main gas chromatography

8/7/2019 MAIN Gas Chromatography

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Gas Chromatography (GC)Introduction

Gas chromatography is a chromatographic technique that can be used to separate volatile

organic compounds. A gas chromatograph consists of a flowing mobile phase, an

injection port, a separation column containing the stationary phase, and a detector. The

organic compounds are separated due to differences in their partitioning behavior

between the mobile gas phase and the stationary phase in the column.

Instrumentation

Mobile phases are generally inert gases such as helium, argon, or nitrogen. The inject ionPort consists of a rubber septum through which a syringe needle is inserted to inject the

sample. The injection port is maintained at a higher temperature than the boiling point of

the least volatile component in the sample mixture. Since the partitioning behavior is

dependant on temperature , the separation column is usually contained in a thermostat-

controlled oven. Separating components with a wide range of boiling points is

accomplished by starting at a low oven temperature and increasing the temperature overtime to elute the high-boiling point components. Most columns contain a liquid stationary

phase on a solid support. Separation of low-molecular weight gases is accomplished with

solid adsorbents. Separate documents describe some specificGC Columns andGCDetectors.

Schematic of a gas chromatograph

Gas Chromatography ColumnsColumnsGas chromatography columns are of two designs: packed or capillary. Packed columns

are typically a glass or stainless steel coil (typically 1-5 m total length and 5 mm inner

diameter) that is filled with the stationary phase. Capillary columns are a thin fused-silica

(purified silicate glass) capillary (typically 10-100 m in length and 250 m inner

diameter) that has the stationary phase coated on the inner surface. Capillary columns


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provide much higher separation efficiency than packed columns but are more easily

overloaded by too much sample.

Gas Chromatography (GC) DetectorsIntroduction

After the components of a mixture are separated using gas chromatography, they must be

detected as they exit the GC column. The links listed below provide the details of some

specific GC detectors. The thermal-conductivity (TCD) and flame-ionization (FID)

detectors are the two most common detectors on commercial gas chromatographs. The

requirements of a GC detector depends on the separation application. For example, one

analysis might require a detector that is selective for chlorine-containing molecules,

another analysis might require a detector that is non-destructive so that the analyte can be

recovered for further spectroscopic analysis.

Specific GC detectors

Atomic-emmision detector (AED)

Chemiluminescence detector

Electron-capture detector (ECD)

The ECD is as sensitive as the FID but has a limited dynamic range and finds its

greatest application in analysis organic molecules that contain electronegative

functional groups, such as halogens, phosphorous, and nitro groups.Flame-ionization detector (FID)

The FID is extremely sensitive with a large dynamic range, its only disadvantage

is that it destroys the sample.

Flame-photometric detector (FPD)

Photoionization detector (PID)

Thermal conductivity detector (TCD)The TCD is not as sensitive as other dectectors but it is non-specific and non-

destructive.

Atomic-Emission Detector (AED)Introduction

As capillary column based gas chromatography takes its place as the major, highest

resolution separation technique available for volatile, thermally stable compounds, the

requirements for the sensitive and selective detection of these compounds increases.

Since more and more complex mixtures can be successfully separated, subsequent

chromatograms (output of a chromatographic separation) are increasingly more complex.

Therefore, the need to differentiate between the sample components using the GCdetector as a means of compounds discriminating is more and more common. In addition,


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each detector has its own characteristics (selectivity, sensitivity, linear range, stability,

cost, etc.) that helps in a decision about which detector to use.

One of the newest additions to the gas chromatographer's arsenal is the atomic emission

detector (AED). This detector, while quite expensive compared to other commercially

available GC detectors, is an extremely powerful alternative. FOR INSTANCE, Instead

of measuring simple gas phase (carbon containing) ions created in a flame as with theflame ionization detector, or the change in background current because of electronegative

element capture of thermal electrons as with the electron capture detector, the AED has a

much wider applicability because it is based on the detection of atomic emissions.

