Gas Chromatography - Tandem Techniques - Examples of GC/MS Spectrometry Applications > Analysis of Anabolic Steroids in Biological Materials > Page 72

The detection limit of an ICP-MS is about 1 part in a trillion, but, as already stated, the device is rather inefficient in ion transport from the plasma to the analyzer. It is estimated that as only about 1% of the ions pass through the sample and skimming cones only a total of about 106 ions will eventually reach the detector. One reason for ion loss is the diverging nature of the beam, but a second is due to space charge effects, which, in simple terms, is the mutual repulsion of the positive ions away from each other.

Unfortunately, there are some disadvantages. To accommodate the high gas temperature and relatively high pressure, the necessary form of the interface is not very efficient and only about 1% of the ions that pass the sample orifice pass through the skimmer orifice. In addition, some molecular ion formation does occur in the plasma, the most troublesome being molecular ions formed with oxygen. These molecular ions can only be reduced by changing the cone position so that only those portions of the plasma where the oxygen population is low, are sampled.

 

The detection limit of an ICP-MS is about 1 part in a trillion, but, as already stated, the device is rather inefficient in ion transport from the plasma to the analyzer. It is estimated that as only about 1% of the ions pass through the sample and skimming cones only a total of about 106 ions will eventually reach the detector. One reason for ion loss is the diverging nature of the beam, but a second is due to space charge effects, which, in simple terms, is the mutual repulsion of the positive ions away from each other.

 

Examples of GC/MS Spectrometry Applications

 

The technical literature has many example of the use of GC/MS for a wide variety of applications that have accumulated over nearly 50 years. GC/MS has been used extensively for forensic work, pollution studies quality control purposes and widely in all aspects of fundamental research. It is impossible to deal systematically with GC/MS applications in a simple monograph. Consequently, a few selected examples will be given to illustrate the versatiity of the technique and the more popular fields of application.

 

Analysis of Anabolic Steroids in Biological Materials

An example of the use of the GC/MS system in metabolite studies is afforded by the work of Schoene et al. (19) who employed a GC/MS tandem instrument to examine the metabolism of 17a-alkyl anabolic steroids in horses after oral administration of the drugs. 100 mg of 17a-methyl testosterone together with the deuterated analog were fed to two horses via their normal food diet. Naturally voided urine was collected over 72 hours, 5 ml samples were extracted with ether and the solvent removed in vacuo to give the free urine fraction. The pH of the urine was adjusted to 6.8 and then heated with E. Coli (2000 U) at 50ūC for three hours to hydrolyze the glucuronide conjugates. A solid phase extraction cartridge, Sep-Pak C18 was pretreated with water and n-hexane before applying the sample.