LECTURE 3. Ionization Techniques for Mass Spectrometry

Transcription

1 LECTURE 3 Ionization Techniques for Mass Spectrometry Jack Henion, Ph.D. Emeritus Professor, Analytical Toxicology Cornell University Ithaca, NY Lecture 3, Page 1

2 Contents Electron ionization (EI) Electrospray ionization (ESI) Nano ESI Atmospheric pressure chemical ionization ( APCI) Atmospheric pressure photoionization ( APPI) Matrix assisted laser desorption ionization (MALDI) LAESI Direct analysis in real time (DART) Desorption electrospray ionization (DESI) Atmospheric sampling analysis probe (ASAP) And there are many more that may or may not mature. Lecture 3, Page 2

3 Many Forms of Ionization have been Reported EI APCI ESI APPI ASAP MALDI DESI RADIO DAPPI REIMS PESI LIAD-ESI DART FAPA LAESI/IR- LDESI/MALDESI ELDI LIAD-ESI Lecture 3, Page 3

4 Electron Ionization (EI) Source (not yet commercially available for LC/MS) High vacuum Lecture 3, Page 4

5 Ionization Processes: Electron Ionization (EI) vs. API Electron ionization (EI) forms radical cations as shown below: M e - M. 2 e - 70 ev electrons The radical cation is very reactive and often is the primary director of the ensuing fragmentation API techniques generate (M1) ions as shown below: Electrospray or APCI: (MH) (MH) (in solution) (ion evaporation or APCI) (gas-phase ion) But how does this occur? Lecture 3, Page 5

6 Positive Ions: Odd-Electron and Even-Electron Ions Odd Electron Ions N H H H e N H H H EE (8) OE (7) 2e Even Electron Ions N H H H H H N H H H EE (8) EE (8) Lecture 3, Page 6 OE and EE Ions

7 Electrospray Ionization Lecture 3, Page 7

8 Electrospray Process A method by which ions present in solution can be transferred to the gas phase High Flow ESI Gas Assisted: Droplet size influenced by high velocity gas & electrostatic repulsion Classical or Pure ESI or NSI: Droplet Size influenced purely by electrostatic repulsion Courtesy ThermoFinnigan Lecture 3, Page 8

9 Ink Jet printing Taylor Cone 8 x 10-3 Torr kv OXIDATION electrons High-Voltage Power Supply electrons 1 kv REDUCTION Lecture 3, Page 9

10 Maximizing Signal. Minimizing Noise. Agilent Animation of S/N Lecture 3, Page 10

11 AB SCIEX Turbo-V Gas Entrainment Source High sensitivity at any flow rate. 1. Increased Thermal Efficiency of Heater 2. Second Heater Double Thermal Load 3. Promote Turbulent Mixing Improved Heat and Mass Transfer Maximize Desolvation 4. Controlled Entrainment Background Reduction 5. Reduced Dispersion of Droplets Signal Enhancement Flow Rate Dependent Lecture 3, Page 11

12 Electrospray Ionization Technology Enhancements: Thermo HESI-II Benefits: Sensitivity across a wider range of flow rates Higher desolvation for >1mL/min flow rates More heated nitrogen auxiliary gas flow aids desolvation at high flow rates One piece metal needle allows for easy replacement Available as low flow and regular (high) flow Contoured tip for enhanced low flow (micro-spray; 5-25uL) stability Generates better electrostatic fields enabling spray stability for lower flow rates * Available on all TSQ products Higher desolvation Contoured tip Metal Needle Standard Lecture 3, Page 12

13 Waters Z-Spray Synapt HDMS System Lecture 3, Page 13

14 Agilent Jet Stream Technology Courtesy of Dr. Alex Mordehai, Agilent Technologies Lecture 3, Page 14

15 H-SRM Operation Enhancing Specificity Lower chemical noise Lower detection limits Collision Cell = only pre-cursor ion transmitted Thermo TSQ Quantum Lecture 3, Page 15

16 Enhancing Detection Limits 90º Collision Cell Design No Line of sight enhanced S/N 5x better SRM transmission efficiency Argon as collision gas for higher sensitivity Exit lens assembly Collision cell housing Entrance lens assembly Square quadrupole collision cell Lecture 3, Page 16

17 A trace of DMSO to Improve Peptide Ionization Reponse Page 17 Jesse G. Meyer, Elizabeth A. Komives, Charge State Coalescence During Electrospray Ionization Improves Peptide Identification by Tandem Mass Spectrometry, J. Am. Soc. Mass Spectrom. (2012) 23:1390Y1399.

18 APPI Atmospheric Pressure Photoionization Lecture 3, Page 18

19 Molecular Weight APPI Source and Compound Strike Zone 100,000 10,000 Electrospray Ionization 1, APPI APCI Non-Polar Polarity Very Polar APPI source configuration employing a VUV light source and orthogonal spray geometry. Qualitative diagram representing ionization of nonpolar compounds by APPI Lecture 3, Page 19

20 Direct vs. Dopant APPI Direct APPI M hv M M S MH S[-H] Analyte molecule M is ionized to a molecular ion M. (If analyte ionization potential is below photon energy) In the presence of protic solvents, M may abstract a hydrogen atom to form MH. Dopant APPI D hv D D M MH D[-H] D M M D A photoionizable dopant is delivered in large concentration to yield many D ions. D ionizes analyte M by proton or electron transfer. This is PI-initiated APCI. Lecture 3, Page 20

21 Benefits of APPI Ionizes a relatively wide range of compounds (e.g., non-polars, electronegative cpds, etc.) Predominantly parent ion signal (minimum fragmentation) Low noise source (minimum solvent signal) Minimum ion suppression Large linear dynamic range Works for positive and negative ionization Lecture 3, Page 21

22 Peak Area Peak Area APPI Is Best For 1.2E05 1.0E05 8.0E04 Dibenzo[a,h]anthracene, 278 > ,000 pg R 2 = E04 Environmental 4.0E04 PAHs 2.0E04 Steroids 0.0E00 Pesticides Injection Amount (pg) Food Safety PAHs 2.E06 >5-decade linear Pesticides response on 2.E06 the ZQ Fish Oil Lipids R 2 = Vitamins 1.E Petroleum/Oils/Biofuels E05 Petroleum 0 Biofuels 0.E00 Triacylglycerol Lipids Inj. Amount (ng) Pharmaceuticals/Small Molecules Steroids Metabolites (especially lacking functional groups) Chiral Separations (i.e., normal phase LC) Analysis of Lipids: Diarachidin (1:1 100 ul/min) R 2 = >10 4 linear response 100 % ESI APPI Progesterone [MH] % Ion m ass Lecture 3, Page 22

23 APPI Very Resistant to Ion/Matrix Suppression APPI TIC Full Scan - Rat Plasma Extract Ratio of signal for FIA/MS vs. LC/MS APPI APCI ESI APPI Average deviation 74% 57% 23% Median deviation 68% 52% 22% APCI Measurement performed on a 29-compound library of unknown purity ESI Determination of ion suppression susceptibility for APPI, APCI, and ESI by post-column addition and detection of fluphenazine while running an LC/MS chromatogram of rat plasma. min These results show minimal ion suppression by APPI. APCI shows less ion suppression than ESI, but APCI signal is noisier than by APPI Lecture 3, Page 23