Analytical Instrumentation
The recovered silicate spikes all lie within the 95% confidence value of 3.33 µg/L SiO2
. The value of the deviation from the
respective spike lies between 0.14 and 2.40 µg/L SiO2 out at 0.93 µg/L SiO2
.
The accuracy of the pre-programmed method is sufficient for many users. Users for whom the error rate of the pre- programmed method is still too large can, however, enhance the accuracy of the method by plotting their own custom calibration curve, thus eliminating batch-specific fluctuations and the user’s own systematic errors.
A calibration curve was plotted for the Spectroquant® for the measurement range 0.50–25.00 µg/L SiO2
Silicate Test , see Figure 2.
In the case of the silicate test, the custom calibration curve was able to improve the performance characteristics obtained acc. to ISO 8466-1, respectively DIN 38402 A51. A comparison of the performance characteristics of the pre-programmed method vs. the custom calibration is presented in Table 2.
At a value of 1.44%, the method coefficient of variation is 2.5 times higher than that of the pre-programmed method. This can be attributed to the fact that deviations have, in relative terms, a stronger effect in the lower measurement range as a result of the custom calibration. Seen in absolute terms, the custom calibration procedure can, however, result in considerably lower method errors, as shown by the values of the standard deviation for the procedure and the confidence interval. The method standard deviation and the confidence interval for P=95 % of the custom calibration are 76% (standard deviation) and 86% (confidence interval) lower than those for the pre-programmed method.
When the standard additions are evaluated using the user-specific calibration function, the deviations can be reduced down to the expected values. On average, the value of the deviation was now 0.29 µg/L, indicating a reduction from the original value of 0.94 µg/L by almost 70%. The measurement values are presented in Table 3.
Method Comparison of GF-AAS and Spectroquant®
Silicate Test
Besides the standard addition experiments, a reference analysis was also performed: the silicate content of the five water samples was quantified by a GF-AAS method. The Limit Of Quantitation (LOQ) of the GF-AAS method was determined using the ten-fold standard deviation of the blank, yielding a value of 1.93 µg/L SiO2
. Table 4 compares the results from the GF-AAS method with those of the photometric determination method, calculated using the pre-programmed method and the custom calibration.
For three of the samples, the silicate concentration measured by the GF-AAS method lay below the LOQ of 1.93 µg/L SiO2
test kit. , a
finding that could also be confirmed by the measurement with the Spectroquant®
The power plant samples lay above the LOQ and the results of the
GF-AAS method are comparable with those of the Spectroquant® test kit. The deviations are all within the 95% confidence interval of the pre- programmed method and that of the custom calibration (see Table 2). As was also the case for the standard addition, it is apparent that a custom calibration can serve to reduce the error even further.
Summary The above results show that the Spectroquant®
Double-distilled water , averaging
11
Figure 2: Calibration curve of Spectroquant®
Silicate Test 101813, measuring range 0.50–25.00 µg/L SiO2
Table 2: Comparison of performance characteristics Sample
Ultrapure water Steam water from power plant Boiler water from power plant DI water
Addition [µg/L SiO2 1.00
5.00
10.00 1.00 5.00
10.00 1.00 5.00
10.00 1.00 5.00
10.00 1.00 5.00
10.00 Table 3: Recovered contents of silicate, evaluated against the custom calibration
re-programmed method 0.25 – 250.00 µg/L SiO2
Method standard deviation [µg/L] Method coefficient of variation [%] Confidence interval (P=95 %) [µg/L]
± 0.790 ± 0.62 ± 3.33
Custom calibration 0.50 – 25.00 µg/L SiO2
± 0.185 ± 1.44 ± 0.45
] Recovered concentration [µg/L SiO2 0.70
4.83
10.28 1.15 5.42
10.13 1.00 5.12 9.54 1.15 4.97
10.28 0.85 6.45
10.57
] Deviation [µg/L SiO2 0.30
0.17 0.28 0.15 0.42 0.13 0.00 0.12 0.46 0.15 0.03 0.28 0.15 1.45 0.57
]
Table 4: Comparison of the results of the Spectroquant® Sample Ultrapure water Silicate Test is
capable of yielding quantitative values for silicate concentrations in process water and deionized water in the lower ppb range. Users for whom the accuracy of the test for the determination of silicate is sufficient can use the pre-programmed method to determine the silicate content of their samples swiftly and effortlessly. In the event that greater accuracy of the method is required, the creation of the user’s own custom calibration curve is recommended, which was shown to be capable of reducing the mean deviation of the spike amounts that were added by almost 70%.
Instruments Used
All measurements were carried out as per the application on a Prove 600 spectrophotometer. The reference method that was
Author Contact Details
Katrin Schwind, Application Scientist, Analytical Point-of-Use R&D, Life Science | Applied SMI | Analytical R&D, Merck KGaA • Frankfurter Str. 250, D-64293 Darmstadt, Germany • E-mail:
katrin.schwind@
merckgroup.com Gunter Decker, Senior Manager, Analytical Sciences Liaision | Spectroscopy, Merck KGaA • Frankfurter Str. 250, D-64293 Darmstadt, Germany • Phone: +49 6151 723183 • E-mail:
gunter.decker@
merckgroup.com
Katrin Schwind Gunter Decker
Steam water from power plant Boiler water from power plant DI water
Double-distilled water
Silicate Test 101813 and the GF-AAS reference analysis
GF-AAS < 1.93 2.25 3.66
< 1.93 < 1.93
used was a graphite furnace atomic absorption spectroscopy method on the SpectrAA 280Z instrument supplied by Agilent.
References 1. Bahadori, A.; Vuthaluru, H.B. APPL THERM ENG 2010, 30, 250-253.
2. The International Association for the Properties of Water and Steam, Technical Guidance Document: Steam Purity for Turbine Operation, 2013.
Concentration [µg/L SiO2] Spectroquant® Pre-programmed method
< 0.25 3.26 4.85 0.29 0.77
Silicate Test
Custom calibration < 0.25 2.18 3.94
< 0.25 < 0.25
3. Zhou, S.; Turnbull, A. Steam Turbine Operating Conditions, Chemistry of Condensates, and Environment Assisted Cracking – A Critical Review, NPL Report MATC (A) 95, 2002.
4. Mustafa, A.I. IJCT 2007, 14, 195-199.
5. Koch,O.G, Koch-Dedic G.A., Handbuch der Spurenanalyse 2, Springer- Verlag, 1974. 6. Package leaflet for the Spectroquant® Cat. No. 101813, April 2016.
Silicate (Silicic Acid) Test,
OCTOBER / NOVEMBER •
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