THE FLAME PROCESS

Atomic absorption is known as a very specific technique with few interferences.

The ultimate analytical method which is absolutely free of any interferences from

the nature of the sample will probably never exist.The next best thing to not having

interferences is to know what the interferences are and how to eliminate them or

compensate for them.The interferences in atomic absorption are well-defined,as

are the means for dealing with them.In order to understand these interferences

thoroughly,we will examine what goes on in the flame atomization process of

atomic absorption.

In order to get the atomic absorption process to occur,we must produce individual

atoms from our sample which starts out as a solution of ions.

.First,by the process of nebulization,we aspirate the sample into the burner chamber, where it mixes as a fine aerosol with the fuel and oxidant gases.At this point,the metals are still in solution in the fine aerosol droplets.As these tiny droplets pass into the heat of the flame,the process of evaporation or desolvation removes the solvent and leaves tiny solid particles of sample material.As more heat is applied, liquefaction will take place,and additional heat will vaporize the sample.At this point the metal of interest, called the analyte,is still

M ++A -(Solution)

1)Nebulization ¯

M ++A -(Aerosol)

2)Desolvation ¯

MA (Solid)

3)Liquefaction ¯

MA (Liquid)

4)Vaporization ¯

MA (Gas)

5)Atomization ¯

M o +A o (Gas)

6)Excitation ¯

M*(Gas)

7)Ionization ¯

M ++e -(Gas)

The flame process."M +"is a metal

cation and "A -"is the associated anion..bound up with some anion to form a molecule which does not exhibit the atomic

absorption phenomenon we wish to measure.By applying still more heat energy,

this molecule is dissociated into the individual atoms which make it up.

Since the thermal energy from the flame is responsible for producing the absorbing

species,flame temperature is an important parameter governing the flame process.

Temperatures for some flames that have been used in atomic absorption are listed

in Table 3-1.Cooler flames are subject to more interference problems resulting

from insufficient energy for complete atomization.The two premix flames now

used almost exclusively for atomic absorption are air-acetylene and nitrous ox-ide-

acetylene.While the air-acetylene flame is satisfactory for the majority of ele-ments

determined by atomic absorption,the hotter nitrous oxide-acetylene flame

is required for many refractory-forming elements.The nitrous oxide-acetylene

flame is also effective in the control of some types of interference.

Temperatures of Premix Flames

Oxidant-Fuel Temp.,°C

Air-Methane 1850-1900

Air-Natural Gas 1700-1900

Air-Hydrogen 2000-2050

Air-Acetylene 2125-2400

N2O-Acetylene 2600-2800

The number of ground state metal atoms formed in step 5 of the flame process

will determine the amount of light absorbed.Concentration is deter-mined

by comparing the absorbance of the sample to that of a known standard con-centration.

The relationship between the number of atoms in the flame and the

concentration of analyte in solution is governed by the flame process.If any con-stituent

of the sample alters one or more steps of this process from the performance

observed for a standard,an interference will exist,and an erroneous concentration

measurement will result if the interference is not recognized and corrected or compensated.

NONSPECTRAL INTERFERENCES

Interferences in atomic absorption can be divided into two general categories,

spectral and nonspectral.Nonspectral interferences are those which affect the formation of analyte atoms.

The first place in the flame atomization process subject to interference is the very

first step,the nebulization.If the sample is more viscous or has considerably dif-ferent

surface tension characteristics than the standard,the sample uptake rate or

nebulization efficiency may be different between sample and standard.If samples

and standards are not introduced into the process at the same rate,it is obvious

that the number of atoms in the light beam and,therefore,the absorbance,will not

correlate between the two.Thus,a matrix interference will exist.

An example of this type of interference is the effect of acid concentration on ab-sorbance.

From Figure 3-2,it can be seen that as phosphoric acid concentration

increases (and the sample viscosity increases),the sample introduction rate and

the sample absorbance decrease.Increased acid or dissolved solids concentration

normally will lead to a negative error if not recognized and corrected.Matrix in-terferences

can also cause positive error.The presence of an organic solvent in a

sample will produce an enhanced nebulization efficiency,resulting in an increased

absorption.One way of compensating for this type of interference is to match as closely as possible the major matrix components of the standard nto those of the sample. Any acid or other reagent added to the sample during preparation should also be added to the standards and blank in similar concentrations.

Method of Standard Additions

There is a useful technique which may make it possible to work in the presence

of a matrix interference without eliminating the interference itself,and still make

an accurate determination of analyte concentration.The technique is called the

method of standard additions.Accurate determinations are made without elimi-nating

interferences by making the concentration calibration in the presence of the

matrix interference.Aliquots of a standard are added to portions of the sample,

thereby allowing any interferent present in the sample to also affect the standard

similarly.

The standard additions technique is illustrated in Figure 3-3.The solid line passing

through the origin represents a typical calibration line for a set of aqueous stand-ards.

Zero absorbance is defined with a water blank,and,as the concentration of

analyte increases,a linear increase in absorbance is observed.

Let us now take equal aliquots of the sample.Nothing is added to the first aliquot;

a measured amount of standard is added to the second;and a larger measured

amount is added to the third.The first volume of added standard is usually selected

to approximate the analyte concentration in the sample,and the second volume is

normally twice the first volume.However,for the method of standard additions to be used ac-curately, the absorbances for all of the solutions must fall within the linear portion of the working curve.

Finally,all portions are diluted to the same volume so that the final concentrations of the original sample con-stituents are the same in each case.Only the amount of added analyte differs,and then by a known amount.

If no interference were present in this sample,a plot of measured absorbance ver-sus

the concentration of added standard would be parallel to the aqueous standard

calibration,and offset by an absorbance value resulting from the analyte present

in the unspiked sample.If some material is present in the sample which causes a

matrix interference,the number of ground state atoms producing atomic absorption

will be affected,as will be the absorbance from the analyte in the unspiked

sample.However,the absorbance increase from added standard will also be

changed by the same proportional amount since the concentration of interferent

is the same in each solution.Therefore,a straight line will still result,but because

of the interference,its slope will be different from that observed for the aqueous

standards.

In this situation,if the absorbance of the unspiked sample were to be compared

directly to the aqueous calibration,an error would result.If,however,the slope

determined by the standard additions to our sample is used as the calibration slope,

an accurate determination of the sample concentration can still be made.By con-tinuing

the concentration calibration on the abscissa backward from zero and ex-trapolating

the calibration line backward until it intercepts the concentration axis,

the concentration responsible for the absorbance of the unspiked sample is indi-cated.

An accurate determination has been made by calibrating in the presence of

the interference.

Properly used,the method of standard additions is a valuable tool in atomic ab-sorption.

The presence of an interference can be confirmed by observing the slope

of the spiked sample calibration and determining whether or not it is parallel to

the aqueous standard line.If it is not,an interference is present.If an interference

is present,the method of standard additions may allow an accurate determination

of the unknown concentration by using the standard additions slope for the cali-bration.

Caution should be used with the technique,however,as it can fail to give

correct answers with other types of interference.The method of standard additions

will not compensate for background absorption or other types of spectral inter-ference,

and normally will not compensate for chemical or ionization types of interference.