Transcript Nanotechnology: From Microelectronics to Health Care

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);
• 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.
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.
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.
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)
Magnetic Sector
• Use magnetic field to separate ions based on their m/z;
• Popular high resolution instrument;
•Bulky.
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.
Example of Ionization Process
Ionization Process
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.
TIC Chromatogram
• Total ion current (TIC) chromatogram;
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)
ESI