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.

Porous layer open tubular nano liquid chromatography directly coupled to electron ionization mass spectrometry.

A 25 µm i.d x 1.2 m length PS-DVB porous layer open tubular column (PLOT) was prepared and assessed in the configuration of a nano liquid chromatography coupled to an electron ionization mass spectrometry system (OT-nanoLC-EI-Ms), via the direct insertion of the column outlet into the ionization source. The developed system's operational parameters were comprehensively studied, and the setup performance was investigated employing both unidimensional and column switching configurations. As a result, the OT-nanoLC-EI-MS system demonstrated competitive applicability in separating non-amenable ESI compounds, such as polyaromatic hydrocarbons (PAHs) and non-amenable GC compounds such as thermolabile pesticides. Furthermore, with excellent chromatographic performance, the PLOT columns can work under more compatible EI-detection conditions - such as the elution with 100% organic solvent. For example, PAHs retention factors ranged between 1.5 and 2.2 for 100% MeCN mobile phase, and more than 33,000 plates per meter for naphthalene at 50 nL/min flow rate. In analyzing thermolabile pesticides, the column switching PLOT-nanoLC-EI-MS system provided LODs of 25 µg/L, demonstrating suitable intra e interday reproducibility (% RSD < 13%, n = 3), and possibilities the direct injection of raw samples with suitable robustness.

An overview of open tubular liquid chromatography with a focus on the coupling with mass spectrometry for the analysis of small molecules.

Open tubular liquid chromatography (OT-LC) can provide superior chromatographic performance and more favorable mass spectrometry (MS) detection conditions. These features could provide enhanced sensitivity when coupled with electrospray ionization sources (ESI-) and lead to unprecedented detection capabilities if interfaced with a highly structural informative electron ionization (EI) source. In the past, the exploitation of OT columns in liquid chromatography evolved slowly. However, the recent instrumental developments in capillary/nanoLC-MS created new opportunities in developing and applying OT-LC-MS. Currently, the analytical advantages of OT-LC-MS are mainly exploited in the fields of proteomics and biosciences analysis. Nevertheless, under the right conditions, OT-LC-MS can also offer superior chromatographic performance and enhanced sensitivity in analyzing small molecules. This review will provide an overview of the latest developments in OT-LC-MS, focusing on the wide variety of employed separation mechanisms, innovative stationary phases, emerging column fabrication technologies, and new OT formats. In the same way, the OT-LC's opportunities and shortcomings coupled to both ESI and EI will be discussed, highlighting the complementary character of those two ionization modes to expand the LC's detection boundaries in the performance of targeted and untargeted studies.

Miniaturized liquid chromatography focusing on analytical columns and mass spectrometry: A review.

The technological advances achieved over the last decades boosted the development of suitable benchtop platforms to work at miniaturized liquid chromatography scale (capillary and nano-LC). Under the right conditions, miniaturized LC can offer higher analysis efficiency resulting in superior chromatographic resolution and overall sensitivity than conventional LC. Among the main advantages are the reduced reagents and sample requirement, the decreasing on analytical column dimensions, and consequently flow rates and the easer coupling to mass spectrometry. This review describes fundamental aspects and advances over miniaturized LC technology with a focus on the last decade. Therefore, relevant characteristics of the most common analytical column, covering both filled (packed and monolithic) and open tubular (PLOT and WCOT) columns, are herein discussed. Alternatively, other modern approaches based on microchip separations or 2D configurations aiming for the sample preparation on the first dimension, are also introduced. Likewise, some positive and negative aspects of these systems over HPLC are underscored. Besides, considering the necessity to developed components to work at capillary or nanoscale, without significant dead-volumes, the most critical features of specially designed instrumentation for benchtop instruments are briefly discussed highlighting connectors, pumping, injections, oven and detection systems. Also, a more detailed section is presented focused on mass spectrometry efforts towards its miniaturization and how this trend can be useful working together with miniaturized LC. Finally, applications of capillary and nano-LC involving bioanalytical, environmental, and food methods are discussed to support the miniaturized LC as a powerful and emergent separation technique for the years ahead.