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BLOOD

المؤلف:  Max M. Houck، Jay A. Siegel

المصدر:  Fundamentals of Forensic Science

الجزء والصفحة:  p373-374

2026-08-15

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BLOOD

Blood is the preferred medium for alcohol measurement because it provides the best surrogate for brain alcohol levels and because of this, most states have statutes that relate sanctions for drunk driving to blood alcohol levels. The ideal blood sample would be arterial blood because it most closely tracks brain alcohol content, whereas venous BAC tends to lag behind. Nonetheless, whole venous blood is most commonly used in drunk-driving cases. The most widely used method for the analysis of BAC for forensic purposes is gas chromatography. This has several advantages, including high specificity, accurate quantitative analysis, and ease of automation. Most forensic toxicology laboratories today use headspace alcohol that is injected into the GC.

In this technique, it is not necessary to separate the alcohol from the blood. A container of blood is put in a sealed vial with a top that can accept a syringe. An airtight syringe is then inserted into the vial and a known volume of the air space above the blood is withdrawn. At a given temperature, the amount of alcohol in the air space will be in equilibrium with the alcohol concentration in the blood. This air is injected into the gas chromatograph along with an internal standard. Most laboratories that do a significant amount of blood alcohol analysis use a gas chromatograph that has an auto sampler, which is essentially a robot that samples the air space above the blood and injects it into the GC. Many vials can be preloaded and analyzed without operator intervention. Typically, the auto sampler will be loaded at the end of the work day and then will run all night. As data are generated, the concentration of alcohol in each blood sample is calculated automatically and printed out, awaiting further review and reports the next morning. A typical chromatogram of alcohol is shown in Figure 1. Besides GC there are also enzymatic methods for the analysis of alcohol. For example, the coenzyme nicotinamide adenine dinucleotide (NAD) will react with alcohol in the presence of alcohol dehydrogenase as a catalyst. The alcohol is con verted to acetaldehyde and the NAD is reduced to NADH. The reaction is monitored by UV/visible spectrophotometry at 340 nm where NADH absorbs, in order to measure the concentration of alcohol.

FIGURE 1 Gas chromatograms of ethyl alcohol. In the chromatograms, the propan-1-ol (isopropyl alcohol) and the t-butanol are internal standards. They are used to help determine the quantity of alcohol in the sample.