Review
Emerging green technologies for the chemical standardization of botanicals and herbal preparations
Ming Yuan Heng a ,Swee Ngin Tan b ,Jean Wan Hong Yong a ,Eng Shi Ong a ,⇑
a Singapore University of Technology and Design,20Dover Drive,Singapore 138682,Republic of Singapore
b
Natural Sciences and Science Education Academic Group,Nanyang Technological University,1Nanyang Walk,Singapore 637616,Republic of Singapore
a r t i c l e i n f o Keywords:Botanical Extraction
孩子的叛逆期Green technology
Chemical standardization
Environmental asssment tool (EAT)Herbal preparation
Life-cycle asssment (LCA)Medicinal plant Quality control Solvent free
川百合a b s t r a c t
As botanicals and many medicinal plants can be procesd to become a food or a health supplement,a drug or cosmetics,chemical standardization is important for their quality control.Hence,the lection of appropriate extraction technologies and analytical techniques is required to provide a solvent-free solution for the chemical standardization of botanicals and herbal preparations.
Ó2013Elvier Ltd.All rights rerved.
Contents 1.Introduction ...........................................................................................................12.
Extraction technologies ..................................................................................................22.1.Ultrasound-assisted pretreatment of solid samples ......................................................................32.2.Supercritical-fluid extraction (SFE)....................................................................................32.3.Microwave-assisted extraction (MAE).................................................................................42.4.Pressurized-liquid extraction(PLE)/accelerated-solvent extraction (ASE).....................................................52.5.Pressurized hot-water extraction (PHWE)..............................................................................53.
Analytical techniques for the chemical standardization of botanicals .............................................................63.1.GC and .63.2.LC and 63.3.Capillary electrophoresis (CE)........................................................................................84.
<9Acknowledgements .....................................................................................................9References ............................................................................................................
9
1.Introduction
Botanicals or medicinal plants are known to contain one or many chemical constituents that may have therapeutic purpos.The class of bioactive compounds prent in medicinal plants include alkaloids,flavonoids,terpenes and saponins.Botanicals or medicinal plants can be procesd to become a food or health sup-plement,a drug or cosmetics.Aromatic plants and spices are com-
monly ud as food flavorings or food supplements,or as a source of esntial oil.Currently,monographs of medicinal plants can be found in the United States Pharmacopeia (USP)[1],Chine Pharmacopeia [2],WHO monographs for medicinal plants [3,4],Japane Pharmacopeia (JP)[5]and others.Approaches for the chemical standardization of botanicals have been covered in a number of previous review papers [6–10].For certain botanicals,a combination of chemical standardization with biological assay has been propod to characterize the synergistic effect of the
dif-ferent constituents prent [6,9].
Due to the complexity of medicinal plants and herbal medi-cines,holistic approaches with a combination of quality control
0165-9936/$-e front matter Ó2013Elvier Ltd.All rights rerved.dx.doi/10.ac.2013.03.012
Corresponding author.Tel.:+6564994513;Fax:+6567795161.
夫妇何求E-mail address:engshi_ong@sutd.edu.sg (E.S.Ong).
(QC),elucidation of the properties of absorption,distribution,metabolism and excretion,and metabonomics evaluation of medicinal plants have been propod [7].The screening strategies of bioactive compounds by biochromatographic methods have been introduced for certain herbal medicines [10].Also,there is a need to approach scientific proof and clinical validation with chemical standardization,biological assays,animal models and clinical trials for botanicals.We note that quality assurance of medicinal plants is the prerequisite of credible clinical trials.From a safety point of view,the misidentification or the mislabeling of plant material can lead to significant toxic effects in hu
mans.The u of a wrong plant may result in unintended intoxication.Hence,standardization and effective control measures are required to monitor the quality of the medicinal plants and to exclude any pos-sible contaminants arising from the misidentification of plants that would badly affect consumers of herbal medicine [8,11].
