At the EMEP WMO-GAW workshop (EMEP, 1997a), inductively coupled plasma mass spectrometry (ICP-MS) was chosen to be the reference technique within EMEP. The exception is mercury where cold vapour atomic fluorescence spectroscopy (CV-AFS) was chosen, but this technique is described in the separate mercury manual. Other techniques may be used, if they are shown to give results of equal quality as obtained with the recommended method. The choice of technique is dependent on the detection limits desired. In Table 4.17.1 the techniques described in this manual are presented with minimum detection limits. 23 countries, within the EMEP network, monitor and report heavy metal data. As shown in Table 4.17.2 various techniques are used (Berg et al, 2000). In this manual methods including the following four techniques are described: ICP-MS, graphite furnace atomic absorption spectroscopy (GF-AAS), flame-atomic absorption spectroscopy (F-AAS) and CV-AFS. The methods described are generally derived from development and experience gained within the EMEP network as well as information provided by EPA through the Ambient Monitoring Technological Information Centre (AMTEC).
Table 4.17.1: Minimum detection limits. These detection limits are the ultimate values since the blank value from the reagents and filter not have been taken into account.
Element |
ICP-MS |
GF-AAS |
F-AAS |
As |
<0.01 |
0.056 |
0.02 |
Cd |
<0.01 |
0.0014 |
0.5 |
Cr |
<0.01 |
0.0038 |
2 |
Cu |
<0.01 |
0.015 |
1 |
Ni |
<0.03 |
0.072 |
2 |
Pb |
<0.001 |
0.007 |
10 |
Zn |
<0.02 |
0.006 |
0.8 |
Hg |
|
0.2 |
0.001 |
a) Fisons
Scientific equipment, VG Instrument Group, Bulletin No.5M/AMSG/390, England
b) Perkin Elmer, new AnalystTM 800
detection limits,technical note,Norwalk,USA,1998
c) Parsons, M.L. and Forster, A.L., Applied Spectroscopy, 37 (1983) 411-418
Table 4.17.2: Analytical techniques for metal determination
within the EMEP network (Berg et al. 2000).
Techniques |
Number of laboratories |
Described in this manual |
NAA |
0 |
|
ICP-MS |
6 |
X |
GF-AAS |
4 |
X |
ICP-AES |
1 |
|
PIXE |
1 |
|
XRF |
1 |
|
F-AAS |
5 |
X |
CV-AFS |
7 |
X |
All reusable labware (glass, teflon, polyethylene etc) should be carefully rinsed before use to avoid contamination of the samples. Sampling cans and bottles should be rinsed with de-ionized water and soaked in 3% HNO3 for 24 hours. After the acid bath the bottles for storing of precipitation samples should be rinsed 3 times and then filled with 1% HNO3 and stopped.
The sampling cans should be rinsed 3 times with de-ionized water, dried, stopped and packed in two clean plastic bags with zip-locks.
Disposable pipette tips should be placed in a plastic bottle filled with 1% HNO3. Turn the bottle upside down a few times to assure that the tips are filled with the acid solution. Leave the tips in the acid solution for minimum 12 hours. Pour out the acid solution and rinse the tips by filling the bottle with de-ionized water 3 times. Shake as much as possible of the water out of the bottle and tips, and keep them in the stopped bottle until use.
The rings and filter supports from the filterpacks should be soaked in 1% HNO3 for 12 hours, rinsed 3 times with de-ionized water.
Autosampler tubes and cups (polystyrene or polyethylene) should be rinsed with de-ionized water, soaked in 1% HNO3 for minimum 12 hours and rinsed 3 times with de-ionized water before use.
ICP-MS is a multi-element technique that is suitable for trace analysis. The technique offers a long linear range and low background for most elements. The detection limits obtained are better or comparable to what is obtained by graphite furnace atomic absorption spectroscopy (GF-AAS). The technique is prone to some interferences that will be described below. Different sample introduction devices may be used in combination with ICP-MS to allow introduction of non-liquid samples such as solid samples, slurries and gaseous samples. In this chapter, only conventional solution introduction will be described.
