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2018.02.15
Rapid and Simultaneous Micro-analysis of Halogens and Sulfur in Organic Compounds by Combustion – Ion Chromatography
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BUNSEKI KAGAKU (Analytical Chemistry) Vol. 66, No. 2, pp. 81-87 (2017)
© 2017 The Japan Society for analytical Chemistry

Special topic: “New development stage for trace organic compound analysis”: Technical papers

Hisomu Nagashima*1 and Yoshimi Dewa*2
(Received October 24, 2016; Accepted December 26, 2016)

Automatic combustion-ion chromatography (CIC) system is composed of auto-sampler, sample combustion furnace, absorption unit and ion chromatograph (IC).
We developed so far 3 kinds of systems. In this paper, some improvements are added in trace organic elements’ system, and considered rapid and simultaneous analysis method for halogens/sulfur in organic compounds.
From view point of device, carrier gas is transformed from argon (helium)/oxygen mixed gases to clarified air, so that analysis cost could be significantly reduced. As well as by adjusting combustion and schedule of chromatogram developing time, analysis time for continuous measurement could be shortened drastically.
On the analysis side, new standard sample (reagent), NAC-st4: C12H7NO2FClBrIS, has developed which contains 4 halogens and sulfur element. This enables to create calibration curve with 5-element (F, Cl, Br, I, S) by organic calibration method. It indicated more than 0.999 of correlation coefficients (r2).
This method can achieve high sensitivity, so it can apply both for micro analysis (sample amount: 1.0 to 10mg) and for ultra-micro analysis (sample amount: 0.1 to 1.0mg).
Measurements for 5 or 6 samples per an hour has attained and processed within allowable error range of organic element analysis (0.3%).

Keywords: rapid and simultaneous micro-analysis; organic halogens; organic sulfur; automatic combustion-ion chromatography

*1 NAC Techno Service Co., Ltd. (Tokyo and Chiba, Japan)
*2 Yanaco LID (Laboratory of Industrial Development) Co., Ltd. (Kyoto, Japan)

1. Introduction

In trace element analysis for organic compounds, automatic analysis of carbon, hydrogen and nitrogen (CHN analysis, hereunder) by thermal conductivity detection method is mainstream, and it is progressed with computer control that accurate analysis is attainable only for weighing sample weight. On the other hand, accurate analysis method for hetero elements becomes its significance year by year. New analysis method, for instance, analysis of halogens/sulfur becomes interesting topics by ion chromatography (IC, hereunder). For these halogens/sulfur analyses, silver absorption method that sample is combusted in the combustion tube and captures on the surface of heated silver has prevailed since 1950s 1), and it is developed to oxygen flask combustion method 3) that is still described in the Japanese Pharmacopoeia 2). However, it has huge obstacle to realize automatization, since it is not effectively connected between measuring method and pre-treatment method that sample is decomposed and becomes to inorganic phase.
Since IC device was developed by Small et.al.4), in 1975, several discussions were executed regarding technology and application, and simultaneous and accurate analysis for most inorganic ion can be determined today. This method, from the standpoint of organic trace element analysis, separation/quantitative determination of halogens/sulfur regarded as difficult, but it becomes possible and automatization process is promptly progressed.
Authors were focused on this IC method and by combination of oxygen flask combustion method and IC method, sole or simultaneous analysis method for halogens and sulfur in organic compounds is established, and eventually routine analysis has been implemented for long years. 5)—7)
Oxygen flask combustion method, however, daily treatment ability becomes high by providing a lot of flasks, but by using abundant of filter papers, chloride content in the filter paper and hydrogen-carbonate produced by combustion, affects on the chromatogram.
Whereas, combustion tube method, that has been utilized and organic sample is combusted and decomposed in the combustion tube, has no prescribing problem, but has lower sample treatment ability. Taking into consideration of such circumstances, in 1990s, by combining sample decomposition device that was used in CHN automatic analysis, and auto-sampler as well as suppressor type IC device, fundamental investigation for the automatization had reported 8) in the purpose of achieving improvement of analysis efficiency and labor saving.
In the wake of retirement from the company, a small company had founded and started reinvestigation of development for automatic combustion analysis system of halogens and sulfur.
In this decade duration, 3 different types of systems were introduced into the market 9)10).
Automatic combustion analysis system of halogens/sulfur; 1) for organic trace elemental analysis, 2) for environmental sample conforming to JIS standard, and 3) high temperature combustion type organic/inorganic compatible system.
In these types, organic trace elemental analysis system11) that was initially-staged developed in 2002 has added several modifications up to now. In this report, background and achievement for organic trace elemental analysis system are described in.
In analysis of iodine, bromine and sulfur, that was regarded as difficult analysis, we realized oxidation/reduction reaction with identical absorbed solution. In conjunction with, 5-element simultaneous analysis method has established by creating organic calibration curve utilizing newly-developed standard reagent that contains 4 kinds of halogens and sulfur.
Subsequently, by pinpointing the cause of carry-over as the blank, its reduction has achieved and consequently accompanying high sensitivity measurement is possible, so that it is applied both for micro analysis (sample amount: 1 to 10mg) and for ultra-micro analysis (sample amount: 0.1 to 1.0mg) in 2-step application. In economical point, changing the combustion supportive gas from helium (argon) / oxygen mixed gases to clarified air enables to reduce cost drastically.
By rapidity for combustion and chromatogram development, analysis time is significantly shortened, and as our target, rapid analysis of 5 or 6 samples per an hour is realized.

