Column selectivity in reversed-phase liquid chromatography. VI. Columns with embedded or end-capping polar groups

Characterization of reversed-phase liquid chromatographic columns containing positively charged functionality

To date, the most commonly used column characterization databases do not determine the relative positive charge associated with new generation RP columns, or they fail to successfully discriminate between RP columns of purportedly low level positive and neutral characters. This paper rectifies this in that it describes a convenient and robust chromatographic procedure for the assessment of the low levels of positive charge on a range of RP columns. The low degree of positive charge was determined by their electrostatic attraction towards the negatively charged 4-n-octylbenzene sulfonic acid (4-OBSA) relative to their retention of the hydrophobic marker toluene (Tol). The new parameter (α4-OBSA/Tol) was determined for 15 commercially available RP-LC columns. When this was combined with existing Tanaka parameters it was possible to guide the chromatographer towards similar columns as "backup / equivalent phases" or dissimilar columns for exploitation in method development strategies. It should be noted that under certain chromatographic conditions the retention mechanism(s) may be too complex to allow direct location of a "backup / equivalent" column(s). The α4-OBSA/Tol results indicate that even the new generation neutral alkyl phases may exhibit a small degree of positive charge at low buffer concentrations. Mobile phases containing low % MeCN were demonstrated to promote mixed mode (anionic exchange / hydrophobic) retention whereas at high % MeCN anionic exchange retention dominated. The measure of electrostatic repulsion between positively charged columns and positively charged bases was assessed by evaluating the relative retention of a range of bases and neutral analytes. The greatest electrostatic repulsion was observed with hydrophilic bases. While there was no correlation between the positive charge associated with the phases assessed by electrostatic attraction or repulsion, the columns could be broadly divided into three subsets (i.e., significant positive character, medium to low positive character and insignificant positive character). Finally, the results were used to highlight the usefulness of the column characterization database containing the new anionic exchange retention parameter (α4-OBSA/Tol) for the selection of an equivalent column possessing a low level of positive character in the analysis of a real-life biopharmaceutical application.

An improved subtraction model applied in supercritical fluid chromatography to characterise polar stationary phases

Herein, an improved subtraction model was proposed to characterise the polar stationary phases in supercritical fluid chromatography (SFC). Fifteen stationary phases were selected, including two types of aromatic columns, Waters Torus and Viridis series columns, as well as silica and amino columns. Ethylbenzene and Torus 1-AA were defined as the reference solute and column, respectively. Identifying the interaction with the maximum contribution to retention in SFC separation and using it as the initial term is a key step in modelling. The dipole, or induced dipole interaction (θ'P), replaced the hydrophobic interaction (η'H) as the starting term. The improved model was expressed as logα=η'H+β'A+α'B+κ'C+θ'P+ε'E+σ'S, where the term ε'E indicated that anion exchange interaction was intentionally supplemented. A 7-step modelling process, including bidirectional fitting and residual analysis, was proposed. The obtained column parameters had reasonable physical significance, with the adjusted determination coefficient (R2adj) greater than 0.999 and the standard error (SE) less than 0.029. Methodological validation was further performed using the other four columns and 12 solutes that were not involved in the modelling. The result revealed good predictions of solutes' retention, as demonstrated by R2adj from 0.9923 to 0.9979 and SE from 0.0636 to 0.1088. This study indicated the feasibility of using the improved subtraction model to characterise polar stationary phases in SFC, with the most crucial being the determination of an initial term, followed by the addition of a new descriptor and the selection of an appropriate reference column. The study expanded the application scope of the subtraction model in SFC, which will help gain an in-depth understanding of the SFC separation mechanism.

Estimating the hydrophobicity extent of molecular fragments using reversed-phase liquid chromatography

