3.1. Effect of desorption temperature on the recovery of isoprene
Carbosieve SⅢ is a porous carbon material that has been widely studied for its potential use in gas separation and purification applications. The main characteristics of Carbosieve SⅢ include its high surface area, narrow pore-size distribution, and high mechanical stability. Its surface area (900 m2 g− 1) and pore volume (0.65 cm3 g− 1) provide a large surface area for gas adsorption, which is important for efficient gas adsorption. The 3 Å-pore size of Carbosieve SⅢ makes it ideal for adsorbing small gas molecules. Its high surface area, narrow pore-size distribution, and good chemical stability make it suitable for industrial processes requiring efficient and selective gas adsorption [37]. However, the adsorption capacity of Carbosieve SⅢ decreases with increasing relative humidity. Therefore, the effect of relative humidity must be carefully considered when using Carbosieve SⅢ to measure VOCs in humid environments [34, 38].
The effectiveness of Carbosieve SⅢ was evaluated through a series of experiments focused on a single substance's adsorption. To assess the isoprene adsorption performance, we compared the results from two sampling traps employing Tenax TA mixed with Carbosieve SⅢ and only Carbosieve SⅢ. Previous studies have reported that a recovery rate of 40% can be achieved using 60 mg of Carbosieve SⅢ [39]. To increase the isoprene adsorption rate, the amount of Carbosieve SⅢ used was increased from 180 mg to 300 mg.
The isoprene recovery rate of Carbosieve SⅢ was evaluated based on two different desorption temperatures. In this study, the sampling flow rate and sampling temperature were 100 ml/min and 20 ℃, respectively. The commonly used desorption temperature in the VOC measurements, namely 250 ℃, was compared with 280 ℃ desorption temperature. The experimental results showed that the recovery rate of isoprene was not strongly affected by the desorption temperature of the adsorbent (Fig. 3). A sufficient amount of adsorbent was added, considering the breakthrough point. However, a recovery rate of about 30% was achieved, regardless of the amount of adsorbent. Besides, when the desorption temperature was increased from 250 to 300 ℃, the recovery rate was slightly higher. When another extra work with 300 mg/350 ℃ was separately conducted, the recovery rate was improved up to 38%. Since the recovery rate was still low, other variables except desorption temperature and the amount of the adsorbent, might directly affect the recovery rate of isoprene. Despite comparison by a rule of thumb on the difference of recoveries, it was found that the effect of the amount of adsorbent and desorption temperature on the recovery rate of isoprene was significant (p = 0.00983 at the significance level of 0.05). On the other hand, a study using a Carbosieve SⅢ (60 mg) trap coupled with Tenax TA (110 mg) followed by this experiment exhibited a recovery rate of 70%, indicating that Tenax TA could improve the recovery rate of isoprene.
In a previous study, it was assumed that losses of 1,3-butadiene and isoprene were caused by both fast reactions on adsorbent surface and irreversible adsorption. After a storage duration of seven days, only 20% of 1,3-butadiene and 26% of isoprene were recovered. The results presented in this paper demonstrated that the adsorptive enrichment of reactive light hydrocarbons such as 1,3-butadiene and isoprene using the carbon molecular sieves (Carboxen 569, Carboxen 1003, and Carbosieve SⅢ) resulted in a significant underestimation of these compounds. The losses increased with increasing storage time. The most remarkable effects were observed for Carbosieve SⅢ. In contrast, no considerable analyte losses were observed using the graphitized carbon black Carbotrap X [39]. is the graphitized carbon black seemed more effective in the enrichment of isoprene than the carbon molecular sieve.
Carbotrap is also a porous carbon material as Carbosieve SⅢ. The adsorption coverage of Carbotrap ranges from C4 to C14, and it exhibits a wide range of application conditions. Carbotrap targets a wide range of VOCs, such as aliphatic hydrocarbon compounds, ketones, and aldehydes. It is advantageous for capturing strong volatile substances [40–42]. The main characteristics of Carbotrap include a high surface area, narrow pore-size distribution, and good mechanical stability. Their surface area (600 m2 g− 1) and pore volume (0.35 cm3 g− 1) provide a large surface area for gas adsorption, which is important for efficient gas adsorption. The 10 Å pore size of Carbotrap makes itself ideal for adsorbing small gas molecules [41, 42]. The experiment’s results using Carbotrap are shown in Fig. 4. The experiment was conducted using a trap composed of Tenax TA and Carbotrap (Table 2). The sampling flow rate and sampling temperature were the same as in the above Carbosieve SⅢ study. Even at a temperature of 250 ℃, which is lower than the recommended temperature of 280 ℃, a recovery rate of 93.8% was achieved. At 280 and 300 ℃, the recovery rates were improved to 99.4% and 99.7%, respectively. It was found that desorption temperatures significantly affected the recovery rate of isoprene (p = 0.03072 at the significance level of 0.05). After this experiment, all other ones were carried out based on 280 ℃ of desorption temperature for the long-term stability of adsorbents. The amount of adsorbents are depicted in Table 2 for the rest of experiments.
