Mass spectrometry requires a sample to be ionised and then subsequent mass analy
ID: 1004732 • Letter: M
Question
Mass spectrometry requires a sample to be ionised and then subsequent mass analysis of the ions produced i) List two different methods of sample ionisation used in mass spectrometry. For each method: briefly describe the principle of the method, state what types of samples it may be used with, and discuss any general advantages and disadvantages of the method. ii) List two different methods of mass analysis of ions used in mass spectrometry. For each method: briefly describe the principle of the method, state what types of samples it may be used with, and discuss any general advantages and disadvantages of the method.Explanation / Answer
1)
I) Electron ionisation
Electron ionsation also referred to as electon ionization,this is the oldest and best charactarised of all the ionization methods .A beam of electrons passes through the gas phase sample.An electon that collides with a neutral analyte molecules can knok off another electron resulting in positive charged ion.The ionization process can either produce a molecular ion which will have the same molecular weight and elemental composition of the starting analyte or it can produce a fragment ion which corresponds to a smaller piece of the analyte molecule.the ionisation potential is the electron energy that will produce a molecular ion.the appearence potential for a given fragment ion is the electron energy that will produce that fragment ion.most mass specrtometers use elecrons with energy 70 eV for EI Decreasing the electron energy can reduce ,but it reduces the number ion formed
Sample introduction .
heated batch inlet . heated direct insertion probe . gas chromatograph . liquid chromatograph (particle-beam interface)
Benefits . well-understood . can be applied to virtually all volatile compounds . reproducible mass spectra . fragmentation provides structural information . libraries of mass spectra can be searched for EI mass spectral "fingerprint"
Limitations . sample must be thermally volatile and stable . the molecular ion may be weak or absent for many compounds. Mass range . Low Typically less than 1,000 Da
II) Chemical Ionization (CI)
Summary Chemical ionization uses ion-molecule reactions to produce ions from the analyte. The chemical ionization process begins when a reagent gas such as methane, isobutane, or ammonia is ionized by electron impact. A high reagent gas pressure (or long reaction time) results in ion-molecule reactions between the reagent gas ions and reagent gas neutrals. Some of the products of these ion-molecule reactions can react with the analyte molecules to produce analyte ions. Example (R = reagent, S = sample, e = electron, . = radical electron , H = hydrogen): R + e ---> R+. + 2e R+. + RH ---> RH+ + R. RH+ + S ---> SH+ + R (of course, other reactions can occur)
Sample introduction
. heated batch inlet . heated direct insertion probe . gas chromatograph . liquid chromatograph (particle-beam interface)
Benefits .
often gives molecular weight information through molecular-like ions such as [M+H]+, even when EI would not produce a molecular ion. . simple mass spectra, fragmentation reduced compared to EI
Limitations . sample must be thermally volatile and stable . less fragmentation than EI, fragment pattern not informative or reproducible enough for library search . results depend on reagent gas type, reagent gas pressure or reaction time, and nature of sample.
Mass range . Low Typically less than 1,000 Da
2)
Fast Atom Bombardment (FAB)
Summary The analyte is dissolved in a liquid matrix such as glycerol, thioglycerol, m-nitrobenzyl alcohol, or diethanolamine and a small amount (about 1 microliter) is placed on a target. The target is bombarded with a fast atom beam (for example, 6 keV xenon atoms) that desorb molecular-like ions and fragments from the analyte. Cluster ions from the liquid matrix are also desorbed and produce a chemical background that varies with the matrix used.
Sample introduction . direct insertion probe . LC/MS (frit FAB or continuous-flow FAB).
Benefits . rapid . simple . relatively tolerant of variations in sampling . good for a large variety of compounds . strong ion currents -- good for high-resolution measurements
Limitations . high chemical background defines detection limits . may be difficult to distinguish low-molecular-weight compounds from chemical background . analyte must be soluble in the liquid matrix . no good for multiply charged compounds with more than 2 charges
Mass range . Moderate Typically ~300 Da to about 6000 Da. Secondary Ion Mass Spectrometry (SIMS
Electrospray Ionization (ESI)
Summary The sample solution is sprayed across a high potential difference (a few kilovolts) from a needle into an orifice in the interface. Heat and gas flows are used to desolvate the ions existing in the sample solution. Electrospray ionization can produce multiply charged ions with the number of charges tending to increase as the molecular weight increases. The number of charges on a given ionic species must be determined by methods such as: . comparing two charge states that differ by one charge and solving simultaneous equations . looking for species that have the same charge but different adduct masses . examining the mass-to-charge ratios for resolved isotopic clusters
Sample introduction . flow injection . LC/MS . typical flow rates are less than 1 microliter per minute up to about a milliliter per minute
Benefits . good for charged, polar or basic compounds . permits the detection of high-mass compounds at mass-to-charge ratios that are easily determined by most mass spectrometers (m/z typically less than 2000 to 3000). . best method for analyzing multiply charged compounds . very low chemical background leads to excellent detection limits . can control presence or absence of fragmentation by controlling the interface lens potentials . compatible with MS/MS methods
Limitations . multiply charged species require interpretation and mathematical transformation (can sometimes be difficult) . complementary to APCI. No good for uncharged, non-basic, low-polarity compounds (e.g.steroids) . very sensitive to contaminants such as alkali metals or basic compounds . relatively low ion currents . relatively complex hardware compared to other ion sources
Mass range . Low-high Typically less than 200,000 Da