Valuable tips when working with digested samples using atomic spectrometry techniques

One of the most frequently asked questions I get is related to the readiness of digested samples to be analyzed in atomic spectrometry techniques. And I totally understand it – it would be amazing for spectroscopists if the digestion instruments could produce solutions ready to be analyzed. Unfortunately, most of the time this is not the case. It is of utmost importance for those who prepare samples to understand the whole analytical process, in which features from the instrument used to determine elemental concentration could be of some help when developing a sample preparation procedure. In this article, I am considering the use of microwave-assisted digestion systems.

Frequently the initial mass of the sample is strictly controlled, so that the efficiency of digestion and security of the system are maintained. The final volume of the solution (after the digestion), on the other hand, is a flexible parameter.

Before starting to digest your samples, you should look at the atomic spectrometry technique available. Some important questions are:

1) What are the limits of quantitation (LOQ) that my instrument is capable of achieving, especially in the sample matrix?

2) How can the instrument deal with %TSD and acidic media?

3) Are there any resources that can be adjusted in the instrument to receive the digested sample?

As for question 1, this is relevant to strategically prepare your samples so that the analyte concentrations are above the limits of quantitation. This is useful when you know the sample composition, or at least have an idea of concentration ranges. If you do not have this information, I suggest using a reference or the manufacturer’s cookbook to start with. And remember – LOQs in the digested sample solution will be different from LOQs in water or diluted acid, usually higher.

Considering different types of atomic spectrometry techniques, the %TDS tolerance may vary. Some frequent problems observed working with high %TSD samples include nebulizer and injector clogging, background shift, and even fast torch deterioration. Question 2 is important so that you can plan the sample digestion based on your instrument operation – and if you must eventually dilute the sample after digestion, this should be accounted for in the final result (dilution from the bench and from the digestion). The same goes for high acid composition – sometimes, if the sample is too acidic, some interferences may arise, especially when working with ICP-MS.

If the sample is correctly dimensioned to be digested, the final volume is decided and LOQs have been checked… What could the spectrometer contribute to work with the digested sample? Time to question 3!

A flame atomic absorption, for example, can be tuned for better sensibility (higher aspiration rate, impact bead position) or to read high concentrations (lower aspiration rate, burner rotation). For emission techniques, such as the MIP OES and ICP OES, we can choose different nebulizers, spray chambers, torches (in the case of ICP OES), and some software parameters such as different emission wavelengths, nebulization flow, and read time to boost or reduce sensibility or to receive more or less sample. In the case of ICP-MS, some models can work with aerosol dilution, in which the aerosol from the spray chamber is diluted with a stream of argon prior to entering the plasma, avoiding the dilution step on the bench. Note that, in aerosol dilution mode, sensitivity is often reduced (the procedure is applied in standards and samples).

See…? Not all spectrometers are bad 😊

Considering that you have taken special care during sample digestion, and your apparatus produced a clear, colorless solution – does it mean that the digestion was successful? It is a great start for sure, but what is going to tell you if it is good or not will be your spectrometer. And how is this evaluated? Normally, three independent portions of the sample are prepared in the same batch, along with some replicates of a certified reference material (CRM), and the analytes are measured. Depending on how well homogeneous your sample is, readings from the three independent replicates of the sample will have a certain agreement (normally expressed in %RSD), as for the CRM, and the values obtained for the CRM should be compared to its certificate so that we can calculate the percentage recovery. Criteria may differ from lab to lab, but a ±10% value for precision between replicates and for percentage recovery is widely acceptable.

I hope this article is useful for your journey in atomic spectrometry, and hopefully, it helps to establish a friendship between digestion systems and spectrometers.