Restek-2015

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GC COLUMNS

Selecting a GC Column

Checking for leaks, using a Restek® electronic

leak detector, is an easy way to protect your

instrument and column from damage.

All Restek columns have published minimum and maximum operating

temperatures that establish the working range for the stationary phase. Note

that these ranges vary with the thickness of the coating.

What Do the Temperature Limits Mean?

Film thickness also directly affects phase ratio (β), which must be accounted for when

changing to a column with a different inner diameter. When inner diameter increases,

film thickness (d

) must also increase in order to provide comparable resolution and

retention. Table III shows β values for common column dimensions; similar values

indicate similar separations on different ID columns.

Efficiency (N)

Column efficiency (N) is the column length divided by the height equivalent of a theo-

retical plate (HETP). The effective number of theoretical plates is affected by how well

the phase has been coated onto the column walls, and it is measured by how narrow

the peaks are when they elute out of the column. Higher column efficiency (N) results

in greater resolution between peaks. Inner diameter also influences efficiency; a simple

rule of thumb is the smaller the column ID, the more efficient the column.

Capillary columns are made in various lengths, typically 10, 15, 30, 60, and 105 meters.

Longer columns provide more resolving power, but will also increase analysis time and

cost more. When column length is doubled, analysis time will increase by as much as

a factor of two. However, doubling the column length increases resolution by only ap-

proximately 40% since the column length term is under the square root function in the

resolution equation. When selecting column length, the increase in resolution obtained

in a longer column must be weighed against the increase in cost and analysis time.

Conclusion

A basic understanding of the resolution equation allows analysts to make more effec-

tive column choices. Phase choice should be influenced primarily by separation fac-

tor, which can be approximated by considering the structures of both the phase and

the analyte, as well as by referencing retention indices or existing applications. Reten-

tion factor and efficiency also affect peak separations and should be considered when

choosing column inner diameter, film thickness, and length. By better understanding

these factors, analysts can simplify the column selection process, optimize separations,

and increase lab productivity.

Table III:

Phase Ratio (β) Values for Common Column Dimensions*

Film Thickness (d

) /

Value

Column ID 0.10 µm 0.25 µm 0.50 µm 1.0 µm 1.5 µm 3.0 µm 5.0 µm

0.18 mm

450

180

0.25 mm

625

250

125

0.32 mm

800

320

160

0.53 mm

1325

530

265

128

= r/2d

(r=internal radius of tubing; d

= phase film thickness)

Rxi®-5Sil MS Columns

(fused silica)

ID d

(µm)

temp. limits

0.25 mm 0.25 -60 to 320/350 °C

0.32 mm 0.50 -60 to 320/350 °C

0.53 mm 1.50 -60 to 320/330 °C

The second temperature is the

maximum

temperature-programmed operating tempera-

ture

, the temperature to which the column can

be heated for short periods of time (i.e., during a

temperature-programmed analysis). If only one

temperature is listed, it is both the isothermal and

the maximum temperature.

Many phases list two maximum operating tempera

tures. The first temperature is the

maximum isother

mal operating temperature

. This is the temperature

to which the columns are guaranteed to meet the

minimum bleed specification (i.e., lowest bleed level).

The

minimum operating temperature

de-

fines the lowest usable temperature before

the stationary phase solidifies. Operating the

column below the minimum temperature will

not harm the phase, but poor peak shape and

other chromatography problems may occur.