Sensitivity Improvement Mechanism of the Mistral Desolvating Sample Introduction to ICP-MS/AES and Its Application to Steel Analysis

Abstract

Mistral Desolvation (MD), a sample introduction method for Inductively Coupled Plasma (ICP)-Atomic Emission Spectroscopy (AES) and Mass Spectrometry (MS), provides sensitivity enhancement over 5 times compared to conventional sample introduction method. Some groups have been proposed different mechanisms of sensitivity enhancement by MD, e.g. inhibition of poly-atomic ion generation derived from solvent, influence of the change of plasma condition, and improvement of sample transportation efficiency in plasma. However, uniform understanding has not been obtained.

In this paper, we have identified the dominant factor of a sensitivity enhancement by MD and examined application to chemical analysis of steel samples. It was found that the MD method provided a decrease of 100–250 K plasma temperature, which led to sensitivity loss. On the other hand, sample transportation efficiency was improved by a factor of 4.7 times owing to an increase in the sum of small droplets. This improvement was comparable to fivefold sensitivity enhancement. Thus, it was concluded that the dominant factor of sensitivity enhancement achieved by the MD method was improvement of sample transportation efficiency with decreasing droplet size.

Besides, the steel certified reference materials have analyzed by MD-ICP-AES. It was found that almost tenth amount of sample consumption and almost 3-fold sensitivity improvement could be achieved, the analyzed value corresponded exactly to certificated value. Thus, in case of a little amount of sample for chemical analysis, for example, sampling from a defected part or a corroded part, this method can be useful due to the high sensitivity and the small sample consumption.

1. Introduction

In recent years, due to the need for weight-saving of, for example, steel plates for automobile, the need to develop high-strength steel plates with excellent formability is increasing rapidly. To meet this need, many steel manufacturers are designing and developing novel steel materials by positively utilizing the technology to control the micro-structure and precipitates of steel.¹⁾ In particular, it has been known that a micro alloy added to steel can have a significant effect on the mechanical characteristics of steel by virtue of the element that is present in the state of precipitate or of a solid solution; so, this is a very important factor to be controlled.^2,3) As the means to obtain the information with regard to a micro alloy in steel, methods to analyze trace elements such as Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES) and Atomic Absorption Spectrometry (AAS) have been widely used. But, the material control technology with a micro alloy has been advanced to the state where the element concentration is required to be equal to or less than quantification limit of these instruments.

On the other hand, Inductively Coupled Plasma-Mass Spectrometry (ICP-MS), which is more sensitive than ICP-AES and AAS by two to three orders of magnitude, has been generally used in ultra-trace level analysis in an order of pg/mL to fg/mL in fields such as semiconductors, food, the environment, etc. When this method is applied to steel analysis, separation and removal of the matrix is indispensable; and examples of the micro alloy analysis in steel have been reported wherein ICP-MS is combined with an ion-exchange separation method,^4,5) an emulsion separation method,⁶⁾ a solid phase extraction separation method,⁷⁾ or other various separation methods.^8,9) However, simplification of the sample preparation as well as further enhancement in sensitivity of ICP-AES is still required in the on-site analysis.

So, many instrument manufacturers and research institutes are studying how to further increase the sensitivity of ICP instruments. Especially in the sample introduction system and the introduction method in ICP-MS and ICP-AES, these have possibilities of improvement in sensitivity because, generally, only several percent of the total sample amount spraied with a nebulizer can be introduced into a plasma,¹⁰⁾ and the S/N ratio can be enhanced by selective removal of the matrix. For example, Inagaki et al.¹¹⁾ reported that the sample introduction efficiency can be improved by inserting a capillary into a conventional concentric double tube nebulizer (triple tube structure) in which the difference between the pressure of the spraied gas in the nozzle and that of the outside atmosphere is utilized, resulting in the generation of further fine droplets (flow focus effect). Many papers have been written with regard to the sensitivity enhancement effect by various desolvation processes (heating-cooling method, gas-exchange method, etc.) of the sample that is transferred to droplets by a nebulizer such as an ultrasonic nebulizer.^12,13)

Mistral Desolvation (MD) is the sample introduction method in which the heating-cooling process is applied to the primary droplets sprayed by a nebulizer with which a solvent is removed by selective evaporation and condensation so that the sensitivity can be improved by several times as compared with conventional methods.¹⁴⁾ It was reported that the mechanism of sensitivity enhancement in this method was influenced by at least the following three hypotheses.