The strength of the AED lies in the detector's ability to simultaneously determine the

atomic emissions of many of the elements in analytes that elute from a GC capillary

column (called eluants or solutes in some books). As eluants come off the capillary

column they are fed into a microwave powered plasma (or discharge) cavity where the

compounds are destroyed and their atoms are excited by the energy of the plasma. The

light that is emitted by the excited particles is separated into individual lines via aphotodiode array. The associated computer then sorts out the individual emission lines

and can produce chromatograms made up of peaks from eluants that contain only aspecific element.

Instrumentation

The components of the AED include 1) an interface for the incoming capillary GC

column to the microwave induced plasma chamber, 2) the microwave chamber itself, 3) a

cooling system for that chamber, 4) a diffraction grating and associated optics to focus

then disperse the spectral atomic lines, and 5) a position adjustable photodiode array

interfaced to a computer. The microwave cavity cooling is required because much of the

energy focused into the cavity is converted to heat.


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Chemiluminescence SpectroscopyIntroduction

Chemiluminescence, like atomic emission spectroscopy (AES), uses quantitative

measurements of the opticalemis s ion from excited chemical species to determine analyte

concentration; however, unlike AES, chemiluminescence is usually emission from

energized molecules instead of simply excited atoms. The bands of light determined by

this technique emanate from molecular emissions and are therefore broader and more

complex then bands originating from atomic spectra. Furthermore, chemiluminescence

can take place in either the solution or gas phase, whereas AES is almost strictly as gasphase phenomenon.Though liquid phase chemiluminescence plays a significant role in laboratories using thisanalytical technique (often in conjunction with liquid chromatography

), we will

concentrate on gas phase chemiluminescence reactions since the instrumental

components are somewhat simpler. These detectors are also often used as detectors forgas chromatography.Like fluorescence spectroscopy, chemiluminescence's strength lies in the detection of


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electromagnetic radiation produced in a system with very low background. And on top of

this, because the energy necessary to excite the analytes to higherelectronic, vibrational,and rotational states (from which they can decay be emission) does not come from an

external light source like a laser or lamp, the problem of excitation source scattering is

completely avoided. The major limitation to the detection limits achievable by

chemiluminescence involves the dark current of the photomultip lier (PMT) necessary to

detect the analyte light emissions.

If the excitation energy for analytes in chemiluminescence doesn't come from a source lamp or laser, where does it come from?

The energy is produced by a chemical reaction of the analyte and a reagent. An example of a reaction of this sort is shown

below:A chemiluminescence reaction

In gas phase chemiluminescence, the light emission (represented as Planck's constant

times nu-the light's frequency) is produced by the reaction of an analyte (dimethyl sulfide

in the above example) and a strongly oxidizing reagent gas such as fluorine (in theexample above) or ozone, for instance. The reaction occurs on a time scale such that the

production of light is essentially instantaneous; therefore, most analytical systems simply

mix analytes and the reagent in a small volume chamber directly in front of a PMT. If the

analytes are eluting from a gas chromatographic column then the end of the column is

often fed directly into the reaction chamber itself. Since as much of the energy released

by the reaction should (in the analyst's eye) be used to excite as many of the analyte

molecules as possible, loss of energy via gas phase collisions is undesirable, and

therefore a final consideration is that the gas pressure in the reaction chamber be

maintained at a low pressure (~ 1 torr) by a vacuum pump in order to minimize the

effects of collisional deactivation.

It must be stated that the ambiguous specification of "products" in the above reaction is

often necessary because of the nature and complexity of the reaction. In some reactions,

the chemiluminescent emitters are relatively well known. In the above reaction the major

emitter is electronically and vibrationally excited HF; however, in the same reaction,

other emitters have been determined whose identities are not known and these also

contribute to the total light detected by the PMT.


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To the analytical chemist the ambiguity about the actual products in the reaction is, in

most case, not important. All the analyst cares about is the sensitivity of the instrument

(read detection limits for target analytes), its selectivity-that is, response for an analyte as

compared to an interfering compound, and the linear range of response.Here is a schematic of the components necessary for a gas phase chemiluminescence

detector interfaced to a capillary gas chromatograph.

