Gas Chromatographic Determination of the Composition of Various Mixtures

BACKGROUND

Please read the relevant chapter on gas chromatography in your textbook before beginning this experiment. Also, please consult the User's Manuals for the gas chromatographs for operating instructions.

PURPOSE

The purpose of this experiment is to use gas chromatography to qualitatively and quantitatively analyze various gas mixtures.

In combination with a mass spectrometer, gas chromatography is probably the most widely used method in modern analytical laboratories. The gas chromatograph (GC) separates the various components of a mixture as it passes through a column inside the instrument; the mass spectrometer ionizes and identifies the components based on the masses of their ions. Gas chromatography can also be combined with other spectrometers, such as a Fourier- transform infrared spectrometer (GC-FTIR). However, we will only be working with gas chromatography in this experiment (but if you want to get your hands on a GC-MS or GC-FTIR, take Instrumental Analysis or Organic Chemistry next year!).

Many GCs have columns for analysis of liquid mixtures, in which case the technique is referred to as “gas-liquid chromatography,” but we will be using a special column developed by Alltech for analyzing gas mixtures.

Procedural Outline

1.The carrier gas (He) flow rate and injection volume will be optimized to obtain a reasonable separation of the gases in a standard (known composition) gas mixture.

2. Various injection volumes of the standard gas mixture will be analyzed to "assign" a peak to each component and to provide data for preparing a calibration curve for each component.

3.Samples of four “unknown” gas mixtures: exhaled breath, laboratory air, laboratory fuel gas and combustion gases sampled from a Bunsen burner flame will be analyzed. You will be able to determine the components of the "unknown" gas mixtures as well as their quantitative composition.

1. * What do you expect the (relative) compositions of exhaled breath, background laboratory air, and combustion gases to be? (Eg. Which would you expect to have the most CO₂ ? The least?).

The identity of a component in an "unknown" mixture can be determined by comparing its retention time to those observed for the components of the standard gas mixture. Quantitative analysis is done by comparing a characteristic (height, area, or mass) of the peak for that component to the slope of calibration curve for that component.

WARNING ‑‑‑ PLEASE READ CAREFULLY BEFORE ATTEMPTING THE EXPERIMENT.

The GC must ALWAYS have a flow of He gas through the detector. Be sure, be very sure, that He gas is flowing through the detector or it will be ruined instantly and you will not be able to finish the experiment or receive a good grade. Also, roasting a $500 detector will prevent others from using the GC for the balance of the semester.

The carrier gas is contained under high pressure in a compressed gas cylinder. Compressed gas cylinders can be very dangerous. Do not touch the carrier gas cylinder or related valves. Ask your instructor for assistance.

You will be using syringes with very sharp needles attached. Make sure that the needle is capped at all times when you are not actually injecting or withdrawing a sample. To avoid injuring yourself or others during injections, keep your fingers away from the injection port (it could be HOT) and the needle. You do not want to inject your sample into your finger (or someone else's)!

The syringes are expensive, and misuse can quickly render them useless. Your instructor will demonstrate the proper needle and syringe technique.

USE OF STANDARD GAS TO OPTIMIZE OPERATING PARAMETERS

Please consult the manufacturer's information regarding the operation of the column and the gas chromatograph. In general, chromatographic column manufacturers give sample chromatograms as well as the temperatures, flow rates and type of detector used to obtain the sample chromatograms. This is true for our column, the CTR I, made by Alltech. The CTR I column is attached to ONLY ONE of the two injection ports (A or B - ask your instructor). You may consult the sample chromatogram to get a starting point for your analysis and to help you identify how many peaks to expect and what components they correspond to. However, fine-tuning on your part will be required for all parameters (flow rate and sample size), since each column and GC have slightly different characteristics (i.e., column backpressure and void volumes). Bubble meters are used to measure the rate of He flow. Make sure that the He flow rate is approximately the same through both exit ports (A and B) so that the carrier gas flow is uniform through the detector.

Once the initial operating conditions are decided on, a sample may be injected. An aerosol can containing the standard gas mixture has been used to fill a polyethylene sample bag. The gas is withdrawn from the sample bag by inserting the needle into the sample bag valve (through the rubber septum), and filling the syringe with the desired amount of gas. See your instructor for a demonstration of this technique.