The different steps ud for the chemical standardization of botanicals will include:
(1)pretreatment that will involve drying and grinding;(2)lection of a suitable method of extraction;
(3)analysis of compounds using suitable chromatographic or
spectroscopic methods;and,辜鸿铭后人
(4)analysis of data bad on bioactive or marker compounds or
pattern-recognition tools.The current methods in various pharmacopeia and other reports may require extensive u of organic solvents and can be time consuming.Moreover,organic solvents are expensive and their dis-posal is very costly.Hence,the u of green technologies to reduce and/or to eliminate the u or production of hazardous materials is highly desirable.The approaches to adapt the principles of green chemistry for the chemical standardization of botanicals are:(1)to reduce the
u of harsh organic solvents;
(2)to encourage the u of emerging extraction technologies;and,(3)to u high-efficiency paration techniques with low usage
of organic solvents.Most important of all,the lection of extraction technologies and analytical techniques will largely be bad on the inherent properties of compounds prent in botanicals (Fig.1).We note that different methods of extraction may affect the medicinal-plant profile and levels of markers or bioactive compounds obtained.Hence,depending on the physical and chemical properties of the target compounds prent in the plant materials,a suitable extrac-tion technology in combination with an analytical technique will be required.Also,a complex matrix may be encountered in herbal extracts for which tedious sample clean-up steps that involve mul-
tiple liquid-liquid extraction (LLE)steps may be required.Hence,approaches that can provide a simple solution for the extraction and analysis of target compounds in botanical extracts will be highly desirable.
Currently,we can find extensive discussion on the sample-preparation and extraction technologies for medicinal plants.However,there has been a limited number of review papers on green approache
s that eliminate or reduce the u of organic solvents for the chemical standardization of botanicals (solid samples),which combine extraction technologies with analytical techniques.Hence,the focus of the current review paper is to iden-tify emerging green approaches for the appropriate extraction technologies for solid samples and analytical techniques to form an energy-efficient and solvent-free solution for the chemical stan-dardization of botanicals.Also,we discuss comparison of the advantages and the disadvantages of the various extraction tech-nologies for solid samples and analytical techniques.
2.Extraction technologies
The traditional extraction techniques that are commonly ud for the chemical standardization of botanicals include Soxhlet extraction,sonication,heating under reflux,blending and solid-liquid extraction.The techniques generally require long extrac-tion times,large amounts of samples and organic solvents that may have potential negative effects on the environment and human health.At the same time,the characteristics mean that sample treatment may become an error-prone part of the method.Hence,there have been proposals to adopt emerging extraction technologies with various potential advantages {e.g.,supercriti-cal-fluid extraction (SFE),microwave-assisted extraction (MAE)and pressurized-liquid extraction (PLE)[12,13]}.The theory,the principles and the applications
of each of the emerging extraction technologies have been covered in the various review papers cited in this prent review.
什么是主板From Fig.2,the extraction mechanism in the various methods involves four quential steps:
(1)first is the desorption of solutes from the active sites in the
sample matrix under the operating conditions of the differ-ent methods of extraction;
(2)the cond may involve the diffusion of extraction fluid into
the matrix;
(3)next,depending on the sample matrix,the solutes may par-tition themlves from the sample matrix into the extraction fluid;and,
(4)finally,the analytes obtained can be analyzed via an appro-priate analytical
technique.
浴室装修设计效果图2M.Y.Heng et al./Trends in Analytical chemistry 50(2013)1–10
Table1compares the various extraction technologies and their energy consumption.We note that emerging extraction tools have lower energy consumption than traditional methods of extraction (e.g.,Soxhlet and heating under reflux).
For the extraction of bioactive or marker compounds in botan-icals,one key challenge is that the target compounds are prent naturally where significant analyte-matrix interaction is prent. Hence,the spiking of target analytes into the plant matrix will not mimic the analyte-matrix interaction
prent naturally. Depending on the sample matrix,high recovery from spiking experiments may not imply that the method is accurate.
