Separation and determination of tocopherols in vegetable oils by solid phase extraction on porous polymers SPE cartridges and capillary gas chromatography analysis

In this study we present the solid phase extraction selectivity of tocopherols from vegetable oils using four porous polymers (Porapak P, Porapak Q, Porapak QS, Porapak N). The tocopherols elution from SPE cartridges was performed using several hexanes:ethyl acetate mixtures (100:0, 95:5, 90:10, 85:15, v/v). Tocopherols (α, γ and δ-tocopherol) were analyzed by gas chromatography without any derivation steep. The amount of NaOH used for triglyceride removal was optimized. Particularly liquid-liquid and solid phase extraction methods for the extraction of tocopherols from vegetable oils were compared. The results confirmed that porous polymers represent promising SPE alternatives for the extraction of tocopherols from oils.     

Simple strategy for the optimization of solid-phase extraction procedures through the use of solid-liquid distribution coefficients application to the determination of aliphatic lactones in wine

A practical strategy for the optimization of solid-phase extraction (SPE) systems is presented. Critical SPE volumes (sample loaded, rinsing and elution solvent) are calculated from solid–liquid extraction coefficients and from basic bed parameters determined in simple experiments, using the Lövkist Jonsson model and other expressions derived from the general theory of chromatography. The agreement between calculated and measured volumes is satisfactory, which makes it possible to consider different sorbents and rinsing and elution solvents in the SPE optimization with a relatively low experimental effort. The strategy has been successfully applied to the optimization of a SPE method directed to the selective extraction of aliphatic lactones from wine. Six different reversed-phase sorbents were studied and the one showing maximum extraction selectivity was selected. Wine (50 ml) is extracted in a 200 mg cartridge filled with Bond Elut-ENV resins. Interferences are removed with 20 ml of methanol–water (40:60) with 1% NaHCO₃. Elution is carried out with 1.8 ml of dichloromethane. The extract is concentrated to 0.15 ml and analyzed by GC–ion trap MS. Eight odor-active aliphatic γ and δ lactones (with 8–12 C atoms) from wine are recovered (R>75%) in an extract free from wine major volatiles. Detection limits are in the 40–300 ng/l range, well below the odor detection threshold of these compounds. Linearity (r²>0.996) and precision (average R.S.D. 3.5%) are satisfactory in all cases. The levels in wine of some of these lactones (γ-octa, undeca and dodecalactones) are reported by first time and results demonstrate that can be present at concentrations above or close to their corresponding odor thresholds.     

Rapid extraction of bioactive compounds from gardenia fruit using new and recyclable deep eutectic solvents

The utilization of deep eutectic solvent as an alternative and environmentally friendly option has gained significant attention. This study first proposed a series of benzylammonium chloride based-deep eutectic systems for the extraction of bioactive compounds from Gardenia jasminoides Ellis. Through the implementation of response surface methodology, the optimal solvent was determined to be dodecyldimethylbenzylammonium chloride–levulinic acid (1:3, mol/mol) with 35% (v/v) water, specifically tailored to extract geniposide, genipin-1-β-d -gentiobioside, crocin-1, and crocin-2 from gardenia fruits with the ratio of solid to liquid of 1:20 at 86°C for 16 min. Their total extraction yields could reach 70.6 mg/g, outperforming those obtained by other solvents and corresponding techniques. Furthermore, the eutectic system was retrieved after first-cycle extraction, and then applied in the subsequent extraction progress, yielding a consistent extraction efficiency of 97.1%. As compared to previous traditional methods, a quick, high-yielding, and green extraction procedure was achieved through simple heating settings that did not constrain the instrument. Therefore, dodecyldimethylbenzylammonium chloride–levulinic acid could serve as a sustainable and reusable solvent for efficient extraction of natural bioactive compounds from plant-based raw materials. The application of deep eutectic solvents has demonstrated their potential as designable solvents with stronger extraction capabilities than traditional organic solvents.     

