An improved interface for universal acoustic flame detection in modified supercritical fluid chromatography

A novel method of interfacing the acoustic flame detector (AFD) with modified supercritical fluid chromatography (SFC) is presented. By applying resistive heating directly to the burner region between the restrictor outlet and the acoustic flame, infrequent severe noise, baseline drifting, and peak deformations that can occasionally be observed with the AFD are eliminated. For example, by increasing the interface temperature only a few hundred degrees Celsius, such sporadic noise in the detector can be reduced nearly ten-fold resulting in smooth stable operation of the AFD. Further, for various levels of methanol modified supercritical carbon dioxide mobile phase examined, the interface was observed to reduce detector noise in each to a common minimal range near 10-25 Hz when an appropriate temperature was achieved. The method is simply assembled, inexpensive to construct, and robust in its daily operation. Overall, the heated interface developed and presented facilitates reliable AFD operation in modified SFC, and supports further exploration and implementation of this sensor as an alternative universal detector in separations requiring an organic cosolvent in the mobile phase.     

Advantages and drawbacks of popular supercritical fluid chromatography/mass interfacing approaches--a user's perspective

Mobile phases in supercritical fluid chromatography (SFC) have low viscosities and high diffusion coefficients with respect to those of traditional high performance liquid chromatography (HPLC). These properties allow higher mobile phase flow rates and/or longer columns in SFC, resulting in rapid analyses and high efficiency separations. In addition, chiral SFC is becoming especially popular. Mass spectrometry (MS) is arguably the most popular "informative" detector for chromatographic separations. Most SFC/MS is performed with atmospheric pressure ionization (API) sources. Unlike LC/MS, the interface between the SFC column and the API source must allow control of the downstream (post-column) pressure while also providing good chromatographic fidelity. Here we compare and contrast the popular interfacing approaches. Some are simple, such as direct effluent introduction with no active back-pressure-regulator (BPR) in high speed bioanalytical applications. The pressure-regulating-fluid interface is more versatile and provides excellent chromatographic fidelity, but is less user friendly. The pre-BPR- split interface and an interface which provides total-flow-introduction with a mechanical BPR are good compromises between user friendliness and performance, and have become the most popular among practitioners. Applications of SFC/MS using these various interfaces are also discussed.     

Influences of temperature,pressure and eluent composition on the behavior of binary eluents at sub- and supercritical conditions. Pentane and diethyleneglycol dimethyl ether

Summary In supercritical fluid chromatography (SFC), studies on binary eluents composed of pentane and diethyleneglycol dimethylether were performed in the concentration rage of 5 to 15% of the ether compound and at pressures of 20 to 50 bar. Influences of temperature, pressure and eluent composition on the behavior of the mobile phase were found to closely resemble those for pentane-dioxane mixtures. Resolution depended on variations in capacity ratios, selectivities, and plate numbers. The critical data for diethyleneglycol dimethylether were estimated by means of different group contribution methods.     

Aufbau und Überprüfung eines neuen Photoionisationsdetektors für die Fluidchromatographie

Zusammenfassung Diese Arbeit beschreibt die Entwicklung eines druckfesten Photoionisationsdetektors (PID) und überprüft seine Anwendung in der Fluidchromatographie (SFC). L?sungen von Benzol in Methanol wurden als Testl?sungen gebraucht. Drei Modelle wurden nacheinander konstruiert und mit Kohlendioxid sowie Argon als mobile Phase getestet. Es zeigten sich grunds?tzlich drei Typen von Problemen: Druckbest?ndigkeit, elektrische Isolation und Anordnung der UV-Lampe (10.2 eV). Obwohl bei einem druckfesten PID die mit der Entspannung des überkritischen Gases verbundenen Probleme wegfallen, bietet er mit Kohlendioxid als mobile Phase wegen seiner geringen Empfindlichkeit und seiner hohen Nachweisgrenze nicht die erhoffte Alternative zu den bisher in der SFC verwendeten Detektoren. Diese kleine Empfindlichkeit ist auf die Lichtabsorption von Kohlendioxid bei dieser Wellenl?nge (121 nm) und haupts?chlich auf einen Quenching-Effekt zurückzuführen.

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Construction and testing of a new photoionization detector for supercritical fluid chromatography

Summary This work describes the development of a pressure resistant photoionization detector (PID) and examines its use in supercritical fluid chromatography (SFC). Solutions of benzene in methanol were used as test solutions. Three models were constructed and tested with carbon dioxide and argon as mobile phase. Three types of problems were principally encountered: pressure resistance, electrical insulation and positioning of the UV-lamp (10.2 eV). Although all problems arising from pressure reduction could be avoided with each PID studied, these “high pressure” PID cannot be used for SFC instead of other usual detectors. With supercritical carbon dioxide as the mobile phase, the sensitivity was too low and the detection limit was too high. this could be explained by light absorption at the emitted wavelength (121 nm) and mainly by a quenching effect by this gas.