ICP-MS is a technique where ions produced in an inductively coupled plasma, are separated in a mass analyser and detected. The sample solution is fed into a nebulizer by a peristaltic pump. The nebulizer converts the liquid sample into a fine aerosol that is transported into the plasma by an Ar gas flow, most often called carrier gas or nebulizer gas. With an ordinary pneumatic nebulizer, only 1-2% of the sample reaches the plasma. In the plasma the sample is evaporated, dissociated, atomised and ionised to varying extent. The produced positive ions and molecular ions are extracted into the mass analyser. A simple quadrupole gives a resolution of 1 amu or more at a peak width of 10% of the peak height. The ions are separated by mass to charge ratio (m/z) and measured by a channel electron multiplier. Detailed description of the ICP-MS technique can be found in various textbooks (Jarvis et al. 1992; Montaser, 1998).
In analysis by ICP-MS, the following interferences should always be considered:
Isobar overlap
Isobar
overlaps exist when two elements have isotopes of essentially the same mass. To
overcome this problem 1) a different isotope of the analyte can be chosen or 2)
by determining the signal for another isotope of the interfering element and by
using the natural abundance information, subtracting the appropriate signal
from the analyte isotope signal.
Isobar overlap by polyatomic ions
Isobar
overlap may occur due to formation of poly-atomic species. As the name
suggests, polyatomic species consist of two or more atomic species, e.g. ArO+.
They are formed by rapid ion-molecule reactions between components of solvent
or sample matrix with the constituents of the plasma. The dominant species in
the plasma and its surrounding are Ar, O, N and H. These elements can combine
with each other to give a variety of polyatomic ions. The main elements of the
solvent or acids used during sample preparation may also participate in these
ion-molecule reactions. A large number of polyatomic species may therefore
cause interference by isobar overlap. To which extent formation of polyatomic
ions occur, depends on several parameters including sampling geometry, plasma
and nebulizer conditions, choice of acids and solvents and the nature of the
sample matrix. By careful optimisation of the ICP-MS instrument, it is possible
to keep the formation of polyatomic species at the minimum and the elemental
sensitivity close to maximum. If interference from polyatomic species cannot be
avoided by selecting alternative isotopes of the analyte, appropriate
corrections should be made to the data.
Isobar overlap by doubly charged ions
Doubly charged ions are detected at half mass (m/2).
Most of the ions produced in the plasma are single charged. The elements that
might produce doubly charged ions are typically the alkaline metals,
alkaline-earth metals and some transition metals. At conventional operating
conditions of the plasma and nebulizer, the level of doubly charged ions is
small (< 1%).
Physical interferences
Physical
interferences are associated with nebulization and transport processes as well
as with ion-transition efficiencies. The efficiency of the nebulization and
transport processes depends on the viscosity and surface tension of the
aspirated solution. Therefore, physical interference (matrix effect) may occur
when samples and calibration standards have different matrix. In addition to
matrix-matching of samples and calibration standards, the use of internal
standard may reduce these problems.
ICP-MS systems are not tolerant to solutions containing significant amounts of dissolved solids. Clogging of nebulizers and salt build-up at the tip of the cones leads to poor sensitivity and considerable signal drift over a short period of time. A level of total dissolved solid (TDS) in the region of 0.1-2 (w/w) % is recommended (Perkin Elmer, 1993)]. High matrix concentration generally leads to poor precision. Memory effects may also be severe and time-consuming washout periods required. The use of flow-injection sample introduction may reduce some of these problems.
Memory effects
If there is a
considerably difference in concentration between samples or standards that are
analysed in sequence, memory effect may occur. The memory effect is caused by
sample deposition on the cones, and in the spray chamber. The effect is also
dependent on which type of nebulizer that is used. The washout time between
samples must be long enough to bring the system down to blank value.
Interference control
It is recommended to determine the concentrations of
the main components in the sample to be able to predict possible interference
effects on the analytes of interest. Following ions should be monitored in the
analysis programs:
25Mg, 24Na 27Al, 31P, 34S, 35Cl, 44Ca, 55Mn and 57Fe (see Table 11.2).
