It produces fast run times with baseline resolution of key components in under 3 minutes. Moreover, it displays enhanced resolutions of ethanol and acetone peaks, which helps with determining the BAC levels. This particular column is known as Zebron-BAC and it made with polyimide coating on the outside and the inner layer is made of fused silica and the inner diameter ranges from.
There are also many other Zebron brand columns designed for other purposes. Another example of a Zebron GC column is known as the Zebron-inferno. Its outer layer is coated with a special type of polyimide that is designed to withstand high temperatures. As shown in figure 6, it contains an extra layer inside. Moreover, it is also used for acidic and basic samples.
The detector is the device located at the end of the column which provides a quantitative measurement of the components of the mixture as they elute in combination with the carrier gas.
In theory, any property of the gaseous mixture that is different from the carrier gas can be used as a detection method. These detection properties fall into two categories: bulk properties and specific properties.
Bulk properties, which are also known as general properties, are properties that both the carrier gas and analyte possess but to different degrees. Specific properties, such as detectors that measure nitrogen-phosphorous content, have limited applications but compensate for this by their increased sensitivity. Each detector has two main parts that when used together they serve as transducers to convert the detected property changes into an electrical signal that is recorded as a chromatogram.
The first part of the detector is the sensor which is placed as close the the column exit as possible in order to optimize detection. The second is the electronic equipment used to digitize the analog signal so that a computer may analyze the acquired chromatogram. The sooner the analog signal is converted into a digital signal, the greater the signal-to-noise ratio becomes, as analog signal are easily susceptible to many types of interferences. An ideal GC detector is distinguished by several characteristics.
The first requirement is adequate sensitivity to provide a high resolution signal for all components in the mixture.
This is clearly an idealized statement as such a sample would approach zero volume and the detector would need infinite sensitivity to detect it. In modern instruments, the sensitivities of the detectors are in the range of 10 -8 to 10 g of solute per second. Furthermore, the quantity of sample must be reproducible and many columns will distort peaks if enough sample is not injected.
An ideal column will also be chemically inert and and should not alter the sample in any way. In addition, such a column would have a short linear response time that is independent of flow rate and extends for several orders of magnitude. Moreover, the detector should be reliable, predictable and easy to operate. Understandably, it is not possible for a detector meet all of these requirements.
Mass Spectrometer MS detectors are most powerful of all gas chromatography detectors. When the sample exits the chromatography column, it is passed through a transfer line into the inlet of the mass spectrometer. The sample is then ionized and fragmented, typically by an electron-impact ion source. During this process, the sample is bombarded by energetic electrons which ionize the molecule by causing them to lose an electron due to electrostatic repulsion. Further bombardment causes the ions to fragment.
Most ions are only singly charged. The Chromatogram will point out the retention times and the mass spectrometer will use the peaks to determine what kind of molecules are exist in the mixture.
A simple quadrupole ion-trap consists of a hollow ring electrode with two grounded end-cap electrodes as seen in figure. Ions are allowed into the cavity through a grid in the upper end cap.
Ions that are too heavy or too light are destabilized and their charge is neutralized upon collision with the ring electrode wall. Emitted ions then strike an electron multiplier which converts the detected ions into an electrical signal. This electrical signal is then picked up by the computer through various programs.
They are rugged, easy to use and can analyze the sample almost as quickly as it is eluted. The disadvantages of mass spectrometry detectors are the tendency for samples to thermally degrade before detection and the end result of obliterating all the sample by fragmentation.
Flame ionization detectors FID are the most generally applicable and most widely used detectors. In a FID, the sample is directed at an air-hydrogen flame after exiting the column.
At the high temperature of the air-hydrogen flame, the sample undergoes pyrolysis, or chemical decomposition through intense heating. Pyrolized hydrocarbons release ions and electrons that carry current. A high-impedance picoammeter measures this current to monitor the sample's elution.
It is advantageous to use FID because the detector is unaffected by flow rate, noncombustible gases and water. These properties allow FID high sensitivity and low noise. The unit is both reliable and relatively easy to use. However, this technique does require flammable gas and also destroys the sample. Thermal conductivity detectors TCD were one the earliest detectors developed for use with gas chromatography.