ThIS fig is belong to ECD..

Electron Capture Detectors(ECD)Introduction

The ECD uses a radioactive Beta emitter (electrons) to ionize some of the carrier gas and

produce a current between a biased pair of electrodes. When organic molecules that


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contain electronegative functional groups, such as halogens, phosphorous, and nitro

groups pass by the detector, they capture some of the electrons and reduce the current

measured between the electrodes. The ECD is as sensitive as the FID but has a limited

dynamic range and finds its greatest application in analysis of halogenated compounds.Schematic of an ECD

Flame-Ionization Detectors (FID)

Introduction

An FID consists of a hydrogen/air flame and a collector plate. The effluent from the GC column passes through the flame, which

breaks down organic molecules and produces ions. The ions are collected on a biased electrode and produce an electrical

signal. The FID is extremely sensitive with a large dynamic range, its only disadvantage is that it destroys the sample.

Flame Photometric GC DetectorIntroduction

The reason to use more than one kind of detector for gas chromatography is to achieve

selective and/or highly sensitive detection of specific compounds encountered in

particular chromatographic analyses. The determination of sulfur or phosphorus

containing compounds is the job of the flame photometric detector (FPD). This device

uses the chemiluminescent reactions of these compounds in a hydrogen/air flame as a

source of analytical information that is relatively specific for substances containing these


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two kinds of atoms. The emitting species for sulfur compounds is excited S2. The lambda

max for emission of excited S2 is approximately 394 nm. The emitter for phosphorus

compounds in the flame is excited HPO (lambda max = doublet 510-526 nm). In order to

selectively detect one or the other family of compounds as it elutes from the GC column

an interference filteris used between the flame and thephotomultiplier tube (PMT) to isolate the appropriate emission band. Thedrawback here being that the filter must be exchanged between chromatographic runs if the other family of compounds is to be

detected.

Instrumentation

In addition to the instrumental requirements for 1) a combustion chamber to house the

flame, 2) gas lines for hydrogen (fuel) and air (oxidant), and 3) an exhaust chimney to

remove combustion products, the final component necessary for this instrument is a

thermal (bandpass) filter to isolate only the visible and UV radiation emitted by the

flame. Without this the large amounts of infrared radiation emitted by the flame's

combustion reaction would heat up the PMT and increase its background signal. The

PMT is also physically insulated from the combustion chamber by using poorly(thermally) conducting metals to attach the PMT housing, filters, etc.

The physical arrangement of these components is as follows: flame (combustion)

chamber with exhaust, permenant thermal filter (two IR filters in some commercial

designs), a removable phosphorus or sulfur selective filter, and finally the PMT.Schematic of a gas chromatographic flame photometric detector


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Photoionization DetectorIntroduction

The reason to use more than one kind of detector for gas chromatography is to achieve

selective and/or highly sensitive detection of specific compounds encountered in

particular chromatographic analyses. The selective determination of aromatic

hydrocarbons or organo-heteroatom species is the job of the photoionization detector

(PID). This device uses ultraviolet light as a means ofionizing an analyte exiting from a

GC column. The ions produced by this process are collected by electrodes. The current

generated is therefore a measure of the analyte concentration.

Theory

If the energy of an incoming photon is high enough (and the molecule is quantum

mechanically "allowed" to absorb the photon) photo-excitation can occur to such an

extent that an electron is completely removed from its molecular orbital, i.e. ionization.

A Photoionization Reaction

If the amount of ionization is reproducible for a given compound, pressure, and light

source then the current collected at the PID's reaction cell electrodes is reproducibly

proportional to the amount of that compound entering the cell. The reason why the

compounds that are routinely analyzed are either aromatic hydrocarbons or heteroatomcontaining compounds (like organosulfur or organophosphorus species) is because these

species have ionization potentials (IP) that are within reach of commercially available

UV lamps. The available lamp energies range from 8.3 to 11.7 ev, that is, lambda max

ranging from 150 nm to 106 nm. Although most PIDs have only one lamp, lamps in the

PID are exchanged depending on the compound selectivity required in the analysis.

