Efficient Use of Spectral Information Through Multiline Analysis in ICP-AES

The classical way to conduct the determination of an element in inductively coupled plasma-atomic emission spectrometry (ICP-AES) is to select a single line with adequate sensitivity and that is free of spectral interferences. When the nature and concentration of the matrix elements are more or less constant, there is no risk of unexpected spectral interferences, and the selection of a single line per element is appropriate.

In marked contrast, when the matrix exhibits changes, the possibility of unexpected spectral interferences becomes significant, and the concentration deduced from the use of a single line may then be wrong. With the use of multichannel detection such as a charge-coupled device (CCD) or charge-injection device (CID), the amount of spectral information has increased significantly compared to the use of photomultiplier tubes, which are inherently single-channel detectors. It is then possible to acquire the full UV-VIS spectra for each element. Selecting a single line per element is therefore a waste of information, since many other sensitive lines may be available for the concentration measurement.

In the development of an ICP-AES instrument, efficient use of the available information must be taken into consideration. One approach is to conduct multiline analysis, i.e., the use of multiple lines per element, in order to handle the unexpected spectral interferences and improve the reliability of the results. Multiline analysis was therefore one of the major specifications for the ACTIVA-M CCD-based ICP-AES instrument (HORIBA Jobin Yvon, Longjumeau Cedex, France). The Czerny-Turner optical system is equipped with a megapixel, low-noise CCD detector, resulting in spectral windows up to 8 nm and a constant resolution over the spectral range of better than 10 pm with a 4343-line/mm holographic grating. Adjacent spectral windows allow the system to cover the entire 120–800 nm range.1

However, multiline analysis can only be performed efficiently if the right tools are available to facilitate multiline selection and to statistically process the data. Two software tools dedicated to this application are the MASTER (Multi-line Analysis, Selection Tool for Enhanced Reliability) tool for multiline selection, and the SOS (Statistical Outliers Survey) tool for data statistical processing (HORIBA Jobin Yvon).

Because of the lack of ICP-dedicated wavelength tables, line selection involves the preparation of synthetic solutions that mimic the matrix. Running numerous samples and line profiles is therefore tedious and time consuming. Multiline selection becomes even more complex. Thus, the company developed a base with a double access: a collection of single-element spectra obtained under standard ICP operating conditions and a spectroscopic database containing wavelengths, sensitivities, limits of detection, line widths, and background values. More than 50,000 lines have been assigned with the corresponding spectroscopic data; details of the proprietary base have been published previously.2

Table 1 - List of elements and their expected concentration range (Clow and Chigh) to be determined in a series of low-alloy steel samples

Basically, all the user has to specify is the list of elements and their expected concentration ranges. Then, the MASTER tool conducts a preselection through a filtering procedure that examines the influence of the highest concentration of the concomitant elements on the lowest concentration of the analyte, i.e., under the most adverse conditions, with a user-defined criterion for spectral interferences. Once a list of lines has been suggested, MASTER provides the ability to display each line window, which includes single-element spectra of the analyte and concomitant elements within the line vicinity. Moreover, the blank spectrum can also be displayed, allowing interactive background correction. Display helps in the ultimate validation or rejection of a line. Lines and backgrounds are then exported, for an automatic setup of the analytical method for analysis.

Once the determination of the concentration has been carried out for each line of a given element, the SOS tool performs a statistical analysis to detect possible outliers. As a result, the remaining concentrations are averaged to provide a reliable concentration per element.3

The use of the MASTER and SOS tools will be illustrated with an example of the determination of niobium (Nb) in low-alloy steel samples. This application is a typical example in which the concentration of the concomitant elements can vary, and then generate unexpected positive bias (if the analyte peak is interfered with) or negative (if the background correction position is interfered with). Reprocessing the results with new background correction positions is possible, but is time consuming and requires some expertise; thus, true multiline analysis is effective along with the supporting tools.

Figure 1 - The 316.340 line of Nb is validated () and the position of the background correction is set up (blue dashed line).

As mentioned above, the analyst must specify the list of elements and their concentration range (see Table 1). The MASTER tool selects lines of Nb that are appropriate for the 1–10 mg/L concentration range and are not interfered with by the concomitant elements when present at the highest concentration. The total number of lines for selection as well as sensitivity and interference filtering criteria are user defined. Each line selected is then displayed by overlapping the 1-mg/L Nb spectrum with the maximum concentration single-element spectra of the concomitant elements. This permits the user to validate the lines for the application and determine the correct background correction positions (Figure 1). In addition, the nonselected lines can be visualized to understand the selection performed by the MASTER tool (Figure 2). Six lines of Nb were selected, among more than 1000 ICP lines that were assigned to Nb in the proprietary ICP-based database. The same procedure is applied to all elements that need to be quantified.

Figure 2 - The 269.706 line of Nb was not selected as interfered with by Fe.

Next, the samples are analyzed. The line concentrations are processed instantaneously by the SOS tool, based on ANOVA statistical tests, to detect any possible outlier. The level of confidence is defined by the user, and was set up at 95% in this example. To illustrate the benefit of the SOS tool, one of the samples was spiked with molybdenum (Mo) at 200 mg/L to replicate an unexpected higher concentration of Mo. This spike leads to a concentration that is 10 times higher than the maximum concentration of Mo originally defined. The results in Table 2 show that two lines were automatically rejected: the 230.208-nm line, which was overestimated due to on-peak interference by Mo, and the 295.088-nm line, which was slightly underestimated because of a Mo peak on the background correction position. From the remaining lines, the final report provides a single reliable concentration value per element.

Table 2 - Outliers assessed by the S.O.S. (concentration values indicated with an *) and rejected for the mean calculation (SD is the standard deviation corrected by the student’s coefficient)

Clearly, multiline analysis can easily manage unexpected spectral interference through this outlier rejection tool. If the nature of the matrix or the concentration ranges of an application change, the MASTER tool is accessible at any time.

Conclusion

An instrument based on the most recent detector technology was described, along with advanced tools that will transform the way users develop analytical methods, enabling them to become proficient and take full benefit of the information available. Operators can increase their confidence in use while ensuring quality of results.

References

  1. Mermet, J.M.; Cosnier, A.; Danthez, Y.; Dubuisson, C.; Fretel, E.; Rogérieux, O.; Vélasquez, S. Design criteria for ICP spectrometry using advanced optical and CCD technology. Spectroscopy 2005, 20(2), 60–8.
  2. Danthez, Y.; Dubuisson, C.; Fretel, E.; Mermet, J.M.; Rogérieux, O. A dedicated spectra database for multiline selection in ICP-AES. Spectroscopy, Special Issue, Applications of ICP and ICP-MS Techniques for Today’s Spectroscopists 2005, 14–19.
  3. Mermet, J.M.; Cosnier, A.; Fretel, E.; Vélasquez, S.; Grigoriev, A.; Dubuisson, C. Multiline analysis: a key technique to enhance reliability in ICP-AES. Spectroscopy, Special Issue, Applications of ICP and ICP-MS Techniques for Today’s Spectroscopists 2006, 34–41.

Dr. Mermet is with Spectroscopy Forever, Tramoyes, France. Ms. Cosnier, Mr. Vélasquez, and Ms. Lebouil are with HORIBA Jobin Yvon, 16-18 rue du Canal 91165 Longjumeau Cedex, France; tel.: +33 1 64 54 13 00; fax: +33 1 69 09 90 88; e-mail: agnes.cosnier@jobinyvon.fr.


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