The liquid sample is one of the most common types of samples submitted for analysis and comprises a vast array of pure compounds and solutions. As an example, several liquids are used in an automobile to sustain critical elements such as lubricating the engine or drive train and to supply mundane functions like washing the windows or generating electrical power.
This paper will describe the analysis of several solutions used in automobiles.
The traditional infrared analysis method for fluid samples is to collect an infrared transmittance spectrum of a thin film of the liquid contained between the windows of an infrared liquid cell. Not all liquids, however, can be analyzed with infrared spectroscopy in this manner. For samples that are aqueous, viscous or chemically reactive, an infrared liquid cell is cumbersome and labor intensive. Frequently, special windows with a reduced spectral range must be used and the cell can be difficult to keep clean to prevent cross contamination.
Infrared analysis using Attenuated Total Reflectance (ATR) accessories like the Golden Gate™ single-reflection micro-ATR require no sample preparation and greatly simplify the collection of FT-IR spectra.
The liquid sample is simply placed onto the ATR crystal and the sample spectrum is collected. A volatiles cover supplied with the Golden Gate™ accessory can be used to cover the sample to prevent evaporation during analysis. The sample is then cleaned from the crystal surface and the accessory is ready to collect additional spectra. ATR analysis methods are less complicated than using infrared liquid cells, are fast and a very small amount of the sample is needed.
The resulting data can be searched against a digital database of ATR spectra for positive identification. Despite changes in the relative peak intensity of the absorption bands, due to the internal reflection mechanism of ATR accessories1, spectra can also be compared to transmission data. As an example, Figure 1 is a plot of the transmission and ATR spectra of methanol.
Experiment and Results
Spectra were collected using a Golden Gate™ micro-ATR accessory equipped with ZnSe lenses (KRS-5 lenses are available to increase the spectral range) and a single-reflection diamond ATR element. Sample volumes of 20 microliters were pipetted onto the ATR crystal surface and the spectra collected. No sample preparation was necessary to obtain the various spectra and the liquid sample is simply wiped from the crystal surface after the data collection. If necessary, a solvent that will remove the sample is used to clean the diamond crystal surface.