•A thin-layer chromatography ambient mass spectrometry method is used.•The Molecular Ionization Desorption Analysis Source (MIDAS) is used.•Quantitative analysis of hop acids is achieved using TLC and MIDAS.•Could extend method to the analysis of isomerized hop extracts and finished beer.A thin-layer chromatography ambient mass spectrometry method was developed for the quantitative analysis of hop acids. Hop acids were extracted from three varieties of hop pellets, Apollo, Cluster and Czech Saaz. Reversed-phase thin-layer chromatography provided the optimum stationary phase for separation and ionization. Using standards, a method was developed to separate α-, iso-α- and β-acids by thin-layer chromatography. Each thin-layer chromatography plate was analyzed directly by the Molecular Ionization Desorption Analysis Source (MIDAS). MIDAS is an atmospheric pressure ambient ionization source for mass spectrometry. Quantitative analysis of the hop extracts was made by successive analysis of known amounts of standards and the determination of a response factor. The expected trend of total α-acid content with Apollo > Cluster > Czech Saaz was observed. Variation in the results was attributed to manual TLC plate spotting and not to the ionization source.Download high-res image (84KB)Download full-size image

UMBC, a diverse public research university, “builds” upon its reputation in producing highly capable undergraduate scholars to create a comprehensive new model, STEM BUILD at UMBC. This program is designed to help more students develop the skills, experience and motivation to excel in science, technology, engineering, and mathematics (STEM). This article provides an in-depth description of STEM BUILD at UMBC and provides the context of this initiative within UMBC’s vision and mission.The STEM BUILD model targets promising STEM students who enter as freshmen or transfer students and do not qualify for significant university or other scholarship support. Of primary importance to this initiative are capacity, scalability, and institutional sustainability, as we distill the advantages and opportunities of UMBC’s successful scholars programs and expand their application to more students. The general approach is to infuse the mentoring and training process into the fabric of the undergraduate experience while fostering community, scientific identity, and resilience. At the heart of STEM BUILD at UMBC is the development of BUILD Group Research (BGR), a sequence of experiences designed to overcome the challenges that undergraduates without programmatic support often encounter (e.g., limited internship opportunities, mentorships, and research positions for which top STEM students are favored). BUILD Training Program (BTP) Trainees serve as pioneers in this initiative, which is potentially a national model for universities as they address the call to retain and graduate more students in STEM disciplines – especially those from underrepresented groups. As such, BTP is a research study using random assignment trial methodology that focuses on the scalability and eventual incorporation of successful measures into the traditional format of the academy.Critical measures to transform institutional culture include establishing an extensive STEM Living and Learning Community to increase undergraduate retention, expanding the adoption of “active learning” pedagogies to increase the efficiency of learning, and developing programs to train researchers to effectively mentor a greater portion of the student population. The overarching goal of STEM BUILD at UMBC is to retain students in STEM majors and better prepare them for post baccalaureate, graduate, or professional programs as well as careers in biomedical and behavioral research.

Molecular ionization-desorption analysis source (MIDAS), which is a desorption atmospheric pressure chemical ionization (DAPCI) type source, for mass spectrometry has been developed as a multi-functional platform for the direct sampling of surfaces. In this article, its utility for the analysis of thin-layer chromatography (TLC) plates is highlighted. Amino acids, which are difficult to visualize without staining reagents or charring, were detected and identified directly from a TLC plate. To demonstrate the full potential of MIDAS, all active ingredients from an analgesic tablet, separated on a TLC plate, were successfully detected using both positive and negative ion modes. The identity of each of the compounds was confirmed from their mass spectra and compared against standards. Post separation, the chemical signal (blue permanent marker) as reference marks placed at the origin and solvent front were used to calculate retention factor (Rf) values from the resulting ion chromatogram. The quantitative capabilities of the device were exhibited by scanning caffeine spots on a TLC plate of increasing sample amount. A linear curve based on peak are, R2 = 0.994, was generated for seven spots ranging from 50 to 1000 ng of caffeine per spot.