Pressurized liquid extraction of lipids for the determination of oxysterols in egg-containing food

Pressurized liquid extraction (PLE, ASE) was compared with the Folch procedure (a solid-liquid extraction with chloroform/methanol 2:1, v/v) for the lipid extraction of egg-containing food; the accuracy of PLE for the quantitative determination of oxysterols in whole egg powder was evaluated. Samples of spray-dried whole egg, an Italian vanilla cake (Pandoro) and egg noodles were used. Two different extraction solvents (chloroform/methanol 2:1, v/v, and hexane/isopropanol 3:2, v/v) were tested at different extraction temperatures and pressures (60 degrees C at 15 MPa, 100 degrees C at 15 MPa, 120 degrees C at 20 MPa). No significant differences in the lipid recovery of the egg powder sample using PLE were found. However, PLE of the vanilla cake and egg noodles with the chloroform/methanol mixture was not selective enough and led to the extraction of a non-lipid fraction, including nitrogen-containing compounds. In the same samples, the pressurized hexane/isopropanol mixture gave a better recovery result, comparable to that obtained using the Folch method. Cholesterol oxidation products of the Folch extract and the pressurized liquid extract of spray dried egg powder (obtained with hexane/isopropanol 3:2, v/v, at 60 degrees C and 15 MPa) were determined by gas chromatography. PLE performed under these conditions is suitable to replace the Folch extraction, because the differences between the two methods tested were not statistically significant. Moreover, PLE shows important advantages, since the analysis time was shortened by a factor of 10, the solvent costs were reduced by 80% and the use of chlorinated solvents was avoided.     

Assessment of different sorbents efficiency for solid phase extraction of aquatic humic acids

In the present study, a simple procedure for the isolation by solid-phase extraction (SPE) and quantification by UV-Vis spectrometry (400 nm) of the humic acids (HAs) in the natural waters was developed. Seven different sorbents: Porapak P (polystyrene-divinylbenzene copolymer), Florisil (chemical composition: 84.0% SiO₂, 15.5% MgO and 0.5% Na₂SO₄), Silica gel C18 (octadecyl silane), Strata X (surface modified polystyrene-divinylbenzene), Strata NH₂ (silica-based trifunctional amino ligand), Strata SAX (silica-based trifunctional quaternary amine) and Strata C18-E (silica-based trifunctional C18 with hydrophobic end-capping of silanols) were tested. The HAs, adsorbed on SPE cartridges, were eluted using: NaOH (0.1 M), sodium dodecyl sulphate (SDS) (20 g L⁻¹), and a 1:1 v/v mixture of SDS (20 g L⁻¹) and NaOH (0.1 M). The extraction efficiency was evaluated by comparing the HAs recovery levels. The repeatability of results was estimated by the relative standard deviation (RSD). The data confirmed that Porapak P, Silica gel C18, Florisil, Strata NH₂ and Strata X could be good alternatives for the traditional isolation of the aquatic HAs with XAD resin. The proposed method was applied for the determination of HAs in some waters sampled from the Western Romanian Plain. The content of HAs was correlated with the arsenic concentration and total organic carbon (TOC) level.      

Solid‐phase extraction of matrine and oxymatrine from Sophora Flavescens Ait using amino‐imidazolium polymer

A new, polymer‐confined, ionic liquid sorbent was developed by a process involving polymerization and modification. The obtained particles were successfully used as a special sorbent in SPE process to isolate matrine and oxymatrine from Sophora Flavescens Ait. Different washing and elution solvents, such as ethanol, methanol, acetonitrile and methanol/triethylamine (90:10, v/v), were evaluated. Compared with the C₁₈ and NH₂ sorbents, the amino‐imidazolium polymer sorbent exhibited higher selectivity. Quantitative analysis was carried out by using a C₁₈ column. The two compounds exhibited good linearity from 5×10^?3 to 0.50 mg/mL (r²>0.99). The bound amounts between target compounds and proteins were obtained by this sorbent. After three recycles of amino‐imidazolium polymer, the extract amounts of the target compounds were not significantly decreased.     