1) Suppression effect in generation of polyatomic ion species derived from a solvent¹⁵⁾

In this mechanism, by selectively removing a solvent before introduction into the plasma, the generation of polyatomic ion species formed between the target element and the solvent is suppressed so that the amount of the target element ion is increased.

2) Effect on sensitivity due to the change in the plasma state¹⁶⁾

Generally, when a solvent vapor is removed by a desolvation process, the amount of hydrogen atoms in plasma is decreased. Because of this, electron density, thermal conductivity inside the plasma, and energy transmission efficiency from the plasma to the sample decrease, so the ionization temperature becomes lower. It was reported that the optimal ionization position and ionization efficiency in the plasma were changed so that the relative sensitivity was affected. However, because the effect on the sensitivity is dependent on the instrument design, there is no unified view.

3) Improving effect of the sample introduction efficiency¹⁷⁾

In the general sample introduction system of ICP, because coarse droplets generated in a nebulizer have an adverse effect on plasma stability, the system design is such that the coarse droplets can be removed in advance in a spray chamber so that only fine droplets can be introduced into the plasma. It is presumed that coarse droplets, which have been removed in a spray chamber in conventional introduction methods, are refined in the desolvation process so that they can be transported to the plasma, which leads to improvement in the sample introduction efficiency.

In the present paper, it was verified the sensitivity enhancement mechanism of ICP-MS/AES by the MD method mentioned above to clarify its dominant factor and the basic information for development of a sample introduction system was obtained. Additionally, as a result of an investigation into the application of this method to steel analysis, it was revealed that high sensitivity could be achieved with smaller sample consumption than before and found the possibilities that the target steel samples for analysis may be expanded. These findings were reported as follows.

2. Experiments

2.1. Reagents

A multi-element standard solution (SPEX Centriprep, XSTC series) and single-element standard solution (SPEX Centriprep, Assurance series) were used for each experiment. The preparation of standard solution for each experiment was conducted adding high-purity nitric acid (Tama Chemicals, TAMAPURE-AA100) and ultrapure water (milli-Q, resistance: 18.2 MΩ, TOC: 8.0 ng/mL) such that the concentration of nitric acid became to 1%.

In analyses of the Japanese Iron and Steel Certified Reference Materials for instrumental analysis, JSS 154-9 and JSS 158-1 were used. The solution for the calibration curve was prepared by dissolving JSS 001-6 (high-purity iron) matrix with hydrochloric acid (Kanto Chemical, reagent grade) followed by the addition of standard metal stock solutions (Kanto Chemical; Cu, Ni, V) to the each concentrations, and the volume was fixed with ultrapure water.

2.2. Instruments

For measurement of trace elements in solution, an ICP-MS spectrometer (Agilent Technologies, Agilent 7500cs) and an ICP-AES spectrometer (Shimadzu, ICPE-9000) were used. For introduction of the sample solution, an MD sample introduction instrument (Elemental Scientific Inc., apex Q) and a micro-flow nebulizer (Elemental Scientific Inc., PFA-400/100 nebulizer) were used (see Fig. 1). As the conventional sample introduction system, a cyclonic spray chamber (Glass Expansion, 50 mL) and a concentric nebulizer (Glass Expansion, Conikal U-Series nebulizer, 1.0 mL/min) were used. Measurement of the uptake flow rate of the sample was made with a sample flow meter (Glass Expansion, Truflo Sample Monitor). For measurement of the sample’s droplet size distribution, a cascade impactor (DEKATI, ELPI Classic) was used. Ar gas with a purity of 99.9995% was used.

Fig. 1.

Scheme of the MD instruments. L: liquid sample, 1: PFA tube, 2: micro-flow nebulizer, 3: chamber with heater (373 K/413 K), 4: cooler (275 K), 5: peristaltic pump, 6: drain, 7: PFA tube, 8: Ar carrier gas, 9: additional gas, 10: plasma torch, 11: carrier gas inlet, 12: additional gas inlet.

2.3. Experimental Method

As for the operating conditions of MD, heating temperature was set at two levels: 373 K and 413 K. To study only the desolvation effect, instrument designs (kind of nebulizer, structure of spray chamber, etc.) need to be identical. In this study, the non-heating condition was set at 298 K (room temperature) for comparison. The cooling temperature after heating was set at 275 K. Measurement of ICP-MS and ICP-AES were made with the measurement conditions summarized in Table 1.

Table 1. Operating conditions for ICP-MS and ICP-AES instruments.

