3.2. Data Evaluation of the Experiments for High Temperature Focus IMS
As shown in previous experiments, the influence of IMS cell temperature and drift gas flow depends on substance group, boiling point and the type of formed proton adducts [
10]. Within the next section, the impact of drift tube temperatures between 80 - 160 °C and different drift gas flows in a range of 100 - 200 mL/min to the peak shape of terpenes and other flavor compounds were evaluated. As the selected substances are common in complex matrices, such as essential oils or essence oils, the quality parameters of peak height, FWHM and tailing factor were evaluated.
While it was already shown that higher IMS cell temperatures have a beneficial effect on peak tailing and peak width for alcohols, ketones and other different contact allergens in the work of Capitain
et al., this effect was also observed for all investigated flavor compounds [
10]. However, the focus drift gas flow resulted in distinct reduced tailing factors by a factor of 2-3, even at lower temperatures of 80 °C and 100 °C in comparison to the classic DTIMS flow design. Comparing to other studies of HS-GC-IMS with terpenes and flavor compounds, peak shape is improved and all analytes indicated acceptable peak shapes even for (
S)-(+)-carvone, citronellol, geraniol or the sesquiterpene β-caryophyllene, as visualized in
Figure 4 [
6,
8,
10].
Figure 4.
GC-IMS spectrum of standard 1 and 2 at 120° C IMS cell temperature and 150 ml/min drift gas flow.
Figure 4.
GC-IMS spectrum of standard 1 and 2 at 120° C IMS cell temperature and 150 ml/min drift gas flow.
The peak shape of geraniol is particularly important for analytics in fields of fragrance and cosmetics, because the substance is listed as an allergenic compound in cosmetic products together with (
R)-(+)-limonene and citral. Since July 2023, this list was further updated with pinene, terpineol, β-caryophyllene, geranyl acetate and carvone, as well as several other substances and peel oils of different
Citrus fruits [
26]. Every cosmetic product containing one of the listed substances must comply with the restrictions after 31
st July 2026 for the new placement on the European market and after 31
st July 2028 for existing products. Thus, reliable analytical procedures have gained importance for the 45 added compounds. With an improved peak shape, analytics of terpenes, terpenoids and other fragrance allergens in complex cosmetic matrices are substantially simplified through HS-GC-IMS approaches, which do not demand for time consuming sample pre-treatment steps and an energy consuming vacuum system, such as GC-MS.
Within the following paragraphs the already mentioned peak quality parameters of the evaluated substances are displayed and discussed. For enhanced readability, 7 of the 13 analytes are visualized exemplarily, while all other graphs have been included in the supplementary data. In
Figure 5, the impact of the drift gas flow to the FWHM and tailing factor on the peaks of (
R)-(+)-limonene, the quantitatively predominate volatile compound of
Citrus peel oils, and γ-terpinene, which represents about nine percent of the volatile fraction of cold pressed lemon oil, at five different drift tube temperatures are visualized [
28]. The peak heights of the monoterpenes (
R)-(+)-limonene and γ-terpinene were observed increased at higher temperatures. While an improved peak height was visible from 80 °C to 140 °C, there was no further improvement from 140 °C to 160 °C. This effect is most likely explainable due to adsorption effects at rather low drift tube temperatures, leading towards peak tailing and higher FWHM values at low drift tube temperatures. FWHM decreased with an increasing drift gas flow, as well as with higher temperatures. However, the standard deviations for both compounds at 80 °C and 160 °C were higher than for the temperatures in between. The tailing factor of (
R)-(+)-limonene and γ-terpinene did not indicate a substantial effect of IMS cell temperature and drift gas flow rates. A good peak symmetry for (
R)-(+)-Limonene was observed at all drift tube temperatures and drift gas flows, within a range of 0.90 to 1.10. However, tailing factors were observed slightly higher at increased temperatures, as well as higher drift gas flow rates.
Figure 5.
