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Determination of silicon, phosphorus, iron and aluminum in biodiesel

by multicommutated stepwise injection analysis with classical least

squares method

Christina S. Vakh ⇑, Andrey V. Bulatov, Andrey Y. Shishov, Andrey V. Zabrodin, Leonid N. Moskvin

Department of Analytical Chemistry, Institute of Chemistry, Saint Petersburg State University, Saint-Petersburg, pr. Universitetskij 26, 198504, Russia

h i g h l i g h t s

 The flow-based procedure for Si, P, Fe,

Al determination in biodiesel is

proposed.

 The proposed procedure is simple,

fast and flexible.

 Applied to biodiesel samples obtained

from castor bean, sunflower, canola

and corn.

g r a p h i c a l a b s t r a c t

a r t i c l e i n f o

 Article history:

Received 8 May 2014

Received in revised form 19 June 2014

Accepted 25 June 2014

Available online 8 July 2014

Keywords:

Biodiesel

Multicommutated stepwise injection

analysis-classical least squares method

Inorganic elements

a b s t r a c t

Recently suggested multicommutated stepwise injection analysis (MCSWIA) coupled with classical least

squares method (CLS) has been developed for silicon, phosphorus, iron and aluminum determination in

biodiesel samples after a microwave digestion. This method allows to carry out four color-forming reac-

tions simultaneously: aluminum (III) and iron (III) with chromazurol S and phosphate and silicate ions

with ammonium molybdate, in two mixing chambers which are commutated into MCSWIA manifold.

The concentrations of analytes were determined by two data matrixes using CLS method. Each of exper-

imental calibration matrixes for CLS were designed with 10 calibration set solutions of 1–10 mg kg1 for

aluminum and 2–20 mg kg1 for iron, silicon and phosphorus. The appropriate experimental conditions

for microwave digestion and automated MCSWIA spectrophotometric determination of analytes were

investigated. Recovery tests show results between 98% and 107% for all type of biodiesel samples. The

limits of detection (LOD) obtained using the MCSWIA-CLS system were 0.3 mg kg

1

for aluminum and0.6 mg kg1 for iron, silicon and phosphorus. The proposed method was successfully applied for the

determination of analytes in different type of biodiesel. The analytical results obtained by MCSWIA-

CLS were compared with the results obtained by reference methods.

  2014 Elsevier Ltd. All rights reserved.

1. Introduction

Biodiesel is a relatively new source of renewable energy the

value of which has recently obtained increasing attention

http://dx.doi.org/10.1016/j.fuel.2014.06.059

0016-2361/  2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel./fax: +7 9291100987.

E-mail address: kristina-fulmes@mail.ru (C.S. Vakh).

Fuel 135 (2014) 198–204

Contents lists available at  ScienceDirect

Fuel

j o u r n a l h o m e p a g e :  w w w . e l s e v i e r . c o m / l o c a t e / f u e l

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worldwide [1]. Biodiesel is composed by the mixture of mono-alkyl

esters of long chain fatty acids, obtained through a transesterifica-

tion reaction of vegetable oils or animal fats with short-chain alco-

hols in the presence of catalysts, such as sodium hydroxide or

potassium [2]. Nevertheless, the final mixture of biodiesel produc-

tion process may contain different kind of contaminants which

have negative impacts for both motor and environment. During

the transesterification process, such undesired elements as silicon,phosphorus, iron and aluminum may also present in a biodiesel

product. The presence of these elements in biodiesel can have a

great damage the engines. Thus, during the combustion of biodie-

sel the organic compounds of silicon, iron and aluminum are trans-

formed into insoluble oxides, which can deposit in combustion

chamber. The raised content of phosphorus in biodiesel causes to

significant harmful influence to the environment and prevent the

operation of catalytic convectors [3].

For determination of silicon, phosphorus, iron and aluminum in

biodiesel the atomic absorption and atomic emission spectroscopy

have been proposed using the procedure described in   [4–6]  and

[7,8]  respectively. For determination of other metals (Cu and Pd)

in biodiesel the electrochemical methods have been proposed

[9,10]. For reducing of matrix influence the atomic absorption

and atomic emission procedures recommend dissolving the biodie-

sel sample in organic solvents, such as xylene, kerosene and other.

The use of these organic solvents requires special attention

because of their toxicity. The electrochemical methods require

the use of less toxic solvents such ethanol or isopropanol but they

have not been proposed for determination of silicon, phosphorus,

iron or aluminum Thus, though atomic absorption and atomic

emission described procedures are highly sensitive and relatively

simple, however, they involve some disadvantages, such as large

consumption of toxic organic solvents and high cost of the analysis.

Taking into account these disadvantages, more preferable for

determination of silicon, phosphorus, iron and aluminum are auto-

mated procedures which are based on principles of flow analysis

and allowed to work with small volumes, saving samples and

reagents and therefore contributing to a green analytical chemistry[11–15]. Despite the simplicity of flow injection analysis (FIA) the

automated flow procedures were developed only for phosphorus

determination in biodiesel. The most of these procedures are based

on the well-known molybdenum blue reaction. The fully auto-

mated flow injection procedure for phosphorus determination in

biodiesel [11]  includes the samples digestion by mixture of acids

in a heating block of FIA system. The procedure described in [14]

proposed to determined phosphorus in biodiesel using a micro-

flow-batch analyzer with solenoid micro-pumps. The samples in

this case were mineralized using an ashing procedure at 550  C fol-

lowed by dissolution of the residue.

