Current Analytical Chemistry - Volume 10, Issue 1, 2014

This review provides an updated discussion on the recent developments and applications of supercritical fluid technology in analytical chemistry. Supercritical mediums are usually used for extraction and separation of various analytes in analytical science. Although, this technology is used for chromatographic aims in the analysis of various samples the main application domain of supercritical fluids is in extraction process. SFE is suitable for recovery of various analytes, especially non-polar organic compounds, from different matrices. However, via some modifications or by the aid of modifiers or a chelating agent, this technique could be successfully applied in the extraction of polar or even ionic analytes. The range of analytes extracted via SFE is wide and mostly includes food and pharmaceutical samples, pollutants extraction and metal cation extraction. The aim of this paper is to review, from analytical chemistry standpoint, recent advances in the use of SC-CO2 for the extraction and separation of analytes. At the same time, a critical assessment will be made of the advantages of SCF technology in comparison with traditional methods. Special emphasis will be given to online coupling of SFE method with chromatographic techniques and FT-IR spectroscopy that make possible the extraction and separation of various analytes from different matrices in order to produce novel analytical techniques. The potentials, advantages, shortcomings, and prospects in employment of supercritical fluids for separation and extraction of various analytes are also considered. The efficiencies of SFE for the extraction of various analytes from solid and aqueous samples are compared to traditional methods such as Soxhlet extraction. Supercritical fluid extraction due to various reasons such as its greenness, rapidity and high efficiency are widely used an alternative to conventional solvent-based extraction methods. Also, SFE can be as an automated and miniaturized extraction technique, since it reduces the amounts of required samples considerably. SFE avoids multi-steps of conventional methods (such as partitioning, clean-up, evaporation) and hence it can reduce the uncertainty in the results. Furthermore, SFE reduces analysis times and is less laborintensive.

Due to the potential for health benefits, the importance of carotenoids is growing. Since this group of compounds is used in medical as well as food and cosmetic applications, a review of the current sources of carotenoids is undertaken. A literature survey is presented related to the methods currently used for the extraction of carotenoids from plants: low-pressure solvent extraction by different techniques and supercritical fluid extraction. The process parameters and their influence on carotenoids yield are considered. The cost of manufacturing carotenoids-rich extracts by supercritical fluid extraction is also discussed.

Extraction with supercritical fluids arises in the last three decades as a promising alternative to conventional analytical methods for lipid extraction, and currently has consolidated its application in extraction processes of raw materials of animal and plant origin. Its advantages over conventional techniques are many and well known, but this technology stands out mainly by environmental factors and quality of products. This article reviews the aspects related to the use of supercritical CO2 for extraction and fractionation of lipids, with special emphasis in triacylglycerols and fatty acids, and summarizes the published literature with the use of SFE to obtain fats and oils with particular interest in the application of the extracts in food products. In this context, marine animals and oilseeds are the most studied raw materials for lipid extraction. A brief review on the separation procedures involving supercritical technology, as supercritical fluid chromatography, is also presented, dealing with separation methods as analytical techniques, or even as fractionation and purification tools. As shown in this paper, natural sources of lipids are abundant and diverse, supercritical CO2 has large potential to be a viable method to extract and purify those substances.

A process based on the use of a semi-continuous countercurrent supercritical fluid extraction (CC-SFE) has been optimized to concentrate minor components, with functional properties, from olive oil. The optimization of the main variables involved in the supercritical extraction process (extraction pressure, temperature and sample flow rate) was performed using a surface response methodology (RSM). A central composite circumscribed design (CCCD) was employed to study the responses selected to describe the process, such as the extraction yield, the concentration of the different minor compounds (squalene, sterols and tocopherols) in the CO2-rich upper current stream, and the recovery of these compounds from the olive oil. Parameters of the model, for each response variable, were simultaneously estimated by Partial Least Squares (PLS) regression. The statistical analysis of the results allowed obtaining mathematical models able to predict the behavior of the different responses selected as a function of the main variables involved in the process. The optimum conditions obtained, that maximized all the responses as a whole, were: extraction pressure, 234 bar; extraction temperature, 35°C and sample flow rate, 82 mL/h, working with a solvent flow rate equal to 2000 mL/h (S/F equal to 28.4 kg CO2/kg oil). The optimum of the model was experimentally confirmed allowing around 90% recovery of squalene without significant changes on the nutritional and physicochemical properties of the oil.