2. Experiment

2-1. System configuration
Photo of Automatic combustion analysis system of halogens/sulfur in this measurement illustrates in . This system is configured from combustion furnace (HNS-15), absorption unit (HSU-20) and auto-sampler (THA-25) were manufactured by Yanaco LID Company.
As for the IC, ICA-2000 (TOA DKK) is used, and for sample’s weighing, ultra-microelectronic balance (Mettler) is used.
This system enables to analyze wide range of samples other than pure organic compounds, by combining constant heating furnace (950 to 1100℃) and transient rapid heating flush heater in the furnace body. Combustion supportive gas is transformed from helium (argon)/oxygen mixed gases to clarified air, and set its flow rate at 1.0L min-1 as high speed flow. By introducing clarified air from the outlet side of the combustion tube, combustion residue materials are not attached on the tip part of the combustion tube. This contriving way enables to achieve reduction of carry-over conspicuously.
Measuring condition of IC and combustion system is indicated in .

fig1table1

2-2. Reagents
Sodium carbonate and sodium hydrogen carbonate for resolving solution (JIS special grade, Wako Chemical) are respectively solved into pure water, then prepared as 3.0mmol/L Na2CO3 — 2.0mmol NaHCO3 solution. For the absorption solution, 30% hydrogen peroxide (JIS special grade, Wako Chemical) 0.8mL and hydrazine — hydrate 0.1mL (ditto) are added and prepares to 2.0L as total amount. Absorption solution is newly prepared on daily basis.
In order to create calibration curve, organic calibration curve method stipulated in ion chromatography general rule (JIS K0127-2013) is applied for. For the standard sample, standard sample for organic elemental analysis (Kishida Chemical) is used, as well as multi-element containing standard sample (NAC-st1, NAC-st2 and NAC-st4) are used, that were collaboratively developed between “Tokyo Metropolitan Industrial Technology Research Institute (TMITRI)” and “NAC Techno Service”. As for the synthesis of NAC-st4, byproduct was produced during synthetic process and so was quite troublesome. However, selective synthesis method was succeeded at TMITRI, and high purity reagent could be obtained only by re-crystallization method.12)
These 3 kinds of standard reagents’ structural formulas are shown in .

fig2

2-3 Analysis operation method
Identical operation is applied not only for ordinal micro-analysis (1 to 10mg), but for ultra-micro analysis (0.1 to 1.0mg). Organic sample, that solely or plurally contains halogens and sulfur, is weighed on the boat made from platinum or ceramic, and set the boat on the auto-sampler.
Sample is introduced automatically into the flush heater part and combustion time is estimated for 3 to 5 minutes.
The sequential processes, exchange of the absorption solution, introduction to the analytical column, washing for the absorption bottle and filling of absorption solution for subsequent sample, are automatically implemented. Meanwhile, chromatogram is developed in parallel, and 4 kinds of halogens ions and sulfur ion produced from organic compound are consecutively eluted within 9 minutes.

