In the analysis the solid-phase microextraction (SPME) and capillary gas chromatography/mass spectrometry (GC/MS) was developed for the identification of volatile compounds (hydrocarbon) in fuel. The samples was used is (kerosene, diesel, thinner and petrol) and one unknown. After the analyte was extracted by SPME in 20min, it directly injected to the GCMS with desorption time 80sec.After the analysis was done, the result was stated at table 4.1, the unknown was identified as a petrol because the hydrocarbon presence in the unknown is the same with the hydrocarbon compound in the petrol sample. Aromatic hydrocarbon was presence in both sample petrol and unknown. Alkanes hydrocarbon was presence in other sample.
The objectives of this experiment are to perform sample preparation by SPME and to identify the components of hydrocarbon in common fuel using SPME-GC-MS.
Solid phase microextraction, a simple, effective adsorption/desorption technique, eliminates the need for solvents or complicated apparatus for concentrating volatile or nonvolatile compounds in liquid samples or headspace. SPME is compatible with analyte separation/detection by gas chromatography or HPLC, and provides linear results for wide concentrations of analytes. By controlling the polarity and thickness of the coating on the fiber, maintaining consistent sampling time, and adjusting several other extraction parameters, an analyst can ensure highly consistent, quantifiable results from low concentrations of analytes.
Analyses of volatile or semivolatile organic environmental pollutants,flavor or fragrance components, and many other samples usually begin with concentrating the analytes of interest through liquid-liquid extraction, purge-and-trap, headspace, or various other techniques. These procedures typically require excessive time, complicated equipment, and/or extravagant use of organic solvents. Solid phase microextraction, or SPME,* an adsorption/desorption technique developed at the University of Waterloo (Ontario, Canada), eliminates the need for solvents or complicated apparatus for concentrating volatile or nonvolatile compounds in liquid samples or headspace. SPME provides linear results over wide concentrations of analytes (1-4), is compatiblewith any packed column or capillary gas chromatograph or gas chromatograph-mass spectrometer system, and can be used with split/splitless or direct/packed injectors. An SPME/HPLC interface allows the technique to be combined with analysis by HPLC, expanding the applications for the extraction technique to detection of surfactants in water, pharmaceuticals in biological fluids, and many other analyses.
An analytical process typically consists of several discrete steps: sampling, sample preparation, separation, quantification and data analysis. For example, in the analysis of semivolatile components in water, the target analytes are first extracted into an organic solvent. The resulting solution is then introduced into an analytical instrument for separation, quantification, and possible identification. Each of these steps affects the precision, accuracy and speed of the analysis. Although multi-dimensional techniques such as gas chromatography/mass spectrometry (GC/MS) have improved separation and quantification, the preparation step is still time consuming and often uses a significant volume of organic.
SPME was developed to simplify the preparation step. SPME is a microextraction technique, which means that the amount of extraction solvent is very small compared to the sample volume. As a result, exhaustive removal of analytes to the extracting phase does not occur, rather an equilibrium is reached between the sample matrix and the extracting phase. To make this approach practical, the extracting phase is permanently attached to rods made out of various materials. In most of the cases, the extracting phase is a polymeric organic phase that is cross-linked and permanently attached to the rod.
In one configuration, the rods consist of an optical fiber made of fused silica, which is chemically inert. A polymer layer is used to protect the fiber against breakage. Two common polymers used are poly (dimethylsiloxane) and polyacrylate. Poly (dimethylsiloxane) behaves as a liquid,which results in rapid extraction compared to polyacrylate, which is a solid. The silica rods have a typical diameter of 100–200 micrometers and a film thickness ranging from
10–100 microns. When the coated fiber is placed into an aqueous matrix (Figure 1), the analyte is transferred from the matrix into the coating. The extraction is considered to be complete when the analyte has reached an equilibrium distribution between the matrix and fiber coating. The equilibrium condition can be described as: n = Kfs Vf Vs Co
Kfs Vf Vs
when n is the amount extracted by the coating Kfs is the distribution coefficient between the fiber coating and the sample matrix, Vf is the volume of the fiber coating, Vs is the volume of the sample, and C0 is the initial concentration of analyte in the sample.
FIGURE 4.1: Microextraction with SPME.
