Mass spectrometry detection/detectors

Test methods that analyze individual compounds (e.g., benzene-toluene-ethylbenzene-xylene mixtures and PAHs) are generally applied to detect the presence of an additive or to provide concentration data needed to estimate environmental and health risks that are associated with individual compounds. Common constituent measurement techniques include gas chromatography with second-column confirmation, gas chromatography with multiple selective detectors, and gas chromatography with mass spectrometry detection (GC/MS) (EPA 8240). 

Peak purity tests are used to demonstrate that an observed chromatographic peak is attributable to a single component. Mass spectrometry is the most sensitive and accurate technique to use for peak purity evaluation because of the specific information derived from the analysis. However, a good number of HPLC methods use mobile phase conditions that are incompatible with mass spectrometry detection. In this case, PDA spectrophotometers using peak purity algorithms may be used to support the specificity of the method. Almost all commercially available diode array detectors are equipped with proprietary software that will perform these calculations. Although this technique is more universal in application to HPLC methods, the data provided is neither particularly... 

All these methods require the use of the refractive index of mass spectrometry (MS) detectors, since the detection of the isotopes is not possible with conventional ultraviolet (UV) detectors. 

In a mass spectrometer, instead of an optical spectrum in the spectrometer, we have a mass spectrum of matter radiation. The necessary condition for using a mass spectrometry detection is ionization of components of the analyzed sample. Thus, we can say that mass spectrometry is a destructive method of analysis. There are many techniques and systems for mass spectrometry, but they all have the same three elements source of ions, ion analyzer, and ion detector (Fig. 1). 

Other detectors include electrochemical, either conductometric or ampero-metric. Also, mass spectrometry detection has become quite popular, using an electrospray-type interface for introduction into a quadrupole mass spectrometer, similar to that used for HPLC. The strong electric field at the end of the capillary creates an aerosol of charged microdroplets, and the solvent evaporates to give gaseous ions. 

After the mass analyser has dispersed the ions in space or in time according to their various m/z values, they may be collected by a detector. In modern mass spectrometry, a detector consists of a planar assembly of small electron multipliers, called an array in one case (spatial separation) and a microchaimel plate in the other (temporal separation). These collectors can either detect the arrival of all ions sequentially at a point (a point ion collector) or detect the arrival of all ions simultaneously (an array or multipoint collector). 

Advanced Ion-trap mass spectrometry Bioaerosol Detector System based on Aerogel UV Fluoresence Detection of BW Agents on Surfaces Parallel Micro Separations-based Detection (PMSD) Taqman PCR-based BW assays Automated Nucleic Acid Extractor Deployable diagnostic kit for biowarfare agents... 

UV detection is used in most chiral analysis by HPLC and other liquid chromatographic modalities. However, some other detectors, such as conductivity, fluorescent and refractive index types, are also used. The choice of detector depends on the properties of the racemic compound to be resolved [41, 144]. Chiroptical detectors, which are based on the principle of polarimetry [145] or circular dichroism [146, 147], are also available. The enantiomer (+)- or (—)-notation is determined by these detectors. Some organochlorine pesticides are not UV-sensitive, and hence they are difficult to detect in liquid chromatography. The detection of these types of pollutant can be achieved by using a mass spectrometry (MS) detector, and therefore LC-MS instruments are now being put on the market for routine use [148, 149]. 

Improvements in the instrumentation, ionization sources, high-resolution mass analyzers, and detectors [67-69], in recent years have taken mass spectrometry to a different level of HPLC-MS for natural product analysis. Mass spectrometry detection offers excellent sensitivity and selectivity, combined with the ability to elucidate or confirm chemical structures of flavonoids [70-72]. Both atmospheric pressure chemical ionization (APCI) and electrospray ionization (ESI) are most commonly used as ionization sources for flavonoid detection [73-76]. Both negative and positive ionization sources are applied. These sources do not produce many fragments, and the subsequent collision-induced dissociation energy can be applied to detect more fragments. Tandem mass spectrometry (MS , n> 2) provides information about the relationship of parent and daughter ions, which enables the confirmation of proposed reaction pathways for firagment ions and is key to identify types of flavonoids (e.g., flavones, flavonols, flavanones, or chalcones) [77-80]. 