Reagents and standards
Nitric
acid (HNO3)
65%
Sodium arsenite (NaAsO2)
Cadmium metal (Cd)
Potassium
chromate (K2CrO4)
Copper
sulfate (CuSO4* 5H2O)
Nickel
sulfate (NiSO4*6H2O)
Lead
nitrate (Pb (NO3)2)
Zinc
metal (Zn)
ICP-MS stock solutions may be purchased or prepared from chemicals of ultra pure quality (99.9% or better). The standards should be dissolved in an appropriate acid (HNO3, HCl, HF) of suprapure quality. In addition to the chemical compound from which the stock solution is made, the acid that is used should be specified. This makes it possible to calculate the contents of ions that may cause problems by interference (Cl, SO42-). HNO3 gives a very simple spectrum and is for this reason considered as the ideal matrix. Only de-ionised water must be used (resistance > 18 MW/cm). Argon gas of high purity grade (99.99% or better) must be used.
Standard stock solutions (1000
mg ml-1)
As 1000 mg
ml-1:
Transfer
9.733 g NaAsO2 to a 1000 ml volumetric flask. Add distilled
de-ionised water to dissolve the salt. Add 5 ml 1:1 HNO3 and dilute
to the mark with distilled de-ionised water. Store the solution in a
polyethylene bottle.
Cd 1000 mg ml-1:
Transfer
1.000 g cadmium metal to a beaker.
Dissolve the metal in 10 ml 1:1 HNO3. Transfer the solution
to a 1000 ml volumetric flask. Dilute to the mark with distilled de-ionised
water. Store the solution in a polyethylene bottle.
Cr 1000 mg
ml-1:
Transfer
3.734 g K2CrO4 to a 1000 ml volumetric flask. Add
distilled de-ionised water to dissolve the salt. Add 5 ml 1:1 HNO3
and dilute to the mark with distilled de-ionised water. Store the solution in a
polyethylene bottle.
Cu 1000 mg
ml-1:
Transfer
3.930 g CuSO4* 5H2O to a 1000 ml volumetric flask. Add
distilled de-ionised water to dissolve the salt. Add 5 ml 1:1 HNO3
and dilute to the mark with distilled de-ionised water. Store the solution in a
polyethylene bottle.
Ni 1000 mg ml-1:
Transfer
4.477 g NiSO4*6H2O to a 1000 ml volumetric flask. Add
distilled de-ionised water to dissolve the salt. Add 5 ml 1:1 HNO3
and dilute to the mark with distilled de-ionised water. Store the solution in a
polyethylene bottle.
Pb 1000 mg ml-1:
Transfer
1.599 g Pb(NO3)2 to a 1000 ml volumetric flask. Add
distilled de-ionised water to dissolve the salt. Add 5 ml 1:1 HNO3
and dilute to the mark with distilled de-ionised water. Store the solution in a
polyethylene bottle.
Zn 1000 mg ml-1:
Transfer
1.000 g zink metal to a beaker. Dissolve the metal in 10 ml 1:1 HNO3.
Transfer the solution to a 1000 ml volumetric flask and dilute to the mark with
distilled de-ionised water. Store the solution in a polyethylene bottle.
Commercially available standard solutions may also be used.
Preparation of secondary stock standard
When making
mixed calibration standards there might be convenient to prepare a secondary
stock standard containing 1000 ng/ml of all the elements of interest, conserved
with 10 % (v/v) HNO3. This secondary stock standard may be stored
and used for 1 year. Care must be taken
in the preparation of mixed stock standards so that the elements are compatible
and stable.
Preparation of mixed calibration standards
Mixed
calibration standards are made by dilution of the secondary stock standard
solution to levels within the linear range for the instrument. The same acid
concentration and method of spiking must be used in calibration standards as in
the samples.
Internal standards
In most
analysis there is an advantage to implement three internal standards; one for
the low mass region, one for the mass region in the middle and one for the high
mass region. Care must be taken in the choice of elements to be used as
internal standards:
The following elements are often used as internal standards:
Sc m/z 45
Rh m/z 103
In m/z 115
Re m/z 185
Sc may be susceptible to isobar overlap from 89Y 2+, 14N216OH+, 28Si16OH+ and 44CaH+. This is a problem only when the concentrations of the mother ions are very high. If there is a risk for isobar overlap, Sc should be omitted when calculating the results.