Selective detection using a PID


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Here is an example of selective PID detection: Benzene's boiling point is 80.1 degrees C

and its IP is 9.24 ev. (Check the CRC Handbook 56th ed. page E-74 for IPs of common

molecules.) This compound would respond in a PID with a UV lamp of 9.5 ev

(commercially available) because this energy is higher than benzene's IP (9.24).

Isopropyl alcohol has a similar boiling point (82.5 degrees C) and these two compounds

might elute relatively close together in normal temperature programmed gas

chromatography, especially if a fast temperature ramp were used. However, since

isopropyl alcohol's IP is 10.15 ev this compound would be invisible or show very poor

response in that PID, and therefore the detector would respond to one compound but not

the other.

Instrumentation

Since only a small (but basically unknown) fraction of the analyte molecules are actually

ionized in the PID chamber, this is considered to be a nondestructive GC detector.

Therefore, the exhaust port of the PIDcan be connected to another detector in series withthe PID. In this way data from two different detectors can be taken simultaneously, and

selective detection of PID responsive compounds augmented by response from, say, anFIDorECD. The major challenge here is to make the design of the ionization chamber

and the downstream connections to the second detector as low volume as possible (read small diameter) so that peaks that

have been separated by the GC column do not broaden out before detection.Schematic of a gas chromatographic photoionization detector


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Thermal Conductivity Detectors (TCD)IntroductionA TCD detector consists of an electrically-heated wire or thermistor. The temperature of

the sensing element depends on the thermal conductivity of the gas flowing around it.

Changes in thermal conductivity, such as when organic molecules displace some of the

carrier gas, cause a temperature rise in the element which is sensed as a change in

resistance. The TCD is not as sensitive as other dectectors but it is non-specific and non-

destructive.

Instrumentation

Two pairs of TCDs are used in gas chromatographs.O

ne pair is placed in the columneffluent to detect the separated components as they leave the column, and another pair isplaced before the injector or in a separate reference column. The resistances of the twosets of pairs are then arranged in a bridge circuit.


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The bridge circuit allows amplification of resistance changes due to analytes passing over the sample thermoconductors and

does not amplify changes in resistance that both sets of detectors produce due to flow rate fluctuations, etc.

Gas Chromatographic InjectorsIntroduction

The great analytical strength of capillary gas chromatography lies in its highres olution.

Capillary columns have 1) more theoretical plates (a measure of column resolving power

or efficiency) per meter as compared to packed columns and 2) since they have less

resistance to flow they can be longer than packed columns. This means that the average

capillary column (30 meters long) has approximately 100,000 theoretical plates while the

average packed column (3 meters) has only 2500 plates.

But with this separation power comes some limitations: 1) Capillary columns, because

they have smaller diameters (0.05 to 0.53 mm) than packed columns (2 to 4 mm), require

relatively specialized injectors and ancillary flow and pressure controllers and 2)

capillary column require a smaller amount of sample than packed columns. While the

average sample mass of each component in a mixture that is separable by packed column

GC can be in the microgram range (10-6 grams) per injection, capillary columns routinelyonly handle 50 nanograms (10-9 grams) of a particular component or less.

Overloaded ChromatographyThis sample size requirement initially meant that if samples contained components that

were too concentrated for a capillary chromatographic analysis, the sample had to be

diluted before it was analyzed. Otherwise the column would beoverload ed by those highconcentrated components. An example of this appears in the first figure below. Theclearly overload peaks are indicated. And while some of the other components are in theresolvable (not overloaded) range, having large masses of components can also distort thepeak shape of some of the lower mass components.

