Chapter 2
Methods
The main method employed in this work involved a GC-ECD system with a custom-built thermal desorption unit (TDU) interface. The methods used for the collection of bromoform measurements at Cape Point and in the laboratory culture experiments are discussed here. The GC was set up with a gas flow rate of 5 mlmin−1 and a 5-step temperature programme between 30 and 200◦C. Nitrogen gas was added at 30 mlmin−1to the ECD which was maintained at 300◦C. Heating the TDU from -10 to 300◦C was achieved in 2 min 30 sec. This resulted in a bromoform detection limit of 0.73±0.09 ppt from 15 minute trapped samples.
trapped sample.
2.1.1 GC oven
Heating in the oven was provided by means of a high resistance heating element. A rotary fan in the back of the oven ensured a uniform heat distribution. The temperature of the element was manually adjusted and was able to achieve a maximum rate of heating of 65◦C min−1. The oven temperature was regulated with an accuracy of±1◦C by a mechanical thermistor and an upper temperature of 300◦C was used for the removal of contaminants from the system. Samples were introduced into the oven through a port on the side of the oven, since the original ports had been blocked.
Tubing connector Electrical connection
Figure 2.1:Schematic drawing showing the layout of the GC-ECD analysis system and associated gas and sample flows. Solid lines denote18" stainless steel tubing with the arrow denoting the direction of gas flow. The system is shown in the ‘inject’ position. Adapted from Kuyper et al. (2012).
Adsorbent trap Adsorbent trap
Sample
Waste
Helium Column
Helium (a) Column
Sample
Waste
Figure 2.2:Gas flow control by means of a Valco 6-port gas valve. Two positions shown: (a) sample loading, (b) sample injection.
2.1. Gas chromatography system 23 2.1.2 Temperature programme
The GC oven temperature was maintained at 30◦C during the injection of a sample and held for 5 minutes.
Thereafter the temperature was increased at a rate of 65◦C min−1every 5 minutes and held at 60, 90, 150, 200 and finally 220◦C respectively (Chapter 3.2.2). This temperature programme, on the DB-624 column, resulted in separating bromoform with a retention time (tR) of approximately 14 minutes. A description of the development of the temperature programme can be found in Chapter 3.2.2.
2.1.3 Detector
A PerkinElmer F-22 ECD was attached to the left side of the GC oven as the original Shimadzu electron capture detector had failed. An external controller modulated the temperature of the ECD to a constant 300◦C via an injection cartridge. Operation of the detector at this temperature resulted in a low stable baseline signal of between 50 - 100 mV. A separate data controller collected and amplified the signal from the ECD. The signal from this controller in analogue format was then transmitted to a personal computer via a digital conversion process (Chapter 3.2.11).
2.1.4 Column
A J&W Scientific DB-624 (30 m x 0.32 mm and 1.8µm, 5 % polarity film) capillary column was used for the separation of bromoform from mixed air samples. The column was housed entirely within the oven. A direct, splitless connection was made between the carrier gas and the column.
2.1.5 Carrier and make-up gas
Helium (He, Grade 5.0, Air Products / AirLiquide, Cape Town, South Africa) was used as the carrier gas within the system for the separation of samples. A constant back pressure of 200 kPa through the column during the elution of samples resulted in carrier gas flow rate of 5 mlmin−1at 30◦C. The back pressure was maintained by means of a TESCOM Dräger regulator fitted to the helium cylinder. Swagelok and Valco gas tight fittings were used to deliver the helium carrier gas directly to the head of the column.
A flow of 30 mlmin−1 nitrogen (N2, Grade 5.0, AirLiquide, Cape Town, South Africa) was added directly to the ECD. This addition of nitrogen was controlled by the back pressure on the cylinder regulator (100 kPa) and a mechanical Porter valve. A Swagelok needle valve (SS-ORS2) was used as a gas shut off.
The effects of variation in carrier and make-up gas flow rate on bromoform detection and chromatography are discussed in Chapter 3.2.5.
2.1.6 Injection valve
As described above, a manually operated Valco 6-port valve was used for the injection of samples (Fig. 2.2).
The valve controlled the direction of carrier and sample gas flow through the system (Fig. 2.1). In the
‘load’ position the carrier gas passed directly onto the column, while sample gas passed through the adsorbent bed and to waste (Fig. 2.2a). During an injection the valve was switched, resulting in the carrier
gas passing through the adsorbent bed, in the opposite direction to sample loading, before passing onto the column. Sample gas was passed directly to waste in the laboratory. Heater tape (RS Components) was added to the tubing between the end of the TDU and the head of the column to minimise condensation of compounds after desorption. The tape was run constantly at its maximum output, maintaining a temperature up to 200◦C. Power for the heater tape was drawn directly from the mains of the building.