Online viscometers (single, dual, or four capillary type viscometers with a symmetrical or asymmetrical bridge) These measure the pressure difference between a sample path and a reference path filled with pure solvent. Viscometers are used to measure specific and intrinsic viscosity, molar masses based on Benoit s universal calibration approach [6] and Mark-Houwink coefficients. Mass spectrometry (MS) detectors (see Chapter 10) Different MS methods have been used in macromolec-ular analysis. They are used to determine absolute molar masses for homo- and copolymers and to detect polymer structures. Matrix-assisted laser desorption ionizationtime of flight (MALDI-ToF) and electrospray ionization (ESI) are the most common instruments used in combination with SEC (Section 9.4.2.4). A recent application summary is available [12]. 

FIA and, in particular, computer-controlled derived techniques still have an important contribution to make to quality control aspects in food analysis. Future trends will accompany the changes observed in analytical laboratories, where mass spectrometry-based detectors are replacing molecular spectrophotometry (e.g., diode array detectors [DAD]). Hence, it is expected the hyphenation of flow injection techniques to mass spectrometry, particularly for sample treatment (extraction, sample matrix removal) using FIA. Other less exploited feature, designated as reversed FIA (Mansour and Danielson, 2012), may also have an important role in future years. In this case, sample is applied as carrier, which allows an enhancement of detection limits. The only constraints are possible sample scarcity or high cost and multiplication of artifacts due to interferences. The future application of FIA is left to the imagination and ingenuity of future food analysts. 

Detectors Most of the detectors used in HPLC also find use in capillary electrophoresis. Among the more common detectors are those based on the absorption of UV/Vis radiation, fluorescence, conductivity, amperometry, and mass spectrometry. Whenever possible, detection is done on-column before the solutes elute from the capillary tube and additional band broadening occurs. 

In modem mass spectrometry, ion collectors (detectors) are generally based on the electron multiplier and can be separated into two classes those that detect the arrival of all ions sequentially at a point (a single-point ion collector) and those that detect the arrival of all ions simultaneously (an array or multipoint collector). This chapter compares the uses of single- and multipoint ion collectors. For more detailed discussions of their construction and operation, see Chapter 28, Point Ion Collectors (Detectors), and Chapter 29, Array Collectors (Detectors). In some forms of mass spectrometry, other methods of ion detection can be used, as with ion cyclotron instmments, but these are not considered here. 

There is potential confusion in the use of the word array in mass spectrometry. Historically, array has been used to describe an assemblage of small single-point ion detectors (elements), each of which acts as a separate ion current generator. Thus, arrival of ions in one of the array elements generates an ion current specifically from that element. An ion of any given m/z value is collected by one of the elements of the array. An ion of different m/z value is collected by another element. Ions of different m/z value are dispersed in space over the face of the array, and the ions are detected by m/z value at different elements (Figure 30.4). 

The use of separation techniques, such as gel permeation and high pressure Hquid chromatography interfaced with sensitive, silicon-specific aas or ICP detectors, has been particularly advantageous for the analysis of siUcones in environmental extracts (469,483—486). Supercritical fluid chromatography coupled with various detection devices is effective for the separation of siUcone oligomers that have molecular weights less than 3000 Da. Time-of-flight secondary ion mass spectrometry (TOF-sims) is appHcable up to 10,000 Da (487). 

The recent development and comparative application of modern separation techniques with regard to determination of alkylphosphonic acids and lewisite derivatives have been demonstrated. This report highlights advantages and shortcomings of GC equipped with mass spectrometry detector and HPLC as well as CE with UV-Vis detector. The comparison was made from the sampling point of view and separation/detection ability. The derivatization procedure for GC of main degradation products of nerve agents to determine in water samples was applied. Direct determination of lewisite derivatives by HPLC-UV was shown. Also optimization of indirect determination of alkylphosphonic acids in CE-UV was developed. Finally, the new instrumental development and future trends will be discussed. 

Every mass spectrometer consists of four principal components (Fig 1) (1) the source, where a beam of gaseous ions are produced from the sample (2) the analyzer, where the ion beam is resolved into its characteristic mass species (3) the detector, where the ions are detected and their intensities measured (4) the sample introduction system to vaporize and admit the sample into the ion source. There is a wide variety in each of these components and only those types which are relevant to analytical and organic mass spectrometry will be emphasized in this survey. The instrumentation... 

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