Quantitative Headspace Analysis of Selected Odorants from Latrines in Africa and India

Abstract

This analytical investigation focuses on the quantification of odorant molecules in the headspace of latrines. Hydrogen sulfide and methyl mercaptan were derivatized under a more stable N-ethyl maleimide conjugate. Since the amount of odorant molecules is very low in the gas phase, we developed a method that had two steps of concentration. The first step consisted of the accumulation of volatiles in buffered water by bubbling 350 L of air in a bottle. The second step consisted of loading the water on a 1 g solid-phase extraction cartridge, shipping it to our laboratories, and desorbing with Et2O, which achieved a total concentration factor of 3.5 × 10(6). The acidification of the water phase gave us access to the acids, and an additional bottle containing an acidic ion-exchange resin gave us access to trimethyl amine. The limits of quantification in the gas phase were 8.7 × 10(-4) μg/L air for hydrogen sulfide, 1 × 10(-4) μg/L air for methyl mercaptan, 1 × 10(-3) μg/L air for butyric acid, 1 × 10(-4) μg/L air for p-cresol, 1 × 10(-5) μg/L air for indole, and 1 × 10(-5) μg/L air for skatole. The system was calibrated by using olfactometers, which can deliver a precisely known quantity of volatiles into the air. We were able to quantify all compounds near their odor detection thresholds (ODTs). All ODTs were measured in our laboratory with the same olfactometry method. This allowed accurate and comparable ODT values for malodorant compounds from toilets.

Simulating latrine conditions to assess perfume performance against malodour

“…We improved the analytical method described by Chappuis et al to quantify hydrogen sulfide, methyl mercaptan, butyric acid, p ‐cresol, and indole in the gas phase. In the previous method, the air sample was pumped through a wash bottle filled with buffered water containing the derivatization agent NEM.…”

“…To force the evaporation of liquids we modified the lower chamber of the olfactometer, as described by Chappuis et al A 150‐L/h nitrogen flow flushed a 500‐ml round‐bottom flask, where liquids were introduced via a polytetrafluorethylene (PTFE) capillary linked to a 1‐ml polypropylene syringe (Figure ). The flow rate of liquids was delivered by a syringe pump and could range from 0.04 to 10 mL/h.…”

“…Using an olfactometer, we created headspaces with known concentrations of butyric acid, indole, p ‐cresol, methyl mercaptan, and hydrogen sulfide to calibrate our analytical method, as described by Chappuis et al Briefly, air with known quantities of compounds was sampled with Oasis cartridges loaded with the derivatization agent NEM (described above) at the outlet of the olfactometer. Methyl mercaptan and hydrogen sulfide were released into the olfactometer from pressurized cylinders containing a mixture of 15 ppm of both sulphur compounds in nitrogen.…”

“…We used dimethyl trisulfide because it is a liquid at room temperature with an odour that approaches that of hydrogen sulfide and methyl mercaptan, two important components of toilet malodour . We developed an analytical method to quantify hydrogen sulfide, methyl mercaptan, butyric acid, p ‐cresol, indole, and skatole in the headspace of latrines in Africa and India . Knowing the gas‐phase concentrations of the major contributors to toilet malodour allowed us to recreate realistic toilet malodour with synthetic compounds by using olfactometers in the laboratory.…”

“…3,4 We developed an analytical method to quantify hydrogen sulfide, methyl mercaptan, butyric acid, p-cresol, indole, and skatole in the headspace of latrines in Africa and India. 5 Knowing the gas-phase concentrations of the major contributors to toilet malodour allowed us to recreate realistic toilet malodour with synthetic compounds by using olfactometers in the laboratory. Consequently, we were able to test perfumes in a stable and precisely defined background of toilet malodour to find optimal perfume formulations and concentrations.…”