[Robertsays]

I've recently learned that glassware can absorb metallic ions and produce errors when analysing solutions with AAS. I was wondering to what degree of error can glass produce?

I'm making up 1 litre 1000mg/L magnesium stock standard and using an analytical balance accurate to around 0.1mg. I presumed the error due the the glass was small but i noticed my instructors used plastic container to prevent it.

Surely the error from the glass would be less then the analytical balance?

[Alpha-Omega]

From what you have written i am not completely sure what problems/intereferences you are experiencing in you analyses. However, the following may be contributory:

AAFS

In flame atomic absorption spectroscopy a liquid sample is aspirated and mixed as an aerosol with combustible gasses (acetylene and air or acetylene and nitrous oxide.) The mixture is ignited in a flame of temperature ranging from 2100 to 2800 degrees C (depending on the fuel gas used.) During combustion, atoms of the element of interest in the sample are reduced to the atomic state. A light beam from a lamp whose cathode is made of the element being determined is passed through the flame into a monochronometer and detector. Free, unexcited ground state atoms of the element absorb light at characteristic wavelengths; this reduction of the light energy at the analytical wavelength is a measure of the amount of the element in the sample.


INTERFERENCES

Solvents, reagents, glassware, and other sample processing hardware may yield artifacts and/or interferences to sample analysis. All of these materials must be demonstrated to be free from interferences under the conditions of the analysis by analyzing method blanks.Specific selection of reagents and purification of solvents by distillation in all-glass systems may
be necessary.

For the determination of trace metals, contamination and loss are of prime concern. Dust in the laboratory environment, impurities in reagents and impurities on laboratory apparatus which the sample contacts are all sources of potential contamination.

For liquid samples, containers can introduce either positive or negative errors in the measurement of trace metals by (a) contributing contaminants through leaching or surface resorption and (b) by depleting concentrations through adsorption. Thus the collection and treatment of the sample prior to analysis requires particular attention.

The sample bottle whether borosilicate glass, polyethylene, polypropylene or Teflon should be thoroughly washed with detergent and tap water; rinsed with 1:1 nitric acid, tap water, 1:1 hydrochloric acid, tap water and finally deionized distilled water in that order.

Chromic acid can be useful in removing organic deposits from glassware; however, the glassware must be thoroughly rinsed with water to remove the last traces of chromium. This is especially important if chromium is to be included in the analytical scheme.

There is a  commercial product--NOCHROMIX--available from Godax Laboratories, 6 Varick St., New York, N.Y. 10013, that can be used in place of chromic acid. (Chromic acid should not be used with plastic bottles.)

If it can be documented through an active analytical quality control program (using spiked samples, reagent and sample blanks) that certain steps in the cleaning procedure are not required for routine samples, those steps may be eliminated from the procedure.

The most troublesome type of interference in atomic absorption spectrophotometry is usually termed "chemical" and is caused by lack of absorption of atoms bound in molecular combination in the flame. This phenomenon can occur when the flame is not sufficiently hot to dissociate the molecule, as in the case of phosphate interference with magnesium, or when the dissociated atom is immediately oxidized to a compound that will not dissociate further at thetemperature of the flame.

The addition of lanthanum will overcome phosphate interference in magnesium, calcium, and barium determinations. Similarly, silica interference in the determination of manganese can be eliminated by the addition of calcium. A nitrous oxide/acetylene gas mixture may be used to help prevent interferences from refractory compounds.

Chemical interferences may also be eliminated by separating the metal from the interfering material. Although complexing agents are employed primarily to increase the sensitivity of the analysis, they may also be used to eliminate or reduce interferences.

The presence of high dissolved solids in the sample may result in an interference from non-atomic absorbance such as light scattering. In the absence of background correction, this can result in false positives and/or falsely elevated values. If background correction is not available, a non-absorbing wavelength should be checked. Signal contribution from uncorrected background can not be diagnosed through the analysis of spike recovery, nor is it compensated for by the application of the method of standard additions (MSA).

If background correction is not available and the non-absorbing wavelength test indicates the presence of
background interference, the sample digestates must be extracted (liquid-liquid or solid phase) prior to analysis, or another analytical method must be selected.

Ionization interferences occur when the flame temperature is sufficiently high to generate the removal of an electron from a neutral atom, giving a positively charged ion. This type of interference can generally be controlled by the addition, to both standard and sample solutions, of a large excess (1,000 mg/L) of an easily ionized element such as K, Na, Li or Cs. Each sample and standard should contain 2 mL KCl/100 mL of solution. This can be prepared using 95 g of potassium chloride in 1 L of reagent water for the KCl solution.

Spectral interference can occur when an absorbing wavelength of an element present in the sample, but not being determined, falls within the width of the absorption line of the element of interest. The results of the determination will then be erroneously high, due to the contribution of the interfering element to the atomic absorption signal. Interference can also occur when resonant energy from another element in a multielement lamp, or from a metal impurity in the lamp cathode, falls within the bandpass of the slit setting when that other metal is present in the sample. This type of interference may sometimes be reduced by narrowing the
slit width.

Be aware that viscosity differences and/or high dissolved or suspended solids may alter the aspiration rate.

All metals are not equally stable in the digestate, especially if it only contains nitric acid and not a combination of acids including hydrochloric acid. The addition of HCl helps stabilize Sn, Sb, Mo, Ba, and Ag in the digestate. The digestate should be analyzed as soon as possible, with preference given to these analytes.

INTERFERENCES FOR SAMPLES CONTAINING MAGNESIUM (PER EPA)

Magnesium -- All elements forming stable oxyanions (P, B, Si, Cr, S, V, Ti, Al, etc.) will complex magnesium and interfere unless lanthanum is added. Addition of lanthanum to prepared samples rarely presents a problem because virtually all environmental samples contain sufficient magnesium to require dilution.

References:

EPA 7000 Series Methods