SPME passively extracts organic compounds and concentrates them onto a thin, fused-silica ﬁber coated with a stationary-phase material. The component in sample was identified by comparing with the mass spectra library. The quality of a component must taken 90% and above. There are three different extraction modes for SPME:
I. Direct: Fiber is placed in the water or air sample and the analytes are adsorbed onto or absorbed into the ﬁber coating directly from the sample matrix.
II. Headspace: Sample of soil or water is placed in a vial. The SPME ﬁber is placed in the air directly above the water or soil, and analytes partition from the sample matrix through the air to the ﬁber coating. The air in the vial serves as a barrier between the
SPME ﬁber and the sample matrix to protect the SPME ﬁber and eliminate fouling by high molecular-weight compounds and other non-volatile interferences in the sample media.
III. Membrane: uses a membrane to protect the SPME ﬁber from heavily contaminated samples that may damage the ﬁber.
Figure 4.2: Schematic diagram of the headspace SPME apparatus.
Unleaded petrol, diesel, paint thinner, kerosene and
SPME holder with 100 μm polydimethylsiloxane (PDMS)
Gas chromatograph (Agilent Technologies 5890 Series II)
Equipped with HP 5971A mass selective detector and a 30m x 0.25μ x 250 μm HP 5 – MS capillary column and glass vials with septum.
Instruments Set up
Injector temperature : 250 oC
Detector temperature : 300 oC
Carrier gas flow rate : 30 ml/s
Column temperature : 60 oC to 170oC at 10oC/min
1. The fiber (PDMS) was conditioned in the GC injection port at 250oC for at least 10 minutes to removed contaminations.
2. Approximately 5 mL of unleaded petrol was added in a glass vial and place the vial on a hot plate. The sample was heated to 50oC.
3. The SPME fiber was exposed to the headspace of the vial for 20 minutes and the temperature was constant at 50oC.
4. The fiber was withdraw into the needle and pulled out from the vial and immediately injected into GC-MS with desorption time 80 seconds.
5. Using the mass spectra library, the major component compound in each sample was identified using the mass spectra library.
6. Step 2 until 5 were repeated for other sample.
The solid phase microextraction process is shown in Figure 4.3. 1cm length of fused silica fiber, coated with a polymer, is bonded to a stainless steel plunger and installed in a holder that looks like a modified microliter syringe. The plunger moves the fused silica fiber into and out of a hollow needle. To use the unit, the analyst draws the fiber into the needle, passes the needle through the septum that seals the sample vial, and depresses the plunger, exposing the fiber to the sample or the headspace above the sample. Organic analytes adsorb to the coating on the fiber. After adsorption equilibrium is attained, usually in 2 to 30 minutes, the fiber is drawn into the needle, and the needle is withdrawn from the sample vial. Finally, the needle is introduced into the gas chromatograph injector, where the adsorbed analytes are thermally desorbed and delivered to the GC column, or into the SPME/HPLC interface. Results compare very favorably to results for other sample preparation.
Figure 4.3: Solid Phase Microextraction
In SPME, equilibria are established among the concentrations of an analyte in the sample, in the headspace above the sample, and in the polymer coating on the fused silica fiber. The amount of analyte adsorbed by the fiber depends on the thickness of the polymer coating and on the distribution constant for the analyte.
Extraction time is determined by the length of time required to obtain precise extractions for the analytes with the highest distribution constants. The distribution constant generally increases with increasing molecular weight and boiling point of the analyte. Selectivity can be altered by changing the type of polymer coating on the fiber, or the coating thickness, to match the characteristics of the analytes of interest. In general, volatile compounds require a thick coating, and a thin coating is most effective for adsorbing/desorbing semivolatile analytes. Desorption of an analyte from an SPME fiber depends on the boiling point of the analyte, the thickness of the coating on the fiber, and the temperature of the injection port. Nonpolar analytes are most effectively extracted with a nonpolar fiber coating and polar analytes are most effectively extracted with a polar coating, just as nonpolar or polar analytes are most effectively analyzed on a gas chromatography column of like polarity.
