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Analytica Chimica Acta 445 (2001) 269–275

Determination of residual carbon by inductively-coupledplasma optical emission spectrometry with axial and

radial view configurations

Sandro T. Gouveiaa, Fernando V. Silvab,c, Letıcia M. Costab,Ana Rita A. Nogueirac, Joaquim A. Nóbregab,∗

a Universidade Federal do Ceará, Fortaleza, CE, Brazilb Departamento de Quımica, Universidade Federal de São Carlos, P.O. Box 676, 13560-970 São Carlos, SP, Brazil

c Embrapa Pecuária Sudeste, São Carlos, SP, Brazil

Received 8 February 2001; received in revised form 3 July 2001; accepted 3 July 2001

Abstract

In this work it was evaluated the performance of inductively-coupled plasma optical emission spectrometers (ICP-OESs)with axial and radial view configurations for residual carbon content (RCC) determination. The effects of carbon compoundsource (urea,l-cysteine, and glucose), sample medium, and internal standards on RCC determination were systematicallyevaluated. All measurements were carried out with two ICP spectrometers using the carbon atomic emission lines: 247.857and 193.025 nm. The results obtained using axial and radial configurations showed that both the carbon source and thesample medium did not affect significantly the emission intensities. The sample medium only caused drastic influencewhen H2SO4 was employed probably due to transport interference that can be corrected employing Y as internal standard.The sensitivity attained using axial view ICP-OES was 20-fold better than that reached using radial view ICP-OES basedon the slopes of the analytical curves at the most sensitive wavelength (193.025 nm). Using radial and axial ICP-OESs,high concentrations of Fe (>100 mg l−1) interfered at 247.857 nm wavelength. An addition-recovery experiment was madeby adding urea to an acid-digested sample and all recoveries were in the 100± 5% range for axial and radial measure-ments. At this wavelength, R.S.D.<2.0% (n = 10) and detection limits of 33 and 34�g ml−1 C, were measured forICP-OESs with radial and axial configurations, respectively. Biological samples were acid-digested using a closed-vesselmicrowave-assisted procedure and RCC was determined using both ICP-OES configurations. © 2001 Elsevier ScienceB.V. All rights reserved.

Keywords:Residual carbon content; Microwave-assisted digestion; Inductively-coupled plasma optical emission spectrometry; Radial andaxial configurations

∗ Corresponding author. Fax:+55-162608350.E-mail address:[email protected] (J.A. Nobrega).

1. Introduction

Frequently instrumental techniques require com-plete sample decomposition before measurements.Usually acid wet digestion is implemented combin-ing oxidant agents and heating for destroying the

0003-2670/01/$ – see front matter © 2001 Elsevier ScienceB.V. All rights reserved.PII: S0003-2670(01)01255-7

270 S.T. Gouveia et al. / Analytica Chimica Acta 445 (2001) 269–275

organic fraction of the sample [1]. However, due tothe high stability of some organic compounds presentin samples or formed during decomposition, incom-plete oxidation is generally observed. Spectroanalyt-ical techniques may not be critically affected by theresidual carbon content (RCC) [2,3]. On the otherhand, the application of electroanalytical techniquescan be severely limited [4–6]. Therefore, RCC is animportant parameter to be controlled depending onthe instrumental technique used. Additionally, RCCmeasurement is an important parameter to evaluatethe efficacy of sample decomposition procedures.

Different approaches were proposed for RCC de-termination [6–8]. Elemental analysis or spectropho-tometric titration of the organic matter with chromicacid can be used for this purpose [6]. In this latterprocedure, controlled reaction conditions and experi-enced analysts are required to reach suitable accuracy.Inductively-coupled plasma optical emission spec-trometry (ICP-OES) was also applied for RCC deter-mination [7,8]. The RCC presents in natural waterswas determined by Emteryd et al. [7] using a flowinjection system coupled to an ICP-OES spectrome-ter [7]. The measurements were made at 193.091 nmwavelength and solutions prepared from citric acidor potassium hydrogen phthalate were used for cali-bration. The obtained results were in agreement withthose determined by elemental analysis. Krushevskaet al. [8] also used the C emission line 193.091 nm toperform RCC determination in milk sample digests.Recoveries of aliphatic and aromatic compounds wereevaluated in different sample media. Recovery val-ues around 100% were achieved using Sc as internalstandard. The authors also mentioned memory effectsin the spray-chamber when measuring aromatic com-pounds. Long washout times were required to reducethis effect.