A fast HPLC method was developed to study the hydrophobicity extent of pharmaceutically relevant molecular fragments. By this strategy, the reduced amount of sample available for physico-chemical evaluations in early-phase drug discovery programs does not represent a limiting factor. The sixteen acid fragments investigated were previously synthesized also determining potentiometrically their experimental log D values. For four fragments it was not possible to determine such property since their values were outside of the instrumental working range (2 < pKa  < 12). An RP-HPLC method was therefore optimized. For each scrutinized method, some derived chromatographic indices were calculated, and Pearson's correlation coefficient (r) allowed to select the so-called "φ0 index" as the best correlating with the log D. Thew s p H ${}_w^spH$ was fixed at 3.5 and a modification of some variables [organic modifier (methanol vs. ACN), stationary phase (octyl vs. octadecyl), presence/absence of the additives n-octanol, n-butylamine, and n-octylamine], allowed to select the best correlation conditions, producing a r = 0.94 (p < 0.001). Importantly, the φ0 index enabled the estimation of log D values for four fragments which were unattainable by potentiometric titration. Moreover, a series of molecular descriptors were calculated to identify the chemical characteristics of the fragments explaining the obtained φ0 . The number of hydrogen bond donors and the index of cohesive interaction correlated with the experimental data.

An attempt to apply a subtraction model for characterization of non-polar stationary phase in supercritical fluid chromatography

This study verified the feasibility of using a subtraction model to characterize the non-polar stationary phases (including C4, C8, and phenyl-type) in supercritical fluid chromatography (SFC). The model with 6 terms was expressed as log α = η'H + θ'P + β'A + α'B + κ'C + σ'S, where a term θ'P indicating dipole or induced dipole interaction was intentionally supplemented. Ethylbenzene and SunFire C8 were respectively defined as the reference solute and column. A 7-step modeling procedure was proposed: in the first 6 steps, except σ'S, by the use of a bidirectional fitting method, other parameters were calculated based on the equation: log α = log (k_i/k_ref) ≈ η'H + θ'P + β'A + α'B + κ'C; and in the 7th step, residual analysis was employed to describe the σ'S term according to the equation: σ'S = log α_exp. - log α_pre. Furthermore, six columns that were not involved in modeling process and 12 compounds with unknown retention were used for methodology validation. It showed good predictions of log k, as demonstrated by adjusted determination coefficient (R²_adj) from 0.9927 to 0.9998 (column) and from 0.9940 to 0.9999 (compound), respectively. The subtraction model emphasized the contribution of dipole or induced dipole interaction to the retention in SFC, and it obtained the σ'S term through residual analysis. Moreover, it made reasonable physical-chemical sense as the linear solvation energy relationship (LSER) model did, with the distinct advantages of better fitting and more accurate prediction. This study provided some new insights into the characterization of non-polar stationary phases in SFC.

Investigation of several chromatographic approaches for untargeted profiling of central carbon metabolism

Monitoring the central carbon metabolism (CCM) network using liquid chromatography/mass spectrometry (LC-MS) analysis is hampered by the diverse chemical nature of its analytes, which are extremely difficult to analyze using single chromatographic conditions. Furthermore, CCM-related compounds present non-specific adsorption on metal surfaces, causing detrimental chromatographic effects and sensitivity loss. In this study, polar reversed-phase, mixed-mode (MMC), and zwitterionic hydrophilic interaction chromatography (HILIC) featuring low-adsorption hardware were investigated towards untargeted analysis of biological samples with a focus on energy metabolism-related analytes. Best results were achieved with sulfoalkylbetaine HILIC with different supports, where polymeric option featured the highest coverage and inert hybrid silica facilitated best throughput and kinetic performance at a cost of less selectivity for small carboxylic acids. MMC demonstrated excellent performance for strongly anionic analytes such as multiresidue phosphates. The obtained experimental data also suggested that an additional hydrophilic modulation might be necessary to facilitate better resolution of carboxylic acids in zHILIC mode, as found during the application of the developed method to study the effect of two different mutations on the energy metabolism of S. aureus.

Separation of Panax notoginseng saponins on modified rosin ester-bonded silica stationary phase and its mechanism

A column prepared using a unique three-membered phenanthrene skeleton of rosin has complementary selectivity to a C18 column for some separation tasks. In this study, propylene pimaric acid (16-hydroxyethyl acrylate-34-n-butyl) ester (BRB) and propylene pimaric acid (16-hydroxyethyl acrylate-34-dodecyl) ester (BRLA) were used as functional ligands to prepare two novel stationary phases, namely BRB@SiO₂ and BRLA@SiO₂, through a "thiol-ene" click chemistry reaction. The characterization results of Fourier transform infrared spectroscopy, thermogravimetric analysis, nitrogen adsorption-desorption measurements, and contact angle tests showed that the BRB@SiO₂ and BRLA@SiO₂ stationary phases were successfully prepared. In addition, the performance of the columns was evaluated using the Tanaka test and hydrophobic subtraction model, which showed that the stationary phases exhibited typical reversed-phase chromatography performance and good hydrophobicity, hydrophobic selectivity, and steric selectivity. The changes in the retention of Panax notoginseng saponins on a column under different chromatographic conditions (acetonitrile content, flow rate, and column temperature) were investigated. The separation effect of BRB@SiO₂ and BRLA@SiO₂ columns on P. notoginseng saponins was better than that of the C18 column and the BRLA@SiO₂ column could replace the C18 column for the detection of P. notoginseng saponins.

