Environmental Sampling and Analysis for Metals - Chapter 8 docx

While the air–acetylene flame is satisfactory for most elements determined by atomic absorption, the hotter nitrous oxide–acetylene flame is required for many refractory-forming elements

Flame Atomic Absorption

Spectrometry (FAAS)

8.1 PRINCIPLE

Flame atomic absorption spectrometry is a rapid and precise method of analysis In this atomic ab-sorption spectrometry technique (see Section 7.6.1), the sample is vaporized and atomized in a high-temperature flame Atoms of the analyte element absorb light of a specific wavelength from a hollow cathode lamp (HCL), passing through the flame The amount of energy absorbed by these atoms is measured and is proportional to the number of atoms in the light path A light beam is directed through the flame into a monochromator and onto a detector that measures the amount of light ab-sorbed by the atomized element in the flame The amount of energy at the characteristic wavelength absorbed in the flame is proportional to the concentration of the element in the sample over a limited concentration range Table 8.1 shows the FAAS concentration ranges

Determinations of analyte concentrations in a milligram-per-liter concentration region are routine for most elements However, trace metal analyses at microgram-per-liter and nanogram-per-liter levels are also needed Because the thermal energy from the flame is responsible for producing the absorbing species, flame temperature is an important parameter governing the flame process The two premixed

flames used almost exclusively for atomic absorption are air–acetylene (2125–2400°C) and nitrous oxide–acetylene(2000–2800°C)

While the air–acetylene flame is satisfactory for most elements determined by atomic absorption, the hotter nitrous oxide–acetylene flame is required for many refractory-forming elements The ni-trous oxide–acetylene flame is also effective in the control of some types of interferences

8.2 DIRECT AIR–ACETYLENE FLAME METHOD

8.2.1 GENERAL DISCUSSION

Because the thermal energy from the flame is responsible for producing the absorbing species, flame temperature is an important parameter governing the flame process Flames used in the AAS tech-nique are discussed in Section 7.3.2 The air–acetylene flame is satisfactory for most elements de-termined by atomic absorption, including Sb, Bi, Cd, Cs, Cr, Co, Cu, Au, Ir, Fe, Pb, Li, Mg, Mn, NI,

Pa, Pt, K, Rh, Ru, Ag, Na, Sr, Ta, Sn, and Zn

8.2.2 INSTRUMENTATION

See Section 7.3

8

8.2.3 REAGENTS

8.2.3.1 Air

The air used should be cleaned and dried through a suitable filter to remove oil, water, and other

for-eign substances Sources include a compressor or commercially bottled gas

8.2.3.2 Acetylene

The acetylene used should be a standard commercial grade Acetone, which is always present in

acety-lene cylinders, can be prevented from entering and damaging the burner head by replacing a cylinder

TABLE 8.1

Atomic Absorption Concentration Ranges, FAAS Technique

Instrument Optimum

Note: A–Ac = air–acetylene; N–Ac = nitrous oxide–acetylene.

a The more sensitive 217.0-nm wavelength is recommended for instruments with background correction capabilities.

when its pressure has fallen to 689 kPa (100 psi) acetylene Because of differences among makes and models of AASs, it is not possible to formulate measurements applicable to every instrument

8.2.3.3 Metal-Free Water

Use metal-free water for preparing all reagents and calibration standards and as dilution water Prepare metal-free water by deionizing and distilling, redistilling, or sub-boiling tap water Always check laboratory pure water to determine whether the element of interest is present in trace amounts

Note: If the source water contains Hg or other volatile metals, single or redistilled water may not

be suitable for trace analysis, because these metals remain in distilled water In such cases, use sub-boiling to prepare metal-free water

8.2.3.4 Stock Metal Solutions

Stock metal solutions are commercially available or can be prepared according to recipes in Appendix H