ICP-MS
Plasma RF power (W)	1140–1400
Ar Plasma gas (L/min)	15
Ar carrier gas (L/min)	0.80–0.85
Ar make-up gas (L/min)	0.46
Ar auxiliary gas (L/min)	0.60
sampling depth (mm)	5–20
sample uptake rate	self-aspiration (ca. 0.40 mL/min)
ICP-AES
Plasma RF power (W)	1200
Ar Plasma gas (L/min)	12
Ar carrier gas (L/min)	0.70
Ar auxiliary gas (L/min)	0.60
Exposure height (mm)	10
sample uptake rate	self-aspiration (ca. 0.40 mL/min)
Exposure time (sec)	15–30
MD
Nitrogen flow	OFF
Nebulizer	PFA-400/100 (Elemental Sci. Inc.)
Spray Chember heater (K)	413/373/298 (No-heated)
Spray Chember cooler (K)	275

2.3.1. Examination of Suppression Effect in Generation of Polyatomic Ion Species Derived from a Solvent — Measurement of the Oxide-ion Generation Ratio in ICP-MS

The cerium standard solution (SPEX Centriprep, Assurance PLCE2) with 10 ng/mL was prepared. By using this solution, the change amount of the generation ratio of an oxide ion of cerium, which is known as one of elements that can most readily produce oxide ion, was examined. The oxide ion generation ratio was calculated from the signal strength ratio of CeO⁺ (m/z=156) to Ce⁺ (m/z=140).

2.3.2. Examination of the Effect to the Sensitivity due to the Change in the Plasma State — Estimation of the Plasma Temperature by the Boltzmann Plot Method

The iron standard solution (SPEX Centriprep, Assurance PLFE2) with 1.0 μg/mL was prepared. To estimate the plasma temperature, by using this solution the atomic line of Fe (I) was measured by ICP-AES to obtain the Boltzmann plot from 13 different emission strengths (Table 2); and from this, the plasma temperatures at each MD heating temperatures were estimated. For making the Boltzmann plot, the Boltzmann equation shown below was used.¹⁸⁾   

$$log\left(\frac{I\lambda }{gA}\right)=-\frac{0.434E_n}{kT}+C\hspace{1em}(C:\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{n}\mathrm{t})$$(1)

  

$$gA=\frac{8{\pi }^2{e}^2}{mc}\cdot \frac{gf}{{\lambda }^2}$$(2)

where I is the emission strength, λ is the wavelength, g is the statistically weighted value of excited level, A is the transition probability, k is the Boltzmann constant, T is the absolute temperature, E_n is the excitation energy, f is the oscillator strength, e is the elementary charge, m is the mass of electron, and c is the velocity of light.

Table 2. Measured Fe (I) emission lines and energy transition.

wave length λ (nm)	Ek (eV)	Lower level	Upper level	gA
370.925	4.257	3d7.(4F).4s	3d7.(4F).4p	1.09E+08
371.993	3.333	3d6.4s2	3d6.(5D).4s.4p.(3P*)	1.78E+08
372.256	3.417	3d6.4s2	3d6.(5D).4s.4p.(3P*)	2.48E+07
372.762	4.284	3d7.(4F).4s	3d7.(4F).4p	1.12E+08
373.332	3.431	3d6.4s2	3d6.(5D).4s.4p.(3P*)	1.94E+07
373.486	4.178	3d7.(4F).4s	3d7.(4F).4p	9.91E+08
373.713	3.369	3d6.4s2	3d6.(5D).4s.4p.(3P*)	1.27E+08
374.826	3.417	3d6.4s2	3d6.(5D).4s.4p.(3P*)	4.58E+07
374.949	4.221	3d7.(4F).4s	3d7.(4F).4p	6.87E+08
375.823	4.257	3d7.(4F).4s	3d7.(4F).4p	4.44E+08
376.379	4.284	3d7.(4F).4s	3d7.(4F).4p	2.72E+08
376.554	6.529	3d7.(2H).4s	3d7.(2H).4p	1.43E+09
381.584	4.734	3d7.(4F).4s	3d7.(4F).4p	7.84E+08

The emission strength I of the raw signal strength measured with the ICP-AES instrument was used. In this study, all experiments were conducted with the same instrument configuration and measurement conditions, so that relative plasma temperatures can be compared even without correction of the sensitivity coefficient of the instrument. Other parameters necessary for calculation are cited from the NIST Atomic Spectra Database Lines Form.¹⁹⁾