The TCD works by measuring the change in carrier gas thermal conductivity caused by the presence of the sample, which has a different thermal conductivity from that of the carrier gas.
Their design is relatively simple, and consists of an electrically heated source that is maintained at constant power. The temperature of the source depends upon the thermal conductivities of the surrounding gases.
The source is usually a thin wire made of platinum, gold or. The resistance within the wire depends upon temperature, which is dependent upon the thermal conductivity of the gas. TCDs usually employ two detectors, one of which is used as the reference for the carrier gas and the other which monitors the thermal conductivity of the carrier gas and sample mixture.
Carrier gases such as helium and hydrogen has very high thermal conductivities so the addition of even a small amount of sample is readily detected. It is mainly due to the analyte that is extracted with cooled condensed solvent.
The extract volume is relatively large its drawback. So, the evaporation step is usually needed to concentrate the analytes before the analysis. The sample size is often 10 g or more, and multiple samples can be extracted on separate Soxhlet units.
An automated Soxhlet extraction Soxtec was approved by the EPA EPA in for the extraction of semivolatile and nonvolatile organic compounds [ 2 ]. Automated Soxhlet extraction is relatively faster than Soxhlet extraction, with lower consuming organic solvents [ 2 ]. In this method, the extraction is performed in three stages: In the first stage, a thimble containing the sample is immersed in the boiling solvent for about 60 min. Since the contact between the solvent and the sample is more vigorous and the mass transfer in a high-temperature boiling solvent is more rapid, extraction here is faster than in Soxhlet.
In the second stage, the sample thimble is placed above the boiling solvent. Then, the condensed solvent drips into the sample and extracts the organics and falls back into the solvent reservoir as well.
This stage is similar to traditional Soxhlet and takes usually 60 min. In the third stage, the solvent is evaporated, and a concentration step happens for 10—20 min.
Li et al. In this study, a method involving four-factor-three-level orthogonal array design was developed. The orthogonal array designs included extracting solvent component, particle size, solvent overflow recycle, and time needed for the optimization of extracting nine organochlorine pesticides from ginseng root, followed by capillary GC-electron capture detector and MS detector [ 20 ]. Ultrasonic extraction, also known as sonication, uses ultrasonic vibration to ensure intimate contact between the sample and the solvent.
Sonication is relatively fast, but the extraction efficiency is not as high as some other techniques. Also, ultrasonic irradiation may decompose some of organophosphorus compounds.
Before the sonication is used for real sample, the selected solvent system and optimum conditions for adequate extraction of the target analytes from reference samples should be investigated. A typical sonication device can be equipped with a titanium tip. The sample is usually dried with anhydrous sodium sulfate and mixed with a certain volume of selected solvent.
The disruptor horn tip is positioned just below the surface of the solvent, yet above the sample. Extraction can be carried out in duration as short as 3 min. After extraction, the extract is filtered or centrifuged, and also some form of cleanup is needed before analysis [ 2 ]. The ultrasonic extraction USE is a very versatile technique due to the possibility of selecting the solvent type or solvent mixture that allows the maximum extraction efficiency and selectivity.
In USE, several extractions can be done simultaneously, and no specialized laboratory equipment is required advantage. But it is not easily automated disadvantage [ 21 ]. The USE technique was used to separate the pesticides from the soil samples [ 21 ]. In supercritical fluid extraction SFE , supercritical fluids possess specific properties which make them facilitate the extraction of organics from solid samples.
Two configuration of SFE operations are on- or off-line mode. Off-line SFE, as its name implies, is a stand-alone extraction method independent of the analytical method to be applied. Off-line SFE is more flexible and easier to perform than that of the online procedure. It allows the Extract to be available for analysis by different techniques [ 2 ]. A supercritical fluid SF is a substance above its critical temperature and pressure. Also, it is an interface between gas and liquid.
In fact it is not a liquid and or a gas, it is a SF. It is nontoxic and nonflammable and also is available at high purity. So, carbon dioxide has become the solvent of interest for most SFE applications. Supercritical CO 2 is nonpolar and without permanent dipole moment; therefore, it can be utilized to extract nonpolar and moderately polar compounds from matrices. But these SFs are not environmentally friendly and they are not used in routine analysis [ 2 ].
