Mass spectrometry universal’ detector

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]. 

The most widely regarded approach to accomplish the determination of as many pesticides as possible in as few steps as possible is to use MS detection. MS is considered a universally selective detection method because MS detects all compounds independently of elemental composition and further separates the signal into mass spectral scans to provide a high degree of selectivity. Unlike GC with selective detectors, or even atomic emission detection (AED), GC/MS may provide acceptable confirmation of the identity of analytes without the need for further information. This reduces the need to re-inject a sample into a separate GC system (usually GC/MS) for pesticide confirmation. Through the use of selected ion monitoring (SIM), efficient ion-trap or quadrupole devices, and/or tandem mass spectrometry (MS/MS), modern GC/MS instruments provide LODs similar to or lower than those of selective detectors, depending on the analytes, methods, and detectors. 

Increasing reliance on mass spectrometry as the universal detector for GC has not solved all the problems of additive identification. Isomer identification is impossible (except for REMPI technology), but is hardly an issue in additive analysis. 

The concept of peak capacity is rather universal in instrumental analytical chemistry. For example, one can resolve components in time as in column chromatography or space, similar to the planar separation systems however, the concept transcends chromatography. Mass spectrometry, for example, a powerful detection method, which is often the detector of choice for complex samples after separation by chromatography, is a separation system itself. Mass spectrometry can separate samples in time when the mass filter is scanned, for example, when the mass-to-charge ratio is scanned in a quadrupole detector. The sample can also be separated in time with a time-of-flight (TOF) mass detector so that the arrival time is related to the mass-to-charge ratio. 

In order to perform qualitative and quantitative analysis of the column effluent, a detector is required. Since the column effluent is often very low mass (ng) and is moving at high velocity (50-100 cm/s for capillary columns), the detector must be highly sensitive and have a fast response time. In the development of GC, these requirements meant that detectors were custom-built they are not generally used in other analytical instruments, except for spectroscopic detectors such as mass and infrared spectrometry. The most common detectors are flame ionization, which is sensitive to carbon-containing compounds and thermal conductivity which is universal. Among spectroscopic detectors, mass spectrometry is by far the most common. 

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... 

Many nonvolatile and thermally labile allelochemicals can be well separated by liquid chromatography (LC). Identification of the separated components on-line by mass spectrometry (MS) is of great value. Fused-silica LC columns of 0.22 mm ID packed with small-particle material are used in the described LC/MS system. The shape of the column end allows direct connection to a electron impact ion source of a magnetic sector mass spectrometer. Separations by LC are reported and LC/MS mass spectra are shown for monoterpenes, diterpene acids, phenolic acids and cardiac glycosides. The LC/MS system provides identification capability and high-efficiency chromatography with a universal detector. 

A 9 mL aliquot from each TIMS sample solution was submitted to the University of Georgia, Laboratory for Environmental Analysis, for inductively coupled plasma-mass spectrometry analysis (ICP-MS). A Perkin-Elmer Elan 6000 ICP-MS with quadrapole chamber mass detector system was used to analyze the solution for Ag, As, Cu, Sb, Sn, Pb, and Zn. Insufficient sample remained for further analysis or replicate samples. However, all appropriate blanks, dilutions, and standards were run. 

LC/MS (Liquid Chromatography/Mass Spectrometry)—Chromatography system in which an HPLC is married to a mass spectrometric detector through an evaporated, ionizing interface. A variety of mass spectrometers are used to produce various LC/MS and LC/MS/MS configurations. MS detectors are universal, mass detectors that provide molecular weight information and can give a definitive identification of separated compounds. 

Mass spectrometry is the most universal of detectors because it detects most organic compounds and is highly selective when selective ionization techniques are employed. Off-line LC-MS where the analyte is collected, concentrated, and analyzed by mass spectrometry is a relatively common practice in the pharmaceutical industry. When the compound in the collected fraction is unstable, on-line LC-MS techniques are preferred. There are eight LC-MS interfaces that have been reviewed [128] and their performance characteristics tabulated (Table 5.2). 

