Phytochemical Contents of Essential Oils from Cymbopogon Species: A Tropical Medicinal Plant

Abstract

Natural resources especially medicinal plants possess the potentials to sustain all existence on earth. Cymbopogon, a globally cultivated herb, possesses high contents of diverse essential oils for medicinal and economic purposes including treatment of malaria and candidiasis. Notable species include Cymbopogon citratus and C. flexosus having citral as the main chemical compound. Numerous compounds of these species include limonene, citronella, geranyl acetic derivatives, elemol, among others. Phytochemical analysis of these essential oils is usually done by the gas chromatography-mass spectrometry (GC-MS) method sequel to obtaining them through solvent extraction, hydrodistillation, supercritical CO2 extraction, chromatography among others. Although the supercritical CO2 extraction method gives greater quality yields void of toxic wastes with preserved thermal stability compared with other methods, its high-working pressure generates issues of safety risks and costs. Quantitative determination is done using spectrophotometric, chromatographic, and Folin-Ciocalteu methods. In comparison with other chromatographic techniques employed, gas chromatography exhibits greater efficiency by quantifying and determining the presence of various components at low concentrations. This prominently economical plant with potent ethnobotanical benefits hinged on the essential oils phytochemicals is faced with diverse extraction challenges; thus, improvement in the extraction and quantification techniques is key to the harvest of pure yields of lemon grass essential oils.

1. Introduction

Medicinal plants play an important role in a healthy society. Restoration of practices and knowledge related to medicinal plant resources is part of an important strategy related to biodiversity conservation, knowledge of new drugs and improving the living standards of rural populations [1]. The Gramineae family includes the genus Cymbopogon, which encapsulates herbs that are globally recognized for possessing high essential oil content. Its species are broadly distributed across the globe where they are utilized for diverse purposes. Both the commercial and medicinal uses of its differential species have been well authenticated [2].