Hyphenated SPE/SFE method was developed for the extraction of phenolic compounds from plant matrices (6 algae species). 20 ml of liquid solvent was injected to the SPE cartridge and extracted at following conditions: 40 MPa, 80°C, restrictor 10.5 cm x 50 μm i.d. for 60 min into 20 ml of MeOH as a trapping solvent. Two different methods were developed for different SPE sorbents (OASIS MCX 3cc, Waters and ADVANTA Spe-ed, Applied Separations) to prove that the principles work at different conditions. Each sorbent needs different modifier: 400 μL of 2M HAc in situ + 50 % MeOH / 5 % ammonia in ultra-pure water on-line is best for OASIS, ADVANTA needs 50 % MeOH and 5 % ether (v/v) on-line. SPE/SFE method has higher recovery, gives cleaner extracts, enables the use of liquid samples via SPE cartridge and higher precision was achieved (no restrictor plugging). Reversed phase liquid chromatography-electrospray mass spectrometry analysis was used for analyte determination: the chromatograph was HP 1100 with single quadruple massselective HP MSD detector and chromatographic column Zorbax SB – C 18 (75 mm x 4.6 mm, 3.5 μm, Agilent Technologies). LOD varied in the range 0.8 – 3.7 ng/l, combined RSDs for both SFE and HPLC were in range 1.4 – 5.1 %. Newly developed methods were applied on real algae samples to evaluate them as a prospective source of functional food additives. 11 of 13 monitored phenolic compounds were found, some of them in concentrations high enough to use the algae as a source of antioxidants.

This paper presents a review of the literature covering separations of enantiomers achieved with supercritical fluids. Supercritical fluid chromatography (SFC) is the main technique represented but chiral extraction (chiral SFE) and simulated moving bed (SMB) are also present. First of all, a historical view of the technique is presented. Then stationary phases, mobile phases and operating conditions are discussed, with a particular focus on the effect of operating parameters on the resolution. Detection issues are presented. The interest of SFC for screening methods and for the preparative scale is demonstrated. Comparison with other chromatographic techniques, namely gaseous-phase chromatography (GC) and high-performance liquid chromatography (HPLC) is provided. While the pharmaceutical industry is the principal application area of enantioselective separations, other examples with agrochemicals or fragrances are also presented. Foreword: This article focuses on main developments in the field of chiral separations with supercritical fluid chromatography and related techniques, as well as representative applications. It is not intended to be a comprehensive review of all papers published on the topic of chiral separations with supercritical fluids. However, it is believed that the publications mentioned here are representative of the evolution of the technique over more than 25 years of use. Several excellent reviews covering other aspects of chiral separations and/or supercritical fluid chromatography are also mentioned where appropriate.

Alzheimer’s disease (AD) is a progressive neurodegenerative disorder that must be diagnosed early to increase drug therapy efficacy. No early definitive diagnostic tool, however, is currently available, thus prompting for the identification of biomarkers of the disease. Isoprostanes (iPs) are prostaglandin-related free radical oxidation products of polyunsaturated fatty acids (PUFA) that are produced under oxidative stress conditions that occur, for instance, in the brain of AD sufferers. iPs are stable lipids known to partition in bodily fluids and are thus interesting potential biomarkers of AD. However, resolution of iPs by reverse phase HPLC is complicated by the existence of multiple regioisomers and enantiomers. Consequently, we have developed two normal phase chiral approaches for the detailed analysis of the F2-iP class of isoprostanes. First, normal phase chiral LC-MS/MS permitted the identification and quantification of four types of F2- iPs in urine samples from control, mild cognitive impairment (MCI) patients and severe AD sufferers. Second, given its suitability for non-polar compounds analysis and chiral separations, supercritical fluid chromatography (SFC) was investigated as an alternative approach. SFC-MS/MS enabled the identification of regioisomers and enantiomers of F2-iPs. Identification of analytes was undertaken by MRM monitoring of diagnostic transitions while the chiral chromatography allowed for the resolution of enantiomers within types. F2-iPs could be reproducibly identified and quantified.

One of the most exciting recent developments in infrared spectroscopy has been the coupling of the spectrometer to an infrared microscope. The combination of the new infrared spectrometer and a microscope was a natural thought of scientists in these fields. This development has been so rewarding and so useful in solving today's chemical problems that infrared microspectroscopy has quickly become a significant subclassification of infrared spectroscopy. Infrared microspectroscopy has a much longer history than the recent enthusiasm would imply, however. The great interest in the use of infrared spectroscopy to solve chemical problems that occurred shortly after World War II spread into the medical and biological fields as well as pharmaceutical and forensic sciences. The objective of this article is to review new developments in applications of FTIR microspectroscopy in analysis, covering the period between 2005 and 2013.