New fractionation scheme for lipid classes based on "in-cell fractionation" using sequential pressurized liquid extraction

A new preparation scheme is proposed to fractionate neutral lipids (acylglycerines, sterol esters, long-chain free fatty acids) from polar phospholipids in biological matrices. This fractionation is mandatory in the microbial community, for the control of bioremediation processes, in the study of phytoplankton growth in lakes and rivers, and in the quality control of processed food. Basically, a two-step pressurized liquid extraction (PLE) scheme is combined with an "in-cell-fractionation" using silica-based sorbents placed at the outlet of the PLE cartridge. The optimized extraction scheme consists of n-hexane/acetone (9:1, v/v) extraction at 50 degrees C (2 cycles, 10 min each) to obtain neutral lipids followed by chloroform/methanol (1:4, v/v) extraction at 110 degrees C (2 cycles, 10 min each). Thermally pre-treated silicic acid and cyanoproyl-modified silica turned out to be appropriate sorbents to ensure clear-cut boundaries between neutral lipids and phospholipids. The proposed protocol is superior to commonly used approaches consisting of an exhaustive lipid extraction followed by off-line lipid fractionation using solid-phase extraction (SPE) regarding fractionation efficiency, time and solvent consumption. In this paper, it is also shown that the transmethylation using trimethylchlorosilane/methanol (1:9, v/v) provides a complete reaction to give methyl esters without artefact formation across the array of different lipid classes even with polyunsaturated fatty acid moieties.     

High performance liquid chromatography–tandem mass spectrometry for the analysis of 10 pesticides in water: A comparison between membrane-assisted solvent extraction and solid phase extraction

This work describes the application of two sample preparation methods: membrane-assisted solvent extraction (MASE) and solid phase extraction (SPE) in combination with high performance liquid chromatography–tandem mass spectrometry (HPLC–MS–MS) for the determination of 10 pesticides in surface and ground water. Optimal extraction conditions for MASE were 60 min extraction time at 30 °C with a solvent volume of 100 μL toluene. 5 μL of the toluene extract were directly injected in the HPLC–MS–MS system. Concerning SPE, two materials were tested and C18 was superior to Oasis HLB. Complete desorption was ensured by desorbing the SPE (C18) cartridge with 3 mL of an acetonitrile/methanol mixture (1:1). After evaporation, the extract was injected in the analytical system. Analyte breakthrough was not found for the investigated compounds. For both methods, high extraction yields were achieved, in detail 71% (metalaxyl) till 105% (linuron) for MASE and 52% (ethiofencarb) till 77% (prometryne) for SPE (C18). Detection limits were in the low ng/L range for both methods and precision, expressed as the relative standard deviation (RSD) of the peak areas was below 13%. Five real water samples were analyzed applying both extraction methods. The results were in good agreement and standard addition proved that no matrix effects (such as ion suppression) occurred. In this comparison SPE has the potential of larger sensitivity whereas faster analysis and slightly better recoveries were achieved with MASE. MASE shows potential to be a promising alternative to the conventional off-line SPE concerning low to medium polar compounds.     

Optimization of the extraction process of flavonoids from Trollius ledebouri with natural deep eutectic solvents

In recent years, natural deep eutectic solvents have been favored greatly due to their environment friendly, mild biological toxicity and simple biodegradability. Natural deep eutectic solvents gradually applied for the extracting bioactive compounds from natural products efficiently. In this study, 20 natural deep eutectic solvents were prepared and their physical and chemical properties were tested. The ultrasonic-assisted extraction method was used to extract flavonoids from Trollius ledebouri and high-performance liquid chromatography-ultraviolet was applied to examine two main bioactive flavonoids (orientin and vitexin). Compared with traditional solvents (water and 60% ethanol solution), natural deep eutectic solvents composed of L(-)-proline and levulinic acid (molar ratio 1:2) show a super extraction efficiency. On this basis, the response surface method was used to optimize the extraction temperature, extraction time, water contents, and solid–liquid ratio. As a consequence, the extraction temperature 60℃, extraction time 18 min, water content 14% (v/v), and the solid–liquid ratio 48 mL·g⁻¹ were chosen as the best extraction process. This study shows that natural deep eutectic solvents can effectively extract flavonoids from T. ledebouri, laying a foundation for the further application of natural deep eutectic solvents to extract bioactive compounds from natural products.     