When spiking the samples with internal standard, the precision of the addition of the spike solution should be better than 1%. Internal standard solution may be added before or after dilution to volume, but equal method of spiking must be used in calibration standards, blanks and samples!
Quality control standard
The
QC-standard (quality control standard) is the initial calibration verification
solution. This standard must be an independent standard made from a certified
reference solutions that are traceable to certified reference samples. An
independent standard is defined as a standard composed of analytes from a
different source than the calibration standard. The QC-standard must be
prepared in the same acid matrix as the calibration standards and contain the
same concentration of internal standard. The concentrations of the QC-standards
are determined of the applications in which the standards are used. A typical
concentration is 10 ng ml-1. The maximum acceptable deviation will
vary from element to element depending on sensitivity, background signal etc.
The measured concentration should be within 3 standard deviations of the mean
value based on results from analysis of a series of the QC standard. If the
measured concentration is more than 3 standard deviations, a re-calibration
must be done.
Blank solutions
Three different blank solutions are required; calibration
blank, procedural blank and rinse blank.
Tuning solution
Tuning
solution is used for tuning and mass calibration of the instrument. The
solution is prepared by diluting the secondary stock solution with 1 (v/v) %
HNO3 to produce a concentration of 10 ng/ml for each element.
Usually the tuning solution contains elements to cover the entire mass range
like Be, Co, In, La, Pb and U.
Sample preparation
Add 100 ml
of 10 ng ml-1 internal standard solution to the appropriate amount
of autosampler tubes. Transfer 10 ml of the sample digest to the tubes. The
samples are now ready for analysis.
Optimisation and stabilising of the instrument
Allow at least 30 min for the instrument to
equilibrate after ignition of the plasma before analysing any element. The
optimisation procedures will vary with varying types of instruments. The
instrument manual should be consulted with regards to optimisation procedures
and specification values. In general all ICP-MS systems shall be optimised to
give maximum sensitivity and minimum level of oxides and doubly charged ions.
The instrument parameters to adjust are as follows;
Aspirate a standard solution containing a suitable concentration (usually 10 ng/ml) of an element in the middle of the mass region (103Rh or 115In). Adjust the instrument parameters mentioned above, as described in the instrument manual, to obtain maximum sensitivity of the aspirated element.
Aspirating a standard solution containing 10 ng/ml 140Ce can check the oxide level at the condition chosen in 1). The ratio between the signal obtained at m/z 156 and m/z 140 should be low (consult instrument specifications for exact value).
Aspirating 10 ng/ml 138Ba can check the level of doubly charged ions. The ratio between the signals obtained at m/z 69 and m/z 138 should be low (consult instrument specifications for exact value).
If equal sensitivity in both the low and the high mass region is required, the lens setting should be adjusted to give equal response for 10 ng/ml 24Mg and 207Pb. This will lead to a minor decrease in the sensitivity obtained in point 1).
If poor sensitivity is obtained, following actions should be taken:
Mass calibration
A masscalibration check should be conducted
to ensure that the masses measured by the instrument, for the tuning solution,
are accurate with respect to the standard spectrum. If a signal shift of more than 0.1 amu is observed, mass
calibration should be adjusted as described in the instrument manual.
Sequence of analysis
Three
calibration blank standards should be analysed to establish a representative
blank level. Then the calibration standards are analysed. After calibration,
the quality control standard should be analysed to verify the calibration.
Flush the sample introduction system with rinse blank, and analyse the blank
solution to check carry-over and blank level. Analyse samples if blank level is acceptable. If blank values are
too high, repeat flushing of the sample introduction system and analysing of
blank solution until acceptable blank level is reached. The calibration blank
value, which is the same as the absolute value of the instrument response, must
be lower than the method detection limit.
Samples having concentration higher than the established linear concentration range should be diluted into range and reanalysed.
Table 4.17.3: Example
of a typical sequence of analysis.
