490 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



Tests for Blood in Stains, etc. 



(1) Peroxidase Reactions. 



The stain is dissolved and tested with guaiacum, benzidine and 

 other reagents. The solution should be boiled so as to exclude the 

 presence of peroxidase and it should be remembered that other com- 

 pounds such as halogens give the reaction. 



This test serves chiefly as a guide. 



(2) Spectroscopic Examination. 



The absorption spectra of haemoglobin and its derivatives afford 

 the best means of detecting blood in the form of spots, stains, etc. 



The spot is scraped off the material after its position has been 

 carefully noted. The material, or if on cloth a piece of the cloth, is 

 soaked in a small quantity of water or '9 per cent, sodium chloride. 

 If the solution is coloured it is examined with a spectroscope, small 

 amounts with a micro-spectroscope, i.e. a spectroscope attached to a 

 microscope. 



If the solution is not coloured or coloured too little to show 

 absorption bands, the material is warmed with a little caustic soda 

 solution. Haematin is formed and shown to be present by conversion 

 by reduction with ammonium sulphide to hremochromogen. Both 

 absorption bands maybe seen, but generally only one ; this band is 

 visible even in very dilute solution. 



(3) Teichmanris Test. Formation of Hcemin. 



This test depends upon the formation of haemin crystals and is 

 carried out in the same way as with a small drop of blood. The blood 

 stain is scraped up and treated with acetic acid as on p. 484. 



Stains on iron work are said not to give the crystals. 



A stain on cloth which cannot be scraped off is dissolved in glacial 

 acetic acid. If the stain be old, it is necessary to add a crystal of 

 sodium chloride, as the chlorides may have been dissolved out. 



Dilute solutions of blood are precipitated by acidifying with acetic 

 acid and adding tannic acid (freshly prepared). The dried precipitate is 

 heated on a slide with a trace of sodium chloride and glacial acetic acid. 



Various modifications have been suggested for this method. 1 



1 See Biochem. J., Vols. VII. and V1I1. 