Optimization and evaluation of a procedure for the gas chromatographic-mass spectrometric analysis of the aromas generated by fast acid hydrolysis of flavor precursors extracted from grapes

A procedure has been developed for the GC-MS analysis of the aromas released in fast acid hydrolysis of precursor fractions from grape musts and skins. Different sorbents for the extraction of the precursors were compared. The best results were obtained with LiChrolut EN polymeric resins which displayed two and six-fold more extraction capacity than Amberlite XAD-2 resins and C18 sorbents, respectively. C18 sorbents are more suitable for selective extraction of less polar precursors. The initial version of the method was imprecise and so the imprecision of the different steps was assessed. The maceration of the solid parts and the liquid-liquid extraction of the aromas released in the acid hydrolysis proved to be the critical steps. Greater crushing of the solid parts and solid-phase extraction (SPE) instead of liquid-liquid extraction (LLE) improved reproducibility. In the method finally proposed about 100 aromatic components belonging to four large groups (lipid derivatives, shikimic acid derivatives, norisoprenoids and terpenes) were determined with good reproducibility. Important aroma compounds, such as cis-rose oxide or wine lactone were detected in non-Muscat grapes.     

Solid-phase extraction of polycyclic aromatic compounds

Solid-phase extraction (SPE) for two groups of polyaromatic compounds--polycyclic aromatic hydrocarbons and naphthalenesulfonates--with completely different problems in the extraction process are reviewed. The sorbents used in each case and the different steps of SPE are studied. Particular problems encountered in the SPE of each group are described. Adsorption problems of PAHs which require an organic solvent or a surfactant to be added to the sample are explained. The need of ion-pair solid-phase extraction for extracting naphthalenesulfonates and the influence of the inorganic species in the extraction are discussed. The on-line systems are described for both group of compounds.     

Phenolic-compound-extraction systems for fruit and vegetable samples

This paper reviews the phenolic-compound-extraction systems used to analyse fruit and vegetable samples over the last 10 years. Phenolic compounds are naturally occurring antioxidants, usually found in fruits and vegetables. Sample preparation for analytical studies is necessary to determine the polyphenolic composition in these matrices. The most widely used extraction system is liquid-liquid extraction (LLE), which is an inexpensive method since it involves the use of organic solvents, but it requires long extraction times, giving rise to possible extract degradation. Likewise, solid-phase extraction (SPE) can be used in liquid samples. Modern techniques, which have been replacing conventional ones, include: supercritical fluid extraction (SFE), pressurized liquid extraction (PLE), microwave-assisted extraction (MAE) and ultrasound-assisted extraction (UAE). These alternative techniques reduce considerably the use of solvents and accelerate the extraction process.     

Extraction and determination of bioactive flavonoids from Abelmoschus manihot (Linn.) Medicus flowers using deep eutectic solvents coupled with high‐performance liquid chromatography

A highly efficient and ecofriendly extraction method using deep eutectic solvents was developed to extract bioactive flavonoids from Abelmoschus manihot (Linn.) Medicus flowers. First, a series of deep eutectic solvents using choline chloride as hydrogen bond acceptor with different hydrogen bond donors was successfully synthesized. Then, the types of deep eutectic solvents and the extraction conditions for bioactive flavonoids (hyperoside, isoquercitrin, and myricetin) were optimized based on the flavonoids extraction efficiencies. The optimized deep eutectic solvent for hyperoside and isoquercitrin extraction was composed of choline chloride and acetic acid with a molar ratio of 1:2. The optimized deep eutectic solvent for myricetin extraction was composed of one mole of choline chloride and two moles of methacrylic acid. The optimal extraction conditions were set as: solid to solvent ratio, 35:1 (mg/mL); extraction time, 30 min; extraction temperature, 30°C. Qualitative and quantitative analysis were performed using ultra high performance liquid chromatography with tandem mass spectrometry and high‐performance liquid chromatography. And the extraction efficiencies of hyperoside, isoquercitrin, and myricetin under optimal extraction conditions were calculated as 11.57, 5.64, and 1.11 mg/g, much higher than those extracted by traditional extraction solvents. Therefore, the prepared deep eutectic solvents can be selected as alternative solvent to extract bioactive flavonoids.     

Development of a solid-phase extraction method for determination of pheophorbide a and pyropheophorbide a in health foods by liquid chromatography

A simple solid-phase extraction (SPE) method was developed for the liquid chromatography (LC) determination of pheophorbide (Phor) a and pyropheophorbide (Pyro) a in health foods such as chlorella, spirulina, etc. The food sample was extracted with 85% (v/v) acetone. The extract was acidified with hydrochloric acid and loaded on a C18 cartridge. After washing with water, Phor a and Pyro a were eluted with the LC mobile phase. Phor a and Pyro a were separated by isocratic reversed-phase LC and quantitated by fluorescence detection. The recoveries for spiked samples of chlorella and the extract were 87.1-102.0%. Commercial health foods (chlorella, spirulina, aloe, kale, Jews mallow, and green tea leaves) were analyzed using the SPE method. The values found for Phor a and Pyro a ranged from 2 to 788 microg/g and from <1 to 24 microg/g, respectively. There was no significant difference between the SPE method and the official method in Japan (spectrophotometry after liquid-liquid extraction). The advantages of the SPE method are the short extraction times, lack of emulsions, and reduced consumption of organic solvents compared with the official method in Japan. The SPE method is considered to be useful for the screening of Phor a and Pyro a in health foods.     

