This ppt describes various sample introduction techniques used in Gas Chromatography

1. Dr. Vrushali Sachinkumar Tambe
Professor (Department of Pharmaceutical Chemistry)
PES Modern College of Pharmacy (for Ladies), Pune,
India
SAMPLE INTRODUCTION
TECHNIQUES USED IN GAS
CHROMATOGRAPHY

2. The analytes moves in the form of vapors on GC column. The analyte
must be introduced as a plug of vapors on the column.
Following techniques are used for sample introduction on GC columns
• Direct injection
• Flash vaporizer
• Split and splitless injection
• Rotary valve injector
• Pyrolysis
• Purge and trap
• Grob injection
• Head space sampling
• Direct thermal extraction (DTE)
• Solid phase microextraction (SPME)
• Autosampler

3. DIRECT INJECTION
• The syringe technique is most common and can be used with both gas and
low viscosity liquid samples by inserting the needle through a rubber septum
to the column inlet port.
• The region into which the needle projects must be heated in order to flash
vaporise the sample.
• However, overheating of the rubber septum must be avoided to prevent
leaching.
• The most popular inlet for capillary GC is the split/splitless injector.
• If this injector is operated in split mode, the amount of sample reaching the
column is reduced (to prevent column overloading) and very narrow initial
peak widths can be obtained. It results in wastage of sample.
• For maximum sensitivity, the injector can be used in splitless mode, then all
of the injected sample will reach the column. Injection may be manual or
automated.

4. FLASH VAPORISER
For optimum column efficiency, the sample should not be too large,
and should be introduced onto the column as a "plug" of vapour. Slow
injection of large samples causes band broadening and loss of
resolution. The most common injection method is where a microsyringe
is used to inject sample through a rubber septum into a flash
vapouriser port at the head of the column. The temperature of the
sample port is usually about 500C higher than the boiling point of the
least volatile component of the sample. For packed columns, sample
size ranges from tenths of a microliter up to 20 microliters. Capillary
columns, on the other hand, need much less sample. For capillary GC,
split/splitless injection is used.

5. ROTARY VALVE INJECTOR
• For quantitative work, more reproducible sample sizes for
both liquids and gases are obtained by means of a rotary
sample valve.
• The sampling loop is filled by injection of an excess of sample.
• Rotation of the valve by 45 deg then introduces the
reproducible volume ACB into the mobile phase.
• Sample valves are convenient for on-line gas stream
analysis.
• In LOAD position, the stream to be sampled flows through a
loop of calibrated volume while the carrier gas alone passes
through the column.
• In INJECTION position, the loop is placed in the carrier gas
stream and the entrapped sample is swept along to the
column.
• Sample valves are becoming more prevalent for quantitative
work employing both liquids and gases to introduce a
reproducible volume of sample onto a column.
• They are typically employed for smaller volumes, e.g., to
prevent over loading of a column > 0.01 ml of a liquid sample
is preferred volume - a precision syringe for this volume is
both expensive and fragile.
• Valves may also be used in split – splitless mode

6. PYROLYSIS GAS CHROMATOGRAPHY (PGC)
• A version of reaction chromatography in
which a sample is thermally decomposed to
simpler fragments before entering the
column.
• Many non-volatile solids can be decomposed
thermally to produce characteristic gaseous
products that can be chromatographed.
• Samples are placed directly on a small coil of
Pt wire where it can be heated to several
hundred degrees in a few milliseconds while
the carrier gas is flowing over it.
• The pyrolysis products are swept directly
onto the column.

7. HEAD SPACE ANALYSIS
• Head space analysis is a technique where the vapours in the gas
above, and in equilibrium with, a solid or liquid is sampled. Typical
samples can include water, effluents, soil, food, and beverages,
blood, urine, human breath and environmental samples. The sample
itself is often in an unsuitable state for direct GC and so without
headspace sampling it would require considerable sample
pretreatment and cleanup before it could be applied to a GC
column.
• The advantage of this approach is that GC can be used instead of
HPLC, thus providing four to five orders of magnitude greater
sensitivity.
• Procedure involves the extraction of a volume of the equilibrium gas
over the sample (usually about 10 ml) by a syringe through either a
vial containing a bed of an appropriate absorbent or a cryogenic
trap.
• The vial/trap may be placed in line with a GC column, heated and
the vaporised sample swept onto the column and the components
separated.
• Used to identify spoiled food, fragrances from botanical material,
determination of plasticizers in plastics and for forensic samples.

8. PURGE AND TRAP
• Can be used for most volatile organic
compounds with boiling points below 200 ºC
and are insoluble or slightly soluble in water.
• Multiple sample aliquots are collected in
sealed containers with minimum headspace
and stored at 4 ºC or less in solvent free area.
• An inert gas is bubbled through aqueous
sample at room or elevated temperature
depending on the target analytes.
• The vapour is swept through a sorbent
column where the analytes are captured.
• After purging, the sorbent column is heated
(thermal extraction) and back-flushed with
inert gas to desorb the components onto a
GC column.

9. SOLID PHASE MICROEXTRACTION
• Solid phase microextraction (SPME) is suitable for sampling
environmental contaminants with a wide range of physical
properties in air, water and soil.
• A fused silica fibre with a polymer coating is exposed to the
sample or the headspace above the sample.
• Organic analytes adsorb to the coating on the fibre. After
adsorption equilibrium is attained, usually in 2 to 30
minutes, the fibre is withdrawn.
• The fibre is introduced into a GC injector, where the adsorbed
analytes are thermally desorbed and delivered to the GC
column.
• The amount of analyte adsorbed by the fibre depends on the
thickness of the polymer coating and on the distribution
constant for the analyte.
• Fibres with a range of different polarities are now
commercially available.

11. DIRECT THERMAL EXTRACTION
• Permits the direct thermal extraction of volatile
and semi-volatile organics directly from small
sample sizes (mg) without the need for solvent
extraction or other sample preparation
requirements.
• The sample maybe trapped on sorbent resin or
placed inside a preconditioned glass-lined
stainless steel desorption tube.
• The desorption tube containing the sample is
then connected to a short path thermal
desorption system.
• The desorption tube is ballistically heated and
carrier gas carries the analytes through the
injection port and onto the GC column for
analysis.

12. GROBS INJECTION
When analysis is performed on small amounts of sample with
some component concentrations in the parts per billion range,
too little material is placed on the column if splitting is used for
these samples. Splitless injection is required for these samples.
The entire sample including any solvent, is injected onto the
open tubular column through a modified heated flash vaporizer.
A large solvent tail is avoided by venting the injection port to the
autosampler at that time, when most of the solvent with sample
have entered the column. The solvent then is liquefied on the
column, due to lower column temperature and helps in the
separation of analyte.

13. AUTOSAMPLER
• It duplicates manual sample
measurement and injection. Sample vials
are glass, throw-away tight septum caps.
The sampler flushes the syringe with a
new sample to remove traces of the
previous sample, pumps new sample to
wet the syringe completely and
eliminates any bubbles, takes in a
precisely measured amount of sample,
and injects the sample into the gas
chromatograph.
• The automatic samplers are machine
reproducible and consistently more
precise than a skilled chromatographer.