8.2.4 OPERATION

Because of differences among makes and models of AASs, it is not possible to formulate instruc-tions applicable to every instrument Follow the manufacturer’s instrucinstruc-tions, but in general proceed

as follows:

1 Install hollow cathode lamp for the desired metal

2 Set wavelength dial as specified by the analytical methodology (see Table 8.1)

3 Set slit width according to manufacturer’s suggested setting

4 Turn on instrument, apply the hollow cathode lamp current suggested by the manufacturer, and let the instrument warm up until energy source stabilizes, usually about 10 to 20 min

5 Readjust current if necessary after warm-up Adjust wavelength dial until optimum energy gain is obtained

6 Align lamp in accordance with manufacturer’s instructions

7 Install suitable burner head, and adjust its position A 10-cm, single-slot burner head is recommended for air–acetylene flames

8 Turn on air, and adjust flow rate according to manufacturer’s instructions to give maxi-mum sensitivity for the metal being measured

9 Turn on acetylene and adjust flow rate to value specified

10 Ignite flame and let it stabilize for a few minutes

11 Aspirate blank and zero instrument

12 Aspirate a standard solution and adjust aspiration rate of nebulizer to obtain maximum sensitivity

13 Adjust burner both vertically and horizontally to obtain maximum response

14 Aspirate blank again and re-zero instrument

15 Aspirate a standard with a concentration near the middle of the linear range and record absorbance

16 The instrument is now ready to operate

17 When analyses are finished, extinguish flame by turning off acetylene first and then air

8.2.5 STANDARDIZATION

1 Select at least three concentrations of each standard metal solution to bracket expected metal concentration of sample (See Table 8.1 for the optimum concentration ranges of metals.)

2 Aspirate blank and zero instrument

3 Aspirate each standard and record absorbances Prepare the calibration curve (Section 6.6) For instruments equipped with direct concentration readout, this step is unnecessary Preparation of standards and complete calibration processes are discussed in Section 6.6

Calibrate Ca and Mg based on concentration of standards before dilution with lanthanum solu-tion; Fe and Mn based on original concentration of standards before dilution with calcium solusolu-tion;

and Cr based on original standards before addition of H2O2 See methodologies for determining these metals in Chapter 18

8.2.6 SAMPLE ANALYSIS

1 Rinse the nebulizer by aspirating water with 1.5 ml of concentrated HNO3/l

2 Aspirate blank and zero instrument Analyze samples

8.2.7 CALCULATIONS

1 Calculate from calibration curve, or read directly from instrument the concentration in milligrams or micrograms per liter according to calibration

2 If the sample has been diluted, multiply concentration readout by the appropriate dilution factor

3 If the sample has been concentrated, divide concentration readout by appropriate concen-tration factor

8.3 DIRECT NITROUS OXIDE–ACETYLENE FLAME METHOD

8.3.1 GENERAL DISCUSSION

The hotter nitrous oxide–-acetylene flame (2600–2800°C) is required for many refractory-forming elements The nitrous oxide–acetylene flame is also effective in the control of some types of inter-ference This method is applicable to the determination of Al, Ba, Be, Mo, Os, Re, Si, Th, Ti, and V

8.3.2 APPARATUS

Use atomic absorption spectrophotometer and associated equipment See Section 7.3

8.3.2.1 Nitrous Oxide Burner Head

Use special burner head as suggested in the manufacturer’s manual At roughly 20-min intervals of operation, it may be necessary to dislodge the carbon crust that forms along the slit surface with a carbon rod or appropriate alternative Usually a special 5-cm head is required when using a nitrous oxide–acetylene flame Burner heads are discussed in Section 7.3.3

8.3.2.2 T-Junction Valve

Use a T-junction valve or other switching valve for rapidly changing from nitrous oxide to air, so that the flame can be turned on or off with air as the oxidant to prevent flashbacks

