Some practical comparisons of the efficiency and overloading behaviour of sub-2 μm porous and sub-3 μm shell particles in reversed-phase liquid chromatography

Evaluating C18 stationary phases with a positively charged surface for proteomic LC-MS applications using mobile phase acidified with reduced formic acid concentration

We have recently demonstrated the ability of a C18 stationary phase with a positively charged surface (PCS-C18) to provide superior chromatographic separation of peptides using mobile phase acidified with a mere 0.01 % formic acid, significantly improving MS sensitivity. Here, we examined three columns packed with different PCS-C18 phases using the MS-favorable mobile phase acidified with low formic acid concentrations to establish the impact of separation performance and better MS sensitivity on peptide identifications. The surface charge interaction was evaluated using the retention of nitrate. The highest interaction was observed for the AdvanceBio Peptide Plus column. A surface charge-dependent shift in the retention time of peptides was confirmed with a change in formic acid concentration in the mobile phase. The separation performance of the columns with MS-favorable mobile phase acidified with low concentrations of formic acid was evaluated using well-characterized peptides. The loading capacity was assessed using a basic peptide with three lysine residues. Good chromatographic peak shapes and high loading capacity were observed for the Acquity Premier CSH C18 column, even when using a mobile phase acidified with 0.01 % formic acid. The extent of improvement in peptide identification when using reduced formic acid concentration was evaluated by analyzing the tryptic digest of trastuzumab and tryptic digest of whole bacteria cell lysate. Each column provided improved MS signal intensity and peptide identification when using the mobile phase with 0.01 % formic acid. The ability of the Acquity Premier CSH C18 column to provide better separation of peptides, even with a reduced formic acid concentration in the mobile phase, boosted MS signal intensity by 65 % and increased the number of identified peptides from the bacterial sample by 19 %. Our study confirms that significant improvement in the proteomic outputs can be achieved without additional costs only by tailoring the chemistry of the stationary phase to the composition of the mobile phase. Our results can help researchers understand the retention mechanism of peptides on the PCS-C18 stationary phases using low-ionic strength mobile phases and, more importantly, select the best-suited stationary phases for their LC-MS proteomic applications.

Practical study on the impact of injection conditions in gradient elution mode for the analysis of therapeutic proteins when using very short columns

The impact of injected sample volume on apparent efficiency has been studied for very short columns in a systematic way. For large molecules such as therapeutic proteins, it was found that relatively large volumes can be injected onto ultra-short RPLC and IEX columns (i.e. L < 50 mm) without significantly affecting the quality of the separation. This favourable behavior is due to the on-off elution mechanism of large molecules and to the fact that therapeutic protein samples are formulated in aqueous-based media, which is the weakest solvent in RPLC and IEX. Therefore, their peak is strongly focused at the column inlet even when large volume is injected, and pre-column peak dispersion is compensated. However, ultra-short HILIC columns do not seem to be favorable, as they require for very low injection volume to avoid detrimental peak splitting and breakthrough effects. Such peak distortion is related to the inherent solvent mismatch between sample diluent (aqueous) and mobile phase strength (highly organic in HILIC). When studying mass load, the ranking of the elution modes was the same, and the largest relative mass could be injected onto IEX columns (as large as 10% sample to sorbent mass), without affecting the separation quality.

A strategy for assessing peak purity of pharmaceutical peptides in reversed-phase chromatography methods using two-dimensional liquid chromatography coupled to mass spectrometry. Part I: Selection of columns and mobile phases

The current study describes the development of a 2D-LC-MS-based strategy for assessing main peak purity in the analysis of pharmaceutical peptides. The focus is on 2D-LC using reversed-phase (RP) separations in both dimensions, and particularly peptide isomer selectivity, since compounds with the same mass to charge ratio are not readily differentiated by mass spectrometry and therefore must be separated chromatographically. Initially, 30 column / mobile phase combinations were evaluated for both general separation performance (i.e., selectivity and peak shape) and isomer selectivity using forcibly degraded peptide samples and mixtures of synthetic diastereomers. A ranking of more than 300 UV and MS chromatograms suggests that when developing a new method, screening a set of four columns and four volatile mobile phases with differing characteristics should be adequate to both cover the selectivity space, and yield good separation performance. When 2D-LC-MS is to be used to evaluate peak purity for a new method, our results show that a second-dimension separation comprising a C8/C18 column possessing no ionic functionality, and an acetic acid / ammonium acetate mobile phase buffered at pH 5, provides good selectivity at 25 °C for peptide isomers with a MW <10 kDa. Retention data for 29 diverse peptides (1 < MW < 14 kDa, 3.7 < pI < 12.5) measured in this study using a variety of column and mobile phase conditions (i.e., 30 in total) are consistent with the classification of these various chromatographic conditions using the previously reported Peptide RPC Column Characterisation Protocol. For the investigated peptides trifluoroacetic acid was found to reduce selectivity differences between columns of diverse properties, probably due to its potential to form ion-pairs with peptides. Trifluoroacetic acid often improves peak shape for very large peptides (i.e. MW > 10 kDa). In the current dataset which also contain smaller peptides it received the highest ranking for 40% of the column and mobile phase combinations due to better selectivity and/or peak shape. The reported work here constitutes part one of a series of two papers. The second paper focuses on the use of retention modelling for rapid and accurate selection of the shallow gradients (i.e., << 1% ACN/min) required to obtain sufficient peptide isomer retention and separation in the second dimension. The overall results presented in this series of papers provides the guidance needed to develop a 2D-LC-MS method from start to finish for the analysis of main peak purity of therapeutic peptides.