Considering the emerging of solid state detectorsand changes of the optical system in ICP-OESs, thepresent work investigated the determination of RCC inacid-digested biological samples using ICP-OESs withaxial and radial view configurations. The main exper-imental parameters and figures of merit were system-atically evaluated and further correlated with the em-ployed configuration. The axial and radial ICP-OESsdeveloped procedures were compared to TOC an-alyzer in order to assess the accuracy of obtainedresults. The developed procedures were also applied to

assess the digestion efficiency of microwave-assisteddigestion procedures.

2. Experimental

2.1. Instrumentation

Axial and radial view simultaneous ICP-OESs(Vista AX and RL, Varian, Mulgrave, Australia)equipped with CCD detectors were used in this study.The spectrometers provided wavelength coveragefrom 167 to 785 nm with the optical system purgedwith argon and the Echelle polychromator ther-mostated at 34◦C. In the axial arrangement the coolplasma tail was removed from the optical path usingan end-on gas to purge the plasma–spectrometer in-terface. An argon snout purge system was employedin the radial configuration to produce an argon purgedenvironment between the pre-optical system and theplasma in order to allow readings below 190 nmwavelength. The operational parameters establishedfor RCC determination in each configuration arelisted in Table 1. All measurements were carried outusing liquid argon to decrease signal blank caused byplasma gas contamination [8].

The RCC was monitored at C I 193.025 and247.457 nm wavelengths. The same instrumental con-ditions and nebulizer system (V-groove) were used inboth ICP-OES spectrometers to facilitate the compari-son of performance of axial and radial configurations.

Table 1Instrumental parameters for RCC determination using axial andradial ICP-OESs

Instrumental parameter Axial and radial

Power (kW) 1.0Plasma gas flow (l min−1) 15.0Auxiliary gas flow (l min−1) 1.5Observation heighta 9Nebulizer gas flow (l min−1) 0.90Spray chamber Sturman–MastersNebulizer V-grooveSample flow rate (ml min−1) 0.80

Analytical wavelengths (nm)C I 193.025C I 247.857Y II 371.022

a Only for radial view configuration.

S.T. Gouveia et al. / Analytica Chimica Acta 445 (2001) 269–275 271

The acid digestions were performed in a microwave-oven (ETHOS 1600, Milestone, Sorisole, Italy)equipped with 10 perfluoralkoxy Teflon® (PFA)closed vessels with calibrated resealing pressure reliefmechanism (maximum operating pressure 110 atm).The vessels were put on a rotating turntable inside themicrowave oven cavity. Before using, the PFA vesselswere acid cleaned and rinsed with deionized water.The heating programs used for acid digestions are de-scribed further on. A sub-boiling apparatus (subPUR,Milestone) was also used to distill the concentratednitric acid. The total carbon determination used toevaluate the accuracy of the proposed procedure wascarried out in a total carbon analyzer (TOC 5000Shimadzu, Japan).

2.2. Reagents and solutions

All solutions were prepared using analytical gradereagents and Milli-Q® distilled and deionized wa-ter (Millipore, Bedford, MA, USA). Sub-boiled dis-tilled nitric acid and hydrogen peroxide (Mallinck-rodt, Mexico) have also been used to perform themicrowave-assisted digestions.

Stock solutions containing 5.0% m/v C in aque-ous medium were prepared from glucose (C6H12O6,Merck, Germany), urea (CH4N2O, Reagen, Brazil)and l-cysteine (C3H7NSO2, Sigma, USA). Testsolutions containing 0.05 and 0.25% m/v C wereprepared in HNO3 (1.4 mol l−1), HNO3 + H2O2(1.4 mol l−1 + 0.30% v/v) and H2SO4 (1.8 mol l−1)media and used to evaluate the influence of carbonsource and sample medium in C emission intensities.Evaluation of Fe interference was carried out using

Table 2Microwave-assisted digestion programs employed to decompose lyophilized bovine liver and soybeans samples

