Mass spectral detector

Environmental monitoring of chloroacetanilides requires methods that have the capability to distinguish between complex arrays of related residues. The two example methods detailed here for water monitoring meet this requirement, but the method for metabolites requires sophisticated mass spectral equipment for the detection of directly injected water samples. In the near term, some laboratories may need to modify this method by incorporation of an extraction/concentration step, such as SPE, that would allow for concentration of the sample, so that a less sensitive and, correspondingly, less expensive, mass spectral detector can be used. However, laboratories may want to consider purchasing a sensitive instrument rather than spending time on additional wet chemistry procedures. In the future, sensitive instrumentation may be less expensive and available to all laboratories. Work is under way to expand the existing multi-residue methods to include determination of additional chloroacetanilides and their metabolites in both water and soil samples. 

The quadrupole MS detector was the first, and is still the most common, detector used for LC/MS, but a number of other mass spectrometers have been adapted to this application. Both three-dimensional spherical (ITD) and linear (LIT) ion trap detectors offer tremendous potential for general, inexpensive LC/MS systems. They both offer the ability to be used as either a mass spectral detector or as a MS/MS detector. The 3D ITD (Fig. 15.5) allows ions to be trapped in the ion trap where they can be fragmented by heavy gas collision and the fragments released by scanning the dc/RF frequency of the trap. 

Figure 11.16 Highly schematic diagram of a mass spectral detector. The sample, perhaps entrained in the mobile phase of an HPLC system, enters an inlet region of the instrument where analyte molecules are ionized and mobile phase (in the case of HPLC) or carrier gas (in the case of GC) are removed. The ions enter the high vacuum region and are separated from one another on the basis of their mass-to-charge (m/ ratio by one or more mass analyzers. The ions are detected and the signal passed to a computer system for storage and analysis (the signal processor).

The Faraday cup described in Figure 7.1 was the earliest mass spectral detector in which ion detection was accomplished by direct charge (current) measurement The Faraday cup is a fixed detector in which mass spectrometers must be scanned to focus ions into the cup. Because mass spectrometric ion beams can be as low as a few fA (1 fA = 6,242 ions/s), 10 to 10 electronic amplification is required. The high input impedance with large feedback resistance required for Faraday cup amplification produces a slow, stable signal but with high electronic noise. Limited by noise and speed, Faraday cup detectors are relatively insensitive and too slow for application to scanning or time-dispersive mass spectrometry. 

Reconstructed total ion gas chromatogram of headspace vapors from Orange Life Savers. The mass spectral detector measures ions above 34 atomic mass units. CO2 and Ar are from air, and CHjCb is the solvent used to clean the syringe. [From R. A. Kjonaas, j. L. Soller, and L. A. McCoy, y. Chem. Ed. 1997, 74,1104.)... 

GC O systems are often used in addition to either a FID or a mass spectrometer. With regard to detectors, splitting column ow between the olfactory port and a mass spectral detector provides simultaneous identi cation of odor active compounds. Another variation is to use an in-line, nondestructive detector such as a TCD [64] or a photoionization detector [65]. Especially when working with GC-0 systems equipped with detectors that do not provide structural information, retention indexes are commonly associated to odor description supporting peak assignment. 

To appreciate the ways in which mass spectral data may be processed to utilize fully the selectivity and sensitivity of the mass spectrometer as a detector for HPLC. 

It should not be concluded that the above examples of the evaluation of qualitative and quantitative data comprise an exhaustive analysis of this particular set of LC-MS data. They have been included primarily for those not used to the analysis of mass spectral data, to show the principles involved, and to demonstrate how powerful the mass speedometer can be as a chromatographic detector. 

The data in Table I are also significant in terms of the type of analysis to determine the presence of NDMA. In all cases analysis was done using gas chromatography coupled with a Thermal Energy Analyzer, a sensitive, relatively specific nitrosamine detector (12). Further, in six of the studies, the presence of NDMA in several samples was confirmed by gas chromatography-mass spectrometry (GC-MS). The mass spectral data firmly established the presence of NDMA in the beer samples. 

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. 

MS detection does not necessarily require as highly resolved GC separations as in the case of selective detectors because the likelihood of an overlapping mass spectral peak among pesticides with the same retention time is less than the likelihood of an overlapping peak from the same element. Unfortunately, this advantage cannot always be optimized because SIM and current gas chromatography/tandem mass spectrometry (GC/MS/MS) methods, it is difficult to devise sequential SIM or MS/MS retention time windows to achieve fast GC separations for approximately > 50 analytes in a single method. 

Principles and Characteristics Mass-spectral analysis methods may be either indirect or direct. Indirect mass-spectral analysis usually requires some pretreatment (normally extraction and separation) of the material, to separate the organic additives from the polymers and inorganic fillers. The mass spectrometer is then used as a detector. Direct mass-spectrometric methods have to compete with separation techniques such as GC, LC and SFC that are more commonly used for quantitative analysis of polymer additives. The principal advantage of direct mass-spectrometric examination of compounded polymers (or their extracts) is speed of analysis. However, quite often more information can be... 

In chromatography-FTIR applications, in most instances, IR spectroscopy alone cannot provide unequivocal mixture-component identification. For this reason, chromatography-FTIR results are often combined with retention indices or mass-spectral analysis to improve structure assignments. In GC-FTIR instrumentation the capillary column terminates directly at the light-pipe entrance, and the flow is returned to the GC oven to allow in-line detection by FID or MS. Recently, a multihyphenated system consisting of a GC, combined with a cryostatic interfaced FT1R spectrometer and FID detector, and a mass spectrometer, has been described [197]. Obviously, GC-FTIR-MS is a versatile complex mixture analysis technique that can provide unequivocal and unambiguous compound identification [198,199]. Actually, on-line GC-IR, with... 

Mass spectrometry can be used to measure the molar mass distribution (MMD) of a polymer sample by simply measuring the intensity, Nt, of each mass spectral peak with mass m . This is due to the fact that mass spectrometers are equipped with a detector that gives the same response if an ion with mass 1 kDa or 100 Da (actually any mass) strikes against it. In other words, the detector measures the number fraction and this implies that Nt also represents the number of chains with mass m,. Thus, the number-average molar mass, Mn, is given by ... 

The most intense peak of a mass spectrum is called base peak. In most representations of mass spectral data the intensity of the base peak is normalized to 100 % relative intensity. This largely helps to make mass spectra more easily comparable. The normalization can be done because the relative intensities are independent from the absolute ion abundances registered by the detector. However, there is an upper limit for the number of ions and neutrals per volume inside the ion source where the appearance of spectra will significantly change due to autoprotonation (Chap. 7). In the older literature, spectra were sometimes normalized relative to the sum of all intensities measured, e.g., denoted as % Lions, or the intensities were reported normalized to the sum of all intensities above a certain m/z, e.g., above m/z 40 (% L 4o)-... 

---