3.2. Effect of sampling temperatures on the recovery of isoprene
VOC sampling, including isoprene, is influenced by the temperature and flow rate [43]. Isoprene is highly volatile and can potentially evaporate at lukewarm (boiling point: 34.067 ℃) conditions. Thus, the sampling temperature should be carefully considered. In this experiment, the effect of the sampling temperature on the recovery rate of isoprene was studied based on the laboratory temperature. The sampling temperature was 25, 35, and 45 ℃, and the experiment was conducted based on these temperature conditions and a sampling flow rate of 100 ml min− 1.
The recovery rates of the Tenax TA/Carbosieve SⅢ and Tenax TA/Carbotrap traps were 63.50% and 87.97%, respectively (Fig. 5). Among sampling temperatures of concern, the first change in adsorption performance was observed at 35 ℃ and, which is near the boiling point of isoprene (34 ℃). The result at 40 ℃ revealed that the recovery rate was significantly lower than that at 35 ℃. However, no significant loss of isoprene was observed at 25 ℃ condition as expected. This result indicates that isoprene is not smoothly adsorbed when the temperature exceeds 35 ℃ during sampling. It was found that sampling temperatures had a significant effect on the performance of adsorbents (Tenax TA/Carbosieve SⅢ and Tenax TA/Carbotrap, p = 6.5643E− 6, 1.9632E− 6 at the significance level of 0.05).
3.3. Effect of flow rates on the recovery of isoprene
In this work, the influence of variations in flow rates on the recovery rate of isoprene was studied. Sampling flow rates were 50, 100, and 200 ml min− 1, which were based on the recommended range of 50–200 ml min− 1 of adsorbent flow rates, and the sampling temperature was 25℃ [44].
The recovery rates of the Tenax TA/Carbosieve SⅢ and Tenax TA/Carbotrap traps were 72.37% and 94.70%, respectively (Fig. 6). The sampling flow rate was expected to influence the adsorption performance, and the recovery rate of isoprene tended to decrease as the flow rate increased.
In the case of Carbosieve SⅢ, the results of the current and previous experiments [21] confirmed that the adsorption capacity for isoprene was lower than that of Carbotrap. Considering the adsorbent performance alone, Carbosieve SⅢ is known to have a larger specific surface area and stronger adsorption than Carbotrap [45]. However, the isoprene-adsorption performance results indicate that the performance of Carbosieve SⅢ was inferior to that of Carbotrap. Therefore, the tendency of the adsorption capacity deteriorated with increasing flow rate was greater in Carbosieve SⅢ than in Carbotrap. It was found that the sampling flow rate had a significant effect on the recovery rate of isoprene (Tenax TA/Carbosieve SⅢ and Tenax TA/Carbotrap, p = 1.5050×10− 4, 8.5377×10− 4 at the significance level of 0.05).
In previous studies, a multi-bed trap, including Carbosieve SⅢ, was used to use the strong adsorption capacity possessed by Carbosieve SⅢ [46–48]. However, due to the strong adsorption capacity of Carbosieve SⅢ, the low flow rate of sampling was not possible in the humid air [49]. For the above reasons, this study has been initiated, and the recovery work on isoprene was carried out. Furthermore, recovery rate experiments were conducted using a double-layer trap (Tenax TA/Carbosieve SⅢ and Tenax TA/Carbotrap) with respect to isoprene sampling temperatures and sampling flow rates. As a result, the recovery rates of two different types of traps (Tenax TA/Carbosieve SⅢ, Tenax TA/Carbotrap) for isoprene were confirmed to be 70% and 90% or more, respectively, and the trap using Tenax TA/Carbotrap showed the better recovery rate than the other.
The recovery rate of isoprene varied according to sampling temperatures and flow rates, especially when the sampling temperature increased. A previous study explains that there could be a loss during sampling at a temperature beyond the boiling point of isoprene, and this study also proved that it was true [46]. In the case of the sampling flow rate, it was found that the recovery rate was low due to the fast flow rate, and this tendency was evident especially in Tenax TA/Carbosieve SⅢ trap. In conclusion, optimizing sampling temperatures and flow rates is critical in ensuring high recovery rates of isoprene. It was also proved that Carbosieve SⅢ was unsuitable for the sorbent of isoprene.