Instrumentation on diode laser atomic fluorescence spectrometry

A diode laser based atomic fluorescence spectrometer for determination of a range of elements has been developed. The initial setup comprised a xenon lamp (instead of diode laser), optics, lab made electrical furnace (400-700 oC) with an igniter ( 1200 oC), lab made chopper, PMT and pre-amplifier, SR510 lock-in amplifier and a lab made acquisition software written in C# language[1]. Various electronic controllers were designed and implemented to control each part individually and a central controller was implemented to control the whole system through the acquisition software[2]. The initial investigation revealed the capability of the system for analyzing hydride forming elements including: Sb, Bi, Hg, Ge, Sn and Te by utilizing interference filters to isolate the appropriate analytical wavelengths. The hydride generation setup and parts were totally designed and implemented in both batch and continuous mode and an electronic interface was designed to incorporate controlling of the setup to central controller[3]. A total consumption burner was used for atomization of non-hydride elements. In next step the radiation source was replaced by a lab-made diode laser system. Three diode laser chips at 780 nm, 850 nm and 405 nm were prepared. The diode laser system including electronics and hardware were designed and implemented targeting current and temperature stabilization and adjustment of the chips. This ensured fine tuning of output wavelength of the diode laser. Furthermore, stabilization on modulated output in frequency range of 100 Hz to 100 kHz is obtained with phase noise less than 0.01o rms at 1 kHz, 100 msec and 12 db TC condition. The output wavelengths were tuned at 780.02 nm, 852.11 nm and 403.4 nm for analyzing Rb, Cs, and Mn respectively. Utilizing the diode laser in modulation mode omitted the optical isolation of wavelength and a dispersive system was obtained. Samples were treated in hydride generator setup and the output vapor was carried to atomization furnace/burner by a purge of high purity argon gas. The furnace ignites the hydrogen of hydride generation process and a mini flame is established as atomization zone. The mini burner, on the other hand, used an optimized ratio of hydrogen-air-argon from separate reservoirs to form a flame as atomization zone. The excitation is followed by the diode laser/xenon lamp radiation across atomization zone and the fluorescence signal is detected via detection system. The detection system included a Hamamatsu photomultiplier tube driven by a Thorlab high-voltage and a pre amplifier board. The background signals and noises are eliminated using the lock-in amplifier. The lock-in amplifier s reference is its internal oscillator for diode laser case and is chopper TTL pulse for xenon lamp case. The output signal carried on the locked frequency is digitized inside the lock-in amplifier and acquired by the acquisition software. The proposed setup is far simple rather than atomic absorption spectrometers due to high intensity radiation of diode laser and purity of the spectrum in which eliminates the need for various optics and monochromator. The detection limits for hydride forming elements are in 0.3-7 ppb range and those for non-hydrides are in 1.5-10 ppm range which are comparable to methods used in atomic absorption spectrometry.