Detection mass spectrometric

Mass spectrometric detectors for capillary electrophoresis are necessarily post-column detectors and must be interfaced to the cathodic end of the capillary. These detectors consist of four main components the interface, that joins the capillary to the ion source, the ion source, that generates ionic fragments from neutral analyte species, the mass analyzer, that distinguishes ions by their mass/charge (m/z) values, and the ion detector, that measures and amplifies the signal. The principles and instrumentation of bioanalytical MS are explained in Chapter 15. 

Soft ionization methods produce few fragments under relatively mild conditions. The ionization method that has received the most attention in terms of its applicability to protein and DNA analysis is the electrospray ionization (ES) technique. This is a soft method that is capable of generating molecular ions from biological macromolecules present in solution. Table 12.3 gives examples of the charge and m/z ranges that have been observed with some biopolymer species in electrospray ionization mass spectrometers. 

TABLE 12.3. Charges and m/z Values Obtained for Biological Macromolecules using ES-MSa 

100-cm long fused ion source RF Quadrupole Quadrupole silica capillary Nj, on, ens mass filter ° n 

Teflon holder Stainless steel (positioner) tubing 

There are numerous ionization methods that allow formation of ions to carry out mass spectrometry however, in this chapter we will only focus on those most common in LC-MS. The challenge in coupling HPLC to mass spectrometry is that the chromatography operates with liquids and under high pressure, while the detector operates under high vacuum. The device between the chromatograph and the mass spectrometer is called the interface. Here, ionization and transition from liquid to gas phase of the compounds occur. The development of the first commercial available interfaces started as early as in the 1970s. Since then numerous interfaces have been introduced. Table 3.6 shows a list of current interfaces and their acronyms. 

The mostly used interfaces in LC-MS are electrospray ionization, atmospheric pressure chemical ionization (APCI), atmospheric pressure photoionization (APPI), and inductively coupled plasma ionization (ICP). While ICP is mostly used in the determination of metals (for elemental analysis) and has its specific applications, 

and APPI are more related to ionization techniques with a wide range of applications. Choosing the suitable interface depends on the nature of the analytes. 

The complexity of chlorinated and brominated POP analyses requires a comprehensive approach for their quantitative determination in environmental samples. Moreover, concentrations of some of them, such as PCDDs and PCDFs in environmental samples are decreasing [72,73] reflecting a general decline in dioxin inputs to the enviromnent owing to tighter controls. The safe values established in different directives and recommendations are more and more restrictive, thus analytical methodologies must be able to achieve the low detection limits now required. 

The PCDD, PCDF and PCB analyses involve detection of multiple congeners at the ppt or ppq level for which isotope dilution techniques using GC-HRMS are currently recommended methods (EN Method 1948, US EPA Method 1613, US EPA Method 8290, US EPA Method 1668) [74-76]. HRMS was used operating in the electron ioiuzation (El) mode at a resolving power of 10000. Under these conditions, different ions (isotopic labelled included) 

GC-NCI-MS and GC-EI-MS are the approaches more frequently used for PBDE analyses. Mass spectra strongly depend on the type of ionization used. NCI mass spectra of all PBDEs were dominated by the bromine ion [Br] and did not show any molecular ion. In contrast, El provided better structural information, giving the molecular ions and the sequential losses of bromine atoms. For NCI-MS experiments, the two ions corresponding to ra/z = 79 and 81 ([Br] ) were monitored, whereas for El-MS experiments, the two most-abundant isotope peaks for each level of bromination, corresponding to the molecular cluster for mono- to tri-BDEs and [M-Br2] for tetra- to hepta-BDEs, were selected. 

NCI-MS eUminated chlorinated interferences but presented different brominated interferences well resolved with the EI-MS approach. When PBBs and PBDEs were simultaneously determined, some important coeluting peaks appear. One critical chromatographic pair is BDE-I54 and PBB-I53 which coelute in many cases [54]. PBB-I53 and TBBPA can also coelute with BDE- 

Since the availabihty of labelled standards for other chlorinated and brominated POPs, such as PCNs and PBDEs, similar isotope dilution techniques using GC-HRMS were developed for an accurate determination of these contaminants [82,83]. 

---

The detection technique can also have an effect upon the angle- and velocity-dependent intensities. Cross sections refer to fluxes of molecules into a given range of velocities and angles. The connnonly employed teclmique of mass spectrometric detection provides a measure of the density in the ionization region. Since density and flux are related by the velocity, we must include a factor of 1/v hr making the transfonnation indicated in equation (B2.3.10) from the CM cross sections to tire measured laboratory intensities. 