8.3.3 REAGENTS

8.3.3.1 Air

See Section 8.2.3

8.3.3.2 Acetylene

See Section 8.2.3

8.3.3.3 Nitrous Oxide

Nitrous oxide is commercially available in cylinders Fit the nitrous oxide cylinder with a special nonfreezable regulator, or wrap a heating coil around an ordinary regulator to prevent flashback at the burner caused by reduction in nitrous oxide flow through a frozen regulator Some AASs have au-tomatic gas control systems that will shut down a nitrous oxide flame safely in the event of a reduc-tion in nitrous oxide flow rate

8.3.3.4 Metal-Free Water

See Section 8.2.3

8.3.3.5 Potassium Chloride Solution

Dissolve 250 g of KCl and dilute to 1000 ml with water

8.3.3.6 Aluminum Nitrate Solution

Dissolve 139 g of Al(NO3)3.9H2O in 150 ml of water Acidify slightly with concentrated HNO3to prevent hydrolysis and precipitation Warm to dissolve completely Cool and dilute to 200 ml

8.3.3.7 Stock Metal Solutions

These solutions are commercially available or can be prepared according to recipes in Appendix H

8.3.4 OPERATION

Instrument operation follows the air–acetylene flame method, as discussed in Section 8.2.4 After steps 1 to 6, proceed as follows:

7 After adjusting wavelength, install a nitrous oxide burner head

8 Turn on acetylene (without igniting flame) and adjust flow rate according to value speci-fied by manufacturer for a nitrous oxide–acetylene flame

9 Turn off acetylene

10 With both air and nitrous oxide supplies turned on, set T-junction valve to nitrous oxide and adjust flow rate according to manufacturer’s specifications

11 Turn switching valve to the air position and verify that the flow rate is the same

12 Turn acetylene on and ignite to a bright yellow flame

13 With a rapid motion, turn switching valve to nitrous oxide The flame should have a red cone above the burner If it does not, adjust fuel flow to obtain red cone After nitrous oxide flame has been ignited, let burner come to thermal equilibrium before starting analysis

14 Atomize water containing 1.5 ml of concentrated HNO3per liter and check aspiration rate Adjust if necessary to a rate between 3 and 5 ml/min

15 Aspirate standard of desired metal with concentration near the midpoint of the optimum concentration range and adjust burner (both vertically and horizontally) in light path to ob-tain maximum response The instrument is ready for analysis

16 To extinguish flame, turn switching valve from nitrous oxide to air and turn off acetylene This procedure eliminates the danger of flashback that may occur on direct ignition or shutdown of nitrous oxide and acetylene

8.3.5 STANDARDIZATION

1 Select at least three concentrations of standard metal solution to bracket expected metal concentration of sample See Table 8.1 for optimum concentration ranges of metals

2 Aspirate blank and zero instrument

3 Aspirate each standard and record absorbances Prepare calibration curve (Section 6.6) For instruments equipped with direct concentration readout, this step is unnecessary Preparation of standards and complete calibration processes are discussed in Section 6.6 For 100 ml of Al, Ba, and Ti standards, add 2 ml of KCl solution (dissolve 250 g of KCl and di-lute to 1000 ml) For 100 ml of Mo and V standards, add 2 ml of Al(NO3)3.9H2O solution (Dissolve

139 g of Al(NO3)3.9H2O in 150 ml of water, acidify slightly with concentrated HNO3, and warm to dissolve completely Cool and dilute to 200 ml.)