2.1.Ultrasound-assisted pretreatment of solid samples
Ultrasound-assisted pretreatment of solid samples is an emerg-ing technique that is clean and energy efficient.Extraction using ultrasound can be performed on solid samples together with a suit-able solvent on a bath sonicator or an ultrasonic probe.The unique conditions provided by acoustic cavitation can enhance solid-sample treatment.The method propod can be performed at atmospheric pressure and room temperature.In most cas,a cer-tain amount of organic solvent will be required to dissolve the tar-get compounds from the medicinal plant.For ultrasonic extraction with an ultrasonic probe,a smaller volume(1–15ml)of organic solvent will be needed[14].Currently,works toward solvent-free approaches with ultrasound-assisted extraction(UAE)are limited. The energy consumption of ultrasound-assisted pretreatment of solid samples is similar to other methods of extraction stated in Table1.UAE has been ud to characterize polyphenolic compounds prent in veral Salvia species.Comparison with other methods of extraction with UAE showed that good method recovery was obrved for certain target compounds in veral Salvia species[15].However,for the analysis of lected polyhalo-genated pollutants in plants,it was obrve
d that the extraction efficiency of UAE was lower than that of other emerging methods [16].Depending on the medicinal plant-sample matrix,the method recovery or extraction efficiency may be lower than other methods of extraction.Hence,as a result of the complexity of botanical extracts,the extraction efficiency of methods using UAE will need to be thoroughly investigated.
2.2.Supercritical-fluid extraction(SFE)
Supercriticalfl,carbon dioxide)are substances above the critical pressure and temperature with properties ranging be-tween liquid and gas.The extreme variability of their solvent power with pressure and temperature,and their low viscosity,en-abling much faster mass transfer than in liquid,are the most important advantages of SFE[17–19].One of the key features of SFE with CO2is that it is non-flammable,cost effective,easily accessible in its high purity and has no negative impact on the environment and human health.The critical point(Tc=31.1°C, Pc=74bar)allows for the extraction of thermally-labile solutes at a lower temperatures than conventional methods of extraction.SFE with CO2is usually carried out continuously or
Fig.2.The extraction mechanism in the methods described.Thefirst step is the
desorption of solutes from the various active sites in the sample matrix under the
operating conditions of the different methods of extraction.The cond step may
involve the diffusion of extractionfluid into the matrix.Next,depending on the
sample matrix,the solutes may partition themlves from the sample matrix into
the extractionfluid.
Table1
Comparison of technologies for the extraction of target compounds from botanicals and medicinal plants
Extraction technologies Advantages Disadvantages Energy consumption
Ultrasound-assisted
extraction with a bath
sonicator
Ultrasound-assisted
extraction with an
ultrasonic probe Extraction can be performed at atmospheric pressure
and room temperature
U of small volume of organic solvent(1–15mL)for
ultrasound-assisted extraction with an ultrasonic probe
Very safe to u
Depending on the plant matrix,the extraction
efficiency may need to be investigated
thoroughly
Moderate
SFE Green extraction technology with CO2as extractant
Process can be scaled up for industrial production
Suitable for thermally-labile substances
Very safe to u High cost for the high-pressure equipment
needed
May be difficult to extract polar components
Moderate
MAE High throughput with the commercially-available
system
Suitable for thermally-labile substances
Organic solvents and water can be ud as an extractant Amount of sample is to the volume of extractant
ud will be important
A challenge to scale up MAE
Potential explosion risks as a result of
pressurization with clod vesl
Moderate
PLE/ASE High throughput with the commercially-available
system for ASE(laboratory scale)
Suitable for thermally-labile substances
Reduction in usage of organic solvent
Very safe to u High cost for the high-pressure equipment
needed
No commercial or high-throughput system for
PLE in the dynamic mode
Moderate
Moderately lower for
PLE at room
temperature
PHWE Green extraction technology using water as extractant
Suitable for thermally-labile substances
Process can be scaled up for industrial production
Ability to perform extraction at lower operating
pressure
Very safe to u
High cost for the high-pressure equipment
needed for operation at higher pressure
No commercial or high-throughput system for
PHWE
Moderate
chemistry50(2013)1–103
mi-continuously.SFE can be operated at an analytical scale or a plant scale.Botanicals will be placed into an extraction vesl and the supercriticalfluid will be fed to the extractor by a high-pres-sure pump at afixedflow rate.Finally,the substances extracted by SFE can be precipitated by temperature and/or pressure changes or by applying a mass-parating agent.Also,SFE equipment has been propod to allow solvent regeneration and recirculation. For the fractionation of the extracts,SFE equipment can be oper-ated with veral parators in ries at different pressures and temperatures.The main drawback of SFE,compared with tradi-tional methods of extraction,is the high cost of the high-pressure equipment needed[17].