The emergence of a sensitive universal detector for HPLC is yet forthcoming. Mass spectrometry is an obvious candidate for such and this research area is currently one of high activity. 

Current IPC detectors are on-stream monitors. HPLC detectors range from (1) non selective or universal (bulk property detectors such as the refractive index (RI) detector), characterized by limited sensitivity, (2) selective (discriminating solute property detectors such as UV-Vis detectors) to (3) specific (specific solute property detectors such as fluorescence detectors). Traditional detection techniques are based on analyte architecture that gives rise to high absorbance, fluorescence, or electrochemical activity. Mass spectrometry (MS) and evaporative light scattering detectors (ELSDs), can be considered universal types in their own right... 

UV and fluorescent spectroscopy can be employed down to 190 nm because there is no solvent interference. Mass spectrometry is easy because the water provides good ionization. Flame ionization detection (FID) is of particular interest because potentially it offers a sensitive and universal detector. A number of different interfaces have been used, including heated capillaries, which have been examined by Miller and Hawthorne [62], Ingelse et al. [63], and others [64, 65], who separated a range of analytes including alcohols, amino acids, and phenols. An alternative method employing a cold nebuliza-tion of the eluent has been introduced by Bone et al. [66]. They were able to detect both aliphatic and aromatic alcohols, polymers, carbohydrates, parabens, and steroids. 

One of the attractions of SFC is that it can use both GC- and LC-like detectors, including the almost universal flame ionization detector (FID) for nonvolatile and volatile analytes after separation on either capillary or packed columns. Selective responses could be also obtained from a number of detectors as NPD, ECD, FPD, ultraviolet, Fourier transform infrared, nuclear magnetic resonance, and mass spectrometry. 

HPLC with MS detection is commonly employed as an alternate detector to UV. Mass spectrometry is generally regarded as a universal detector, but the response per unit weight depends greatly on the ionization type (e.g., positive or negative electrospray, atmospheric pressure chemical ionization, etc.) and on the ionization efficiency of the analyte under the given conditions. 

The ideal detector is universal yet selective, sensitive and structurally informative. Mass spectrometry (MS) currently provides the closest approach to this ideal. The combination of multi-dimensional gas chromatography with high resolution MS or mass-selective detectors in the single ion monitoring (SIM)-mode is currently the most potent analytical tool in enantioselective analysis of chiral compounds in complex mixtures [29]. Nevertheless, it must be pointed out that the application of structure specific detection systems like MS [51] or Fourier transform infrared (FT-IR) [52] cannot save the fundamental challenges to optimum (chiral) resolutions and effective sample clean-up [53]. 

Mass spectrometry is, however, a destructive technique—it consumes sample, albeit very small amounts. It is not a universal detector. There are entire classes of compounds that respond differentially in the mass spectrometer under varied conditions, poorly in general, or not at all. The inference of structure from mass spectra is also highly empirical. Regardless of effort, the theoretical prediction of the mass spectrum from a given structure has not been... 

Besides the universal detector systems, for example electron capture, flame ionisation and thermal conductivity usually coupled with gas chromatographic columns, various other detectors are now being used to provide specific information. For example, the gas chromatograph/mass spectrometer couple has been used for structure elucidation of the separated fractions. The mechanics of this hybrid technique have been described by Message (1984). Other techniques used to detect the metal and/or metalloid constituents include inductively coupled plasma spectrometry and atomic absorption spectrometry. Ebdon et al. (1986) have reviewed this mode of application. The type and mode of combination of the detectors depend on the ingenuity of the investigator. Krull and Driscoll (1984) have reviewed the use of multiple detectors in gas chromatography. 

Unfortunately no truly universal detector which satisfies these criteria has as yet been developed for HPLC although mass spectrometry and electrochemical detection systems arguably approach this ideal. 

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