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3.3 Trapping of samples
Air samples were trapped on a cooled adsorbent bed held at -10◦C with a sample flow rate of 100 mlmin−1 (Sec. 3.2.7). Local (UCT, Fig. 1.2) air sample volumes of between 1.0 – 3.0lwere trapped. Variations in trapped volume were analysed to establish the ideal sampling volume that resulted in a good pre- concentration of bromoform and limited chance of breakthrough. Analysis of the chromatography generated from varying sample volumes suggested that medium sized samples achieved the best chroma- tography (Fig. 3.15). Pre-concentration of bromoform in 1.0lair samples was low, resulting in greater uncertainty within the measurement (Fig. 3.15). There was no noted benefit in trapping 3.0lover 1.5l samples (Fig. 3.15). The chromatography was not significantly better in the larger trapped sample volumes.
Also the larger sample volumes required longer to trap, limiting the number of samples that could be performed in a day. Consequently, sample volumes of 1.5lwere used for the analysis of bromoform in this study. This achieved sufficient pre-concentration of bromoform for detection, while limiting the
3.3. Trapping of samples 51 chance of breakthrough (see below). Breakthrough of compounds from the adsorbent was more likely to occur at larger sample volumes, thus losing part of the sample.
(a) (b)
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Figure 3.15:Variations in volume trapped (a) 1.0lBromoform not indicated (b) 1.5lBromoform (1) (c) 3.0l Bromoform not indicated.
Trap breakthrough occurs when the adsorbent bed is no longer able to retain or adsorb a compound from the passing air stream (Poole, 2003). Verification of the breakthrough volume of bromoform on this system was achieved by passing 1, 3, and 5 l environmental air samples through two TDU adsorbent traps connected in series. The samples were collected in multiple cylinders filled at the same location in immediate succession. This suggests that the mixing ratios within the cylinders should be consistent. The traps were flushed at 100 mlmin−1with marine air collected from False Bay (Fig. 1.2).
Environmental marine samples were selected for the breakthrough analysis over pure laboratory standards as the breakthrough volume may vary within a complex mixture compared to a single species chemical standard (Palmer, 2006). The environmental samples were collected in polished stainless steel flasks on a research cruise transect in False Bay (Philibert, 2010).
Figure 3.16:Trapping of varying volumes testing for breakthrough of packed adsorbent beds. The volume loaded on the primary trap and the cumulative peak area observed from the secondary trap are displayed on the X and Y-axes, respectively. Bromoform (tR ≈14 min) is denoted by the dot-dash line, while the other lines (dashed, dotted and solid) indicate unidentified volatile compounds (tR= 2.4, 2.7 and 3.0 min, respectively), included here for comparison. Adapted from Kuyper et al., 2012.
The two TDU adsorbed traps connected in series allowed the testing of the secondary trap for break- through analytes without altering the setup. The primary trap, upstream of the secondary trap, was maintained at laboratory temperature (20 – 25 ◦C), whilst the secondary was cooled to -10◦C. This arrangement allowed the testing of the secondary trap to be performed without changing tubes. Air flow through the traps was stopped after 5 minutes and the secondary trap was analysed for breakthrough analytes using the thermal desorption unit and GC-ECD following the procedure described in Sec. 2.3.
After the analysis was complete this secondary trap was cooled and the gas flow through both tubes resumed. Repeated flushing of the tubes provided conditions for the build up of up to 5lon the primary trap, while the secondary was cleaned after each analysis. The volume of the sample that had to be passed through the primary trap in order to be detectable on the secondary tube defines the breakthrough volume of this system under these conditions. For this system, breakthrough of bromoform occurred at a volume of 5l(Fig. 3.16). Two unidentified atomically lighter compounds had smaller breakthrough volumes than bromoform. Consequently additional work needs to be done to validate the system before sampling for lighter compounds. Note, however, that the 1 – 3lsample volumes for bromoform exhibited no breakthrough.