An Overloaded Chromatogram


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The following figure shows a little better chromatography with fewer overloaded peaks.The second eluting peak (about 6 minutes) is clearly not overloaded while the groupbetween 10 and 14 minutes still shows overloading characteristics: long drawn-out tailingand much less than baseline separation with peaks that elute nearby (the 11 and 12minute peaks, for instance).


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Normal Packed Column Injector

The normal sequence of events in a GC injection is as follows. We will assume in this

explanation that some analytes are dissolved in a (liquid) solvent although much of thisprocess also holds for gas GC injections too: A small amount of liquid (microliters) is

injected through a silicon rubber septum (using a special microliter syringe) into the hot

(usually 200+ degrees C) GC injector that is lined with an inert glass tube. The injector is

kept hot by a relatively large, metal heater block that is thermostatically controlled. The

sample is immediately vaporized and a pressurized, inert, carrier gas-which is continually

flowing from a gas regulator through the injector and into the GC column-sweeps the

gaseous sample, solvent, analyte and all, onto the column. In the packed column injector,

ALL the vaporized sample enters onto the column. This is how all packed column

injectors work; everything that is injected goes onto the column. One modification of this

is a small ancillary flow of carrier gas that bathes the underside of the injector's septumso that hot vaporized sample gases can't interact and possibly stick to the septum. This

improves peak shape and reproducibility. This last feature is called the septum purge. The

following figure is a schematic of a packed column injector, sometimes called a direct or

flash injector. The septum purge is not shown here although the carrier gas regulator and

inlet, a septum, and an injection port liner ARE detailed. Last by not least, the packed GC

column itself is connected at the bottom of the injector via metal fittings.


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Schematic of packed GC column injector with septum purge

One last subtle point about the configuration of the carrier gas inlet: notice that it enters

the injector at about the middle of the heating block and its gas has to travel along the

outside of the injector before it enters the injection port liner AT THE TOP. This is so the

carrier is preheated before it enter the liner where the sample is vaporized. This helps toprevent a cold spot at the top of the injector where the carrier gas enters.

Now remember that the size of the capillary column limits the amount of analyte that can

be injected, otherwise, chromatographic overloading occurs. Therefore this packed

column injector design, if used with a capillary column, would require that samples with

high concentrations of analytes be diluted. Unless... what other alternative is there to get

the amount of analytes that are injected onto the column smaller without having to dilute

concentrated samples? The solution is the split/splitless capillary GC injector.

The Split/Splitless Capillary Injector

OK so far so good. But how does the split/splitless injector work? It starts with the same

requirements as the packed column injectors: carrier gas inlet, a septum, septum purge,

injector insert, heater block, and column connection; but the heart of this technological

feat is another set of gas linesout of the injector-another path that the vaporized sample

can take. This is called the split line or vent. The manufacturers of these systems design

them so that the carrier gas flow onto the column is constant-to maintain the

chromatographic requirements of the column and yield reproducible retention times for

analytes. At the same time, the amount of gas that goes out the split vent controls the

amount of sample that enters the column. If the split vent is closed, via a computer


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controlled split valve, then all of the sample introduced into the injector goes on thecolumn. If the split vent is open then most of the vaporized sample is thrown away towaste via the split vent and only a small portion of the sample is introduced to thecolumn. The following diagram illustrates a split/splitless injector with the split ventonso that only a small portion of the sample injected goes on the column

And finally, a very neat aspect of this is that the amount of gas exiting the split vent can

be varied while keeping the flow onto the column constant. This means that the

AMOUNT of the split (called the split ratio) can be varied. A common split ratio is 50 to

1. That is, for every 50 units of gaseous sample that are thrown away to waste, 1 unit

goes on the column.

The analyst keeps careful control of the split ratio so that results from thechromatography can still be quantified. Chromatographic peaks that show up as, say 2.5

ng of compound X really represent 2.5 x 50 = 125 ng of analyte X in the original sample.

Also notice that this mass (125 nanograms) would have overloaded the column if all of it

ended up on the capillary column. Voila! A split injection, and no sample dilution

required.


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main gas chromatography

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