Separation of eight selected flavan-3-ols on cellulose thin-layer chromatographic plates

The potential of microcristaline cellulose as sorbent in the separation of eight compounds: (+)-catechin (C), (-)-epicatechin (EC), (-)-gallocatechin (GC), (-)-epigallocatechin (EGC), (-)-epicatechin gallate (ECg), (-)-epigallocatechin gallate (EGCg), procyanidin B1 and procyanidin B2 was studied. Cellulose HPTLC plates prewashed in water (not necessary, when water was used as developing solvent) and dried with a hair dryer, bandwise application and development in horizontal developing chamber (sandwich configuration) gave the best results. Detection was performed using vanillin-H3PO4 reagent. Four new developing solvent systems were proposed: water, 1-propanol-water (20:80, v/v), 1-propanol-water-acetic acid (4:2:1, v/v) and 1-propanol-water-acetic acid (20:80:1, v/v), and at least two of them were needed for the differentiation between all eight compounds. Surprisingly, water enabled the separation of epimers C from EC and GC from EGC, as well as the dimers procianidin B1 and B2. Additionally, C, EGC, B1 and B2 were separated from all the other compounds. The best choice for developing solvent is given for each of the studied compounds. The best separation of the five main catechins (EC, GC, EGC, ECg, EGCg) present in green tea extract was achieved using 1-propanol-water-acetic acid (20:80:1, v/v). The chromatograms of oak bark extract developed in solvents with higher water content (1-propanol-water (1:4, v/v) and 1-propanol-water-acetic acid (20:80:1, v/v)) showed less bands than chromatograms developed in solvents with higher organic modifier content (e.g. 1-propanol-water-acetic acid (4:2:1, v/v)). It was proved that such behavior was due to the presence of procyanidins beside the main component catechin.     

Combined microwave-assisted isolation and solid-phase purification procedures prior to the chromatographic determination of phenolic compounds in plant materials

A fast, sensitive and selective procedure employing a combination of microwave-assisted extraction (MAE) and solid phase extraction (SPE) was applied prior to liquid chromatographic identification and quantification of phenolic compounds in plant materials. MAE has been tested and optimized for the isolation of phenolic acids (gallic, protocatechuic, p-hydroxybenzoic, chlorogenic, vanilic, caffeic, syringic, p-coumaric, ferulic, sinapic, benzoic, m-coumaric, o-coumaric, rosmarinic, cinnamic acids) and 3,4-dihydroxybenzaldehyde, syringaldehyde, p-hydroxybenzaldehyde, and vanillin in various plants. The effects of experimental conditions on MAE efficiency, such as solvent composition, temperature, extraction time, have been studied. The extraction efficiencies were compared with those obtained by computer-controlled, two-step Soxhlet-like extractions. Plant extracts were purified and phenolic compounds were pre-concentrated using SPE on polymeric RP-105 SPE sorbent prior to HPLC analysis. Chromatographic separation was carried out on a Hypersil BDS C₁₈ column using a mobile phase consisted of 0.3% (v/v) acetic acid in water (solvent A) and methanol (solvent B) at flow rate 0.6 ml min⁻¹ and column temperature 30 °C with gradient elution.     

Extractions with superheated water

As the temperature of liquid water is raised under pressure, between 100 and 374 degrees C, the polarity decreases markedly and it can be used as an extraction solvent for a wide range of analytes. Most interest has been in its application for the determination of PAHs, PCBs, and pesticides from environmental samples, where it gives comparable results to Soxhlet extraction but more rapidly and without the use of significant volumes of organic solvents. Unlike SPE, n-alkanes are not extracted unless the pressure is reduced and steam is used. Other applications have included the extraction of essential oils from plant material where it preferentially extracts the economically more important oxygenated components compared to steam distillation. The aqueous extract has been concentrated in a number of different methods (solvent extraction, SPE, SPME, extraction disc) or the extraction can be linked on-line to LC or GC. In many cases the superheated water extraction is cleaner, faster and cheaper than the conventional extraction methods.     

