20.2 Ion Sources. ions electrospray uses evaporation of a charged liquid stream to transfer high molecular mass compounds into the gas phase as MH n

Transcription

1 20.2 Ion Sources electron ionization produces an M + ion and extensive fragmentation chemical ionization produces an M +, MH +, M +, or M - ion with minimal fragmentation MALDI uses laser ablation to transfer high molecular mass compounds into the gas phase as MH + or MH 2 2+ ions electrospray uses evaporation of a charged liquid stream to transfer high molecular mass compounds into the gas phase as MH n n+ ions, where n is large, e.g. 10 to : 1/14

2 Overview Sources ionize a neutral molecule by electron ejection, electron capture, protonation, deprotonation, adduct formation, or charge transfer. High energy ionization methods produce fragmentation, while low energy ionization methods produce primarily the. For thermally stable and volatile compounds, two methods will be given - electron ionization and chemical ionization. For thermally unstable or non-volatile compounds, two methods will be given - matrix assisted laser desorption ionization (MALDI) and electrospray ionization. Both methods are useful for biological samples. Not covered are methods for inorganic compounds. The most common are glow discharge and : 2/14

3 Electron Impact Ionization (1) The heated filament produces electrons which are accelerated by the potential on the sample chamber. Once in the chamber the electrons travel in a to the anode collector. The sample is introduced through a small opening and travels across the chamber toward the mass analyzer. filament heater current electron accelerating potential filament electron beam ion beam gaseous sample inlet to mass analyzer anode extracting lens focusing lens accelerating lens A high-energy electron will knock a second electron off a neutral molecule. The resultant has excess energy and fragments. The molecular ion and the fragments continue traveling toward the outlet. At this point they are attracted by the extracting lens potential, focused, and accelerated toward the mass analyzer : 3/14

4 Electron Impact Ionization (2) One electron accelerated with 1 V has an energy of 1 ev, which is J or 96.3 kj mol -1. The maximum efficiency of ionization by electron impact occurs at ~ ev. At this accelerating voltage, about ev (~2,000-3,000 kj mol -1 ) are transferred to the molecular ion. This amount of internal energy is high enough to break several bonds, and fragment the ion. At a given potential and temperature, the number of ions produced per unit volume is proportional to the sample and electron current. For 70 ev electrons, on average one ion is produced for every 1000 molecules entering the source chamber : 4/14 Hoffmann and Stroobant, "Mass Spectrometry," 2nd edition, Wiley, 2002, figure 1.2.

5 Electron Impact Ionization (3) The extent of fragmentation depends upon the. The 70 ev spectrum of β-lactam has many more than the 15 ev spectrum. This can be seen by the relative height of the fragment ions and the molecular ion. However, the intensity of the molecular ion is in the 15 ev spectrum because of the efficiency curve shown on the previous slide. Hoffmann and Stroobant, "Mass Spectrometry," 2nd edition, Wiley, 2002, figure : 5/14

6 Chemical Ionization (1) In chemical ionization a reagent gas is ionized by the electron beam. The reagent gas transfers its charge to the sample through a variety of reactions. The reagent gas is, so the sample is not ionized by the electron beam. The resultant molecular ion contains little excess energy, thus there is. The figure shows the butyl methacrylate mass spectrum obtained with electron ionization (top), methane, and isobutane (bottom). Electron ionization eliminates virtually all the molecular ion at 142 Th. Methane produces some MH + through proton transfer and high mass fragments, while isobutane produces primarily. H 2 C O C C O CH 2 CH 2 CH 2 CH 3 CH : 6/14

7 Chemical Ionization (2) : the ionized reagent gas transfers a proton to the sample, thereby transferring its charge. The reagent gas can also extract a hydride from the sample. M + CH 5+ MH + + CH 4 MH + CH 5+ M + + CH 4 + H 2 : polar molecules will often form adducts. Self association is also possible. M + CH 3+ (M+CH 3 ) + MH + + M (2M+H) + : inert gases with high ionization potentials react by transferring charge to the sample. Xe + + M M + + Xe 20.2 : 7/14 Negative ion formation: if the sample has acidic groups or electronegative elements, it can form a negative ion by capturing a slow electron from the reagent plasma. AB + e - AB - AB + e - A + B - AB + e - A + + B - + e -