A simple and fast on-line chromatographic procedure has been developed for separation of ¹³⁴Cs, ¹³⁵Cs, ¹³⁷Cs and ⁸⁷Sr ions from interfering isotopes of ¹³⁴Ba, ¹³⁵Ba, ¹³⁷Ba and ⁸⁷Rb. This analytical method offers the possibility to measure the chromatograms of Rb, Cs, Sr and Ba isotopes using ICP-MS technique in isocratic conditions in one run. For the separation, the oxalic acid neutralized with ammonia could be used as the main component of the mobile phase. For specialized ion chromatography (IC) column IonPac CS5A and standard HPLC column Zorbax SCX300, an optimal concentration of oxalic acid has been fixed at 0.1 mol/L and eluent pH range has been fixed at 4.5-5.5. Under these conditions, the retention time of all four elements did not exceed 15 minutes. Our observations demonstrate that the sample injection volume can be increased up to 1 mL. The application of the method for the determination of ¹³⁴Cs, ¹³⁵Cs, ¹³⁷Cs and some isotopes of Rb, Sr and Ba in sample from spent nuclear fuel storage has been demonstrated. Activity of ¹³⁴Cs and ¹³⁷Cs calculated from ICP-MS data is comparable to the gamma spectroscopy data.

Flucloxacillin (FLU) is a semi-synthetic penicillin active against Gram-positive bacteria such as streptococci and penicilinase-producing staphylococci, including methicillin-susceptible S. aureus. A simple, rapid and reproducible stability-indicating micellar electrokinetic chromatography (MEKC) method was developed and validated for analysis of flucloxacillin sodium in capsules, using nimesulide as internal standard. The MEKC method was performed on a fusedsilica capillary (50 μm id; effective length of 40 cm). The background electrolyte consists of 50 mM borate and 80 mM anionic surfactant SDS quantities at pH 8.5. The separation was achieved at 20 kV applied voltage and 27.5 ºC. The injection was performed using the hydrodinamic mode at 50 mbar for 4s, with detection at 210 nm. The method was linear in the range of 20-100 μg mL^-1 (r = 0.9996) with adequate results for the precision (≤1.60%) and accuracy (99.45%). The specificity and stability-indicating capability were demonstrated through forced degradation studies and with placebo solution, which showed that there is no interference of the excipients. LOD and LOQ were 3.20 and 9.70 μg mL-1, respectively. The method proved to be robust by a fractional factorial design evaluation. The method was successfully applied for the drug analysis and the results were compared to liquid chromatographic method, showing non-significant difference (p>0.005). The proposed method might be applied in routine quality control in the pharmaceutical industries.

A derivative UV spectrophotometric method for determination of paliperidone in OROS^® tablets was validated and compared to a previously validated HPLC. The parameters specificity, linearity, precision, accuracy, LOD and LOQ were studied according to official guidelines. The first-order derivative spectra were obtained at N = 5,Δλ= 1.0 nm, and determinations were made at 288 nm. The drug was extracted from osmotic tablets by using 0.1 M HCl as solvent, and ultrasonic bath. The proposed method showed specificity and linearity in the concentration range of 40.0 to 80.0 μg mL^-1 The intra and interday precision data demonstrated that the method has good reproducibility. The drug content on the formulation was ranged from 101.84 to 102.76%. Accuracy was also evaluated and results were found to be satisfactory. The detection and quantitation limits were 0.85 and 2.59 μg mL^-1, respectively. When compared to HPLC, both methods proved to be adequate for determination of paliperidone in tablets.

This work proposes an alternative method for determination of sodium, potassium, calcium, and magnesium in biodiesel by flame atomic absorption spectrometry (FAAS), using dissolution in ethanol for sample preparation. The ethanolic medium enabled the use of aqueous standards, and adjustment of the viscosity of the solutions was achieved using samples of washed biodiesel, free of the analytes. The analytical curves showed linear correlation coefficients (r) higher than 0.99, and the slopes obtained using different matrices indicated that the matrix had no influence on quantification of the analytes. The instrumental limits of detection (LOD) and quantification (LOQ) for all metals were in the ranges 0.05- 0.31 mg kg^-1 and 0.17-1.02 mg kg^-1,respectively. The recovery percentages were in the range 99-114%, with relative standard deviations (RSD) lower than 15%. The accuracy of the method was also evaluated by comparison with the Brazilian official method, and no significant differences were observed (p=0.05). These results suggest that ethanol dissolution can be used as an alternative method of sample preparation for determination of Na, K, Ca, and Mg in biodiesel samples by FAAS.

The purpose of this study is to investigate the retention behavior of weakly ionizable basic compounds in ionsuppression reversed-phase liquid chromatography (IS-RPLC) using ammonia or triethylamine as ion-suppressor in a binary hydroorganic mobile phase. The apparent n-octanol-water partition coefficient (Papp) was proposed to adjust the noctanol- water partition coefficient (Po/w) of weakly basic compound. LogPapp was found to have a better linear correlation with logkw, the logarithm of hypothetical retention factor corresponding to neat aqueous fraction of the mobile phase. This straightforward relationship has been applied to effective prediction of retention behavior of weakly basic compounds, and offered a promising medium for direct determination of Po/w data for these solutes. In addition, the relationship between mobile phase pH and retention behavior of weakly basic compounds was studied in this IS-RPLC mode, and the possible reasons for the biphasic effect of pH on retention have been explained.