3.3. Trapping of samples 53 3.3.1 Injection
Various heating methods, depending on the trapping mechanism used, have been suggested to thermally desorb trapped samples for analysis (Sec. 3.2.7). The use of heated oil and water have been reported for
‘freezeout-loops’, and electronic heating coils with an adsorbent bed have also been used (Wevill and Carpenter, 2004; Poole, 2003). The desorbed sample was passed to the column. A sample might be passed directly to the column through a splitless injector (directly) or a split injector. The splitless injector passes the entire sample directly to the head of the column. A split injector contains an extra gas stream, which splits the sample, both diluting it and reducing the gas volume injected to the column. The split injector has been used to dilute samples where high concentrations have been recorded. When a large volume is injected it can be reduced through a split injector as only a portion of the sample volume reaches the head of the column. A switching valve that swaps the direction of gas flow to the TDU has been used to inject samples and the carrier gas is passed through the TDU while heating. In automated systems, an electronic switching valve is used. The use of an electronic valve ensures that the sample volume injected is exactly replicated. The rate of switching is also faster in the electronic valves. Manual switching valves are cheaper and simpler to maintain; however, they make it more difficult to achieve exactly consistent injection volumes.
An injection procedure aims to deliver the trapped sample efficiently to the head of the column for separation. This should result in minimal band broadening and high quality chromatography. Following the trapping of samples on the cooled adsorbent bed, the TDU was heated and the sample was desorbed. The injection valve was switched to the ‘inject’ position (Fig. 2.2b) once the TDU had attained a temperature of 15◦C, transferring the sample to the GC for separation and analysis. A back pressure of 200 kPa helium carrier gas was maintained through the trap to ensure all the elutants were swept to the head of the column.
Once the internal temperature had reached 300◦C (2 min 30 sec) the injection valve was returned to the
‘load’ position, returning N2flow through the trap. This cut off the He flow through the trap, which was passed directly to the column to maintain the carrier gas flow. The long injection times that occurred as a result of slow heating of the TDU produced poor resolution of the peaks in the chromatography. The poor chromatography was mainly due to band broadening effects (Poole, 2003). The resultant chromatograms showed little shape (Fig. 3.17a). The use of injection windows (Table 3.4) and second stage cryo-focusing (Fig. 3.17c) methods were explored as methods to address the problems of poor chromatography.
Injection windows limit the injection time to a short time-interval between two temperatures (Poole, 2003). The injection valve was switched for 10 seconds around the boiling point of bromoform (149◦C).
The short window limits the size of the slug injected, improving the chromatography. Since lighter halocarbons would be flushed as the trap is being heated, this method precluded the chance of sampling for anything other than bromoform. Bromoform might also be mobilised prior to the valve switchover and thus lost. Since the loss would be non-quantifiable, it could bias the measurements and impact on accuracy.
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Figure 3.17:Improved chromatography as result of second stage cryo-focusing. (a) Salt-water∼-20◦C, (b) N2(l) and chloroform mixture∼-60◦C, (c) N2(l)-196◦C.
Desorbed compounds are re-trapped and desorbed at the head of the column, prior to separation and analysis, in a process known as second stage cryo-focusing (Poole, 2003; Wevill and Carpenter, 2004).
The head of the column is cooled for the duration of the primary injection. This offered only limited improvement where the coolants were salted ice water (±-20◦C; Fig 3.17a) and a liquid nitrogen (N2(l)) and chloroform mixture (±-60◦C; Fig. 3.17b), which were not sufficiently cold to trap the desorption slug of elutants. The mixture was expensive, consuming (N2(l)) and chloroform. The use of liquid nitrogen as the cryo-focusing coolant resulted in the retention of the entire desorbed volume and minimised band broadening (Fig 3.17c). The N2(l)was held in place around the stainless steel loop for the duration of the initial desorption and rapidly replaced with boiling water to re-desorb the trapped sample. The concurrent use of injection windows and cryo-focusing to achieve excellent chromatography was not necessary (Table 3.4). The cryo-focusing was used as the method to limit band broadening with a full temperature injection from the TDU ensuring a full sample volume.