8 Chemical Ionization (3) A dual electron ionization (EI)/chemical ionization (CI) source is shown. The left diagram is operation in the EI mode, with behavior as described earlier : 8/14 The right diagram shows the CI mode with the reagent gas box lowered into the electron beam. In this configuration the sample enters the box where the reagent gas is held at a pressure of ~ torr. This pressure creates a mean free path for collisions of ~ mm. For a box with centimeter dimensions each ion undergoes ~ collisions. Pumps keep the pressure outside the box at the 10-5 torr level. 1, IE/CI switch; 2, microswitch; 3, reagent gas entrance; 4, flexible capillary; 5, diaphragm; 6, filament; 7, path of ions toward analyzer; 8, hole for ionizing electrons; 9, sample inlet; 10, box with holes, called the "ion volume."

9 Matrix-Assisted Laser Desorption Ionization, MALDI (1) The compound to be analyzed is dissolved in a solvent containing a small organic molecule that the laser radiation. The mixture is placed on the probe tip and the solvent allowed to evaporate. The analyte is then dissolved in a solid solution of matrix crystals. desorption desolvation H + + protonation + The mixture is irradiated by a pulsed laser producing ~20 mj cm -2. The matrix absorbs the laser and the mixture explosively ablates from the surface. The sample molecules are surrounded by matrix as they leave the surface. After a short distance the sample begins to "desolvate." During the desolvation process a is transferred from the matrix to the analyte forming the MH + ion. The matrix is present in large excess, keeping the sample molecules away from each other and minimizing optical damage to the sample. The laser frequency determines the and not the sample! 20.2 : 9/14

10 MALDI (2) MALDI is most often used to put species into the gas phase. It is a major tool in proteomics, where proteins can initially be separated by 2D-PAGE then analyzed by MALDI. The top spectrum is from a monoclonal antibody with a mass of ~150,000 Da. Note that the charge state is due to addition of to the basic residues of the protein. The bottom spectrum is a mixture of poly(methyl methacrylate) polymers with a nominal mass of 7100 Da. This is far from a monodispersed polymer! 20.2 : 10/14

11 MALDI (3) Infrared and ultraviolet laser desorption give essentially the same mass spectrum. With infrared desorption there can be less fragmentation if the sample absorbs in the ultraviolet. The table below shows some common UV matrices and typical applications. Matrix α-cyano-4-hydroxycinnamic acid 3,5-dimethyl-4-hydroxycinnamic acid 2,5-dihydroxybenzoic acid 3-hydroxypicolinic acid trihydroxyacetophenone 5-chlorosalicylic acid Application peptides, proteins, organics higher mass biopolymers peptides, proteins, carbohydrates oligonucleotides oligonucleotides, peptides water-insoluble polymers With infrared lasers, matrices include urea, carboxylic acids, alcohols and water : 11/14

12 Electrospray Ionization (1) A solution containing the sample is pushed through a metallic capillary at 1-10 μl min -1. The solution leaves at atmospheric pressure. The capillary is charged 3-6 kv across a distance of cm producing an electric field of ~10 6 V m -1. As the liquid leaves the capillary the electric field causes it to charge with. The multiple charges cause droplets to form. The analyte can pick up some of the charge as it is desolvated. are used to isolate the sample from the solvent. High capacity pumping is necessary to keep the spectrometer at low pressure. The ionization efficiency is times greater than chemical ionization. Like MALDI, electrospray is used to introduce high molecular weight molecules into the gas phase : 12/14

13 Electrospray Ionization (2) Electrospray ionization is due to the addition of protons. Since the droplets contain many protons, the sample can become charged. When m/z is measured, multiple charges permit high molecular weight compounds to be observed with " " instruments. Obtaining the actual mass requires some arithmetic. Let M be the actual mass, m 1 be a peak in Thomsons with charge z 1, and m p be the m/z of a proton. The measured mass can be written as a sum. z 1 m 1 = M + z 1 m p A second m/z is chosen that appears j peaks higher in the spectrum. Solving one gets: (z 1 - j)m 2 = M + (z 1 - j)m p z 1 = j(m 2 - m p )/(m 2 - m 1 ) and 20.2 : 13/14

14 Electrospray Ionization (3) m 1 = 939.2, m 2 = , and j = 6 z 1 ( ) ( ) = = 19 M = = ( ) 20.2 : 14/14