Automatic flow system for evaluation of polystyrene-divinylbenzene sorbents applied to preconcentration of phenolic pollutants

In this work, an evaluation of commercially available polystyrene-divinylbenzene sorbents for solid-phase extraction (SPE) of eleven phenolic compounds is intended. Considering the particle size and cross-linking degree, Amberlite XAD-4 (commercial or grounded), Macronet MN-200 and Lichrolut EN were tested. The SPE protocol was performed by an automatic system, providing repeatable experimental conditions for assessment of sorbent capacity, breakthrough volume and enrichment factor (EF). A positive correlation between EF and log?K _ow was found for Amberlite XAD-4 while a negative correlation was observed between EF and molecular weight of analyte for Macronet MN-200 and for Lichrolut EN. This indicates a prevalence of hydrophobic interactions or molecular exclusion depending upon the polymer cross-linking degree. Despite the similar repeatability (RSD?<?4.7%, n?>?6) and recovery values attained (97.6–102.7%, using 50?mL of sample) for all sorbents, Lichrolut EN is the best choice for analytical application as higher EF and lower LOD values (between 18 and 207?ng) were attained for this sorbent.     

Determination of organochlorine and pyrethroid pesticides in fruit and vegetables using solid phase extraction clean-up cartridges

A method to determine six organochlorine and three pyrethroid pesticides in grape, orange, tomato, carrot and green mustard based on solvent extraction followed by solid phase extraction (SPE) clean-up is described. The pesticides were spiked into the sample prior to analysis, extracted with ethyl acetate, evaporated and reconstituted with a solvent mixture of acetone:n-hexane (3:7). Three different sorbents (Strong Anion Exchanger/Primary Secondary Amine (SAX/PSA), Florisil and C18) were used for the clean-up step. Pesticides were eluted with 5mL of acetone:n-hexane (3:7, v/v) and determined by gas chromatography and electron-capture detection (GC-ECD). SAX/PSA was the sorbent, which provided chromatograms with less interference and the mean recoveries obtained were within 70-120% except for captafol. The captafol recoveries for grape were within acceptable range with C18 clean-up column.     

QuEChERS-超高效液相色谱-高分辨串联质谱技术检测鸡肉中6种抗球虫药物

建立了鸡肉中二硝托胺、尼卡巴嗪、地克珠利、妥曲珠利、莫能菌素及盐霉素6种抗球虫药物的超高效液相色谱-高分辨串联质谱多残留检测方法。经QuEChERS样品净化,首先使用含有1%(v/v)三氯乙酸的乙腈-水(3 : 7, v/v)溶液提取样品中的被测物,再加入氯化钠,使用50 mg/mL N-丙基乙二胺(PSA)+50 mg/mL中性氧化铝(Alumina-N)的混合分散固相萃取(dispersive solid phase extraction, DSPE)粉末净化提取,过0.22 μ m滤膜后以超高效液相色谱-高分辨串联质谱检测。选择Waters Acquity UPLC^® BEH C8色谱柱(100 mm×2.1 mm, 1.7 μ m),以甲醇-5 mmol/L醋酸铵水溶液为流动相进行梯度洗脱。使用正、负离子同时扫描模式,基质外标法定量。研究表明,6种目标化合物的线性范围为:二硝托胺,1.0~30.0 μ g/L;尼卡巴嗪,0.2~6.0 μ g/L;地克珠利、妥曲珠利,2.0~60.0 μ g/L;莫能菌素、盐霉素,4.0~120.0 μ g/L。空白样品中添加低、中、高3个水平的混合标准溶液,回收率在67.7%~126.8%之间,相对标准偏差(RSD)≤10.4%。6种抗球虫药物的定量限分别为:二硝托胺,2.50 μ g/kg;尼卡巴嗪,0.50 μ g/kg;地克珠利、妥曲珠利,5.00 μ g/kg;莫能菌素、盐霉素,20.00 μ g/kg。该方法操作简便,灵敏度高,且能够满足日常检测要求。