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Table of Contents

Gas Chromatograph/Mass Spectrometer

  • Manufacturer - Hewlett Packard
  • Model Number - HP 5890 II
    • Shown with Autosampler (on right)

Theory

  • Six Basic Principles in Performing Gas Chromatography:
    • Flow Controller
    • Sample Introduction Device
    • Column
    • Oven
    • Detectors
    • Computer Interface (Data Management)

Gas Chromatography

  • Process involves the physical separation of compounds in a mixture by injecting the mixture into a tube (column) that contains a silica based inert compound. The mixture is called the mobile phase and the silica compound is known as the stationary phase. A carrier gas is used to flush the mobile phase through the column. The stationary phase will pass some compounds and slow down others.
  • Compounds in mixture are identified by their retention times.
    • Retention time is the time it takes for the individual compound to pass through the entire column.

Mass Spectrometry

  • Mass spectrometry deals with shattering compounds into component ions by passing sample through an ionization (high voltage) source.
  • The ions are separated by a large magnetic or electric field. The ions are then collected in “traps” and then analyzed. A charge to mass ratio is calculated and a mass spectra is generated.

Interface and Equipment

GC Column

  • Capillary tube of length between 1 to 100 m and thickness range of 0.25 to 5 mm
  • Column is either packed with silicone based stationary phase or inside is coated with the material.

Hardware

  • Nitrogen Gas Cylinder
    • Carrier gas for mobile phase, non-reactive and inert to sample being tested
  • Hydrogen Gas Cylinder
    • Used to combust sample at the end of the analysis
  • Plumb Lines
    • To connect cylinders to GC/MS
  • Vacuum Pump
    • Used for MS which operates under low pressure conditions

Computer Interface

  • The GC/MS can either be run automatically or manually.
    • Both require use of computer program.
      • PC and software are Windows based.
      • Software contains program that controls oven temperature such that the oven starts at a low temperature and ramps up to a higher temperature at different heating rates.

Calibration

  • Calibration standard is made of a pure compound (e.g. bromofluorobenzene) and is used for quality control of the GC/MS.
  • The compound has a specific retention time and a specific mass spectra.
  • Quality control is usually done at after a fixed number, usually 20, of samples have been analyzed.
  • Quality assurance is done whenever the instrument cannot be calibrated.
    • QA steps include:
      • Changing the check standard - an aged check standard can affect accuracy of instrument
      • Conditioning the column - bake the column to a temperature of 300oC to remove contaminants
      • Replacing the column - cracks and fissures
      • Checking the carrier gas flow rate - mobile phase may be going faster or slower than normal
      • Replacing the entire instrument - GC oven may be irreparable, internal plumbing damaged, etc

Purpose and Performance

  • A combined GC/MS has the ability to detect minute quantities of a solute contained within a larger volume of solvent. For example, a properly calibrated GC/MS can quantify organic contaminants, such as volatile organic carbons (VOC) on the order of parts per billion present within large volumes of soil, water or air.
    • This is useful in environmental sampling especially when looking for compounds that adversely affect human health.
    • Liquid or air samples on the order of milliliters can be sampled.
      • If using the direct inject method, microliters of sample are usually used.
  • Sample Injection
    • Non-Direct Inject
      • Usually done with help of autosampler.
      • Nitrogen gas is bubbled into a tube containing sample to draw out the volatile parts of the mixture.
        • Semi or non-volatile compounds are not usually sampled by GC.
      • Mixture is held in a trap until GC signals the computer to start the oven program.
      • Mixture is injected into the inlet of both the GC and MS at the same time. Both instruments collect data at the same time.
      • As the compound is sent through and out of the column, the MS shatters each compound into ions as they are ejected and quantifies their mass spectra.
      • At the end of the column, there is a flame ionizing detector, FID, which combusts the sample with the aid of hydrogen gas.
        • The GC quantifies the amount of burn and the time of ionization to render a retention time and concentration.
    • Direct Inject
      • Analyst will start computer program and inject sample at same time rather than using autosampler.

Data

  • Top picture shows the chromatogram.
    • Chromatogram shows peaks for each compound in the mixture according to retention time. It also shows concentrations of the compounds.
    • Integrated area of peak is the concentration.
  • Bottom picture shows the mass spectra.
    • Using the reference spectra library that comes with the GC/MS software, an analyst can identify each peak by clicking on the highest point of the peak.
    • At the end of the mass spectra is the molecular weight of the ion.
      • For example, chloroform is a chlorinated VOC with molecular weight of 119.5 g/mol. The ion molecular weight will be somewhere between 116 and 118 g/mol. The discrepancy is due to isotopes of carbon and chlorine.

Usage and Deployment

  • Liquid samples are collected in bottles and air samples are collected in bags.
    • Air samples can be direct injected into GC/MS from the bag instead of by autosampler or syringe as with liquid samples.
    • Does not perform real time sampling.
    • In a production laboratory setting, a GC/MS can be dedicated to performing emergency sampling which requires a fast turn around.
    • Most samples that are being tested for VOCs have an average sampling time of 30 minutes from injection to the end of the analysis.
  • GC/MS is designed to be used in a stationary environment.
    • Complex setup of GC, MS, computer interface, carrier gas cylinders, and plumbing make it difficult to use in a mobile setting (e.g. aircraft).
    • Less sophisticated instruments can be used in mobile platforms.
      • Martian land rover has onboard GC/MS but uses much less sophisticated sampling techniques due to space constraints.
  • Fields Other Than Environmental and Space Science GC/MS Is Used for
    • Forensic Science
    • Toxicology
    • Drug Screening
    • Pesticides
  • Power Consumption
    • GC/MS can be used on standard 120 V (220 V if 50 Hz cycling alternating current) power line but requires high current for MS.
    • 800 W power consumption
  • Cost
    • $40,000 for GC/MS combination
    • $1000 for PC and software
    • $500 for gas cylinder - lasts approximately three weeks with 24 hour consumption

Acknowledgments and References

atmos/535/projects/hewlett_packard_5890_series_ii_gc-ms.txt · Last modified: 2020/01/29 17:25 by 127.0.0.1