SFE has gained increased attention as a good candidate instead of conventional liquid solvent extraction. This is mainly due to significant properties of supercritical fluids SFs such as their high diffusivity and low viscosity which make them extract selectively different chemicals without additional cleanup steps and so use little sample amounts [ 22 ].
Rissato et al. In this study, SFE procedure was used to separate some pesticides from honey samples, and it was compared with liquid-liquid extraction method [ 22 ].
Supercritical fluid extraction is matrix dependent and usually needs the addition of organic modifiers. ASE was developed to overcome these limitations. Although it was expected that conventional solvents would be less efficient than supercritical fluids, the results turned out to be quite the opposite. In many cases, extraction was faster and more complete with organic solvents at elevated temperature and pressure than with SFE [ 2 ]. The elevated pressure and temperature used in ASE affect the solvent and sample properties and their interactions as well.
ASE properties include the following [ 2 ]: Under higher pressure, the extraction would be performed at higher temperature values. This is mainly due to the increase of the solvent boiling point. At higher pressures, the solvent penetration into the sample medium would be increased, and so the extraction of the interested analyte may be facilitated from the matrix. The elevated temperature can reduce the power of analyte-sample bonds like dipole, hydrogen, and van der Waals interactions.
High temperature decreases the solvent viscosity and surface tension and so enhances solvent penetration into the matrix medium. Therefore, faster extractions and better analyte recoveries can be achieved by ASE procedures.
ASE process has some steps mentioned below: The extraction cell is filled with the sample medium. And, the cell temperature and pressure are increased to the desired level.
The above steps are referred to the prefill method. If before addition of solvent the sample is warmed, the process is mentioned as preheat method.
In comparison of the two procedures with each other, the prefill method is usually preferred [ 2 ]. Pastor et al.
It should be noted that microwave-assisted extraction MAE is different from microwave-assisted acid digestion. The former uses organic solvents to extract organic compounds from solids, while the latter uses acids to dissolve the sample for elemental analysis with the organic contents being destroyed. MAE is applied for the extraction of semivolatile and nonvolatile compounds from solid samples.
In general, organic extraction and acid digestion use different types of microwave apparatuses, as these two processes require different reagents and experimental conditions. The basic components of a microwave system include a microwave generator magnetron , a waveguide for transmission, a resonant cavity, and a power supply. There are two types of laboratory microwave units: Closed extraction vessels under elevated pressure. In the liquid and solid states, molecules do not rotate freely in the microwave field, despite of gaseous molecules; therefore, no microwave spectra can be observed.
Liquid- and solid-state molecules respond to the radiation differently, and this is where microwave heating comes in. During microwave heating procedure, electromagnetic energy would be changed to heat.
This is mainly due to the ionic conduction and dipole rotation of the molecules which are imposed. Ionic conduction is concluded from the ion mobility in a solution under an electromagnetic field, and then, the heat is produced. Dipole rotation means that the directions of dipole rotations are changed under microwave irradiation.
When a polarized molecule is imposed in an electromagnetic field, it can rotate around its axis at a rate of 4. So, with the larger molecular dipole moments, the more vigorous oscillations of molecules are obtained under a microwave field. The proper choice of solvent is the key to successful extraction in MAE. In general, three types of solvent system can be used in MAE: Solvent s of high dielectric coefficient.
Zhou et al. The cleanup step was performed by passing the extracts through a non-deactivated silica gel column [ 24 ]. From an analytical point of view, volatile organic compounds VOCs are organic materials whose vapor pressures are greater than or equal to 0. Many VOCs are environmental pollutants, and in most cases of their analyses, the analytes are transferred to a gas-vapor phase and then analyzed by GC techniques [ 2 ]. Generally, the analysis of pure volatile compounds is simple, and the volatile analyte can be injected directly into a GC column [ 25 ].
However, the challenge is to extract the analytes from the matrix samples such as soil, food, cosmetics, polymers, and pharmaceutical raw materials. Headspace extractions are approaches to this and are divided into two categories: static headspace extraction SHE and dynamic headspace extraction purge and trap [ 2 ].
