Tia K.
Anderlini†
and
Diane
Beauchemin
*
Department of Chemistry, Queen's University, Kingston, Ontario K7L 3N6, Canada. E-mail: diane.beauchemin@chem.queensu.ca; Fax: +1-613-533-6669; Tel: +1-613-533-2619
First published on 4th December 2017
In the search for greater sample introduction efficiency and enhanced analytical performance with inductively coupled plasma optical emission spectrometry (ICPOES), a conventional ultrasonic nebuliser was modified to replace the heater/condenser with an infrared heated pre-evaporation tube. In continuation from previous works with pre-evaporation, the current work investigated the effects of infrared heating with ceramic block and ceramic beaded rope heaters. Heating temperatures were varied. By monitoring changes to sensitivity, detection limit, precision, and robustness, and analyzing two certified reference materials, a sample introduction system operating at 300 °C and 0.8 mL min−1 sample uptake rate was established, which provided improved sample introduction efficiency and comparable analytical performance to a previous system operating at 400 °C and 1.5 mL min−1. These conditions are robust, with a Mg 280.270 nm/285.213 nm ratio of 12.4 ± 0.3 (n = 6), which enabled the accurate analysis of two certified reference materials with a simple external calibration without internal standardization.
One of the leading sample introduction systems for nearing this goal is the ultrasonic nebuliser (USN) with desolvation system. By greatly reducing the noise caused by variable droplet size, and increasing the transport efficiency, the USN generally results in a 10-fold improvement in detection limits over conventional pneumatic nebulisation with ICPOES. Through vaporising and condensing the sample aerosol with the USN's heater and condenser, respectively, solvent is removed to pre-concentrate the analyte and increase the sample introduction efficiency to 30%.6,7 However, the matrix is concurrently pre-concentrated.8 Moreover, some elements may be lost during the process of desolvation, and memory effects may become exacerbated by the long path involved in the heater-condenser (HC) system, as well as the membrane desolvator used for organic solvents.9,10 Finally, by removing water in the HC system, the beneficial attributes of water as a load buffer in the plasma are lost. Water typically minimizes matrix effects11 and acts as the main source of hydrogen in the plasma. Having a high thermal conductivity, hydrogen facilitates the transfer of energy between the bulk and central channel of the plasma.12
By replacing the HC of a USN with a pre-evaporation tube (PET), water is preserved while the benefits of ultrasonic nebulisation are still achieved.13,14 Using heating tape to heat the PET, improvements in detection limit in comparison to a conventional PN were achieved while preserving robustness. This method also allowed for the successful determination of Hg, which otherwise would have been lost in the desolvation system.14 However, the plasma extinguished at sample uptake rates above 0.3 mL min−1, whereas insignificant improvements in detection limit and a degradation of instrumental precision were obtained compared to the standard USN system.
Alternatively, by using a ceramic block infrared (IR) heater, the benefits of a fast heating rate and uniform temperature were realized.15,16 A 10–25 fold improvement in detection limit was achieved over conventional pneumatic nebulization,13 proving that pre-evaporating the sample aerosol improves both the plasma excitation conditions and robustness by increasing the amount of water vapour entering the plasma. The objective of this work was to improve on these past USN-PET approaches using two types of infrared heaters at lower temperatures. While 400 °C had been used in the past work, it was the lowest temperature tested with the IR heater,13 which had been selected based on the fact that 400 °C was the highest temperature that could be used with the heating tape and provided the best results.14 The present goal was to see if a lower heating temperature with the IR heater would provide sufficient pre-evaporation to improve sample introduction efficiency and if a beaded rope could replace the bulkier block heater.