Step Microwave digestion programsa

1 2 3 4 5 6

Time(min)

Power(W)

Time(min)

Power(W)

Time(min)

Power(W)

Time(min)

Power(W)

Time(min)

Power(W)

Time(min)

Power(W)

1 1 250 1 250 1 250 1 250 1 250 1 2502 1 0 1 0 1 0 1 0 1 0 1 03 3 250 3 250 3 250 3 250 3 250 3 2504 5 400 10 400 5 400 5 400 5 400 5 4005 5 600 5 800 2.5 600 7.5 600 7.5 800

a A sixth step consisting of 5 min of ventilation without any applied power was implemented in all tested programs.

solutions containing 0.05% m/v C plus 10, 100 and500 mg l−1 Fe, respectively. Carbon addition-recoveryexperiments were performed using 0.05% m/v C refer-ence solutions and additions of 0.10% m/v C as urea,glucose,l-cysteine and citric acid. Addition of carbonto an acid-digested sample was also performed.

For RCC determination the analytical curve used(0.05, 0.10 and 0.25% m/v C) was prepared in1.4 mol l−1 HNO3 using urea stock solution. Yt-trium as internal standard was added to all referencesolutions and samples in a final concentration of1.0 mg l−1.

2.3. Samples

Standard reference materials NIST-1577b Bovineliver, NIST-8435 Whole milk powder, NIST-1515 Ap-ple leaves, and NIST-1570a Spinach leaves (NationalInstitute of Standards and Technology, Gaithersburg,MD, USA) were digested and RCC was determinedusing TOC analyzer in order to check the accu-racy of the proposed procedures. Lyophilized bovineliver and soybeans samples were microwave-assistedacid-digested using different heating programs. TheRCC in all diluted digests was determined usingICP-OESs with axial and radial view configurations.

2.4. Sample preparation

The microwave-assisted digestions were carried outusing 250 mg of sample and an oxidant mixture con-taining 2 ml of HNO3 plus 1 ml of H2O2. Accordingto the procedure recommended by Krushevska et al.the digests were transferred to 10 ml glass beakers and


evaporated gently at 120◦C to remove the volatile car-bon compounds [8]. After, the digests were quantita-tively transferred to 10 ml volumetric flasks and thevolume was made up with H2O. It was used the mi-crowave digestion program 1 described in Table 2.

The time and power parameters in the last stepof the microwave heating program 1 were systemat-ically modified to evaluate its effect on the digestionof lyophilized bovine liver and soybeans samples. Allevaluated programs are presented in Table 2. The di-gestion efficiency was evaluated by determining theRCC in the digests.

2.5. Total carbon analyzer

The accuracy was evaluated comparing the obtainedresults with those established using a total carbon an-alyzer (TOC). In the comparative method, the residualcarbon presents in the sample digests was thermallyconverted into CO2 and detected by an infrared sen-sor. The analytical curve was obtained using referencesolutions containing 1.0, 5.0, 11 and 17 mg l−1 C pre-pared using potassium hydrogen phthalate (C8H5O4K,Nacalai Tesque, Japan) in aqueous medium. The sam-ple digests were diluted according to the analyticalcurve concentration range.

3. Results and discussions

3.1. Carbon source and sample medium evaluation

The influence of carbon source on C emissionintensities was evaluated using reference solutions

Table 3Analytical curves parameters obtained for glucose, urea andl-cysteine reference solutions established by ICP-OESs with axial and radialview configurations

Reference solution λ (nm) Slope Linear coefficient Linear correlation coefficient

Axial Radial Axial Radial Axial Radial

Glucose 193.025 115 6.48 23928 481 0.9993 1.0000247.857 8.24 0.37 774 39.7 0.9995 0.9999

Urea 193.025 95.9 6.12 24058 554 0.9948 1.0000247.857 6.86 0.35 950 44.4 0.9946 1.0000

l-Cystein 193.025 115 6.31 25911 469 0.9990 1.0000247.857 8.30 0.36 782 36.9 0.9996 1.0000

Table 4Ratio of carbon emission intensities in axial and radial view con-figurations ICP-OESs

λ (nm) Ratio (axial/radial)

