The MAX ABS value will always be higher than the absorbance for

concentration C1 but you should not assume that the working range can be

arbitrarily extended up to MAX ABS. Remember that it is analytically preferable to operate on that portion of the calibration graph which is substantially linear.

Calibration Check

It is good analytical practice to check the calibration at particular intervals during your analytical program. Frequency of calibration will depend on the accuracy and precision required, and the type of samples being analyzed. Guidance regarding the optimum frequency of calibration is usually given in the methodology. Use the RESLOPE facility where this is analytically valid. Otherwise, re-calibrate against all standards. For IVO analyses, work strictly in accordance with all instructions given in the methodology.

Standards

For standard additions methods, refer to the guidelines given under 'Complex Unknown Aqueous Matrices'. For normal concentration calibration the following guidelines may be helpful in selecting standards:

 Only one standard is needed when the working range is strictly within the linear portion of the calibration graph.

Note that there is one special case where a single standard is almost always adequate. This is when working at an absorbance of 0.1 or lower, but where the sample concentration is at least ten times the detection limit.

 If the working curve is non-linear, but the deviation from linearity is small, two or three standards should be used. Four or five standards can be used when the working curve is nonlinear, or where the maximum working range is required. Note, however, that analysis time will be extended, and analytical accuracy may not always be improved.

 There is a special case for the use of two standards. This is the 'bracketing standards' method in which the concentrations of the standards are slightly lower and higher than the expected sample concentration.

 Remember that some solution schemes may be such that standards and samples do not contain identical volumes of blank or identical volumes of chemical modifier. In these circumstances, the contribution from blank or modifier to the analytical signal will not be the same for all solutions. The difference must be established and allowed for.

Simple Aqueous Matrices

It is usually sufficient to prepare a series of aqueous standard solutions - nitric acid standards for nitric acid digested samples, for example. Peak height or peak area measurements may be used although for some high temperatur9 elements it is preferable to use peak height measurements (refer to Chapter Three). The calibration will be automatically computed and displayed on the video screen. Multiple injection procedures can be performed manually to enhance the analyte signal when using the hand-injected technique, or they can be performed

automatically (see also Chapter Six). A few points to remember are:

 Method development will be required for each type of sample - refer to Chapters Three and Four.

 For manual operation, the multiple injections must be carried out at the appropriate point in the program, and suitable time periods must be built into the program to provide time for the injections. This period will vary according to analytical circumstances and the dexterity of the operator (it may need to be determined experimentally).

 When multiple injections are being placed manually in the graphite tube, any errors are additive. Careful placement of the droplet will aid precision, but analysts should remember that an additional error is likely to occur with each injection.

 As described in Chapter Three, a step is required to allow the gas flow to stop prior to the atomization stage.

Complex Known Aqueous Matrices

When samples such as steels, ores and other defined samples are to be analyzed, interferences between elements can occur. In a quality control situation, where the matrices are known, the standards can be chemically matched with the samples so that the behavior of standards and samples will be similar. For example, when steel is being analyzed for its aluminum content, the standards should include

constituents such as nickel, chromium, copper and iron at approximately the same concentrations as the sample.

Even for some complex samples, it is sometimes possible to use simple aqueous standards. Generally, the steps prior to atomizing the sample will completely remove the remainder of the sample matrix. Complexing reagents can be used for this purpose (see Chapter Seven), but each sample must be considered on its own merits.

Complex Unknown Aqueous Matrices

With complex samples, it is unlikely that the chemical and physical properties of standards prepared by normal methods will match those of the sample.

Consequently, the behavior of sample and standards during drying, ashing and atomization will not be identical, and analytical errors may occur. In these circumstances the standard additions technique should be used.

With the standard additions technique, mismatch between samples and standards is minimized because the standards are prepared from the actual samples. The general procedure is to take several aliquots of sample, add different quantities of analyte element to each aliquot and dilute to the final volume. These solutions then form a series of standards of different concentration. One aliquot is used without the addition of analyte element.

The quantities of analyte added are based on the expected concentration in the sample. If, for example, you use three standards, typical additions would be 50%, 100% and 150% of the concentration predicted in the sample.