Parameter | PN | USN-HC | USN-PET 300 °C, IR block | USN-PET 160 °C, IR block | USN-PET 250 °C, IR rope |
---|---|---|---|---|---|
RF power (kW) | 1.40 | ||||
Plasma observation height (mm) | 11.0 | ||||
Plasma gas flow rate (L min−1) | 12.00 | ||||
Auxiliary gas flow rate (L min−1) | 1.0 | 1.0 | 2.50 | 2.50 | 2.50 |
Sample uptake rate (mL min−1) | 2.0 | 2.0 | 0.8 | 0.5 | 0.8 |
Drain removal rate (mL min−1) | 11.0 | 15.0 | 15.0 | 15.0 | 15.0 |
Aerosol carrier gas flow rate (L min−1) | 1.00 | 0.75 | 0.33 | 0.33 | 0.43 |
Sheathing gas flow rate (L min−1) | — | — | 0.40 | 0.40 | 0.40 |
USN heater temperature (°C) | — | 140 | — | — | — |
USN condenser temperature (°C) | — | 3 | — | — | — |
Certified reference waste water EU-L-3 (SCP Science, Baie d'Urfé, Québec, Canada) was analyzed directly for method validation, while SRM 8433 Corn Bran (National Institute of Standards and Technology, Gaithersburg, MD, USA) was digested on a hot plate prior to analysis. About 0.3 g of SRM 8433 was weighed into a Teflon decomposition vessel (Savillex, Minnetonka, USA) and 2.5 mL HNO3 and 0.5 mL H2O2 (30 m/m% in H2O, ACS reagent, Sigma-Aldrich, Steinhein, Germany) were added prior to placing the vessel on a hot plate at 50 °C for 2 h. Digested samples were diluted to 60 mL with DDW, and all standards and blanks for calibration were prepared with matching final acid concentrations.
Experiment number | IR temperature (°C) | Sample uptake rate (mL min−1) | Sheath gas flow rate (L min−1) | Aerosol carrier gas flow rate (L min−1) |
---|---|---|---|---|
1 | 300 | 0.75 | 0.45 | 0.3 |
2 | 300 | 0.5 | 0.4 | 0.4 |
3 | 230 | 0.75 | 0.45 | 0.3 |
4 | 160 | 0.5 | 0.5 | 0.2 |
5 | 160 | 0.5 | 0.4 | 0.4 |
6 | 230 | 0.75 | 0.45 | 0.3 |
7 | 230 | 0.75 | 0.4 | 0.3 |
8 | 300 | 0.5 | 0.5 | 0.2 |
9 | 230 | 0.75 | 0.45 | 0.3 |
10 | 160 | 0.75 | 0.45 | 0.3 |
11 | 230 | 0.75 | 0.45 | 0.4 |
12 | 160 | 0.5 | 0.5 | 0.4 |
13 | 230 | 0.75 | 0.5 | 0.3 |
14 | 300 | 1 | 0.5 | 0.2 |
15 | 160 | 1 | 0.4 | 0.2 |
16 | 230 | 0.75 | 0.45 | 0.3 |
17 | 230 | 0.75 | 0.45 | 0.3 |
18 | 230 | 0.75 | 0.45 | 0.3 |
19 | 230 | 0.75 | 0.45 | 0.3 |
20 | 160 | 0.5 | 0.4 | 0.2 |
21 | 230 | 1 | 0.45 | 0.3 |
22 | 230 | 0.75 | 0.45 | 0.2 |
23 | 160 | 1 | 0.4 | 0.4 |
24 | 230 | 0.5 | 0.45 | 0.3 |
25 | 300 | 1 | 0.4 | 0.2 |
26 | 160 | 1 | 0.5 | 0.4 |
27 | 160 | 1 | 0.5 | 0.2 |
28 | 300 | 1 | 0.4 | 0.4 |
29 | 300 | 0.5 | 0.5 | 0.4 |
30 | 300 | 1 | 0.5 | 0.4 |
31 | 300 | 0.5 | 0.4 | 0.2 |
According to the contour plots (not shown) that resulted from the multivariate optimization, a decreased sample uptake rate of 0.8 mL min−1 along with 0.33 L min−1 aerosol carrier gas flow rate at 300 °C provided the best compromise in terms of signal intensity and robustness versus the 1.5 mL min−1 sample uptake rate and 0.25 L min−1 aerosol carrier gas flow rate previously used at 400 °C (ref. 13) (all other operating conditions were identical). With a decreased temperature, a lower sample uptake rate is required to ensure all sample aerosol is vaporized in the PET. This trend is also observed in the 160 °C experiment, whereby a 0.5 mL min−1 sample uptake rate was selected. Increasing the sample uptake rate much higher than this would result in a buildup of un-vaporized sample and condensation at the base of the torch, which would inevitably extinguish the plasma. Because the ceramic beaded rope heater provided both convective and IR heating, the area immediately above that in direct contact with the IR heaters was found to be 30–60 °C higher, according to an IR temperature gun, than the temperature measured by the thermocouple at the base of the torch, resulting in a lower optimum temperature when compared to the block heater temperature. Hence, the vapour entering the plasma was of higher temperature than the surface of the base of the torch due to the added convective heating. This is supported by the fact that the same sample uptake rate of 0.8 mL min−1 as for the block heater at 300 °C was optimal with the beaded rope at 250 °C.
