Lecture 6CM3024

Spectral Interferences

• Emission Interference is controlled by:

• Correct alignment of the furnace

• Cleanliness of the furnace and spectrometer sample compartment windows.

• avoidance of extreme high atomisation temperatures

• Background Absorption:

• Attenuation of the incident line source radiation due to the light scattering by smoke particles or molecular absorption by matrix components in the graphite furnace, can cause false analytical signal unless it is corrected.

Spectral Interferences

• Background reduction techniques:

• Effective use of Pyrolysis step permits removal of many matrix components.

• However, it is not always possible to remove all of the matrix at this stage due to the relative volatilities of the analyte and matrix.

• It is possible to control the relative volatilities by use of chemical modifiers.

• A Chemical Modifier is a reagent which is added to a sample to

(a) increase the volatility of the matrix or

(b) decrease the volatility of the analyte

Spectral Interferences

• A wide range of modifiers have been used

• (a) Most samples are made up in a dilute solutions of nitric acid.

• This promotes the formation of volatile HCl

• (b) One of the more problematic matrices is NaCl. This is a non-volatile compound, to remove it requires pyrolysis temperatures that would result in the loss of many analytes.

• By adding ammonium nitrate the sample matrix is converted to a more volatile form

• Reaction: NaCl + NH4NO3 NaNO3 + NH4Cl

Spectral Interferences• Other classical modifiers convert the analyte to a less

volatile form, these modifiers include Ni and Pd.

• The effect of Ni as a modifier in the determination of Se is illustrated below.

Spectral Interferences

• Palladium retains analyte when in zero valence state, reduced by graphite, H2 or ascorbic acid.

• Pd can also be used in a ‘mixed modifier’ with Magnesium nitrate.

• Using reduced Pd, char temperatures can be increased by an average of 400K.

• Mechanism may be the formation of a stoichiometric complex of Pd-analyte.

• The physical occlusion of the analyte in the Pd ‘melt’ has also been postulated

Spectral Interferences

• Several other techniques can be used to minimise background interferences:

–use of smaller sample volumes

–modification of the atomisation temperature

–use of alternative wavelengths.

• It is usually not possible to completely eliminate background, especially when a complex matrix is present

• To eliminate background interference is is necessary to measure background alone as well as background plus analyte signal

Non-Spectral Interferences• The basis of AA is that free analyte atoms are present in

the light path.

• Non-spectral interferences occur when matrix components interfere with the formation of free atoms.

• Depletion of free atom concentration is due to the following mechanisms;

• Loss of volatile species during the dry and char stages.

• Occlusion of the analyte in a refractory matrix.

• Suppression of analyte chloride dissociation in the gas phase.

• Expulsion of the analyte by rapid evolution of vapourised matrix.

• Formation of stable analyte compounds.

• Standard Addition: Add known quantity of analyte to boost the signal.

• Only viable if the matrix component affecting formation of atoms from the analyte originally present affects the added analyte in exactly the same way.

• Graphite Tube Surface: The tube surface itself can lead to certain non-spectral interferences, especially carbide formation.

• To reduce this, pyrolytically coated graphite tubes are used. The denser surface on these tubes reduce analyte ‘soakage’ reducing analyte interaction with the carbon and therefore carbide formation.

• Chemical Modifiers: Modifiers delay of analyte atoms. This delay allows additional time for the furnace to reach constant temperature before atoms are produced

Control of Non-Spectral Interferences

The L’vov Platform

• To ensure freedom from non-spectral interferences, it is desirable to delay the appearance time of the analyte until the furnace has reached the designated atomisation step.

• This favours free atom formation, maximises sensitivity and produces constant sensitivity regardless of matrix.

• When the sample is placed directly onto the surface wall, it is heated quickly as the wall temperature increases.

• The temp. of the gas inside the furnace lags behind that of the wall and the gas is cool relative to the wall.

• When the wall reaches the temp. at which atoms are produced, atoms are released from the hot tube surface into the cooler gas phase.