In SPME however, because only 1cm of fiber is exposed to the sample matrix, the fiber coating must be either nonpolar or strongly polar in nature. The small differences in stationary phase polarity that are useful in gas chromatography (a 5% diphenylsiloxane/95% dimethylsiloxane phases versus a 100% dimethylsiloxane phase, for example) will not produce appreciable selectivity differences in SPME. The polydimethylsiloxane (PDMS) is the non- polar stationary phase that has been used in this experiment. The –R groups are all –CH3, giving a liquid that is relatively nonpolar. In general, polar fibers are used for polar analytes and nonpolar fibres for non-polar analytes. Before proceed the analysis, the sample must be heated to make the sample attach to the fibre. The fibre was put into the vial that contains the sample. The vial was heated by using the water bath at 500C this was done because if the temperature was not strictly controlled, the components of interest from the sample that we wanted to collect will not be able to obtain.
We controlled the temperature by adding cold water inside the water bath system if the temperature goes high, reduced the amount of water, and also increased the hot plate’s temperature if the temperature goes low than 500C.Before placing the fibre into the sample vial through its septum, the fibre was first injected into the injection port of the GCMS at temperature of 2500C for 10 minutes so that any interferences and contaminants can be effectively removed. By doing this, we actually want to make sure that no impurities existed on the fibre. Thus, we can be sure that we are actually injecting pure vapor of sample’s components obtained from exposing the fibre to the headspace of the vial. After the extraction time was completed, we transferred the fibre immediately to injection port for injection purpose, as we did not want any of the components of sample that we collected to be disappeared into the surrounding.
After the extraction time was completed, we transferred the fibre immediately to injection port for injection purpose, as we did not want any of the components of sample that we collected to be disappeared into the surrounding. After 80 seconds of injection, the fibre
was pulled out from the injection port. The 80 seconds period is known as desorption time. The factors that effects on determining desorption times are carrier gas linear flow and temperature. These factors will influence the carryover experience on the fibre. There are several factors that effects on precision of the fibre such as condition of the fiber, GC injector (fibre positioning), desorption time, sample volume, agitation, extraction time and temperature. During the experiment, there were source of error occurred such as personal and methods error. For example, when heated, the temperature was not maintained at 50oC. Besides, the fibre was not quickly injected into the GC-MS which can cause the sample to vaporize and in the end the analytes disappear. To get the best result, ensuring the fibre and sample is heated longer and maintain the temperature at 50oC so that the compounds of sample can attached and coated to the fibre.
SPME has several important advantages compared to traditional sample preparation techniques. The advantages of SPME discuss. The SPME method for semivolatile analysis consists of inserting the fiber device into the aqueous sample matrix, pushing the plunger to expose the fiber, retracting the fiber into the needle when equilibrium has been reached, and finally introducing the fiber into the analytical instrument. During desorbtion of the analyte, the polymeric phase is cleaned and therefore ready for reuse. The absence of solvent in SPME is an important feature, as it is not only environmentally friendly but makes the separation faster, which increases throughput and allows for the use of simpler instruments. Another important feature of SPME is its small size, which is convenient for design. Another important feature of SPME is its small size, which is convenient for designing portable devices for field work.
Since the amount of extracting phase is small, the equilibrium of the system is not disturbed and can therefore be studied. Very small objects can be studied with miniature fibers, such as a single flower or even a single cell. The sensitivity and limit of determination is comparable to techniques that rely on liquid extraction. Although only a small portion of analyte is extracted from the matrix, all extracted analytes are transferred to the analytical instrument. This is in contrast to liquid extractions, where the majority of analyte is transferred from a given sample to the organic phase but only a small portion (1/100 or 1/1000) of the extracted analyte is introduced to the analytical instrument. The analyte if the sample non volatile the other extraction method can achieve by placing the SPME fiber directly into the aqueous sample.
In conclusion, since hydrocarbon is volatile compound, the extraction procedure by placing the SPME fiber in the headspace above the sample can extract the analyte and directly injected to the GCMS, the components in the sample can be identified using the GCMS where we can compare the obtained spectra with the instrument’s library. From the experiment we are successfully determine the compounds that present in the thinner, kerosene, diesel and petrol. The hydrocarbons that present in unknown are similar to those in kerosene because it is quite similar in chromatography diagram.Besides, the sample are not quality so that the result that we obtain is not accurate a bit because of oftenly used.
1. Washing the injector properly so that, no contaminate will occurred. References