I work on the determination of the three component system of heavy metal ions (Pb2+, Cu2+, Cd2+) in water by an anodic stripping voltammetry technique on a thin film of mercury on the glassy carbon electrode. According to my observations, there is no interference between the determination of such metal ions. I have some samples that were in contact with an adsorbent. After adsorption equilibrium, the solution was separated. I want to know how much of the mentioned metals are retained in the solution. My advisor insisted on the standard addition of three metals in sperate runs, and it will be so tricky because of the high number of samples. Do I want to know what if, all three metals added simultaneously in the standard addition process?
Hi Mohammad,
My point is as M. Toner's, do as your advisor indicates... no need to explain why, there are multiple reasons...
Related to the standard addition methodology, it is very useful to avoid matrix interferences. If you know there are no this type of interferences you can use external calibration. However, you can have different matrices, so then it is better to use standard additions.
About adding all-at-once (Pb, Cd, Cu) or one-by-one, obviously analysis time is an important parameter and it would be desirable to add all in the same addition. Initially, I don't see any inconvenience, but it will depend on the concentration level. If the concentration is very high you can "overlap" peaks (e.g. Pb and Cd are very close) and make measurement difficult. As C.Radu says, there are a lot of bibliography on ASV and probably this is well documented.
On the other hand, although ASV is an excellent technique, you can also consider the proposal of M. Farooq and use AAS as a validation technique, if the species to be determined are the same (maybe different results respond to different species of the same element).
Apart from this, society requires low-cost and decentralised analyses and the use of gases, flames, expensive and technologically complicate instrumentation (not portable) is not in this trend. In any case, both enter in the toolbox of analytical techniques and have specific excellent applications.
Best regards and good luck
What is the composition of your solution, is it a real water sample or synthetic standard solutions? There is a simple way to test your supervisor's suggestion. Do a control test. Do a standard addition method as well as a external calibration. Compare the results are same (statistically similar), you can take the easy path of external calibration, otherwise try an analytical technique which is not that sensitive to your matrix.
If you have access to an atomic absorption spectrophotometer (AAS), you may not need to do standard additions and voltammetric analysis (which is potentially a headache anyway). AAS is extremely selective unlike electrochemical approaches.
Thank you, Dr. Wahab (M. Farooq Wahab).Your respond is wise. My samples at the moment are synthetic. I will use the paired sample t-test to find: "Is there any significant difference between the two standard addition method?", However, I disagree with you about higher selectivity and sensitivity of AAS in comparison to voltammetry in all cases. AAS technique is more straightforward of course, but when we have more than one component, it is time-consuming, and in some cases, metal oxide formed in the flame and due to lack of proper ignition of metal ions that is a real concern.
The method (ASV for Pb2+, Cu2+, Cd2+ ) is well studied and the possible interfering species are known. Cd signal may be lowered if Cu is present in higher concentration (an intermetallic compound is formed). The concentrations of the standard additions must be adapted to be not higher than twice the sample concentration (for each analyte). Commonly the added standard contains all 3 species. Try to read more papers about the method.
Here is the thing. If your advisor is insisting you do something a certain way that is the way to do it. Arguing will not endear yourself to him.
I do cyclic voltametry stripping, something a bit different from what you are doing and I always build the standards with each component changed separately rather than all three at once to check the calibration. Main reason. To check the conditions to make sure they are correct for the analyte in question.
Hi Mohammad,
My point is as M. Toner's, do as your advisor indicates... no need to explain why, there are multiple reasons...
Related to the standard addition methodology, it is very useful to avoid matrix interferences. If you know there are no this type of interferences you can use external calibration. However, you can have different matrices, so then it is better to use standard additions.
About adding all-at-once (Pb, Cd, Cu) or one-by-one, obviously analysis time is an important parameter and it would be desirable to add all in the same addition. Initially, I don't see any inconvenience, but it will depend on the concentration level. If the concentration is very high you can "overlap" peaks (e.g. Pb and Cd are very close) and make measurement difficult. As C.Radu says, there are a lot of bibliography on ASV and probably this is well documented.
On the other hand, although ASV is an excellent technique, you can also consider the proposal of M. Farooq and use AAS as a validation technique, if the species to be determined are the same (maybe different results respond to different species of the same element).
Apart from this, society requires low-cost and decentralised analyses and the use of gases, flames, expensive and technologically complicate instrumentation (not portable) is not in this trend. In any case, both enter in the toolbox of analytical techniques and have specific excellent applications.
Best regards and good luck
Dear Mohammad,
I hope the attached files were useful for you.
The Best.
I did that in the multi-element determination by atomic emission spectroscopy
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