Ramifications and Insights on the Role of Water in Chiral Sub/Supercritical Fluid Chromatography

More than 40 cosolvents have been used with carbon dioxide to alter its solvation strength. Among the most interesting systems is the subcritical/supercritical CO₂/alkanol eluents. Using small amounts of water in CO₂/MeOH is known to be beneficial in chiral subcritical/supercritical chromatography. However, the ramifications of introducing water as a cosolvent component is not entirely understood. In this work, we demonstrate important aspects of the CO₂/MeOH/H₂O system on nine chiral stationary phases with very different surface chemistries, encompassing derivatized polysaccharides, macrocyclic glycopeptides, iso-butylmercaptoquinine, isopropyl macrocyclic oligosaccharides, and π-electron acceptor/π-electron donor phases. A hydrophilicity scale has been shown to be useful in predicting if a given chiral column chemistry would show a significant enhancement in separation efficiency in the presence of water in the CO₂/MeOH system. We demonstrate up to 8-fold enhancements in plate counts of chiral separations with a concomitant decrease in retention times, as predicted by the qualitative test. The same chiral analysis can now be completed in almost a third of the time with the addition of small amounts of water, thereby decreasing organic solvent consumption by a considerable amount. Hydrophobic stationary phases show a minimal increase in efficiency and decrease in analysis times and optimized separations show much larger reduced plate heights, compared to more hydrophilic stationary phases. Furthermore, the presence of water can alter the nature of the adsorption isotherm under nonlinear conditions. Small amounts of water can be used to tune nonlinear tailing peaks into fronting ones, significantly improving preparative enantiomeric separations.

Preliminary kinetic evaluation of an immobilized polysaccharide sub-2μm column using a low dispersion supercritical fluid chromatograph

The performance of a 3×50mm, 1.6μm d_p column with an immobilized polysaccharide stationary phase (ChiralPak IA-U) was evaluated for efficiency, and pressure drop, with respect to flow rate and modifier concentration using supercritical fluid chromatography (SFC). This appears to be the first such report using such a column in SFC. A unique low dispersion (ultra-high performance) SFC was used for the evaluation. The minimum reduced plate height of 2.78, indicates that the maximum efficiency was similar to or better than coated polysaccharide columns. Selectivity was different from ChiralPak AD, with the same chiral selector, as reported by many others. At high flows and high methanol concentrations, pump pressures sometimes approached 600bar. With 5% methanol, pressure vs. flow rate was non-linear suggesting turbulent flow in the connector tubing. The optimum flow rate (F_opt) at 40% methanol was ≈0.8mL/min, where the column efficiency was highest. At 5% methanol, F_opt increased to ≈1.6mL/min, but efficiency degraded noticeably. The differences in F_opt suggests that the solute diffusion coefficients are a strong function of modifier concentration. Several sub-1min separations, including a 7.5s separation, are presented.

Kinetic performance of a 50mm long 1.8μm chiral column in supercritical fluid chromatography

Reduced plate heights (hr) of <2 were observed for the first time during the chiral separation of enantiomers, on sub-2μm particles with supercritical fluid chromatography (SFC). The enantiomers of trans-stilbene oxide, were separated on a 4.6×50mm, 1.8μm R,R-Whelk-O1 column, with hr as low as 1.93. The plumbing of a commercial SFC instrument was modified to create a low dispersion version. Without the modification performance was considerably worse. vanDeemter like plots of reduced plate height vs. flow rate, for trans-stilbene oxide, indicate that the optimum flow varied with% modifier. On a 4.6×250mm, 5μm R,R- Whelk-O1 column, the optimum flow was >4mL/min for 5% methanol in CO2, decreasing to <2mL/min for 40% methanol (more than a factor of 2). For a 4.6×50mm column packed with 1.8μm particles the optimum appeared to be near, or >5mL/min with 2.5%, 5%, and 10% methanol, decreasing to between 3 and 3.5mL/min at 40% methanol. This is the first time such shifts have been characterized. Since the solutes were the same in all cases, the differences are likely due to changes in solute diffusion coefficients caused by changes in modifier concentration, and pressure. Pump pressure requirements sometimes exceeded 500bar. It is shown that a 5mL/min flow rate is inadequate for use with 1.8μm particles in a 4.6mm ID column format. Instead, it is suggested to decrease the ID of the column to 3mm, where the optimum flow rates are on the order of 2mL/min with decreased tubing variance. Nevertheless, a number of sub-1min chromatograms are presented.