Additionally, the ethnopharmacology corroboration reveals the presence of an expansive array of properties possessed by these species, which establishes their utilization for pest control for cosmetics and anti-inflammatory media. Species of Cymbopogon may also envelope potentials as potent antitumor and chemopreventive drugs [3]. Cymbopogon flexuosus and Cymbopogon citratus are the two main species vastly farmed for their essential oils in various parts of the world [4]. It is cultivated in the subtropical and tropical regions of the world and widely used in the agriculture, cosmetics, flavor, food, pharmaceutical industries [1]. It is a member of the aromatic grasses containing essential oils with lemon flavor. Its species are tufted perennial C4 grasses with several hard stems emerging from a short, rhizomatous base [5] with a citrus flavor, dried to powder or freshly used. The name Cymbopogon is derived from the Greek words “kymbe” (boat) and “pogon” (beard), referring to the flower spike arrangement [5]. The species C. citratus is identified by many international common names, such as West Indian lemon grass or lemon grass (English), citronelle or verveine des indes (French), hierba limon or zacate de limón (Spanish), xiang mao (Chinese), capimcidrao, or capim-santo (Portuguese), and locally, there are more than 28 indigenous names identified from different countries of the world [4]. Other common names of C. citratus include lemongrass, barbed wire grass, citronella grass, fever grass, and tanglad [6]. C. citratus thrives best in sunny, warm, humid conditions of the tropics and grown in a wide range of soil types, from rich loam to poor laterite. Although calcareous and water-logged soils adversely affect growth [7], those cultivated on sandy soils have higher leaf oil yields and higher citral content [8]. C. citratus is believed to have originated from Malaysia, and it is now widely grown in Central and South America, regions in Africa, Southeast Asia, and the Indian Ocean Islands, both on subsistence and commercial scales particularly in South-east Asia. It is an aromatic, evergreen, perennial grass that produces multiple stiff stems emerging from a short rhizome-like rootstock and grows to approximately 1.5 m tall. Although it rarely produces florets, the leaflets are blue-green, erect, and linear and exude a characteristic lemon flavor when crushed [9]. The C. citratus is positioned as one of the most globally distributed genera that are usually utilized in all parts of the globe [3]. The plant, which can be dried and powdered, or used fresh, has been employed in diverse activities that include food flavoring, in teas, soups, with poultry, fish, beef, seafood, and curries. Reports have validated its global diverse health benefits, including the fact that lemongrass leaves and other parts can be infused to treat nausea, stomach aches, constipation, and a variety of stomach infections as well as to prevent ulcers [4]. C. flexuosus (Poaceae) is described as a native, tall perennial aromatic grass (sweet smelling sedge) with growth confined to specific patches of subtropical parts of Asia, Africa, and America. Cymbopogon flexuous, also known as the Cochin or Malabar grass, is native to Sri Lanka, India, Thailand, and Burma. It is naturalized in numerous parts of the tropical and subtropical Southeast Asia and Africa [10, 11]. Consequently, it has received significant global demand due its varied range of applications in differential industries. Reports reveal that Cymbopogon flexosus include more than 140 with 52 of them growing in Africa, 45 in India, 6 in Australia, 6 in South America, 4 in Europe (only in Montenegro), 2 in North America, and the others in South Asia. It is utilized as a medicinal tea, in preparation of soups, curries, and starting agent for vitamin A synthesis and has been known to be both perfumery and flavorful with the therapeutic characteristics [10]. C. flexosus is a C₄ grass endowed with industrial importance and abundant medicinal properties, and utilized for its essential oil (EO) production [11]. India is a significant exporter and the major producer of lemongrass oil. The essential oil comprises citral (i.e., a mixture of neral and geranial), geraniol, limonene, and geranyl acetate among others and is well-recognized for their antimicrobial, anticancer, and allelopathic activities [11, 12]. These essential oils are employed in the production of eco-friendly pesticides [3, 11]. In addition, lemongrass is an important source of several vitamins (A, B1, B2, B3, B5, B6, folate, and vitamin C) and essential minerals (calcium, copper, iron, magnesium, manganese, potassium, phosphorous, zinc) [11]. The above-listed properties cause lemongrass to be an industrially preferred crop due to its enormous potential in the fields of medicine, cosmetics, food, and biotechnology [13]. In furtherance, a couple of studies published that lemongrass essential oil can be utilized as biofuel; thus, it is regarded as an energy grass [14].

2. Phytochemicals of essential oils of Cymbopogon species

The essential oils of Cymbopogon are identified by monoterpene constituents including citral, limonene, geraniol, citronellol, elemol, b-carophyllene, citronellal, 1,8 cineole, linalool, methylheptenone, geranylformate, and geranyl acetic acid derivation. Essential oils are typically chemically characterized by GC-MS [1, 15, 16, 17]. The plant C. citratus is abundant in bioactive substances. Flavonoids, alkaloids, saponins, tannins, and phenolic compounds, such as quercetin, luteolin, apiginin, isoorientin 2’-O-rhamnoside, and kaempferol, have been isolated and identified from the plant’s leaves [18, 19]. These phytochemicals have been reported to be beneficial, especially in the pharmaceutical, food, health, and agricultural industries [20, 21]. Alcohols, aldehyde, ketones, esters, and terpenes are predominantly the other compounds found in C. citratus [20]. It also consists of 1–2 percent essential oil on a dry basis with the chemical composition varying greatly depending on the habitat, genetic diversity, and agricultural treatment of the crops. Longifolene (V4) (56.67%) and selina-6-en-4-ol (20.03%) are the constituents of volatile oil from the roots [22]. Although the primary chemical constituent of lemongrass essential oil is citral, borneol, geranial, geraniol, β-myrcene, limonene, neral, geranyl acetate, alpha-terpeniol, estragole, methyleugenol, citronellal, careen-2, farnesol, (+)-cymbodiacetal, proximadiol, methyl heptenone, terpinolene, pinene, linalool, linalyl acetate, and β-caryophyllene have also been reported [5, 22]. Citral (3, 7-dimethyl-2, 6-octadien-2-al) refers to the natural mixture of two isomeric acyclic monoterpene aldehydes, that is, geranial (citral A or trans citral) and neral (citral B or ciscitral) [20], which have same molecular formula (C₁₀H₁₆O) but different structures [23, 24, 25, 26]. The various components of C. flexosus are significantly recognized due to the high concentration of aromatic essential oil, which contain many secondary metabolites, particularly monoterpenes (citral) and sesquiterpenes (caryophyllene) [11, 27]. Lemongrass is used in a variety of traditional Asian dishes and beverages, and also in high-end perfumes, pharmaceuticals, and biomedical applications [28]. The antibacterial, insecticide, larvicide, antitumoral, and cytotoxic characteristics of C. flexuosus’ essential oil make it popular in alternative medicine [20]. The main constituents of the essential oil of C. flexuosus are Z-citral (-citral), geraniol, and -geranial (-citral), with citral contributing significantly to the oil’s antibacterial properties.