Investigation of stationary phases performance for eicosanoids profiling in RP-HPLC

Eicosanoids - oxidative derivatives from arachidonic acid - represent biologically active lipid mediators in inflammatory processes. Different analytical methods treat eicosanoid analysis. Among which, reverse phase liquid chromatography figures as the appropriate method for eicosanoid profiling. RP-HPLC for eicosanoid analysis is often conducted on C18 columns. Some studies focused on profiling one family of eicosanoids; others considered all eicosanoid families. In both cases, co-elution remained a major issue and detection in mass spectrometry partially resolves this problem. In fact, the mass transitions used to monitor eicosanoid species are not specific enough and many isobars can be listed. For this, optimizing the RP-HPLC separation remains important. Based on the parameter F_s - deriving from the hydrophobic-subtraction model - and radar plots, we chose columns with different selectivities. The hydrophobic-subtraction model guided our interpretation of molecular interactions between eicosanoids and stationary phases. We founded our approach for selectivity optimization on peak capacity per minute and time needed values. Herein, we screened seven stationary phases and evaluated their chromatographic performances in RP-HPLC. Stationary phases presented different chemistry, type of silica, length, and particle size. Superficially porous particle columns registered better chromatographic profiles than classical stationary phases; and columns with embedded polar group did not serve our purpose. The stationary phase Accucore C30 - even being the least retentive - revealed the best selectivity and efficiency, and recorded the shorter duration for eicosanoid analysis.

Benefits of Innovative and Fully Water-Compatible Stationary Phases of Thin-Film Microextraction (TFME) Blades

Octadecyl (C₁₈) groups are arguably the most popular ligands used for preparation of solid phase microextraction (SPME) devices. However, conventional C₁₈-bonded silica particles are not fully compatible with the nearly 100% aqueous composition of typical biological samples (e.g., plasma, saliva, or urine). This study presents the first evaluation of thin-film SPME devices coated with special water-compatible C₁₈-bonded particles. Device performance was assessed by extracting a mixture of 30 model compounds that exhibited various chemical structures and properties, such as hydrophobicity. Additionally, nine unique compositions of desorption solvents were tested. Thin-film SPME devices coated with C₁₈-bonded silica particles with polar end-capping groups (10 µm) were compared with conventional trimethylsilane end-capped C₁₈-bonded silica particles of various sizes (5, 10, and 45 µm) and characteristics. Polar end-capped particles provided the best extraction efficacy and were characterized by the strongest correlations between the efficacy of the extraction process and the hydrophobicity of the analytes. The results suggest that the original features of octadecyl ligands are best preserved in aqueous conditions by polar end-capped particles, unlike with conventional trimethylsilane end-capped particles that are currently used to prepare SPME devices. The benefits associated with this improved type of coating encourage further implementation of microextractraction as greener alternative to the traditional sample preparation methods.

Analysis of Nucleosides and Nucleotides in Plants: An Update on Sample Preparation and LC-MS Techniques

Nucleotides fulfill many essential functions in plants. Compared to non-plant systems, these hydrophilic metabolites have not been adequately investigated in plants, especially the less abundant nucleotide species such as deoxyribonucleotides and modified or damaged nucleotides. Until recently, this was mainly due to a lack of adequate methods for in-depth analysis of nucleotides and nucleosides in plants. In this review, we focus on the current state-of-the-art of nucleotide analysis in plants with liquid chromatography coupled to mass spectrometry and describe recent major advances. Tissue disruption, quenching, liquid-liquid and solid-phase extraction, chromatographic strategies, and peculiarities of nucleotides and nucleosides in mass spectrometry are covered. We describe how the different steps of the analytical workflow influence each other, highlight the specific challenges of nucleotide analysis, and outline promising future developments. The metabolite matrix of plants is particularly complex. Therefore, it is likely that nucleotide analysis methods that work for plants can be applied to other organisms as well. Although this review focuses on plants, we also discuss advances in nucleotide analysis from non-plant systems to provide an overview of the analytical techniques available for this challenging class of metabolites.

