Fiber properties
PAL SPME Arrow is based on a stabilizing stainless steel inner rod that runs continuously through the entire fiber, carrying the cylindrically shaped sorption phase and connecting the upper parts of the device to its solid tip, which is shown in Fig. S2 of the ESM, and specially designed to allow gentle penetration of injector and sample vial septa. This tip also retains the sorption phase, which is attached to the inner rod, and furthermore enables PAL SPME Arrow’s capability to enclose this sorption phase during transfer processes. This is an important difference to the traditional SPME fiber, which only allows for the retraction of the latter, with its outer capillary more open to external, potentially adverse influences such as contaminations from ambient air. Furthermore, an open capillary faces significant resistance during penetration processes, in contrast to a PAL SPME Arrow in its closed state. Its outer capillary rests against the solid tip, resulting in a homogeneously closed fiber since both parts possess the same diameter.
A sketch of a conventional SPME fiber and a PAL SPME Arrow is shown in Fig. 1. Pictures comparing the points and sorption phases of both instruments are included in the ESM (Figs. S1 and S2).
Classical SPME fibers can cause coring of injector septa due to their open tubular tip [2]. Based on own experiences, exchange of the septa of gas chromatographic systems, which are subject to regular SPME measurements, is required after approximately 100 injections to avoid leakages and introduction of septum material into the liner.
Using PAL SPME Arrow, the wear of injector septa diminished due to the specially designed tip. Despite the enlarged diameter compared to the classical fiber, at least 200 injections without coring, abrasion, or leakage are possible.
PAL SPME Arrow demonstrated faultless mechanical reliability over the entire course of these studies. In our experience, classical SPME fibers are more fragile, typically requiring replacement after 100 to 200 injections due to bending of the fibers (an exemplary picture is included in the ESM). These values seem to be typical and are also encountered in literature [7, 9, 10]. Active agitation of the sample vial (instead of the liquid sample via stirring) by the standard PAL agitator may decrease this value even further since the fiber material is weakened by being constantly bent into alternating directions.
The main reason for this change in mechanical reliability is the increased diameter of the fibers’ outer capillary, which is 1.5 mm in contrast to approx. 0.7 mm in the case of the classical gauge 23 SPME fiber. In addition, the tip of PAL SPME Arrow not only conserves septa during penetration but thereby also lowers the resistance, which has to be overcome.
Extraction optimization
In general, PAL SPME Arrow and classical SPME fibers require the same optimization procedure. For the here applied direct immersion (DI) extraction, the important optimization steps are evaluation of extraction time and stirring velocity [6].
In Fig. 2, the influence of stirring rate and extraction time is shown exemplarily for four of the 16 EPA PAHs, with achieved results confirming expectations according to literature [2, 15, 21].
For the optimized PAL SPME Arrow method, an extraction time of 70 min was chosen. Apparently, this technique represents a reasonable compromise in this context. Classical SPME fibers typically require approx. 30 min [15] of extraction time in order to reach an equilibrium state, and alternative extraction techniques with larger sorption phases such as SBSE may require timeframes of up to 14 h [22].
In Fig. 2b, the influence of the stirring rate between 0 and 1500 rounds per minute (rpm) is shown. In accordance with the SPME extraction theory [2], an increased stirring rate leads to a higher mass transfer in the system, since the diffusion layer around the fiber coating is minimized and thus the equilibrium is attained faster. For the optimized method, the maximum possible stirring rate of 1500 rpm was used.
Since the typical behavior of decreasing extraction yields at higher temperatures caused by smaller partition coefficients of the analytes between the extraction phase and the sample matrix [2] could be observed in our preliminary measurements as well, the lowest possible temperature of 35 °C was used for all sample extractions. See Fig. S5 in the ESM for the corresponding optimization results.
Extraction efficiency
To determine the effects of the enlarged sorption phases in the case of PAL SPME Arrow, a comparison with classical SPME fibers was performed. Prior to sample measurements, theoretically extracted analyte amounts were calculated with Eq. (1) [21]:
$$ {m}_f=\frac{K_{fs}{V}_f{V}_s{c}_0}{K_{fs}{V}_f+{V}_s} $$
(1)
where m
f
is the extracted mass of analyte in the polymeric sorption phase under equilibrium conditions and V
f
and V
s
are the volumes of the polymer and the aqueous sample, respectively. The initial amount of each analyte present in the aqueous samples with a volume of 19 mL and an initial analyte concentration (c
0) of 10 ng L−1 was 190 pg.