Effect of drift gas flow and drift tube temperature on peak height (a and d), FWHM (b and e) and tailing factor (c and f) of (R)-(+)-limonene and γ-terpinene.
Figure 5.
Effect of drift gas flow and drift tube temperature on peak height (a and d), FWHM (b and e) and tailing factor (c and f) of (R)-(+)-limonene and γ-terpinene.
For γ-terpinene, the tailing factor also indicated a symmetric peak shape with values between 1.0 and 1.15 (
Figure 5f). The tailing factor is marginally decreasing with higher IMS cell temperatures and higher drift gas flow rates. Similar effects were observed for the monoterpenes sabinene and β-pinene, with an improved peak height up to an IMS cell temperature of 140 °C. The FWHM of β-pinene decreased notably for higher drift tube temperatures and an increasing drift gas flow, while the FWHM of the sabinene peak only featured a decrease at temperatures of 140 °C and 160 °C and higher drift gas flows of 175 mL/min and 200 mL/min (
Figure S1b,e). However, the signal of β-pinene exhibited a slight peak fronting with a tailing factor of below 0.9.
For ethyl butyrate monomer and dimer, a decreased peak height at higher temperatures was visible and was observed minimal at 160 °C (
Figure S2a,d). In contrast to terpenes, ethyl butyrate is a smaller molecule with a boiling point of 120 - 121 °C, while the boiling points of (
R)-(+)-limonene and γ-terpinene range around 180 °C. Thus, adsorption effects are less likely to have a distinct influence in peak height of ethyl butyrate monomer and dimer. Peak height was particularly affected with the significantly decreased RIP height and less proton water clusters at higher drift tube temperatures, resulting in less intense peaks. FWHM of the peaks, was observed smaller at higher temperatures for both signals (
Figure S2b,e). Further, there was a decrease in FWHM at higher drift gas flows, but the effect of temperature to FWHM was more perceivable. An increase of the drift tube temperature to 160 °C at a drift gas flow of 150 mL/min led to a FWHM of 4.40 ± 0.08 s for the monomer and 5.34 ± 0.12 s for the dimer, in comparison to a FWHM of 8.83 ± 0.08 s and 6.58 ± 0.02 s at a temperature of 80 °C. The tailing factors of ethyl butyrate monomer and dimer decreased with higher drift tube temperatures, as well as with higher drift gas flows, resulting in more symmetric peaks (
Figure S2c,f).
In
Figure 6, peak height, FWHM and tailing factors of the monoterpene alcohol α-terpineol and the monoterpene allyl alcohol geraniol are displayed. For those similar trends were observable, with an increased peak height at high drift tube temperatures. However, there was a decrease in peak height observable with higher drift gas flow rates, presumably due to dilution effects.
While FWHM of geraniol at 80 °C was in a range of 13 - 19 s at the evaluated drift gas flows, FWHM at 160 °C was in a range of approximately 6.5 - 7 s. For α-terpineol, at 80°C the FWHM was approximately 20 s and an increase in temperature to 100 °C was already reducing the FWHM by more than a factor of 2 (
Figure 6b,e). Drift gas flow only affected the FWHM at 175 mL/min and 200 mL/min at 80 °C drift tube temperature. Peak symmetry of both substances was particular dependent on the temperature. While the tailing factor of geraniol did show a decrease from 3.5 to 2.0 with an increased drift gas flow at 80 °C, this effect was not observable at α-terpineol. There, the Tailing factor was near to 3.0 at 80 °C and 2.4 at 100 °C, respectively. Further, drift tube temperatures of 120 °C or even higher featured a beneficial effect on peak symmetry. Tailing factors of geraniol at 140 °C and 160 °C were close to 1.5 and for α-terpineol, tailing factors at a drift tube temperature of 140 °C and 160 °C were decreased, by a factor of two, to less than 1.2, in comparison to a drift tube temperature of 100 °C at all drift gas flow rates.
