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 Capillary Electrophoresis
The capillaries used in CE and related techniques areas almost invariably made from drawn fused silica externally coated with polyimide for strength and mechanical stability. They normally have no coating on their internal walls, however, certain substances can be bonded to the inner surface, either to eliminate sample to wall interactions or to modify the magnitude or direction of the electro-osmotic flow (EOF).

Bare Silica Capillaries
The capillary inner wall has a large number of surface silanol groups which, when in contact with an electrolyte solution, dissociate leading to a negative charge on the capillary wall. Consequently, in order to preserve overall electrical neutrality, there must be an excess of cations in the electrolyte immediately adjacent to the inner surface (see diagram below). When a voltage is applied, these cations migrate in the direction of the cathode and in so doing exert a force on the surrounding fluid causing it to flow in the same direction. This electrically induced flow is known as electro-osmotic flow (EOF). In bare silica capillaries, i.e. uncoated silica, the electro-osmotic flow is high when the electrolyte pH is greater than 6 and effectively drops to zero for pH values of 3 or less.

For reproducible analyses it is important that the surface density of free silanol groups remains constant from one analysis to the next. For this reason, the capillary is often rinsed with sodium hydroxide solution (or alternatively a strong acid) followed by a further rinsing with the run buffer.

For coated capillaries, the rinsing procedures are generally less harsh in order to avoid the possibility of stripping off of the inner coating of the capillary.

In order to obtain a consistent EOF, new uncoated capillaries are flushed with 0.1M NaOH solution for 20minutes prior to their initial use. This procedure is carried out in order to reactivate the surface silanols. The Procedure must no be carried out with wall coated capillaries.

Electrical Double Layer - Origin of Electro-Osmotic Flow (EOF)


Capillary Regeneration


Coated Capillaries
The nature of the inner wall of the capillary can be modified by covelant bonding of appropriate substances. This may generate a neutral or a charged surface layer.

Neutral Coatings
Neutral Capillaries are used either to reduce analyte adsorption or in situations where it is considered desirable to eliminate EPF. Usually the capillaries are covelantly coated with polymers such as celluloses or polyacrylamide. These types of coatings completely eliminate EOF. Prior to the bonding procedure, the silica surface is normally derivatised with an alkyls silane.

Charged Coatings
The capillaries can be coated either with a positive or negatively charged substance in order to manipulate both the direction and the magnitude of the EOF and/or to reduce analyte adsorption. For example, polyamine coated capillaries produce a highly positive charged surface that generates, in contrast to the usual situation, a substantial EOF towards the anode. The positive charge on the surface repels positively charged basic compounds and their tendency to absorb in the surface is considerably reduced.

Alternatively, the capillary can be treated to give an increased negative charge on the surface relative to an uncoated capillary. This increase in EOF can be useful in significantly reducing analysis times.

In uncoated capillaries the EOF is strongly dependent on the buffer pH and is not present at all at low pHs. This makes it impossible, for example to carry out micellar work at low pH values. Moreover, small pH changes in the pH range 4-6 can result in relative large changes in electro-osmotic flow, which can be a problem in terms of repeatability and precision of a method.

However, if the capillary coating is fully ionised over a wide pH range, the EOF rate is then largely independent of the pH ( pH independent capillary). For example, the pH independent capillary allows the use of SDS based MECC at lower pH values than those normally used. Low pH MECC is a particularly useful technique for the analysis of acidic solutes, which at high pH are negatively charged and therefore repelled from the negatively charged micelle.

Controlled manipulation of the surface charge density on the pH independent coating allows a tight control of the EOF obtained for the capillary. For example, a medium charge density produces a moderate EOF rate (medium flow capillary) whilst a low level of coating generates a low EOF (low flow capillary)

Bare Silica Capillaries
Uncoated CE capillaries are made from high quality drawn fused silica with a high degree of UV transparency. All Capital Analytical capillaries have an outer diameter of 375µm and are available with internal diameters of 50µm and 75µm

For convenience of handling and easy installation, the majority of CE instruments use a cartridge, which houses the coiled capillary. In most cases there are a number of standard lengths for each instrument type. These standard lengths are given by the way in which the capillary is coiled, i.e., each length increment should be a multiple of the coiled circumference. Capillary lengths are typically 25-75cm. Longer length capillaries i.e., 100cm or more are sometimes used for special applications such as interfacing of commercial CE instruments with a mass spectrometer.

The injection end of the capillary should be cleanly cut to give a flat surface. Poorly cut capillaries can lead to loose polyimide coating and a cracked or shattered fused silica edge which will lead to tailing peaks, sample carry over and a reduction in peak area precision. It is therefore of considerable benefit for the CE user to obtain expertly prepared capillaries. Capital Analytical prepared capillaries are cut to a high degree of precision with the UV detection window accurately prepared and positioned. A small length of the polyimide coating is also removed from the injection end in order to improve sample introduction precision.

Capital Analytical can also supply prepared capillaries in non standard lengths. For customers who wish to supply their own capillaries, we offer a precision preparation service (please contact our sales office for details).

Description

Cat. No.

