2024 Lecture 4 - Elemental Analysis Exercises PDF

Lecture 4 - Analytical Techniques I: Elemental Analysis Exercises Atomic Absorption Spectroscopy 1. What is the function of the Flame (or Furnace) in Atomic Absorption Spectroscopy? The function of the Flame or Emission system in AAS is to reduce the sample into the atomic state....

Lecture 4 - Analytical Techniques I: Elemental Analysis Exercises Atomic Absorption Spectroscopy 1. What is the function of the Flame (or Furnace) in Atomic Absorption Spectroscopy? The function of the Flame or Emission system in AAS is to reduce the sample into the atomic state. 2. AAS is a single-element technique which is a huge disadvantage, why would analysts still want to use it? Less expensive in both initial purchasing and operating costs Very easy to use and set up Relatively few interferences The AAS is much quicker than the ICP in equilibration time For elements such as Mercury the detection limit is equal for both AAS and ICP- MS Inductively Coupled Plasma techniques 3. In what types of industries is trace elemental quantification important? Name five. Some examples of correct answers: Food and beverage testing Environmental monitoring (e.g. waste water, soil analysis etc.) Materials manufacturing Catalyst testing Pharmaceutical monitoring Clinical research Consumer product testing Geological, Cosmochemistry and marine science 4. How are lasers used as a means of sampling solids for ICP-MS? Lasers are used for sampling in ICP-MS by exposing a solid sample to an intense, pulsed, laser beam, which rapidly vaporizes (Ablates) the sample. The resulting vapor is carried into the ICP torch where atomization and ionization occurs. The resulting gaseous mixture then enters the MS for analysis. 5. Which elements cannot be measured with ICP-MS? The only elements that ICP-MS can’t measure are H and He (which are below the mass range of the mass spectrometer), Ar, N, and O (which are present at high level from the plasma and air), and F and Ne (which can’t be ionized in an argon plasma). Of these “impossible” elements, even F can be analyzed indirectly using a triple quadrupole ICP-MS. Halogens, carbon and noble gasses are notoriously difficult to measure and thus, likely not feasible for the average laboratory set-up; It is however, possible in the right instrumental set-up or with the use of MS-MS. Additionally, you will find elements >85 mass of which it is uncertain whether you can measure them effectively. 6. Describe the influence of spectral interference, matrix, and ionization effects in ICP-MS  Spectral interferences: Both isobaric and polyatomic interferences are cause by other ionic species having the same or very similar mass to the analyte. In a typical Quadrupole based instrument this results in direct overlap of an interference on an element of interest. For HRMS this is often not an issue. Overall spectral interferences are present but not of the highest concern in ICP-MS measurements as the influence can be limited or even negated with proper instrumentation and settings. Additional information on spectral interferences: Isobaric interferences occur when there are naturally occurring isotopes for more than one element at the same mass. Polyatomic interferences occur when elements present combine with each other or with plasma gases forming molecular ions at the same mass as an analyte. Ionization effects: The Argon plasma (high concentration of electrons) in ICP is a highly efficient ion source with little interferences. Most metals are ionized at >80-95% efficiency and even high ionization potential metals have ionization efficiencies above 40%. Therefore, Ionization effects/interferences (not caused by the matrix) are only observed to a limited extend in ICP-MS. Matrix effects: Matrix effect present the largest challenge and interferences when using ICP based instruments. Matrix elements result in the suppression of analyte signals by both spectral interferences (overlap) and by reducing the ionization efficiency of the analyte in the plasma. An relevant example for ICP-MS is the influence of total dissolved solids (TDS) and organic matrices, which can lower the energy of the plasma and decrease the ionization efficiency and thus affecting the sensitivity in comparison to non-matrix standards. Note that there are also less common examples where matrix effects enhance the signal in ICP-MS measurements. 7. Can these influences (spectral interference, matrix and ionization effects) be corrected for in the ICP-MS measurements? The easiest way to compensate for analyte signal suppression (or enhancement) due to matrix effects is the use of an internal standard. This can be extended to compensate for matrix effects by matching the internal ionization potential and mass to the analyte. Preferably, the ISTD is not present in the samples or only at a very low concentration. The ITSD also compensates for random instrumental errors that arise from fluctuations in the output of the plasma source. 