Equation for peak capacity estimation in two-dimensional liquid chromatography

Comprehensive two-dimensional liquid chromatography — practical impacts of theoretical considerations. A review

A theory of comprehensive two-dimensional separations by liquid chromatographic techniques is overviewed. It includes heart-cutting and comprehensive two-dimensional separation modes, with attention to basic concepts of two-dimensional separations: resolution, peak capacity, efficiency, orthogonality and selectivity. Particular attention is paid to the effects of sample structure on the retention and advantages of a multi-dimensional HPLC for separation of complex samples according to structural correlations. Optimization of 2D separation systems, including correct selection of columns, flow-rate, fraction volumes and mobile phase, is discussed. Benefits of simultaneous programmed elution in both dimensions of LCxLC comprehensive separations are shown.

Experimental setup, modulation of the fraction collection and transfer from the first to the second dimension, compatibility of mobile phases in comprehensive LCxLC, 2D asymmetry and shifts in retention under changing second-dimension elution conditions, are addressed. Illustrative practical examples of comprehensive LCxLC separations are shown.

Recent advances on multidimensional liquid chromatography-mass spectrometry for proteomics: from qualitative to quantitative analysis--a review

With the acceleration of proteome research, increasing attention has been paid to multidimensional liquid chromatography-mass spectrometry (MDLC-MS) due to its high peak capacity and separation efficiency. Recently, many efforts have been put to improve MDLC-based strategies including "top-down" and "bottom-up" to enable highly sensitive qualitative and quantitative analysis of proteins, as well as accelerate the whole analytical procedure. Integrated platforms with combination of sample pretreatment, multidimensional separations and identification were also developed to achieve high throughput and sensitive detection of proteomes, facilitating highly accurate and reproducible quantification. This review summarized the recent advances of such techniques and their applications in qualitative and quantitative analysis of proteomes.

Combination of off-line two-dimensional hydrophilic interaction liquid chromatography for polar fraction and two-dimensional hydrophilic interaction liquid chromatography×reversed-phase liquid chromatography for medium-polar fraction in a traditional Chinese medicine

Two-dimensional liquid chromatography (2-D LC) has been widely used for the analysis of complex samples owing to its great improvement in separation selectivity and peak capacity. However, one 2-D LC system may not be enough to meet the separation requirements due to the complexity of certain samples and respective limitations of two separation modes. In this work, water extract of Scutellaria barbata D. Don, a traditional Chinese medicine, was fractionated into polar fraction and medium-polar fraction by means of solid phase extraction (SPE). The fraction preparation made it easy to select the corresponding combination of 2-D LC method from hydrophilic interaction chromatography (HILIC) and reversed-phase liquid chromatography (RP-LC). An off-line 2-D HILIC×HILIC to analyze the polar fraction and an off-line 2-D HILIC×RP-LC to analyze the medium-polar fraction were developed, respectively. In total, 749 peaks were detected: 206 peaks from the polar fraction by the 2-D HILIC×HILIC and 543 from the medium-polar fraction by the 2-D HILIC×RP-LC. The practical peak capacities obtained in both systems were 2698 and 2879, and the orthogonality reached 63.18% and 90.62%, respectively. The results demonstrated that the two systems were both highly orthogonal, and the peak capacities greatly increased.

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.

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

The experimental effects of sampling time on the resolving power of on-line LC×LC were investigated. The first dimension gradient time ((1)t(g)) and sampling time (t(s)) were systematically varied ((1)t(g)=5, 12, 24 and 49 min; t(s)=6, 12, 21 and 40s). The resolving power of on-line LC×LC was evaluated in terms of two metrics namely the numbers of observed peaks and the effective 2D peak capacities obtained in separations of extracts of maize seeds. The maximum effective peak capacity and number of observed peaks of LC×LC were achieved at sampling times between 12 and 21s, at all first dimension gradient times. In addition, both metrics showed that the "crossover" time at which fully optimized 1DLC and LC×LC have equal resolving power varied somewhat with sampling time but is only about 5 min for sampling times of 12 and 21s. The longest crossover time was obtained when the sampling time was 6s. Furthermore, increasing the first dimension gradient time gave large improvements in the resolving power of LC×LC relative to 1DLC. Finally, comparisons of the corrected and effective 2D peak capacities as well as the number of peaks observed showed that the impact of the coverage factor is quite significant.

Recent developments in the HPLC separation of phenolic compounds

Phenolic compounds represent a class of highly complex naturally occurring molecules that possess a range of beneficial health properties. As a result, considerable attention has been devoted to the analysis of phenolics in a variety of samples. HPLC is the workhorse method for phenolic separation. However, conventional HPLC methods provide insufficient resolving power when faced with the complexity of real-world phenolic fractions. This limitation has been traditionally circumvented by extensive sample fractionation, multiple analysis methods and/or selective detection strategies. On the other hand, there is an increasing demand for improved throughput and resolving power from the chromatographic methods used for phenolic analyses. Fortunately, during the last decade, a number of important technological advances in LC have demonstrated significant gains in terms of both speed and resolution. These include ultra high-pressure liquid chromatography (UHPLC), high-temperature liquid chromatography (HTLC), multi-dimensional separations as well as various new stationary phase chemistries and morphologies. In recent years, these technologies have also found increasing application for phenolic analysis. This review seeks to provide an updated overview of the application of recent advances in HPLC to phenolic separation, with the emphasis on how these methodologies can contribute to improve performance in HPLC analysis of phenolics.