Sequence no. |
Sample type |
|
1-3 |
Calibration blank |
Establish blank level |
4-9 |
Calibration standards |
Calibration |
10 |
Quality control standard |
Calibration verification (accuracy) |
11 |
Calibration blank |
Check for carry-over |
12-41 |
Samples |
|
42 |
Quality control standard |
|
43 |
Calibration standard |
|
44 |
Calibration standard |
|
Table 4.17.4: Isotopes of the priority heavy metals and some possible interferences.
Element |
Isotope mass |
Relative abundance |
Isobar
overlap |
Poly-atomic species |
Cr |
52 |
83.76 |
|
ArC+, 35ClOH+, |
|
53 |
9.55 |
|
37ClOH+ |
Ni |
58 |
67.88 |
58Fe |
42CaO, |
|
60 |
26.23 |
|
44CaO, |
|
61 |
1.19 |
|
|
|
62 |
3.66 |
|
46CaO |
|
64 |
1.08 |
|
48CaO |
Cu |
63 |
69.09 |
|
TiO+, ArNa+, PO2+ |
|
65 |
30.91 |
|
ArMg+ |
Zn |
64 |
48.89 |
64Ni (1.8) |
SO2+, SS+, ArMg+ |
|
66 |
27.81 |
|
ArMg+ |
As |
75 |
100 |
|
Ar35Cl+ |
Cd |
111 |
12.75 |
|
93MoO+ |
|
114 |
28.86 |
114Sn (0.66) |
|
Pb |
204 |
1.48 |
204Hg (6.85) |
|
|
206 |
23.6 |
|
|
|
207 |
22.6 |
|
|
|
208 |
52.3 |
|
|
Graphite furnace atomic absorption spectroscopy (GF-AAS) is a powerful technique suitable for trace analysis. The technique has high sensitivity (analyte amounts 10-8-10-11 g absolute), the ability to handle micro samples (5-100 ml), and a low noise level from the furnace. Matrix effects from components in the sample other than the analyte are more severe in this technique compared to flame-AAS. The precision is typically (5-10) % using GF-AAS.
A graphite tube is located in the sample compartment of an AA spectrometer with the light from an external light source passing through it. A small volume of sample is placed inside the tube, which then is heated by applying a voltage across its ends. The analyte is dissociated from its chemical bonds and the fraction of analyte atoms in the ground state will absorb portions of light. The attenuation of the light beam is measured. As the analyte atoms are created and diffuse out of the tube, the absorption raises and falls in a peak-shaped signal. Beer-Lamberts law describes the relation between the measured attenuation and concentration of analyte. A detailed description of the GF-AAS technique can be found in various textbooks (Montaser, 1998).
Background absorption
Background
absorption is non-specific attenuation of radiation at the analyte wavelength
caused by matrix components. To compensate for background absorption,
correction techniques such as continuous light source (D2-lamp),
Zeeman or Smith-Hieftje should be used. Enhanced matrix removal due to matrix
modification may reduce background absorption.
Non-spectral interference (Matrix effect)
Non-spectral
interference arises when components of the sample matrix alter the vaporization
behaviour of the particles that contains the analyte. To compensate for this
kind of interference, method of standard addition can be used. Enhanced matrix
removal by matrix modification or the use of a L“vov platform may also lead to
a reduction of non-spectral interferences.
Atomic absorption spectrophotometer single- or double-beam instrument having a grating monochromator, photomultiplier detector, adjustable slits, equipment for flameless atomization (graphite furnace) and a suitable recorder or PC. The wavelength range must be 190-800 nm.
Hollow cathode lamps for As, Cu, Cr, Ni, Pb and Zn. Single-element lamps are preferred, but multi-element lamps may be used if no spectral interference can occur. Electrodeless discharge lamps may be used if available.
Pyrolytically coated graphite tubes.
All chemicals must be of analytical grade or better.