2.3.3. Examination of Improving Effect of the Sample Introduction Efficiency — Estimation of the Sample Introduction Efficiency by Collecting Sample Transported from MD Instrument Using the Cascade Impactor

To examine the change of the sample introduction efficiency, the cerium standard solution with a concentration of 50 μg/mL was used for MD system, sample droplets from an outlet of the MD instrument (heating conditions: 298 K and 413 K) were sieved and collected directly on the filter for a period of 30 minutes using the cascade impactor (Fig. 2). The cerium contained in the droplets that were collected on the filter was recovered with the 1% HNO₃ solution and the volume was fixed. The collected amount of cerium was analyzed using ICP-MS, and the size distribution of the droplets and the sample transportation efficiency were compared.

Fig. 2.

Experimental procedure of the size distribution analysis of sample droplets by using cascade impactor.

2.3.4. Analysis of the Japanese Iron and Steel Certified Reference Materials for Instrumental Analysis

For MD-ICP-AES analysis, the samples were prepared by the following flow.

Firstly, after 0.5 g of samples (Japanese Iron and Steel Certified Reference Materials shown in Table 3) was weighed, the samples were decomposed with 20 mL of hydrochloric acid (HCl) in a 200 mL PFA beaker by heating at 200°C on a hot-plate. Then, this solution was fixed to 200 mL adding ultrapure water.

Table 3. Certified values of CRMs. (Asterisk is attached to reference value.)

unit: mass%
JSS No.	JSS154-9	JSS158-1	JSS001-6
C	0.11	0.14	0.00024
Si	0.60	0.30	0.0001
Mn	1.15	0.47	0.000003*
Ni	0.52	0.048	0.00002*
Cu	0.20	0.16	0.000036
V	0.30	–	<0.00003*
P	0.0045	0.006	0.00005*
S	0.0045	0.007	0.00015
Cr	1.98	0.042	<0.00006*
Mo	0.37	–	<0.00002*
Al	0.009	–	<0.0001*
N	0.0117	–	0.00021
Co	–	0.30	0.000032
Ti	–	0.10	<0.00002*
As	–	0.092	<0.0003*
Sn	–	0.050	0.00003*
Nb	–	0.088	<0.00003*
B	–	–	0.00002*
Ca	–	–	<0.0002*
Mg	–	–	<0.00006*
Pb	–	–	0.000018
W	–	–	0.00001*
Zn	–	–	0.00019

Secondly, the standard solution was prepared as follows. Similarly to above, 0.5 g of high purity iron (JSS 001-6) was weighed. Then, this sample was decomposed with 20 mL of HCl in a 200 mL PFA beaker by heating at 200°C on a hot-plate. And after, the standard solution of Cu, Ni, and V was added to this decomposed solution, and then the volume was fixed to 200 mL so as to give the Cu, Ni, and V concentrations of 0–30 μg/mL adding ultrapure water. The sample preparation mentioned above was independently carried out twice and measured with MD-ICP-AES under the measurement conditions shown in Table 1. Each average values of Cu, Ni, and V was calculated.

3. Experimental Results

3.1. Examination of Suppression Effect in Generation of Polyatomic Ion Species Derived from a Solvent — Measurement of the Oxide-Ion Generation Ratio in ICP-MS

As reported by N. Jakubowski et al., we focused on the Ce ion, which is one of indicator of the oxide ion formation, and compared the oxide-ion generation ratio (CeO⁺/Ce⁺) at the MD heating temperature of 298 K and 413 K. As a result, the ratios were 0.75% at 298 K (without heating) and 0.51% at 413 K (with heating). A slight decrease in the Ce oxide-ion generation ratio by the MD heating was confirmed, but the decrease was very small, only about 0.24%.

3.2. Examination of the Effect on the Sensitivity due to the Change in the Plasma State - Estimation of the Plasma Temperatures by the Boltzmann Plot Method

The MD instrument was connected to the ICP-AES instrument, and the Boltzmann plot was obtained by measuring the emission lines of Fe (I) to estimate the plasma temperature. Because the plasma source of the ICP-AES instrument used in this experiment is different from that of the ICP-MS, the plasma temperatures must be different to be exact. However, in this experiment, it was expected that a relative comparison could be made by examining the effect of the heating temperature of the MD instrument using the same plasma source of ICP-AES instrument.

