The GC device is generally a reliable analytical instrument. The GC instrument is effective in separating compounds into their various components. However, the GC instrument can not be used for reliable identification of specific substances. The MS instrument provides specific results but produces uncertain qualitative results. When an analyst uses the GC instrument to separate compounds before analysis with an MS instrument, a complementary relationship exists. The technician has access to both the retention times and mass spectral data. Many scientists consider GC/MS analysis as a tool for conclusive proof of identity. i
GC/MS analysis, where the effluent to the GC instrument is the feed to the MS instrument, is in wide use for confirmation testing of substances. Drug testing, manufacturing quality control, and environmental testing are some typical uses.
Although many consider GC/MS to be the "gold standard" in scientific analysis, GC/MS does have some limitations. Because great faith is maintained in GC/MS analysis, erroneous results are not expected and hard to dispute. However, false positives and false negatives are possible.
Some problems with GC/MS originate in improper conditions in the GC portion of the analysis. If the GC instrument does not separate the specimen's compounds completely, the MS feed is impure. This usually results in background "noise" in the mass spectrum. If the carrier gas in the GC process is not correctly deflected from entering the MS instrument, similar contamination may occur.
Also, the MS portion suffers from the inexact practice of interpreting mass spectra. An analyst must correlate computer calculations with system conditions. The typical memory bank for MS identification contains about 5000 spectra for a particular group of compounds. ii Even if a competent analyst could find conclusive results pointing to one substance out of 5000 substances, this does not rule out the remaining over 200,000 known existing chemicals. iii For the 5000-spectra memory bank, the typical computer result is limited to as many as six possible identifications. iv
In one instance, erroneous GC/MS results may have been responsible for a criminal defendant receiving a death sentence. John Brown killed a police officer and wounded two bar patrons in a shoot-out on June 7, 1980 in Garden Grove, California. v Mr. Brown's diminished capacity defense to capital murder relied on the assertion that Mr. Brown was under the influence of narcotics at the time of the shooting. vi The prosecution introduced GC/MS evidence that showed Mr. Brown's blood to be free of narcotics. vii The California Supreme Court overturned the jury's death sentence because the prosecution never introduced evidence from a radioactive immunoassay ("RIA") test that detected phencyclidine (PCP) in Mr. Brown's blood. viii Obviously, an example like this demonstrates that analytical evidence, including GC/MS, should always be confirmed with another reliable technique.
A more advanced analytical method is MS/MS, a tandem series of instruments, which has the advantage of increased sensitivity. ix One court states that MS/MS analysis has never produced a false positive in the FBI laboratory. x However, MS/MS is not widely used yet as the instrument's cost is prohibitive. xi
i Bruce Stein et al., An Evaluation of Drug Testing Procedures Used by Forensic Laboratories and the Qualifications of Their Analysts, in Scientific and Expert Evidence 433, 483 (Edward J. Imwinkelried ed., 2d ed. 1981).
ii James W. Robinson, Undergraduate Instrumental Analysis, 742 (5th ed. rev. & expanded 1995).
v San Diego Daily Transcript Website http://www.sddt.com/files/library/98/04/02/tbb.html accessed April 28, 1999.
viii In Re Brown, 14 Cal. 4th 873, 876-77 (1998).
ix U.S. v. Bush, No. FR247-23-7903, at http://www.armfor.uscourts.gov/opinions/1996Term/96- 1239app.htm accessed on April 28, 1999 (USAF Trial Judiciary, Eastern Circuit 21 DEC 1994).