The various conditions that may affect the extraction efficiency of SFE are thefl,CO2),operating temperature(40–90°C) and pressure(100–400bar).The other important factors in SFE in-clude particle size and shape of the plant material,moisture of the solid materials and solventflow-rate.To increa the extraction efficiency of more polar substances from plant materials,a small amount of modifi,methanol,ethanol and water)may be added.
Currently,a number of botanicals have been ud as sources of bioactive compounds using SFE.Bad on the operating conditions ud,we note that the extracts or the active fractions are obtained with specific characteristics.The extracts obtained by SFE maintain the bioactivity of extracts obtained by traditional extraction meth-ods,becau SFE promotes a lective extraction and results in an extract enriched in the desirable compounds.Thefinal result is that the SFE extract may be free of organic solvents and without loss of compounds due to degradation.SFE is very suitable for non-polar components.However,for certain class of compounds that include the more polar constituents in medicinal plants,a lower yield or lower extraction efficiency than traditional methods may be obrved.
Currently,SFE has been applied for the extraction of esntial oils,phenolic compounds,carotenoids,tocopherols,and tocotrie-nols[17–22].Respon-surface methodology was a
pplied to opti-mize the supercritical carbon-dioxide extraction of esntial oil from Cyperus rotundus Linn.It was noted that the yield of Cyperus rotundus Linn by SFE was significantly higher than Soxhlet extrac-tion with n-hexane[20].Similarly,SFE was ud to extract non-polar ,fatty acids from Borago officinalis L.flower) and it was obrved that SFE was more effective than the conven-tional hydrodistillation method in extracting fatty acids and pre-rving its quality[21].Despite the weakness stated,to eliminate the usage of organic solvents,SFE without any additives will remain the method of choice for the extraction of non-polar components in medicinal plants.
2.3.Microwave-assisted extraction(MAE)
MAE is one of the emerging techniques that have been widely employed for the extraction of bioactive and marker compounds from medicinal plants.Microwaves are non-ionizing electromag-netic waves that compri an electricfield and a magneticfield oscillating perpendicularly to each other in a frequency range (0.3–300GHz).In addition,microwaves penetrate into certain materials and interact with the polar components to generate the heat needed for extraction.The heating of microwave energy acts directly on the compounds by ionic conduction and dipole rotation. The result in only lective,targeted materials that can be heated bad on their dielectric constant.The e
fficiency of the microwave heating depends on the dissipation factor of the material,which measures the ability of the sample to absorb microwave energy and dissipate heat to the surrounding molecules.MAE has at-tracted significant attention in the analysis of medicinal plants due to its special heating mechanism,moderate capital cost, high-throughput capability and good performance under atmo-spheric conditions[12,23,24].
In general,MAE can be classified as clod or open vesl.For MAE with clod vesl,the extractions are carried out in a aled vesl with different modes of microwave radiation.The uniform microwave heating with the high working pressure and tempera-ture of the system allows fast,efficient extraction of bioactive com-pounds in botanicals.The pressure inside the clod extraction vesl is controlled in such a way that it would not exceed the working pressure of the vesl and the temperature can be regu-lated above the normal boiling point of the extraction solvent.
To counter the shortcomings of clod ,safety is-sues)and to extract thermally-labile compounds,open vesl MAE was developed.For MAE with open vesl,more solvent can be added at a suitable point during the extraction process and the system has higher throughput.In addition,the upper part of the vesl is connected to a reflux unit to conden any volatile sol-vent[23
–25].
The factors that may affect the efficiency of MAE are power and frequency of the microwaves,duration of the microwave radiation, moisture content of the botanical sample,concentration of solvent, ratio of solid to liquid,extraction temperature,extraction pressure and maybe the number of extraction cycles.Solvent and tempera-ture are the most important parameters for MAE that affect the sol-ubility of the bioactive or marker compounds.MAE can be applied to extract thermally-labile ,gastrodin in Gastrodia elata and stevioside and rebaudioside A from Stevia rebaudiana Ber-toni)through optimization of the applied temperature[26,27].On the whole,the choice of solvent takes into account not only its affinity with the target compound but also its ability to absorb microwave energy[22–24].
