สารบัญ

Thai name  โกฐน้ำเต้า (KOT NAM-TAO)

Category  Laxative.

       Rhubarb consists of the dried underground parts of Rheum palmatum L.; R. tanguticum Maxim. Ex Balf., R. officinale Baill. or their interspecific hybrids, or mixture of these (Family Polygonaceae), separated from the stem, rootlets and most of the bark, frequently divided.  It contains not less than 0.25 per cent of sennoside A, C₄₂H₃₈O₂₀ calculated on the dried basis.

Origin of plant  Rhubarb is native to Asia, specifically China.  It is also widely cultivated in China, North Korea and South Korea.

Constituents  Rhubarb contains hydroxyanthracene derivatives including emodin, physcione, aloe-emodin, and chrysophanol glycosides. The dimeric sennosides A-F are also its major active principles.

Description  Odour, aromatic and characteristic; taste, bitter and slightly astringent.
Macroscopical  Ovoid, oblong-ovoid to cylindrical rhizome, often cut transversely or longitudinally, up to 15 cm long, up to 10 cm wide; surface shows a pinkish tinge with a reticulum of darker lines; transversely cut surface shows narrow outer zone; fracture granular.
Microscopical  Transverse section of the rhizome shows parenchyma, medullary ray, phloem, xylem vessels.  Parenchyma:  thin-walled round or polygonal cells with abundant simple or compound starch granules, star-shaped hilum or cluster of calcium oxalate crystals.  Medullary ray:  1- to 4-celled cells containing yellowish red or brownish orange amorphous mass.  Phloem:  sieve tubes, frequently obliterated.  Xylem vessel, in group of 2 to 5, reticulate, thickened.  Sclereids and fibres absent.

Other relevant information  According to its geographical origin, the appearance of rhubarb varies from subcylindrical, barrel-shaped, or irregularly formed pieces to cubes or rectangular pieces known as “rhubarb fingers”. It is gritty between the teeth and colours the saliva yellow upon chewing.

Packaging and storage  Rhubarb shall be kept in well-closed containers, protected from light and moisture.

Identification
A.  To 50 mg of the powdered drug, add 25 mL of 2 M hydrochloric acid, heat on a water-bath for 15 minutes, allow to cool, shake with 20 mL of ether, separate the ether layer and shake it with 10 mL of 6 M ammonia:  the aqueous layer turns red.
B.  Carry out the test as described in the “Thin-Layer Chromatography” (Appendix 3.1).
Standard solution  Dissolve 5 mg of emodin in 5 mL of ether.
Test solution  Heat 50 mg of the powdered drug in a water-bath for 15 minutes with a mixture of 1 mL of hydrochloric acid and 30 mL of water.  Allow to cool, add 25 mL of ether and shake well.  Dry the ether layer over anhydrous sodium sulfate and filter.  Evaporate the ether layer to dryness and dissolve the residue in 0.5 mL of ether.  
Adsorbent  Silica gel G.
Mobile phase  Anhydrous formic acid, ethyl acetate, and light petroleum (1:15:75).
Application  Apply 20 µL each of Standard solution and Test solution as 10-mm bands.
Development and drying  Allow the solvent front to ascend 10 cm above the line of application.  Dry the developed plate in air.
Detection  Examine the plate under ultraviolet light (366 nm), spray the plate with a 10 per cent w/v solution of potassium hydroxide in methanol and examine under visible light.  Observe the result.  
Results  When examined under ultraviolet light (366 nm), the test solution shows an orange fluorescent band due to emodin in the middle of the chromatogram corresponding in R_f to the band shown by the standard solution; above the emodin band, two bands of similar fluorescence (physcione and chrysophanol, in order of increasing R_f value); below the emodin band, also two bands of of similar fluorescence (rhein and aloe-emodin, in order of decreasing R_f value).  All the bands become red to violet after spraying with a 10 per cent w/v solution of potassium hydroxide in methanol.

Rheum rhaponticum  Carry out the test as described in the “Thin-Layer Chromatography” (Appendix 3.1).
Standard solution  Dissolve 10 mg of rhaponticin in 10 mL of methanol. 
Test solution  Heat 200 mg of the powdered drug, with 2 mL of methanol and boil under a reflux condenser for 5 minutes.  Allow to cool and filter.  Use the filtrate.
Adsorbent  Silica gel G.
Mobile phase  Methanol and dichloromethane (20:80).
Application  Apply 20 µL each of Standard solution and Test solution as 3-mm bands.
Development and drying  Allow the solvent front to ascend 12 cm above the line of application.  Dry the developed plate in air.
Detection  Spray the plate with phosphomolybdic acid TS and examine under visible light.  Observe the result.  
Results  The chromatogram obtained with the test solution does not show a blue band near the line of application corresponding to that shown in the chromatogram obtained with the standard solution.  

Loss on drying  Not more than 10.0 per cent w/w after drying at 105° to constant weight (Appendix 4.15).

Acid-insoluble ash  Not more than 2.0 per cent w/w (Appendix 7.6).

Total ash  Not more than 13.0 per cent w/w (Appendix 7.7).

Assay  Carry out the test as described in the “Liquid Chromatography” (Appendix 3.5). 
Mobile phase  Diluted acetic acid (1 in 80) and acetonitrile (4:1).
Standard preparation  A solution containing 50 µg per mL of Sennoside A RS in a 0.1 per cent w/v solution of sodium hydrogen carbonate.
System suitability preparation  Dissolve 1 mg each of Sennoside A RS and naringin in a 0.1 per cent w/v solution of sodium hydrogen carbonate to make 10 mL.
Assay preparation  Weigh accurately about 500 mg of the powdered Rhubarb, add 50 mL of a 0.1 per cent w/v solution of sodium hydrogen carbonate, shake for 30 minutes and filter.
Chromatographic system
DETECTOR  Ultraviolet light (340 nm). 
COLUMN  A stainless steel column (15 cm × 4 to 6 mm), packed with octadecylsilane chemically bonded to porous silica or ceramic microparticles (5 µm).
COLUMN TEMPERATURE  40°
FLOW RATE  Adjust so that the retention time of sennosides A is about 15 minutes. 
System suitability
Sample  System suitability  preparation
Suitability requirements
RESOLUTION  Not less than 3 between sennoside A and naringin peaks. 
RELATIVE STANDARD DEVIATION  Not more than 1.5 per cent determined from the sennoside A peak.
Procedure  Separately inject equal volumes (about 10 µL) of Standard preparation and Assay preparation into the chromatograph, record the chromatograms and measure the responses for the major peaks.
Calculation  Calculate the content of C₄₂H₃₈O₂₀ in the portion of the Rhubarb taken, using the declared content of C₄₂H₃₈O₂₀ in Sennoside A RS.

 RHUBARB, POWDERED

Thai name โกฐน้ำเต้าผง  (KOT NAM-TAO PHONG)

Complies with the requirements for content of sennoside A, Packaging and storage, Identification, Rheum rhaponticum, Loss on drying, Acid-insoluble ash, and Total ash stated under Rhubarb and with the following requirement.

Description  Orange or yellow-brown.  Diagnostic structures:  calcium oxalate cluster crystals, up to 100 µm or more in diameter, or their fragments; unlignified, reticulately thickened vessels, 20 to 175 µm in width, mostly 60 to 100 µm; starch granules abundant; sclereids and fibres absent.