Table 4 Typical standard additions dilution scheme*

Solutions mL Sample mL Standard (0.1 µg/mL)

mL Solvent Final Volume (mL)

No. 1 5.0 — 5.0 10

No. 2 5.0 1.0 4.0 10

No. 3 5.0 2.0 3.0 10

No. 4 5.0 3.0 2.0 10

Table 5 Concentration of Analyte

Solutions Concentrations Present in Solutions Prepared

No. 1 Original sample concentration (µg/mL) x 1/2

No. 2 Elemental concentration (µg/mL) x 1/2 + 0.01 (µg/mL) No. 3 Elemental concentration (µg/mL) x 1/2 + 0.02 (µg/mL) No. 4 Elemental concentration (µg/mL) x 1/2 + 0.03 (µg/mL) *Expected elemental concentration about 0.02 μg/mL.

By constructing a linear plot as shown In Figure 22 and extrapolating to zero absorbance, the concentration of the sample can be calculated.

When a sample type is being analyzed for the first time it is good analytical practice to compare the standard additions method with the normal calibration procedure. If the gradients of the two plots are equivalent, it can be assumed that there is negligible chemical interference and the standard additions technique would be unnecessary. Figure 22 illustrates an analytical situation where the standard additions curve and the calibration curve are parallel. This indicates that the standard additions method is not required.

Figure 22. Standard additions graph

Prepare the appropriate addition standards in accordance with the following criteria:

 The standards MUST be within the linear portion of the calibration graph. The automatic calibration program is based on linear regression and an accurate result cannot be calculated from non-linear calibration points.

 You may use up to five standards plus the zero addition.

 You will also need a reagent blank to establish the baseline.

In the most rigorous application of this technique, each individual sample must be analyzed separately against a set of addition standards which are specific for that sample. Under some circumstances, however, it is possible to analyze a number of samples against a single set of addition standards. This method can, of course, only be used where the samples are chemically and physically similar. The similarity must be such that the regression calibration established from the single set of addition standards is valid for all of the samples in the program.

The SpectrAA system enables you to use either of these methods automatically. In addition, the system can be programmed to use either addition standards prepared in your laboratory (SAMPLER PREMIXED), or, addition standards prepared automatically by the sample dispenser (SAMPLER AUTOMIXING).

Sampler Premixed

This is explained more fully in Chapter Six. Complete operational details are given in the Operation Manual.

This mode is intended to be used where it is preferable to prepare the addition standards in the laboratory.

The complete carousel load must consist of a series of batches, each batch containing blank, addition standards and sample (addition 0). All batches must have the same number of standards.

You may include more than one sample in each batch if this is analytically valid. In this circumstance, each batch must include the same number of samples as well as the same number of standards.

If you are proposing to use a modifier solution, you do not need to include the modifier when preparing your addition standards in the laboratory. The sample dispenser can be programmed to add the required volume of modifier solution. The analytical program can be performed completely automatically, or semi- automatically.

Sampler Automixing

This is explained more fully in Chapter Six. Complete operational details are given in the Operation Manual.

This mode is intended to be used where it is preferable to have the addition standards prepared automatically by the sample dispenser.

The addition standards will be prepared in accordance with the solution scheme that you compile on the SAMPLER page.

The analytical program can be performed completely automatically, or semi- automatically.

Organic Matrices

To establish an accurate calibration it is necessary to use organic standards (56). Organometallic standards are now commercially available in two forms:

 Oil-soluble organometallic powders

 Organometallic compounds in oil.

Suppliers of these standards include Conostan, B.D.H Chemicals and National Spectrographic Laboratories Inc. (57-69). The appropriate range of standards can be prepared by diluting with the required solvent.

It is important to ensure that samples and standards have closely similar

viscosities. For example, ensure that the required amount of blank oil is added to the standards. Always quote the method used to prepare the standards, that is, weight per volume, volume per volume or weight per weight. Conversion is possible if the specific gravity of the diluting solvent is known (70, 71).

The rules applicable to aqueous standards are equally applicable to organic standards. A reagent blank must be prepared and corrected for in all analyses. Once the blank level has been established the absorbance working range should be from 0.1 to 0.8 to obtain the best precision.

When standards are to be stored for any length of time, a stabilizer should be added to maintain the correct concentration levels.

Organic solvents that can be used for furnace analysis include 4-methyl-pentan-2- one (MIBK), xylene and kerosene. Tetrahydrofuran (THF) can also be used for furnace analysis. This has the further advantage of being miscible in both water and oils. When using organics over a prolonged period, ensure that the containers are well sealed since evaporation will occur, and thus increase the relative elemental concentration.