The heating temperature of the MD instrument was changed in three temperature levels, and the difference in the plasma temperatures in each MD heating temperature was compared (Fig. 3). Because the sufficient linearity was obtained in all the obtained Boltzmann plots with the correlation coefficient R of about 0.97 to 0.98, the plasma temperatures T_p were calculated from the slopes of these straight lines (Fig. 4). As a result, it was found that the plasma temperature in each condition was about 6000 K, and that as the MD heating temperature goes up, the plasma temperature was prone to go down by about 100 to 250 K, which was contrary to expectations.

Fig. 3.

Boltzmann plot upon heating temperature of MD process. ■, 298 K; ○, 373 K; △, 413 K.

Fig. 4.

Results of plasma temperature estimation upon heating temperature of MD process.

3.3. Examination of Improving Effect of the Sample Introduction Efficiency — Measurement Result of the Sample Introduction Efficiency by the Cascade Impactor

To examine the change in the droplet size distribution, the droplets of the cerium standard solution transported from the MD instrument were directly sampled in each droplet size using the cascade impactor and quantified using ICP-MS. In this experiment, to eliminate influences on the evaluation of size distribution by the effects of performance of the nebulizer, structure of the spray chamber, and transportation path of the droplets etc., the size distributions of the droplets were compared by changing the heating temperature of the MD instrument from 298 K to 413 K (Fig. 5(a)). As a result, it was revealed that the total amounts of cerium in the droplets with the size of less than the cut-off diameter of the spray chamber (ca. 10 μm) was significantly increased by heating of the MD instrument, whereas the average size of the droplets were not substantially changed as they were 0.26 μm at 298 K and 0.41 μm at 413 K. The average size of these droplets is depending on the nebulizer, flow rate of the Ar carrier gas, uptake rate of the sample, etc. For example, K. Kahen et al.²⁰⁾ reported that the Sauter average diameter (D_3.2) of the droplets by the laser scattering method was decreased from 12.9 μm to 5 μm or less by combining the micro-flow nebulizer (PFA-100) with the Scott-type spray chamber. Also, P. E. Walters et al.²¹⁾ reported that when desolvation was applied at 150°C using an ultrasonic nebulizer without a spray chamber, the droplet size measured with the laser diffraction was decreased to about 3 μm. In this study, because the cyclone-type chamber was used and the cut-off diameter of the cascade impactor was measured, measurement conditions and measurement methods of the droplet size were different from previous reports, so that a comparison cannot be made strictly; however, it was found that the relative tendencies were the same.

Fig. 5.

Effect of heating temperature of MD process for droplet size distribution and introduction efficiency. (a) Droplet size distribution upon heating temperature of MD process (b) Results of sample introduction efficiency. ■, 298 K; △, 413 K.

Additionally, when the sample introduction efficiency was defined as Eq. (3), the efficiency was enhanced from 2.6% at 298 K to 12.1% at 413 K, i.e., by a factor of about 4.7 times. Thus, it was revealed that the desolvation enabled the transportation of more droplets than the conventional method (Fig. 5(b)).   

$$\begin{array}{l}\mathrm{S}\mathrm{a}\mathrm{m}\mathrm{p}\mathrm{l}\mathrm{e}\mathrm{\hspace{0.17em} }\mathrm{i}\mathrm{n}\mathrm{t}\mathrm{r}\mathrm{o}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{\hspace{0.17em} }\mathrm{e}\mathrm{f}\mathrm{f}\mathrm{i}\mathrm{c}\mathrm{i}\mathrm{e}\mathrm{n}\mathrm{c}\mathrm{y}(\%)=\\ \frac{\mathrm{A}\mathrm{m}\mathrm{o}\mathrm{u}\mathrm{n}\mathrm{t}\mathrm{\hspace{0.17em} }\mathrm{o}\mathrm{f}\mathrm{\hspace{0.17em} }\mathrm{c}\mathrm{o}\mathrm{l}\mathrm{l}\mathrm{e}\mathrm{c}\mathrm{t}\mathrm{e}\mathrm{d}\mathrm{\hspace{0.17em} }\mathrm{s}\mathrm{a}\mathrm{m}\mathrm{p}\mathrm{l}\mathrm{e}\mathrm{\hspace{0.17em} }\mathrm{b}\mathrm{y}\mathrm{\hspace{0.17em} }\mathrm{C}\mathrm{I}}{\mathrm{A}\mathrm{m}\mathrm{o}\mathrm{u}\mathrm{n}\mathrm{t}\mathrm{\hspace{0.17em} }\mathrm{o}\mathrm{f}\mathrm{\hspace{0.17em} }\mathrm{u}\mathrm{p}\mathrm{t}\mathrm{a}\mathrm{k}\mathrm{e}\mathrm{d}\mathrm{\hspace{0.17em} }\mathrm{s}\mathrm{a}\mathrm{m}\mathrm{p}\mathrm{l}\mathrm{e}}\times 100\end{array}$$(3)

