DUI Blood Testing
You will find a brief discussion of blood draws in Michigan’s drunk driving cases in the blog entitled: Michigan’s Use of Expired Blood Draw Kits in DUI Cases Should Be Discouraged. This blog specifically discusses the very basic “nuts and blots” of a forensic blood draw.
This topic is also covered in detail in the two volume treatise entitled Defending Drinking Drivers, which is available for purchase on-line. What follows is a small portion of chapter two:
§243 Gas Chromatography – Whole Blood Testing
The method used to test the driver’s blood will depend on a variety of circumstances, but as a general rule, when the motorist’s blood is tested for the presence and amount of alcohol the state will analyze the blood using headspace gas chromatography. If the blood sample is analyzed for the presence and/or amount of a controlled substance, then the state lab will usually use mass spectrometry. Both methods are considered “whole blood testing.” If the blood is tested for the presence and amount of alcohol at a hospital, then enzymatic analysis is usually used. Enzymatic analysis is considered “serum” blood testing.
Hospital blood draws are most commonly encountered with accident cases. Consequently, the majority of cases encountered by defense attorneys will involve whole blood gas chromatography.
§243.1 Theory and Operation
The process of gas chromatography involves the use of an instrument called a gas chromatograph. This instrument separates and then measures the amount of alcohol in the driver’s blood. To accomplish this testing the analyst first removes a very small amount of the driver’s blood from the blood draw vial and places it, along with a very small amount of an internal standard, into a separate testing vial. This testing vial is called a “headspace” vial. This headspace vial is then shaken to mix the chemicals and heated to produce a gas. This “headspace gas” is then drawn off via a headspace sampling syringe. On its way into the headspace vial, the syringe passes through a rubber gasket. The headspace gas that is drawn off is then introduced into a very thin column. The gas is pushed through this column by way of an inert carrier gas, such as helium. Because it is important for this column to be kept at a constant temperature throughout the test, it is located inside a part of the instrument called the “oven.”
At the end of the column is a flame ionization detector. As the chemicals exit or “elute” from the end of the column the flame incinerates them, and this combustion produces an electronic charge in the form of ions. These ions are then measured by the detector and subsequently converted by the instrument’s computer into the reported blood alcohol level. Thus, it is helpful to understand that because only the headspace gas is measured, gas chromatography does not actually test the blood directly. In this regard, whole blood testing by gas chromatography is like breath testing because both are indirect testing methods. However, there is certainly an agreement in the scientific community that a result produced by way of gas chromatography is typically much more accurate and precise than one produced by way of breath testing.
The gas chromatographic system includes the following:
- The sample.
- The carrier gas.
- The tubing (also called the coil or column).
- The solid support (glass beads or diatomaceous earth).
- The liquid phase (coating covering the solid support material).
Gas chromatography uses only a small quantity of the sample, usually 0.001 to 100 micrograms. The sample is placed into the headspace vial and mixed with a few microliters of solvent (internal standard). The analyst operating the gas chromatograph uses a syringe to introduce the mixture into the chromatograph. This is done through an injection port which consists of a silicon rubber diaphragm. The injection port temperature must be maintained above the sample’s boiling point to create vaporization, thereby creating the headspace gas. The resulting gas is then swept into the gas chromatograph column by the carrier gas. When the sample is converted into a gas and swept into and through the column by the carrier gas stream, that phase is known as the mobile phase. A high pressure gas cylinder serves as the source of the carrier gas. There are several carrier gases that can be used, including helium, nitrogen, hydrogen and carbon dioxide. Because the material inside the column is inert it avoids any interaction with either the sample or the liquid phase.
The column is housed in copper, stainless steel, glass or aluminum tubing. While the column is straight it is packed with an inert substance, usually glass beads or diatomaceous earth. After packing, the tubing is coiled or bent. The column length varies from a few inches to more than 50 feet. The diameter of the column also varies from 0.01 to 2.0 inches. The solid support portion of the column provides an inner base for spreading the liquid phase. It plays no active role in the process. The solid support can be made up of a number of substances, including microglass beads, aluminum, powdered teflon, or diatomaceous earth. The liquid phase, otherwise known as the stationary phase, is a thin layer of coating. This layer of coating covers the solid support material and consists of polyesters, silicone polymers and polyethylene glycols.
The separation process begins when the mobile phase sweeps the sample into the column. Components of the sample mixture will have differing affinities for the liquid phase. Components travel through the column only when they are in the mobile phase. Therefore, a component with a greater affinity for the liquid phase will take longer to traverse the column than components with less affinity. Consequently, the components will emerge from the column at different times because of their differing affinities for the liquid phase. Since each type of molecule has a different rate of progression, the various components of the blood mixture are separated as they progress along the column and reach the end of the column at different times (retention time). The detector monitors and measures the various chemicals as they elute from the column. Generally, substances are identified by the order in which they emerge (elute) from the column and by the retention time of the chemical in the column.
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