Vitamin D has a well-known role in the calcium homeostasis associated with the maintenance of healthy bones. It increases the efficiency of the intestinal absorption of dietary calcium, reduces calcium losses in urine, and mobilizes calcium stored in the skeleton. However, vitamin D receptors are present ubiquitously in the human body and indeed, vitamin D has a plethora of non-calcemic functions. In contrast to most vitamins, sufficient vitamin D can be synthesized in human skin. However, its production can be markedly decreased due to factors such as clothing, sunscreens, intentional avoidance of the direct sunlight, or the high latitude of the residence. Indeed, more than one billion people worldwide are vitamin D deficient, and the deficiency is frequently undiagnosed. The chronic deficiency is not only associated with rickets/osteomalacia/osteoporosis but it is also linked to a higher risk of hypertension, type 1 diabetes, multiple sclerosis, or cancer. Supplementation of vitamin D may be hence beneficial, but the intake of vitamin D should be under the supervision of health professionals because overdosing leads to intoxication with severe health consequences. For monitoring vitamin D, several analytical methods are employed, and their advantages and disadvantages are discussed in detail in this review.

Chromatographic methods have become popular in clinical analysis in both routine and research laboratories. The purpose of this review article is to provide an overview of the current state of chromatographic methods, i.e., high-performance liquid chromatography (HPLC), gas chromatography (GC), and supercritical fluid chromatography (SFC), in clinical analysis. The aspects related to method sensitivity, selectivity, analysis time, and throughput have been discussed in detail. Adequate solutions to improve these features have also been presented. HPLC is the most widely used method among the chromatographic methods, whereas GC is dedicated to several specific applications, and SFC is used only marginally certainly due to its only recent comeback to the analytical scene. Based on the literature search, the application fields in clinical analysis are divided into the following groups: drugs, hormones, drugs of abuse, metabolomics, lipidomics, volatile organic compounds, biomarkers and endogenous compounds, proteomics, multi-analyte approches, and others. The important features of these applications have been emphasized.

Plants and food byproducts contain high-value bioactive compounds like carotenoids, tocopherols, flavonoids, and other phenolic compounds that can potentially be extracted prior to usage of the lower-ranked biomass, all in line with a circular bioeconomy. Extraction is typically performed by leaching using an organic solvent, or alternatively with more advanced techniques such as supercritical fluid extraction (SFE), pressurized liquid extraction (PLE), microwave assisted extraction (MAE) or ultrasound assisted extraction (UAE). With the exception of SFE, mass transfer is enhanced and solvent consumption is minimised by the use of elevated temperature, which has a negative impact on the extraction yield and structural preservation for thermally labile bioactive compounds. Hence, in this study we aim to explore the use of CO₂ in extraction solvent mixtures as a viscosity-lowering entrainer. Furthermore, we aim to disregard the instrumental boundaries between SFE and PLE by extending the design space for extraction method development, using solvent mixtures of pressurized carbon dioxide, water, and either ethanol or ethyl lactate, covering a uniquely large area of the solvent's relative permittivity. Such a strategy will inherently enable extraction method optimisation for target compounds differing a lot in polarity, here demonstrated for β-carotene, α-tocopherol, and quercetin in sea buckthorn berry pomace. Our results show that by the use of experimental design, optimum extraction conditions were found for each of the three compounds: supercritical CO₂ with 10 vol% of ethanol at 300 bar and 80 °C for β-carotene, liquid CO₂/ethanol/water (50/44/6, v/v/v) at 210 bar and 60 °C for α-tocopherol, and liquid CO₂/ethyl lactate/water (33/54/13, v/v/v) at 200 bar and 62 °C for quercetin. Extraction rates were fast in all three cases, giving complete extractions within less than 20 min at the highest flow rate explored, 3 mL min⁻¹. The developed extraction methods gave significantly higher extraction yields than published optimised extraction methods based on PLE. Moreover, the optimized methods for the less polar compounds, β-carotene, and α-tocopherol, enabled a significantly lower consumption of organic solvents (30–52%), which makes them greener in comparison with the more conventional PLE extraction methods.

Within this review, we thoroughly explored supercritical fluid chromatography (SFC) columns used across > 3000 papers published from the first study carried out under SFC conditions in 1962 to the end of 2022. We focused on the open tubular capillary, packed capillary, and packed columns, their chemistries, dimensions, and trends in used stationary phases with correlation to their specific interactions, advantages, drawbacks, used instrumentation, and application field. Since the 1990s, packed columns with liquid chromatography and SFC‐dedicated stationary phases for chiral and achiral separation are predominantly used. These stationary phases are based on silica support modified with a wide range of chemical moieties. Moreover, numerous unconventional stationary phases were evaluated, including porous graphitic carbon, titania, zirconia, alumina, liquid crystals, and ionic liquids. The applications of unconventional stationary phases are described in detail as they bring essential findings required for further development of the supercritical fluid chromatography technique.