Figure 6.
Effect of drift gas flow and drift tube temperature on peak height (a and d), FWHM (b and e) and tailing factor (c and f) of α-terpineol and geraniol.
Figure 6.
Effect of drift gas flow and drift tube temperature on peak height (a and d), FWHM (b and e) and tailing factor (c and f) of α-terpineol and geraniol.
Citral, the mixture of the isomers geranial and neral featured similar results: for both substances, the monomer and dimer were evaluated. While the neral monomer and dimer showed an increase in peak height up to a temperature of 160 °C, the geranial monomer and dimer signals increased up to a temperature of 140 °C, but showed a moderate decrease at a temperature of 160 °C (
Figure S3a,d). The FWHM of the geranial monomer peak decreased by a factor of 3.5 with an increase in drift tube temperature of 80 °C to 140 °C and 160 °C. Further, a higher drift gas flow indicated a positive effect regarding FWHM, limited to a drift tube temperature of 80 °C. FWHM of the geranial dimer did show comparable effects, but is reduced by a factor of two. For the tailing factors of the geranial monomer and dimer, a decent peak symmetry was observed at drift tube temperatures of 120 °C and above (
Figure S3c,f). In comparison to the peak tailing at 80 °C, there is a reduction by a factor of three and two, respectively for the monomer and dimer. The dimer indicated a smaller tailing factor with an increased drift gas flow at 80 °C and 100 °C drift tube temperature, while this effect is negligible at higher temperatures. In comparison to the monomer of geranial, FWHM of the neral monomer at 80 °C drift tube temperature is substantially smaller, while the FWHM of the dimer were observed in a similar range (
Figure S4b,e). Tailing factors of neral monomer featured a decrease with higher drift gas flows of 175 mL/min and 200 mL/min at 80 °C. Again, a significant impact of drift tube temperature to peak symmetry was observable for the neral monomer and dimer. This effect was previously described by Capitain
et al. for citral, citronellol, and geraniol [
10]. However, the focus drift gas flow architecture depicted a beneficial impact on peak symmetry, in comparison to previous studies. There for instance, the peak shape of geranial monomer indicated a severe tailing [
8,
10]. In the previous evaluation of an high temperature drift tube IMS, tailing factors of neral dimer were observed distinct above 15.0 and in range of 7.5 for the monomer, at 80 °C and a drift gas flow of 150 mL/min [
10]. These values were observed significantly reduced with the focus flow design to 3.10 ± 0.25 for the neral dimer and 2.73 ± 0.11 for the neral monomer, resulting in a reduction by a factor of five and three, respectively. At a drift tube temperature of 160 °C and a drift gas flow of 200 mL/min, the dimer of neral was not detected, supposedly to dilution effects of the higher drift gas flow and the increased background noise, as already described in the previous sections. The peak height of citronellol monomer and dimer did show a maximum at 120 °C drift tube temperature. For the monomer, at 140 °C and 160 °C, peak height is not significantly below the heights at 120 °C, however, the citronellol dimer featured a substantial decrease in peak height with temperatures above 120 °C (
Figure S5a,d). The corresponding spectra indicated a different behavior of citronellol at temperatures above 120 °C, with forming a monomer and only a very small dimer. Similar effects were already observed for the dimer of cinnamal in previous research [
10]. While there is an explicit description of the dependency of analyte concentration on monomer and dimer formation in a number of studies, the influence of high temperatures for monomer and dimer formation were not part of research yet and should be evaluated in further studies [
8,
20]. The monomer and dimer of citronellol displayed a considerably lower FWHM at higher temperatures in the IMS drift tube, while the impact of the drift gas flow was limited (
