1) In what pressure range are mass spectrometers normally operated?

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1 Exercises Ionization 1) In what pressure range are mass spectrometers normally operated? Mass spectrometers are usually operated in the high vacuum regime to ensure mean free paths significantly longer than the mass analyzer's dimensions. 2) What is a metastable ion? What is a base peak? and a molecular ion? Some fragment ions, undergo secondary fragmentations in the analyzer tube of the mass spectrometer; the resulting signals or peaks represent neither the m/e of the first ion nor that of the second ion; instead, metastable ion peaks are observed For a reaction F 1 + F 2 + (m/z) 1 (m/z) 2 a metastable ion peak, m*, is observed m* = m 2 2 /m 1 metastable ion peaks require a special type of spectrometer; they give valuable information about fragmentation patterns of molecular ions. 3) How are the ionizing electrons generated in electron ionization (EI)? The beam of ionizing electrons is produced by thermionic emission from a resistively heated metal wire or filament typically made of rhenium or tungsten. The filament reaches up to 2000 C during operation 4) How are samples usually introduced into an EI ion source? Sample introduction systems (also sample inlet systems or inlets) suitable for the combination with EI are: reservoir inlets, gas chromatographs, and direct insertion probes. Which of these inlet systems is to be preferred depends on the type of sample going to be analyzed. Whatever type the inlet system may be, it has to manage the same basic task, i.e., the transfer of the analyte from atmospheric conditions and to vaporize it into the high vacuum of the EI ion source (Chap. 5.2). 5) What ions are normally created from a neutral molecule M upon EI? Positive radical ions, M +.. 6) Do EI spectra in general exhibit a high or a low degree of ion fragmentation? Most of the molecular ions are in highly excited states and undergo rapid fragmentation within the ion source. Therefore, EI mass spectra exhibit a high degree of ion fragmentation.

2 7) Can you enumerate all techniques or sorts of inlet systems, if you like, for sample introduction into EI ion sources? The most common sample introduction techniques in EI are direct insertion probe and gas chromatograph. Occasionally, samples are introduced via a reservoir inlet. Liquid chromatographs can be coupled to EI but are nowadays used with APCI or ESI ion sources. Direct exposure probes aren't common any more (Chap. 5.2). 8) Suggest suitable sample introduction for a pure volatile liquid, e.g., xylene: reservoir inlet mixture of many volatile compounds, e.g., gasoline: GC moderately volatile solid sample, e.g., benzopyrene: DIP solution of several moderately volatile organics, e.g., extract of N- heterocycles from coffee beans: GC 9) What sort of analytes are promising candidates for EI? Analytes for electron ionization should be accessible to evaporation or sublimation without significant thermal decomposition. These requirements are usually met by saturated and unsaturated aliphatic and aromatic hydrocarbons and their derivatives such as halides, ethers, acids, esters, amines, amides etc. Heterocycles generally yield useful EI spectra, and flavones, steroids, terpenes and comparable compounds can successfully be analyzed by EI, too. Whereas low- and medium-polarity analytes are usually well suited for EI, highly polar or even ionic compounds should not be subjected to EI 10) What is the upper limit for the analyte's molecular weight to use EI? There is - as with any other ionization method - no strict upper limit for molecular mass, nevertheless a range of up to u is a realistic estimate. Exceptions up to 1300 u are observed, if the analyte is perfectly nonpolar. 11) Name four processes of positive ion generation in CI and write down the corresponding general equations. Chemical ionization may involve the transfer of an electron, proton, or other charged species between the reactants. There are four general pathways to form ions from a neutral analyte M in CI (Chap ): M + [BH]+ [M+H]+ + B proton transfer M + X+ [M+X]+ electrophilic addition M + X+ [M A]+ + AX anion abstraction M + X+ M+ + X charge exchange 12) What role does a reagent gas play in CI? In chemical ionization new ionized species are formed when gaseous molecules (the neutral analyte M) interact with ions from a reagent gas. The large excess of reagent gas also shields the analyte molecules effectively from ionizing primary electrons which is important to suppress competing direct EI of the analyte.

3 13) Which of the following attributes apply to CI? 1. Soft ionization technique: YES 2. Old method: YES 3. Very difficult to apply: NO 4. Still highly relevant method: YES 14) What kind of particles are impacting onto the surface in case of FAB and what kind in LSIMS? In FAB, the surface is bombarded chiefly by neutral atoms of argon or preferably xenon; LSIMS uses Cs + ions instead 15)Describe some uses of ambient MS techniques (DESI and DART) Ambient MS enables the detection of surface materials like waxes, alkaloids, flavors, or pesticides from plants as well as explosives, pharmaceuticals, or drugs of abuse from luggage or banknotes. These and many more analytical applications are readily accessible by exposing the corresponding items to the ionization region of the interface. 16) DART uses a fluid medium to achieve ionization of samples. What medium is used? Typically, helium is employed. Nitrogen can also be used. 17) Which mechanism initiates ionization in DART? A Penning ionization process is initiated by a corona discharge in helium. Metastable He atoms are then emerging from the source and effect ionization of atmopheric nitrogen and water. Ion-molecule reactions in the ambient then lead to the ionization of analyte molecules 18) Think about operator safety when measuring DART spectra. Is there high voltage applied in proximity to the sample stage, are there other potential hazards? In DART, there is no high voltage at the sample stage, which makes it even safer than DESI. Nonetheless, a regular daily exposure of skin to the ionizing medium should probably be avoided. The use of tweezers and gloves seems appropriate when exposing samples to a DART source. 19) What is the acronym MALDI? What type of molecules is used for? Matrix-assisted laser desorption/ionization Allowing the analysis of biomolecules and biopolimers( DNA, RNA, proteins ) and large organic molecules like macromolecules, polymers.

4 20) Right or wrong? 1. DART yields only positive ions. NO 2. DART always produces radical ions. NO 3. DART is suitable for samples on surfaces. YES 4. DART can be used to analyze samples in the gas phase. YES 5. The acronym DART reflects the mechanism of ionization. NO 21) Briefly describe the following ionization methods: 1. atmospheric pressure chemical ionization 2. electrospray ionization. 3. Atmopheric pressure photoionization In atmospheric pressure chemical ionization (APCI) ion-molecule reactions at atmospheric pressure are employed to generate analyte ions in the gas phase by a corona descharge. The gas phase is obtained by evaporating the solvent in a stream of hot nitrogen. electrospray ionization (ESI) also uses high voltage to spray an electrolytic solution that already contains preformed analyte ions. Ionization take place by ion transfer. Atmospheric pressure photoionization ( APPI) use photons to ionize gas-phase molecules.vaporization is taking place by a heater nebulizer similar to the one use in APCI. 22) What polarity and what kind of ions do you expect to predominate with the following analytes in ESI? Adjust the second column correctly. Analyte and Solvent polystyrene in toluene protein in water/acetonitrile 1: % TFA oligosaccharide in water/methanol 5:1 anionic transition metal complex in chloroform polycyclic aromatic hydrocarbons in hexane Predominating Ions does not spray [M+H] +, [M+alkali] + plus [M+alkali] + [A] - (plus cluster ions, e.g., [2A+C] - ) does not spray oligonucleotide in [M-H] -, [M-2H+alkali] - plus water/isopropanol + amines 50 mm

5 peptides in water/acetonitrile 1: % acetic acid [M+H] +, [M+alkali] + plus