On-line in-tube solid phase microextraction coupled to capillary liquid chromatography-diode array detection for the analysis of caffeine and its metabolites in small amounts of biological samples

Open tubular liquid chromatography: Recent advances and future trends

Nano-liquid chromatography (nanoLC) is gaining significant attention as a primary analytical technique across various scientific domains. Unlike conventional high-performance LC, nanoLC utilizes columns with inner diameters (i.ds.) usually ranging from 10 to 150 μm and operates at mobile phase flow rates between 10 and 1000 nl/min, offering improved chromatographic performance and detectability. Currently, most exploration of nanoLC has focused on particle-packed columns. Although open tubular LC (OTLC) can provide superior performance, optimized OTLC columns require very narrow i.ds. (< 10 μm) and demand challenging instrumentation. At the moment, these challenges have limited the success of OTLC. Nevertheless, remarkable progress has been made in developing and utilizing OTLC systems featuring narrow columns (< 2 μm). Additionally, significant efforts have been made to explore larger columns (10-75 μm i.d), demonstrating practical applicability in many situations. Due to their perceived advantages, interest in OTLC has resurged in the last two decades. This review provides an updated outlook on the latest developments in OTLC, focusing on instrumental challenges, achievements, and advancements in column technology. Moreover, it outlines selected applications that illustrate the potential of OTLC for performing targeted and untargeted studies.

The association of salivary caffeine levels with serum concentrations in premature infants with apnea of prematurity

The purpose of this paper is to verify whether the concentrations of caffeine in saliva are comparable to serum concentrations in preterm infants who are treated with caffeine for apnea of prematurity. This is a prospective observational study. Eligible participants were newborn infants < 37 weeks of gestational age treated with oral or intravenous caffeine for apnea of prematurity. Two paired samples of saliva and blood were collected per patient. Tube solid-phase microextraction coupled online to capillary liquid chromatography with diode array detection was used for analysis. A total of 47 infants with a median gestational age of 28 [26-30] weeks and a mean of 1.11 ± 0.4 kg of birth weight. Median postmenstrual age, when samples were collected, was 31 [29-33] weeks. Serum caffeine median levels of 19.30 μg/mL [1.9-53.90] and salivary caffeine median levels of 16.36 μg/mL [2.20-56.90] were obtained. There was a strong positive Pearson's correlation between the two variables r = 0.83 (p < 0.001).

CONCLUSION

The measurement of salivary caffeine concentrations after intravenous or oral administration offers an alternative to serum caffeine monitoring in apnea of prematurity. Measurement of salivary concentration minimizes blood draws, improves blood conservation, and subsequently minimizes painful procedures in premature infants.

WHAT IS KNOWN

• Salivary sampling may be useful when is applied to extremely low birth weight infant, in whom blood sampling must be severely restricted.

WHAT IS NEW

• The measurement of caffeine salivary concentrations after intravenous or oral administration offers an alternative to serum caffeine monitoring in apnoea of prematurity. • Salivary sampling may be a valid non-invasive alternative that could be used to individualize and optimize caffeine dose.

Amr AEG, Almehizia AA, Naglah AM, H. Kamel A. New Potentiometric Screen-Printed Platforms Modified with Reduced Graphene Oxide and Based on Man-Made Imprinted Receptors for Caffeine Assessment