Advancements in the preparation and application of monolithic silica columns for efficient separation in liquid chromatography

Fast and efficient separation remains a big challenge in high performance liquid chromatography (HPLC). The need for higher efficiency and resolution in separation is constantly in demand. To achieve that, columns developed are rapidly moving towards having smaller particle sizes and internal diameters (i.d.). However, these parameters will lead to high back-pressure in the system and will burden the pumps of the HPLC instrument. To address this limitation, monolithic columns, especially silica-based monolithic columns have been introduced. These columns are being widely investigated for fast and efficient separation of a wide range of molecules. The present article describes the current methods developed to enhance the column efficiency of particle packed columns and how silica monolithic columns can act as an alternative in overcoming the low permeability of particle packed columns. The fundamental processes behind the fabrication of the monolith including the starting materials and the silica sol-gel process will be discussed. Different monolith derivatization and end-capping processes will be further elaborated and followed by highlights of the performance such monolithic columns in key applications in different fields with various types of matrices.

Column Characterization and Selection Systems in Reversed-Phase High-Performance Liquid Chromatography

Reversed-phase high-performance liquid chromatography (RP-HPLC) is the most popular chromatographic mode, accounting for more than 90% of all separations. HPLC itself owes its immense popularity to it being relatively simple and inexpensive, with the equipment being reliable and easy to operate. Due to extensive automation, it can be run virtually unattended with multiple samples at various separation conditions, even by relatively low-skilled personnel. Currently, there are >600 RP-HPLC columns available to end users for purchase, some of which exhibit very large differences in selectivity and production quality. Often, two similar RP-HPLC columns are not equally suitable for the requisite separation, and to date, there is no universal RP-HPLC column covering a variety of analytes. This forces analytical laboratories to keep a multitude of diverse columns. Therefore, column selection is a crucial segment of RP-HPLC method development, especially since sample complexity is constantly increasing. Rationally choosing an appropriate column is complicated. In addition to the differences in the primary intermolecular interactions with analytes of the dispersive (London) type, individual columns can also exhibit a unique character owing to specific polar, hydrogen bond, and electron pair donor-acceptor interactions. They can also vary depending on the type of packing, amount and type of residual silanols, "end-capping", bonding density of ligands, and pore size, among others. Consequently, the chromatographic performance of RP-HPLC systems is often considerably altered depending on the selected column. Although a wide spectrum of knowledge is available on this important subject, there is still a lack of a comprehensive review for an objective comparison and/or selection of chromatographic columns. We aim for this review to be a comprehensive, authoritative, critical, and easily readable monograph of the most relevant publications regarding column selection and characterization in RP-HPLC covering the past four decades. Future perspectives, which involve the integration of state-of-the-art molecular simulations (molecular dynamics or Monte Carlo) with minimal experiments, aimed at nearly "experiment-free" column selection methodology, are proposed.

High-selectivity profiling of released and labeled N-glycans via polar-embedded reversed-phase chromatography

N-Glycosylation is the most complex post-translational modification of proteins and involved in many physiological processes and is therefore of major interest in academic research and in the biopharmaceutical industry. Reliable, robust, reproducible, and selective analysis of N-glycans is essential to understand the multitude of biological roles of N-glycosylation. So far, hydrophilic interaction liquid chromatography analysis of 2-AB or 2-AA derivatized N-glycans has been the standard method. In this work, the superiority of reversed-phase chromatography for complex N-glycosylation analysis is demonstrated. Separation of N-glycans derivatized with anthranilic acid on polar-embedded stationary alkyl phases with sub-2-μm particles results in outstanding selectivity and resolution. In combination with the highly mass spectrometry–compatible mobile phase, even very complex glycan mixtures can be separated, identified, and quantified precisely and accurately. The presented methodology can be applied broadly from basic research to analytical control and release testing of biological drug products and can be implemented in analytical laboratories with minimal effort.