• This sudden cooling inhibits atomisation of the vapourised molecular species and non-spectral interference results

• The L’vov platform is a small piece of pyrolytic graphite which is place in the bottom of the graphite tube.

• The sample is pipetted into a shallow depression in the bottom of the tube.

The L’vov Platform

• By contrast to wall atomisation, when the sample is placed on the platform it follows the temp. of the platform and not the wall.

• The platform is in poor physical contact with the wall and is heated by radiative rather than conductive energy.

• Therefore the platform temp. is far closer to that of the gas phase.

• When atoms are produced from the platform, they experience a hotter, more isothermal gas phase.

The L’vov Platform

Tube Wall and Platform Temp. Profiles

Time →

Tem

pera

tur

eInternal GasTemp.

AtomizationFrom Wall

AtomizationFrom Platform

Hydride Generation and Cold Vapour Techniques

• Alternative to flame atomisation for group IVb, Vb and VIb elements that form volatile hydrides; namely

Ge As Se

Sn Sb Te

Pb Bi

• These elements have resonance wavelengths from 193 - 283nm and show poor atomisation efficiences - detection limits obtained are poor.

• Hydride formation removes analyte from matrix and hydrides decompose at ~ 1000oC thus improving atomisation efficiency.

• Samples are reacted in an external system with reducing agent (usually sodium borohydride)

• Gaseous reaction products are carried to sampling cell in light path of the AA spectrometer.

• Reaction products are volatile;

GeH4 AsH3 SeH2

SnH4 SbH3 TeH2

PbH4 BiH3

Hydride Generation and Cold Vapour Techniques

Reduction Process

• Efficient hydride formation requires analyte in correct oxidation state, e.g., As can exist as As (III) or as As (V).

• Hydride formation is better for As (III). A preliminary reduction step is required often by addition of KI solution.

• Hydride formation results on addition of NaBH4 solution.

• Need to optimise acid concentration (use HCl, a non-oxidising acid) and NaBH4 concentration.

• Use ~1% w/v NaBH4 in 1M NaOH (for stability) is sufficient for most elements.

Hydride Generation Apparatus

1 Reduction Cell

2 Collection System

3 Atomisation Cell

1 Reduction Cell

• Usually glass cell of 50ml volume, typically sample is 10ml. Place sample in cell first, add HCl and thenNaBH4.

• Rapid generation of hydrides can be brought about by complete mixing in the reduction cell

• Hydride and H2 vapours evolved. For some elements,

however, the kinetics are less favourable and a collection system is required

Collection System• Gas is flushed from the reaction chamber using inert gas into a

cold trap in liquid nitrogen.• Trap is PTFE or silanised glass loop with packing material to

absorb hydride. H2 passes through.

• Heating trap with water bath results in rapid hydride release.

• Removal of H2 has conflicting effects, H2 is produced erratically therefore the H2/air flame can become erratic.

• However H2 is necessary for atom formation e.g.,

AsH3 + H. AsH2

. + H2

AsH2

. + H

. AsH +H2

AsH + H. As + H2

• If there is no H2, dimers form, not atoms, so steady stream of H2 added to purge gas

Atomisation Cell

• Once evolved from trap, hydride passes into atomisation cell. Variety of cell options;

• 1 H2 / air flame

• 2 Quartz tube heated in flame

• 3 Graphite Furnace

• 4 Electrically heated quartz tube

Mercury Cold Vapour AAS

• Mercury is a difficult element for which to obtain accurate analyses.

• Samples can easily be contaminated during handling, storage, and preparation for analysis.

• Mercury can also be lost from samples during the drying process or while in storage.

• Mercury is volatile even at room temperature, therefore samples require refrigeration at 4oC.

• This method is applicable for drinking, ground, surface, sea and brackish water, domestic and industrial wastes.

Mercury Distribution in US

• An aliquot of a water sample is transferred to a BOD bottle or equivalent closed-system container.

• The sample is digested with a dilute potassium permanganate-potassium persulfate solution for two hours at 950C.

• The digestion oxidizes all forms of mercury to Hg(II).