Using USN-PET with IR block at 300 °C instead of the regular USN-HC resulted in an average factor of improvement in sensitivity for 32 emission lines of 3.2 ± 4.2, which is not significant according to a paired t test. Similarly, the average factor of improvement of 1.1 ± 2.0 in detection limits against the regular USN-HC is not significant according to a paired t test. Hence, a similar performance is obtained at 0.8 mL min−1 sample uptake rate using USN-PET with IR block at 300 °C as with USN-HC operated at over twice the sample uptake rate (2.0 mL min−1). This demonstrates the greater efficiency of the IR-heated PET system.
To compare the effects of heating temperature and method of IR heating (i.e. block heaters vs. beaded rope heaters), experiments were conducted at 300 °C and 160 °C with block heaters and at 250 °C with a beaded rope heater. Heating at 300 °C instead of 160 °C resulted in average improvement factors of 3.5 ± 1.9 for sensitivity and 3.6 ± 2.6 for detection limit, which paired t tests indicated was only significant for sensitivity. Unsurprisingly, heating to 160 °C at a reduced sample uptake rate was not as efficient at vaporizing the aerosol and therefore degraded sensitivities through a concurrent reduction in transport efficiency. The fact that detection limits were not significantly degraded stems from the poorer precision observed at 300 °C than at 160 °C (Table 5). Heating at 300 °C with block heaters instead of 250 °C with the beaded rope heater at the same sample uptake rate resulted in average improvement factors of 1.3 ± 0.3 for sensitivity and 2.6 ± 3.0 for detection limit, neither of which is significant according to paired t tests. Hence, because beaded rope heaters use both convective and IR heating, a lower heating temperature may be used to attain similar performance as with block heaters. Indeed, the area downstream of the rope heater was actually 30–60 °C higher than that measured by the thermocouple, indicating that the true temperature used for pre-evaporation may in fact be the same for both heater types.
In any case, because compromise operating conditions were used for each sample introduction system, degradation in performance resulted for some elements while an enhancement occurred for others. To better assess improvements in analytical performance, atomic and ionic lines were compared element-by-element. Generally speaking, sensitivity enhancements are greater for ionic lines than atomic lines, due to lines with a higher total excitation potential being more sensitive to changes in the ICP's excitation conditions.19 For instance, the factors of improvement for detection limit when using recommended ionic lines over recommended atomic lines for Al, Be, and Mg are 120, 22, and 4.2, respectively, when comparing USN-PET at 300 °C to USN-HC. In terms of sensitivity, these factors of improvement are 7.1, 2.7, and 3.1, for Al, Be, and Mg respectively.