It is well known that the electron-impact ionization mass spectrum contains both the parent and fragment ions. The observed fragmentation pattern can be usefiil in identifying the parent molecule. This ion fragmentation also occurs with mass spectrometric detection of reaction products and can cause problems with identification of the products. This problem can be exacerbated in the mass spectrometric detection of reaction products because diese internally excited molecules can have very different fragmentation patterns than themial molecules. The parent molecules associated with the various fragment ions can usually be sorted out by comparison of the angular distributions of the detected ions [8]. 

Figure B2.3.7. Schematic apparatus of crossed molecular beam apparatus with synclirotron photoionization mass spectrometric detection of the products [12], To vary the scattering angle, the beam source assembly is rotated in the plane of the detector. (By pemrission from AIP.)...

Instead of shifting the detector position, as indicated in figure B2.5.1 one often varies the location of the reactant mixing region using moveable injectors. This allows complex, possibly slow, but powerfril, analytical teclmiques to be used for monitoring gas-phase reactions. In combination with mass-spectrometric detection. 

B) The multiphoton excitation of electronic levels of atoms and molecules with visible or UV radiation generally leads to ionization. The mechanism is generally a combination of direct, Goeppert-Mayer, and quasi-resonant stepwise processes. Since ionization often requires only two or tln-ee photons, this type of multiphoton excitation is used for spectroscopic purposes in combination with mass-spectrometric detection of ions. 

Mass Spectrometer. The mass spectrometer is the principal analytical tool of direct process control for the estimation of tritium. Gas samples are taken from several process points and analy2ed rapidly and continually to ensure proper operation of the system. Mass spectrometry is particularly useful in the detection of diatomic hydrogen species such as HD, HT, and DT. Mass spectrometric detection of helium-3 formed by radioactive decay of tritium is still another way to detect low levels of tritium (65). Accelerator mass spectroscopy (ams) has also been used for the detection of tritium and carbon-14 at extremely low levels. The principal appHcation of ams as of this writing has been in archeology and the geosciences, but this technique is expected to faciUtate the use of tritium in biomedical research, various clinical appHcations, and in environmental investigations (66). 

J. A. Lippeit, B. Xin, N. Wu and M. L. Lee, East ultraliigh-pressure liquid chromatography on-column UV and time-of-flight mass spectrometric detection , 7. Microcolumn. Sep. 11 631 (1999). 

E. Pocurull, C. Aguilar, E. Borrull and R. M. Marce, On-line coupling of solid-phase extraction to gas cliromatography with mass spectrometric detection to determine pesticides in water , 7. Chromatogr. 818 85-93 (1998). 

G. S. Frysinger and R. B. Gaines, Comprehensive two-dimensional gas chromatogr aphy with mass spectrometric detection, (GC X GC/MS) applied to the analysis of petr oleum , /. High Resolut. Chromatogr. 22 251 -255 (1999). 

For off-bead analysis, coupling between chromatographic separation and mass spectrometric detection has proven especially powerful. The combination between high performance liquid chromatography (HPLC) and electrospray ionisation mass spectrometry has the advantage that purity of product mixtures can be coupled on-line with the product identification. 

The ion spray liquid chromatography/mass spectrometry (LC-MS) interface coupled via a postsuppressor split with an ion chromatography (IC) has been used in the analysis of alcohol sulfates. The IC-MS readily produces the molecular weight while the tandem mass spectrometric detection IC-MS-MS provides structural information [305]. 

Amirav A, Jing H. 1998. Simultaneous pulsed flame photometric and mass spectrometric detection for enhanced pesticide analysis capabilities. J Chromatogr 814 133-150. 

Revilla, 1. et al.. Identification of anthocyanin derivatives in grape skin extracts and red wines by liquid chromatography with diode array and mass spectrometric detection, J. Chromatogr. A, 847, 83, 1999. 

Kugler, F. et ah. Determination of free amino compounds in betalainic fruits and vegetables by gas chromatography with flame ionization and mass spectrometric detection, J. Agric. Food Chem., 54, 4311, 2006. 

Laser-Induced Thermal Desorption with Fourier Transform Mass Spectrometric Detection... 

Bertani, R., Michelin, R.A., Mozzon, M., Traldi, P., Seraglia, R., Busetto, L., Cassani, M.C., Tagliatesta, P. and D Arcangelo, G. (1997) Mass Spectrometric Detection of Reactive Intermediates. Reaction Mechanism of Diazoalkanes with Platinum(O) and Gold (I) Complexes. Organometallics, 16(14), 3229-3233. 