Glucose Urea l-Cystein

193.025 18 16 18247.857 22 20 23

prepared from different organic compounds. Glucose,urea andl-cysteine were used to prepare referencesolutions in the 0.50–2.0% m/v C concentration range.This concentration range showed a non-linear behav-ior in axial configuration at the 193.025 nm wave-length. Probably, the elevated carbon concentrationcaused self-absorption effects at the most sensitivewavelength. Thus, measurements were repeated usingsolutions containing from 0.05 up to 0.25% m/v C.The parameters of the analytical curves obtained byICP-OES axial and radial view configurations foreach evaluated carbon source are shown in Table 3.The sensitivity of the measurements was not affectedby carbon source in both studied configurations. Therelative standard deviations of the slopes for curvesobtained using glucose, urea andl-cysteine referencesolutions were around 10 and 3% for axial and radialview configurations, respectively. We can concludethat any tested compound could be used for calibra-tion owing to the low differences observed, however,it should be mentioned that aromatic compounds cangenerate memory effects and therefore aliphatic com-pounds are recommended for preparation of standardsolutions [8]. All further measurements were car-ried out using urea. Table 4 shows that axial viewimproved sensitivities for all C sources.


The sample medium can influence the analytical sig-nal either by physical effects, such as changes in neb-ulization efficiency, or by chemical processes, such asalteration of excitation mechanisms in the plasma. Thecarbon emission intensities were evaluated in differentmedia (HNO3, HNO3/H2O2 and H2SO4). All resultswere compared to those obtained using reference so-lutions prepared in aqueous medium. In axial and ra-dial view configurations, deviations minor than 10%were observed when HNO3 or HNO3/H2O2 mediumwas used. Sulfuric acid medium caused a pronounceddecrease in C emission intensities with both config-urations. This could be related to the higher viscos-ity of this solution that affects the efficiency of sam-ple transport to the plasma. This undesired effect ofH2SO4 can be corrected employing Y as internal stan-dard. The analytical curve for RCC determination wasprepared in 1.4 mol l−1 HNO3 taking into account thefinal acid concentration of diluted digests.

3.2. Iron interference

The measurements of carbon at 247.857 nm emis-sion line for RCC determination can be spectrallyinterfered by Fe II 247.857 nm. This Fe ionic lineis two-fold more intense than the C atomic line atthis same wavelength. Based on this effect, the in-fluence of Fe on carbon measurements at 193.025and 247.857 nm wavelengths in both configurationswas evaluated. For axial configuration, at 247.857 nmwavelength, iron caused positive interferences(Table 5). At 193.025 nm emission line, the signalvariation was lower than 10%. The same behaviorwas observed for radial configuration, however, atobservation height of 14 mm the Fe interference on

Table 5Effect of Fe on C recoverya

Iron (mg l−1) Recovery (%)

Axial Radial

193.025 nm 247.857 nm 193.025 nm 247.857 nm

9 mma 17 mma 9 mma 17 mma

10 97.2 99.2 98.5 100 104 105100 97.6 117 99.5 101 112 123500 103 178 104 103 145 167

a Observation heights.

Table 6LOD and BEC for axial and radial view configurations

λ (nm) Axial Radial

BEC(mg l−1)

LOD(mg l−1)

BEC(mg l−1)

LOD(mg l−1)

193.025 149 34.0 126 33.0247.857 251 19.0 90.5 33.0

247.857 nm wavelength was slightly more pronouncedthan that observed at 9 mm.

3.3. Limits of detection and memory effects

The limits of detection (LOD) at 193.025 and247.857 nm wavelengths were determined consider-ing the background equivalent concentration (BEC)[9] and the results for both configurations are pre-sented in Table 6. The background repeatability wassimilar in both plasma views, but the measurements at247 nm in radial view configuration presented higherR.S.D. due to the low sensitivity at this wavelength.

Despite the highest intensities obtained using axialconfiguration, the highest background equivalent con-centration also increased and affected negatively theLOD. Therefore, the LODs were similar using axialand radial configurations. It should be pointed out thatthe detection limits with both arrangements could beslightly deteriorated by C contamination of the plasmagas despite of the use of liquid argon.