Effects of first dimension eluent composition in two-dimensional liquid chromatography

Comprehensive two-dimensional liquid chromatography (LC×LC) has received a great deal of attention during the past few years because of its extraordinary resolving power. The biggest advantage of this technique is that very high peak capacities can be generated in a relatively short time. Numerous approaches to maximize the peak capacity in LC×LC have been employed. In this work we investigate the impact of the first dimension mobile phase on selectivity. LC×LC has several potential advantages over one-dimensional LC (1DLC) in that unconventional solvents, at least in reversed-phase LC, can be used. For example, solvents which strongly adsorb in the UV in the first dimension are not problematic in LC×LC. This so because the UV detector is placed after the second dimensional column, as pulses of the first dimension eluent arrive at the second dimensional column, they elute well before the solutes of interest and therefore do not interfere at all with detection of solute peaks. So far, the most widely used solvents in reversed-phase 1DLC are methanol and acetonitrile. However, the "UV advantage" of 2DLC allows us to employ UV active solvents, such as acetone. We compare their differential selectivities to that of acetonitrile for the separation of 23 indole acetic acids of interest in plant biology. We also apply them to the separation of a maize seed extract, a very complex sample. In both sample sets, mobile phase composition can be an important parameter to increase the orthogonality of the two dimensions and thus, to increase the effective peak capacity of LC×LC.

Peak capacity optimization in comprehensive two dimensional liquid chromatography: a practical approach

In this work we develop a practical approach to optimization in comprehensive two dimensional liquid chromatography (LC x LC) which incorporates the important under-sampling correction and is based on the previously developed gradient implementation of the Poppe approach to optimizing peak capacity. The Poppe method allows the determination of the column length, flow rate as well as initial and final eluent compositions that maximize the peak capacity at a given gradient time. It was assumed that gradient elution is applied in both dimensions and that various practical constraints are imposed on both the initial and final mobile phase composition in the first dimension separation. It was convenient to consider four different classes of solute sets differing in their retention properties. The major finding of this study is that the under-sampling effect is very important and causes some unexpected results including the important counter-intuitive observation that under certain conditions the optimum effective LC x LC peak capacity is obtained when the first dimension is deliberately run under sub-optimal conditions. In addition, we found that the optimum sampling rate in this study is rather slower than reported in previous studies and that it increases with longer first dimension gradient times.

The impact of sampling time on peak capacity and analysis speed in on-line comprehensive two-dimensional liquid chromatography

Comprehensive two-dimensional liquid chromatography (2DLC) offers a number of practical advantages over optimized one-dimensional LC in peak capacity and thus in resolving power. The traditional "product rule" for overall peak capacity for a 2DLC system significantly overestimates peak capacity because it neglects under-sampling of the first dimension separation. Here we expand on previous work by more closely examining the effects of the first dimension peak capacity and gradient time, and the second dimension cycle times on the overall peak capacity of the 2DLC system. We also examine the effects of re-equilibration time on under-sampling as measured by the under-sampling factor and the influence of molecular type (peptide vs. small molecule) on peak capacity. We show that in fast 2D separations (less than 1h), the second dimension is more important than the first dimension in determining overall peak capacity and conclude that extreme measures to enhance the first dimension peak capacity are usually unwarranted. We also examine the influence of sample types (small molecules vs. peptides) on second dimension peak capacity and peak capacity production rates, and how the sample type influences optimum second dimension gradient and re-equilibration times.

A study of the precision and accuracy of peak quantification in comprehensive two-dimensional liquid chromatography in time

Simulated chromatographic data were used to determine the precision and accuracy in the estimation of peak volumes (i.e., peak sizes) in comprehensive two-dimensional liquid chromatography in time (LCxLC). Peak volumes were determined both by summing the areas in the second dimension chromatograms and by fitting the second dimension areas to a Gaussian peak. The Gaussian method is better at predicting the peak volume than the moments method provided there are at least three second dimension injections above the limit of detection (LOD). However, when only two of the second dimension signals are substantially above baseline, the accuracy and precision of the Gaussian fit method become quite poor because the results from the fitting algorithm become indeterminate. Based on simulations in which the modulation ratio (M(R)=4(1)sigma/t(s)) and sampling phase (phi) were varied, we conclude for well-resolved peaks that the optimum precision in peak volumes in 2D separations will be obtained when the M(R) is between two and five, such that there are typically four to ten second dimension peaks recorded over the eight sigma width of the first dimension peak. This sampling rate is similar to that suggested by the Murphy-Schure-Foley criterion. This provides an RSD of approximately 2% for the signal-to-noise ratio used in the present simulations. The precision of the peak volume of experimental data was also assessed, and RSD values were in the range of 4-5%. We conclude that the poorer precision found in the LCxLC experimental data as compared to LC may be due to experimental imprecision in sampling the effluent from the first dimension column.

Off-line comprehensive 2-dimensional hydrophilic interaction x reversed phase liquid chromatography analysis of procyanidins

The development of an off-line comprehensive 2-dimensional liquid chromatography (2-D-LC) method for the analysis of procyanidins is reported. In the first dimension, oligomeric procyanidins were separated according to molecular weight by hydrophilic interaction chromatography (HILIC), while reversed phase LC was employed in the second dimension to separate oligomers based on hydrophobicity. Fluorescence, UV and electrospray ionisation mass spectrometry (ESI-MS) were employed for identification purposes. The combination of these orthogonal separation methods is shown to represent a significant improvement compared to 1-dimensional methods for the analysis of complex high molecular weight procyanidin fractions, by simultaneously providing isomeric and molecular weight information. The low correlation (r(2)<0.2100) between the two LC modes afforded a practical peak capacity in excess of 2300 for the optimal off-line method. The applicability of the method is demonstrated for the analysis of phenolic extracts of apple and cocoa.