3. Economic importance of essential oils of Cymbopogon

C. citratus Stapf. (lemongrass) is a spice commonly used in tropical regions, particularly in Southeast Asia. The primary compounds identified in C. citratus essential oil include α-citral, β-citral, geraniol, nerol, citronellal, myrcene, terpinolene, geranyl acetate, and terpinol methylheptenone. Terpenes, alcohols, ketones, and certain flavonoids and phenolics have also found in the plant [29]. Scientific research has described the antibacterial, anticarcinogenic, anti-inflammatory, antifungal, antioxidant, antiprotozoal, antirheumatic and cardioprotective effects of C. citratus [30, 31, 32]. It has shown a marked suppression of fungal infections including athlete’s foot, itching, ringworm, and yeast infections and has a synergistic effect by suppressing the growth of filamentous fungi by inactivating yeast cells [6, 33]. Citral, myrcene, and citronellal are secondary metabolites that have been isolated from lemongrass and characterized as antimalarials. They showed remarkable activity against Plasmodium sp. [34]. In HIV/AIDS patients, oral candidiasis brought on by Candida albicans has been demonstrated to be successfully treated with lemongrass essential oil in 1 to 5 days [35].

LGEO’s pharmaceutical potential has been reported in rodents in a well-designed trial involving oral administration of EO’s key ingredient, citral, in combination with the nonsteroidal anti-inflammatory drug naproxen to experimental rats. The combination of naproxen and citral showed comparable anti-inflammatory effects compared with naproxen alone, but with much less stomach adverse effects [36]. Citral from C. citratus is used as an additive in creams and ointments to treat local inflammation as it significantly inhibits inflammatory mediators. It has also been shown to inhibit neutrophil attachment generated by tumor necrosis factor (TNF)-α at a dose of 0.1% and lipopolysaccharide (LPS)-induced nitric oxide synthase (iNOS) and monooxidation-induced signaling pathways co-bind to receptors, thereby blocking the nuclear factor Kappa B (NFƙB) pathway, COX2 and peroxisome proliferators. It suppresses activated receptor alpha (PPARα) by 60–70% and inhibits oral and tissue inflammation [6].

It has been reported to inhibit platelet composition and treat anxiety, gastrointestinal infections, diabetes, malaria, and pneumonia [25]. Tea made from lemongrass essential oil has been proven to have sedative, analgesic, anti-inflammatory, antipyretic, and antispasmodic properties. It has also been used as massage oil for relief of joint and muscle pain [37]. Diarrhea, stomach aches, and digestive issues can all be treated with lemongrass tea [38]. Lipid-lowering and hypoglycemic drugs may also contain lemongrass. In folk remedies and Ayurvedic medicines, it is used to control serum glucose, fat, and lipid levels and prevent obesity and high blood pressure. This plant has been used to keep blood sugar levels stable by secreting insulin (hyperinsulinemia). It lowers blood pressure that may result in hypertension [5]. It has been reported that citral (geranial and neral), the main constituent of C. citratus essential oil, is cytotoxic to a number of human leukemia cell lines. This occurs by the activation of procaspase 3. It has also been proven to inhibit the proliferation of pathogenic food-borne bacteria including Listeria monocytogenes and Salmonella Typhimurium [4].

