Fast gradient elution reversed-phase high-performance liquid chromatography with diode-array detection as a high-throughput screening method for drugs of abuse. I. Chromatographic conditions

Instrument parameters controlling retention precision in gradient elution reversed-phase liquid

The precision of retention time in RPLC is important for compound identification, for setting peak integration time windows and in fundamental studies of retention. In this work, we studied the effect of temperature (T), initial (ϕo) and final mobile phase (ϕf) composition, gradient time (tG), and flow rate (F) on the retention time precision under gradient elution conditions for various types of low MW solutes. We determined the retention factor in pure water ( [Formula: see text] ) and the solute-dependent solvent strength (S) parameters of Snyder's linear solvent strength theory (LSST) as a function of temperature for three different groups of solutes. The effect of small changes in the chromatographic variables (T, ϕo, ϕf, tG and F) by use of the LSST gradient retention equation were estimated. Peaks at different positions in the chromatogram have different sensitivities to changes in these instrument parameters. In general, absolute fluctuations in retention time are larger at longer gradient times. Drugs showed less sensitivity to changes in temperature compared to relatively less polar solutes, non-ionogenic solutes. Surprisingly we observed that fluctuations in temperature, mobile phase composition and flow rate had less effect on retention time under gradient conditions as compared to isocratic conditions. Overall temperature and the initial mobile phase composition are the most important variables affecting retention reproducibility in gradient elution chromatography.

Improved synthesis of carbon-clad silica stationary phases

Previously, we described a novel method for cladding elemental carbon onto the surface of catalytically activated silica by a chemical vapor deposition (CVD) method using hexane as the carbon source and its use as a substitute for carbon-clad zirconia.1,2 In that method, we showed that very close to exactly one uniform monolayer of Al (III) was deposited on the silica by a process analogous to precipitation from homogeneous solution in order to preclude pore blockage. The purpose of the Al(III) monolayer is to activate the surface for subsequent CVD of carbon. In this work, we present an improved procedure for preparing the carbon-clad silica (denoted CCSi) phases along with a new column packing process. The new method yields CCSi phases having better efficiency, peak symmetry, and higher retentivity compared to carbon-clad zirconia. The enhancements were achieved by modifying the original procedure in three ways: First, the kinetics of the deposition of Al(III) were more stringently controlled. Second, the CVD chamber was flushed with a mixture of hydrogen and nitrogen gas during the carbon cladding process to minimize generation of polar sites by oxygen incorporation. Third, the fine particles generated during the CVD process were exhaustively removed by flotation in an appropriate solvent.

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.

Rapid in situ detection of street samples of drugs of abuse on textile substrates using microRaman spectroscopy

Trace amounts of street samples of cocaine hydrochloride and N-methyl-3,4-methylenedioxy-amphetamine (MDMA) on natural and synthetic textiles were successfully detected in situ using confocal Raman microscopy. The presence of some excipient bands in the spectra of the drugs did not prevent the unambiguous identification of the drugs. Raman spectra of the drugs were readily obtained without significant interference from the fibre substrates. Interfering bands arising from the fibre natural or synthetic polymer structure and/or dye molecules did not overlap with the characteristic Raman bands of the drugs. If needed, interfering bands could be successfully removed by spectral subtraction. Also, Raman spectra could be acquired from drug particles trapped between the fibres of highly fluorescent textile specimens. The total acquisition time of the spectra of the drug particles was 90 s accomplished non-destructively and without detachment from their substrates. Sample preparation was not required and spectra of the drugs could be obtained non-invasively preserving the integrity of the evidential material for further analysis.

Perspectives on recent advances in the speed of high-performance liquid chromatography

Perhaps the most consistent trend in the development of high-performance liquid chromatography (HPLC) since its inception in the 1960s has been the continuing reach for ever faster analyses. The pioneering work of Knox, Horvath, Halasz, and Guiochon set forth a theoretical framework that was used early on to improve the speed of HPLC, primarily through the commercialization of smaller and smaller particles. Over the past decade, approaches to improving the speed of HPLC have become more diverse, and now practitioners of HPLC are faced with the difficult task of deciding which of these approaches will lead them to the fastest analysis for their application. Digesting the rich literature on the optimization of HPLC is a difficult task in itself, which is further complicated by contradictory marketing messages from competing commercial outlets for HPLC technology. In this perspectives article we provide an overview of the theoretical and practical aspects of the principal modern approaches to improving the speed of HPLC. We present a straightforward theoretical basis, informed by decades of literature on the problem of optimization, that is useful for comparing different technologies for improving the speed of HPLC. Through mindful optimization of conditions, high-performance separations on the subminute timescale are now possible and becoming increasingly common under both isocratic and gradient elution conditions. Certainly the continued development of ultrafast separations will play an important role in the development of two-dimensional HPLC separations. Despite the relatively long history of HPLC as an analytical technique, there is no sign of a slow-down in the development of novel HPLC technologies.