Caffeine is a psychoactive drug that is administered as a class II psychotropic substance. It is also considered a component of analgesics and cold medicines. Excessive intake of caffeine may lead to severe health damage or drug addiction problems. The assessment of normal caffeine consumption from abusive use is not conclusive, and the cut-off value for biological samples has not been established. Herein, new cost-effective and robust all-solid-state platforms based on potentiometric transduction were fabricated and successfully utilized for caffeine assessment. The platforms were modified with reduced graphene oxide (rGO). Tailored caffeine-imprinted polymeric beads (MIPs) based on methacrylic acid (MAA) and ethylene glycol dimethacrylate (EGDMA) were prepared, characterized, and used as recognition receptors in the presented potentiometric sensing devices. In 50 mM MES buffer, the sensors exhibited a slope response of 51.2 ± 0.9 mV/decade (n = 6, R2 = 0.997) over the linear range of 4.5 × 10−6−1.0 × 10−3 M with a detection limit of 3.0 × 10−6 M. They exhibited fast detection of caffeinium ions with less than 5 s response time (<5 s). The behavior of the presented sensors towards caffeinium ions over many common organic and inorganic cations was evaluated using the modified separate solution method (MSSM). Inter-day and intra-day precision for the presented analytical device was also evaluated. Successful applications of the presented caffeine sensors for caffeine determination in commercial tea and coffee and different pharmaceutical formulations were carried out. The data obtained were compared with those obtained by the standard liquid chromatographic approach. The presented analytical device can be considered an attractive tool for caffeine determination because of its affordability and vast availability, particularly when combined with potentiometric detection.

Determination of caffeine in dietary supplements by miniaturized portable liquid chromatography

In this study three miniaturized liquid chromatography (LC) instruments have been evaluated and compared for the analysis of caffeine in dietary supplements, namely a benchtop capillary LC (capLC) system, a benchtop nano LC (nanoLC)system and a portable LC system. Commercial products derived from different sources of caffeine have been analyzed. Under optimized conditions, the methods based on benchtop systems were superior in terms of sensitivity. The limits of detection (LODs) found with the capLC and nanoLC systems were 0.01 and 0.003 µg mL^-1, respectively, whereas the LOD obtained with the portable LC instrument was of 1 µg mL^-1. The portable LC-based method was superior in terms of simplicity and throughput (total analysis time < 15 min). On the basis of the results obtained, a new method for the rapid measurement of caffeine in dietary supplements by portable miniaturized LC is presented. This method provided good linearity within the 1-20 µg mL^-1 interval, and it allowed the quantification of caffeine even in products derived from decaffeinated green coffee extracts. The contents of caffeine found with the proposed portable LC method in the real samples analyzed ranged from 1.38 to 7 mg per gram of product, which were values statistically equivalent to those found with the benchtop capLC and nanoLC methods, being the precision, expressed as relative standard deviation (RDS), of 2 -14% (n = 3). The proposed portable LC based method can be used as a simple and rapid alternative to estimate the quality, effectiveness and safety of dietary supplements, regarding their caffeine content.

UPLC-MS/MS method for the simultaneous quantification of caffeine and illicit psychoactive drugs in hair using a single-step high-speed grinding extraction - Insights into a cut-off value for caffeine abuse

Caffeine is a commonly consumed psychoactive substance whose addictive potential has long been reported. Excessive caffeine intake may lead to severe health damage or drug addiction problems; however, studies on the surveillance of caffeine abuse by the Chinese population are lacking. This study aimed to propose a concentration value for caffeine based on hair analysis to distinguish excessive intake from normal consumption, and provide an analytical tool for forensic toxicology investigations of caffeine and other frequently abused drugs. A sensitive and accurate ultra-high performance liquid chromatography-tandem mass spectrometry method was developed to detect caffeine and 13 illicit drugs and their metabolites in hair. Thereafter, this method was employed to test 479 real samples. Briefly, the hair samples were washed with water and acetone, and subsequently extracted by one-step high-speed grinding with acetonitrile-buffer solution. The lower limit of quantifications of 0.05 ng/mg for caffeine and THC, and 0.005 ng/mg for others, were achieved for all substances. The results revealed a mean caffeine concentration of 0.78 (range 0.008-3.5 ng/mg) based on 24 healthy volunteers, 55.0 (range 3.07-292.2 ng/mg) based on 52 self-reported caffeine abuse participants, and 5.78 (range 0-140.34 ng/mg) based on 403 drug addicts. The mean caffeine concentration in hair from self-reported caffeine abusers was 70-fold higher than that in hair from healthy volunteers. A tentative cut-off level of 5.5 ng/mg as an indicator of excessive caffeine consumption was developed based on receiver operating characteristic analysis. Additionally, the assessment of 403 hair samples from drug addicts indicated that illicit drug abusers had potential for caffeine abuse, especially polydrug users. This hair analysis method serves as a useful tool for the large-scale surveillance of caffeine and illicit drug abuse.