MAE has been widely reported as a good,reliable method in preparing samples for medicinal plants.We note that the extrac-tion yield of MAE is higher and the extraction time needed is short-er than traditional methods of extraction[23].With the availability of commercial MAE equipment,MAE has been widely ud to ex-tract a wide variety of ,flavonoids,and saponins from botanicals)[23–27].Although organic ,metha-nol or ethanol,with or without the addition of water)are com-monly ud,we have reported that MAE with water shows higher extraction efficien
7言古诗
cy than heating under reflux with water for gastrodin in Gastrodia elata Blume[26]and stevioside and rebaudioside A from Stevia rebaudiana Bertoni[27].
Also,ionic liquids(ILs)are gaining wide recognition as novel, environment-friendly solvents in chemistry.Due to their excellent solvent ,negligible vapor pressure,wide liquid range,good thermal stability,tunable viscosity,miscibility with water and organic solvents,good solubility and extractability for various organic compounds,room-temperature ILs are gaining attention as extracting solvents.MAE with ILs has been propod as an alternative to conventional organic-solvent extraction for the extraction of components from medicinal plants[23,25,28]. Compared with water and common organic ,metha-nol and ethanol),the availability of lected ILs for the extraction of target compounds in medicinal plants may prent a challenge. Also,the environmental impact of the lected ILs may need to be thoroughly evaluated.
Despite of the newer instrumental design of ,nitro-gen-protected MAE and dynamic MAE),MAE with clod or open vesl systems will remain the preferred mode for the extraction of target compounds in botanicals.Lastly,to reduce reliance on or-ganic solvents,it is clear that MAE with an environment-friendly ,water,aqueous surfactants and ILs)is an emerging trend for the extraction of botanicals.
4M.Y.Heng et al./Trends in Analytical chemistry50(2013)1–10
2.4.Pressurized-liquid extraction(PLE)/accelerated-solvent extraction (ASE)
PLE/ASE wasfirst introduced in1995at the Pittcon Conference by Dionex Corporation and it is also known as pressurized solvent extraction and enhanced solvent extraction.The technique is re-ferred to as pressurized hot-water extraction(PHWE),subcritical water extraction(SWE)or superheated water extraction when water is ud as the extractant.
PLE involves extraction of target compounds from medicinal plants using solvents at elevated temperature and pressure.The elevated temperature with pressure enhances the method perfor-mance compared to traditional methods of extraction carried out near to room temperature and atmospheric pressure.The advanta-ges of using organic solvents at temperatures above their atmo-spheric boiling point are enhanced solubility and mass transfer. At the same time,methods using PLE significantly reduce the usage of organic solvent during the extraction process.For PLE,the pres-sure applied will increa the boiling point of the solvent ud and allows the extraction to be carried out at temperature above the boiling point of the solvent[21,29].
For extraction by PLE,depending on the water content,the plant material is typically disperd in a dr
ying or inert sorbent (e.g.,sodium sulfate,diatomaceous earth or others).The mixture of inert sorbent and plant sample is packed in a stainless-steel cell and inrted in a clodflow-through system.There are two main t-ups for PLE,namely static and dynamic instruments.For PLE in the dynamic mode,the extraction solvent is continuously pumped through the extraction cell.The operation involves theflow rate t during the static time and the pump delivers the solvent at a con-stantflow rate for a certain ,1.0–1.5mL/min for20–30min).Currently,there is no commercial dynamic PLE system available in the market.
叶欣的故事By contrast,for PLE in static mode,once the t parameters of the extraction temperature and pressure are reached,the extraction is performed for a predetermined time.A common range is5–15min that is done in different cycles.Compared to PLE in dynamic mode, the extraction process compris one or veral extraction cycles with replacement of the solvent between cycles in the static mode. The sample cell is purged with an inert gas to wash off the solvent from the cell and the tubing into the collection vial at the end of the last extraction cycle to avoid any loss or memory effects.A wide range of extraction temperatures from room temperature to200°C and the applied pressure range of35–200bar can be applied for PLE.