The distribution constants K
fs
for the analytes’ phase transition from the aqueous solution into the PDMS sorption phase were calculated from literature parameters [23] and Eq. (2), yielding the results included in Table 1. The letters E, S, A, B, and V thereby denote the solute descriptors according to the Abraham model for excess molar refraction, dipolarity/polarizability, overall hydrogen bond acidity, overall hydrogen bond basicity, and McGowan volume, respectively [24].
Table 1 Calculated log K
fs
and m
f
values for ten exemplary PAHs included in this work, determined for a SPME fiber (100 μm × 10 mm, 0.6 μL), a PAL SPME Arrow (250 μm × 20 mm, 10.2 μL), and an SBSE bar (500 μm × 20 mm, 47 μL) for a c
0 of 10 ng L−1, sorted by ascending log K
fs
value, based on solute descriptors from literature [23]
$$ \log\;{K}_{fs}=0.246+0.568E-1.305S-2.565A-3.928B+3.573V $$
(2)
According to Table 1, PAL SPME Arrows can be expected to exhibit improved extraction yields when compared to classical SPME fibers with a ratio of up to 12.2 for PAHs. In the case of the SBSE bars, the further improvement in relation to PAL SPME Arrow has a ratio of up to 2.2. Especially for molecularly larger compounds with a log K
fs
of approx. 5 or larger, differences in extraction efficiency between PAL SPME Arrow and SBSE are negligible. Obviously, the effect of a further increase in sorption phase dimensions peaks in the range where PAL SPME Arrow is situated. The critical relation here is the phase ratio between sample and sorption phase. While these results were calculated for 20-mL vials, the SBSE technique is probably better suited for analysis of larger sample volumes, which are however less straightforward to automate.
Further investigation on the extraction behavior of PAL SPME Arrow was conducted by calculating the recoveries that are to be expected theoretically from PDMS-based extraction techniques with different phase volumes. We selected a commonly available variant of classical SPME fibers, a PAL SPME Arrow, and an SBSE device as representative examples. Using K
fs
values from literature [23], we calculated the theoretically extracted percentages for the aforementioned extraction phases and three model analytes under equilibrium conditions (Fig. 3).
In order to evaluate these calculated values, the depletion SPME method [25] was used to determine the extracted percentages of analytes out of a sample with an initial concentration of 50 ng L−1 for a single extraction. The latter was carried out either by a classical SPME fiber (100 μm × 10 mm, 0.6 μL) or a PAL SPME Arrow (250 μm × 20 mm, 10.2 μL). This method is based on performing depletion extractions by extracting and measuring samples multiple times. The declining, logarithmical peak areas are then plotted against the number of consecutive extractions, yielding a linear regression, whose slope b then enables calculation of the extraction ratio E from log(1 − E) [25].
The results of these measurements can be seen in Table 2 and are in good agreement with literature [8], as well as the previously calculated values in Table 1. This is also visible when plotting calculated against measured results with a linear trend line. An example for such plots can be found in Fig. S7 in the ESM, exhibiting a correlation coefficient of 0.9777. Depletion curves of these measurements and their corresponding linear correlations and trend lines are shown in Figs. 4 and S6. The slope of the logarithmic depletion curves and their linear correlations are also included in Table 2, demonstrating sufficiently good correlations (>0.98) for all analytes. These measurements were also carried out for the largest available PAL SPME Arrow sorption phase variant (250 μm × 30 mm, 15.3 μL), and the results are included in the ESM (Table S2).
Table 2 Slopes, correlation coefficients, and extracted percentages of the performed depletion experiments according to Zimmermann et al. [25] for samples containing 19 mL of water and an initial concentration of 50 ng L−1 PAHs for the first extraction by a classic SPME fiber (100 μm × 10 mm, 0.6 μL) and a PAL SPME Arrow (250 μm × 20 mm, 10.2 μL)
The increased extraction yields of PAL SPME Arrow are also visible in Fig. 4 by comparing the depletion curves from these experiments that were either generated using a classical SPME fiber (a) or PAL SPME Arrow (b), since the latter showed a more rapid depletion of analytes in the samples.