8. Are these interferences of similar magnitude and effect in ICP-OES and XRF? Compare the type of interferences and the impact on applicability Spectral interferences are more present in ICP-OES than in ICP-MS, however, ICP- OES typically suffers less from matrix effects such as TDS. General XRF suffers from both spectral and background interferences and Matrix effects. Due to the lower spectral resolution this is more an issue for XRF measurements. Note that inhomogeneous samples makes this even more difficult. 9. In the slides you can find both ICP-AES and ICP-OES mentioned. What is the difference between the two techniques? People use both names intertwined, there are little to none differences in principle. Inductively coupled plasma optical emission spectroscopy (ICP-OES) and Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) effectively refer to the same methodology.  10. An analytical chemistry lab in the environmental analysis sector requires a new machine for their elemental analysis. The detection limits required are expected to be in the ppm/ppb range and the machine is expected to operate daily with a high sample throughput. Should the laboratory in question invest in a ICP-MS or a ICP-OES system, argue both sides? (Think cost, sample prep, speed, resolution etc.) ICP-MS ICP-OES (AES) Wide dynamic range Analytical grade reagents are sufficient Cost ~ 150K (quadrupole) Simpler method development (less expertise required)_ Isotope detection Cost ~ 50K Detection: 1-10 ppt Detection: 1-10 ppb High throughput High throughput Multi-element detection Multi-element detection Spectral interferences: insignificant and Spectral interferences: much greater than predictable ICP-MS and more complicated A good case can be made for both instruments and many arguments are viable. ICP-OES can measure samples with high TDS or suspended solids. These cause significant matrix effects which are less influential in ICP-OES than in ICP-MS measurements. Especially for routine environmental analysis this can result in a more robust method. Additionally, it is cheaper and with easier maintenance and method development. However, the ICP-MS has a wider dynamic range and can reach lower detection limits. The mass resolution also reduces the spectral interferences and can differentiate between isotopes of similar mass. Is the matrix a potential issue? E.g. ground water, wastewater, soil and soil waste. Then it is likely that ICP-OES is preferable. Trace analysis in a clean(er) matrix? ICP- MS would be preferable. On a tight budget? ICP-OES. If we remember Peters lecture companies will often have only one ICP instrument in which the versatility and low detection range of the ICP-MS wins out over the ICP-OES. However, both are present you are likely to only use the ICP-MS when the ICP-OES is not sufficient. Thus? Get both :) Nuclear Activation Analysis 11. XRF is non-destructive, what about NAA? NAA is non-destructive, however, it is of course radioactive afterwards. It might take a while before they can return the sample. 12. How does the detection limit of NAA compare to ICP-MS and XRF? Can we objectively compare these detection limits? NAA is quite sensitive and in that sense close to the detection limits of ICP-MS (and less so of XRF). However, you cannot directly compare the techniques as they are in different units. 13. Given the low throughput of NAA techniques, the requirements in operating (safety) and the cost of the system, it is unlikely to see NAA in a lab. However, for method development the technique offers a distinct advantage when multiple techniques are required. Explain the added benefit of NAA in method development for complex problems. Most sources of systematic and random error (e.g. interfering nuclear reactions, overlap of spectral lines and dead-time losses) are identifiable, since the physical principles of NAA are well understood and described. The distinct advantage of NAA is that the technique is based on principles fundamentally different from other analytical techniques. It is prone to completely different systematic biases and can therefore be useful in the analysis of reference materials or in assessing the comparability of measurement results between analysis methods. Part of a previous exam questions: Breaking bad impurities In the pre-clinical stage of your drug-development process, your colleague organic chemists are experimenting with ways to produce the drug candidate in larger quantities. You as analytical chemist have the responsibility to confirm that the product is safe for testing and evaluate which impurities are