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Food Analysis Lecture 25 (4/26/2005)

Food Analysis Lecture 25 (4/26/2005). Mass Spectrometry. Qingrong Huang Department of Food Science Read Material: Chapter 26, page 423 Final Exam: April 29. Mass Spectrometry Basics. Ionization of molecules; Separation of ions based on mass-to-charge ratio (m/z);

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Food Analysis Lecture 25 (4/26/2005)

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  1. Food Analysis Lecture 25 (4/26/2005) Mass Spectrometry Qingrong Huang Department of Food Science Read Material: Chapter 26, page 423 Final Exam: April 29

  2. Mass Spectrometry Basics • Ionization of molecules; • Separation of ions based on mass-to-charge ratio (m/z); • Detected by mass analyzer under electrostatic field; • Routine in analytical labs; • GC-MS: interface of mass spectrometer with GC; • LC-MS: interface of mass spectrometer with HPLC.

  3. Sample Introduction • Direct Injection: for a gas or a volatile liquid; • Direct Insertion: for somewhat volatile solid, the sample is placed • in a small cup at the end of a stainless steel rod or probe; • For mixtures, we can use GC-MS or LC-MS through an • interface which removes excess GC carrier gas or HPLC solvent.

  4. Ionization • In ion source, the compound is exposed to a beam of electrons • emitted from a filament composed of rhenium or tungsten metal. • When a direct current is applied to the filament (~70 eV), it heats • and emits electrons that move across the ion chamber toward a • positive electrode; • As the electron pass through the source region, they come in close • proximity to the sample molecule and extract an electron, forming • an ionized molecule; • The ionized molecules can further fragment into smaller molecular • fragments.

  5. Mass Analyzer • Heart of a MS; • Separating charged fragments based on their m/z; • Dictating the mass range, accuracy, and sensitivity; • Five types of mass analyzer: • - quadrupoles • - ion traps • - time-of-flight (TOF) • - magnetic sectors • - Fourier-transform ion cyclotrons (FT-ICR)

  6. Magnetic Sector • Use magnetic field to separate ions based on their m/z; • Popular high resolution instrument; • Bulky.

  7. Quadrupole Mass Analyzers • “Four rods” are used to generate two equal but out-of-phase electric • potentials; • One is alternating current (AC) frequency of applied voltage that falls • in radiofrequency (RF) range; the other is direct current (DC); • The potential difference creates an oscillating electric field between • two of the opposite rods, resulting in their having equal but opposite • charges; • By adjusting the potentials on the rod, only selected ions can be made • stable and detected.

  8. Example of Ionization Process

  9. Ionization Process

  10. GC-MS • GC column is connected directly to the MS source via a heated • capillary transfer line, which is kept hot enough to avoid • condensation of the volatile component eluting from GC column; Total ion current GC chromatogram methyl esters of fatty acids.

  11. TIC Chromatogram • Total ion current (TIC) chromatogram;

  12. LC-MS • A LC-MS interface facilitates desolvation by application of heat, • followed by a rapid expansion of this vapor in an area of reduced • pressure; • The heat energy applied in evaporation does not contribute to • degradation of any thermally labile species present in LC eluant. • Two commonly used interface: • - Electrospray interface (ESI) • - Atmospheric pressure chemical ionization interface (APCI)

  13. ESI

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