Sarafraz-Yazdi A. Liquid-phase microextraction. Liquid phase microextraction techniques as a sample preparation step for analysis of pesticide residues in food. Sep Purif Rev. Liquid-phase microextraction combined with graphite furnace atomic absorption spectrometry: A review.
Anal Chim Acta. Rasmussen K.
Developments in hollow fibre-based, liquid-phase microextraction. Pedersen-Bjergaard S. Lee J.
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Environmental and bioanalytical applications of hollow fiber membrane liquid-phase microextraction: a review. Anal Lett 45 8 , Alsharif A. J Chromatogr Sci. Application of magnetic solvent bar liquid-phase microextraction for determination of organophosphorus pesticides in fruit juice samples by gas chromatography mass spectrometry. Food Chem. Lambropoulou D. Application of hollow fiber liquid phase microextraction for the determination of insecticides in water.
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Zhao L. Anal Chem. Determination of widely used non-steroidal anti-inflammatory drugs in water samples by in situ derivatization, continuous hollow fiber liquid-phase microextraction and gas chromatography-flame ionization detector. Abulhassani J. Hollow fiber based-liquid phase microextraction using ionic liquid solvent for preconcentration of lead and nickel from environmental and biological samples prior to determination by electrothermal atomic absorption spectrometry.
Effect-Directed Analysis of Complex Environmental Contamination
J Hazard Mater. Farahani H. Green chemistry approach to analysis of formic acid and acetic acid in aquatic environment by headspace water-based liquid-phase microextraction and high-performance liquid chromatography. Toxicol Environ Chem. Zanjani M. A new liquid-phase microextraction method based on solidification of floating organic drop. Han D. Trends in liquid-phase microextraction, and its application to environmental and biological samples. 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.
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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.
Figure Arrangement of the poles in Quadrupole and Ion Trap Mass spectrometers. 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. 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.
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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.