3.4. Analysis Results of the Japanese Iron and Steel Certified Reference Materials for Instrumental Analysis

The quantified values of Cu, Ni, and V in the Japanese Iron and Steel Certified Reference Materials using the MD-ICP-AES method were summarized in Table 4. All measured values agreed well with the certified values, and thus, it was confirmed that this method could be valid for chemical analysis of steel samples. Further, the comparison of the calibration curves between the MD method and the conventional sample introduction method (concentric nebulizer + cyclone-type chamber) was shown in Fig. 6. It was found that the slope of the Cu (213.598 nm) calibration curve was increased by a factor of 3.3 times in the MD method. This trend was confirmed in other calibration curves about V and Ni. The slopes of the V (290.882 nm) and the Ni (231.604 nm) calibration curve were increased by a factor of 3.1 times, 2.7 times, respectively (data was not shown). Additionally the amount of sample uptake using the concentric nebulizer was 1020 μL/min, while that using micro-flow nebulizer was 116 μL/min (Fig. 7). Thus, it was revealed that micro-flow nebulizer decreased the amount of sample uptake to 11.4%.

Table 4. Analytical results of CRMs by MD-ICP-AES. (a) JSS 154-9, (b) JSS 158-1.

(a)

unit: mass%
n	Cu	Ni	V
n=1	0.195	0.519	0.294
n=2	0.201	0.533	0.300
Average	0.198	0.526	0.297
Certified value	0.200	0.520	0.300

(b)

unit: mass%
n	Cu	Ni
n=1	0.159	0.049
n=2	0.163	0.050
Average	0.161	0.050
Certified value	0.160	0.048

Fig. 6.

Comparison of Cu (213.598 nm) calibration curve between MD and conventional sample introduction system.

Fig. 7.

Comparison of sample uptake amounts between concentric nebulizer and micro flow nebulizer.

4. Discussion

As shown below, the experimental results were compared to three hypotheses with regard to the factors for sensitivity enhancement by MD. The dominant factor about sensitivity enhancement by MD was considered. And their applicability for steel analysis was examined.

4.1. Suppression Effect in Generation of Polyatomic Ion Species Derived from a Solvent

Firstly, the suppression effect on the oxide-ion generation due to solvent removal was suggested. A solvent (water in this study) in each droplet was separated from the droplet by evaporation, and concentrated in the cooling section, so that it was selectively removed. Accordingly, the total amount of the solvent that is transported to the plasma must be decreased. Therefore, the reaction between the target element and O₂ in water could be suppressed, resulting in a decrease in the oxide ion and an increase of the target ion. From our experimental results, it was confirmed that the oxide-ion generation ratio decreased slightly by 0.24% due to heating with the MD method. Contrary to this, G. Zhu et al.²²⁾ reported that the ratio of CeO⁺/Ce⁺ was decreased from 2.5–4% to about 1–2% by lowering the spray chamber temperature from 35°C to 0°C. It could be explained as the effect of solvent removal due to condensation of the solvent vapor by cooling the spray chamber. Also, N. Jakubowski et al. reported that the ratio of YO⁺/Y⁺ was decreased from 0.54% to 0.14% using a desolvation with a heating section and a cooling section. This result was almost same as our observed result. The effect of heating and cooling in the sample introduction system was summarized as follows.

If only the heating section is equipped, because the transportation amount of the solvent vapor to the plasma is more than the conventional method, oxide ion is generated more; and thus, it is suggested that equipping the cooling section is indispensable to reduce the oxide ion.

There is no report with regard to the extensive study on the relation of oxide-ion generation to the sensitivity; however, the decrease in the oxide-ion generation ratio in these reports was very small, so it was presumed that the sensitivity enhancement effect of more than a factor of 5 times could not be explained only by the suppression effect in oxide-ion generation. Thus, it was suggested that the dominant factor for the sensitivity enhancement effect by the MD method was a factor other than this mechanism.