Graphical abstract

Separation of highly charged compounds using competing ions with hydrophilic interaction liquid chromatography - Application to assay of cellular nucleotides

Separation of highly charged compounds has always been a challenge in chromatography. Ion-pair reversed phase chromatography has been the most successful approach to date. Although polar reversed phase and HILIC columns have been introduced, they have limitations with highly charged compounds. Competing ions have been used, in addition to ion-pair reagent, to achieve better resolution with reversed phase columns. Herein, we explored the use of competing ions with HILIC columns to demonstrate the effects on retention and separation of mono-, di-, and tri-nucleotides, introducing a new tool to improve resolution with HILIC columns. HILIC columns that had irreversibly retained highly charged tri-nucleotides became capable of successfully separating the same compounds, by using this approach. The optimised method was used to successfully resolve a mixture of 12 nucleotides with charges ranging from 1^- to 3^-. The method was applied to quantify nucleotides in blood cell extracts.

Optimizing separations in online comprehensive two-dimensional liquid chromatography

Online comprehensive two-dimensional liquid chromatography has become an attractive option for the analysis of complex nonvolatile samples found in various fields (e.g. environmental studies, food, life, and polymer sciences). Two-dimensional liquid chromatography complements the highly popular hyphenated systems that combine liquid chromatography with mass spectrometry. Two-dimensional liquid chromatography is also applied to the analysis of samples that are not compatible with mass spectrometry (e.g. high-molecular-weight polymers), providing important information on the distribution of the sample components along chemical dimensions (molecular weight, charge, lipophilicity, stereochemistry, etc.). Also, in comparison with conventional one-dimensional liquid chromatography, two-dimensional liquid chromatography provides a greater separation power (peak capacity). Because of the additional selectivity and higher peak capacity, the combination of two-dimensional liquid chromatography with mass spectrometry allows for simpler mixtures of compounds to be introduced in the ion source at any given time, improving quantitative analysis by reducing matrix effects. In this review, we summarize the rationale and principles of two-dimensional liquid chromatography experiments, describe advantages and disadvantages of combining different selectivities and discuss strategies to improve the quality of two-dimensional liquid chromatography separations.

A simple method for HPLC retention time prediction: linear calibration using two reference substances

BACKGROUND

Analysis of related substances in pharmaceutical chemicals and multi-components in traditional Chinese medicines needs bulk of reference substances to identify the chromatographic peaks accurately. But the reference substances are costly. Thus, the relative retention (RR) method has been widely adopted in pharmacopoeias and literatures for characterizing HPLC behaviors of those reference substances unavailable. The problem is it is difficult to reproduce the RR on different columns due to the error between measured retention time (t_R) and predicted t_R in some cases. Therefore, it is useful to develop an alternative and simple method for prediction of t_R accurately.

METHODS

In the present study, based on the thermodynamic theory of HPLC, a method named linear calibration using two reference substances (LCTRS) was proposed. The method includes three steps, procedure of two points prediction, procedure of validation by multiple points regression and sequential matching. The t_R of compounds on a HPLC column can be calculated by standard retention time and linear relationship.

RESULTS

The method was validated in two medicines on 30 columns.

CONCLUSION

It was demonstrated that, LCTRS method is simple, but more accurate and more robust on different HPLC columns than RR method. Hence quality standards using LCTRS method are easy to reproduce in different laboratories with lower cost of reference substances.

Design of peptide standards with the same composition and minimal sequence variation to monitor performance/selectivity of reversed-phase matrices

The present manuscript extends our de novo peptide design approach to the synthesis and evaluation of a new generation of reversed-phase HPLC peptide standards with the same composition and minimal sequence variation (SCMSV). Thus, we have designed and synthesized four series of peptide standards with the sequences Gly-X-Leu-Gly-Leu-Ala-Leu-Gly-Gly-Leu-Lys-Lys-amide, where the N-terminal is either N(α)-acetylated (Series 1) or contains a free α-amino group (Series 3); and Gly-Gly-Leu-Gly-Gly-Ala-Leu-Gly-X-Leu-Lys-Lys-amide, where the N-terminal is either N(α)-acetylated (Series 2) or contains a free α-amino group (Series 4). In this initial study, the single substitution position, X, was substituted with alkyl side-chains (Ala Collapse

Key Words Collapse

MESH Headings

• Amino Acid Sequence
• Chromatography, High Pressure Liquid/standards
• Chromatography, Reverse-Phase/standards
• Peptides/chemistry
• Reference Standards