Essential oils from C. citratus have been used to control infections and insects. It is efficient against Aedes aegypti, Phenacoccus solenopsis, Dermatophagoides sp., and Musca domestica. C. citratus is used in herbal soaps to cure swelling, itching skin, and rashes [6]. It has also been demonstrated that lemongrass essential oil inhibits Microsporum canis. Shampoos containing citral were efficient against Malassezia furfur, a fungus found in dandruff [39]. Lemongrass essential oil has been noted to exhibit considerable resistance to pathogenic fungi that interfere with the release of mycotoxins during preservation of grains and other food products [40]. Cymbopogon is a common herb in tropical regions [30]. It is frequently used as a food ingredient for human consumption. Lemongrass is frequently used in Asian cuisine for its aroma. Industrially, they are important as part of beverages, baked goods, fragrances, pesticides, and preservatives [6, 41]. They can serve as deodorants for perfumes, local samples, candle repellents, and other insect repellents. It has been used as a repellent against snakes and other reptiles in some Asian and African countries [42, 43]. The potential of lemongrass as an effective substitute to antibiotic growth promoters was evaluated [44].

4. Detection of phytochemicals in essential oils in Cymbopogon species—lemongrass

An understanding of the chemical components in plants is important for the discovery of beneficial phytochemicals useful in the synthesis of therapeutic agents and other useful chemical compounds. Higher plants are crucial sources of bioactive compounds essential for the maintenance of human health. C. citratus contains many pharmacologically active essential oils, flavonoids, phenolic compounds, and other bioactive constituents [6]. The methods of extraction of these essential oils include the following:

1. Solvent extraction: The solvent is combined with the plant material and then heated to extract the essential oil (EO). The plant extract is filtered and concentrated by evaporation of the solvent. The oil is extracted from the concentrate by combining it with pure alcohol and distilling it at low temperatures. However, because this method takes a long time to complete, the oils are more expensive than other methods [4, 45, 46].

2. Distillation methods: In hydrodistillation, the plant material absorbs water during the boiling process. The oil present in the oil cells diffuses via osmosis from the cell walls [46]. During steam distillation, at about 100°C, the combined vapor pressure corresponds to the ambient pressure. This allows volatile components with boiling temperatures between 150 and 300°C to evaporate at temperatures near 100°C [45]. The use of this simple technique for extraction of LGEO has been reported [47, 48]. Hydrodistillation yields less oil because incomplete oil extraction takes place due to a variable rate of distillation caused by heat [49]. Steam distillation has various disadvantages including possible loss of certain volatile chemicals, low extraction efficiency, and unsaturated compound degradation may occur [46].

3. Supercritical CO₂ extraction: CO₂ was chosen for use in extraction for various reasons, including inertness, non-toxicity, non-flammability, high solubility, availability at low cost, ideal for thermolabile compound extraction, and ease of removal from the extract. Supercritical CO₂ extraction is strongly suggested at low temperatures to prevent any damage to desirable EO constituents [50]. The use of this technique results in products that are free of toxic waste and of greater quality preserving thermal stability compared with conventional methods. However, the high working pressure necessitates the use of sophisticated equipment, which raises safety risks and costs [51].

