COSMOSIL C₁₈-OPN

Conventional reversed phase C₁₈ packing materials are restricted to about 30-50% water in the mobile phase. The COSMOSIL C₁₈-OPN is a new "Water-Wet" C₁₈ packing material developed for reversed phase open column chromatography. The C₁₈-OPN material can be used in 100% aqueous effluents.

Characteristics

The external surface of the C₁₈-OPN gel is coated with hydrophilic group to increase wettability of the gel, and octadecyl group is bonded in the pore of the gel. This physical characteristic of the gel makes the reversed phase open column chromatography possible with 100% water.

Separation of Theobromine and Theophylline

The below figure shows the samples are clearly separated by reversed-phase open column chromatography with 70% water in the mobile phase.

Chromatogram of theobromide and theophylline separated on C₁₈-OPN

Conditions
Packing material	COSMOSIL 75C₁₈-OPN	Temperature	Room temperature
Column size	20 mm I.D. x 250 mm bed height	Detection	UV 254 nm
Mobile phase	Methanol:water = 30:70	Sample	Theobromide (100 mg) Theophylline (100 mg)
Flow rate	0.2 ml/min	Sample	Theobromide (100 mg) Theophylline (100 mg)

Influence of Particle Size

Condition
Packing material	COSMOSIL 75C₁₈-OPN	Detection	UV 254 nm
Column size	10 mm I.D. x 250 mm bed height	Sample	Theobromide (TB) Theophylline (TP) Caffeine (CF)
Mobile phase	Methanol:water = 20:80
Temperature	room temperature

Table: Comparison between 75 µm and 140 µm particle size silica

Particle size (µm)	Flow rate (ml/min)	Theoretical plate number			R_S		Separation time (min)	Solvent concentration (ml)
Particle size (µm)	Flow rate (ml/min)	TB	TP	CF	TB/TP	TP/CF	Separation time (min)	Solvent concentration (ml)
75	0.25	400	390	340	1	1.74	240	60
140	0.6	300	280	260	0.9	1.4	100	60

Flow Rate

Concentration of methanol vs. flow rate
Column: 10 mm I.D. x 180 mm bed height (gravitational liquid flow)

Since reversed phase chromatography generally employs high viscosity solvents such as water and methanol, the flow rate is lower than that of normal phase chromatography. The flow rate of reversed phase depends on the mobile phase composition. The flow rate of the COSMOSIL 140C₁₈-OPN (140 µm in particle size) is about 2.5 times higher than that of the COSMOSIL 75C₁₈-OPN.

Separation of p-Cresol and p-Ethyphenol in Normal and Reversed Phase Mode

Since the structural difference between p-cresol and p-ethylphenol is only one methyl group, it is difficult to separate such samples under normal phase conditions. On the other hand, the samples are clearly separated under reversed phase conditions with COSMOSIL C₁₈-OPN packing material.

Condition
Packing material	Silica gel 120 (150-325 mesh)
Column size	10 mm I.D. x 180 mm bed height
Mobile phase	n-Hexane : ethyl acetate = 90 : 10
Flow rate	0.7 ml/min
Temperature	Room temperature
Detection	UV 254 nm
Sample	p-Cresol p-Ethylphenol

Condition
Packing material	COSMOSIL 75C18-OPN
Column size	10 mm I.D. x 180 mm bed height
Mobile phase	Methanol:water = 60:40
Flow rate	0.2 ml/min
Temperature	Room temperature
Detection	UV 254 nm
Sample	p-Cresol p-Ethylphenol

Packing Material by Column Size

Column size and required amount of C₁₈-OPN packing material

Column I.D. (mm)	Bed height (mm)	Amount of C₁₈-OPN (g)
10	150	4
10	250	7
20	150	17
20	250	28
30	150	38
30	250	63

Reproducibility and Washing

Wash the COSMOSIL C₁₈-OPN packing material with tetrahydrofuran, chloroform, or other solvents to remove the impurities. This packing material has excellent reproducibility and can be used repeatedly.

CAUTION

Do not wash with basic solvents of pH 7 or greater, which will dissolve the silica gel, or pH 2 or less, which will cleave the C₁₈ stationary phase. Dry the packing material at 50°C or less.