experiment of gas chromatography

TITLE: Gas Chromatography

OBJECTIVES OF THE EXPERIMENT:

1) To conduct quantitative determination by using gas chromatography.

2) To conclude the relation between tR and the number of atomic carbon in homolog series.

3) To explain the Van Deemter equation.

4) To describe the effect of flow rate on the efficiency of column.

INTRODUCTION:

Gas chromatography specifically gas-liquid chromatography - involves a sample being

vaporized and injected onto the head of the chromatographic column. The sample is transported

through the column by the flow of inert, gaseous mobile phase. The column itself contains a

liquid stationary phase which is adsorbed onto the surface of an inert solid. In gas

chromatography (GC), the stationary phase is a high-boiling liquid and the mobile phase is an

inert gas. In the organic chemistry teaching labs at CU Boulder, GC is used as an analytical tool

to find out how many components are in a mixture. It can also be used to separate small amounts

of material. In gas chromatography, the moving phase ( "mobile phase") is a carrier gas, usually

an inert gas such as helium or an unreactive gas such as nitrogen.

The stationary phase is a microscopic layer of liquid or polymer on an inert solid support,

inside a piece of glass or metal tubing called a column (a homage to the fractionating column

used in distillation). The instrument used to perform gas chromatography is called a gas

chromatograph (or "aerograph", "gas separator"). The gaseous compounds being analyzed

interact with the walls of the column, which is coated with different stationary phases. This

causes each compound to elute at a different time, known as the retention time of the compound.

The comparison of retention times is what gives GC its analytical usefulness. Gas

chromatography is in principle similar to column chromatography (as well as other forms of

chromatography, such as HPLC, TLC), but has several notable differences. Firstly, the process of

separating the compounds in a mixture is carried out between a liquid stationary phase and a gas

moving phase, whereas in column chromatography the stationary phase is a solid and the moving

phase is a liquid. Secondly, the column through which the gas phase passes is located in an oven

where the temperature of the gas can be controlled, whereas column chromatography (typically)

has no such temperature control. Thirdly, the concentration of a compound in the gas phase is

solely a function of the vapor pressure of the gas.

Gas chromatography is also similar to fractional distillation, since both processes

separate the components of a mixture primarily based on boiling point (or vapor pressure)

differences. However, fractional distillation is typically used to separate components of a mixture

on a large scale, whereas GC can be used on a much smaller scale (microscale). Gas

chromatography is also sometimes known as vapor-phase chromatography (VPC), or gas-liquid

partition chromatography (GLPC). The carrier gas is an inert gas, helium. The flow rate of the

gas influences how fast a compound will travel through the column; the faster the flow rate, the

lower the retention time. Generally, the flow rate is held constant throughout a run. (The GCs at

CU Boulder are set at a flow rate of 55 mL/min).

The gas chromatography can be divided into column, detector, gas (flow rate) and

temperature. The detector can be divided into FID, ECD, TCD, FTD that functioning based on

the type of sample. The detector will combined all the compound in the peak form. Gas as

mobile phase, for example helium and nitrogen. Column known as capillary which are different

based on the polarity of sample used. The longer size of column, more better separation will

occur. It also based on the polarity of sample to known the time taken for separation. Other than

that, the column can be divided into polar, semi polar and non polar. In the column has stationary

phase, the sample will go through it and this phase will separated it.

RESULTS:

Part I Determination of tR for the homolog series

Homolog Series Number of Carbon

Mr (g/mol) Boiling Point (⁰C)

log tR

Methanol 1 32.04 64.7 -0.0969

Ethanol 2 46.07 78.1 -0.0980

Butanol 4 74.12 117.7 0.0326

Mixture of alcohol

Mixture tR1 (min) tR2 (min) log tR1 log tR2 No. of C Mr (g/mol)

Butanol +

Methanol

0.803 1.035 -0.096 0.015 5 106.16

Butanol +

Ethanol

0.802 1.002 -0.0958 0.001 6 120.19

Part II The effect of flow rate on efficiency column

Flow Rate ( mL/min ) tR (min) w (min) n HETP (cm)

20 1.278 0.294 302.33 9.9

30 1.078 0.183 555.21 5.4

40 0.918 0.471 60.78 49.4

50 0.815 0.179 331.69 9.0

80 0.650 0.130 400.00 7.5

Effect of temperature on retention time of 2-propanol

Temperature ( ⁰C ) tR (min)

90 0.807

120 0.765

Part III Quantitative procedure in GC analysis

A. Standard Sample

Sample Weight (mg) tR (min) Peak Area Concentration (M)

Octane 6.6 8.323 42232 12.5346

Decane 3.3 11.43 20997 6.2321

Napthalene 3.1 15.688 27143 8.0562

Hexadecane 5.0 21.282 31051 9.21620

B. Unknown Sample

Sample tR (min) Peak Area Concentration (M)

1 8.318 13261 4.5452

2 11.428 17575 6.024

3 15.695 30400 10.42

4 21.275 18560 6.3618

CALCULATION:

Part II

1) =

2) n = 16 ( tR/w )2

3) HETP = ( L/n )

i. When flow rate = 20 mL/min

1) =

w = 0.294 min

2) n = 16 2 = 302.33

3) HETP = = 9.9 cm

ii. When flow rate = 30 mL/min

1) =

w = 0.183 min

2) n = 16 2 = 555.21

3) HETP = = 5.4 cm

iii. When flow rate = 40 mL/min

1) =

w = 0.471 min

2) n = 16 2 = 60.78

3) HETP = = 49.4 cm

iv. When flow rate = 50 mL/min

1) =

w = 0.179 min

2) n = 16 2 = 331.69

3) HETP = = 9.0 cm

v. When flow rate = 80 mL/min

1) =

w = 0.130 min

2) n = 16 2 = 400

3) HETP = = 7.5 cm

Part III

A. Standard Sample

Peak Ratio from Standard =

1) For naphthalene:

Peak ratio = = 1.00

2) For octane:

Peak ratio = = 1.56

3) For decane:

Peak ratio = = 0.77

4) For hexadecane:

Peak ratio = = 1.14

B. Unknown Sample B

a) Peak Ratio from Unknown Sample B =

b) Concentration of Unknown Sample:

= × Concentration of Sample (standard)

1) By referring tR of standard for naphthalene:

a) Peak ratio = = 1.00

b) Concentration = × 8.0562 = 8.0562 M

2) By referring tR of standard for octane:



3) By referring tR of standard for decane:



4) By referring tR of standard for hexadecane:



DISCUSSION:

In the experiment, retention time plays big role in analyzing the sample in the gas

chromatography. For the first part of the experiment, retention time are differentiated according

to the number of carbon, molecular weight and the boiling point of the samples. Graphs plotted

from these readings that obtained from the chromatography shown that log tR is proportionally

increase when number of carbon is increased. It goes the same with molecular weight and boiling

point, log tR increase when the molecular weight and boiling point of the compound increased.

This data concluded to the relationship that when the number of carbon in samples tested

increased, their molecular weight also increased, and so with their boiling point. Therefore, the

retention time, time for a maximum of symmetrical peak to occur is also increased.

In Part II, the effect of flow rate on column efficiency were determined by relating the

retention time with the HETP (height equivalent to a plate). The graph of flow rates versus

HETP plotted is inversely proportional which meant that the efficiency of column is greater

when slower flow rate is used. Other than that, retention time is lower when the temperature of

the same compound tested is increased. In Part III, quantitative procedure in gas

chromatography analysis was carried out. In this part, the concentration of the unknown samples

are determined by using an internal standard as reference. The peak ratio from the standard

solution prepared are used to calculate the unknown samples.

The process of gas chromatography is carried out in a specially designed instrument. A very

small amount of liquid mixture is injected into the instrument and is volatilized in a hot injection

chamber. Then, it is swept by a stream of inert carrier gas through a heated column which contains the

stationary, high-boiling liquid. As the mixture travels through this column, its components go back and

forth at different rates between the gas phase and dissolution in the high-boiling liquid, and thus separate

into pure components. Just before each compound exits the instrument, it passes through a detector. When

the detector “sees” a compound, it sends an electronic message to the recorder, which responds by

printing a peak on a piece of paper.

The relationship between tR. and the number of atomic carbon in homolog series is supposed to be

more increase the number of atomic carbon, more increase the tR. The retention time is corrected by

subtracting the value of actual retention time with the retention time of air. Unfortunately, the peak for air

is not visible when ionization flame detector is used.

As can be expected, the retention timed decreases with the increase of temperature. When the

temperature used is 90⁰C, the retention time is 0.807 min while for 120⁰C, the retention time is 0.765

min. Even though the high temperature is needed to speed the analysis, lack of efficiency would cause the

peaks to close together.

The effect of flow rate on the efficiency of column can be determine by the Van deemter equation

which is :

The form taken by f2(k') was considered by Van Deemter to be,

where H is a column plate height (also known as height equivalent to one theoretical plate, H.E.T.P.), u is

average velocity of a carrier gas, and B and C are constants that depend on column dimensions, carrier

gas type, and, to a smaller degree, on other factors. Explicit equations are derived for the B and C terms in

the Van Deemter equation for liquid chromatography, involving diffusion coefficients in the mobile zone

(eluent outside the particles), the stagnant mobile phase (eluent inside the particles) and the stationary

phase. Longitudinal diffusion rates (B term) are determined by the arrested elution method for simple

aromatic solutes in methanol and methanol-water mixtures with ODS-Hypersil as packing material. When

compared to diffusion rate for the same solutes in eluent, measured by the open-tube method, the rates of

diffusion in the stationary ODS phase are about half those in methanol-water mixtures.

C values are obtained by measuring the band-dispersion when the same solutes are eluted from

50-μm and 540-μm ODS-bonded silica gels at high reduced velocities. After allowing for dispersion due

to processes in the mobile zone, the C values are obtained and found to be of the magnitude predicted by

the non-equilibrium theory of Giddings, taking into account the actual rates of diffusion of the solutes in

the stationary zone.

PRECAUTIONARY STEPS:

1. Shake all the solution adequately to ensure dissolution and degas for 5 min before

injection into chromatograph.

2. Make sure that the column is not clogged, it maybe cause by precipitation of proteins in

the column caused by removal of stabilizing agents during fractionation.

3. Make sure rinse and clean the syringe before and after each sample to prevent any

contaminant and others solution that will affect the process.

4. Avoid formation of bubbles during the experiment because it will affect the reading in the

column while we taken the measurement.

CONCLUSION:

1) The concentration of unknown samples in this experiment were determined by using gas

chromatography quantitative determination.

2) The retention time increased when the number of carbon is increased.

3) The Van Deemter equation can be used to explain the HETP and the effect of flow rate on the

efficiency column.

4) The faster the flow rate, the less the column’s efficiency. The HETP decreased when the flow

rate increased.

experiment of gas chromatography

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