     

Construction of a new interface for comprehensive supercritical fluid chromatography x reversed phase liquid chromatography (SFC x RPLC)

The design of a new interface to hyphen high efficiency supercritical fluid chromatography (SFC) and fast RPLC in a comprehensive configuration is described. SFC x RPLC is a viable alternative to normal phase (NP) LC x RPLC and is characterized by high orthogonality. Compared to NPLC x RPLC an additional advantage is the expansion of supercritical carbon dioxide (CO(2)) when exposed to atmospheric pressure leading to fractions consisting of solvents that are miscible with the second dimension RPLC mobile phase. The interface consists of a two-position/ten-port switching valve equipped with two packed octadecyl silica (C(18)) loops for effective trapping and focusing of the analytes after elution from the SFC dimension. The addition of a water make-up flow to the SFC effluent prior to entering the loops is of fundamental importance to efficiently focus the solutes on the C(18) material and to reduce interferences of expanded CO(2) gas on the second dimension separation. The features of the system are illustrated with the analysis of a lemon oil sample.     

Stochastic resonance in surface catalytic oxidation of carbon monoxide induced by colored noise

Noise is generally considered as a disadvanta-geous factor, which would smear weak signals, there-fore, people always try to reduce its influence. How-ever, recent studies show that, for a nonlinear system, under certain circumstances, noise can enhance sys-tem抯 response to a signal. At specific noise intensity, the response of a system to a weak signal may reach its maximum, which is called 搒tochastic resonance?(SR). The concept of SR was originally put forward by Benzi and his collaborat…     

On-line packed column supercritical fluid chromatography--microwave-induced plasma atomic emission

Interfacing and evaluation of packed column supercritical fluid chromatography (SFC)-microwave-induced plasma atomic emission detection (AED) is described. Via a flow splitter and an integral restrictor, efficient transfer of solutes from column to detector without band broadening is obtained. Variation of CO2 flow-rate during pressure gradients has little influence on both AED signal and baseline drift while it provides similar sensitivity as in capillary SFC. Continuous introduction of CO2 in the plasma reduces the available range of emission domains; nevertheless the region of detection which is free of CO2 interferences allows selective detection of C1 and Br as reported in this paper. reserved.     

饲料中D-泛酸钙的超临界流体色谱测定

采用超临界流体色谱(SFC)测定了饲料中D-泛酸钙的含量.在二氧化碳流动相中添加14.85%甲醇改性剂及0.15%三氟乙酸添加剂,流速2 mL/min,在Kromasil ODS柱上分离,检测波长为210 nm.结果表明D-泛酸钙能在3 min内完成分离,在1.0 ～25.0 mg/L范围内,样品的质量浓度与色谱峰面积呈良好的线性关系(r=0.999 5).方法的精密度良好,相对标准偏差在2.3%以内,回收率为89% ～98%.方法操作简单,结果准确,适合普及应用.     

Enhancement of sensitivity in capillary supercritical fluid chromatography through optimization of injection and detection techniques

The reproducibility of peak areas as a function of the technique used for sample injection was investigated in capillary supercritical fluid chromatography (SFC). An injection technique has been developed to increase the volume of sample introduced into the capillary column. Using a modified time-split injection technique, long injection duration times were successfully applied to achieve lower detection limits. Analytes were effectively focused at the head of the analytical column using a unique pressure trap program. Because this on-column focusing was performed only by pressure and temperature programming, no instrumental modifications were necessary. Up to 1.0 μL of sample solution was injected onto 50 μm i.d. columns using this technique, with no observable peak splitting. Dual detection using ultraviolet (UV) absorption and flame ionization detection (FID) was performed in series, thereby avoiding the necessity of splitting the column effluent. For the compounds investigated (five nitroaromatics and one phthalate ester), the absolute sensitivity of the UV detector was significantly greater than that of the FID.     

Use of isopycnic plots in designing operations of supercritical fluid chromatography. III: reason for the low column efficiency in the critical region

This paper discusses the origins of efficiency loss in supercritical fluid chromatography (SFC) when analyses are carried out in the low pressure supercritical region of carbon-dioxide, close to its critical point. Recent publications have shown strong evidence of radial thermal heterogeneity inside an SFC column and suggested that it leads to peak-shape distortion and greatly reduces the column efficiency. We demonstrate that the physico-chemical properties of CO(2) close to the critical point are such that formation of thermal heterogeneity inside the column is highly probable and could cause the observed efficiency loss. Consideration of isopycnic plots of CO(2) permits clear identification of the problematic region and explains why these properties of CO(2) are primarily responsible for the often perplexing efficiency losses taking place during the SFC operations.     

