2.4.2.1 HG 3000 hydride generator
The flow manifold and set up for the HG 3000 hydride generator is shown in Figure 2.7. Arsine gas (AsH3) that was produced during the reaction of NaBH4 with the As sample was
separated from the liquid phase by the gas-liquid separator and transported to the graphite furnace by a stream of argon. The AsH3 was accumulated on a pre-deposited Pd modifier,
located within the graphite furnace, by inserting a glass capillary (which is connected to the gas-liquid separator via Tygon tubing) manually into the dosing hole of the furnace.
Figure 2.7 Schematic diagram of HG3000 hydride generation set-up used in the work for this thesis. All tubing used was supplied with the HG3000 except that used to
deliver AsH3 into the graphite furnace. The delivery tube was constructed using Ismatec
tygon tubing (2.06 mm internal diameter) which was connected to a glass capillary so that the sample could be introduced into the graphite furnace.
The gas-liquid separator that was supplied with the HG 3000 hydride generator was modified in-house to prevent solution from entering the Tygon delivery tubing and causing a blockage. Figure 2.8 shows the gas-liquid separator that was supplied with the HG 3000 hydride generator, and the supplied gas-liquid separator that had been modified by including an extra connector between the separator cap and the neck of the gas-liquid separator.
Figure 2.8 Photos of a) gas-liquid separator supplied with the HG3000 hydride generator, and b) modified gas-liquid separator used in the work for this thesis.
Sodium acetate NaBH4 Sample Reaction coil 2 mL min-1 10 mL min-1 2 mL min-1 Peristaltic pump Waste Argon Gas-liquid separator Graphite furnace
a)
b)
2.4.2.2 ETAAS and graphite furnace parameters
The instrumental parameters and the graphite furnace heating program for As measurements by HG-AAS are shown in Tables 2.6 and 2.7, respectively.
Parameter Value
Lamp manufacturer Photron
Lamp current/mA 10-16
Wavelength/nm 193.7
Slit width/nm 1.0
Slit height Reduced
Table 2.6 Instrumental parameters for As measurements by HG-AAS.
Step Final
Temperature/oC
Ramp time/s Hold time/s Inert gas Read
1 45 0.1 0 Yes No 2 110 10 10 Yes No 3 150 10 15 Yes No 4 800 10 15 Yes No 5 300 5 5 Yes No 6 300 5 30 No No 7 300 0 5 No No 8 300 5 15 Yes No 9 150 3.3 10 No No 10 2400 1.2 2 No Yes 11 2600 0.2 2 Yes No
Table 2.7 Graphite furnace heating program for As measurements by HG-AAS.
The PdII modifier (20 µL) was deposited in the graphite furnace in step 1 and thermally pre- treated at 800 oC in step 4. The AsH3 was accumulated at 300 oC in the graphite furnace in
step 6.
2.4.2.3 HG-AAS chemical and physical parameters
The chemical and physical parameters used for determination of As by HG-AAS are summarized in Table 2.8.
Parameter Value
[NaBH4] 0.12 mol L-1
[NaOH] in NaBH4 solution 0.05 mol L
-1
[PdII] 100 ppm
Volume of PdII solution 20 µL
[HCl] in sample 0.25 % v/v
[Sodium acetate] 0.25 mol L-1
[L-cysteine]* 0.25 % w/v
NaBH4 flow rate 2 mL min
-1
Sodium acetate flow rate 2 mL min-1
Sample flow rate 10 mL min-1
Argon flow rate 28 mL min-1
Pd pre-treatment temperature 800 oC
Accumulation temperature 300 oC
Accumulation time 30 s
Atomization temperature 2600 oC
Table 2.8 Chemical and physical parameters used for the analysis of As by HG-AAS.
*
Note that L-cysteine was only added to samples for total As analyses.
When using the parameters described in Table 2.8, and the ETAAS and graphite furnace parameters in Tables 2.6 and 2.7, respectively, the AsIII and total As detection limits were 0.01 and 0.005 ppb, respectively. Note that peak height was used to quantify all As measurements unless stated otherwise.
2.4.2.4 Preparation of reagents for HG-AAS
The preparation of reagents that were used for the generation and accumulation of AsH3 are
detailed below.
A 0.12 mol L-1 NaBH4 solution in 0.05 mol L-1 NaOH was prepared by dissolving 1 g of
NaOH (Analar, BDH) in ~ 350 mL of Milli-Q water, then 2.27 g of NaBH4 (GPR, BDH) was
added and made up to 500 mL. This solution was prepared daily to counter decomposition of NaBH4.
Sodium acetate buffer (0.25 mol L-1, pH 5.0) was prepared by dissolving 41.0 g of sodium acetate (Analar, Scharlau) in ~ 1 L of Milli-Q water; 16.5 mL of concentrated acetic acid (Analar, BDH) was added and then the solution was diluted to 2 L.
A 100 ppm PdII solution was prepared by adding 0.0166 g of PdCl2 (Merck) and 2.5 mL of
concentrated HNO3 (Aristar, BDH) to ~ 80 mL of Milli-Q water; this solution was heated to
assist the dissolution of PdCl2. The solution was made up to 100 mL.
A 0.1 mol L-1 L-cysteine solution was prepared by dissolving 6.25 g of L-cysteine in 500 mL of Milli-Q water. This solution was prepared daily.
2.4.2.5 Preparation of standards and samples for HG-AAS
For AsIII analyses, AsIII standards in the concentration range 0 to 0.8 ppb were prepared by dilution of a 100 ppb AsIII working stock solution (which was prepared by sequential dilution of the 1000 ppm AsIII stock solution). The required AsIII aliquot was added to ~ 20 mL of Milli-Q water, which contained 0.25 mL of concentrated HCl (Aristar, BDH), and made up to 100 mL.
For total As analyses, AsV standards in the concentration range 0 to 0.4 ppb were prepared by dilution of a 100 ppb AsV working stock solution (which was prepared by sequential dilution of the 1000 ppm AsV stock solution). L-cysteine (> 99 %, Applichem) was added to the AsV standards to reduce AsV to AsIII. The required AsV aliquot was added to 20 mL of 0.1 mol L-1 L-cysteine and left for 45 to 60 min; 0.25 mL of concentrated HCl (Aristar, BDH) was then added and the solution made up to 100 mL. The concentration of L-cysteine in the AsV standards was 0.02 mol L-1, which is equal to 0.25 % w/v.
Arsenic samples were prepared in the same way as above (depending on the particular As analysis) except that if the sample had already been acidified then the volume of HCl added was such that both standards and samples contained 0.25 % v/v HCl. In some circumstances the volume of the As sample added resulted in a HCl concentration greater than 0.25 % v/v; for these analyses the amount of HCl in the standards was increased so that both standards and samples contained the same concentration of HCl.
At pH 5.0, AsV does not form significant amounts of AsH3 from its reaction with NaBH4
whereas AsIII does. This allowed the concentrations of AsIII and AsV to be determined in samples containing both As species. Samples containing both AsIII and AsV were first analyzed selectively for AsIII, then for total As by adding L-cysteine. Addition of 0.25 % w/v L-cysteine to an AsV solution gives the same analytical response as addition of 0.25 % w/v L-
cysteine to an AsIII solution of the same As concentration. The concentration of AsV is determined by the difference between the total As and AsIII measurements.