2.1.7 Sample pre-concentration
A custom-built TDU (UniTemp, Cape Town) was used for the pre-concentration of air samples (Fig. 2.3).
Two adsorbents, Carbopack X and Carboxen 1016 (9mg each) were housed in a glass tube restrained by glass wool. The adsorbents were packed such that an air sample was introduced to the less adsorbent Carbopack X first. During an injection the direction of gas flow was reversed ensuring samples were completely swept onto the column.
A Lauda RM-6 recirculation water chiller was used to circulate cooled glycol (-15◦C) through the external water jacket, maintaining a temperature of -10◦C within the adsorbent bed. Heating of the TDU was achieved using a high resistance coil regulated by a customised external alternating current (AC) controller. The TDU was heated from -10 to 300◦C in 2 min 30 sec.
Figure 2.3:Plan view of the thermal desorption unit.
2.1.8 Mass flow controller
An ASM AFC-260 mass flow controller was connected in series after the magnesium perchlorate drying trap. The mass flow controller (MFC) limited the gas flow rate through the TDU adsorbent bed to a rate of 100 mlmin−1.
2.1. Gas chromatography system 25 2.1.9 Air pump
A Rocker 400 piston air pump was used to draw external samples into the system and had a maximum air displacement rate of 400 mlmin−1. The pump was connected in-line to draw samples down the sampling line. Air samples were then pushed through the drying trap, mass flow controller and adsorbent bed. Excess air drawn down the sampling line was vented to the laboratory upstream of the mass flow controller and regulated by means of a Swagelok needle valve.
2.1.10 Drying trap
A custom-made glass trap with 8 mm glass tubes and a volume of approximately 20 ml filled with magnesium perchlorate was used to dry air samples. The magnesium perchlorate was held in place by a 1 cm plug of glass wool at each end of the trap. For a complete account of the chemicals and volumes tested see Chapter 3.2.10. Gas tight connections were achieved utilising silicone rubber ferrules with metal Swagelok reducing unions (Chapter 3.2.7.2).
2.1.11 Computer interface
Data output from the ECD was converted to a digital signal and recorded on a personal computer. A custom-built graphical user interface (GUI) was used to regulate the flow of data into the computer and record these data (Fig. 2.4). MATLAB provided the working support of the GUI and an internal script polled the serial (RS-232) port on the computer for data every 0.25 seconds. The digital signal read into the computer was displayed in real-time on a fixed set of axes. Time was displayed on the x-axis with detector response in millivolts on the y-axis. The data was recorded immediately and a figure was saved at the end of the sample measurement for later analysis.
0 5 10 15 20 25 30 35
50 100 150 200 250 300 350 400 450 500
Figure 2.4:Custom graphical user interface developed in MATLAB, used to collect, display and save data from the ECD. The status of system figures was included in this GUI.
2.1.12 Permeation oven
A custom-built permeation oven was used for calibration of the GC-ECD system as per Wevill and Carpenter (2004). A commercially purchased bromoform permeation tube (HRT-003.00-4096/70, A&J Scientific, Cape Town) was housed in a glass trap within a heating block (Fig. 2.5). The aluminium heating block was kept at 70◦C by four injection cartridges (Hotwatt, Superwatt: HS37-1.5) and a temperature controller. The flow of nitrogen (Grade 5.0, AirLiquide Cape Town) was regulated through the oven system at a pressure of 100 kPa and rate of 100 mlmin−1by means of the cylinder pressure regulator and a mechanical Porter valve. A Swagelok needle valve (SS-ORS2) was added in-line as a shut off.
Loop injection position TDU
TDU TDU Loop loading
position
Key:
Cylinder and regulator
Waste Tubing Sample loop
Tubing connector Heater block and
permeation tube
Thermal desorption unit
Figure 2.5:Plan view of the permeation oven unit used for the external calibration of the GC system.
Gas from the glass trap was continuously swept through a 100µlsample loop (Valco) before passing onto a halocarbon trap. Heater tape (RS Components) was wrapped around all tubing downstream of the glass trap to ensure no loss of bromoform occurred through condensation. An electric 6-port sample valve (Valco) was used to inject samples from the 100µlsample loop to the TDU adsorbent bed (Fig. 2.5).