Spectrum search (library search) is a procedure often used in contaminant analysis. It consists of using the search function included in Fourier transform infrared (FTIR) software or specialized search software to compare the measured contaminant spectrum with spectra in a library installed on the hard drive. The comparison result is indicated with a score that represents the degree of similarity. Spectra with a high score are displayed in a list (commonly called the hit list). Spectrum search is an extremely convenient function because it allows in-depth searching of similar contaminant spectra among a great number of spectra contained in the library.
While researchers have relied on spectrum search for successful contaminant analysis, the procedure is not flawless. Although good results can be obtained when the substance in the library is the same as a contaminant, a spectra hit list is displayed even if that same substance is not in the library. Furthermore, the spectra in most commercial libraries have been obtained from analysis of pure samples of each substance, even though contaminants always occur in mixtures. Because it is only natural that a low score might be obtained when comparing a contaminant spectrum with a pure single-constituent spectrum, the contaminant principal constituent may not necessarily be ranked highest in the hit list even if it is in the library. As a result, it is necessary to conduct an in-depth search in the hit list of spectra that are similar to that of the contaminant.
In addition, information cannot be obtained without conducting other processing such as generating a difference spectrum, for example, in situations in which there are second and third subconstituents in the mixture that have a low possibility of appearing in the hit list together with the principal constituent. Thus, difficulty arises in conducting contaminant analysis when the correct answer is not obtained using a simple spectrum search.
A solution to this often cumbersome task can be found with the Contaminant Analysis Program (patent pending) (Shimadzu Scientific Instruments, Columbia, MD). The program combines spectrum search and peak matching to not only identify the principal constituent, but also to indicate the possible existence of subconstituents from very small characteristic peaks that also appear in the spectrum.
The Contaminant Analysis Program contains spectra for over 300 highly selected inorganic substances, organic substances, and polymers that are often detected as contaminants in Shimadzu’s Analytical Applications Department. It incorporates algorithms that focus on spectral characteristics, rather than performing simple spectrum searches, and allows the automation of the process, including searching, judgment, and report creation. Therefore, even analysts with limited experience in infrared spectroscopic analysis will find the tool useful in the most difficult aspect of spectral analysis: identification and qualitative analysis.
For example, when an analysis is conducted after the contaminant spectrum file and measurement method are specified, the results for the main constituent are shown in an upper field, and those for the subconstituents are displayed in a lower field. Identifying marks (+++) at the left of the constituent names indicate the analysis result, with the greater the number of + marks (maximum 3) indicating the highest match.
Figure 1 - Contamination Analysis Program screen.
Clicking on a substance name in a results field allows comparison with the contaminant spectrum. Figure 1 shows a screen of the overlay display of the principal contaminant spectrum with the spectra of the principal constituent and subconstituents, including polyvinyl chloride, calcium carbonate (CaCO3), and aluminum hydroxide (Al(OH)3). From these analytical results, it is probable that the contaminant consists of a mixture of these substances.
While the Contaminant Analysis Program cannot replace spectra libraries, because a list of common contaminants are compiled in one easy-to-use program, it is easier for analysts, from beginners to experienced personnel, to quickly perform identification checks of anything from raw materials to finished products. Equipped standard with Shimadzu IRsolution software, the program provides users with another tool to improve productivity and work efficiency.
Dr. Monti is Spectroscopy Product Manager, Shimadzu Scientific Instruments, 7102 Riverwood Dr., Columbia, MD 21046, U.S.A.; tel.: 800-477-1227; fax: 410-381-1222; e-mail: [email protected].