Shortly after the development of the mass spectrograph it was discovered that bromine consists of two stable isotopes (Aston 1920a, 1920b). Later it was confirmed that the two isotopes have abundances which are bout equal (Aston 1931). More precise measurements on the ratio between the two stable bromine isotopes were done by Blewett (1936) who was able to analyse variations with a precision of about 25‰. After the development of a new type of mass spectrometers by Nier (1940), Williams and Yuster (1946) were able to measure the bromine isotope composition with a precision of about 4‰. The first negative ion thermal ionization mass spectrometry study (Cameron and Lippert 1955), in which attempts were made to observe natural bromine variations with a precision of (also) approximately 4‰, did not find natural bromine isotope variations. Techniques improved to about a precision of 1.8‰ in the early 1960s (Catanzaro et al. 1964), while the first study showing that very precise measurements are possible was published in 1993 (Xiao et al. 1993), using positive ion thermal ionization mass spectrometry on the Cs2Br+ ion. Since Willey and Taylor (1978), who were the first to test bromomethane as mass spectrometer gas, it is expected that this may be the most promising gas for gas source isotope ratio mass spectrometry (both for dual inlet and continuous flow techniques) and it was using this gas that the first natural variations were finally determined by Eggenkamp and Coleman (2000). The techniques that were used by Eggenkamp and Coleman (2000) in their first study to determine natural bromine isotope variations are also described by Eggenkamp (2004) in De Groot (2004). They are repeated here due to the importance of these methods for the development of bromine isotope analysis techniques. Especially the method to separate bromide from a large excess of chloride (Dechan 1886; Friedheim and Meyer 1892; Boeke 1908) is still almost universally used in laboratories that study bromine isotope variations in groundwaters.