Thin film molecularly imprinted polymer (TF-MIP), a selective and single-use extraction device for high-throughput analysis of biological samples

Enhancing selectivity, reducing matrix effects and increasing analytical throughput have been the main objectives in the development of biological sample preparation techniques. A thin film molecularly imprinted polymer (MIP) is employed for extraction and analysis of tricyclic antidepressants (TCAs) as a model class of compounds in human plasma for the first time to reach the abovementioned goals. The thin film MIPs prepared on a metal substrate can be used directly for extraction from biological matrices with no sample manipulation steps and no pre-conditioning. This method was validated with good linearity (R² > 0.99 in 1.0-500.0 ng mL^-1 range), excellent accuracy (90% -110%) and precision (RSD % value less than 15%) in pooled human plasma samples (N = 3). The limits of quantitation (LOQ) for TCAs in plasma samples were between 1.0-5.0 ng mL^-1 which are lower than the therapeutic ranges of these drugs. Kinetic and isotherm studies showed the superior performance of MIP sorbent compared to a non-imprinted polymer (NIP) sorbent in extracting TCAs from a bovine serum albumin (BSA) solution. The optimized and validated method for pooled human plasma was utilized for monitoring the concentration of TCAs in three patient samples who had been prescribed TCAs. These selective single-use thin film extraction devices are promising for efficient and fast procedures for analyzing biological samples.

Recent applications of microextraction sample preparation techniques in biological samples analysis

Analysis of biological samples is affected by interfering substances with chemical properties similar to those of the target analytes, such as drugs. Biological samples such as whole blood, plasma, serum, urine and saliva must be properly processed for separation, purification, enrichment and chemical modification to meet the requirements of the analytical instruments. This causes the sample preparation stage to be of undeniable importance in the analysis of such samples through methods such as microextraction techniques. The scope of this review will cover a comprehensive summary of available literature data on microextraction techniques playing a key role for analytical purposes, methods of their implementation in common biological samples, and finally, the most recent examples of application of microextraction techniques in preconcentration of analytes from urine, blood and saliva samples. The objectives and merits of each microextration technique are carefully described in detail with respect to the nature of the biological samples. This review presents the most recent and innovative work published on microextraction application in common biological samples, mostly focused on original studies reported from 2017 to date. The main sections of this review comprise an introduction to the microextraction techniques supported by recent application studies involving quantitative and qualitative results and summaries of the most significant, recently published applications of microextracion methods in biological samples. This article considers recent applications of several microextraction techniques in the field of sample preparation for biological samples including urine, blood and saliva, with consideration for extraction techniques, sample preparation and instrumental detection systems.

In-tube solid-phase microextraction: Current trends and future perspectives

In-tube solid-phase microextraction (IT-SPME) was developed about 24 years ago as an effective sample preparation technique using an open tubular capillary column as an extraction device. IT-SPME is useful for micro-concentration, automated sample cleanup, and rapid online analysis, and can be used to determine the analytes in complex matrices simple sample processing methods such as direct sample injection or filtration. IT-SPME is usually performed in combination with high-performance liquid chromatography using an online column switching technology, in which the entire process from sample preparation to separation to data analysis is automated using the autosampler. Furthermore, IT-SPME minimizes the use of harmful organic solvents and is simple and labor-saving, making it a sustainable and environmentally friendly green analytical technique. Various operating systems and new sorbent materials have been developed to improve its extraction efficiency by, for example, enhancing its sorption capacity and selectivity. In addition, IT-SPME methods have been widely applied in environmental analysis, food analysis and bioanalysis. This review describes the present state of IT-SPME technology and summarizes its current trends and future perspectives, including method development and strategies to improve extraction efficiency.

