Capillary column selection method - Database & Sql Blog Articles

1.1

Stationary phase selection

When faced with an unknown, try the existing one first.

GC

Column, if the column separation is not ideal, according to your understanding of the sample, the basic principle is that the analyte will interact with the stationary phase when it has similar chemical properties. This means that the more you know about the sample, the easier it is to find the right stationary phase.

Non-polar molecule

——

Usually only by

C

with

H

Composition and no dipole moment, direct (n-alkane) is an example of a common non-polar compound.

Polar molecule

——

Mainly by

C

with

H

The composition also has other atoms, such as:

N

,

O

,

P

,

S

Or halogen. Samples include alcohols, amines, thiols, ketones, organic halides, and the like.

Polarizable substance

——

Mainly by

C

with

H

The composition contains both unsaturated bonds. There are usually: alkyne and aromatic compounds.

If your sample is a mixture of non-polar components with similar chemical properties, such as hydrocarbons in most petroleum fractions, you can try

OV-1

Capillary column, which is separated in boiling order. If you suspect that there is an aromatic compound, try using a phenyl group.

SE-52

or

SE-54

column.

Polar or polarizable component samples are capable of medium polarity and

/

Or can be analyzed on a polarizable stationary phase column, such as a phenyl or similar group stationary phase, such as

OV-17

or

OV-225

column. If you need higher polarity, you can use polyethylene glycol (

PEG

The stationary phase, which is commonly referred to as

WAX

Stationary Phase.

1.2

Film thickness selection

The film is faster than the thick film eluting component, the peak separation is good, and the temperature is low.

In general, the film thickness of the column is

0.25

To

0.5μm

. For outflows

300 ° C

Most of the samples (including waxes, triglycerides, steroids, etc.) are well analyzed. For higher elution temperatures, you can use

0.1μm

Liquid film. The thick liquid film is advantageous for low boiling compounds, for the outflow temperature

100 ° C

~

200 ° C

Material between

1

~

1.5μm

The liquid film works better. Ultra thick film

3

~

5μm

) Used to analyze gases, solvents, and pursable materials to increase the interaction of sample components with the stationary phase. Another reason for choosing a thick film is to maintain resolution and retention time when using large diameter columns. For this reason, large diameter columns have only thick films. The loss of thick film is large, and the temperature limit must decrease as the film thickness increases.

1.3

Length selection

generally,

15m

Columns are used to quickly screen for simple mixtures or compounds with very high molecular weight.

30m

Columns are the most common column length. Extra long column

50

,

60

or

100m

,

150m

) for very complex samples.

Column length is not an important parameter in column performance, for example: double column length, constant temperature analysis time is doubled but peak resolution is only increased

40%

. If the analysis is better but not particularly good, there are better ways to improve the analysis than increasing the length of the column, such as considering thinner membranes, optimizing carrier gas flow or using programmed temperature.

Analysis of highly active components is a special case. If the sample is in contact with the column material, the peak will be severely tailed. Thicker membranes, relatively short columns can mask the surface of the active surface due to fewer pillars and thicker fixed liquids

,

Thereby reducing the chance of interaction.

1.4

Inner diameter selection

Increasing the diameter means that more stationary phase is needed, even if the thickness is not increased, there is a larger sample capacity. It also means that the separation ability is reduced and the loss is large. Small-bore columns provide the required separation for complex samples, but usually require split injection because of the low column capacity. Large diameter columns can avoid this if the reduction in resolution is acceptable. When sample capacity is a primary consideration, such as: gas, strong volatile samples, purge and trap or headspace injection, large internal diameter or even

PLOT

The column may be suitable.

At the same time, the limitations and requirements of the instrument should be considered in the selection of the inner diameter of the column. Large-bore capillary columns can be used for the inlet of the packed column (

0.53mm

The inner diameter), while the small diameter column may not be able to be connected to the instrument. The inlet of the capillary column can generally be used for capillary columns of all internal diameter ranges. (

0.1mm

,

0.25mm

,

0.32mm

,

0.53mm

Directly combined

GC/MSD

with

MSD

A small bore column is required because the vacuum pump cannot handle large flows of large diameter columns. Find out your entire system and look at the columns that fit your inner diameter.