Spectrometric detector

A number of analytical techniques such as FTIR spectroscopy,65-66 13C NMR,67,68 solid-state 13 C NMR,69 GPC or size exclusion chromatography (SEC),67-72 HPLC,73 mass spectrometric analysis,74 differential scanning calorimetry (DSC),67 75 76 and dynamic mechanical analysis (DMA)77 78 have been utilized to characterize resole syntheses and crosslinking reactions. Packed-column supercritical fluid chromatography with a negative-ion atmospheric pressure chemical ionization mass spectrometric detector has also been used to separate and characterize resoles resins.79 This section provides some examples of how these techniques are used in practical applications. 

GC nitrogen-phosphorus detector (NPD), flame photometric detector (FPD), electron capture detector (BCD), flame ionization detector (FID), mass-spectrometric detector (MS)... 

On the other hand, if only specific GC detectors, e.g. the electron capture, nitrogen-phosphorus or flame photometric detectors, are tested, the argument of lack of GC method sensitivity is not acceptable. In most cases mass spectrometric detectors provide the sensitivity and selectivity needed. Unfortunately, tandem mass spectrometry (MS/MS) or MS" detectors for GC are still not widely used in official laboratories, and therefore these techniques are not always accepted for enforcement methods. 

For multi-analyte and/or multi-matrix methods, it is not possible to validate a method for all combinations of analyte, concentration and type of sample matrix that may be encountered in subsequent use of the method. On the other hand, the standards EN1528 andEN 12393 consist of a range of old multi-residue methods. The working principles of these methods are accepted not only in Europe, but all over the world. Most often these methods are based on extractions with acetone, acetonitrile, ethyl acetate or n-hexane. Subsequent cleanup steps are based on solvent partition steps and size exclusion or adsorption chromatography on Florisil, silica gel or alumina. Each solvent and each cleanup step has been successfully applied to hundreds of pesticides and tested in countless method validation studies. The selectivity and sensitivity of GC combined with electron capture, nitrogen-phosphorus, flame photometric or mass spectrometric detectors for a large number of pesticides are acceptable. 

Bloxam et al. [482] used liquid chromatography with an inductively coupled plasma mass spectrometric detector in speciation studies on ppt levels of mercury in seawater. 

Fluorescence detection can be up to four orders of magnitude more sensitive than UV absorbance, especially where laser induced excitation is used, mass detection limits being as low as 10-20—10 21 mole. Pre- and post-column derivatization methods are being developed to extend the applicability of fluorescence detection to non-fluorescent substances. Several types of electrochemical and mass spectrometric detector have also been designed. Detector characteristics are summarized in Table 4.21. 

Even though detectors used for GC are generally more sensitive and provide unique selectivity for many types of samples, the available HPLC detectors offer unique advantages in a variety of applications. In short, it is a good idea to recognize the fact that HPLC detectors are favored for some samples, whereas GC detectors are better for others. It should be noted that mass spectrometric detectors have been used effectively for both GC and HPLC. 

Nuclear magnetic resonance and mass spectrometric detectors HPLC/NMR and HPLC/MS are popular techniques that combine the versatility of HPLC with the identification power of NMR or MS (see Chapters 11, 12, and 18). 

In a well-equipped laboratory it is mandatory that these three techniques be coupled with a mass spectrometric detector in order to achieve a combination of resolution of mixtures, positive identification of separated organics and the high sensitivity that is essential when dealing with environmental samples. The penetration of mass spectrometers in recent years is indicated by the fact that of the 50 types of organic compound that have been determined by gas chromatography in 21 cases mass spectrometric detection is discussed. This trend will, no doubt, continue. 

Karasek et al. [1] determined hydrocarbons in benzene water extracts (pH7) of soil and in incinerator or fly ash by a variety of techniques including gas chromatography with flame ionization, electron capture and mass spectrometric detectors. Benzene water extractants were adjusted to pH4, 7 and 10 before the extraction in order to selectively extract various types of acidic and basic organic compounds in addition to hydrocarbons. 

Lee [42] determined pentachlorophenol and 19 other chlorinated phenols in sediments. Acidified sediment samples were Soxhlet extracted (acetone-hexane), back extracted into potassium bicarbonate, acetylated with acetic anhydride and re-extracted into petroleum ether for gas chromatographic analysis using an electron capture or a mass spectrometric detector. Procedures were validated with spiked sediment samples at 100,10 and lng chlorophenols per g. Recoveries of monochlorophenols and polychlorophenols (including dichlorophenols) were 65-85% and 80-95%, respectively. However, chloromethyl phenols were less than 50% recovered and results for phenol itself were very variable. The estimated lower detection limit was about 0.2ng per g. 

