# Online Determination of Boron Isotope Ratio in Boron Trifluoride by Infrared Spectroscopy
## 1 Introduction  
Boron in naturally occurring compounds is composed of two isotopes, one of mass 10 and the other of mass 11. \({}^{10}\)B at a relative abundance of 18.8%, has an unusually large cross section for the capture of low energy neutrons [1]. Therefore, in high concentrations, it is particularly valuable for neutron shielding [2, 3].  
The design and realization of the isotopes separation processes of boron began as early as the Second World War. A large industrial plant with a productivity of up to 300 kg per year of elemental boron with 95% \({}^{10}\)B content was put into operation in 1944 [4]. At present, the most common methods for the separation of boron isotopes include the distillation method [5], chemical exchange rectification [6, 7], ion exchange [8, 9], and laser isotope separation [10, 11], among others. Chemical exchange rectification is the simplest and most productive method for the separation of \({}^{10}\)B and \({}^{11}\)B isotopes at concentrations of 95% and over [12]. Figure 1 shows the principal scheme of the apparatus for the separation of boron isotopes using the chemical exchange rectification method. \({}^{10}\)B and \({}^{11}\)B can be extracted from the separation and the complex parts of the apparatus, respectively [13]. The internal variations in this separation process tend to be very complex, making it necessary to frequently track any changes in the isotope ratio obtained.  
The mass spectrometry (MS) method is one of the most important ways of analyzing the boron isotope ratio. Analysis comes from the peaks with mass-to-charge ratios (m/e\({}^{+}\)) equal to 10 and 11 [14]. [PERSON] [15] et al. determined the isotopic ratio of boron in samples of groundwater by inductively coupled plasma mass spectrometry (ICP-MS) to evaluate possible levels of boron pollution from anthropogenic inputs into natural aqueous systems. The matrix effect was reduced by the preconcentration and ion-exchange of the sample. Compared with the standard material, the precision of the method achieved 0.13% (natural abundance). However, the sampling and preparation for this method are very complex, so it is difficult to achieve real-time tracking for this process during actual production. Infrared spectroscopy (IR) is a traditional technology that has been used for on-line detection. For instance, [PERSON] [16] et al. established a method using online FTIR spectroscopy to determine the siloxane content in bio-gas. Both the precision and the detection limit for siloxane were satisfactory when using this online FTIR technology.  
The vibrational spectra of BF\({}_{3}\) has been studied by [PERSON] [17; 18] et al.and [PERSON] [19] et al. The results were obtained by dissolving BF\({}_{3}\) in liquid Argon. The infrared absorption bands changed depending on the change in the boron isotope ratio. Thus, the bands could be used to measure the boron isotopic ratio in BF\({}_{3}\). Despite having a good signal-noise ratio in the \(\
u_{3}\) region, such a high dilution factor was difficult to realize via changing optical paths in the industrialized process. Although the 2\(\
u_{3}\) is a weak IR absorption band for BF\({}_{3}\), the difference in absorbance peaks was about 112 cm\({}^{-1}\) between \({}^{10}\)BF\({}_{3}\) and \({}^{11}\)BF\({}_{3}\) at the 2\(\
u_{3}\) bands, which means that there is good resolution for this measurement. In this paper, the IR method was applied to determine the boron isotope ratio based on calculating the BF\({}_{3}\) infrared spectrum at 2\(\
u_{3}\). Further, an infrared spectrophotometer was installed in a boron separation device to monitor the separation process online.