3. Result and consideration

3-1. Simultaneous oxidation/reduction with identical absorption solution
The combustion ion chromatography (CIC) method is the method that gases produced by combustion decomposition of the sample are absorbed into the absorption solution, and it is analyzed by the ion chromatograph. By combustion, halogens (F, Cl, Br, I) and sulfur (S) in the sample are transformed to halides and halogen gas, while sulfur becomes to sulfuric acid and sulfurous acid gas. Thus far, simultaneous analysis of reduction-required bromine and iodine, and oxidation-required sulfur, is regarded as in difficult way, and analysis was implemented respectively. Here, a few amount of hydrogen peroxide (here used as oxidation agent, and it indicates reduction agent in H2O2-alkaline solution, too) and hydrazine hydrate (as reduction agent) are simultaneously added into the absorption solution, and in the identical absorption solution, simultaneous reactions of oxidation (SO32- → SO42-) and reduction (Br2, I2 → Br, I) are executed.
Consequently, both of respective reactions in oxidation/reduction absorption solution are confirmed in progress stoichiometrically. For the absorption solution, aqueous solution adding a few amount of hydrogen peroxide and hydrazine hydrate, such as in , or 2mM sodium carbonate solution, is appropriately used according to the purpose. As described later in 3-5, 2mM sodium carbonate solution is desirable for the sample containing bromine and sulfur simultaneously. By utilizing oxidation/reduction absorption solution, 5-elements simultaneous analysis for 4 kinds of halogens and sulfur can be realized, that was regarded as quite difficult analysis in long duration, and its patent has obtained.13)14)

table1

3-2. Separation condition for ion species
In organic micro elemental analysis, tungsten oxide (WO3) is added as combustion supportive agent for halogen/sulfur analysis of the sample that contain special element like metal. By adding tungsten oxide, partial tungsten acid is produced during combustion process, and other ion’s separation is affected by eluting close retention time. Therefore effect of column temperature, that affects separation of anion ion species, has considered (). As a result, retention time fluctuation accompanying with temperature rising is insignificant for divalent anions, while for monovalent iodide ion, its retention time is drastically decreased by endothermic reaction accompanying with temperature rising. At 35℃ as the column temperature, ions of SO42-, I and WO42- are separated in adequate interval. All ion species are eluted within 9 minutes, and rapid separation was achievable even in addition of combustion supportive agent (WO3).

fig3

3-3. Reduction of carry-over
The point taking into consideration most in implementing speeding-up and ultra-micronization of CIC method, is contamination in the carrier gas and the influence by carry-over (previous sample is remained in the phase as residue). At upstream side, impurity in the carrier gas is absorbed and removed by washing with 2 washing bottles. Also combustion product is adhered at the tube path close to exhaustion outlet of the combustion furnace at downstream side, and it may cause conspicuous disordering of the baseline and its reason is specifically determined. Thus, in order to prevent adsorption to the combustion part, the method is adopted that considerable amount of clean air is introduced from the outside of the needle tube15), too.
Standard reagent, NAC-st4, is used for simultaneous test for the carry-over. This method enables to obtain the carry-over values for total 5 elements of F, Cl, Br, I and S at just one measurement. Chromatograms of NAC-st4 and its carry-over measurement are illustrated in , and test results are shown in , respectively. In any respective element, carry-over quantity just after NAC-st4 measurement is less than 0.2%.