Comparison to literature
To enable a statistical comparison of achievable detection limits for PAL SPME Arrow with classical SPME fibers, we determined the method detection limits (MDL) according to Keith et al. [26], as well as relative standard deviations.
Using PAL SPME Arrow (250 μm × 20 mm, 10.2 μL), it was possible to calibrate in concentration ranges as low as 0.5 to 2.5 ng L−1 for all 16 EPA PAHs. Results are displayed in Table 3 in terms of LOD and RSD values for calibrations performed in ultrapure water and filtrated groundwater. Linear ranges and correlation coefficients for these calibrations can be found in the ESM.
Table 3 Calibration results obtained with PAL SPME Arrow (250 μm × 20 mm, 10.2 μL) in ultrapure water and groundwater: MDL values (calculated with a 99 % confidence interval) and relative standard deviations (RSD)
In accordance to literature [27], it was impossible to determine freely dissolved PAHs via SPME fiber or PAL SPME Arrow in groundwater samples with a significant content of particulate organic matter (POM).
After removal of POM (along with sorbed compounds) via filtration, spiking of groundwater samples enabled determination of PAHs from the freely dissolved fraction with the following exceptions due to matrix interference: phenanthrene, anthracene, pyrene, fluoroanthene, and indeno(1,2,3 cd)pyrene, as indicated by the dashes in Table 3.
Table 4 displays LODs and RSDs for PAL SPME Arrow and comparable techniques. While Cheng et al. [15] extrapolated the LOD values for their classical SPME fibers from the standard deviation of their results at the lowest calibration point (10 ng L−1), the results presented herein were calculated from measurements at 0.5 ng L−1 for reagent water-based samples and at 5 ng L−1 for groundwater samples.
Table 4 MDL and RSD results obtained with PAL SPME Arrow (250 μm × 20 mm, 10.2 μL) for PAHs in water in comparison with literature data for classical SPME fibers and SBSE bars
Carrera et al. [22] achieved LODs that are similar to the ones generated with PAL SPME Arrow, by extracting a 100-mL water sample for 14 h with a 500-μm × 20-mm SBSE bar. We calculated the sorption phase volume on these bars to be 47 μL, which would be approx. threefold larger as the largest available PAL SPME Arrow phase.
Determined MDLs for PAL SPME Arrow are generally more similar to those generated with the SBSE bars and approximately one order of magnitude better than those of the classical SPME fibers. In contrast to SBSE though, these results have been achieved with a fully automated method. The corresponding RSD values are thereby in the range of 5–12 % which is acceptable in such small concentration ranges and in good agreement with literature.
Exemplary leaching experiment
For the roofing felt samples, naphthalene and acenaphthylene were the only EPA PAHs that could be measured from the freely dissolved fraction of the sample. This was expected since the material pieces inside the vials act as a second organic, hydrophobic phase. Since the sorptive properties of PAHs increase with their molecular weight, larger compounds are difficult to remove from this phase without a solvent extraction step. In addition to the two abovementioned PAHs, further compounds have been tentatively identified via their mass spectral information in the NIST library. These compounds and their estimated concentrations (converted from 300 mg to 1 g) are summarized in Table 5.
Table 5 Results for roofing felt extractions with ultrapure water, measured with a PAL SPME Arrow (250 μm × 20 mm, 10.2 μL): leached concentrations (estimated from calibrations for naphthalene and acenaphthylene for all other compounds) and relative standard deviations (RSD) at calculated concentrations
Latter concentrations can be expected to be leached into 1 L of water, which is exposed to 1 g of roofing felt under the extraction conditions given above. Since the used calibration standards contained the 16 EPA PAHs, these results were estimated using the calibration functions of naphthalene (for naphthalene and 2-vinylnaphthalene) and acenaphthylene (for all other compounds). It should however be noted that only PAHs and structurally similar substances such as heterocycles or substituted PAHs were taken into account during these measurements.
Despite the minor concentrations recovered in this small-scale experiment, the large quantities of, e.g., bitumen-based waterproofing materials that are applied globally could still account for a significant contribution to the overall anthropogenic discharge of PAHs into the environment. Further assessment of these contributions should involve influences by temperature, acidity, and UV radiation.