Characterizing and Classifying Water-Based Lubricants Using Direct Analysis in Real time®–time of Flight Mass Spectrometry
Emily C. Lennert
Keywords: lubricants, sexual assault, condom, personal lubricant, water-based, DART-MS, mass spectrometry, high resolution
Article to be reviewed:
1. Maric, M.; Bridge, C. “Characterizing and classifying water-based lubricants using direct analysis in real time®–time of flight mass spectrometry.” Forensic Science International. 2016, 266, 73–79.
Disclaimer: The opinions expressed in this review are an interpretation of the research presented in the article. These opinions are those of the summation author and do not necessarily represent the position of the University of Central Florida or of the authors of the original article.
Sexual assault is a common crime, the authors of this study cite a survey conducted by the National Center for Injury Prevention and Control, which stated that approximately 1 in 5 women and 1 in 71 men in the U.S.A. will experience a sexual assault in their lifetime. In addition to the frequency of sexual assault, condom use has become more common in assaults as perpetrators attempt to prevent the transfer of biological evidence to their victim or to nearby items, such as clothing. In these cases especially, where biological evidence cannot be recovered, sexual lubricant evidence must be focused on to link a suspect to the victim or crime scene. Characterization of lubricants allows an investigator to examine an unknown lubricant sample and identify a smaller group of lubricants that the unknown sample likely came from, narrowing the scope of the investigation. Lubricants typically belong to one of two categories: condom lubricants or bottled personal lubricants. While relatively few studies have been conducted in lubricant analysis, even fewer have been focused on bottled personal lubricants. Four main categories exist for bottled personal lubricants: water based, silicone based, oil/petroleum based, and organic/edible lubricants. Previous research has focused primarily on classifying an unknown lubricant into one of these four categories, based on major ingredients, or components. This study focused on the characterization of water based, bottled personal lubricants, to create smaller groupings, or classes, based on both major and minor components.
Water based lubricants may share similar major components: glycerol, poly(ethylene glycol), methyl/propyl paraben, water, carboxymethyl cellulose, and propylene glycol. However, minor components can vary and may also serve as differentiating factors that allow for classification of these water based lubricants. Some components serve specific purposes, such as anesthetics, i.e. numbing agents (lidocaine and benzocaine), warming sensation (capsaicin), cooling sensation (menthol), and flavor/scent (maltol and ethyl maltol). These components have not been used to characterize lubricants prior to this research. Current techniques in lubricant analysis, e.g. gas chromatography-mass spectrometry (GC-MS) or Fourier transform infrared spectroscopy (FTIR), have been used to identify major components for lubricant analysis. However, these techniques fall short in detecting minor components, which are difficult to isolate and identify by these methods due to the high concentration of the major components. The authors state that the characterization of the personal lubricant samples into sub-groupings based on chemical composition, particularly including minor components, would be applicable to characterization of unknown lubricants from sexual assault cases, which would provide a more specific determination of the unknown lubricant’s possible identity than current classification methods.
Direct analysis in real time®-time of flight-mass spectrometry (DART®-TOFMS) was used in this research to address the issue of more detailed characterization using minor components. DART®-TOFMS allows for rapid analysis of small amounts of sample with little to no sample preparation, and allows for high resolution of peaks, accurate mass measurements of trace, or small, amounts of a compound within the sample. With the advantages of DART®-TOFMS in mind, the authors of this study set out to develop an analytical method for characterization of personal lubricants by DART®-TOFMS. The DART® ion source works by producing excited state atoms or molecules, known as metastables. When the sample is introduced in the sample gap, the metastables interact with the sample and produce an ionized, or charged, sample. (See Fig. 1) The charge of the ionized sample, positive or negative, will depend on the ionization mode selected. After the sample is ionized, the sample travels through the inlet and into the mass spectrometer.
Figure 1. The DART® ion source. Only metastables leave the ion source to interact with the sample prior to traveling into the mass spectrometer. (Image source: http://www.jeolusa.com/PRODUCTS/Mass- Spectrometers/AccuTOF-DART/AccuTOF-DART-Technology)
Figure 2. The DART® ion source and mass spectrometer inlet.
In this study, 33 water based personal lubricants were analyzed by DART®-TOFMS. Analysis was performed in positive and negative mode. Calibration standards were used to ensure that mass measurements were accurate. To perform the sample analysis, the closed end of a small glass tube, a capillary tube, was dipped into the sample then placed in the sample gap and waved in front of the ion source. Each sample was analyzed five times, and each analysis contained 5 replicates to allow for averaging of each analytical run. After samples were analyzed, mass data was processed and exported for statistical analysis and characterization.
Processing and analysis of mass data for each lubricant revealed that results obtained by positive ionization mode contained more significant compounds than mass data obtained from negative ionization mode. Therefore, statistical analysis was performed on positive ionization mode results primarily. Sub-groups were identified using principal component analysis (PCA). PCA minimizes similarities within the data in order to maximize the differences of the peaks observed and their intensities within the group. The goal is that samples with the same differences will group together, thus providing trend information about those groups and subsequently give characterization information for that group. Based on PCA, six groups were created. A summary of the grouping of the samples can be seen in Table 4 within the study. Groups were primarily based on the presence or absence of only seven compounds: glycerol, ethoxydiglycol, phenoxyethanol, propylene glycol, butylene glycol, lidocaine, and benzocaine. To verify and test the classification system created during PCA, the authors utilized linear discriminant analysis (LDA). LDA showed clear distinction between groupings, and that each sample would be classified into 1 of the 6 groups, correctly. Thus giving 100% classification accuracy. Based on the results of the PCA groupings and LDA classification testing, the authors conclude that DART®-TOFMS is a viable option for classification of lubricants based on major and minor components. The authors theorize that groups will expand as the sample set expands, and additional groups may be created as additional data is analyzed.
- Traditionally, lubricant analysis has only been able to differentiate between major components: water based, silicone based, oil/petroleum based, and organic/edible.
- DART®-TOFMS has the capability to detect minor, i.e. trace, components within a sample.
- Larger groupings of lubricants, e.g. water based, can be broken down into sub-groupings, enhancing theevidential value of lubricant analysis.
- The authors of this study created and verified a classification scheme for water based lubricants by DART®-TOFMS.Relevance: The research presented in this study sets the framework for further research and application of lubricant analysis for forensic casework.Potential conclusions:
- DART®-TOFMS is an instrument that allows for efficient and accurate analysis of trace evidence.
- DART®-TOFMS is an ideal method for lubricant analysis based on minor components.
- Classification of water based lubricants based on DART®-TOFMS data is accurate and the groupings are distinct.