•The mechanism of indirect pulsed electrochemical detection is explained.•A single pulsed potential-time waveform can be used for a wide range of analytes.•Sorbitol as a mobile phase additive produces the best results.•Detection limits for aliphatic carboxylate-containing compounds are 2–0.05 ppm.•Over-the-counter formulations are assayed for biotin and proline.The mechanism of detection in pulsed electrochemical detection (PED) requires preadsorption of the analyte to the working electrode prior to its subsequent oxidation. Indirect detection is accomplished by the addition of a PED-active reagent to the mobile phase, whose signal is attenuated by an analyte that more strongly adsorbs to the electrode surface. Here, indirect PED (InPED) is applied to the determination of aliphatic carboxylate-containing compounds separated using high performance anion-exchange chromatography (HPAEC). Limits of detections of 0.05–2 ppm (10–400 pmol) are found for most analytes tested using an optimized potential-time waveform at a gold working electrode. The analytical utility of InPED is demonstrated for assays of gabapentin, biotin, proline and several over-the-counter formulations.

•Reviews the fundamental and basic tenants of pulsed electrochemical detection.•The latest advances in pulsed electrochemical detection technology and its applications.•Contains a bibliography of applications since 1997.Pulsed electrochemical detection (PED) has progressed as a highly sensitive and selective detection technique following aqueous-based separation systems over the past three decades. The application of on-line pulsed potential cleaning to electrocatalytic noble metal electrodes has significantly increased the number of applications formerly achieved with conventional electrochemical (EC) detection. Electrochemical cells are easily miniaturized, providing the ability to apply detection by PED at microelectrodes and onto microchips utilizing electrophoretic separations. In addition, recent advances in PED waveforms and instrumentation have enabled the detection technique to be easily coupled with high pressure separation systems which require rapid detection to maintain separation integrity. As a result, advanced applications for the determination of carbohydrates as well as the expansion of PED for the detection of other organic aliphatic compounds have been recently accomplished. This review will focus on developments and methods utilizing PED following liquid chromatography (LC) and capillary electrophoresis (CE). Publications are reviewed in chronological order to emphasize the advancement of the detection method and the sustained relevance of its applications.

Water cluster ion intensity and distribution is affected by source conditions in direct sample analysis (DSA) ionization. Parameters investigated in this paper include source nozzle diameter, gas flow rate, and source positions relative to the mass spectrometer inlet. Schlieren photography was used to image the gas flow profile exiting the nozzle. Smaller nozzle diameters and higher flow rates produced clusters of the type [H + (H2O)n]+ with greater n and higher intensity than larger nozzles and lower gas flow rates. At high gas flow rates, the gas flow profile widened compared with the original nozzle diameter. At lower flow rates, the amount of expansion was reduced, which suggests that lowering the flow rate may allow for improvements in sampling spatial resolution.

Highly reactive N-acylating solid-phase reagents based on macroporous polystyrene-bound 1-hydroxybenzotriazole (P-HOBt) and silica-bound 1-hydroxybenzotriazole (Si-HOBt) were prepared and compared for reactivity by synthesis of small combinatorial libraries of acetamides and benzamides.

Post-column photochemical reaction systems have developed into a common approach for enhancing conventional methods of detection in HPLC. Photochemical reactions as a means of ‘derivatization’ have a significant number of advantages over chemical reaction-based methods, and a significant effort has been demonstrated to develop an efficient photochemical reactor. When coupled to electrochemical (EC) detection, the technique allows for the sensitive and selective determination of a variety of compounds (e.g., organic nitro explosives, beta-lactam antibiotics, sulfur-containing antibiotics, pesticides and insecticides). This review will focus on developments and methods using post-column photochemical reaction systems followed by EC detection in liquid chromatography. Papers are presented in chronological order to emphasize the evolution of the approach and continued importance of the application.

Pulsed electrochemical detection (PED) following liquid chromatographic separation has been applied to the indirect determination of amino acids and proteins. Here, the adsorption of these analytes at noble metal electrodes is exploited to suppress the oxidation of polyols and carbohydrates under alkaline conditions to elicit an indirect response. Of the reagents tested, gluconic acid gave the best overall signal-to-noise values for the indirect detection of amino acids following high performance anion-exchange chromatography (HPAEC). Limits of detection of amino acids were found to be 2–30 pmol using optimized potential–time waveforms at an Au electrode. Indirect PED provided much greater detection sensitivity toward amino acids than direct PED. Analytical sensitivity of indirect PED is a function of both the analyte's ability to adsorb to the electrode surface and its molecular size, which was demonstrated by the separation and detection of bovine serum albumin, ovalbumin, and myoglobin following gel-filtration chromatography (GFC).