4.2. Effect to the Sensitivity due to the Change in the Plasma State

Secondly, the effect of the change in the plasma state due to the solvent removal was suggested. To confirm the change in the plasma state due to a decrease in the hydrogen atom in the plasma by desolvation, the plasma temperature was estimated by the Boltzmann plot with ICP-AES.

In past papers, the following findings have been reported. S. E. Long et al. was reported about 900 K decrease in the ionization temperature in case of introducing a mixture of droplets and steam, compared to introducing a mixture of ETV and steam. Besides, N. Jakubowski et al. reported that the solvent load effect was not dependent on the temperature of heating but more on the temperature of cooling, claiming that 80% or more of the solvent can be removed by cooling at 273 K.

To this information, according to the our experimental result, it was found that the plasma temperature tended to drop by about 2°C while raising the MD heating temperature by 1°C. This agrees with the past information; and the reason is discussed as follows. When the heating temperature in the MD instrument was raised, it was presumed that solvent in all droplets evaporates to cause decrease in size of the droplets. At that time, the total amounts of fine droplets with a size of less than the cut-off diameter of the spray chamber must be increased. This enables more droplets to be introduced into plasma per unit time. Therefore, the decrease in the plasma temperature can be well explained. Namely, it was presumed that with raising the heating temperature in the MD, the total introduction amount of droplets increases to cause lowering of the plasma temperature. Besides, this experiment was carried out with the constant temperature of the cooling section at 275 K. Therefore, most of water that was evaporated by desolvation in the heating section was presumably condensed in the subsequent cooling section so as to be transported to the drain section. Thus, it was presumed that the effect of the cooling section on the plasma temperature could be neglected.

In the past reports, dependence of the sensitivity on the change in the plasma temperature has not been discussed in detail; however, because all the present experiments were carried out with the constant cooling temperature, it was presumed that the plasma temperature did not change so much that effect on the sensitivity enhancement was small.

4.3. Improving Effect on the Sample Introduction Efficiency

Finally, it was suggested that due to the solvent removal effect of the MD method, the amount of the fine droplets increases. Usually, if a sample solution is sprayed by a nebulizer, droplets having a wide size distribution are formed. The design of sample introduction system is made such that the plasma can be stabilized by reducing the solvent load. Namely, the large droplets removed at the spray chamber, only fine droplets could be introduced into the plasma. Therefore, it is presumed that when the droplets become smaller by desolvation, more droplets can be introduced into the plasma to cause enhancement of the sample introduction efficiency, which leads to an increase in the sensitivity.

From the measurement result of the droplet size distribution, it was found that the absolute amount of the droplets with a size of 10 μm or less was drastically increased by heating in the MD instrument while the average size of the droplets hardly changed. The droplets measured in this experiment were the droplets which have passed through the spray chamber so that it could be actually introduced into the plasma. As a whole, the amount of the droplets with a size of 10 μm or less was increased by heating in MD; and thus, it was presumed that the droplets which were removed in the spray chamber in advance and not measured were desolvated to cause a size reduction to less than 10 μm so that they can be measured.

Besides, it was confirmed that the sample introduction efficiency was enhanced by a factor of about 4.7 times by heating of the MD instrument, showing that the introduction amount of the droplets increased. This result almost agrees with the increase by a factor of 2.2 times in the introduction of a slurry sample reported by J. H. D. Hartley et al. And the sensitivity enhancement of cerium in the ICP-MS was a factor of 4.6 times, which is about the same improving effect of the sample introduction efficiency due to the heating in the MD method. Given these facts, this presumably constitutes the largest contribution to sensitivity enhancement.

These examination results were summarized in Table 5. In past reports, comparisons were made mainly between the desolvation sample introduction methods (MD method (heating-cooling), or only heating, or only cooling) and usual sample introduction systems (without heating, without cooling); on the other hand, in this report, the effects on the ICP sensitivity with or without heating are compared under the cooling condition at 275 K (constant), so that the comparison condition is different to be exact. As mentioned before, according to the report of N. Jakubowski, most of the solvent vapor is removed only by the effect of cooling; and thus, it is presumed that the influence of polyatomic ion generation ratio and the plasma temperature are improved sufficiently. Accordingly, it is presumed that the improving effect of the sample introduction efficiency due to the heating in the MD method constitutes the largest contribution to sensitivity enhancement.

Table 5. Experimental results and influence for ICP sensitivity.

