Controlled pore glass and polystyrene resins are available commercially (with aminoalkyl groups attached) with loadings of 20—100 micromoles per gram. These resins are suitable for oligonucleotide synthesis provided that they have pores of 500 Å or more in diameter to allow the necessary reagents to diffuse, and the individual particles are more than 500 Å (50 nm) in diameter. Smaller particles will not permit rapid flow of solvents and reagents through the synthesis column and can block filters. The nucleoside attachment chemistry is shown in Figure 24 . Four separate resins are needed for general oligonucleotide synthesis − one functionalized with each base (dA, dG, dC and dT).
AsH 3 is also well known in forensic science because it is a chemical intermediate in the detection of arsenic poisoning. The old (but extremely sensitive) Marsh test generates AsH 3 in the presence of arsenic.  This procedure, published in 1836 by James Marsh ,  is based upon treating an As-containing sample of a victim's body (typically the stomach contents) with As-free zinc and dilute sulfuric acid : if the sample contains arsenic, gaseous arsine will form. The gas is swept into a glass tube and decomposed by means of heating around 250–300 °C. The presence of As is indicated by formation of a deposit in the heated part of the equipment. On the other hand, the appearance of a black mirror deposit in the cool part of the equipment indicates the presence of antimony (the highly unstable SbH 3 decomposes even at low temperatures).