The occurrence of memory effects was investigatedby continuous monitoring of the emission signals ofglucose, urea andl-cysteine solutions intercalatedwith blank solution aspiration. For all carbon sourcesthe C emission intensities decreased quickly after


Table 7RCC determined using ICP-OESs with axial and radial view configurations and TOC analyzer in biological sample digestsa

Sample RCC (wt.%)

Axial Radial TOC

NIST-1577b bovine liver 9.82± 0.53 10.6± 0.01 10.2± 0.3NIST-8435 whole milk powder 14.5± 2.0 16.6± 1.1 15.3± 1.0NIST-1515 apple leaves 7.79± 0.14 8.70± 0.10 7.23± 0.12NIST-1570a spinach leaves 8.42± 0.32 8.98± 0.12 6.89± 0.15

a Standard deviation based on sample in quadruplicate (n = 4).

Table 8Effect of microwave-assisted heating program on RCC in acid digests of lyophilized bovine liver and soybeansa

Microwave program RCC (wt.%)

Lyophilized bovine liver Soybean

Axial Radial Axial Radial

1 10.5± 0.8 9.04± 0.49 12.3± 0.6 10.7± 0.62 11.0± 0.4 10.8± 0.5 13.2± 1.0 14.1± 0.83 6.64± 0.34 5.55± 0.16 8.24± 0.58 6.68± 0.504 9.56± 0.75 9.15± 0.94 10.5± 0.6 10.9± 0.65 6.20± 0.75 5.32± 0.79 7.71± 1.02 7.27± 0.876 3.52± 0.48 3.35± 0.28 5.91± 1.6 3.99± 0.84

a Standard deviation based on sample in quadruplicate measurements (n = 4).

stopping their nebulization. Thus, the Sturman–Masterschamber was effective for avoiding memory effects.

3.4. Carbon recovery

Additions of 0.05% m/v C to reference solutionsgenerated recovery values around 100± 5% for alltested carbon compounds in both ICP-OESs config-urations. Similar results were obtained when C wasadded to a lyophilized bovine liver digest, indicatingthe absence of matrix effects.

3.5. Residual carbon content determination

The RCCs for biological samples digested usingthe microwave digestion programs showed in Table 2were determined by ICP-OESs with axial and radialview configurations and TOC analyzer. All measure-ments with ICP-OESs were carried-out at 193.025 nm.Table 7 summarizes the obtained results. Accordingto a pairedt-test all results are in agreement at 95 or99% confidence levels.

The results obtained for lyophilized bovine liver andsoybeans samples digested using different microwaveheating programs are shown in Table 8.

It can be seen that when nominal power was in-creased from 400 to 800 W in the last step (programs2 and 3), the RCC decreased 45% in both samples. Onthe other hand, when the heating time was increasedfrom 2.5 to 5 min (programs 1 and 4), the same pro-nounced effect was not observed. Increasing the heat-ing time from 2.5 to 7.5 min (programs 4 and 5), theRCC decreased 38 and 26% for lyophilized bovineliver and soybeans samples, respectively. These re-sults indicate a more pronounced effect of nominalpower than heating time on the efficiency of decom-position. Lower RCCs were reached using simultane-ously higher nominal power and longer heating time.

4. Conclusions

The developed procedures were suitable for RCCdetermination in biological sample acid digests. When


compared to RCC determination using TOC analyzer,axial and radial ICP-OESs procedures reduced theanalysis time and decreased sample manipulation.Additionally, the ICP-OES multi-elemental charac-teristics enable the simultaneous monitoring of otheranalytes. Both configurations evaluated presented re-sults in agreement with those obtained using TOCanalyzer. However, the determination carried out withaxial view configuration presented higher sensitiv-ity and similar deviations compared to the radialone. Considering the procedures tested to performmicrowave-assisted acid digestions, it was observeda more pronounced effect of applied power on theefficiency of organic compounds decomposition.

Acknowledgements

The authors are grateful to Fundação de Amparoà Pesquisa do Estado de São Paulo by the financialsupport (98/10814-3) and by the fellowship provided

to F.V.S. (00/00997-4). A.R.A.N., J.A.N. and L.M.C.are grateful to CNPq by researchships and fellowshipprovided. S.T.G. is grateful to CAPES-PICDT by fel-lowship provided.

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