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Grants

• R01 GM061855 NIGMS NIH HHS
• R01 GM061855-10 NIGMS NIH HHS
• R01 GM61855 NIGMS NIH HHS

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Affiliation(s)

Colin T. Mant Department of Biochemistry and Molecular Genetics, University of Colorado Denver, School of Medicine, Aurora, CO 80045, USA
Robert S. Hodges Department of Biochemistry and Molecular Genetics, University of Colorado Denver, School of Medicine, Aurora, CO 80045, USA

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Silica-based, hyper-crosslinked acid stable stationary phases for high performance liquid chromatography

A new family of hyper-crosslinked (HC) phases for use under very aggressive acid conditions including those encountered in ultra-fast, high temperature two-dimensional liquid chromatography (2DLC) has been recently introduced. This type of stationary phase shows significantly enhanced acid and thermal stability compared to the most acid stable, commercial RPLC phases. In addition, the use of "orthogonal" chemistry to make surface-confined polymer networks ensures good reproducibility and high efficiency. One of the most interesting features of the HC phases is the ability to derivatize the surface aromatic groups with various functional groups. This has led to the development of a family of hyper-crosslinked phases possessing a wide variety of chromatographic selectivities by attaching hydrophobic (e.g. -C₈), ionizable (e.g. -COOH, -SO₃H), aromatic (e.g. -toluene) or polar (e.g. -OH) species to the aromatic polymer network. HC reversed phases with various degrees of hydrophobicity and mixed-mode HC phases with added strong and weak cation exchange sites have been synthesized, characterized and applied. These silica-based acid-stable HC phases, with their attractive chromatographic properties, should be very useful in the separation of bases or biological analytes in acidic media, especially at elevated temperatures. This work reviews prior research on HC phases and introduces a novel HC phase made by alternative chemistry.

HPLC determination of cefprozil in tablets using monolithic and C18 silica columns

Cefprozil (CPZ) is a second-generation semi-synthetic cephalosporin antibiotic that commonly exists as the mixture of Z and E diastereoisomers, at the ratio of approximately 9:1. A novel reversed-phase HPLC method for the determination of CPZ in tablets was described. The separation of CPZ diastereoisomers and caffeine (internal standard) was carried out by applying the same analytical and instrumental conditions on two stationary phases, which have different surface chemistries. The columns used in the study were monolithic silica Merck Chromolith Performance RP-18e and conventional C18 silica Phenomenex Synergi Hydro RP columns. In total, 10 μL aliquots of samples were injected into the system and eluted using water-acetonitrile (90:10, v/v) solution, which was pumped through the column at a flow rate of 1.0 mL/min. The analyte peaks were detected at 200 nm using diode array detector with high specificity. CPZ diastereoisomers and caffeine were measured within 13 min using the C18 column, whereas <5 min was required for the monolithic one. Validation studies were performed according to official recommendations. Value of a monolithic column for the assay of diastereoisomers in pharmaceutical tablets was evaluated for the first time and found as a powerful alternative to highly efficient C18 columns.

The design and use of a simple System Suitability Test Mix for generic reverse phase high performance liquid chromatography-mass spectrometry systems and the implications for automated system monitoring using global software tracking

The development of a seven-component test mixture designed for use with a generic gradient and a reversed-phase high performance liquid chromatography-mass spectrometry (RP-HPLC-MS) system is discussed. Unlike many test mixtures formulated in order to characterise column quality at neutral pH, the test mixture reported here was designed to permit an overall suitability assessment of the whole liquid chromatography-mass spectrometry (LCMS) system. The mixture is designed to test the chromatographic performance of the column as well as certain aspects of the performance of the individual instrumental components of the system. The System Suitability Test Mix can be used for low and high pH generic reverse phase LCMS analysis. Four phthalates are used: diethyl phthalate (DEP), diamyl phthalate (DAP), di-n-hexyl phthalate (DHP) and dioctyl phthalate (DOP). Three other probes are employed: 8-bromoguanosine (8-BG), amitryptyline (Ami), and 4-chlorocinnamic acid (4-CCA). We show that analysis of this test mixture can alert the user when any part of the system (instrument or column) contributes to loss of overall performance and may require remedial action and demonstrate that it can provide information that enables us to document data quality control.