4. Ultrasound extraction (UAE): Plant cell walls are disrupted using ultrasound at frequencies greater than 20 kHz, which aid the ability of the solvent to permeate the cells and improve extraction yields. The UAE can process at a low temperature while maintaining good extract quality. The UAE is considered one of the easiest extraction methods due to the use of basic experimental equipment such as ultrasonic baths. The temperature and extraction time are controlled, while the ground sample is mixed with the appropriate solvent and placed in an ultrasonic bath [52]. Unfortunately, there are two major drawbacks to using an ultrasonic probe, both of which are connected to experimental reproducibility [53]. An advantage of this technique is that it is a green technology and its use in extraction of phenolic compounds has risen in popularity in the recent years [54].

5. Microwave extraction: Microwave extraction has a similar principle to hydrodistillation except that heating is achieved using microwaves. This method reduces the number of biological components lost during extraction. It reduces time and extraction solvent volume. It has been employed as an alternative to traditional antioxidant extraction techniques by reducing extraction time and being environmentally friendly [4, 54].

6. Spectroscopy: Data from variety of spectroscopic techniques are used to determine the structure of substances including ultraviolet (UV)-visible, infrared (IR), nuclear magnetic resonance (NMR), and mass spectroscopy. The fundamental premise of spectroscopy is that electromagnetic energy is passed through organic molecules that absorb part of it. A spectrum can be generated by measuring the amount of electromagnetic energy absorbed. Spectra are unique to each bond in the molecule, and this can be used to elucidate the structure of the organic molecule can be determined using these spectra. Since aromatic compounds are potent ultraviolet chromophores, UV-visible spectroscopy is preferred for quantitative study. When compared with other procedures, this technique takes less time and costs less. Fourier transform infrared spectroscopy (FTIR) is a non-destructive, high-resolution, quick analytical tool for identifying chemical components and determining the structure of compounds [54].

7. Chromatography: TLC is a rapid, inexpensive, solid-liquid chromatography technique for determining the presence of components in a mixture. Silica gel (SiO2 x H2O) and alumina (AL2O3xH2O) are the most used solids in chromatography [55]. Bio-autography is a technology that may be used to identify bioactive compounds with antibacterial activity in plant extracts. The bioautographic TLC method combines chromatographic separation with in situ activity measurement to facilitate identification and separation of active elements in a compound. High-performance liquid chromatography (HPLC) is an established technology for separating bioactive compounds. For optimal isolation, it is crucial to select the right mobile phase, flow rate, detectors, and columns and other conditions. The identifying peak should have adequate retention time and be well separated from unrelated peaks [56]. Liquid chromatography-mass spectrometry (LC/MS) is also a useful tool for the analysis of phenolic compounds [56]. When pure standards are not available, the combination of HPLC with MS can quickly and reliably identify the chemical composition of herbs [57]. Overpressured layer chromatography (OPLC) is the bridge between thin-layer chromatography (TLC) and high-performance liquid chromatography (HPLC). OPLC is particularly attractive for separating antimicrobial components from various matrices [58]. Gas chromatography is often combined with a flame ionization detector or an electron capture detector. This technique can quantify and determine the presence of materials present at low concentrations [59]. The gas chromatography section divides sample’s compounds into pure chemical pulses based on their vaporization.

8. Immunoassays: This method uses monoclonal antibodies to identify pharmaceuticals and natural low-molecular-weight bioactive substances. Monoclonal antibodies are produced by hybridoma technology. They are becoming increasingly relevant in the study of bioactive compounds. They have been shown to have high specificity and sensitivity. MAb-based enzyme-linked immunosorbent assays (ELISA) have been shown to be more sensitive than traditional HPLC procedures in many cases [56].

5. Quantification of phytochemicals of essential oils of Cymbopogon species

Refs. [60, 61] have documented the quantitative determination of phytochemicals (total alkaloids, flavonoids, phenol, saponins and tannins) of extracts of leaves using spectrophotometric and Folin-Ciocalteu methods, expressed per gram of the sample dry matter [60, 61]. These were majorly initiated by ethanolic and methanolic of acidified methanolic extractions as necessitated by the phytochemical of interest [62], followed by centrifugation and storage at −20°C until analysis was done.

