GC× GC, excelling in its separation ability, is generally applies for compounds with complicated components, and often allocated into the separation of group composition and target compounds^[3]. The separation of target compound is to isolate the target component from other ones. However, the group separation is to extract the components with same characteristics (molecular structure and functional group) from the other components with different characteristics^[4]. The qualitative method of GC× GC is almost same as the one of one-dimensional GC, completing to match between unknown compound and known samples, as performed by GC/MS. The quantitative analysis of GC× GC is also similar to GC, conducting through external and internal tagging and normalization treatment to obtain the calculation of the width, the area of total peak and its volume^[5-^6].

GC× GC possesses obvious advantages over the traditional separation choices. First, good separation ability. The peak volume of traditional GC is from only one chromatographic column, whereas GC× GC makes a direct connection of two chromatographic columns, making its peak volume the product of two chromatographic columns’ so that the separating ability increases tremendously. It was recently found that the peak volume of GC× GC is 10 times of traditional GC as indicated in the comparison between them under same conditions^[7]. Second, high sensitivity and resolution. For trace substance, the sensitivity and resolution are crucial of an instrument. A sensitivity comparison has been made between the traditional GC and GC× GC, showing that the sensitivity of GC× GC is 20 to 50 times higher than the traditional one^[8]. Third, accurateness. Complicated compound is difficult to separate with traditional ways as the peak easily becomes bumpy. However, the huge peak volume of GC× GC may easily separate the complicated peaks, achieving a clear pattern and better accuracy. Fourth, high separating rate. As mentioned above that the peak volume of GC× GC is 10 times larger than the traditional one, hence the traditional GC, if gained the same effect as GC× GC’ s, should have had its chromatographic column 100 times longer, column pressure 10 times higher and analysis time 1000 times lengthier than the current^[7].

Ignitable liquid is indispensable in arson cases, usually divided into five types by American society for testing and materials (ASTM): petroleum distillate (the 1st level), gasoline (the 2nd level), intermediate distillates (the 3rd level), kerosene (the 4th level), and the 5th level that includes heavy distillate, diesel and domestic heating oil. Besides, the 0 level consists of some other ignitable liquids, and those used as triggers because they contain oxygen, isoparaffin, naphthenic acid, or aromatic solvent^[9]. The common means to test ignitable liquid are: UV spectroscopy, TLC, GC, HPLC, GC/MS and IR spectroscopy. Among them, the most commonly used are GC, HPLC and GC/MS. GC has strong separation yet poor qualitative ability; GC/MS has its limitations for detecting the more complex organic compounds or volatile substances of high boiling point; and for complex or volatile substances, HPLC is a good choice though GC can be still used to test the mildly volatile substances such as gasoline, diesel oil.

GC× GC, outperforming the above instruments, has high peak capacity, strong separation and good qualitative ability. Using GC× GC, Glenn S. Frysinger^[10] identified the band-different products that burned from paint diluent, turpentine and fuel oil on various objects. And then he distinguished kinds of flammable liquid with slight differences in chemical components out of the burning products. Gasoline is the most common incendiary agent, accounting for 90% of the arson cases^[11]. Yet few stories were covered of gasoline tested by GC× GC until Frysinger^[12] had tested the quantity of component from gasoline. Petroleum fraction in the medium boiling range plus the symbolic components of gasoline such as Benzene, toluene, xylene, trimethylbenzene, tetrabenzenes and other aromatic hydrocarbons, were the main separation research targets for some time. Bruckner and colleagues have applied GC× GC into separation among a mixture of toluene, ethylbenzene, xylene and propylene in gasoline^[13]. Moreover, Frysinger’ s team realized not only the separation among benzene, ethylbenzene, xylene and aromatic hydrocarbons but also the quantification among them^[12].

Biodiesel is an environment-friendly bio-fuel material by vegetable oil and/or waste oil^[14]. Maria Silvana A and colleagues conducted feature analysis upon biodiesel and general diesel along with their compounds through GC× GC, establishing a qualitative and quantitative method, thereby conducting a comparison between GC× GC-FID and GC× GC/TOFMS^[15]. They concluded that GC× GC/TOFMS possesses higher selectivity and sensitivity at its price being taken no account. In China, Lu Xin’ s team analyzed the hydrocarbon components in diesel fraction, studied the composition distribution of nitrogenous compounds and sulfur-containing compounds^[16], and conducted analysis on origin place of different diesel samples so that a quantitative identification had been established for non-aromatic and aromatic compounds. A relative comprehensive method of analyzing diesel is believed as the comparison between quantitative identification and ASTM D2425.