Distilled de ionized water | |
Nitric acid | (HNO3) 65% |
Sodium arsenite | (NaAsO2) |
Cadmium metal | (Cd) |
Potassium chromate | (K2CrO4) |
Copper sulphate | (CuSO4* 5H2O) |
Nickel sulphate | (NiSO4*6H2O) |
Lead nitrate | (Pb (NO3)2) |
Zinc metal | (Zn) |
Palladium nitrate | (Pd(NO3)2) |
Magnesium nitrate | (Mg(NO3)2) |
Lanthanum nitrate | (La(NO3)2*6 H2O) |
Ammonium phosphate | ((NH4)3PO4) |
Argon (Ar) as purge gas. |
Standard stock solutions (1000
mg ml-1)
As 1000 mg
ml-1:
Transfer
9.733 g NaAsO2 to a 1000 ml volumetric flask. Add distilled
de-ionised water to dissolve the salt. Add 5 ml 1:1 HNO3 and dilute
to the mark with distilled de-ionised water. Store the solution in a
polyethylene bottle.
Cd 1000 mg ml-1:
Transfer
1.000 g cadmium metal to a beaker.
Dissolve the metal in 10 ml 1:1 HNO3. Transfer the solution
to a 1000 ml volumetric flask. Dilute to the mark with distilled de-ionised
water. Store the solution in a polyethylene bottle.
Cr 1000 mg
ml-1:
Transfer
3.734 g K2CrO4 to a 1000 ml volumetric flask. Add distilled de-ionised water to dissolve
the salt. Add 5 ml 1:1 HNO3 and dilute to the mark with distilled
de-ionised water. Store the solution in a polyethylene bottle.
Cu 1000 mg
ml-1:
Transfer
3.930 g CuSO4* 5H2O to a 1000 ml volumetric flask. Add
distilled de-ionised water to dissolve the salt. Add 5 ml 1:1 HNO3
and dilute to the mark with distilled de-ionised water. Store the solution in a
polyethylene bottle.
Ni 1000 mg ml-1:
Transfer
4.477 g NiSO4*6H2O to a 1000 ml volumetric flask. Add
distilled de-ionised water to dissolve the salt. Add 5 ml 1:1 HNO3
and dilute to the mark with distilled de-ionised water. Store the solution in a
polyethylene bottle.
Pb 1000 mg ml-1:
Transfer 1.599
g Pb(NO3)2 to a 1000 ml volumetric flask. Add distilled
de-ionised water to dissolve the salt. Add 5 ml 1:1 HNO3 and dilute
to the mark with distilled de-ionised water. Store the solution in a
polyethylene bottle.
Zn 1000 mg ml-1:
Transfer
1.000 g zink metal to a beaker. Dissolve the metal in 10 ml 1:1 HNO3.
Transfer the solution to a 1000 ml volumetric flask and dilute to the mark with
distilled de-ionised water. Store the solution in a polyethylene bottle.
Commercially available standard solutions may also be used.
Calibration standards
Calibration
standards are prepared by single or multiple dilutions of the stock metal
solutions. Prepare a reagent blank and at least 3 calibration standards in
graduated amount in the appropriate range of the linear part of the curve. The
calibration standards must contain the same acid concentration as in the
samples following processing. For precipitation samples, that would be 1% (v/v) HNO3 and for suspended
particulate matter10% (v/v) HNO3. The calibration
standard should be transferred to polyethylene bottles.
Table 4.17.5: Calibration
range.
|
As |
Cd |
Cr |
Cu |
Pb |
Ni |
Zn |
Calibration range (mg l-1) |
0-100 |
0-5 |
0-40 |
0-50 |
0-50 |
0-40 |
0-5 |
The operating instructions will vary between various brands and models of satisfactory instruments, making it virtually impossible to give precise details of a proposed GF-AAS method that is guaranteed to reduce interference effects on all commercial instruments. The instrument manual should be confirmed in regards of operating instructions. A careful interference study and calibration procedure as given in the particular instrument manual must be carried out by the analyst. Some general guidelines are given below.
A temperature programme consists most commonly of four steps: Drying, pyrolysis, atomization and cleaning.
Drying step: A quick ramp (5 s) to 15oC below the boiling point of the solvent. Then a slow ramp (25 s) to reach a temperature just above the solvents boiling point. This provides a gentle evaporation without sputtering. Hold the furnace at the selected temperature until drying is complete (5- 10 s). The drying time will vary with sample volume and salt content. A purge gas flow of 250-300 ml min-1 is normally used.
