how to use shilling aparatus and chromotograph

8/3/2019 How to Use Shilling Aparatus and Chromotograph

1/8

Gas Chromatography and gas characteristics.

Objectives:1. Investigate the properties of gaseous fuels using various methods.2. Use and understand techniques for accurate chromatographic analysis.

ChromatographyThe Gas Chromatography unit consists of three elements:-

1. The column oven/injection port/outlet port (A large yellow cube with lots of knobs and dialson it)

2. A chart recorder (Black box with paper coming out of it.)3. An integrator/recorder (Yellow box with a key pad, and paper coming from it)

All of these elements have a large number of knobs and buttons. The machine has alreadybeen set up so DO NOT TOUCH any of them unless specifically told to within the

experimental protocol provided.

Analysis of gas samples using chromatography.

The object of this test is to determine what components are present in unknown gassamples and in what concentrations.

The sample is injected as a 'plug' into a stream of high purity carrier gas. The sampleis swept by the carrier gas through a tube packed with a porous solid (the solid phase)which adsorbs and desorbs the different components of the sample gas at different

rates. The time of the desorbtion of a component on the packing is a characteristic ofthat component. The final effect is that some components take longer to pass throughthe column than others with the result that, if the column is long enough, a completeseparation of components occurs. The accuracy of separation depends upon thecarrier gas flow rate, column length and packing type and temperature.

The emergence of each component from the end of the column is sensed by a'detector', in this case a thermal conductivity detector.

Apparatus.

With this Gas Chromatograph (Shimadzu GC-8A), separation of the componentgases in a mixture of gases is achieved using an aluminium silicate packing known as'molecular sieve 5A (mesh size 40-60)' in column 1 and 'Chromosorb 101 (mesh size80-100)' in column 2. (Both columns are 2 m long stainless steel tubes with an internaldiameter of 3 mm.) The molecular sieve can separate H2, O2, CH4, N2 and CO. The

Chromosorb 101 can separate CO2 and higher hydrocarbons. Clearly, a sample hasto be injected into each column in turn for full resolution of its makeup.

High purity helium is used as the carrier gas. Helium has a higher thermal conductivitythan most other gases. This makes detection of the gases at low concentrations in thesample, and therefore in the carrier gas, easier. This is because changes in the

thermal conductivity of the emerging gases are consequently larger. The carrier gasstream is split into two before entering the column, the sample being injected into onlyone of the streams. Both streams pass over the thermal conductivity detectors as theyemerge, the presence of a sample component being detected when the thermal


2/8

conductivity of the two streams differs. The change in thermal conductivity is measuredby an electronic bridge circuit and, ideally, should be displayed as a positive peak onthe recorder/integrator for gases whose thermal conductivity is LESS than that of

helium.

Data inscribed on the chart recorder.

As the separated components of the gas sample emerge into the detector, the change

in voltage in the detecting bridge circuit causes a representative peak to be drawn ona chart recorder. The position of the peak along the time axis of the chart measuresthe components retention time, which identifies the component. This is directly relatedto carrier gas flow rate, temperature and column packing and dimensions. The areaunder each peak is proportional to the concentration of the component of the sample.The area of the peaks inscribed on the chart recorder can be determined bymultiplying the height of the peak in mm, by the width of the peak in mm at the peakheight. The calibration curves for use with the chart recorder are therefore peak areaplotted against concentration.

Knowing what components are being looked for, standard samples of eachcomponent can be run through the columns and the peak area for a given amountrecorded and used to construct a standard curve. The peak areas of the variouscomponents from the unknown gas samples can be compared to the standard curvesto determine their absolute quantity, (e.g. ml of component). Standard curves for aselection of possible gases are provided.

Data generated by the Integrator.

The integrator (Shimadzu C-R6A Chromatopac) works on the same principle as describedabove but the electronics do all the measurements and maths for you. The integrator willalready be programmed with a standard curve. Data such as peak height, peak area andcompassion to standards will be calculated and printed out automatically. The chart recorderand the integrator can be used simultaneously, but for this experimental session the data willbe used separately. The processed data obtained from these recording devices arecomparable but the raw data cannot be directly compared.