Exploring hand-portable nano-liquid chromatography for in place water analysis: Determination of trimethylxanthines as a use case

Analytical performance and optimization of figures of merit of a portable nano liquid chromatograph (NanoLC) with UV detection at 255 nm have been established for in place analysis. Methylxanthines: caffeine, theophylline and theobromine were selected as target analytes. A fast lab method based on IT-SPME coupled on line with capillary liquid chromatograph (CapLC) with diode array detection (DAD) was employed for comparative studies. IT-SPME and solid phase extraction were coupled off-line to NanoLC for improving instrumental parameters, mainly detection capacity and selectivity. IT-SPME or SPE/portable NanoLC based methods were superior in terms of chromatographic resolution and organic solvent consumption per sample, around 200 μL vs 10 mL for IT-SPME-CapLC-DAD. Limits of detection (LODs) obtained with the SPE/portable NanoLC were 2-10 ng/mL, which can be suitable for testing the presence of the analytes in several environmental waters in the field. As predictable, the lab method provided better LODs, between 0.1 and 0.5 ng/mL. Good linearity was achieved for both methods and precision was similar for them (≤7%). Both systems were tested for the analysis of real water samples with suitable results.

Determination of 4 psychoactive substances in tea using ultra high performance liquid chromatography combined with the quadrupole time-of-flight mass spectrometry

In this study, a method for the qualification and quantification of 4 psychoactive substances in tea using ultra high performance liquid chromatography coupled with the quadrupole time-of-flight mass spectrometry (UPLC-Q-TOF/MS) has been developed. Tea samples were extracted by a 50% (v/v) methanol-water solution, and then separated by an ACQUITY UPLC BEH Shield RP18 column using a binary solvent system by gradient elution. The analytes were determined by Q-TOF/MS in TOFMS and information-dependent acquisition (IDA)-MS/MS mode. The results showed that the mass accuracy error of the 4 psychoactive substances were lower than 5.0 × 10-6, and a good linear relationship was observed in the range of 0.5-500 μg L-1 and correlation coefficient was higher than 0.9990. The LOD was in the range of 0.005-0.020 mg kg-1 and the LOQ was in the range of 0.010-0.040 mg kg-1. The recovery of the method was in range of 80.14-93.25% with spike levels of 0.010-0.400 mg kg-1, and relative standard deviations were lower than 10%. The method was simple, specific and reliable. It has been successfully used for the detection of 4 psychoactive substances in tea samples.

Innovations in Extractive Phases for In-Tube Solid-Phase Microextraction Coupled to Miniaturized Liquid Chromatography: A Critical Review

Over the past years, a great effort has been devoted to the development of new sorbents that can be used to pack or to coat extractive capillaries for in-tube solid-phase microextraction (IT-SPME). Many of those efforts have been focused on the preparation of capillaries for miniaturized liquid chromatography (LC) due to the reduced availability of capillary columns with appropriate dimensions for this kind of system. Moreover, many of the extractive capillaries that have been used for IT-SPME so far are segments of open columns from the gas chromatography (GC) field, but the phase nature and dimensions are very limited. In particular, polar compounds barely interact with stationary GC phases. Capillary GC columns may also be unsuitable when highly selective extractions are needed. In this work, we provide an overview of the extractive capillaries that have been specifically developed for capillary LC (capLC) and nano LC (nanoLC) to enhance the overall performance of the IT-SPME, the chromatographic separation, and the detection. Different monolithic polymers, such as silica C₁₈ and C₈ polymers, molecularly imprinted polymers (MIPs), polymers functionalized with antibodies, and polymers reinforced with different types of carbon nanotubes, metal, and metal oxide nanoparticles (including magnetic nanoparticles), and restricted access materials (RAMs) will be presented and critically discussed.

Bioanalytical HPLC Applications of In-Tube Solid Phase Microextraction: A Two-Decade Overview

In-tube solid phase microextraction is a cutting-edge sample treatment technique offering significant advantages in terms of miniaturization, green character, automation, and preconcentration prior to analysis. During the past years, there has been a considerable increase in the reported publications, as well as in the research groups focusing their activities on this technique. In the present review article, HPLC bioanalytical applications of in-tube SPME are discussed, covering a wide time frame of twenty years of research reports. Instrumental aspects towards the coupling of in-tube SPME and HPLC are also discussed, and detailed information on materials/coatings and applications in biological samples are provided.