Advanced Analytical Tools Based on Optical Spectroscopy with Applications in the Biosciences and Petrochemistry

The development of modern quantitative analytical techniques based on optical spectroscopy is described with relevant applications in the biosciences and petrochemistry. Two Raman spectrometers were constructed with a red and green laser excitation source and a simple monochromator for detection. The systems are compact and portable. Surface-enhanced Raman spectroscopy (SERS) is introduced with silver nanoparticles (AgNP). Hydroxylamine hydrochloride reduction of AgNO3 obtained nanospheres with average diameter 50 nm. Reducing AgNO3 using hydroxylamine to obtain seed crystals and trisodium citrate to attach branches gave nanostars. The successful synthesis was confirmed by TEM images and UV/vis spectroscopy. AgNP SERS of aniline achieved Raman scattering enhancement factors of about 1000 for nanospheres and 10 for nanostars. SERS spectra of Gram-negative and positive bacteria and strains were obtained. The spectra may be used to confirm the presence of bacteria, but they are very similar making distinction of species difficult. SERS enhancement is dependent on aggregation of colloidal AgNP. NaCl and phosphate buffer were characterised as aggregation agents, with optimum concentrations of 150 mM NaCl or 100 mM phosphate buffer. SERS spectra and calibration curves were obtained for several purine degradation products. For adenine a limit of detection (3σ limit) of about 3 x 10-7 M with phosphate buffer or 2 x 10-8 M with NaCl is estimated. By comparing SERS of E. coli coincubated for 3 h in AgNP solution with spectra of the supernatant, it is shown that SERS arises from purine degradation pathways of E. coli starvation
during sample preparation, and are not directly from the bacteria. The metabolism of E. coli was studied where time dependent concentrations of cysteine as substrate are followed by SERS, and the product formation of H2S and ammonia by FTIR spectroscopy with home-built White cells. Ammonia from the
growth medium is distinguished after 15N isotopic labelling. An almost complete conversion of cysteine to H2S and ammonia within ca. 2 h was found. Analysis of
cysteine by colourimetry as an alternative to SERS failed due to interferences. In a final application of advanced optical spectroscopy in chemical analysis, toxic H2S was detected in natural gas samples using the 6 m White cell. Dominant CH4 in natural gas has to be removed by pumping off under liquid N2. FTIR analysis
allows the detection of ethane, propane and CO2, and trace levels of H2S down to a 1σ detection limit of 170 ppm in 1 bar