Price (£)

  

0-35cm Fused Silica Capillary (bare surface) 7m

EKA1125

0-35cm Fused Silica Capillary (bare surface) 7m - Pack of Three

EKA1125P3

40-65cm Fused Silica Capillary (bare surface)7m

EKA1150

40-65cm Fused Silica Capillary (bare surface)7m - Pack of Three

EKA1150P3

70-100cm Fused Silica Capillary (bare surface) 7m

EKA1175

70-100cm Fused Silica Capillary (bare surface) 7m - Pack of Three

EKA1175P3

0-35cm Fused Silica Capillary (bare surface) 50µm

EKB1125

0-35cm Fused Silica Capillary (bare surface) 50µm - Pack of Three

EKB1125P3

40-65cm Fused Silica Capillary (bare surface) 5m

EKB1150

40-65cm Fused Silica Capillary (bare surface) 50µm - Pack of Three

EKB1150P3

70-100cm Fused Silica Capillary (bare surface) 50µm

EKB1175

70-100cm Fused Silica Capillary (bare surface) 50µm - Pack of Three

EKB1175P3


Capillary Replacement After the polyimide coating is removed to form the detection window, the exposed fused silica becomes extremely fragile and should be subjected to minimal handling. Man users will have experienced the frustration of breaking the capillary whilst fitting it into the cartridge unit. In order to overcome this problem, Capital Analytical can supply prepared capillaries already fitted in the cartridge aligned with the detection aperture. The exact alignment of the capillary detection window with the cartridge detection aperture is essential in order to ensure maximum UV sensitivity.

Capital Analytical offers a capillary replacement service for al popular cartridge formats such as Hewlett-Packard, Beckman and TSP. For other instrument formats please contact our sales office.

Description

Cat. No.

Price (£)

  

Beckman – 25cm Fused Silica Capillary (bare) 7m

EKA1225

Beckman – 25cm Fused Silica Capillary (bare) 7m - Pack of Three

EKA1225P3

Beckman – 50cm Fused Silica Capillary (bare) 7m

EKA1250

Beckman – 50cm Fused Silica Capillary (bare) 7m - Pack of Three

EKA1250P3

Beckman – 75cm Fused Silica Capillary (bare) 7m

EKA1275

Beckman – 75cm Fused Silica Capillary (bare) 7m - Pack of Three

EKA1275P3

Beckman 25cm Fused Silica Capillary (bare) 50µm

EKB1225

Beckman 25cm Fused Silica Capillary (bare) 50µm - Pack of Three

EKB1225P3

Beckman 50cm Fused Silica Capillary (bare) 50µm

EKB1250P3

Beckman 50cm Fused Silica Capillary (bare) 50µm - Pack of Three

EKB1250

Beckman 75cm Fused Silica Capillary (bare) 50µm

EKB1275

Beckman 75cm Fused Silica Capillary (bare) 50µm - Pack of Three

EKB1275P3

Fitted to HP Cartridge

HP 25cm Fused Silica Capillary (bare) 75µm

EKA1325

HP – 25cm Fused Silica Capillary (bare) 7m - Pack of Three

EKA1325P3

HP 50cm Fused Silica Capillary (bare) 75µm

EKA1350

HP – 50cm Fused Silica Capillary (bare) 7m - Pack of Three

EKA1350P3

HP 75cm Fused Silica Capillary (bare) 75µm

EKA1375

HP – 75cm Fused Silica Capillary (bare) 7m - Pack of Three

EKA1375P3

HP – 25cm Fused Silica Capillary (bare) 5m

EKB1325

HP 25cm Fused Silica Capillary (bare) 50µm - Pack of Three

EKB1325P3

HP – 50cm Fused Silica Capillary (bare) 5m

EKB1350

HP 50cm Fused Silica Capillary (bare) 50µm - Pack of Three

EKB1350P3

HP – 25cm Fused Silica Capillary (bare) 5m

EKB1375

HP 25cm Fused Silica Capillary (bare) 50µm - Pack of Three

EKB1375P3

Fitted to TSP Cartridge

TSP 25cm Fused Silica Capillary (bare) 75mm

EKA1425

TSP 25cm Fused Silica Capillary (bare) 75mm - Pack of Three

EKA1425P3

TSP 50cm Fused Silica Capillary (bare) 75mm

EKA1450

TSP 50cm Fused Silica Capillary (bare) 75mm - Pack of Three

EKA1450P3

TSP 75cm Fused Silica Capillary (bare) 75mm

EKA1475

TSP 75cm Fused Silica Capillary (bare) 75mm - Pack of Three

EKA1475P3

TSP – 25cm Fused Silica Capillary (bare) 50mm

EKB1425

TSP – 25cm Fused Silica Capillary (bare) 50mm - Pack of Three

EKB1425P3

TSP – 50cm Fused Silica Capillary (bare) 50mm

EKB1450

TSP – 50cm Fused Silica Capillary (bare) 50mm - Pack of Three

EKB1450P3

TSP – 75cm Fused Silica Capillary (bare) 50mm

EKB1475

TSP – 75cm Fused Silica Capillary (bare) 50mm - Pack of Three

EKB1475P3

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