present. Due to experimentation with different catalytic processes, you want to ensure that there are no residual metals in the sample that might result in unsafe testing conditions. In your lab, you have access to an ICP-MS system. a. Briefly explain the basic principle of ICP. (3 pts) Inductively coupled plasma (ICP) in which electromagnetic induction is used to supply energy to create a plasma. Typically Argon gas is used for generation of the plasma and it will reach temperatures around 10000 K. when a sample is nebulized into this hot plasma, the molecules will atomize and ionize. Excited atoms will release light of a characteristic wavelength as they transition back to a lower energy level. Alternatively, the ions can directly be detected by mass spectrometry (MS). (note you will not lose points on small details such as the temperature or the type of gas) b. To obtain the best sensitivity for detecting trace elements, is ICP-MS a better option than ICP-AES? (4 pts) Typical detection limit of ICP-MS is 1-10 ppt while that of ICP-AES is 1-10 ppb. This is mainly a result of spectral interferences. In MS the spectral interference is insignificant and predictable while in AES the spectral interference is much greater and more complicated.  c. What problems do you expect in relation to matrix effects? (3 pts) ICP-MS is more susceptible to matrix effects from the sample as opposed to ICP- AES. We might be dealing with residual solid material and we clearly have an organic matrix. Both of these can lower the energy of the plasma and decrease the ionization efficiency and thus affect the sensitivity of the method. However, we are dealing with relatively clean samples and we would know what elements specifically to look for. If this poses to be a problem, it will be straightforward to develop a sample clean-up strategy to get rid of most of the matrix. Coating the competition Typically metals and minerals are not expected to be present in polymer adhesives. However, a colleague suspects that part of the “claimed superiority” of the new competitor product is related to a small concentration of manganese (Mn) present in the adhesive. The colleague proposes to use neutron activation analysis for the measurements. a. Do you agree with your colleague that neutron activation analysis is a good approach to measure the concentration of Mn in the adhesive? Explain why or why not. (4 pts) Although the technique is capable of measuring Mn (with a good detection limit), there are multiple reasons why it is a very inconvenient idea and should not be done. Some examples: Complexity and availability of the technology (Not all research institutions or laboratories have access to the necessary neutron sources or equipment for NAA.) Time-consuming (NAA typically involves irradiating the sample with neutrons, followed by a period of waiting for radioactive isotopes to form through neutron activation. This process can take hours to days, depending on the elements being analyzed and their concentrations.) Safety reasons, even though it is technically non-destructive your sample remains radioactive. Your project leader, Ton Beters, suggest that perhaps it is easier to measure the samples with X-ray fluorescence since those instruments are readily available in the lab. b. Provide two advantages of using XRF for the analysis of the adhesive material. (4 pts) Some examples of good answers: Quantitative technique Simple in use and data processing Quick measurements Non destructive Understanding the basic principles of XRF and being aware of potential issues, you quickly ask your colleague in what concentration range they expect the Mn to be present. They reply that if you dissolve the adhesive, the estimated final concentration is about 0.05-0.1 ppb Mn. c. Give the new information on the expected concentration and the possibility to dissolve the sample. Which technique would you suggest to measure the Mn in the adhesive? Briefly explain your answer. (6 pts) For most XRF setups, these types of detection limits will be challenging or unreasonable. Therefore, it would be wise to switch to an alternative technique, especially now that you have learned that the adhesives can be dissolved. Alternative techniques could include ICP-AES, ICP-MS, or GFAAS. In the case of ICP, you should be aware that due to matrix effects, it is likely still difficult to measure at such low concentrations. (Destruction approaches would solve this issue, but at the cost of being time-consuming.) You will receive all the points for this particular question if you rationally assess the sensitivity of XRF (or argue why you think it can be measured with XRF) and suggest a reasonable alternative technique, along with reasoning why you believe this technique would be sufficient or better equipped than XRF.