Miniaturization of breath sampling with silicon chip: application to volatile tobacco markers tracking

Abstract

This work presents the performances of silicon micro-preconcentrators chips for breath sampling. The silicon chips were coupled to a handheld battery powered system for breath sampling and direct injection in a laboratory gas chromatography mass spectrometry system through thermal desorption (TD). Performances of micro-preconcentrators were first compared to commercial TD for benzene trapping. Similar chromatographic peaks after gas chromatographic separation were observed while the volume of sample needed was reduced by a factor of 5. Repeatability and day to day variability of the micro-preconcentrators were then studied for a 500 ppb synthetic model mixture injected three times a day four days in a row: 8% and 12% were measured respectively. Micro-preconcentrator to micro-preconcentrator variability was not significant compared to day to day variability. In addition, micro-preconcentrators were tested for breath samples collected in Tedlar bags. Three analyses of the same breath sample displayed relative standard deviations values below 16% for eight of the ten most intense peaks. Finally, the performances of micro-preconcentrators for breath sampling on a single expiration were illustrated with the example of volatile tobacco markers tracking. The signals of three smoking markers in breath, benzene, 2,5-dimethylfuran, and toluene were studied. Concentrations of benzene and toluene were found to be 10 to 100 higher in the breath of smokers. 2,5-dimethylfuran was only found in the breath of smokers. The elimination kinetics of the markers were followed as well during 4 h: a fast decrease of the signal of the three markers in breath was observed 20 min after smoking in good agreement with what is described in the literature. Those results demonstrate the efficiency of silicon chips for breath sampling, compared to the state of the art techniques. Thanks to miniaturization and lower sample volumes needed, micro-preconcentrators could be in the future a key technology towards portable breath sampling and analysis.

Recent developments and trends in miniaturized gas preconcentrators for portable gas chromatography systems: A review

“…In the last decades, MEMS technology has been widely used for the fabrication of PCs [1,5,20,26,27,[33][34][35][36][37][38][39][40]; therefore, most of these devices are made of materials related to the electronics industry. So far, silicon has been the most employed material due to its relatively high thermal conductivity and the variety of existing micromachining techniques.…”

“…Once the cavities are etched, the substrate is bonded to another plate usually made of glass resulting in a sealed chip. Anodic bonding is the most common technique for silicon-glass bonding [6][7][8]19,34,36,37,[51][52][53]. In this process, a DC voltage ranging from 500 to 2000 V is applied between the substrates whereas they are heated to temperatures varying from 300 to 600 °C [48].…”

“…These systems consist of layers of resistive and conductive metals to form the resistive heaters and the contact pads, respectively. Techniques such as e-beam evaporation [38] and sputtering [27,29] are commonly used for metal deposition of a wide range of metals. Among them, platinum is the most used material in micro PC's heating systems [1,5,24,27,[68][69][70].…”

“…Techniques such as e-beam evaporation [38] and sputtering [27,29] are commonly used for metal deposition of a wide range of metals. Among them, platinum is the most used material in micro PC's heating systems [1,5,24,27,[68][69][70]. A resistive heater made of platinum is presented in Figure 6b.…”

“…Similarly to Ni-Cr, platinum has a relatively high resistivity and constant TCR, being suitable to act both as a heater and a temperature sensor. Heating rates ranging from 3.5 to 375 °C s -1 and power consumptions between 1 and 10.5 W have been reported in micro preconcentrators with platinum heaters [20,26,27,33,34,36,40]. Although this metal presents better adherence to substrates than other metals, an adhesive layer of titanium [24,27,33,68] or chrome [40] is generally deposited beforehand to improve its adhesion to the substrate.…”