Gas chromatography coupled with mass spectrometric detection has also been applied to the detection of... 

Selley et al. (1992) have recently employed gas chromatography combined with mass spectrometric detection to determine levels of the cytotoxic monounsaturated aldehyde 4-hydroxy-/7 t-2-nonenal in the blood plasma of healthy human subjects, and patients with rheumatoid and osteoarthritis. Intriguingly, this lipid peroxidation end-product is present at a concentration ofc. lx 10 mol/dm in healthy and osteoarthritic human plasma samples (but significantly elevated in those collected from rheumatoid arthritis patients). Although at least some of this could originate from the oxidative degradation of PUFAs invm, there may be a relationship existing between these levels and the frequency of thermally/... 

Creed JT, and Brockhoff CA (1999) Isotope dilution analysis of bromate in drinking water matrixes by ion chromatography with inductively coupled plasma mass spectrometric detection. Anal Chem 71 722-726. 

Confirmation of suspected residue findings relies on the various chromatographic principles of cleanup and determination (GPC, NP-LC, GC), and is further supported by re-analysis of the final extract(s) on a GC stationary phase of different polarity, providing modified selectivity, or by the use of GC with specific mass spectrometric detection [GC/MS or gas chromatography/tandem mass spectrometry (GC/MS/MS)]. 

Zheng, J., Kosmus, W., Pichler-Semmelrock, F., and Kock, M., Arsenic speci-ation in human urine reference materials using high-performance liquid chromatography with inductively coupled plasma mass spectrometric detection, /. Trace Elements Med. Biol., 13, 150, 1999. 

Simonsick, Jr., W. J. and Prokai, L., Size-exclusion chromatography with electrospray mass spectrometric detection, in Chromatographic Characterization of Polymers, Hyphenated and Multidimensional Techniques, Provder, T., Barth, H. G. and Urban, M. W., Eds., American Chemical Society, Washington, D.C., 1995, chap. 4. 

Ludlow, M., Louden, D., Handley, A., Taylor, S., Wright, B., and Wilson, I.D., Size-exclusion chromatography with on-line ultraviolet, proton nuclear magnetic resonance, and mass spectrometric detection and on-line collection for off-line Fourier transform infrared spectroscopy, /. Chromatogr. A, 857,89,1999. 

Taylor, M.R. and Teale, P., Gradient capillary electrochromatography of drug mixtures with UV and electrospray ionisation mass spectrometric detection, /. Chromatogr. A, 768, 89, 1997. 

Alternative approaches consist in heat extraction by means of thermal analysis, thermal volatilisation and (laser) desorption techniques, or pyrolysis. In most cases mass spectrometric detection modes are used. Early MS work has focused on thermal desorption of the additives from the bulk polymer, followed by electron impact ionisation (El) [98,100], Cl [100,107] and field ionisation (FI) [100]. These methods are limited in that the polymer additives must be both stable and volatile at the higher temperatures, which is not always the case since many additives are thermally labile. More recently, soft ionisation methods have been applied to the analysis of additives from bulk polymeric material. These ionisation methods include FAB [100] and LD [97,108], which may provide qualitative information with minimal sample pretreatment. A comparison with FAB [97] has shown that LD Fourier transform ion cyclotron resonance (LD-FTTCR) is superior for polymer additive identification by giving less molecular ion fragmentation. While PyGC-MS is a much-used tool for the analysis of rubber compounds (both for the characterisation of the polymer and additives), as shown in Section 2.2, its usefulness for the in situ in-polymer additive analysis is equally acknowledged. 

Table 6.2 Main characteristics of mass-spectrometric detection...

Principles and Characteristics Ionisation processes are the basis for mass-spectrometric detection. Each of the ionisation techniques occupies its own position in mass spectrometry. The optimum performance of any ionisation method (and therefore the result) will depend critically on the characteristics and reliability of the mass spectrometer. Ionisation may occur in the gas, liquid or condensed phase, and may be either hard or soft , i.e. with or without extensive... 

Undoubtedly, the technique most suited to tackle polyatomic multichannel reactions is the crossed molecular beam (CMB) scattering technique with mass spectrometric detection and time-of-flight (TOF) analysis. This technique, based on universal electron-impact (El) ionization coupled with a quadrupole mass filter for mass selection, has been central in the investigation of the dynamics of bimolecular reactions during the past 35 years.1,9-11 El ionization affords, in principle, a universal detection method for all possible reaction products of even a complex reaction exhibiting multiple reaction pathways. Although the technique is not usually able to provide state-resolved information, especially on a polyatomic... 

---