Application of silica-based hyper-crosslinked sulfonate-modified reversed stationary phases for separating highly hydrophilic basic compounds

The separation and determination of hydrophilic basic compounds are of great importance in many fields including clinical and biological research, pharmaceutical development and forensic analysis. However, the most widely used analytical separation technique in these disciplines, reversed-phase liquid chromatography (RPLC), usually does not provide sufficient retention for several important classes of highly hydrophilic basic compounds including catecholamines, many drug metabolites and many drugs of abuse. Commonly eluents having little or no organic modifier and/or strong ion pairing agents must be used to achieve sufficient retention and separation. Use of highly aqueous eluents can lead to column failure by dewetting, resulting in poor retention, low selectivity and irreproducibility and slow recovery of performance. The use of a strong ion pairing agent to increase retention renders the separation incompatible with mass spectrometric detection and complicates preparative separations. This paper describes the successful applications of a novel type of silica-based, hyper-crosslinked, sulfonate-modified reversed stationary phase, denoted as (-)SO(3)-HC-C(8)-L, for the separation of highly hydrophilic cations and related compounds by a hydrophobically assisted cation-exchange mechanism. Compared to conventional reversed-phases, the (-)SO(3)-HC-C(8)-L phase showed significantly improved retention and separation selectivity for hydrophilic amines. Concurrently, due to the presence of both cation-exchange and reversed-phase retention mechanisms and the high acid stability of hyper-crosslinked phases, the separation can be optimized by changing the type or concentration of ionic additive or organic modifier, and by varying the column temperature. In addition, gradients generated by programming the concentration of either the ionic additive or the organic modifier can be applied to reduce the analysis time without compromising resolution. Furthermore, remarkably different chromatographic selectivities, especially toward cationic solutes, were observed upon comparing the (-)SO(3)-HC-C(8)-L phase with conventional reversed-phases. We believe that the combination of these two types of stationary phases will be very useful in two-dimensional liquid chromatography.

Fast, comprehensive two-dimensional liquid chromatography

The absolute need to improve the separating power of liquid chromatography, especially for multi-constituent biological samples, is becoming increasingly evident. In response, over the past few years, there has been a great deal of interest in the development of two-dimensional liquid chromatography (2DLC). Just as 1DLC is preferred to 1DGC based on its compatibility with biological materials we believe that ultimately 2DLC will be preferred to the much more highly developed 2DGC for such samples. The huge advantage of 2D chromatographic techniques over 1D methods is inherent in the tremendous potential increase in peak capacity (resolving power). This is especially true of comprehensive 2D chromatography wherein it is possible, under ideal conditions, to obtain a total peak capacity equal to the product of the peak capacities of the first and second dimension separations. However, the very long timescale (typically several hours to tens of hours) of comprehensive 2DLC is clearly its chief drawback. Recent advances in the use of higher temperatures to speed up isocratic and gradient elution liquid chromatography have been used to decrease the time needed to do the second dimension LC separation of 2DLC to about 20s for a full gradient elution run. Thus, fast, high temperature LC is becoming a very promising technique. Peak capacities of over 2000 and rates of peak capacity production of nearly 1 peak/s have been achieved. In consequence, many real samples showing more than 200 peaks with signal to noise ratios of better than 10:1 have been run in total times of under 30 min. This report is not intended to be a comprehensive review of 2DLC, but is deliberately focused on the issues involved in doing fast 2DLC by means of elevating the column temperature; however, many issues of broader applicability will be discussed.

Requirements for bioanalytical procedures in postmortem toxicology

The application of analytical techniques in postmortem toxicology is often more difficult than in other forms of forensic toxicology owing to the variable and often degraded nature of the specimens and the diverse range of specimens available for analysis. Consequently, analysts must ensure that all methods are fully validated for the particular postmortem specimen(s) used. Collection of specimens must be standardized to minimize site-to-site variability and should if available include a peripheral blood sample and at least one other specimen. Urine and vitreous humor are good specimens to complement blood. In some circumstances solid tissues such as liver are recommended as well as gastric contents. Substance-screening techniques are the most important element since they will determine the range of substances that were targeted in the investigation and provide initial indication of the possible role of substances in the death. While immunoassay techniques are still commonly used for the most common drugs-of-abuse, chromatographic screening methods are required for general unknown testing. These are still predominately gas chromatography (GC) based using nitrogen/phosphorous detection and/or mass spectrometry (MS) detection, although some laboratories are now using time-of-flight MS or liquid chromatography (LC)-MS(MS) to cover a sometimes more limited range of substances. It is recommended that laboratories include a second chromatographic method to provide coverage of acidic and other substances not readily covered by a GC-based screen when extracts do not include all physiochemical types. This may include a gradient high-performance liquid chromatography (HPLC) photodiode array method, or better LC-MS(MS). Substance-specific techniques (e.g., benzodiazepines, opiates) providing a second form of identification (confirmation) are now divided between GC-MS(MS) and LC-MS(MS) procedures. LC-MS(MS) has taken over from many methods for the more polar compounds previously used in HPLC or in GC methods requiring derivatization. Analysts using LC-MS will need to obtain clean extracts to avoid poor and variable sensitivity caused by background suppression of the signal. Isolation techniques in postmortem toxicology tend to favor liquid extraction; however solid-phase extraction and solid-phase microextraction methods are available for many analytes.

Fast gradient elution reversed-phase liquid chromatography with diode-array detection as a high-throughput screening method for drugs of abuse. II. Data analysis

In Part I of this work, we developed a method for the detection of drugs of abuse in biological samples based on fast gradient elution liquid-chromatography coupled with diode array spectroscopic detection (LC-DAD). In this part of the work, we apply the chemometric method of target factor analysis (TFA) to the chromatograms. This algorithm identifies the target compounds present in chromatograms based on a spectral library, resolves nearly co-eluting components, and differentiates between drugs with similar spectra. The ability to resolve highly overlapped peaks using the spectral data afforded by the DAD is what distinguishes the present method from conventional library searching methods. Our library has a mean list length (MLL) of 1.255 and a discriminating power of 0.997 when both retention index and spectral factors are considered. The algorithm compares a library of 47 different compounds of toxicological relevance to unknown samples and identifies which compounds are present based on spectral and retention index matching. The application of a corrected retention index for identification rather than raw retention times compensates for long-term and column-to-column retention time shifts and allows for the use of a single library of spectral and retention data. Training data sets were used to establish the search and identification parameters of the method. A validation data set of 70 chromatograms was used to calculate the sensitivity (correct identification of positives) and specificity (correct identification of negatives) of the method, which were found to be 92% and 94%, respectively.