Static headspace extraction is known as equilibrium headspace extraction or simply as headspace. This technique has been available more than 30 years, so its instrumentation is both mature and reliable. In this technique, the extraction method includes the following [ 2 ]: A sample, either solid or liquid, is put in a headspace autosampler HSAS or vial.
The sample vial is brought to a constant temperature and pressure, and the volatile analytes diffuse into the headspace vessel. When the analyte concentration in the headspace part of the vessel reaches to an equilibrium level with respect to its concentration in the sample, the vial is connected to the GC column head, and then, a portion of the headspace is introduced into a GC for detection.
This analyte transfer is due to a pressure drop between the vessel and the GC inlet pressure. The vial is again isolated.
For automated systems, this sampling procedure can be repeated by the same or the next vial. The advantage of static headspace extraction is the ease of initial sample preparation. Usually for qualitative analysis, the sample can be placed directly into the headspace vial and analyzed with no additional preparation procedures.
But for quantitative analysis, it may be vital to know the optimized matrix effects to gain good sensitivity and accuracy. For large solid samples, it may be needed to change the physical state of the sample matrix. Two approaches in differentiating the sample state are to powder the solid sample and to disperse it into a liquid. By crushing the solid sample, the surface area available for the volatile solute to distribute into the headspace phase is enhanced.
So, the solute is distributing between a solid and the headspace phases. But in the second procedure, dispersing the solid into a liquid is preferred because the analyte partitioning process into the headspace often reaches the equilibrium faster. Therefore, by choosing a suitable solvent with high affinity toward the volatile analytes, the problems with sample and standard transfer from volumetric flask to headspace vials can be eliminated [ 2 ]. Some experimental factors affecting SHE should be optimized to improve extraction efficiency, sensitivity, quantitation, and reproducibility.
These experimental variables include vial and sample volume, temperature, pressure, and the form of the matrix itself. For the analysis of trace amounts of analytes, or where an exhaustive extraction of the analyte is required, purge and trap or dynamic headspace extraction DHE is more preferred than SHE.
This technique is used for both solid and liquid samples. The samples can be biological, environmental, industrial, pharmaceutical, and agricultural. In DHE, there is no equilibrium between its concentration in the gas and matrix phases. Instead, they are removed continuously from the sample by a gas flow. This provides a concentration gradient between two mentioned phases which makes the exhaustive extraction of the volatile analytes.
A typical purge and trap system consists of the following: A purge vessel. A purge and trap cycle consists of several steps: 1 purge, 2 dry purge, 3 desorb preheat, 4 desorb, and 5 trap bake. The mentioned steps in a purge and trap cycle can be explained as follows: An aqueous sample is introduced into the purge vessel.
The valve is set to the purge position. A purge gas typically, helium breaks through the sample continuously and sweeps the volatile organics to the trap, where they are retained by the sorbents. Then, the gas is vented to the atmosphere. The purging step consists of purge, dry purge, and preheating. After purging, while the trap is at the ambient temperature, the purge gas is transferred directly into the trap without passing through the sample.
This step is called dry purge. The main objective of this step is to remove the water which has been accumulated on the trap. Dry purging often takes place between 1 and 2 min. Preheat makes the subsequent desorption faster. On the other hand, it is back-flushed with the GC carrier gas. So, the preheat temperature is reached, and the six-port valve is rotated to the desorb position to initiate the desorption step. Desorption time is about 1—4 min and depends on the carrier flow rate in GC instrument.
For instance, the trap desorption time is short at the high flow rate, and so, a narrowband injection is achieved. The flow rate of the desorb gas should be selected in accordance with the type of GC column used. On the other hand, the operational conditions of the purge and trap must be compatible with configuration of GC system. With a packed GC column, higher carrier gas desorb gas flow rates can be applied. Capillary columns require lower flow rate and are often preferred over the packed one for better resolution.