Evaluation of numerical sediment quality targets for the St. Louis River Area of Concern

Numerical sediment quality targets (SQTs) for the protection of sediment-dwelling organisms have been established for the St. Louis River Area of Concern (AOC), 1 of 42 current AOCs in the Great Lakes basin. The two types of SQTs were established primarily from consensus-based sediment quality guidelines. Level I SQTs are intended to identify contaminant concentrations below which harmful effects on sediment-dwelling organisms are unlikely to be observed. Level II SQTs are intended to identify contaminant concentrations above which harmful effects on sediment-dwelling organisms are likely to be observed. The predictive ability of the numerical SQTs was evaluated using the matching sediment chemistry and toxicity data set for the St. Louis River AOC. This evaluation involved determination of the incidence of toxicity to amphipods ( Hyalella azteca) and midges (Chironomus tentans) within five ranges of Level II SQT quotients (i.e., mean probable effect concentration quotients [PEC-Qs]). The incidence of toxicity was determined based on the results of 10-day toxicity tests with amphipods (endpoints: survival and growth) and 10-day toxicity tests with midges (endpoints: survival and growth). For both toxicity tests, the incidence of toxicity increased as the mean PEC-Q ranges increased. The incidence of toxicity observed in these tests was also compared to that for other geographic areas in the Great Lakes region and in North America for 10- to 14-day amphipod (H. azteca) and 10- to 14-day midge (C. tentans or C. riparius) toxicity tests. In general, the predictive ability of the mean PEC-Qs was similar across geographic areas. The results of these predictive ability evaluations indicate that collectively the mean PEC-Qs provide a reliable basis for classifying sediments as toxic or not toxic in the St. Louis River AOC, in the larger geographic areas of the Great Lakes, and elsewhere in North America.

High-speed screening of combinatorial libraries by gradient packed-column supercritical fluid chromatography

Packed-column supercritical fluid chromatography (pSFC) with a fast composition gradient is used as a rapid screening tool for combinatorial chemistry. The advantages of fast analysis speed and fast recovery to initial conditions are demonstrated. Retention time reproducibility is similar to isocratic analyses by pSFC and ranges from 0.37 to 0.64% relative standard deviation. A mixture of beta-blockers illustrates the 'normal phase' retention mechanism. For these solutes and a range of analytes, the peak width is relatively constant. Such behavior permits the classical peak capacity equation to be reduced to a simple, straightforward form. Using this as a performance metric, pSFC is shown to be 5-10 times faster than reversed phase HPLC for library screening.

The development of a semi-preparatory scale supercritical-fluid chromatograph for high-throughput purification of 'combi-chem' libraries

A unique separator was developed which allowed automatic separation and peak collection using semi-preparative supercritical fluid chromatography (SFC). A peak detector switched the effluent between waste and special collection cassettes. Up to 50 mg of various solutes were injected onto a 21-mm I.D. Cyano column. The entire flow path was contained and no aerosols were generated. Collection efficiency was as high as 95%. Peak purity was often greater than 99. 9%. Typical run times were less than 10 min. An analytical SFC was used to screen the performance of a wide range of mobile and stationary phases for the elution of more than 60 miscellaneous small drug compounds. The best 'universal' gradient employed 0.4% isobutyl or isopropylamine dissolved in methanol, then mixed from 5 to 55% into carbon dioxide at 10%/min. Flow rate was 50 ml/min. The analytical SFC was shown to be a good predictor of the semi-prep instrumental performance.