Figure S5b,e). The tailing factors are decreased substantially at higher IMS cell temperatures, similar to geraniol. Further, tailing factor indicated that low drift tube temperatures, 80 - 100 °C, featured a higher standard deviation of the tailing factors for citronellol monomer peaks, while higher drift gas flows, 175 mL/min and 200 mL/min, led to increased standard deviations of tailing factors for the dimer peak (
Figure S5c,f). For (
S)-(+)-carvone monomer and dimer, peak height is maximum at 120 °C and 140 °C drift tube temperature and indicates a decrease at 160 °C, similar to the data of geranial. The monomer did show an FWHM at 80 °C and 150 mL/min is 27.64 ± 1.76 s, and a tailing factor of 2.89 ± 0.33. These values were decreased significantly at a drift tube temperature of 160 °C and drift gas flow of 150 mL/min to 6.97 ± 0.17 and 1.03 ± 0.06, respectively. For the dimer peak this is also observable. However, there is no significant optimization of FWHM and tailing factor with a further increase of temperature from 140 °C to 160 °C (
Figure S6b,c,e,f). The peak height and peak shape quality parameters of the late eluting compounds geranyl acetate, methyl-
n-methylanthranilate and β-caryophyllene are visualized in
Figure 7. Peak height is increased for all displayed analytes with higher temperatures, while the difference between 140 °C and 160 °C was again observed only minimal. Further there is a decrease in peak height with an increase in drift gas flow for all of the late eluting compounds.
Figure 7.
Effect of drift gas flow and drift tube temperature on peak height (a, d and g), FWHM (b, e and h) and tailing factor (c, f and i) of Geranyl acetate, methyl-n-methylanthranilate and β-Caryophyllene.
Figure 7.
Effect of drift gas flow and drift tube temperature on peak height (a, d and g), FWHM (b, e and h) and tailing factor (c, f and i) of Geranyl acetate, methyl-n-methylanthranilate and β-Caryophyllene.
The sesquiterpene β-caryophyllene did feature an improved FWHM at higher temperatures,
Figure 7h. While FWHM at 80 °C is in range of 25 s, an increase in drift tube temperature to 120 °C already led to a decreased FWHM between 15 s and 20 s, for all the experimental drift gas settings. Drift tube temperature of 140 °C again led to similar results, regarding FWHM, such as a drift tube temperature of 160 °C. FWHM of geranyl acetate and methyl-
n-methylanthranilate featured a similar decrease by more than a factor of two, with an increase in drift tube temperature to 140 °C (
Figure 7b,e). However, for these high-boiling VOC, a small increase of FWHM at higher drift gas flows was observed. While the tailing factors of geranyl acetate and β-caryophyllene were not affected by the drift gas flow, a decrease in tailing factor was observed for methyl-
n-methylanthranilate with an increased drift gas flow,
Figure 7c,f,i. Even though the peak shape of these compounds featured comparatively large FWHM, prior studies show peaks with a length of approximately 100 s for β-caryophyllene and methyl-
n-methylanthranilate [
6,
9]. Thus, there is a significant improvement regarding peak shape due to the focus flow design in combination with a high-temperature drift tube.
Complex signal patterns, such as found in Citrus peel or essence oils require optimization of DTIMS systems. The broad spectrum of eluting compounds, including alcohols, esters, ketones as well as terpenes, terpenoids and sesquiterpenes, demands for optimal separation and peak shape for substances with a boiling point range of approximately 80 °C up to 260 °C.
In
Figure 8, the complexity of an exemplary grapefruit essence oil GC-IMS spectrum is visualized, at settings of 150 mL/min drift gas flow and a cell temperature of 140 °C. The spectrum features well-separated, narrow and symmetric peaks even for high-boiling VOC, such as the sesquiterpenes. This is in particular beneficial for the analysis of highly complex samples such as the aforementioned
Citrus peel and essence oils, but also for cosmetics, complex food and beverages.
Figure 8.
GC-IMS spectrum of a pink grapefruit essence oil at 140 °C and 150 mL/min.
Figure 8.
GC-IMS spectrum of a pink grapefruit essence oil at 140 °C and 150 mL/min.