Finally, the USN-PET with IR block at 300 °C is compared to that from the previous work at 400 °C.13 The average sensitivity ratio (sensitivity at 300 °C divided by the one at 400 °C) of 1.1 ± 0.4 (for 22 emission lines) indicates no significant difference in sensitivity, which was confirmed by a paired t test. On the other hand, the average detection limit ratio (detection limit at 400 °C divided by the one at 300 °C) of 0.4 ± 0.3, indicates a degradation of detection limits by reducing the heating temperature to 300 °C, which was confirmed by a paired t test. However, the previous work was carried out at 1.5 mL min−1, i.e. almost twice the sample uptake rate used in this work. If the work at 400 °C had been carried out at 0.8 mL min−1, the corresponding figures of merit would have been degraded (because sensitivity increased with sample uptake rate up to 1.5 mL min−1),13 which may then have translated into no degradation or perhaps even an improvement in detection limits upon heating to 300 °C instead of 400 °C. The detection limits under the two sets of conditions (Table 4) are in fact similar for several elements (notably, As, Be and Si). Hence, the 300 °C system is more efficient, as it provides similar sensitivities and nearly the same detection limits as the 400 °C system used in the previous work, while operating at nearly half the sample uptake rate. Similar observations were made with the beaded rope heater at 250 °C. However, the range of detection limit ratios for 22 emission lines of 0.5–2.1 at 250 °C versus 0.7–2.8 at 300 °C suggests that the latter may be better.
Instrumental precisions, represented as % relative standard deviation (RSD) of 10 replicates of a 100 μg L−1 multi-element standard, are listed in Table 5. While USN-PET at 400 °C had an average RSD of 1.2 ± 0.7% (n = 22 emission lines), USN-PET at 160 °C had a greater RSD of 2.1 ± 0.7%, and USN-PET at 300 °C had an even larger RSD of 3.6 ± 1.0%, according to values listed in Table 5 (n = 32 emission lines). USN-HC provided a comparable RSD to the beaded rope heater, with average values of 2.3 ± 2.3% and 2.4 ± 1.3% for USN-HC and USN-PET rope at 250 °C, respectively. Finding the reason for the slight degradation in precision at 300 °C will be the aim of a future investigation. In any case, the higher RSD obtained with the USN-PET with IR block at 300 °C constitutes a very small sacrifice when considering that it provides similar detection limits for many elements at 0.8 mL min−1 to those with USN-HC at 2.0 mL min−1 with drastically improved robustness, as shown in the next section.
Element line (nm) | PN | USN-HC | USN-PET 300 °C, IR block | USN-PET 160 °C, IR block | USN-PET 250 °C, IR rope | USN-PET 400 °C, IR block13 |