Pyrolysis step: A pyrolysis curve should be made to find the appropriate temperature to use in this step without losing any analyte. Consult the instrument manual for the procedure of making a pyrolysis curve. In a pyrolysis step a typical ramp will vary between 20-50 oC s-1. Too steep ramp may cause sputtering. A purge gas flow of 250-300 ml min-1 is normally used.
Atomization step: An atomization curve should be made to find the appropriate temperature to use in this step. Consult the instrument manual for the procedure of making an atomization curve. The lowest temperature that still gives maximum signal should be used in order to extend the lifetime of the graphite tube. Zero ramp time is used in this step. Gas stop during atomization is recommended.
Cleaning step: A tube cleaning cycle after the analyte measurement should be done to remove any remains of sample and thereby avoid memory effects. A purge gas flow of 250-300 ml min-1 is normally used.
All times and temperatures are guidelines only.
The characteristic mass (sometimes called sensitivity) is defined as the absolute mass of an element that will absorb 1% of the incoming radiation. This equals a signal of 0.0044 absorbance units (AU). The characteristic mass may be used as an indicator of instrument optimisation. Values of the characteristic masses are most often given in the instrument documentation. Experimental values for comparison can be determined by measuring the absorbance signal (area) of a known mass of analyte and calculate using the following formula:
mo = Vs * Cs*0.0044 AU / observed peak area
mo: Characteristic mass (ng)
Vs : Standard volume injected (ml)
Cs : Standard concentration (ng ml-1)
Table 4.17.6: Proposed instrument parameters.
|
l, nm |
slit |
Drying temp |
Pyrolysis temp |
Atomization temp |
Chemical modifier |
Pyrolysis temp. |
Atomization temp. |
As |
193.7 |
0.7 |
120 |
500 |
2300 |
Pd(NO3)2+ |
1300 |
2300 |
Cd |
228.8 |
0.7 |
120 |
350 |
1800 |
|
|
|
Cr |
357.9 |
0.7 |
120 |
1350 |
2660 |
Mg(NO3)2 |
1650 |
2500 |
Cu |
324.7 |
0.7 |
120 |
900 |
2600 |
|
|
|
Pb |
217.0 |
0.7 |
120 |
550 |
2000 |
(NH4)3PO4 or La(NO3)2 |
700 |
1800 |
Ni |
232.0 |
0.2 |
120 |
1200 |
2600 |
|
|
|
Zn |
213.9 |
0.7 |
120 |
350 |
1800 |
Mg(NO3)2 |
700 |
1800 |
Other operating parameters should be set as specified by the particular instrument manufacturer.
In order to achieve better separation between analyte and matrix prior to atomisation, a chemical modifier can be used. The role of the modifier is most often to stabilise the analyte making higher temperatures in the pyrolysis step possible without any loss of analyte. The concentration level of most modifier mixtures is usually in the ppm level. The injection volume most often is in the 5-20 µl region. The modifier mixture should be injected and dried prior to sample injection. For suggestions of chemical modifiers for the various elements see Table 4.17.6.
F-AAS is a very specific technique prone to few interference effects. F-AAS is a single element technique with analyte determinations in the mg l-1 region as routine for most elements.
A liquid sample is nebulized to form a fine aerosol, which is mixed with fuel and oxidant gasses and carried into a flame. In the flame the sample is dissociated into free ground state atoms. A light beam from an external light source emitting specific wavelengths passes through the flame. The wavelength is chosen to correspond with the absorption energy of the ground state atoms of the desired element. The measured parameter in F-AAS is attenuation of light. Lambert-Beers law expresses the relationship between the attenuation of light and concentration of analyte.
F-AAS is known as a technique with few problems related to interference effects. The interferences that occur are well defined, as are the means of dealing with them. For analysis of a few elements the type and temperature of the flame are critical; with improper conditions ionisation and chemical interferences may occur.
Ionisation
Ionisation
of the analyte atoms in the flame depletes the levels of free ground state
atoms available for light absorption. This will reduce the atomic absorption at
the resonance wavelength and lead to erroneous results. The degree of
ionisation of a metal is strongly influenced by the presence of other ionisable
metals in the flame. By addition of an excess of a very easily ionised element
to the blanks, standards and samples the effect of ionisation can usually be
eliminated. Ionisation is most common in hot flames such as nitrous oxide-
acetylene flames. In an acetylene-air flame ionisation is most often limited to
be a problem in analysis of the alkali- and alkaline earth metals.