fig4table2

3-4. Creation of calibration curve
In ion chromatography general rule (JIS K0127-2013), the item of “Combustion pre-treatment of organic compounds” is added, and 2 methods are described as inorganic calibration curve method and previously described organic calibration curve method. Either method is used along with measuring purpose and calibration curve is created and quantitative calculation is executed.
Especially, in the field of organic micro elemental analysis, compatibility is considered with organic standard sample calibration curve method commonly used in CHN analysis, as well as organic calibration curve method is widely applied for in halogens/sulfur analysis that organic standard sample is combusted in step and create calibration curve accordingly. However it is required several times of standard sample measurement complying with respective element, hence simultaneous multiple elements analysis takes long time and effort. In NAC-st4, it contains total 5 elements of 4 kinds of halogens and sulfur in one compound, and it enables to analyze simultaneously, creation time of calibration curve as well as analysis time for respective element can be shortened drastically. As in , organic calibration curves of 5 kinds of elements accompanying with combustion of NAC-st4, have good correlation with correlation coefficient (r2) more than 0.999.

fig5

3-5. Br/I analysis in S-containing sample
There is no problem in bromine sole analysis in organic compound, but in case of simultaneous analysis for sulfur and bromine, it is pointed out that bromine’s analysis value becomes at negative and for this reason leads a lack of accuracy16). 3 standard reagents (NAC-st1; contains Br and S, NAC-st2 and NAC-st4; contains Br, S, I) are used, and bromine analysis is implemented for the standard of p-bromo-acetanilide, that is standard sample for elemental analysis, and analysis results are shown in . In this method, even 3 compounds contain sulfur, normal bromine contents are confirmed. Reasons that good result was not obtained with the method that pointed out the problem16) are assumed that most bromines exist as Br2 in acidic combustion gas accompanying with sulfur combustion, also since Br2 is not absorbed into water and sufficient reduction is not performed in the absorption solution, a part of Br2 is volatilized to out of phase.
In this paper, by adding to the absorption solution with hydrogen peroxide along with hydrazine hydrate, that indicates stronger reduction interaction, it is extremely effective means, not only for the reduction of iodine, but for the reduction of bromine. Also, the method introducing purified air from the tip of the combustion tube that is quite effective to the reduction of carry-over, is admitted prominent effect of dilution diffusion (Br2 → Br) of combustion gas. Therefore, usage of oxidation/reduction absorption solution as well as gas dilution diffusion method introducing clarified air through the tip as previously mentioned, can contribute to Br2 → Br reduction reaction under existence of SO2 through both of gas and liquid phase. By contrast, in iodine analysis, more than 99% quantitative yield is obtained along with stoichiometrical theory with/without the existence of sulfur, by using oxidation/reduction absorption solution.

table3

3-6. Analysis results for organic samples
For standard samples for elemental analysis which contain F, Cl, Br, S and S solely or simultaneously in organic compounds respectively, respective quantitative analysis is implemented within from micro to ultra-micro sample amount range. Measuring result that NAC-st4 was used for creation of calibration curve is shown in . Analysis results illustrate clearly that appropriate measurement is achievable for any respective analysis within allowable error range (absolute value, 0.3%) of organic elemental analysis

table4

4. Conclusion

The system for organic micro elemental analysis in this paper has been modified and improved during 15 years lapse, and eventually micro and ultra-micro rapid, simultaneous analysis system has completed for total 5 elements of 4 kinds of halogens and sulfur which are targeted elements in organic compounds. To comply with high accuracy required field of organic micro elemental analysis, the system has been utilized day by day for analyses that have a variety of field of samples in commissioned request.
Organic micro elemental analysis has a long history in the society, development of CHN automatic analyzer was started in 1960s, and was achieved total automatization with introduction of computer technology in 1980s, and they are prevailed as conventional analytical instrument widely now.
On the other hand, as for total automatization analysis for 4 kinds of halogens and sulfur as hetero elements, several researchers were challenged including the author during 1980 to 1990s, but even some progress were admitted, current situation has still some researching subjects.
Through consecutive research in this paper, it is quite significant at the points that simultaneously, rapidly, cost effectively and easily implementing analysis for hetero elements is attainable, similarly to mainstream CHN analysis.
For phosphorous analysis in organic compound, it is not achievable as total analysis system yet. Therefore, even with organic phosphorous element, development of systemized pre-treatment and analysis method as similar identical operation for halogens/sulfur is anticipated.

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