In the trap baking step, after the desorption step, the valve is readjusted in the purge position. The objective of this step is to remove possible contaminants and eliminate sample transport. After the trap baking step, the trap temperature is diminished and the next sample can be extracted. In each step, the conditional parameters such as temperature, time, and flow rate should be the same for all of the samples and calibration standards. The trap is usually a stainless steel tube 3 mm in inside diameter ID and 25 mm long packed with multiple layers of adsorbents, and it should do the following steps: Retain the analytes of interest, but do not introduce impurities.
The sorbents are often arranged in layers to increase the trapping capacity. During purging process, the purge gas reaches the weaker sorbent at first, and only less volatile organics are retained. But more volatile compounds just pass through this layer and then are trapped by the other stronger adsorbent layers. During desorption process, the trap is heated and back-flushed with the GC carrier gas. However, the less volatile compounds have never been in contrast with the stronger adsorbents, and so, the reversible adsorptions can be achieved.
To trap volatile organic compounds, the substances such as Tenax, silica gel, activated charcoal, graphitized carbon black GCB or Carbopack , carbon molecular sieves Carbosieve , and Vocarb are usually used [ 2 ]. GC , 11 , —]. Three factors determine how we introduce a sample to the gas chromatograph. Second, the analytes must be present at an appropriate concentration.
Finally, the physical process of injecting the sample must not degrade the separation. Each of these needs is considered in this section. Not every sample can be injected directly into a gas chromatograph. A solute of low volatility, for example, may be retained by the column and continue to elute during the analysis of subsequent samples. A liquid—liquid extraction of analytes from an aqueous matrix into methylene chloride or another organic solvent is a common choice.
An attractive approach to isolating analytes is a solid-phase microextraction SPME. The fiber, which is coated with a thin film of an adsorbent material, such as polydimethyl siloxane, is lowered into the sample by depressing a plunger and is exposed to the sample for a predetermined time. After withdrawing the fiber into the needle, it is transferred to the gas chromatograph for analysis. Two additional methods for isolating volatile analytes are a purge-and-trap and headspace sampling.
In a purge-and-trap , we bubble an inert gas, such as He or N 2 , through the sample, releasing—or purging—the volatile compounds.
These compounds are carried by the purge gas through a trap that contains an absorbent material, such as Tenax, where they are retained. Heating the trap and back-flushing with carrier gas transfers the volatile compounds to the gas chromatograph.
In headspace sampling we place the sample in a closed vial with an overlying air space. After allowing time for the volatile analytes to equilibrate between the sample and the overlying air, we use a syringe to extract a portion of the vapor phase and inject it into the gas chromatograph.
Alternatively, we can sample the headspace with an SPME. Thermal desorption is a useful method for releasing volatile analytes from solids. We place a portion of the solid in a glass-lined, stainless steel tube. After purging with carrier gas to remove any O 2 that might be present, we heat the sample. Volatile analytes are swept from the tube by an inert gas and carried to the GC.
Because volatilization is not a rapid process, the volatile analytes often are concentrated at the top of the column by cooling the column inlet below room temperature, a process known as cryogenic focusing. Once volatilization is complete, the column inlet is heated rapidly, releasing the analytes to travel through the column.
The reason for removing O 2 is to prevent the sample from undergoing an oxidation reaction when it is heated. To analyze a nonvolatile analyte we must convert it to a volatile form. For example, amino acids are not sufficiently volatile to analyze directly by gas chromatography.
Reacting an amino acid, such as valine, with 1-butanol and acetyl chloride produces an esterified amino acid. A side benefit of many extraction methods is that they often concentrate the analytes.
If an analyte is too concentrated, it is easy to overload the column, resulting in peak fronting see Figure Injecting less sample or diluting the sample with a volatile solvent, such as methylene chloride, are two possible solutions to this problem. In Chapter We also introduce an additional source of band broadening if we fail to inject the sample into the minimum possible volume of mobile phase.
There are two principal sources of this precolumn band broadening: injecting the sample into a moving stream of mobile phase and injecting a liquid sample instead of a gaseous sample. The top of the column fits within a heated injector block, with carrier gas entering from the bottom. Injecting the sample directly into the column minimizes band broadening because it mixes the sample with the smallest possible amount of carrier gas. In a split injection we inject the sample through a rubber septum using a microliter syringe.