Chemical interference
The
most common type of chemical interference occurs when the sample contains
components that forms thermally stable compounds with the analyte and thus
reduce the rate at which it is atomised. Adding an excess of a compound that
form thermally stable compounds with the interfering element eliminates
chemical interference. For example, calcium phosphate does not dissociate
completely in the flame. Addition of Lanthanum will tie up the phosphate
allowing calcium to be atomised. A second approach to avoid chemical
interference is, if possible, to use a hotter flame. Using the method of
standard addition can also control chemical interference.
Physical interference
If the
physical properties as viscosity and surface tension vary considerably between
samples and standards, the sample uptake rate or nebulization efficiency may be
different and lead to erroneous results. Dilution of samples or method of
standard addition or both can be used to control these types of
interferences.
Background absorption and light scattering
Matrix
components that are not 100% atomised and that has broadband absorption spectra
may absorb at the analytical wavelength. Tiny solid particles in the flame may
lead to scattering of the light over a wide wavelength region. The background
absorption can be accounted for by using background correction techniques such
as continuous light source (D2-lamp) or Smith-Hieftje.
Atomic absorption spectrophotometer single- or double-beam instrument having a grating monochromator, photomultiplier detector, adjustable slits, equipped with a air-acetylene burner head and a suitable recorder or PC. The wavelength range must be 190-800 nm.
All reagents must be of analytical grade or better.
Distilled de-ionized water
Nitric acid
(HNO3)
Zinc metal
(Zn)
Acetylene gas
(99,99% or better)
Air supply
Standard stock solutions (1000
mg ml-1)
Zn 1000 mg
ml-1:
Transfer
1.000 g zink metal to a beaker.
Dissolve the metal in 10 ml 1:1 HNO3. Transfer the solution
to a 1000 ml volumetric flask and dilute to the mark with distilled de-ionised
water. Store the solution in a polyethylene bottle.
Commercially available standard solutions may also be used.
Calibration standards
Calibration
standards are prepared by single or multiple dilutions of the stock metal
solution. Prepare a reagent blank and at least 3 calibration standards in
graduated amount in the appropriate range of the linear part of the curve. The
calibration standards must contain the same acid concentration as will result
in the samples following processing. For precipitation samples, that would be
1% (v/v) HNO3 and for suspended particulate matter10% (v/v) HNO3.
The calibration standard should be transferred to polyethylene bottles.
The operating procedure will vary between instrument brands, so the instrument manual should be followed carefully. The position of observation and the fuel:oxidant ratio must be optimised. Some general guidelines are outlined below
The characteristic concentration (sometimes called sensitivity) is defined as the concentration of an element (mg l-1) that will absorb 1 % of the incoming radiation. This equals a signal of 0.0044 absorbance units (AU). The characteristic concentration is instrument dependent and is calculated as follows:
C = (S * 0.0044 AU) / measured absorbance
C: Characteristic
concentration (mg l-1)
S: Concentration
of measured standard (mg l-1)
Knowing the characteristic concentration allows the analyst to check if the instrument is correctly optimised and performing up to specifications.
Berg, T., Hjellbrekke, A.G. and Larsen, R. (2000) Heavy metals and POPs within the EMEP region 1998. Kjeller, Norwegian Institute for Air Research (EMEP/CCC-Report 2/2000).
EMEP (1997) EMEP-WMO workshop on strategies for monitoring of regional air pollution in relation to the need within EMEP, GAW and other international bodies. Aspenäs, Sweden, 2-4 June 1997. Kjeller, Norwegian Institute for Air Research (EMEP/CCC-Report 10/97).
Jarvis, K.E., Gray, A.L. and Houk, R. S. (1992) Handbook of inductively coupled plasma mass spectrometry. Glasgow, Blackie.
Montaser, A. (1998) Inductively coupled plasma mass spectrometry. New York, Wiley.