The gas flow-rate for both columns has been set prior to the experiment:

Column 1 30cm3

min-1

.

Column 2 35cm3

min-1

.

The injection port temperature (80oC) and column oven temperature (40oC) have been pre-set pressing the blue buttons marked INJ & COL, beneath the temperature display can checkthese values.These parameters of gas flow-rate and temperatures are pre-set so as to allow thechromatograph to stabilise ready for use.

The gas samples to be analysed are AIR, NATURAL GAS AND A MYSTERY GAS.These are injected separately into Column 1 and then Column 2.


3/8

METHODOLOGY

COLUMN 1

1. Check that the blue polarity button is out (+) for detection on column 1.

2. Check that the chart recorders sensitivity range is set to 1.

3. Zero the pen on the recorder to a line 1cm from the edge of the chart paper to form the

baseline for the chromatograph.

4. Flush the syringe out with air three times (using atmospheric air). Then fill the syringe

with air to the 1cm3

mark (this represents 100%).

5. Set the chart recorder to 20mm/minute.

6. Set the sensitivity to 2mV/cm for Air.

7. Insert the syringe into column 1, holding the plunger with a finger so the carrier gas

flowing through the column does not push it out. Inject the gas sample into the columnand at the same time press the marker button on the chart recorder. This sets the point

to measure the retention time from.

8. For the air sample allow for two full peaks to appear and then repeat the sample

analysis.

9. For natural gas take a sample from the gas tap which has a piece of rubber tubing

attached. Turn on the tap and allow gas to flow for approximately 5 seconds, then

insert the empty syringe into the rubber tubing and fill the syringe then flush the syringe

to atmosphere. Do this three times before taking a final sample for analysis.

10. Set the sensitivity to 2mV/cm. Repeat procedures 1-6. Allow for the appearance of

one major peak and then again repeat for a second sample.

11. For the mystery gas, a gas sampling vessel, charged with a sample of the gas, will be

available from which to take the samples for analysis. Insert the syringe through the top

of the rubber septum and withdraw a sample of gas and flush to atmosphere. Do this

three times before taking a final sample for analysis.

12. Set the sensitivity to 1mV/cm, then repeat the procedures 1-6. Allow for the

appearance of two major peaks and then repeat for a second sample.

MARK THE CHROMATOGRAPH TRACE WITH:-

(i) COLUMN USED (ii) THE GAS SAMPLE

(iii) THE PAPER SPEED (iv) VOLTAGE SENSITVITY


4/8

COLUMN 2

1. Check that the blue polarity button is in (-) for detection of sample on Column 2.

2. Check that the chart recorder sensitivity range is set to 1.

3. Zero the pen on the recorder to a line 1cm from the edge of the chart paper to form the

baseline for the chromatograph.

4. Flush the syringe out with air three times (using atmospheric air). Then fill the syringe

with air to the 1cm3

mark (this represents 100%).

5. Set the chart recorder to 20mm/minute.

6. Set the chart recorded at a sensitivity of 5mV/cm, the peaks that appear are previously

analysed gases, ignore them they are not going to be used in any determination or

calculations. After their emergence on the chart is complete, the sensitivity must be

changed to detect the further emerging peaks used in the determination of the gases.Lift the pen off the chart paper and then change to the required sensitivity. When the

sensitivity is re-set, re-zero the pen on the baseline as in (3).

7. For air the sensitivity is changed from 5mV/cm to 0.2mV/cm and allow for the

appearance of one very small peak and then again repeat for a second sample.

8. Insert the syringe into column 2, holding the plunger with a finger so it is not push out by

the carrier gas flowing through the column. Inject the gas sample into the column and at

the same time press the marker button on the chart recorder. This sets the point tomeasure the retention time from.

9. For natural gas take a sample from the gas tap which has a piece of rubber tubing

attached. Turn on the tap and allow gas to flow for approximately 5 seconds, then

insert the empty syringe into the rubber tubing and fill the syringe then flush the syringe

to atmosphere. Do this three times before taking a final sample for analysis and then

again repeat for a second sample

10. Set the sensitivity to 5mV/cm to 0.5 mV/cm and repeat procedures 1-6. Allow the

appearance of two medium peaks and then again repeat for a second sample.