In situ FTIR spectroscopic study of the effect of CO2 sorption on H-bonding in PEG-PVP mixtures

A study of the H-bonding between poly(ethylene glycol) (PEG) and polyvinylpyrrolidone (PVP) in the presence of supercritical carbon dioxide at various temperatures, pressures, different M(w) of PEG and PVP and different PEG/PVP ratios is presented. In situ attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy was used to investigate H-bonding by examining changes in the relative intensities and positions of peak maxima of the 2nd derivative ν(C=O) bands associated with 'free' and H-bonded C=O groups. In general, relative intensities of bands associated with H-bonded CO groups decreased upon CO(2) sorption and was accompanied by an increase in intensity of bands associated with 'free' C=O groups. At the same time, these bands were shifted to higher wavenumbers. These shifts were attributed to the shielding effect of CO(2) molecules on H-bonding interactions between PEG and PVP. The magnitude of the effects of CO(2) shielding generally increased with decreasing polymer M(w) and increasing CO(2) content. However, upon CO(2) venting the extent of the H-bonding between PEG and PVP reappeared. The extent of H-bonding recovery was greatest for blends with low M(w) PEG (M(w): 4×10(2)) and PVP (M(w): 9×10(3)) and PEG content ≥0.54 wt% under mild conditions of pressure (80 bar) and temperature (35°C). For the same low M(w) blends, increasing pressure to 150 bar, or temperature to 50°C, showed poor H-bond recovery upon CO(2) venting. Overall, it was shown that supercritical CO(2)-induced shielding of H-bonding interactions in polymer blends is reversible upon CO(2) venting, and the magnitude of both was influenced by processing conditions and blend composition.     

Retention behaviour of carboxylic acid methyl esters in supercritical fluid chromatography

The study on retention behavior in supercritical fluid chromatography (SFC) is necessary to understand the mechanism of the various interactions in SFC. The retention of SFC in carboxylic acid methyl ester/polymethylsiloxane/CO2 system was studied systematically and the retention behavior of this kind of compounds under various typical operation conditions was described using the method of an alternative unified theory of chromatographic retention. The results illustrated that expression: Ink.= a + b/T + cp + dp/T + ep2/T can be used to describe quantitatively the retention behavior of carboxylic acid methyl ester/polymethylsiloxane/CO2 system in the ranges of reduced density from 0.549 to 1.411. It was also found that the entropy of solute in stationary phase is dependent on the density of supercritical fluid (SF) under typical operating conditions of SFC.     

Packed microcolumn SFC coupled with photodiode array UV detector and inductively coupled plasma detector (SFC-photodiode array UV-ICP)

Hyphenation of packed microcolumn SFC, a photodiode array UV detector, and an inductively coupled plasma spectrometer has been developed in order to evaluate the stability of acetylacetone complexes under supercritical conditions.     

Separation of divalent transition metal beta-diketonates and their adducts by supercritical fluid chromatography

A method for separation and detection of divalent transition metal beta-diketonates by adduct formation/supercritical fluid chromatography (SFC) with an open-tubular capillary column and a FID detector is described. The crystal structures of copper (Cu)-hexafluoro-acetylacetone (HFA) and Cu bis(2,2,6,6-tetramethyl-3,5-heptanedionato) (THD) complexes have been determined by X-ray crystallography. The SFC behavior of Cu beta-diketonates shows a strong correlation with the structure of the complexes. The hydrated cu beta-diketonate complexes usually exhibit strong intermolecular interactions or decomposition in SFC. Formation of adducts with a neutral donor, such as tributyl phosphine oxide (TBPO), can greatly improve the SFC behavior and detection sensitivity of Cu(II) and Mn(II) beta-diketonates. The stoichiometry and thermal stability of the adducts Cu(II) and Mn(II) beta-diketonates with TBPO in supercritical CO(2) have also been investigated. The implications of utilizing adduct formation for supercritical fluid extraction (SFE) of divalent transition metals and for on-line coupled SFE/SFC analysis of divalent transition metals are discussed.     

Demonstration of High Speeds with Low Pressure Drops Using 1.8 μm Particles in SFC

The literature contains few, if any, references to the use of sub-2 μm particles in supercritical fluid chromatography (SFC). In this study, the use of 1.8 μm particles in SFC is demonstrated, producing high efficiency chromatograms, less than 1 min long for each of a diverse range of solute families, including steroids, sulfonamides, profens, nucleic acids and xanthenes. Most of the solutes eluted from bare silica with surprisingly good peak shapes. The 3 × 100 mm column packed with 1.8 μm spherical silica particles, produced as many as 22,400 plates (80% of theoretical). Column head pressure did not exceed 410 bar and pressure drops did not exceed 240 bar, even at flows of 3 mL min^?1, or modifier concentration as high as 65%. Such performance produced the fastest solvation based chromatography (LC–SFC) reported to date. The chromatographic hardware requirements are met by older (400 bar max.) LC equipment. The speeds observed exceed even ultra high performance liquid chromatography (UHPLC), which requires dramatically higher pressure capability. Sub 2-μm particles appear suitable for routine use, employing common 400 bar LC equipment, only slightly modified to perform SFC, although faster detectors and smaller flow cells enhance performance.     