Development and evaluation of consensus-based sediment quality guidelines for freshwater ecosystems

Numerical sediment quality guidelines (SQGs) for freshwater ecosystems have previously been developed using a variety of approaches. Each approach has certain advantages and limitations which influence their application in the sediment quality assessment process. In an effort to focus on the agreement among these various published SQGs, consensus-based SQGs were developed for 28 chemicals of concern in freshwater sediments (i.e., metals, polycyclic aromatic hydrocarbons, polychlorinated biphenyls, and pesticides). For each contaminant of concern, two SQGs were developed from the published SQGs, including a threshold effect concentration (TEC) and a probable effect concentration (PEC). The resultant SQGs for each chemical were evaluated for reliability using matching sediment chemistry and toxicity data from field studies conducted throughout the United States. The results of this evaluation indicated that most of the TECs (i.e., 21 of 28) provide an accurate basis for predicting the absence of sediment toxicity. Similarly, most of the PECs (i.e., 16 of 28) provide an accurate basis for predicting sediment toxicity. Mean PEC quotients were calculated to evaluate the combined effects of multiple contaminants in sediment. Results of the evaluation indicate that the incidence of toxicity is highly correlated to the mean PEC quotient (R(2) = 0.98 for 347 samples). It was concluded that the consensus-based SQGs provide a reliable basis for assessing sediment quality conditions in freshwater ecosystems.

In vitro and in vivo persistence of reticulocytes from donor red cells

BACKGROUND

Reticulocytes are important in the phenotyping of transfused patients. Reticulocytes can persist in blood units for the shelf life of the unit.

STUDY DESIGN AND METHODS

Temperature dependence of reticulocyte persistence was examined in vitro at 4, 24, and 37 degrees C by using thiazole orange staining and flow cytometric analysis. Two-color flow cytometric analysis was used to evaluate the persistence of donor reticulocytes in transfused patients.

RESULTS

Flow cytometric analysis using thiazole orange demonstrated that persistence of reticulocytes in units of stored CPDA-1 blood was temperature-dependent. Reticulocytes disappeared over 13 and 6 days at 24 degrees C and 37 degrees C, respectively, but at 4 degrees C the reticulocyte count changed little over 35 days. Two-color flow cytometric analysis of reticulocyte antigens was used to follow donor reticulocytes in 14 transfusion events in nine different patients. Donor reticulocytes persisted through 24 hours in 75 percent of the patients and were detectable at 48 hours in three patients.

CONCLUSION

This study demonstrates that reticulocytes persist during refrigerated storage; they are detectable in the circulation of most recipients for the first 24 hours after transfusion and in the circulation of a few recipients after 48 hours. These findings may have relevance for separation techniques based on reticulocyte density in samples drawn shortly after transfusion and for evaluation of reticulocyte counts in patients with hematologic abnormalities.

Separation of basic drugs by packed-column supercritical fluid chromatography. 3. Stimulants

A wide range of stimulants was rapidly and efficiently separated with symmetrical peak shapes with a tertiary supercritical mobile phase and a cyanopropyl liquid chromatography column. Compared with other drug families studied in this series, the stimulants exhibited a much broader range of retention. Several strong bases, including primary aliphatic amines, were easily eluted with a modified mobile phase. The stimulants were generally more retained than antipsychotic drugs. Compared with antidepressant drugs, a few stimulants behaved similarly, but most were more strongly retained. Modifier concentration proved to be the most effective means for changing both retention and selectivity. An over-the-counter eye drop solution (an aqueous buffer) was analyzed for tetrahydrozoline. The positive results with good peak shapes indicate that packed-column supercritical fluid chromatography is compatible with at least small aqueous samples.

