Packed column supercritical fluid chromatography with light-scattering detection. I. Optimization of parameters with a carbon dioxide/methanol mobile phase

Summary Evaporative light scattering detectors have, in recent years, gained acceptance in chromatography with dense mobile phases i.e. liquid and supercritical fluid chromatography. In the present work an instrument of this type has been used in packed column supercritical fluid chromatography with carbon dioxide/methanol mixtures. Detector response and signal-to-noise ratios have been determined using squalane as test compound. Nebulizer gas flow, evaporator temperature, photomultiplier sensitivity, and mobile phase composition were found to have an influence on instrument performance. With this type of detector the field of packed column SFC applications can be extended to include non-UV-absorbing substances even when mixed mobile phases or composition gradients are necessary for the separation.     

Packed column supercritical fluid chromatography with light-scattering detection. II. Retention behaviour of squalane and glucose with mixed mobile phases

Summary Evaporative light scattering detectors can be used to detect organic substances without chromophoric groups in packed column supercritical fluid chromatography (SFC). A detector of this type has been used to detect squalane and glucose after SFC with various packed columns and binary mobile phases. In this study, the amount of organic modifier in carbon dioxide/modifier mixtures was varied. The results give further insight into the mechanisms that influence retention behaviour in packed column separations with super- and subcritical mobile phases. Squalane is an ideal non-polar test solute which shows long retention times on non-polar columns while its elution can be accelerated by non-polar modifiers in carbon dioxide. Glucose is an extremely polar solute containing hydroxyl groups. Elution of this sugar can be improved with polar modifiers. Column packings with polar end groups lead to high capacity ratios and long retention times for glucose. Most columns used in this study contained silica-based packing materials. For purposes of comparison, a polymeric packing (HEMA RP-18) was also employed.     

The analysis of lubricating oil additives by supercritical fluid chromatography with packed and open tubular capillary columns

Lubricating oil additives have been analyzed by supercritical fluid chromatography on open tubular and packed capillary columns. Carbon dioxide and modified carbon dioxide were used as mobile phases and detection was accomplished by flame ionization and micro UV. Rapid and efficient analysis of the lubricating oil additives was demonstrated.     

Modifier effects on packed and capillary columns in supercritical fluid chromatography

Summary The separation of polar thermally labile solutes is one of the potentially most rewarding fields of SFC application. A presupposition for such applications is, however, mobile phases having relatively high solvent strengths. A promising approach to achieve this is the use of mobile phases consisting of carbon dioxide with a polar additive. In this work, the chromatographic effects of different concentrations of an additive, isopropanol, in carbon dioxide have been studied on capillary and packed columns. A series of antibiotics was used as test substances. Best results were obtained with carbon dioxide/8% isopropanol as mobile phase on a capillary column coated with a cyanopropyl-substituted polysiloxane stationary phase.     

Current technological challenges in capillary supercritical fluid chromatography

In cases where high efficiency is required to resolve complex mixtures of either thermally labile or nonvolatile organic compounds, capillary supercritical fluid chromatography may be the most desirable analytical method. While great strides in this new technology have been made over the last few years, several problem areas are requiring increased attention. These include sample introduction systems, pressure reduction at the end of the column, column stability in various supercritical mobile phases, and migration of polar solute molecules. This paper describes the state-of-the-art in capillary SFC with emphasis on the progress made and future needs in the solutions to these specific problems.     

Retention in capillary supercritical fluid chromatography

Supercritical fluid chromatography (SFC) sometimes exhibits GC-like behavior and sometimes LC-like behavior, depending on conditions. However, it is not always clear whether one of these types of behavior, or a combination, operates for a particular set of conditions for every solute in a mixture. For example, some components may be partitioned mostly by their vapor pressures, while others, in the same mixture, are partitioned predominantly by solvent-like properties of the mobile phase. Plots of retetion (as log of the capacity factor) vs. reciprocal temperature at constant pressure reveal a clear change in the character of the separation of well-behaved solutes. A thermodynamic explanation of the observed behavior is given, based on the assumption that partitioning is controlled by the heats of solution of solute in the mobile and stationary phases. A model of SFC retention as it deviates from pure-GC behavior on the same column is presented.     

