Improving Sensitivity and Sample Introduction Efficiency in ICP-OES and ICP-MS

Although inductively coupled plasma–optical emission spectrometry (ICP-OES) and ICP-mass spectrometry (MS) are generally considered to be mature techniques, researchers continue to investigate the fundamentals of the techniques and improve their capabilities. Diane Beauchemin, a professor at Queens University in Kingston, Ontario, is engaged in that challenge. She recently spoke to us about methods she has developed for simultaneous speciation and her work to improve sample introduction efficiency, to improve sensitivity and detection limits.

A recent paper of yours addressed the risk assessment of food safety-specifically, the importance of simultaneous speciation analysis of certain potentially toxic elements in organically grown white and brown rice, and basmati rice (1). Can you comment on why simultaneous speciation is important? What did the study tell you about the effect of various food preparation steps on the levels of harmful species present?

For speedy risk assessment of food safety, which is required for screening, fast methods for the determination of many elements are best. Unfortunately, speciation analysis by chromatographic techniques is not fast, as the separation of different chemical species takes at least 10 minutes (in the case of the speciation analysis of As, for example). Thus, enabling the simultaneous speciation of several elements makes sense. It makes use of the additional separation dimension afforded by ICP-MS. Chromatographic separation of the species of different elements is not necessary, because ICP-MS separates the elements from each other. The speciation analysis of several elements thus becomes possible without greatly increasing the time required for the chromatographic separation.

How did using a modified version of ion-exchange chromatography (IEC) with detection by ICP-MS enable simultaneous speciation?

We first applied a method that we previously developed for the speciation analysis of As to the speciation analysis of Se and found that it was adequate. This result was not surprising, as As and Se behave similarly and others had reported this possibility in the past, albeit with different chromatographic separation conditions. We then thought of adding Cr to the list, which turned out to be more challenging. Indeed, the method that my group previously developed for the separation of Cr(III) and Cr(VI) only required a guard column. With the much longer AS7 analytical column that is required in addition to the AG7 guard column for the separation of As and Se species, Cr species took too much time to elute, thus requiring the addition of a third and stronger eluent. With the new gradient elution program, the speciation analysis of As (As[III], As[V], monomethylarsonic acid and dimethylarsinic acid), Se (Se[IV] and Se[VI]), and Cr (Cr[III] and Cr[VI]) is accomplished within 12 minutes.

In another recent paper, you discussed the improvement of sample introduction to inductively coupled plasma–optical emission spectrometry (ICP-OES) using an ultrasonic nebulizer with an infrared-heated pre-evaporation tube (2). What advantages did this technique provide over conventional ICP-OES without the improved sample introduction? What types of applications are suitable for this technique and why?

Compared to a conventional nebulization system consisting of a concentric nebulizer and a spray chamber, an infrared-heated pre-evaporation tube with an ultrasonic nebulizer (instead of the heater and condenser) improves sensitivity and detection limits by at least an order of magnitude, depending on the emission line, the improvement being in general larger for ionic emission lines than atomic ones. The improvement in sensitivity and detection limit is in fact similar to that achieved using an ultrasonic nebulizer with a desolvation system. However, the pre-evaporation tube does not remove water but introduces it in vapor form in the plasma, which is very beneficial. Indeed, water acts as a load buffer in the plasma, which helps to minimize matrix effects. In contrast, removing water through desolvation effectively pre-concentrates the matrix along with the analytes, thereby exacerbating matrix effects. Furthermore, water acts as a source of hydrogen in the plasma, which has a higher thermal conductivity than argon and thus facilitates energy transfer between the bulk of the plasma and the central channel where the analytes are located. This effect further improves robustness, to the point where samples with complex matrices can be accurately analyzed (in this specific paper, wastewater and a food digest) by a simple external calibration without internal standardization.

In that same study, how did replacing a block heater with a ceramic beaded rope heater provide improved sample introduction efficiency?

The ceramic block mostly provides infrared heating whereas both infrared heating and convective heating occur with the ceramic beaded rope heater. This combination of heating types translated into similar benefits being obtained at a lower heating temperature for a given sample uptake rate than with the block heater. This approach is also much more compact than the bulky ceramic block heater. As a result, my group has mostly switched to this type of infrared heater.

What are your next steps for using ICP in your research and studies?

There is no lack of studies to carry out with ICP-OES and ICP-MS! Here are a few examples of ongoing studies:

• Work toward alleviating the Achilles' heel of atomic spectrometry, with the goal to improve the sample introduction efficiency without jeopardizing robustness while keeping the sample introduction system compact. Infrared heating of a standard spray chamber using the ceramic beaded rope heater shows promise to achieve an order of magnitude improvement in detection limit.

• Explore more forensic analysis applications of solid sampling electrothermal vaporization (ETV) coupled to ICP-OES, because this hyphenation is an excellent match for the multielement analysis of minute samples found at crime scenes. This research will include expanding our work on hair, to infer gender and ethnicity, and on paint scrapings left in hit-and-run cases, to infer the vehicle manufacturer, year of production, model, and color. The simultaneous introduction of water vapor (from an infrared-heated system) or hydrogen or nitrogen in the plasma to increase robustness may facilitate the analysis of a variety of samples by electrothermal vaporization coupled with inductively coupled plasma–optical emission spectrometry (ETV-ICP-OES).

• Improve single-particle ICP-MS for the analysis of nanoparticles. For example, mixed-gas plasmas may be advantageous for the analysis of nanoparticles of elements with a high ionization potential.

• Continue to use on-line leaching coupled to ICP-MS for risk assessment of food safety, along with speciation analysis of elements whose toxicity depends on their chemical form. Work is ongoing on wheat and rice. Other staple foods, such as corn flour and bread, need to be looked at.

References

(1) N.W. Sadiq and D. Beauchemin, Anal. Chem. 89(24), 13299–13304 (2017).

(2) T.K. Anderlini and D. Beauchemin, J. Anal. At. Spectrom. 33, 127–134 (2018).

This interview has been edited for length and clarity. To read more spectroscopy-related interviews, please visit: www.spectroscopyonline.com/autolist/20/more