For instance, the 1,10-phenanthroline method of total alkaloids content (TAC) estimation, as described by Ref. [63], entails the oxidation of alkaloids by iron (III) and subsequent complexation of iron (II) with 1,10-phenanthroline, to form a red-colored complex having the maximum absorbance at 510 nm. The reaction mixture containing 1 ml plant extract, 1 ml of 0.025 M FeCl₃ in 0.5 M HCl, and 1 mL of 0.05 M of 1, 10-phenanthroline in ethanol is usually incubated for 30 min in hot water bath with maintained temperature of 70 ± 2°C, before the measurement of the absorbance of red-colored complex at 510 nm against reagent blank. Alkaloid contents are then estimated and expressed as the standard curve of quinine (0.1 mg/ml, 10 mg dissolved in 10 ml ethanol, and diluted to 100 ml with distilled water) and the values expressed as g.100 g^–1 of dry weight. This simple, sensitive, and economically viable spectrophotometric method has been used in the determination of some Rauwolfia alkaloids (ajmaline, ajmalicine, reserpine, and yohimbine HCl) in tablets of pharmaceutical formulations, with reports showing that the common excipients do not interfere with the proposed method. A statistical comparison of these results with the results of the reported approach shows good agreement and no significant difference in accuracy [61, 62, 63].

Phenolic compounds inhibit lipid oxidation by scavenging free radicals, chelating metals, activating antioxidant enzymes, reducing tocopherol radicals, and inhibiting enzymes that cause oxidation reactions. These may provide the basis for the rapidly growing interest in the use of natural antioxidants and antimicrobials [64]. Quantification of total flavonoids content (TFC) has been according to the aluminum chloride method reported by [65], involving the dispense of 0.5 ml of extract into test tube, followed by the addition of 1.5 ml of methanol, 0.1 mL of aluminum chloride (10%), 0.1 ml of 1 M potassium acetate, and 2.8 ml of distilled water in a reaction mixture. The absorbance read at 514 nm, after allowing to stand at room temperature for 30 min, is expressed as quercetin equivalent (QE) in mg/g material. In the same vein, the total phenolic content (TPC) quantification of samples extracts can be determined according to the Folin-Ciocalteu method of [62], where 1.5 ml of a 1 in 10 dilution of Folin-Ciocalteu reagent is added to 300 ml of leaf sample extract, followed by 1.2 ml of Na₂CO₃ solution (7.5 w/v). The absorbance read, at 765 nm against a blank after allowing to stand at room temperature for 30 min, is expressed as gallic acid equivalent (GAE) in mg/g material.

Variations in the extraction yields could arise from the different extraction methods and solvents. Other factors could be the evaluated variety, harvest year, processing, and storage [66]. Using walnut leaf as a case study, Ref. [67] investigated several solvents with different polarities such as hexane, chloroform, ethyl acetate, methanol, and ethanol for the evaluation of the cytotoxicity of walnut leaf extract on human cancer cell lines. They reported that the resulting methanol extract had the highest amount of TPC and TFC (120.28 ± 2.32 and 59.44 ± 0.87 mg/g DE, respectively) using colorimetric methods. For the ethanolic extracts, the concentration of the phenolic compounds in young leaves was substantially greater than those in the mature leaves. Employing the response surface methodology of optimization of the ultrasound assisted hydroalcoholic extraction of phenolic compounds of walnut leaves, Ref. [68] tried to establish the optimum conditions and the maximum predicted TPC, using 61% ethanol concentration, 51.28 min extraction time, and the 4.96 v/w liquid-to-solid ratio to obtain 10,125.4 mg GAEs/l, while 2925 mg quercetin equivalents (QEs)/l as maximum TFC was achieved by using ethanol with 67.83% concentration, v/w liquid-to-solid ratio of 4.96, and 49.37 min of extraction time. Under these conditions, the experimental results were reasonably similar to the values predicted by the polynomial response surface model equation. Ref. [69] compared the antioxidant and antimicrobial activities of the prepared ethanol and water extracts from the leaves of three plants, namely P. aphylla, Persian walnut, and oleander. They showed that the ethanol extracts had the highest amount of total phenolics and flavonoids in all assays, as well as highest antioxidant and antimicrobial activities when compared with the water extracts.