Evaluation of a thermionic detector for capillary supercritical fluid chromatography of nitrated polycyclic aromatic compounds

A thermionic detector was evaluated for capillary supercritical fluid chromatography of polar nitro-containing polycyclic aromatic compounds (nitro-PAC). Three modes of detector operation were studied. The best performance was obtained using a nitro-selective mode of detection. Although linearity was confined to a narrow range for a given source current, the sensitivity was excellent; 20 pg injected for p-nitrophenol gave a signal-to-noise ratio of 3. Conventional thermionic detection also produced good sensitivity; however, serious baseline drift was observed at high operating temperatures using density programming. A third mode, flame thermionic detection, was not acceptable because of low sensitivity. A number of nitro-PAC were successfully chromatographed using density programming at 101°C. Hydroxynitropyrenes, nitropyrene quinones, and 9-hydroxy-2-nitrofluorene, which could not be eluted in capillary gas chromatography, were successfully chromatographed here. Retention of these compounds increased sequentially on 50% n-octyl-, 5% phenyl-, and 25% biphenyl polysiloxane stationary phases, respectively. Stationary phase interactions appeared to be more a function of the polar functional groups on the aromatic rings than of the hydrocarbon character of the compounds. Solute solubility in the mobile phase and volatility were additional factors contributing to the elution of these molecules. Finally, these results were used to identify a number of nitro-PAC in a polar subfraction of a diesel particulate extract.     

Splitless injection in capillary supercritical fluid chromatography

A splitless injection technique, allowing 0.5 μl injections on 50 μm i.d. columns, has been developed.     

Abnormal composition of commerical waxes revealed by supercritical fluid chromatography

Summary The analysis of natural waxes is a complicated process because they occur as complex mixtures. A study using supercritical fluid chromatography with packed columns made it possible to observe the behavior of each family of hydrocarbons, fatty esters, alcohols, acids and triglycerides. The latter were then separated according to their function type and alkyl chain lengths using octadecyl silica as stationary phase. With a polar modifier added to the supercritical fluid, it was possible to analyse certain waxes. Composition anomalies were observed which related to an overabundance of certain compounds which, in excess, are harmful to the quality of waxes intended for use in cosmetics.     

Ion mobility detection after supercritical fluid chromatography

Using a unidirectional flow ion mobility detector, non-selective detection, tunable selective detection, and complete Fourier transformed ion mobility spectra were successfully obtained after supercritical fluid chromatography on compounds with higher molecular weights than have been previously investigated. In the most selective mode, single oligomers from polymeric material could be independently detected. Using the Fourier transform capabilities of this instrument, complete ion mobility spectra for each oligomer could be obtained in a single chromatographic separation. The collection of individual ion mobility spectra of the components of polymeric material has not been possible prior to the technique described in this paper. Only complex ion mobility spectra of polymeric mixtures are available in the literature. The spectra obtained in this study are all simple, uncomplicated spectra consisting of only one or two product ion peaks. K_o values reported in this work range from 0.633 to 1.61, which are some of the lowest values ever reported in ion mobility spectrometry. With the unidirectional flow design of the detector, the supercritical fluid mobile phase, carbon dioxide, was efficiently eliminated from the detector so that the ion mobility spectrometer could be operated in its normal manner. The fact that CO₂ did not interfere with normal ion mobility operation indicates that other supercritical fluids may also be compatible with this sensitive and versatile detection method.     

Peroxyoxalate chemiluminescence detection with packed column supercritical fluid chromatography

Summary A detection scheme based upon peroxyoxalate chemiluminescence, which utilizes two post-column pumps and two stages of depressurization is investigated. The chemiluminescent detection limit for perylene is 23 times lower than determined by fluorescence, and is in the attomole range. This detection technique is investigated for packed column supercritical fluid chromatography (SFC). Due to the interface design used and the chemical band narrowing effects of chemiluminescence, an apparent increase in efficiency is observed. The interface design affords a wide range of pressures to be used for a separation. During pressure programming the column effluent changes flow rate. Because of a back-pressure regulator, the reaction and detection take place at nearly constant pressure. Therefore pressure gradient work is possible without concern for post-column reagent solubility (which is a concern for high-performance liquid chromatography). The effects of the expanded CO₂ from the SFC on the chemiluminescence signal and background are studied. The post-column detection is optimized for pH, photomultiplier voltage, concentrations and flow rates of the peroxide and oxalate ester.     