Counteranions at Peripheral Sites Tune Guest Affinity for a Protonated Hemicryptophane

The affinity of small molecules for biomolecular cavities is tuned through a combination of primary and secondary interactions. It has been challenging to mimic these features in organic synthetic host molecules, however, where the cavities tend to be highly symmetric and nonpolar, and less amenable to chemical manipulation. Here, a host molecule composed of a TREN ligand and cyclotriveratrylene moiety was investigated. Size-matched polar guests were encapsulated within the cavity via triple protonation of the TREN moiety with various sulfonic acids. X-ray crystallography confirmed guest encapsulation and identified three methanesulfonates, p-toluenesulfonates, or 2-naphthalenesulfonates hydrogen-bonded with H3TREN at the periphery of the cavity. These structurally diverse counteranions were shown by 1H NMR spectroscopy to differentially regulate guest access at the three portals, and to undergo competitive displacement in solution. This work reveals "counteranion tuning" to be a simple and powerful strategy for modulating host-guest affinity, as applied here in a TREN-hemicryptophane.

Recent Trends in Fast Liquid Chromatography for Pharmaceutical Analysis

Background:

Liquid chromatography is the workhorse of analytical laboratories of pharmaceutical companies for analysis of bulk drug materials, intermediates, drug products, impurities and degradation products. This efficient technique is impeded by its long and tedious analysis procedures. Continuous efforts of scientists to reduce the analysis time resulted in the development of three different approaches namely, HTLC, chromatography using monolithic columns and UHPLC.

Methods:

Modern column technology and advances in chromatographic stationary phase including silica-based monolithic columns and reduction in particle and column size (UHPLC) have not only revolutionized the separation power of chromatographic analysis but also have remarkably reduced the analysis time. Automated ultra high-performance chromatographic systems equipped with state-ofthe- art software and detection systems have now spawned a new field of analysis, termed as Fast Liquid Chromatography (FLC). The chromatographic approaches that can be included in FLC are hightemperature liquid chromatography, chromatography using monolithic column, and ultrahigh performance liquid chromatography.

Results:

This review summarizes the progress of FLC in pharmaceutical analysis during the period from year 2008 to 2017 focusing on detecting pharmaceutical drugs in various matrices, characterizing active compounds of natural products, and drug metabolites. High temperature, change in the mobile phase, use of monolithic columns, new non-porous, semi-porous and fully porous reduced particle size of/less than 3μm packed columns technology with high-pressure pumps have been extensively studied and successively applied to real samples. These factors revolutionized the fast high-performance separations.

Conclusion:

Taking into account the recent development in fast liquid chromatography approaches, future trends can be clearly predicated. UHPLC must be the most popular approach followed by the use of monolithic columns. Use of high temperatures during analysis is not a feasible approach especially for pharmaceutical analysis due to thermosensitive nature of analytes.

Repeatability of gradient ultrahigh pressure liquid chromatography-tandem mass spectrometry methods in instrument-controlled thermal environments

The impact of viscous friction on eluent temperature and column efficiency in liquid chromatography is of renewed interest as the need for pressures exceeding 1000bar to use with columns packed with sub-2μm particles has grown. One way the development of axial and radial temperature gradients that arise due to viscous friction can be affected is by the thermal environment the column is placed in. In this study, a new column oven integrated into an ultrahigh pressure liquid chromatograph that enables both still-air and forced-air operating modes is investigated to find the magnitude of the effect of the axial thermal gradient that forms in 2.1×100mm columns packed with sub-2μm particles in these modes. Temperature increases of nearly 30K were observed when the generated power of the column exceeded 25W/m. The impact of the heating due to viscous friction on the repeatability of peak capacity, elution time, and peak area ratio to an internal standard for a gradient UHPLC-MS/MS method to analyze neurotransmitters was found to be limited. This result indicates that high speed UHPLC-MS/MS gradient methods under conditions of high viscous friction may be possible without the negative effects typically observed with isocratic separations under similar conditions.

Are sub-2 μm particles best for separating small molecules? An alternative

Superficially porous particles (SPP) in the 2.5-2.7 μm range provide almost the same efficiency and resolution of sub-2 μm totally porous particles (TPP), but at one-half to one-third of the operating pressure. The advantage of SPP has led to the introduction of sub-2 μm SPP as a natural extension of this technology. While short columns of both SPP and TPP sub-2 μm particles allow very fast separations, the efficiency advantages of these very small particles often are not realized nor sufficient to overcome some of the practical limitations and disadvantages of such small particles. Advantages and disadvantages of columns packed with sub-2 μm particles are described for comparison with the characteristics of larger particles. The authors conclude that while sub-2 μm particles have utility in research studies, columns of larger particles are often better suited for most applications. A suggested 2.0 μm superficially porous particle diameter retains many of the advantages of sub-2 μm particles, but minimizes some of the disadvantages. The characteristics of these new 2.0 μm SPP are described in studies comparing some present sub-2 μm SPP commercial columns for efficiency, column bed homogeneity and stability.