• The Hg(II) in the digested water sample is reduced with tin chloride to elemental mercury which is sparged from the sample and detected by atomic absorption.

Mercury Cold Vapour AAS

Automated Cold Vapour System

Sample Definitions

• Analytical Sample (AS) - Any sample in which mercury is being determined, excluding standards, method blanks, or QC reference samples.

• Calibration Blank (CB) - A volume of reagent water fortified with the same matrix as the calibration standards, but without the analytes.

• Calibration Standard (CAL) - A solution prepared from the primary dilution standard solution or stock standard solutions. The CAL solutions are used to calibrate the instrument response with respect to analyte concentration.

• Field Reagent Blank (FRB) - An aliquot of reagent water or other blank matrix that is placed in a sample container in the laboratory and treated as a sample in all respects, including shipment to the sampling site, exposure to the sampling site conditions, storage, preservation, and all analytical procedures. The purposes of the FRB is to determine if contamination is occurring in the field environment

• Field Duplicates (FD) - Two separate samples collected at the same time and placed under identical circumstances and treated exactly the same throughout field and laboratory procedures. Analyses of field duplicates indicate the precision associated with sample collection and storage as well as with laboratory procedures.

Sample Definitions

• Instrument Performance Check Solution (IPC) - A standard containing the analytes of interest which is used to verify the accuracy of the analysis and monitor instrument drift. It is analyzed periodically throughout an analysis sequence.

• Laboratory Duplicate (LD) - An aliquot of sample prepared and analyzed separately with identical procedures. Analysis of the sample and LD indicates precision associated with the laboratory procedures, but not with sample collection, preservation or storage procedures.

Sample Definitions

• Laboratory Fortified Blank (LFB) - An aliquot of reagent water or other blank matrix to which known quantities of the method analyte is added in the laboratory. The LFB is analyzed exactly like a sample, and its purpose is to determine whether the methodology is in control and whether the laboratory is capable of making accurate and precise measurements.

• Laboratory Fortified Sample Matrix (LFM) - An aliquot of an analytical sample to which known quantities of the method analyte is added in the laboratory. The LFM is analyzed exactly like a sample, and its purpose is to determine whether the sample matrix contributes bias to the analytical results.

Sample Definitions

• The background concentrations of the analyte in the sample matrix must be determined in a separate aliquot and the measured values in the LFM corrected for background concentrations.

• Laboratory Reagent Blank (LRB) - An aliquot of reagent water or other blank matrix that is treated exactly as a sample. The LRB is used to detect sample contamination resulting from the procedures used to prepare and analyze the samples in the laboratory environment.

• Linear Dynamic Range (LDR) - The concentration range over which the instrument response to an analyte is linear.

Sample Definitions

• Quality Control Sample (QCS) - An independent solution of the method analyte of known concentration. The QCS is obtained from a source external to the laboratory and different from the source of calibration standards. It is used to check either laboratory or instrument performance with externally prepared test materials.

• Standard Addition (SA) - The addition of a known amount of analyte to the sample in order to determine the relative response of the detector to an analyte within the sample matrix. The relative response is then used to assess either an operative matrix effect or the sample analyte concentration.

• Stock Standard Solution (SSS) - A concentrated solution containing one or more method analytes prepared in the laboratory using assayed reference materials or purchased from a reputable commercial source.

The Zeeman Effect

• Complete elimination of background interferences is not always possible due to matrix affects.

• However, electromagnets can be employed to isolate the signal of interest from background absorption by interferents

• The magnets are used to induce the Zeeman Effect in the analyte atoms during the atomisation process.

• Analyte atoms undergo splitting of the atomic absorption line which prevents atomic absorption momentarily at the wavelength of interest.

Background Correction

• The timing of the background correction is illustrated below.

• The background and analyte absorbance and the background only (BG) are measured alternately.

• Therefore, the background is not measured at exactly the same time as the analyte absorbance

analyte analyte

BG BG BG BGBG

analyte

BGBG

analyte

Spectral Interferences (Contd.)

• To compensate for this, an interpolation technique exists.