An experimental study of sampling time effects on the resolving power of on-line two-dimensional high performance liquid chromatography

Likelihood of total resolution in selective comprehensive two-dimensional liquid chromatography with parallel processing: Simulation and theory

The probability Pr(sLC×LC) that all peaks are separated by a resolution of 1.5 or more in selective comprehensive two-dimensional liquid chromatography (sLC × LC) is computed for simple model systems of 5 to 60 peaks and first-dimension (¹D) gradient times of 100 to 2000 s. The computations include mimics of a commercial instrument, whose fixed second-dimension (²D) gradient time and use of one cycle time for initialization reduces Pr(sLC×LC) relative to an earlier report. For serial sLC × LC, in which a single device collects and transfers ¹D multiplets to the second dimension, Pr(sLC×LC) under practical conditions is predicted to be only slightly larger than the probability of total resolution in LC × LC for separations of the same duration in each case. To increase Pr(sLC×LC), two model systems are proposed based on parallel processing, in which one device collects multiplets from the first separation while a second device simultaneously transfers fractions from previously collected multiplets to the second dimension for further separation. A sum of probabilities guideline is proposed by which optimal fixed ²D gradient times, ranging from 9.5 to 12 s, are found for both serial and parallel models. The increases of Pr(sLC×LC) based on parallel processing are modest; the largest is only 0.062 for one system and 0.106 for the other, relative to the serial model. A theory is derived that rationalizes the modesty of the increase, which was unexpected. It shows that Pr(sLC×LC) equals the probability of total resolution in the first dimension, plus the product of the probability that all ¹D multiplets are transferred to the second dimension and the probability that all multiplets are separated in the second dimension. The theory shows that, although parallel processing is better than serial processing for multiplet transfer, the ability to leverage this gain is offset by the limited probability that all multiplets are then actually separated in the second dimension, which is only about 0.55 for conditions where the change from serial to parallel processing is most beneficial. With these findings in hand, two scenarios are examined for future consideration: one in which the ²D peak capacity is doubled, and another in which multiplets are always transferred to the second dimension. The latter shows considerable promise for increasing Pr(sLC×LC) substantially beyond its counterpart in LC × LC. For example, a 50% probability of separating all peaks in a 15-component mixture can be reached in 1150 s using LC × LC. The same probability can be reached in the same time for a sample with nearly twice as many components (27) in the case of sLC × LC, assuming transfer of all multiplets to the second dimension. These findings will be useful to those considering systematic approaches to developing 2D-LC methods for moderately complex mixtures, and to those interested in instrument development for 2D-LC.

Validation of a two-dimensional liquid chromatography method for quality control testing of pharmaceutical materials

Despite the advantages of 2D-LC, there is currently little to no work in demonstrating the suitability of these 2D-LC methods for use in a quality control (QC) environment for good manufacturing practice (GMP) tests. This lack of information becomes more critical as the availability of commercial 2D-LC instrumentation has significantly increased, and more testing facilities begin to acquire these 2D-LC capabilities. It is increasingly important that the transferability of developed 2D-LC methods be assessed in terms of reproducibility, robustness and performance across different laboratories worldwide. The work presented here focuses on the evaluation of a heart-cutting 2D-LC method used for the analysis of a pharmaceutical material, where a key, co-eluting impurity in the first dimension (¹D) is resolved from the main peak and analyzed in the second dimension (²D). A design-of-experiments (DOE) approach was taken in the collection of the data, and the results were then modeled in order to evaluate method robustness using statistical modeling software. This quality by design (QBD) approach gives a deeper understanding of the impact of these 2D-LC critical method attributes (CMAs) and how they affect overall method performance. Although there are multiple parameters that may be critical from method development point of view, a special focus of this work is devoted towards evaluation of unique 2D-LC critical method attributes from method validation perspective that transcend conventional method development and validation. The 2D-LC method attributes are evaluated for their recovery, peak shape, and resolution of the two co-eluting compounds in question on the ²D. In the method, linearity, accuracy, precision, repeatability, and sensitivity are assessed along with day-to-day, analyst-to-analyst, and lab-to-lab (instrument-to-instrument) assessments. The results of this validation study demonstrate that the 2D-LC method is accurate, sensitive, and robust and is ultimately suitable for QC testing with good method transferability.

Recent advances in 2D-LC for bioanalysis

This article summarizes the most important developments in the use of 2D-LC for bioanalysis in the last 5 years. While several interesting and powerful applications have been developed recently, this work has been supported by continued, significant development of theoretical concepts, instrument development and practical aspects of method development. Some of the most exciting applications have been focused on the use of 2D-LC and characterize proteins both as biotherapeutic drug substances, and in formulations. These materials are inherently complex, difficult to resolve chromatographically and present problems that are essentially unknown (e.g., aggregation) in the small molecule world, thus 2D-LC can be leveraged very effectively to address these challenges.