Separation of drugs by packed column supercritical fluid chromatography. 2. Antidepressants

Antidepressants can be rapidly and efficiently separated by packed-column supercritical fluid chromatography with a tertiary mobile phase. Ten antidepressants were nearly baseline resolved in < 6 min without programming. Calculated detection limits with a 5-microL injection volume were as low as 88 ppb. The effects of temperature, pressure, and modifier concentration on retention and selectivity were studied. Changing modifier concentration caused significant changes in selectivity. Several relationships between solute structure and selectivity changes were noted.

Separation of drugs by packed column supercritical fluid chromatography. 1. Phenothiazine antipsychotics

Studies of speed, resolution, and sensitivity indicate packed column supercritical fluid chromatography is a viable technique for the analysis of antipsychotic drugs. Fourteen such drugs (mostly phenothiazines) were studied from a cyanopropyl liquid chromatography packed column with a tertiary mobile phase of carbon dioxide, methanol, and isopropylamine. An arbitrary mix of 10 components was baseline resolved in approximately 11 min. Detection limits were as low as 125 ppb, with a feasible 10-fold decrease. Without the isopropylamine in the mobile phase, none of the solutes eluted. With the additive, all produced symmetrical peaks and high efficiency (i.e., > 80% of theoretical). Changing modifier concentration was the most effective physical parameter for changing retention but had little effect on selectivity. Temperature changes had a modest effect on retention but provided surprisingly large changes in selectivity, with numerous peak reversals occurring over only a 30 degrees C temperature range. In addition, temperature programming was the most effective means to optimize resolution of the 10-component mix. No noticeable loss in efficiency accompanied the temperature programs.

A flow cytometric method for phenotyping recipient red cells following transfusion

BACKGROUND

Reticulocyte phenotyping is used for transfused patients, who have red cell antibodies, to match blood for subsequent transfusion. Current methods are labor-intensive and require a significant amount of sample.

STUDY DESIGN AND METHODS

A simple dual-color flow cytometry method developed for antigen typing of reticulocytes in mixed red cell populations is reported. Antigens were labeled by an indirect immunofluorescence technique using undiluted reagent sera as the primary label, biotinylated goat anti-human IgG as the secondary label, and avidin-phycoerythrin as the fluorescent stain. Reticulocytes were labeled with a thiazole orange fluorescent stain. Reticulocyte identification and antigen typing were performed on 319 samples to establish the validity of the procedure. Mixed red cells were prepared in all possible c antigen combinations to simulate transfusion concentrations of 25, 50, and 75 percent.

RESULTS

The anti-c flow cytometry profiles readily distinguished between antigen-positive and antigen-negative populations and allowed the detection of reticulocytes at all simulated transfusion concentrations. Similar results were obtained in experiments using C, K, s, Fya, Fyb, Jka, or Jkb sera against equal volumes of antigen-positive and -negative cells. Anti-S gave inconsistent results. The in vitro results were confirmed in 19 transfused patients who had received red cells antigenically different from their own as well as cells from 1 chimera blood donor.

CONCLUSION

This method provides a simpler, safer, less labor-intensive, and less subjective technique requiring far less sample volume than current methods for antigen typing of reticulocytes in mixed red cell samples from recently transfused patients.

Gradient separation of PTH-amino acids employing supercritical CO2 and modifiers

Twenty-four PTH-amino acids are rapidly and efficiently separated on a packed cyanopropyl Zorbax column by gradient elution of supercritical CO2 and tetramethylammonium hydroxide-modified methanol. Complete or partial resolution of 22 derivatives is observed with only valine coeluting with norleucine and lysine coeluting with asparagine. A wide variety of stationary phases and modifiers are investigated with supercritical CO2 in attempting to achieve the separation in less than 15 min. Critical to achieving a rapid and efficient separation is the control of modifier flow into the CO2. No modifier is required for elution of neutral PTH-amino acids. The addition of base plays a major role in the elution of acidic and basic PTH-amino acids. Peak tailing is minimized and the elution order of several peaks is altered upon incorporation of reagent into the mobile phase.