The anatomy of a gas chromatograph

 

The results produced by a gas chromatograph are usually the difference between innocence and guilt in a DUI case.  The prosecution’s purported blood alcohol concentration (BAC) is typically the “end-all be-all” of their case. Let’s take a look at how this machine creates such a critical measurement.

Big Pictures Thoughts

  • If done properly, gas chromatography is a reliable way to measure the amount of blood in an alcohol sample.  However, automobiles are also reliable, but there are still thousands of car wrecks every day.  There is no presumption of reliability simply because a gas chromatograph was used.
  • The measurement process has both human components and machine components.  All steps in the process must be done correctly for the measurement to be trusted.
  • The goal of is to produce a measurement, which is both accurate and reliable.

General Principles

  • Gas chromatography is an indirect measurement.  The machine does not test liquid portion of a blood sample.  In headspace gas chromatography, the machine converts substances to a gas, and then it must separate the different types of molecules in the sample.  After separation, a microscopic amount of the gas is measured by software.
  • The machine must demonstrate it is able to separate different types of molecules before it can measure them.  If it cannot properly separate different categories of molecules, then its measurements may be artificially higher.
  • Gas chromatography is done in manner like a production line.  Multiple samples (usually over 100 vials) are being processed in a “batch.”  It is essential to safeguard against the wrong information being assigned to the wrong sample.

Vocabulary

  • Gas chromatograph - a machine that separates molecules, and then measures, the amount of the various components in a sample.
  • Gas Chromatography - the scientific process performed by a gas chromatograph.
  • Chromatogram – the graphical representation of the data produced by the gas chromatograph.  This is where you will find the final measurement.  A chromatogram is the machine’s conclusion.

The Human Part

The measurement process starts long before the gas chromatograph is actually turned on.  The blood must be collected, identified, stored and transferred properly before the sample is put into the gas chromatograph.  Even the best machines cannot account for, or identify, that a sample has been corrupted.  The principle of garbage in garbage out must be kept in mind.  That is, incorrect (or poor quality) input will always produce wrong output.  

Human are also responsible for teaching the machine a specific alcohol concentration.  The machine does not come out of the box knowing any specific alcohol concentration.  Typically a lab will purchase approximately four (4) different alcohol concentrations from a vendor.  For example, .01, .10, .20, .40 are often used to calibrate the machine. 

These samples are put into the machine and the analyst programs the machine’s software to use these values.  If the analyst tells the machine a sample is a .40 but it is really a .30, the machine cannot tell the difference.  Ensuring a calibrator is what it purports to be is known as traceability.

The Machine Part

The machine starts its analysis after a small portion (less than the size of the a single M&M) of each blood sample is put into a headspace vial. The headspace vials (usually over 100) containing the samples are loaded into a part of the machine called the autosampler.  

The samples are then heated (in a headspace tube) forcing molecules in the liquid portion of the sample to rise.  After the molecules are vaporized, a needle punctures the top of the headspace vial and extracts a microscopic portion of the gas above the liquid.

These vaporized molecules are pushed through long thin columns by a carrier gas (hydrogen or helium).  These thin columns have a chemical coating inside them designed to interact with the molecules passing through them.  The carrier gas moves at a constant pressure.  This results in different molecules in the gas to group together (e.g. ethanol with ethanol, methanol with methanol).  Each molecule group, such as ethanol, has a unique rate of speed.  This accounts for the separation of the each substance in the columns.

After each molecule group is pushed out of the column, they will be pushed to a detector.  The time when is substance exits the column is called the time it elutes.  The detector’s software has been programmed to identify different substances by the time they elute from the column.  The Flame Ionization Detector, as the name implies, then burns each molecule group and then measures how much is burned.

The software gathers the “raw data” and then processes it.  The “process data” is graphically represented in something called a chromatogram.    The measurement is found here.

The above summary just scratches the surface of the measurement process using a gas chromatograph.  If you are going to rely upon the measurement produced by this technique, then every step in process (both the human and machine) must be shown to have been done correctly.

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