	Our results	Influence for sensitivity
1. Inhibition of poly-atomic ion generation	Approximately 0.2% reduced	Slightly effective
2. Plasma temperature change	100–250 K reduced	Slightly negative
3. Improvement of sample transportation efficiency	Almost 4.7% improved	Most effective

4.4. Effect of Application of the MD Method to Steel Analysis

Additionally, the results of the Japanese Iron and Steel Certified Reference Materials quantified by the MD-ICP-AES method will be discussed below. When the conventional concentric nebulizer and the micro-flow nebulizer of the MD method are simply compared, the sample uptake amount of the MD method is about one ninth, suggesting the same degree of decrease in the sensitivity. However, as a result of sensitivity comparison between these nebulizers by using the same cyclonic spray chamber, it was revealed that the decrease amount of the relative sensitivity in the micro-flow nebulizer of the MD method is 42% as compared with the conventional concentric nebulizer, which is significantly lower than the expected value of 88.6% (Fig. 8). The reason is presumably because the nebulization efficiency of the micro-flow nebulizer is superior to that of the concentric nebulizer. Thus the fine droplets can be generated more efficiently to cause improvement in the introduction efficiency to the plasma.

Fig. 8.

Comparison of sensitivity between concentric nebulizer and micro flow nebulizer measuring Cu 327.396 nm emission line.

In addition, the sample transportation amount increases by a factor of about 4.7 times by the MD method. Therefore, assuming that the sensitivity of the conventional sample introduction method (concentric nebulizer + cyclone-type chamber) is 1, the relative sensitivity of the MD method is expected to be (1−0.42)×4.7≒2.7 times, which is almost the same as the measured value of 3.0 times. Thus, it can be concluded that by combining the micro-flow nebulizer with the desolvation process of the MD method, not only the sample consumption amount can be reduced to one ninth relative to the conventional method but also the sensitivity can be enhanced by a factor of more than 3 times relative to the conventional method. If this method is applied to the steel analysis for which 0.5 g of the sample dissolution amount was generally required in the past, the amount can be reduced to about 18 mg, about one twenty-seventh relative to the conventional method. Meanwhile, the micro-flow nebulizer used in the MD method employs the same nebulization method as the conventional method, and thus, problems of fouling and clogging due to the iron matrix, having been problematic in the ultrasonic nebulizer, etc. can be avoided. In addition, as mentioned before, the difference in the sensitivity could be explained as only the difference in the sample introduction efficiency, and it was not confirmed that the co-existing iron matrix gives the matrix effect to other element. If the MD method is applied, even in the case that only several milligrams to several tens of milligrams is sampled, for example, a defected part, or a corroded part, etc., of a steel material, wherein the dissolved sample for measurement is small, a high-sensitivity analysis is possible while reducing the sample consumption amount.

5. Summary

Sensitivity enhancement mechanism upon application of the MD method to ICP-MS/AES was studied, and three factors for the sensitivity enhancement mechanism were examined.

(1) It was confirmed that the oxide ion generation ratio in ICP-MS was slightly decreased (0.24%) by the heating process of the MD method.

(2) The plasma temperature of ICP estimated from the Boltzmann plot method was about 6000 K, and by raising the chamber heating temperature in the MD method, the plasma temperature dropped by about 100 to 250 K.

(3) The droplets were refined by the MD method, so that the droplets with a diameter of 10 μm or less, the cut-off diameter of the spray chamber, were drastically increased. Because of this, the transportation efficiency of the droplets was increased by a factor of about 4.7 times, and thus, it was revealed that this constitutes the largest contribution to the sensitivity enhancement effect.

From the above, it was revealed that the improving effect of the sample introduction efficiency by refinement of the droplets in (3) mentioned above is a dominant factor for sensitivity enhancement by the MD method.

In addition, it was revealed that the measurement result by MD-ICP-AES agrees well with the certified values of the Japanese Iron and Steel Certified Reference Materials. These results indicate that MD-ICP-AES enables accurate analysis for steel samples. Additionally, MD-ICP-AES provides that the high sensitivity of a factor of about 3 times can be achievable even under the condition that the sample consumption amount is reduced to one ninth. From these results, it is presumed that the sample dissolution amount may be reduced to about one twenty-seventh relative to the conventional method.

Thus, MD method can be applied for the case of sampling only several milligrams to several tens of milligrams from a defected part or a corroded part, etc., further study about sample introduction technique is expected to enable the application for various steel samples.

In preparing this paper, we received guidance and valuable comments from Dr. Koji Kanehashi of Advanced Technology Research Laboratories of Nippon Steel & Sumitomo Metal Corporation. The authors wish to give their sincere thanks to him.

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