Using selective detectors

Variations in the composition of a copolymer can cause substantial differences in the properties of the copolymer. Compositional information about copolymers may be acquired using selective detectors. Figure 3.9 shows the separation of an ethylene-vinyl acetate (EVA) copolymer by FfPSEC using IR detectors. One IR detector monitors the vinyl acetate carbonyl at 5.75 /u,m, and the other IR detector monitors the total alkyl absorbance at 3.4 /cm. 

Also, it is possible to use selective detectors e.g., a polarimetric detector or a mass spectrometer) to acquire separately the elution profiles of the two compounds. In the case of mixtures of different compounds with different response factors, it is possible to collect fractions and analyze them separately. This procedure eliminates the calibration problem but may cause other problems by introducing errors on the time of the various data points and by causing some amount of back-mixing. Proper design of the fraction collection experiments allows minimization of this error [118]. 

Sensitivity With a flame ionisation detector, the measurement of parts-per-million (ppm) level of almost all volatile organic compounds can be easily achieved. When using selective detectors such as electron-capture and nitrogen-phosphorus detectors, levels as low as parts-per-bilhon (ppb) have been routinely measmed. 

This technique allows to use selective detectors even in the case of zones which can only poorly be distinguished by these detectors It permits a better orienta-... 

OP), and carbamate pesticides are available as standard methods by the AO AC covering both nonfatty and fatty foods. These methods are based on solvent extraction and a variety of column chromatographic cleanup procedures with determination by GC using selective detectors. The International Dairy Federation has approved similar methods for OC and OP pesticide residues in milk and milk products. The AOAC have standard methods for specific pesticides or groups of pesticides that are approved for certain foodstuffs these methods involve colorimetric, spectrophotometric, or gas chromatographic determination. In the case of me-thylcarbamate pesticides, a liquid chromatographic method is approved. 

Of the many available detectors, the most common (Table 3) are thermal conductivity detector (TCD), flame ionization detector (FID), electron-capture detector (ECD), alkali-flame ionization detector (AFID or NPD), flame photometric detector (FPD), and mass selective detector. The TCD and FID are usually considered universal detectors as they respond to most analytes whereas the ECD, AFID, and FPD are the most useful selective detectors and give differential responses to analytes containing different functional groups. Note that this does not imply that the magnitude of the response of the universal detectors is constant to all analytes. The mass selective detector has the advantage of operation in either universal or selective detection mode whilst an infrared detector is a powerful tool for distinguishing isomers. 

When using selective detectors, one must consider the possible change in the response factor for a particular component due to the presence of the background matrix. For example, a selective detector for sulfur and phosphorus-containing compounds is the flame photometric detector, but it shows a diminished response... 

Gas chromatography was applied, using selective detectors electron-capture detector for organochlorine compounds, and alkali flame-ionisation detector for organophosphorus compounds. We determined ... 

Selectivity Because it combines separation with analysis, gas chromatography provides excellent selectivity. By adjusting conditions it is usually possible to design a separation such that the analytes elute by themselves. Additional selectivity can be provided by using a detector, such as the electron capture detector, that does not respond to all compounds. 

Fast concentration and sample injection are considered with the use of a theory of vibrational relaxation. A possibility to reduce a detection limit for trinitrotoluene to 10 g/cnf in less than 1 min is shown. Such a detection limit can by obtained using selective ionization combined with ion drift spectrometry. The time of detection in this case is 1- 3 s. A detection technique based on fluorescent reinforcing polymers, when the target molecules strongly quench fluorescence, holds much promise for developing fast detectors. 

Identification of stmctures of toxic chemicals in environmental samples requires to use modern analytical methods, such as gas chromatography (GC) with element selective detectors (NPD, FPD, AED), capillary electrophoresis (CE) for screening purposes, gas chromatography/mass-spectrometry (GC/MS), gas chromatography / Fourier transform infra red spectrometry (GC/FTIR), nucleai magnetic resonance (NMR), etc. 

Select the detector. To acquire molecular weight distribution data, use a general detector such as a refractive index detector. To acquire structural or compositional information, employ a more selective detector such as an ultraviolet (UV) or infrared (IR) detector. Viscometric and light-scattering detectors facilitate more accurate molecular weight measurement when appropriate standards are not available. 