11. For the mystery gas, a gas sampling vessel will be given from which to take the

samples for analysis. Insert the syringe through the top of the rubber septum and

withdraw a sample of gas and flush to atmosphere. Do this three times before taking a

final sample for analysis.

12. For mystery gas the sensitivity is changed from 5m/cm to 0.2mV/cm and repeat

procedures 1-6. Allow for the appearance of two medium peaks and then again repeat

for a second sample.


5/8

MARK THE CHROMATOGRAPH TRACE WITH:-

I. COLUMN USED

II. THE GAS SAMPLE

III. THE PAPER SPEED

IV. VOLTAGE SENSITVITY


6/8

Use of the Integrator for analysis.

When using the integrator the same procedures as described in sections 1 & 7 and the samegas sampling techniques (sections 4, 8 & 10) are used for each of the three gases to beanalysed in columns 1 and 2. All the necessary parameters have been programmed into theintegrator so that the only keypads needed are the large orange START/RUN pad, the grey

STOP pad beside it and the grey FEED pad.

a) On injection of the sample onto the column press the orange START pad. The

integrator will begin clicking as the chart paper starts to travel. The date, time and theword Start will be printed onto the integrator paper.

b) At the end of the run press the grey STOP pad. The integrator will print the word stop at

the end of the trace then go on to printout the sample run data.

c) To run the recorder paper out of the integrators printer area press the FEED pad unto

sufficient has run through the press FEED pad again to stop the flow.

WHEN ANALYSING THE GASES ON THE INTEGRATOR, DO ONE SAMPLE RUN

ONLY FOR EACH OF THE GASES. LABEL THE CHROMATORGRAPH PRINT OUTS

WITH THE GAS AND COLUMN USED.

Specimen example

Ethane - recorded at 0.5mV/cm sensitivity and measured peak height of 10cm.

Thus : 5cm x 0.5mV/cm = 2.5mV.

Therefore reading 2.5mV from the calibration chart would give a reading of 0.2mls

volume.

A 1ml sample was used, therefore 0.2mls x 100 = 20% volume of ethane in the gassample analysed.

NB: If a peak occurs from any gas mixture which does not correspond to any of the

combustion gases, it is possible to determine the amount of this gas by knowing that the totalvolume of all the gases must add up to 1 ml.


7/8

SPECIFIC GRAVITY OF GASES USING THE SIGMA APPARATUS

This test is based on comparing the time taken for a given volume of gas to effuse through asmall hole compared with the time taken for the same volume of air. The method is called theSchilling test.

Specific Gravity of gas =

(Time in seconds ) for gas

(Time in seconds ) for air

2

2

The three gases to be determined are Air, a mystery gas and natural gas.

To conduct a test of the specific gravity of a gas by the Schilling method, the followingprocedure should be adopted.

The sample gas is admitted through the inlet valve of the gas holder.

The effusion orifice should be fitted onto the outlet side of the gas holder. (Use as short a

length of rubber tubing as possible).

I. Turn on the inlet supply valve and allow the pressure of the air to raise the holder to the toplimit of its travel. Turn off the gas supply and then remove the orifice temporarily and purgethe holder by allowing the gas to disperse. Replace the orifice and re-fill holder. Do thisthree times before beginning the test.

II. Fill the holder as in (1), leaving the orifice in place for the tests, turn off the inlet supply valve

and accurately time the fall of the holder as the pointer passes from the top graduation tothe lowest graduation of the fixed scale.

III. Recharge the holder again and time the fall two more times.

Repeat steps 1, 2 and 3, using the mystery gas and natural gas.

The results are then calculated as in the following example:

The mean of the 3 times for the air 312.3 secThe mean of the 3 times for the gas 215.5 sec

The S.G. of the gas compared to air (unity) = (215.5/312.3)2 = 0.476

SPECIAL NOTES.

To obtain very accurate results the test times should agree within 0.2 seconds.

The air pump should be used gently to avoid raising the air sample temperature.

The effusion orifice must be kept thoroughly clean.


8/8

how to use shilling aparatus and chromotograph

Documents