Krock and Wilkins [4] have used multidimensional gas chromatography with infrared and mass spectrometric detection to determine organics in soil. Direct acetylation followed by gas chromatography with flame ionization, electron capture and mass spectrometric detectors has been used to determine phenolic residues in soil [5]. Llopart-Visoso et al. [6] have used direct acetylation followed by headspace gas chromatography to determine phenolic and cresolic components of soil. 

River sediment Alkyltin Reverse phase hplc with ICP mass spectrometric detector 3-I6 pg [83] ... 

River sediment Alklytin Microwave digestion, glc with ICP mass spectrometric detector 50pg [8I] ... 

Conversion to tetra-alkyl lead compounds using nBu MgCI then gas chromatography with atomic absorption spectrometric detector Petroleum ether extraction, glc... 

With regard to quantitative measurements of APG surfactants in, e.g. environmental samples, the authors stressed that it was of crucial importance to promote the formation of the desired molecular (or adduct) ion in order to obtain reproducible mass spectra. If tuning of the ESI interface parameters did not suffice to yield abundant ions of the selected species, acquisitions of the mass spectrometric detector after negative ionisation in conjunction with appropriate selection of the mobile phase composition were used as an alternative despite the lower sensitivity in this mode [1,2],... 

Several methods are available for analyzing disulfoton in biological media some of the commonly used methods are reported in Table 6-1. A variety of detectors may be used for the gas chromatographic analysis of disulfoton, but flame photometric detectors are superior because of low background interference and good reproducibility (Holstege et al. 1991). Mass spectrometric detectors show high specificity (Kawasaki et al. 1992) and may also be used to confirm detection by other methods. 

GC analyses of the pupal secretion of E. borealis have indicated the presence of vitamin E acetate and other tocopherol derivatives [49,50]. However, in tests with ants, these compounds proved to be essentially inactive, whereas the secretion itself was potently deterrent. To find and identify the active components in the pupal Epilachna borealis secretion, NMR spectroscopic studies on the fresh secretion were carried out. One and two-dimensional NMR experiments revealed that the tocopheryl acetates account for only a relatively small percentage of the beetles5 total secretion (20%), whereas the major components represented a group of previously undetected compounds. By analysis of the COSY, HSQC and HMBC spectra of the mixture, these components were shown to be esters and amides derived from three (co-l)-(2-hydroxyethylamino)alka-noic acids 44-46. HPLC analyses coupled to a mass spectrometric detector revealed that the secretion contain a highly diverse mixture of macrocyclic polyamines, the polyazamacrolides (PAMLs) 47-52 (Fig. 8). 

As a volatile material, bromomethane is readily determined by gas chromatographic analysis. The selectivity and sensitivity of detection are increased by the use of an electron capture detector or a halide-specific detector, both of which are very sensitive for organohalides such as bromomethane. Specificity in detection is achieved with mass spectrometric detectors. 

The mass spectrometric detector separates gas phase ions according to their m/z (mass to charge ratio) value. Introduction of the sample from an HPLC system to an MS detector is usually done under atmospheric conditions and requires special considerations with regard to flow rate, pH, and mobile phase constituents. This is discussed further in the chapter of this handbook by Sloley et al. 

If the separation mechanisms are independent, then the probability of component overlapping is quite small components, unresolved in first separation according to one parameter, will probably be separated during the second separation. In the case where the two chromatographic separations are preceded by a solvent extraction step, the latter should be considered as a part of the multidimensional system, yielding a 3D system [1], An additional dimension is also represented by a mass spectrometric detector and hence, even 4D systems are achievable. 

Several different types of chromatography have been coupled with atomic spectrometric detectors. Most applications involving chromatography coupled with atomic spectrometry yield speciation data, i.e. they separate different chemical forms of an analyte. 

Gas chromatograph equipped with a split/splitless injector, OV-1701 coated fused silica column (25 m, 0.25 mm i.d.) and chemical ionisation mass spectrometric detector. 

CE detection is similar to detectors in, and include absorbance, fluorescence, electrochemical, and mass spectrometric detectors. The capillary can also be filled with a gel, which eliminates the electroosmotic flow. Separation is accomplished as in conventional gel electrophoresis but the capillary allows higher resolution, greater sensitivity, and on-line detection. In CE, low picogram amounts of analytes can be detected using glass fiber optics. However, this does not mean low limits of detection since only a few nanoliters can be injected. 

Critical separations in chromatography should be investigated at an appropriate level. For critical separations, selectivity can be demonstrated by the resolution of the two components that elute closest to each other. Peak purity tests using diode array or mass spectrometric detectors may be useful to show that the analyte chromatographic peak is not attributable to more than one component. 

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