The geometry of capillary columns is fairly simple, consisting of length, internal diameter, and stationary phase thickness. Nevertheless, there are endless

The geometry of capillary columns is fairly simple, consisting of length, internal diameter, and stationary phase thickness. Nevertheless, there are endless possible combinations of these three factors that could be used for optimizing chromatography.

Capillary Columns

Doubling the column length effectively doubles the number of theoretical plates but the resolution between any two compounds is proportional to the square root of the plate number so doubling the column length only increases resolution by about 40%

Column Length

Column Diameter

Smaller Diameter: Faster ChromatographyHigher Number of Theoretical PlatesLower Sample CapacityHigher Detection Limit (lower mass injected)

Linear Gas Velocity (cm/sec)

Column Diameter

Film Thickness

Thinner FilmsHigher NUsed for High BP CompoundsLow Sample Capacity

Thicker FilmsUsed for Low BP compoundsLess Peak Tailing for Polar Compounds

Linear Gas Velocity (cm/sec)

The

oret

ical

Pla

te H

eigh

t

Film Thickness

Compound Polarity Max Temp oC Column ID ManufacturerPoly(methyl siloxane) Low 300-350 HP-1

AT-1DB-1, SE-30OV-1ZB-1RTx-1BP-1SPB-1CP-Sil 5 CB

AgilentAlltechJ&WOhio ValleyPhenomenexRestekSGESupelcoVarian

95% Dimethyl, 5%phenylPoly(methyl siloxane)

Low 300 HP-5AT-5, EC-5DB-5, SE-54OV-5ZB-5RTx-5BP-5SPB-5,MDN-5CP-Sil 8 CB


Polyethylene glycol Medium 250 HP-20MAT-WaxDB-WaxCarbowax 20MZB-WaxStabilwaxBP20Supelcowax 10CP-Wax 52 CB


Stationary Phases

Mobile Phase

Lighter gasses are best for fast analysis (hydrogen)Fast analysis produces narrow peaks with better detectibility

Heavier gasses have slightly higher N (but only at low velocity)(not used too much)

Mobile PhaseEffect of Mobile Phase Gas Type and Velocity

Mobile Phase

pi/po is the ratio of the inlet velocity(pi) to the outlet velocity (po).

There is a drop in pressure as the gas moves through the column. This drop is pressure causes the gas to expand which can result in peak broadening. It also causes the gas velocity to increase as

it moves through the column

As the column oven is heated, the viscosity of the mobile phase increases. Therefore as the column is heated during the

temperature ramp, the flow rate goes down. In order to keep a constant flow ( and reduce peak spreading of later eluting peaks) a process of pressure programming is used. The constant flow mode increases the pressure at the head end of the column, and keeps the mobile phase velocity constant. Pressure programming can also be used to increase mobile phase velocity as the temperature increases, further decreasing analysis time and increasing peak height.

Mobile Phase

The retention time of an analyte provides some information on the chemistry of the compound. However, retention time is dependant on many operational factors such as temperature, column length, column diameter, coating thickness, etc.

The use of a relative retention value compensates for many of these variations. The Kovats retention index is used to calculate relative retention values based on a scale defined by the elution of a series of n-alkanes. An index value calculated for an analytes should be the same for any chromatographic run as long as the same stationary phase is used.

Information on the Kovats Index for many compounds can be found in the literature.

Kovats Retention IndexKovats Retention Index


where X refers to the adjusted retention volumes or times, z is the numberof carbon atoms of the n-alkane eluting before and (z + 1) is the numberof carbon atoms of the n-alkene eluting after the peak of interest:

Kovats Retention IndexKovats Retention IndexThe classical Kováts retention index is measured under isothermal conditions. However, in the case of temperature-programmed gas chromatography a similar value can be calculated utilizing direct numbers instead of their logarithm. In other words, an equation for the Kovats index can be developed for a polynomial regression of a series of alkanes vs. their retention times.

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

Time - Minutes

05

101520253035404550

Res

pons

e -

Mill

iVol

tsKovats Retention IndexKovats Retention Index

MinutesMinutes

Det

ecto

r R

espo

nse

Det

ecto

r R

espo

nse

The output from the GC is a series of peaks.The output from the GC is a series of peaks.While the output does not tell you what theWhile the output does not tell you what thepeaks, the retention time is useful information.peaks, the retention time is useful information.

Using a series of alkane standards, the retention Using a series of alkane standards, the retention time can be converted to a retention index whichtime can be converted to a retention index whichcan be compared to literature values.can be compared to literature values.

1100

1000

DB-5 OV-17 CM-20 COMPOUND AROMA 1094 1085 1442 pentyl butanoate banana 1096 1086 1345 Z-3-nonenal cucumber 1100 1100 1100 undecane alkane 1100 1197 1444 Z,Z-2,6-nonadienal fatty 1100 1202 1537 linalool lemon 1104 1195 1385 nonanal soapy 1105 1246 1767 2-acetyl-2-thiazoline roasty 1107 1347 2205 sotolon curry


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The geometry of capillary columns is fairly simple, consisting of length, internal diameter, and stationary phase thickness. Nevertheless, there are endless