Detection of a drug and its glucuronide metabolite(s) is of great importance in interpretive forensic and clinical toxicology. Until recently, glucuronides were determined by cleavage of the glucuronide with an enzyme (e.g., β-glucuronidase) to yield the parent compound, which was subsequently detected, or via derivatization to a more volatile or detectable analogue. Direct detection of the glucuronide conjugates overcomes the critical limitations of approaches that involve enzymatic cleavage procedures and/or derivatization. This review will focus on direct methods to determine glucuronides of various potentially abused drugs in human biological matrices. More specifically, this review will be restricted to methods that involve liquid chromatography (LC) coupled with various detectors. Papers are presented in chronological order for each compound class to emphasize the evolution of methodology and continued importance of the application.

A direct, versatile method for the determination of ethyl glucuronide (EtG), a biomarker of ethanol consumption, in urine has been developed using reversed-phase liquid chromatography with pulsed electrochemical detection (PED). EtG and methyl glucuronide (MetG), which serves as an internal standard, are readily separated using a mobile phase consisting of 1% acetic acid/acetonitrile (98/2, v/v). Post-column addition of NaOH allows for the detection of all glucuronides using PED at a gold working electrode. Upon optimization, EtG was found to have a limit of detection of 0.03 μg/mL (7 pmol; 50 μL injection volume) and repeatability at the limit of quantitation of 1.7%R.S.D. (relative standard deviation). Solid-phase extraction (SPE) using an aminopropyl phase was used to remove interferents in urine samples prior to their analysis. Compound recovery following SPE was approximately 50 ± 2%. The forensic utility of this method was further validated by the analysis of 29 post-mortem urine specimens, whose results agreed strongly with certified determinations.

Pulsed electrochemical detection (PED) following reversed-phase liquid chromatography (LC) has been applied recently to the detection of ethyl glucuronide (EtG) in the urine of live and deceased individuals. In this paper, several key improvements to the method are made to enhance sensitivity, reproducibility, and accuracy. These improvements include (i) further optimization of the sample preparation procedure that has increased the recovery from ca. 50% to 84 ± 3% in synthetic urine matrix; (ii) changing the internal standard from methyl glucuronide (MetG) to propyl glucuronide (ProG), which does not elute within the interference of the matrix; and (iii) altering the mobile phase of the separation from acetonitrile to t-butanol to virtually eliminate signal suppression in PED. As a consequence, detection limits have been reduced to 0.01 μg mL−1, reproducibility has been improved by a factor of two, and sample size has been reduced five-fold. Blind studies in synthetic urine showed no significant difference between the amount recovered and the true value determined at the 95% confidence level for all samples. Importantly, PED requires no derivatization, and it can detect virtually all glucuronides.

Pulsed electrochemical detection (PED) following liquid chromatographic separation has been applied to the direct (i.e., without derivatization) determination of two major sulfur-containing compounds used as pharmaceutical additives, isopropyl-thio-β-d-galactopyranoside (IPTG) and monothioglycerol (MTG). Limits of detection of IPTG and MTG were found to be 1 ppb (0.2 pmol, 50 μL) and 0.2 ppb (0.1 pmol, 50 μL), respectively, using optimized potential-time waveforms applied to a Au electrode. In addition to high sensitivity as compared to optical detection, the simultaneous detection of free thiols and disulfides can be used to study the kinetics of these conversions, as is shown for MTG. A practical application of HPLC–PED is demonstrated in determining MTG in a pharmaceutical formulation. The high selectivity of PED for thiocompounds reduces sample preparation and produces simpler chromatograms in a variety of matrices.

High-performance liquid chromatography with photo-assisted electrochemical detection (HPLC–PAED) is used in conjunction with ultraviolet absorbance (UV) detection for determining explosives in environmental samples. The system utilizes an on-line solid-phase extraction technique for sample pretreatment (i.e., fractionation and concentration), thus reducing the required ground water sample size from 1 L to 2 mL and minimizing sample handling. Limits of detection for explosives using solid-phase extraction and PAED range from 0.0007 to 0.4 μg/L, well below those achieved with UV detection for several important explosives (e.g., RDX). The method has demonstrated good accuracy, precision, and recovery for all tested explosives, thus proving that the method is suitable for evaluation of explosives in ground water with competitive advantages over the U.S. Environmental Protection Agency (EPA) Method 8330. A system adaptable for the on-site environmental analysis of explosives has been developed and validated.