The use of ammonium formate as a mobile-phase modifier for LC-MS/MS analysis of tryptic digests

A major challenge facing current mass spectrometry (MS)-based proteomics research is the large concentration range displayed in biological systems, which far exceeds the dynamic range of commonly available mass spectrometers. One approach to overcome this limitation is to improve online reversed-phase liquid chromatography (RP-LC) separation methodologies. LC mobile-phase modifiers are used to improve peak shape and increase sample load tolerance. Trifluoroacetic acid (TFA) is a commonly used mobile-phase modifier, as it produces peptide separations that are far superior to other additives. However, TFA leads to signal suppression when incorporated with electrospray ionization (ESI), and thus, other modifiers, such as formic acid (FA), are used for LC-MS applications. FA exhibits significantly less signal suppression, but is not as effective of a modifier as TFA. An alternative mobile-phase modifier is the combination of FA and ammonium formate (AF), which has been shown to improve peptide separations. The ESI-MS compatibility of this modifier has not been investigated, particularly for proteomic applications. This work compares the separation metrics of mobile phases modified with FA and FA/AF and explores the use of FA/AF for the LC-MS analysis of tryptic digests. Standard tryptic-digest peptides were used for comparative analysis of peak capacity and sample load tolerance. The compatibility of FA/AF in proteomic applications was examined with the analysis of soluble proteins from canine prostate carcinoma tissue. Overall, the use of FA/AF improved online RP-LC separations and led to significant increases in peptide identifications with improved protein sequence coverage.

On-line comprehensive two dimensional separations of charged compounds using reversed-phase high performance liquid chromatography and hydrophilic interaction chromatography. Part II: application to the separation of peptides

In this second paper of a two-part series, on-line RPLC×HILIC is compared to on-line RPLC×RPLC through the separation of peptides. Our choices regarding the conditions are discussed. Injection effects and overloading effects are evaluated in both configurations. It is shown that whereas large volumes can be injected in the second dimension in RPLC×RPLC under HT-UHPLC conditions (>20% of the column dead volume), even small injection volumes (8% of the column the dead volume) have a detrimental effect on peak shapes in RPLC×HILIC. Advantages and limits of the two 2D-systems are compared through the 2D-separation of a tryptic digest of three proteins. A ten-fold gain in analysis time along with a significant gain in peak capacity are obtained with both systems compared to the most efficient one-dimensional separation of peptides recently published.

Overloading study of basic compounds with a positively charged C18 column in liquid chromatography

While tailing and overloading of basic compounds remain problematic on most RP columns, a new kind of positively charged RP column named XCharge C18 was found to be superior good for the separation of alkaloids in our practical use. In this work, the surface charge property of the XCharge C18 column was evaluated by the retention of NO(3)(-) under different pH values and buffer concentrations. A considerable and pH-dependent positive charge was confirmed on the column. Then overloading behaviors of bases were systematically studied using amitriptyline as a basic probe. Good peak shapes (Tf<1.5) and extra high loadability with a C(0.5) of about 30,000 mg/L were observed on the column, with commonly used 0.1% formic acid as mobile phase additive. However, increasing the ionic strength of buffer with phosphates led to tailing peaks at high sample amount and sharp decline in loadability (C(0.5) of 2000-3000 mg/L), although it brought higher column efficiency at low sample amount. Higher pH also induced worse performance and lower loadability. The overall results demonstrated the importance of an appropriate level of ionic repulsion for the XCharge C18 column to achieve the good performance for bases, which could be explained by the multiple-site adsorption theory as ionic repulsion would shield the solute from occupying high-energy sites deeper in C18 layer.

Kinetic performance comparison of fully and superficially porous particles with sizes ranging between 2.7 μm and 5 μm: Intrinsic evaluation and application to a pharmaceutical test compound

The reintroduction of superficially porous particles has resulted in a leap forward for the separation performance in liquid chromatography. The underlying reasons for the higher efficiency of columns packed with these particles are discussed. The performance of the newly introduced 5 μm superficially porous particles is evaluated and compared to 2.7 μm superficially porous and 3.5 and 5 μm fully porous columns using typical test compounds (alkylphenones) and a relevant pharmaceutical compound (impurity of amoxicillin). The 5 μm superficially porous particles provide a superior kinetic performance compared to both the 3.5 and 5 μm fully porous particles over the entire relevant range of separation conditions. The performance of the superficially porous particles, however, appears to depend strongly on retention and analyte properties, emphasizing the importance of comparing different columns under realistic conditions (high enough k) and using the compound of interest.