FIGURE 3.9 Selective detectors can be used to provide structural information about copolymers. 

Recently, multidimensional GC has been employed in enantioselective analysis by placing a chiral stationary phase such as a cyclodextrin in the second column. Typically, switching valves are used to heart-cut the appropriate portion of the separation from a non-chiral column into a chiral column. Heil et al. used a dual column system consisting of a non-chiral pre-column (30 m X 0.25 mm X 0.38 p.m, PS-268) and a chiral (30 m X 0.32 mm X 0.64 p.m, heptakis(2,3-di-(9-methyl-6-(9-tert-butyldimethylsilyl)-(3-cyclodextrin) (TBDM-CD) analytical column to separate derivatized urinary organic acids that are indicative of metabolic diseases such as short bowel syndrome, phenylketonuria, tyrosinaemia, and others. They used a FID following the pre-column and an ion trap mass-selective detector following the... 

The most common selective detectors in use generally respond to the presence of a characteristic element or group in the eluted compound. This is well illustrated by the thermionic ionisation detector (TID) which is essentially a flame ionisation detector giving a selective response to phosphorus- and/or nitrogen-containing compounds. Typically the TID contains an electrically heated rubidium silicate bead situated a few millimetres above the detector jet tip and below the collector electrode. The temperature of the bead is maintained... 

One great advantage of GC is the variety of detectors that are available. These include universal detectors, such as flame ionization detectors and selective detectors, such as flame photometric and thermionic detectors. The most generally useful detectors, excluding the mass spectrometer are described in the following sections. 

The application areas for LC-MS, as will be illustrated later, are diverse, encompassing both qualitative and quantitative determinations of both high-and low-molecular-weight materials, including synthetic polymers, biopolymers, environmental pollutants, pharmaceutical compounds (drugs and their metabolites) and natural products. In essence, it is used for any compounds which are found in complex matrices for which HPLC is the separation method of choice and where the mass spectrometer provides the necessary selectivity and sensitivity to provide quantitative information and/or it provides structural information that cannot be obtained by using other detectors. 

Unreacted monomers In AN/M latex solutions were measured similarly. Methylacrylate Is a relatively strong UV absorber In the 225-250 nm range while acrylonitrile does not significantly absorb UV radiation at wavelengths above about 220 nm. At 230 nm, the wavelength used, the UV detector responded only to methylacrylate. And the refractive Indices of the two monomers are such that. In the 75/25 AN/MA proportions used, the refrac-tometer was Insensitive to methylacrylate. The reiErac tome ter therefore functioned as a selective detector for acrylonitrile In the presence of methylacrylate. 

Conventional ion-selective electrodes have been used as detectors for immunoassays. Antibody binding measurements can be made with hapten-selective electrodes such as the trimethylphenylammonium ion electrode Enzyme immunoassays in which the enzyme label catalyzes the production of a product that is detected by an ion-selective or gas-sensing electrode take advantage of the amplification effect of enzyme catalysis in order to reach lower detection limits. Systems for hepatitis B surface antigen and estradiol use horseradish peroxidase as the enzyme label and... 

Chromatographic systems were finally coupled with relatively inexpensive, yet powerful, detection systems with the advent of the quadrupole mass selective detector (MSD). The operational complexity of this type of instrumentation has significantly declined over the last 15 years, thus allowing routine laboratory use. These instruments... 

Universal and selective detectors, linked to GC or LC systems, have remained the predominant choice of analysts for the past two decades for the determination of pesticide residues in food. Although the introduction of bench-top mass spectrometers has enabled analysts to produce more unequivocal residue data for most pesticides, in many laboratories the use of selective detection methods, such as flame photometric detection (FPD), electron capture detection (BCD) and alkali flame ionization detection (AFID) or nitrogen-phosphorus detection (NPD), continues. Many of the new technologies associated with the on-going development of instrumental methods are discussed. However, the main objective of this section is to describe modern techniques that have been demonstrated to be of use to the pesticide residue analyst. 

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