Impact of reversed phase column pairs in comprehensive two-dimensional liquid chromatography

A major issue in optimizing the resolving power of two-dimensional chromatographic separations is the choice of the two phases so as to maximize the distribution of the analytes over the separation space. In this work, we studied the choice of appropriate reversed phases to use in on-line comprehensive two-dimensional liquid chromatography (LC×LC). A set of four chemically different conventional bonded reversed phases was used in the first dimension. The second dimension column was either a conventional bonded C18 phase or a carbon-clad phase (CCP). The LC×LC chromatograms and contour plots were all rather similar indicating that the selectivities of the two phases were also similar regardless of the reverse phase column used in the first dimension. Further, the spatial coverage seen with all four first dimension stationary phases when paired with a second dimension C18 phase were low and the retention times were strongly correlated. However, when the C18 column was replaced with the CCP column much improved separations were observed with higher spatial coverages, greater orthogonalities and significant increases in the number of observed peaks.

Development of a carbon clad core-shell silica for high speed two-dimensional liquid chromatography

We recently introduced a new method to deposit carbon on fully porous silicas (5 μm) to address some of the shortcomings of carbon clad zirconia (C/ZrO(2)), which has rather low retention due to its low surface area (20-30 m(2)/g). The method enables the introduction of a thin, homogeneous layer of Al(III) on silica to serve as catalytic sites for carbon deposition without damaging the silica's native pore structure. Subsequent carbon deposition by chemical vapor deposition resulted in chromatographically useful carbon phases as shown by good efficiencies and higher retentivity relative to C/ZrO(2). Herein, we use the above method to develop a novel carbon phase on superficially porous silica (2.7 μm). This small, new form of silica offers better mass transfer properties and higher efficiency with lower column back pressures as compared to sub 2 μm silica packings, which should make it attractive for use as the second dimension in fast two-dimensional LC (LC × LC). After carbon deposition, several studies were conducted to compare the new packing with C/ZrO(2). Consistent with work on 5 μm fully porous silica, the metal cladding did not cause pore blockage. Subsequent carbon deposition maintained the good mass transfer properties as shown by the effect of velocity on HETP. The new packing exhibits efficiencies up to ∼5.6-fold higher than C/ZrO(2) for polar compounds. We observed similar chromatographic selectivity for all carbon phases tested. Consequently, the use of the new packing as the second dimension in fast LC×LC improved the peak capacity of fast LC × LC. The new material gave loading capacities similar to C/ZrO(2), which is rather as expected based on the surface areas of the two phases.

Improving peak capacity in fast online comprehensive two-dimensional liquid chromatography with post-first-dimension flow splitting

The use of flow splitters between the two dimensions in online comprehensive two-dimensional (2D) liquid chromatography (LC × LC) has not received very much attention, in comparison with their use in 2D gas chromatography (GC × GC), where they are quite common. In principle, splitting the flow after the first dimension column and performing online LC × LC on this constant fraction of the first dimension effluent should allow the two dimensions to be optimized almost independently. When there is no flow splitting, any change in the first-dimension flow rate has an immediate impact on the second dimension. With a flow splitter, one could, for example, double the flow rate into the first dimension column and perform a 1:1 flow split without changing the sample loop size or the sampler's collection time. Of course, the sensitivity would be diminished, but this can be partially compensated through the use of a larger injection; this will likely only amount to a small price to pay for this increased resolving power and system flexibility. Among other benefits, we found a 2-fold increase in the corrected 2D peak capacity and the number of observed peaks for a 15-min analysis time, using a post-first-dimension flow splitter. At a fixed analysis time, this improvement results primarily from an increase in the gradient time, resulting from the reduced system re-equilibration time, and, to a smaller extent, it is due to the increased peak capacity achieved by full optimization of the first dimension.

Effect of first dimension phase selectivity in online comprehensive two dimensional liquid chromatography (LC×LC)

In this study, we examined the effect of first dimension column selectivity in reversed phase (RP) online comprehensive two dimensional liquid chromatography (LC×LC). The second dimension was always a carbon clad metal oxide reversed phase material. The hydrophobic subtraction model (HSM) and the related phase selective triangles were used to guide the selection of six different RP first dimension columns. Various kinds of samples were investigated and thus two different elution conditions were needed to cause full elution from the first dimension columns. We compared LC×LC chromatograms, contours plots, and fcoverage plots by measuring peak capacities, peak numbers, relative spatial coverage, correlation values, etc. The major finding of this study is that the carbon phase due to its rather different selectivity from other reversed phases is reasonably orthogonal to a variety of common types of bonded reversed phases. Thus quite surprisingly the six different first dimension stationary phases all showed generally similar separation patterns when paired to the second dimension carbon phase. This result greatly simplifies the task of choosing the correct pair of phases for RP×RP.