---|---|---|---|---|---|---|
Al II 167.019 | 1 | 1.1 | 6.9 | 0.9 | 3.1 | — |
As I 189.042 | 0.5 | 0.88 | 4.9 | 1.6 | 1.1 | 1.4 |
Be II 313.042 | 1.3 | 3.2 | 4.7 | 2 | 4.7 | 0.7 |
Cd II 214.439 | 0.8 | 2.5 | 4.3 | 2.8 | 4.8 | 0.3 |
Co II 228.615 | 0.53 | 1.8 | 4.1 | 3 | 2.2 | 2.4 |
Cr II 267.716 | 0.73 | 1.8 | 3.9 | 3.4 | 2 | 1.7 |
Cu II 224.700 | 0.63 | 1.6 | 3.4 | 1.6 | 1.5 | 2 |
Cu I 324.754 | 0.54 | 1.5 | 3.3 | 2.2 | 1.8 | 1.4 |
Eu II 420.505 | 0.6 | 3 | 2.6 | 1.3 | 3.6 | — |
Fe II 238.204 | 0.73 | 1.8 | 3.6 | 2.2 | 2.3 | 2.1 |
Ga I 294.364 | 0.61 | 0.5 | 2.2 | 2.3 | 2 | — |
In II 230.606 | 0.42 | 0.38 | 2.7 | 2.3 | 2.3 | — |
La II 408.672 | 0.5 | 2.1 | 2.4 | 1.7 | 2.5 | — |
Li I 670.783 | 0.49 | 0.83 | 2.4 | 1.7 | 2 | 1.6 |
Mg II 280.270 | 0.67 | 3.6 | 4 | 1.8 | 4.9 | 0.2 |
Mg I 285.213 | 0.45 | 1.3 | 3 | 1.7 | 2.4 | 1.5 |
Mn II 257.610 | 0.62 | 2.1 | 3.7 | 2.1 | 4.7 | 0.7 |
Mo II 202.032 | 0.83 | 0.86 | 4.1 | 1.2 | 2.3 | — |
Ni II 231.604 | 0.74 | 1.7 | 4.3 | 3.2 | 2 | 1.8 |
P I 177.434 | 0.69 | 1.9 | 3.4 | 2.6 | 1.2 | — |
Pb II 220.353 | 0.99 | 0.86 | 4 | 2.2 | 0.72 | — |
S I 180.669 | 0.73 | 0.79 | 2.2 | 1.7 | 0.92 | — |
Sb I 217.582 | 0.41 | 0.46 | 3 | 1.4 | 0.78 | 1.4 |
Se I 196.026 | 0.55 | 1.9 | 5.6 | 1.4 | 0.62 | — |
Si I 251.611 | 0.63 | 1.5 | 3.2 | 3.3 | 1.4 | 2 |
Sr II 421.552 | 1.4 | 3 | 3.8 | 1.3 | 3.7 | 0.2 |
Ti II 334.941 | 0.7 | 2 | 3.3 | 1.7 | 4.4 | 1 |
V II 292.464 | 0.63 | 1.7 | 3.7 | 3.9 | 1.4 | 1.6 |
Y II 371.029 | 0.81 | 3.2 | 3.2 | 1.2 | 4.4 | 0.9 |
Zn II 206.200 | 0.6 | 11 | 4.3 | 2.9 | 2.1 | 0.8 |
Zn I 213.857 | 0.56 | 10 | 3.6 | 2.1 | 2.1 | 0.2 |
Zr II 339.198 | 0.55 | 2 | 2.9 | 1.8 | 2 | 0.2 |
Element | Waste water, low (EU-L-3) (mg L−1) | Corn bran SRM 8433 (mg kg−1) | ||
---|---|---|---|---|
Measured | Certified ± confidence limit | Measured | Certified ± confidence limit | |
Al | 6.50 ± 0.04 | 6.28 ± 0.19 | ||
As | 8.45 ± 0.10 | 8.40 ± 0.12 | ||
Be | 1.23 ± 0.01 | 1.23 ± 0.02 | ||
Cu | 10.58 ± 0.21 | 10.6 ± 0.2 | ||
Cd | 2.29 ± 0.03 | 2.28 ± 0.05 | ||
Cr | 6.15 ± 0.07 | 6.26 ± 0.15 | ||
Fe | 5.60 ± 0.11 | 5.80 ± 0.09 | 15.47 ± 0.32 | 14.8 ± 1.8 |
K | 202.6 ± 4.2 | 207 ± 5 | 475 ± 85 | 566 ± 75 |
Mn | 11.83 ± 0.16 | 12.2 ± 0.2 | 2.30 ± 0.15 | 2.55 ± 0.29 |
Mo | 3.97 ± 0.05 | 3.97 ± 0.08 | ||
Na | 471 ± 65 | 430 ± 31 | ||
Ni | 8.29 ± 0.06 | 8.34 ± 0.12 | ||
Pb | 4.13 ± 0.01 | 4.18 ± 0.06 | ||
S | 659 ± 84 | 860 ± 150 | ||
Sb | 1.78 ± 0.03 | 1.84 ± 0.07 | ||
Se | 2.84 ± 0.05 | 2.79 ± 0.16 | ||
Zn | 2.90 ± 0.06 | 3.05 ± 0.21 | 17.09 ± 0.73 | 18.6 ± 2.2 |
Footnote |
† Current address: Seastar Chemicals, 10005 McDonald Park Road, Sidney, BC V9L 5Y2, Canada. |
This journal is © The Royal Society of Chemistry 2018 |