After filling the syringe, the sample is injected into the GC through the working injection port. This should be done as quickly as possible after filling the syringe with standard gas to avoid sample loss from the syringe. Also, the sample should be injected as rapidly as practically possible (near instantaneously) to reduce band broadening. At least three injections (maybe more) will be necessary to optimize the parameters of flow rate and injection size. Do not forget to write down all pertinent information for the composition of the gas standard (see manufacturer's spec sheet), and values for flow rate, detector current, ambient temperature and pressure and injection volume for each injection. It is advisable to record the values in the lab notebook as well as on the chromatogram itself.

Several different injection volumes are necessary for establishing a calibration curve for each component of the standard gas mixture.

After standardization, you can begin analyzing samples of the 'unknowns.' It is desirable to do your standardization, calibration, and unknown analyses all in a single lab session because of daily fluctuations in temperature, gas pressure, etc.

For analysis of precision of results you will have to inject at least three samples of each unknown. You should also think about and DISCUSS (in the appropriate section) your sampling technique: are you sure the syringe has been purged of previous samples AND that you are filling it with a uniform representative sample of the gas mixture to be analyzed?

CALCULATIONS

For this experiment, there are a number of legitimate ways of analyzing the data that will yield the same or similar results. Your instructor may suggest another (perhaps simpler) way to analyze the data.

Dalton's law of partial pressures asserts that each component in a perfect gas mixture exerts a partial pressure , given by

(1)

with the number of moles of component j, n the total number of moles of gas, the mole fraction of j, P the total pressure, R the universal gas constant, T the temperature and V the volume. Since the mole fractions must add up to one, the sum of partial pressures is equal to the total pressure;

(2)

In the present context, P is the atmospheric pressure of the laboratory in any convenient pressure units, and T id the temperature of the laboratory in degrees Kelvin (say, 293.15K = 20°C). For an injection size of, say, 5ml, the total number of moles (n) can be easily determined from . Since the mole fractions of the standard are known (they are easily calculated given the mass fractions on the canister), you can easily calculate the number of moles of each component which are introduced onto the column in a particular injection.

We expect that the total peak area in a chromatogram for a particular component, , will be proportional to the number of moles of that component; i.e., that

, (3)

with the proportionality constant for the jth component and n_j the number of moles of that component. Since we expect the area for a particular component to approach zero as the number of moles approaches zero, there is no y-intercept parameter in this equation.

(Note: if a particular peak consists of 2 or more components, it cannot be used as part of the "total peak area" and should simply be excluded from the analysis.) The area of a peak can be estimated by the triangular formula, i.e., area = 0.5 * base * height. If there is no inhomogeneous broadening of the peaks, the height of the peak can be used instead of the area for this analysis; however this is almost never the case for a basic instrument such as this.

Each proportionality constant can be obtained by carrying out a linear least-squares fit of the peak area as a function of n_j. (what Excel refers to as "drawing a trendline"). This can be done either by writing your own least-squares fit program or by using Excel, KaleidaGraph, MatLab, or some other numerical analysis package. Also, most scientific calculators have this capability built-in. The least-squares fit line can then be used to figure out how many moles of a particular component of an unknown gas are injected in an analysis of the unknown. From this the number of grams of each component can be calculated, and then it is simple to calculate the mass percentage of each component. One measure of the quality of your results will be the predicted y-intercept value for each component; if it is not very close to zero, your data is probably not quite correct.

THE REPORT

Your instructor may make changes to what is reported and how it is reported.

For each of the "unknowns," give the mole percent of each component. Estimate the uncertainty in each percentage. Also include the literature values (if available) for the mole percent composition of every “unknown,” remembering to properly cite your reference sources. Include plots of your peak areas as a function of the number of moles of each component (or alternatively, as a function of the injection volume). Also show your calibration curve (the line you obtain from your linear least-squares fit of the data). Carry out any and all appropriate uncertainty analyses. Give some thought as to how to estimate the uncertainty in the numbers you extracted from a linear least-squares plot; ask your instructor if you need help with this.

Make sure you include the following:

· Calibration curves for each component of the reference gas (remember 0,0 is a valid point): show
value of slope with units.

· A table showing the known composition of the reference gas.

· Moles of that component vs. mass of cut-out peak

· P and T conditions in the lab.

· Chart speed.

· He flow rate and all injection volumes.

· For each injected volume of reference gas mixture:

- x_i

- n_i

- moles_total

- retention time_i

- mass of peak_i (cut-out)

- slope (units!) of calibration curve for each components of reference gas

• For each component of each unknown gas mixture:

- Retention time

- Identity of component

- Mass of cut-out peak

- moles of component

- mole % of that component

- comparison with literature values (if available)

· How do your results compare with what you expected? (Refer to question 1)