Instead of injecting the sample directly into the column, it is injected into a glass liner where it mixes with the carrier gas. At the split point, a small fraction of the carrier gas and sample enters the capillary column with the remainder exiting through the split vent.
By controlling the flow rate of the carrier gas as it enters the injector, and its flow rate through the septum purge and the split vent, we can control the fraction of sample that enters the capillary column, typically 0. In a splitless injection , which is useful for trace analysis, we close the split vent and allow all the carrier gas that passes through the glass liner to enter the column—this allows virtually all the sample to enters the column. Because the flow rate through the injector is low, significant precolumn band broadening is a problem.
For samples that decompose easily, an on-column injection may be necessary. In this method the sample is injected directly into the column without heating. The column temperature is then increased, volatilizing the sample with as low a temperature as is practical.
In an isothermal separation we maintain the column at a constant temperature. To increase the interaction between the solutes and the stationary phase, the temperature usually is set slightly below that of the lowest-boiling solute.
One difficulty with an isothermal separation is that a temperature that favors the separation of a low-boiling solute may lead to an unacceptably long retention time for a higher-boiling solute. Temperature programming provides a solution to this problem. As the separation progresses, we slowly increase the temperature at either a uniform rate or in a series of steps.
The final part of a gas chromatograph is the detector. The ideal detector has several desirable features: a low detection limit, a linear response over a wide range of solute concentrations which makes quantitative work easier , sensitivity for all solutes or selectivity for a specific class of solutes, and an insensitivity to a change in flow rate or temperature.
Because of its high thermal conductivity, helium is the mobile phase of choice when using a thermal conductivity detector TCD. Thermal conductivity, as the name suggests, is a measure of how easily a substance conducts heat.
A gas with a high thermal conductivity moves heat away from the filament—and, thus, cools the filament—more quickly than does a gas with a low thermal conductivity. When a solute elutes from the column, the thermal conductivity of the mobile phase in the TCD cell decreases and the temperature of the wire filament, and thus it resistance, increases.
The detector also is non-destructive, which allows us to recover analytes using a postdetector cold trap. One significant disadvantage of the TCD detector is its poor detection limit for most analytes. Applying a potential of approximately volts across the flame creates a small current of roughly 10 —9 to 10 —12 amps. When amplified, this current provides a useful analytical signal. Most carbon atoms—except those in carbonyl and carboxylic groups—generate a signal, which makes the FID an almost universal detector for organic compounds.
Most inorganic compounds and many gases, such as H 2 O and CO 2 , are not detected, which makes the FID detector a useful detector for the analysis of organic analytes in atmospheric and aqueous environmental samples. Advantages of the FID include a detection limit that is approximately two to three orders of magnitude smaller than that for a thermal conductivity detector, and a linear response over 10 6 —10 7 orders of magnitude in the amount of analyte injected.
The sample, of course, is destroyed when using a flame ionization detector. The electron capture detector is an example of a selective detector. The emitted electrons ionize the mobile phase, usually N 2 , generating a standing current between a pair of electrodes. The stationary phase is assembled as a compacted material packed column or as a wall-coating film capillary column.
The carrier gas is introduced from a gas cylinder into the gas chromatograph. It moves through the column at a constant flow rate and exits at the detector outlet. Unlike other methods, the mobile phase in GC does not interact with chemicals and only serves to carry them.
Because of this, the carrier gas must be inert. Some examples are helium, nitrogen, and argon. The type of detector on the GC usually determines which gas is used.
It is not something that you would have to decide since a working GC should already have a gas tank connected to it. Like any other oven, a GC oven provides heat. But instead of baking goods, what this oven gives you is vaporized material right after injection. In addition, it keeps the column heated so that you continue to have gaseous molecules traveling through. A temperature program can be set electronically to maintain a constant temperature or to gradually increase ramping.
The program that you select will depend on the nature of the sample. This is a device at the end of the column that senses each compound as it comes out. The data recorded by the detector is transmitted into a computer and produces a two-dimensional plot, called chromatogram. There are several types of detectors with varying detection methods and limits. A particular powerful detector is the mass spectrometer MS.
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