table Liquid sample analysis using XRF can be the easiest or most difficult depending upon the composition and stability of the liquid1. Liquid samples can initiate several problems such as evaporation, precipitation, wicking, stratification and most commonly spillage. On the positive side, solutions are essentially homogeneous that gives them a theoretical edge over the conventional solid samples regarding data reproducibility. If the above mentioned problems associated with the liquid samples are eliminated with careful sample preparation, XRF could be the answer to the challenge of cumbersome and rigorous methods of wet chemical analysis as the chance of human error is least. Though it is possible to analyze any element in XRF theoretically, for our purpose we have chosen heavier elements such as Cu, Zn and Pb to avoid the effect of the solvent contribution and low emission intensity.
Materials and methods:
As suggested by the relevant literatures, WD-XRF or T-XRF would have been the conventional tool for liquid sample analysis2-4. But recent advances in the instrumentation area prompt us to use the bench top ED-XRF ARL QUANT’X, Thermo Fisher Scientific5. The ARL QUANT’X EDXRF spectrometer used for this application is equipped with a 50 kV, 50 W silver target X-ray tube and a silicon drift detector (SDD) of the latest generation. Five samples of Cu, Zn and Pb ores each were analyzed using conventional wet chemical analysis following the method SOP:TPM/MSK/47/A, issue no 1, dated 1/1/2013; SOP: TPM/MSK/48/A, issue no 1, dated 1/1/2013 and SOP: TPM/MSK/46/A, issue no 1, dated 1/1/2013 respectively. Three of the samples of each ore were used to prepare the calibration curve for the XRF spectrometer and the rest two were used as samples to be analyzed. A Perkin Elmer Optima 7000 DV ICP-OES instrument was used to further confirm the results obtained from XRF and wet chemical analysis. Results obtained from these three techniques are summarized in table 1.
Table 1: Summary of results obtained from different analytical techniques
|Analyte/Sample ID||Concentration (%) XRF||Concentration (%) Wet Chemical method||Concentration (%) ICP OES|
Sample preparation for XRF:
5 ml of samples were transferred to a sample cup of 32 mm outer diameter, sealed with a 4 micron polypropylene film. After waiting for 5 minutes to check for possible spillages, the solutions were analyzed using predefined excitation conditions as summarized in table 2.
Table 2 below shows the optimized excitation condition used to perform the analysis. Two different filter conditions have been used to cover the 3 elements. Live times of 100 s or 150 s were chosen accordingly to enhance the data quality.
Table 2: Excitation conditions:
|Condition||Filter||Voltage (KV)||Current (mA)||Atmosphere||Live time (s)||Analytes|
|Mid Zb||Medium Pd||20||Auto||Air||100||Cu|
|Mid Zb||Medium Pd||20||Auto||Air||150||Zn|
|Mid Zc||Thick Pd||28||Auto||Air||150||Pb|
Linear calibration curves relating net intensities to concentrations were set up using three randomly chosen samples whose concentrations were determined using wet chemical methods. These concentrations were also verified using ICP-OES and found to be consistent with results obtained from wet chemical analysis.The concentration levels used to set up the curves are summarized in table 3. Figures 1a,1b and 1c show the calibration curves obtained for Cu, Zn and Pb respectively.
Table 3: Calibration results
|Analyte/Sample ID||Given Concentration (%)||Calculated Concentration (%)||Uncertainty|
To check the repeatability of the results each sample was measured three times and the results of the measurement are summarized in table 3.
Table 3: Data repeatability check results
|Analyte/Sample No||Conc.(%)/1st Run||Conc.(%)/ 2nd Run||Conc.(%)/3rd Run|
The unique capabilities of the EDXRF spectrometer provide fast, accurate and repeatable monitoring of various elements in solution phase. Analysis of such samples can be done under ambient air which is convenient, shortens analysis time and offers savings on costly helium gas consumption. The results are compared with conventional wet chemical technique and ICP-OES results and are found to be in order. Further validation of XRF technique towards liquid sample analysis may allow us to reduce the dependence on conventional wet chemical analysis and provide an answer to the challenge of proper homogeneity associated with solid samples although a rigorous and thorough understanding of various aspects of the interaction between X-Rays and a liquid sample containing different elements is much needed.
- E. Marguí, B. Zawisza, R. Sitko,Trends in Analytical Chemistry 53 (2014) 73–83.
- E. Marguí, M. Hidalgo, I. Queralt, K. Van Meel, C. Fontas, Spectrochim. Acta B 67 (2012) 17–23.
- E. Marguí, I. Queralt, M. Hidalgo, J. Anal. At. Spectrom. 28 (2013) 266–273.
- K. Kocot, B. Zawisza, E. Marguí, I. Queralt, M. Hidalgo, R. Sitko, J. Anal. At. Spectrom. 28 (2013) 736–742.
- Thermoscientific application note, AN419050.
Contributed by: Arijit Goswami under guidance of Prof. Barun Gupta