On-line multidimensional open tubular column supercritical fluid chromatography using a valve-switching interface

Summary An on-line two-dimensional open tubular column supercritical fluid chromatograph was constructed and evaluated. A rotary valve interface allowed independent flow control of two 50-m i.d. open tubular columns, providing maximum versatility for heartcutting. A solvent-venting injection technique was incorporated in the system that enabled single or multiple 2.0-L volumes to be injected into an uncoated, yet deactivated, length of capillary precolumn without flooding of the analytical column. A cold trap was employed to refocussolutes from single or multiple fractional cuts after being transferred to the second dimension. The performance of the system was demonstrated with the analysis of mixtures of polycyclic aromatic compounds and steroids. Efficiencies of 4,500 plates m^–1 were preserved in the second column after heartcutting.     

Fast supercritical fluid chromatography using capillaries packed with nonporous particles

Summary Packed columns containing microparticles provide high column efficiency per unit time and strong retention characteristics compared with open tubular columns, and they are favored for fast separations. Nonporous particles eliminate the contribution of solute mass transfer resistance in the intraparticle void volume characteristic of porous particles, and they should be more suitable for fast separations. In this paper, the evaluation of nonporous silica particles of sizes ranging from 5 to 25 μm in packed capillary columns for fast supercritical fluid chromatography (SFC) using neat CO₂ is reported. These particles were first deactivated using polymethyl-hydrosiloxanes and then encapsulated with a methylphenylpolysiloxane stationary phase. The retention factors, column efficiencies, column efficiencies per unit time, separation resolution, and separation resolution per unit time for fast SFC were determined for various length capillaries packed with various sizes of polymerencapsulated nonporous particles. It was found that 15 μm nonporous particles provided the highest column efficiency per unit time and resolution per unit time for fast packed capillary SFC. Under certain conditions, separations were completed in less than 1 min. Several thermally labile silylation reagent samples were separated in times less than 5 min. Presented at the 21^st ISC held in Stuttgart, Germany, 15^th–20^th September, 1996     

Direct on-line coupling of small subcritical and supercritical fluid extractors with packed column supercritical fluid chromatography

Summary A mini extractor of 85 L void volume and a micro extractor of 3–4 L void volume have been coupled directly with a packed column SFC and used under sub- and supercritical conditions. The mini extractor is suitable for holding adsorbates which can be on-line extracted and the extract chromatographed (direct SFE-SFC). The micro extractor can be used for direct sample introduction of liquid and solid materials under SF conditions. Thus any solvent interference with the sample and the chromatographic conditions is excluded. Standard samples of wood tar residue, engine oil, and metal organic compounds have been tested.     

Polyacrylate liquid crystalline stationary phases in supercritical fluid chromatography with carbon dioxide mobile phase

Summary New polyacrylate liquid crystalline compounds were coated onto glass or fused-silica capillary columns as stationary phases and applied to supercritical fluid chromatography. These stationary phases, were very stable: no bleeding was observed at 200°C and up to 200kg/cm² pressures of carbon dioxide mobile phase. The wide working range of the capillary column was extended below the g-n transition temperature. Isomeric compounds such as - and -methoxynaphthalene, anthracene and phenanthrene and several phenolic compounds were separated.     

Supercritical fluid chromatography of petroleum products using flameless sulfur chemiluminescence detection

Supercritical fluid chromatography using flameless sulfur chemiluminescence detection has been investigated for the analysis of sulfur compounds in petroleum products. The chromatography and detection system was easy to implement and exhibited good precision, linearity, selectivity, and sensitivity. A minimum detectable limit of 0.3 pg sulfur/s was obtained, and response to sulfur in different sulfur species was nearly equimolar.     

Feasibility of screening large aqueous samples for thermally unstable pesticides using high efficiency packed column supercritical fluid chromatography with multiple detectors

Summary Nearly 100 pesticides were eluted with good peak shapes from silica columns using methanol/carbon dioxide mobile phases. The pesticides included organophosphorus, organochlorine, phenylurea, sulfonylurea, triazine, carbamate and phenoxyacid pesticides, demonstrating the wide applicability of SFC to pesticide analysis. A subset of 31 pesticides were on-line extracted from large water samples, typically 10 to 20 ml, but up to 100 ml. The extracts were separated on a 1.6 meter long column packed with 5 μm particles, then detected simultaneously with a photodiode array UV detector plus an electron capture (ECD) and a nitrogen-phosphorus (NPD) detector. Recovery, reproducibility, linearity, response factors and detection limits were determined. Detection limits from a 5 ml water sample were in the range of 100 to 500 parts per trillion (1/10¹²). Larger samples appear capable of lowering these limits to below 10 ppt.