Initially, the formation of stable and persistent foam on the liquid surface for approximately 15 min represented the presence of saponins [70]. However, the quantification of plant saponins is usually performed by spectrophotometry and chromatography. The major difference in quantitative expression between the two techniques is that spectrophotometry quantifies the total saponin value, while chromatography quantifies specific saponin compound [71].

5.1 Spectrophotometric method of phytochemical quantification

Regarded as a simple, rapid, and inexpensive approach, total saponins assay, also known as vanillin-sulfuric acid assay, involving the reaction of oxidized triterpene saponins with vanillin is one of the spectrophotometric methods used to quantify saponins. It uses sulfuric or perchloric acid as oxidant, resulting in a distinctive purple coloration of the reaction system [71]. Being the most commonly utilized spectrophotometric method for quantifying plant saponins and providing an excellent reference for future experimental design, reports suggest that in order to allow full color development, few criteria, such as choice of standards and wavelength, should be taken into account when selecting this method [70]. However, researchers have reported 544-nm wavelength, with a majority selecting wavelengths in the range of 480–610 nm (excluding 473 nm and 283 nm). This is likely due to the maximum absorption of purple color that falls within this range [72, 73].

Hemolytic method is a spectrophotometric method for determining the saponin concentration of a plant material [74]. This is based on the release of oxy-hemoglobin when saponins react with blood reagent producing measurable color changes detected by the spectrophotometer. Ref. [75] have quantified the saponin content in bitter gourd varieties with the hemolytic technique, with their result showing that white bitter gourd types had much lower levels of saponin (0.25%) than the green varieties (0.67%). In furtherance, the saponin extract was dissolved in distilled water, before incubation of 100 μl of this solution with 1 ml fresh EDTA-blood at 30°C for 30 min. Hemoglobin was measured in the supernatant photometrically at 545 nm, after centrifugation for 10 min, and the result is expressed in hemolytic saponins.

5.2 Chromatographic method of phytochemical quantification

Thin-layer chromatography (TLC), high-performance liquid chromatography (HPLC), and ultra-pressure liquid chromatography (UHPLC) are the most common chromatographic methods employed [73, 76, 77]. Multiple reaction monitoring (MRM)-based UHPLC-ESI-MS/MS technology, according to studies, was developed to provide more precise and sensitive measurement of main saponins and sapogenins in conjunction with chlorogenic acid [78]. Refs. [79, 80] described the use of ultra-high-performance liquid chromatography coupled to single-stage Orbitrap high-resolution mass spectrometry (UHPLC-Orbitrap-MS) to simultaneously detect and quantify phytochemicals in green tea and walnut leaves-derived nutraceuticals. Ref. [81] submit that by combining LC separation and MS detection, a high selectivity could be attained since the MS detector’s selectivity allows for more precise identity by confirmation by comparing fragmentation patterns and observing qualifier and quantifier ion transitions.

Additionally, although sensitivity—the connection between analyte signal and concentration—may not be a crucial parameter for evaluation during method validation, it provides information about the instrument signal and might be helpful during method optimization. In light of these, Ref. [78] suggest that this method can be used to quantitatively measure bioactive compounds in crude plant materials and other related products, while also determining the same compounds in other biological sample matrices such as plasma, potentially minimizing matrix effect. Reports of Refs. [82, 83] show chromatographic methods to allow separation and purification of various saponin biotypes from plant materials to identify a specific saponin compound and investigate its pharmaceutical property. Refs. [84, 85] suggest that the main goal of all the studies using HPLC technique is the quantification of specific saponin components. The specific saponin content detected serves as an excellent data reference source to future researchers, in addition to providing a reliable scientific reference to pharmaceutical manufacturers interested in further processing of their respective plant sources [71].

