made these liabilities tolerable. Also, cross-linked PEG phase is able to overcome these deficiencies. Under GC temperature condition, PEG stationary phases must be liquids. For example, alcohols, ethers, aldehydes and other compounds with low boiling points butt end be separated by a suitable sorbent called PEG 400. Carbowax 20M can be used for the separation of polar compounds with higher boiling points. Other polar compounds such as amino alcohols, hydroxyl acids, dibasic acids, amines, nitrile, fatty acids, fatty acid methyl esters (FAMEs), aromatic volatile compounds, and nitrosamine s can also be separated using PEG editorials.Arylene-modified polysiloxanes are also known as aryl-poly or arylene stationary phase. They are similar to standard polysiloxane except having phenyl groups in the polymer backbone. This stationary phase has several advantages including lower column hightail it and higher temperature limits than their polysiloxane counterparts.Diagram 1 Structure of arylene-modified polysiloxaneIn order to prevent column bleed during GC analysis, most of the stationary phases used today are of arylene-modified polysiloxane. These stationary phases have been designed to be equivalent to a familiar stationary phase such as 5% phenylmethyl polysiloxane (BD-5ms and DB-5). They have lissom differences although both the stationary phases have similar separation characteristics.Chiral stationary phases are also used in Gas Chromatography analysis. These stationary phases are typically used to separate individual enantiomers, stereoisomers which except diff er in the spatial arrangement of their atoms and in their ability to rotate the plane of polarized light. Separation of two substances can only occur when their standard energy of distribution differ, which means that their standard enthalpies and/or their standard entropies of distribution also differ. In general, the standard enthalpy indicates the difference in the interactive forces such as polar, dispersive and noggin interactive on the molecule in the two phases whereas the standard entropy indicates their spatial disposition. Hence, to separate chiral solutes, the stationary phase chosen must differ significantly in the spatial arrangement of its composite atoms results in the probability or proximity of interaction between the two enantiomers to be separated. Many chiral compounds are used in the preparation of chiral stationary phase (CSP). Cyclodextrin (CD) and their derivatives are the most commonly used chiral compounds. Cyclodextrin is a cyclic oligomer substituted int o a conventional siloxane stationary phase. A strong interaction with the cavity in the CD is achieved when fundamental molecules of correct size and shape are present. Hence, these organic molecules will be more strongly maintained on the capillary column. Furthermore, modified CDs are used since they are capable of resolving chiral solutes over a high range of GC temperatures. Chiral stationary phase plays a vital role in separation especially in pharmaceutical industry because pharmaceutical compounds usually exist as enantiomers. Some ungainly estimations about the target compounds that are generally well dissolved into their enantiomers by using specific chiral stationary phase are illustrated below.Product NameStationary PhaseAnalytesCyclodextrin E2,6-Pentyl-3-Butyryl-gamma-Cyclodextrinoxygenated terpenes, alcohols, epoxidesCyslodextrin G6-Methyl-2,3-Pentyl-gamma-Cyclodextrinmonoterpene hydrocarbons, volatile/low temperatureCyclodextrin H2,6-Methyl-3-Pentyl-gamma-Cyclodextr interpenes, alcohols, alkenesCyclodextrin 3P2,6-Methyl-3-Pentyl-beta-Cyclodextrinterpenes, alcohols, alkenesCyclodextrin TM6-TBDMS-2,3-Methyl-beta-CyclodextrinPCB, polycyclic or chlorinated aromatics, pesticidesCyclodextrin TE6-TBDMS-2,3-Ethyl-beta-Cyclodextrinpharmacopeia separations of essential oilsCyclodextrin TA6-TBDMS-2,3-Acetyl-beta-Cyclodextrinoxygenated terpenes, aromatics, low volatileCyclodextrin PM2,3,6-Methyl-beta-Cyclodextrinlegacy phase for many analytes.In Gas Chromatography, there are generally two different types of column used which are packed columns and capillary columns.Packed columns contain finely divided unemployed solid maintain material that is densely packed in the inside of the column in which the material is coated with a liquid stationary phase. This stationary phase is 3-10% by weight of the solid support and will form a thin liquid film on the surface of the material where the mobile phase will flow over and around the coated material as it travels down the column. The solid support material used is usually diatomaceous earth. To improve resolution and speed, the particles size should be undersized enough, ranging from less than 100-300mm and are uniform in size. Small size of particles is necessary as it increases the surface area for easier partition and separation of solutes. Besides that, the material should be inert to avoid any chemical reaction between the solutes and solid support material. However, packed columns have limited resolution where NDiagram 2 jump section of packed columnSolid support materialPacked columns are 1.5 10 m in length and have an internal diameter of 2 4mm. They are normally constructed from clear steel but can be glass such as Pyrex glass if a less reactive surface is desired. Pyrex glass is chosen when thermally labile solutes are being separated. Unfortunately, glass has mechanical press limitations and for long packed columns, stainless steel columns are chosen since they possess hig h pressure tolerance. The temperament of the coating material which is the liquid stationary phase determines what type of solutes will be most strongly adsorbed onto it. Hence, various columns are available that are designed to separate specific types of compounds.Open tubular columns or rather known as capillary columns are characterized by a small narrow disruption in the centre of the column through which the mobile phase will travel as it moves foregone the stationary phase. There is no packing of solid support material unlike packed columns. hairlike column is constructed by fused silica which is a highly purified and inert material. There is a preventative coating on the outside of the column, called polyamide that affords strength and flexibility in order to wind into small coil.Diagram 3 Cross-section of capillary column capillary columns have a very small internal diameter, on the order of a few tenths of millimeters, are between 25-60 meters in length. Capillary col umns can be divided into three classes which are wall-coated open tubular (WCOT) columns, support-coated open tubular (SCOT) columns and porous tier open tubular (PLOT) columns. For WCOT columns, the inside column walls are coated with a thin bed of liquid stationary phase. The thickness of liquid coating is 0.25 0.5 m thick leading to very fast and cost-effective separations (up to 300,000 plates). Other types of capillary columns exist with the stationary phase contained in different formats. These columns are typically expeditious but they have a small sample capacity due to their low surface area. For SCOT columns, the inner wall of capillary columns are lined with approximately 30m of a porous support material in order to allow a higher loading of stationary phase, resulting higher column capacity. Then, a thin film of liquid stationary phase is then coated on this layer of support material, providing SCOT columns a larger surface area. For PLOT columns, they are similar t o SCOT columns except solid support materials are attached to the inner column wall where the particles themselves are the stationary phase. There support materials can be glass powder or microcrystalline materials rather than particulate support.Diagram 4 cross section of WCOT, SCOT and PLOT columnsGenerally, capillary columns are favored over packed columns and WCOT columns are more economical than SCOT columns in Gas Chromatography. The table below shows further comparison of capillary (WCOT) and packed columns.ParameterCapillary ColumnPacked columnEfficiency (plates/m)100000Sample size (ng)10-7510-1000000Realtive pressureLowHigh sexual congress speedFastSlowChemical inertnessBestPoorestColumn flexibilityYesNoResolutionGoodPoor