Nucleophilic displacements in supercritical carbon dioxide under phase-transfer catalysis conditions. 2. Effect of pressure and kinetics

The nucleophilic displacement on n-octylmesylate (n-C(8)H(17)OSO(2)CH(3), 1) with four different anions (I(-), Br(-), N(3)(-), and SCN(-)) is investigated under liquid-supercritical phase-transfer catalysis (LSc-PTC) conditions, i.e. in a biphase system of supercritical carbon dioxide (scCO(2)) and water, in the presence of both silica supported and conventional onium salts. The CO(2) pressure greatly affects the concentration of 1 in the sc-phase and plays a major role on its conversion. For example, at 50 degrees C and with a supported PT-catalyst, the conversion of 1 into n-octyl iodide drops by a factor of 5 as the CO(2) pressure is increased from 80 to 150 bar, while in the same pressure range, the solubility of n-octylmesylate in scCO(2) shows a 6-fold increase, indicating that the reagent is desorbed from the catalyst. Under LSc-PTC conditions, pseudo-first-order kinetic rate constants, evaluated for the investigated reactions, show that the performance of scCO(2) as a PTC solvent and the relative nucleophilicity order of the anions (N(3)(-) > I(-) > or = Br(-) > SCN(-)) are comparable to those of toluene and n-heptane. The behavior of conventional phosphonium salts in the scCO(2)/H(2)O biphase system suggests that the reaction may take place either within small droplets of PT-catalyst containing water or in a separate third liquid phase of the PT-catalyst itself.     

Addition of modifier in supercritical fluid chromatography using a microbore reciprocating pump

Summary Solubilities of polar analytes in supercritical CO₂ can be enhanced by the addition of polar modifiers. The addition of modifier into an SFC system using a low cost reciprocating pump has been studied. Two different mixing chambers were evaluated for mixing the supercritical CO₂ with modifier. It appeared that a mixing chamber with a packed bed was enough to reduce baseline noise from the modifier pump. Results from the effect of pressure and temperature with various modifier flow rates were obtained. High percentages of modifier (>15%) at a low CO₂ pressure (2000 psi) caused baseline instability. In addition, different I.D. columns were tested with the system and the effect of modifier compressiblity on detector noise was also studied. Several pharmaceutical compounds were separated to demonstrate system performance.     

Development of capillary supercritical fluid chromatography/Fourier transform infrared spectrometry

Summary The feasibility of a new technique, supercritical fluid chromatography/Fourier transform infrared spectrometry (SFC/FTIR), for separation and identification of nonvolatile and labile compounds in complex mixtures is discussed in this report. The design of a high pressure (flow cell) interface is described. The results of a continuing study of possible mobile phases for SFC/FTIR are presented. Finally, SFC/FTIR was applied to a standard test mixture. The current detection limits of one component of the mixture, 2,6-di-t-butylphenol, are presented.     

An attempt to extend ordinary capillary gas chromatography to supercritical fluid chromatography (SFC)

Since capillary columns with well immobilized stationary phases are expected to withstand contact with supercritical fluids, we wished to study their amenability to SFC. Simultaneously, we wished to learn how far SFC can be accomplished with the ordianary tools of capillary GC. The study demonstrates that truly supercritical, not just relatively high, pressure is required to ensure the typical effects of SFC. Results obtained with sub-and supercritical pressure are compared and discussed. A comprehenshive study of the parameters permitting SFC with capillary GC equipment showed a clear preference for CO₂ as a carrier, FID detection, and oncolumn sampling. While no additional equipment is required, a critical feature is the flow restrictor to be mounted on the end of the column. The production and properties of this restrictor are discussed in detail. It is reasonable to hope that SFC with 0.1 mm id capillary columns can be realized in the pressure range of 100–150 bar, where substances which cannot be eluted from a capillary colum under GC donditions are expected to be analyzed.     

一种测量超临界条件下苯酚吸附等温线方法

超临界吸附相平衡是超临界吸附／色谱分离过程设计的基础,通常,研究超临界吸附相平衡的实验不仅需要在高压下操作,而且需使用耐高压的检测器,本文提出一种测量超临界条件下吸附相平衡关系的“扩容减压吸收法”方法,它不需要耐高压检测器,以“苯酚－活性炭－超临界二氧化碳流体”为体系,测定了苯酚在活性炭－超临界二氧化碳流体之间的吸附相平衡关系,测定了苯酚在两种活性炭上的超临界吸附等温线,比较了苯酚在超临界条件和常