One drawback of using pressurized-fluid technologies is that the higher applied pressure requires ex
pensive equipment.How-ever,we demonstrated that a laboratory-made dynamic PLE sys-tem at a lower applied pressure of10–20bar could be successfully applied to extract bioactive compounds in medicinal plants[26,27].It was obrved that the effect of pressure on the recovery or extraction efficiency of most substances in medicinal plants is usually negligible.Depending on the configuration of the extraction cell ud,the volume of solvent required for dy-namic PLE is comparable with PLE using static mode.Lastly,Table2 compares static and dynamic PLE.
The main factors that will affect the extraction efficiency of PLE include the nature of the solvent ud,applied temperature and number of cycles for static mode or time of extraction for the dy-namic mode[29].Optimization of the applied temperature for PLE is of key importance for the extraction of thermally labile compounds from botanicals.Bad on various reports,PLE has been successfully applied for the extraction of phenolic com-pounds,alkaloids,lignans,carotenoids and others from botanicals [29,30].The extraction efficiency of bioactive compounds by PLE is comparable with traditional methods of extraction[29,30].For the extraction of thermally labile compounds,such as gastrodin and vanillyl alcohol in Gastrodia elata Blume,PLE at room temper-ature with a laboratory-asmbled system was applied and the extraction efficiencies of the target compounds were found to be comparable with heating under reflux.For certain medicinal plants,PLE at room temperature wa
s found to be rapid and highly energy efficient as heating was not required[31].For the extraction of target compounds in certain botanicals where organic solvents are required,the significant reductions in time for sample prepara-tion and organic solvents make it an attractive option.
2.5.Pressurized hot-water extraction(PHWE)
To eliminate the u of organic solvents,PHWE is a feasible op-tion for the extraction of target compounds in food and herbal plants.The same equipment as PLE in the dynamic mode can be ud for PHWE.However,a commercially-available ASE system was also applied for the extraction of ginnosides in ginng using water as the extractant[32].Although a higher applied pressure at 50bar was propod in the earlier works for SWE[33],we ob-rved that applied pressure of10–20bar for PHWE will give a method recovery comparable with traditional methods of extrac-tion[26,27].Hence,PHWE can be carried out using a simpler instrumental t-up for operation at lower applied pressure.
For PHWE,the plant sample needs to be disperd with a cer-tain quantity of sand or other inert material.This additional step is required as plant materials have a higher tendency to adsorb water during the cour of extraction.Hence,the ground plant materials must be dispend evenly with sand to prevent any po-tential blockage of the system.
By contrast,this step will not be required for PLE using an or-ganic ,methanol)[26,27,33–35].Hence,the develop-ment of methods where plant samples need not be dispend by sand or other inert materials will be very desirable.
The parameters that may affect the extraction efficiency in PHWE include the applied temperature,extraction time and addi-tion of a small percentage of organic solvents or surfactants [34,35].Similar to PLE,optimization of the applied temperature will be critical for the extraction of thermally-labile compounds
Table2
Comparison of static and dynamic PLE(for which the plant material is typically disperd in a drying or inert ,sodium sulfate diatomaceous earth,sand or others).The mixture of inert sorbent and plant sample is packed in a stainless-steel cell and inrted in a clodflow-through system.
Static PLE Dynamic PLE
The extraction is performed in a static mode in the extraction cell for a
predetermined time
The extraction solvent is continuously pumped through the extraction cell
The extraction process consists of one or veral extraction cycles with replacement of the solvent between cycles in the static mode The operation involves theflow rate t during the static time and the pump delivers the solvent at a constantflow rate for a certain ,1.0–1.5mL/min for20–30min)
The sample cell is purged with an inert gas to wash off the solvent from the cell
and the tubing into the collection vial at the end of the last extraction cycle to
avoid any loss or memory effects
No inert gas is needed
35–2008bar can be applied A lower pressure(10–50bar)may be applied
M.Y.Heng et al./Trends in Analytical chemistry50(2013)1–105