Most modern instruments are equipped with microprocessors and digital readouts that permit calibration in the direct concentration range

8.3.6 ANALYSIS OF SAMPLES

1 After standardization, rinse atomizer by aspirating water with 1.5 ml of concentrated HNO3per liter and zero instrument

2 Analyze samples When analyzing Al, Ba, and Ti, add 2 ml of KCl solution to a 100-ml sample before analysis When analyzing Mo and V, add 2 ml of Al(NO3)3solution to a 100-ml sample

8.3.7 CALCULATIONS

See Section 8.2.7

8.4 INTERFERENCES, SAFETY, AND QUALITY CONTROL REQUIREMENTS

IN FAAS

See Sections 7.7, 7.8, and 7.9, respectively

8.5 MAINTENANCE OF FAA SPECTROPHOTOMETER

Maintenance activities for the FAAS are listed in Table 8.2

8.6 PERFORMANCE CHECK OF FAA SPECTROPHOTOMETER

Performance of the AAS is checked every time a different metal is analyzed The performance check

is an indicator of deterioration of the lamps or the spectrophotometer and reveals the optimal oper-ating condition of the instrument Performance is measured by a “sensitivity check standard” based

on a concentration specific to the method for each metal The absorbance of this standard should be 0.200 If it differs by more than 10%, the instrument is not performing correctly and has to be cor-rected Metal concentrations used in performance checks for the FAAS technique are presented in Table 8.3 Standard conditions for the FAAS technique are summarized in Table 8.4

TABLE 8.2 Maintenance of FAAS

Check tubing, pump, and lamps D Clean quartz windows W Check electronics SA (I) (C) Checks optics A (I) (C)

Note: D = daily; W = weekly; SA = semiannually; A = annually; I =

instrumentation specialist; C = on contract.

TABLE 8.3

FAAS Performance Check

Element Concentration of Sensitivity Standard (mg/l)

Barium (Ba) 20

Calcium (Ca) at 422.7 nm 4.0 Calcium (Ca) at 287.4 nm 60 Cadmium (Cd) at 228.8 nm 1.5 Cadmium (Cd) at 368.4 nm 850 Cobalt (Co) 7.0

Copper (Cu) 4.0 Iron (Fe) 5.0 Lead (Pb) 20

Nickel (Ni) 7.0 Silicon (Si) 100 Sodium (Na) 0.5

Tin (Sn) 150

Zinc (Zn) 1.0

Note: Performance of the FAA should be checked every time a metal is analyzed by

using a sensitivity check standard The sensitivity check data here pertain to the metal

concentration (mg/l) in aqueous solution, which will give a reading of approximately

0.20 absorbance units

Metal Fuel Oxidant SBW (nm)

Wavelength (nm)

Optimum Range (mg/l)

Sensitivity Check (mg/l)

Detection Limit (mg/l)

Addition of Suppressant

Ba Ac–N 2 O 0.4 553.6 1.0–20 20.0 0.1 2 ml KCL per

100-ml sample

Ca Ac–N 2 O 0.7 422.7 0.2–7 4.0 0.01 1 ml LaCl 3 per

10-ml sample

Cr Ac–N 2 O 0.7 357.9 0.5–10 4.0 0.05 2 ml KCL per

100-ml sample

Mg Ac–air 0.7 285.2 0.02–0.05 0.3 0.001 1 ml LaCl 3 per

10-ml sample

V Ac–N 2 O 0.7 318.4 2–200 90 0.2 2 ml Al(NO 3 ) 2

per 100-ml sample

TABLE 8.4

Standard Conditions for Flame AAS

Note: Potassium chloride (KCl) solution: Dissolve 95 g of KCl in analyte-free water and dilute to 1 liter Lanthanum chloride

(LaCl 3 ) solution: Dissolve 29 g of La 2 O 3 in 250 ml of concentrated HCl (be careful, because reaction is violent!), and dilute to

500 ml with analyte-free water Aluminum nitrate (Al(NO 3 ) 2 ) solution: Dissolve 139 g of aluminum nitrate nonahydrate (Al(NO 3 ) 2 9H 2 O) in 150 ml of analyte-free water Heat After dissolution, cool to room temperature, and dilute to 200 ml Suppressant should be added to the blanks, standards, and samples Addition of alkali salt is recommended to control ioniza-tion Aluminum is added to improve sensitivity and linearity Lanthanum is added to improve sensitivity.

a The use of an impact bead will improve sensitivity by about 2%.