Spectroscopy {mass spectroscopy} (MS) can find atomic and molecular mass. Mass spectroscopy is fast, reliable, expensive, and delicate.
sensitivity
Mass spectroscopy can detect masses from 1 to 400,000 daltons, at concentrations as low as 10^-12 M.
uses
Mass spectroscopy can find molecule functional groups by fragment mass differences. For example, methyl groups have mass 15, CO groups have mass 28, water has mass 18, ammonia has mass 17, and phenyl groups have mass 77.
Mass spectroscopy measures leak detection, blood gases, and tracers and analyzes petroleum, plastics, fertilizers, and insecticides. Mass spectroscopy can detect illegal drugs, impurities, pollutants, reaction products, and toxins in gases, liquids, and solids. Mass spectroscopy determines age, quantifies chemical composition, studies metabolism, detects molecular changes, and monitors chemicals. Mass spectroscopy can sequence peptides.
process
Mass spectroscopy measures mass-to-charge ratio of ionized atoms or molecules.
process: vaporization
A 350-C vacuum chamber heats sample to make gas, which expands into ionization chamber through pinhole.
process: ionization
70-eV electron beams ionize sample molecules to make ions with charge +1. Ionization can use proton transfer from ionized methane. Vaporized molecules can ionize {desorption, mass spectroscopy} by californium-252, secondary ions, lasers, high electric field on thin film, or electrospray from high-voltage needle.
process: electric field
Source positive charge repels positively charged ions into analyzer. Ions enter strong electric field and accelerate through slits to collimate.
process: magnetic field
Ions enter magnetic field and arc in semicircle. Typically, magnetic field or voltage sweeps. Big ions move slower and have big radius. Small ions move faster and have small radius.
process: detector
Ions hit surface with applied voltage, causing charge cascade {electron multiplier}. Ions make current, which can be as small as 10^-15 A. Detector measures ion energy and location. Location indicates ion mass. Energy, converted to electric current or light, indicates ion number.
process: types
Double-focusing mass spectroscopy separates masses first by radial electric field and then by radial magnetic field.
Sector detector uses electric field to focus ions, then magnetic field to spread ions. Larger masses need stronger fields to focus them on detector. Smaller masses need smaller field to bring them to detector. Detector plate converts collision energy into ions or electrons, detected as current, or photons, detected by photomultiplier.
Quadrupole mass filter uses four parallel plates or rods with constant direct-current electric field between two plates and varying radio-frequency electric field between two plates.
Ion traps use two parallel plates and ring electrode to trap ions that have mass range. Electric field increases to eject ions toward detector.
High-resolution detectors can measure dalton fractions. Fourier-transform ion-cyclotron resonance (FT-ICR) can trap ions between electrodes in a magnetic field. Radio-frequency electric field makes ions orbit. Orbiting ions create electromagnetic frequency measured by detector plates.
Time-of-flight (TOF) methods send ions accelerated to constant velocity across distance. Detector measures time and so mass.
Two-stage mass spectroscopy (MS/MS) can separate compound from mixture or separate compound constituents to analyze compound structure.
process: results
If electron-beam energy is high enough, sample has unique ion-fragment pattern {cracking, mass spectroscopy}. Highest-mass peak is original molecule with one electron missing. Pattern depends on chemical-bond strength, atoms, total molecular mass, and ionization potential. Chemical bonds break most easily where molecules branch. Double bonds can break. Saturated ring compounds break at side-chain alpha carbon. Carbonyls break so carbonyl ionizes. Aromatic compounds do not break.
process: isotopes
Isotopes cause peak doubling or tripling. Elements have definite peak-doubling or peak-tripling ratios. F, P, and I have no isotopes. 2H, 15N, and 18O [2, 15, and 18 are superscripts] have negligible amounts.
process: isotope ratios
First, find carbon number. 13C [13 is superscript] is 1.11% of total carbon, so first peak to second peak ratio shows carbon number. Next, find oxygen, sulfur, chlorine, and bromine numbers by isotope ratios. Find nitrogen number. If molecular mass is even, nitrogen number is 0 or even. If molecular mass is odd, nitrogen number is odd. You can also use compound spectra tables.
Physical Sciences>Chemistry>Analytical Chemistry>Spectroscopy
5-Chemistry-Analytical Chemistry-Spectroscopy
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Date Modified: 2022.0224