Poisoning cases generally happened in remote and underdeveloped regions in China. Recently, the rate of these cases has been declining due to the control of poisons and improvement of people’ s legal awareness. However, such cases always draw very bad influence once happening with the common toxic as pesticide and medicine (with excessive consumption or not semeiological). LC/MS and GC/MS are common to test poisons. But LC/MS has not yet set up general standards in terms of mobile phase composition, chromatographic column properties, ionization voltages and other specific technical parameters, nor has a standard mass spectrogram database been established. Therefore, it is difficult for unknown toxicants to be qualitatively screened. GC/MS is mainly unable to analyze the non-volatile, polar and thermally unstable compounds. GC× GC should be a good solution to the above problems once widely used.

Professor Shin Miin Song^[17] conducted separation into compounds with 78 medicine samples through GC× GC, along with the qualitative analysis by FID and TOF-MS. GC× GC is indeed more suitable in forensic medical identification based on a comparison between GC× GC and traditional GC/MS. Although presently few reports of GC× GC were found in forensic investigation because of rare toxic cases happened in China, there are plenty of researches to be conducted in the environment protection, agriculture, forestry and medicine. For instance, Jiang Jun’ s group analyzed 64-pesticide residues in vegetables through GC× GC/TOF-MS, providing limits of detection (LODs) and limits of quantification (LOQs) by optimization of their experimental conditions.

Drug controlling has been becoming a key task as drug abuse can lead to infinite harm. Common drugs include crystal meth, ecstasy, ketamine and heroin. GC× GC has been gradually being used for drug identification, with cannabis and its metabolite also being added into test generally. Among the commonly used methods of testing drugs, spectroscopy requires a high purity of the drug, so the spectral analysis has been limited in testing mixed drugs. GC/MS or HPLC/MS, also the common-used test means, is difficult to attain accurate attribution when the sample is too complicated. GC× GC can be used to achieve separation of samples, accurate characterization, so as to have laid the foundation for the traceability of the sample.

Professor R Andrews^[19] analyzed metabolite of THC, CBD, CBN and THC-COOH in human blood by GC× GC, generating linearity between concentration of metabolite and time, and successfully having his method applied into time confirming in 54 autopsy reports. GC× GC can qualified THC-COOH in hair after smoke of marijuana, with LOD of 0.05 pg/mg, reaching the level in Federal Register’ s^[20]. Professor Rodger D. Scurlock and his fellows optimized GC× GC, raising the extract rate of THC and THCA through solid-extraction^[21]. Professor Blagoj Mitrevski’ s team ^[22]identified volatile organic compounds in 24 ecstasies, conducted clustering analysis though analyzing main components, and identified its source country. Safrole, the by-product of sassafras oil, is generally used as precursors of MDMA production. Sassafras is quite common in American. Yet only pure and minor compounds in specific essential oil could be identified through traditional GC. Professor M Schä ffer conducted quality and quantity analysis to safrole through GC× GC, and testified some impurities by comparison with standard substances. The impurities can identify the source of safrole, therefore GC× GC can be considered as a common and available method in forensic science^[23].

Explosion would severely threaten the social stability, hence identifying the explosion source is the priority for public security, generally being realized through the determination of volatile marker from it. There are many ways to test explosives. Among them, the terahertz spectroscopy, a cutting-edge technology, is still in the laboratory trial stage; the Raman spectrum is weak and susceptible to fluorescence interference; and a variety of new hand-held sensor technologies can though achieve quick on-site inspection, yet such devices have poor mechanical strength and weak resistance against impact; GC/MS is currently the most commonly-used testing technique, but is still limited by the peak capacity. GC× GC assembles the advantages of GC, able to solve the problems.

Professor Pierre-Hugues Stefanuto and colleagues^[24] identified PETN, TNT and RDX to analyze the retention time of GC× GC under the direct injection of SPME and liquid, and they also conducted positive and negative comparison to verify the reproducibility of the method, concluding that GC× GC can tentatively identify nitrocellulose explosives. Furthermore, Professor Pierre-Hugues Stefanuto developed and evaluated fast chromatograph to improve the volatile characteristics of explosive samples by conducting headspace sampling on SPME and then analyzing through GC and GC× GC. Fast chromatograph will lower eluting temperature of each analyte, avoiding thermal degradation of sensitive compounds (e.g. nitrocellulose explosives). Fast GC× GC, based on modulator’ s cryogenic focusing effect, will lower the LOD, optimize explosive sample extraction and chromatographic separation, capable of being applied in testing commercial explosive samples. GC× GC was proved to be voluble in anticrime by safely obtaining evidence.