Standardization and purification of complex extracts are still problematic since the mixtures are more toxic than individual components and present more difficulty in detoxification than a single molecule [86, 87]. More so, isolation, synthesis, or formulation processes could be slow and expensive, but relatively inexpensive for plant essential oils. In light of these, Ref. [88] allude that simultaneous quantification and qualitative analyses of phytochemicals could easily be achieved via quick and conventional methods such as non-destructive near-infrared spectroscopy and isocratic high-performance liquid chromatography. These improved methods can support rapid and precise content evaluation and confirmation [86].

5.3 Spectroscopic techniques of phytochemical quantification

Spectroscopic techniques such as ultraviolet (UV-visible), infrared (IR), mass spectroscopy, and nuclear magnetic resonance (NMR) provide sufficient information for quantitative as well as qualitative analysis of phytochemicals. Basically, depending on its structure, the organic molecule absorbs electromagnetic radiations in certain regions and produces a spectrum. The obtained spectrum helps in the identification and quantification of the molecule, since they are specific to functional groups [89].

The spectroscopic techniques include the following:

1. Ultraviolet-visible spectroscopy can be used to identify various phytochemicals using maximum absorption (λ max) values that correspond to their structural characteristics, such as total phenolic extract (280 nm), flavones (320 nm), and phenolic acids (360 nm) [54]. This technique is cheaply available and requires less time for observation [90].

2. Infrared IR spectroscopy also referred to as vibrational spectroscopy exploits the vibrational modifications associated with elongation or bending of certain molecules upon exposure to the infrared region of electromagnetic radiations [89]. The various functional groups or chemical bonds in the molecule have different vibrational frequencies depending on the force constant (bond strength) values and decreased masses [54], which aid structural determination of a bioactive compound by capturing the characteristic frequency absorption bands for the functional groups present in the IR spectrum. Higher resolutions of chemical composition and elucidation of molecular structure can further be achieved with the Fourier transform infrared spectroscopy (FTIR) [91].

3. Mass spectroscopy involves the conversion of organic molecules to highly energetic charged species through bombardment with electrons or lasers [89]. As such, the molecular formula of the bioactive compound can be predicted by the estimated relative molecular weight of the fragmented ions adjacent to the places of fragmentations [54]. According to the published reports, mass spectroscopy has proven to be a highly effective analytical instrument in the structural elucidation of phenolic compounds when used in conjunction with electrospray ionization (ESI), a preferred method of producing charged species from macromolecules [91].

4. Nuclear magnetic resonance (NMR) spectroscopy reveals the magnetic properties of some nuclei including 1H, 13C, 19F, and 31P [90]; thus, a signal is produced when the molecule’s intrinsic frequency of the active nuclei resonates with the oscillation frequency of the external magnetic field applied. This is determined by measuring the chemical shift arising from variations in the strength of the applied magnetic field, the chemical environment, and changes in the magnetic characteristics of the various nuclei [54, 89]. The quantitative concentrations of the most abundant compounds in the oily fractions of hemp leaves, hurds, and roots were revealed by the NMR method [92, 93, 94].

6. Conclusion

This chapter discusses two of the most common species of Cymbopogon and their essential oils. The chapter showcases diverse phytochemicals of the oils, economic importance of Cymbopogon essential oils, detection, and quantification techniques of the phytochemicals of these essential oils obtained from Cymbopogon species. Additionally, different extraction and quantification methods for obtaining the essential oils were explicated. Findings from this chapter portray the different challenges associated with extraction and yields of these essential oils. Although chromatographic methods are commonly employed for extracting and quantifying these essential oils, there is need for improvement via the use of combined and more sensitive techniques for greater as well as pure yields of essential oils of lemon grass.

Acknowledgments

We wish to acknowledge Covenant University for sponsoring this publication.

Conflict of interest

The authors declare no conflict of interests.