This present work describes a new procedure for simultaneous

determination of silicon, phosphorus, iron and aluminum in bio-

diesel after a microwave digestion using recently suggested multi-commutated stepwise injection analysis  [16].

The MCSWIA is an automated system which integrates the con-

cepts of batch and fl0ow methods. The MCSWIA manifold consists

of several mixing chambers and programmed multi-commutated

valves, peristaltic pumps and detector. The mixing chambers are

often used for mixing of the sample with solutions of color-forming

reagents by gas bubbles. The number of mixing chambers is deter-

mined by the number of analytes and the number of corresponding

color-forming reactions, which are necessary for their determina-

tion. Previously MCSWIA has been successfully implemented for

determination of zinc (II), copper (II) and nickel (II) in wet aerosols

[14]. In this procedure three mixing chambers were used for per-

forming of three selective color-forming reactions. Nevertheless,

the MCSWIA offers a number of disadvantages such as the neces-sity to increase the number of mixing chambers for performing

of corresponding color-forming reactions for each new analyte.

This significantly increased the time of analysis, memory effects

and complicated the analysis. Moreover the number of mixing

chambers is limited by the number of ports of a valve.

These disadvantages can be overcome by coupling of MCSWIA

with classical list squares method (MCSWIA-CLS). The CLS method

has been successfully implemented for multicomponent flow-

injection   [17,18]   and sequential injection   [19]   analysis. In thepresent work, CLS method has been used for data analysis of simul-

taneously MCSWIA determination of four analytes. For this pur-

pose, MSWIA manifold includes two similar mixing chambers

where the color-forming reactions were performed: aluminum

(III) and iron (III) with chromazurol S and phosphate and silicate

ions with ammonium molybdate.

2. Experimental

 2.1. Reagents and solutions

All solutions were prepared from analytical reagent grade. Ultra

pure water from Millipore Milli-Q RG (Millipore, USA) was used for

preparation of solutions and dilution. The aqueous stock solutionsof 0.04 mmol L 1 silicon (IV) and phosphorus (V) were prepared by

diluting of 1 gL 1 standard solutions of SiO32 and PO4

3 (Merk,

Germany) with deionized water. To prepare the stock solutions

of organic forms of silicon and phosphorus (10 mg kg1) the appro-

priate amount of silicon oil and triphenylphosphine were dissolved

in the biodiesel. The solutions of ammonium molybdate

(0.2 mol L 1), oxalic acid (0.01 mol L  1), ascorbic acid

(0.05 mol L 1) and sulfuric acid (0.02 mol L 1) were prepared by

dissolving the corresponding amount in deionized water.

The stock aqueous solutions of 0.04 mmol L 1 of aluminum (III)

and iron (III) were prepared by dissolving of corresponding amount

of Al(NO3)3   and FeCl3  in 0.1 mol L 1 of HCl. To prepare the stock

solutions (10 mg kg1) of organic forms of iron (III) and aluminum

(III) in biodiesel 2-ethylhexanoates of Al and Fe (ABCR GmbH &

CO.KG) were dissolved in the biodiesel. The 0.2 mmol L 1 of the

chromazurol S solution was prepared immediately before the

experiment by mixing the solution of 0.4 mmol L 1 of color-form-

ing reagent and acetate buffer (pH 5) solution in a ratio of 1:1,

respectively. All chemicals of analytical reagent grade were used

without further purifications.

 2.2. Samples

Four different samples of biodiesel were obtained from different

sources: castor bean, sunflower, canola and corn. These samples

were previously prepared using alkaline catalyzer sodium hydrox-

ide (NaOH) [18,20].

 2.3. Sample preparation

The sample preparation was performed by microwave diges-

tion. Biodiesel sample (1 g) was placed into a digestion vessel.

Then, 5 mL of 10 mol L 1 HNO3  and 2 mL of 9 mol L 1 H2O2  were

added to this vessel. The sample was heated according to the opti-

mized program presented in Table 1. After cooling the solution was

 Table 1

Optimized program for microwave-assisted acid digestion of biodiesel samples.

Step Applied power (W) Time (min) Pressure (MPa)

1 400 5 0.5

2 400 5 1

3 400 25 15

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transferred to a 10 mL volumetric flask, made up to the pH 5.0 by

addition of ammonia solution (1:1) and diluted to 10 mL with

deionized water. The prepared solution was analyzed by

MCSWIA-CLS.

 2.4. Manifold and apparatus

The MCSWIA-CLS manifold (Fig. 1) includes: two solenoidvalves (Cole–Parmer, USA), two peristaltic pumps MasterFlex L/S

(Cole–Parmer, USA) ensuring a reverse flow (flow rate is from 0.5

to 5 mL/min), two identical thermostated mixing chambers (MC)

(cylindrical-shaped glass tubes with a funnel-shaped inlet at the

bottom, 20 mm in height and 10 mm in i.d.) and communication

tubes (PTFE, 0.5 mm in i.d.). It is equipped with a source of visible

light LS-1 and fiber-optic spectrometer USB 4000 (Ocean Optics,

USA) with a 50 mm path length flow cell FIA-Z-SMA-50-TEF (Fia-

lab, USA) and a single-channel membrane separator with a PTFE

membrane. Mathematical operations were performed by the pro-

gram Excel (Microsoft Office).

Acidity of the solution was measured by a pH meter pH-410

(Aquilon, Russia). MDS-6 microwave digestion system (Shandong,

China) was used for samples digestion.

ICPE-9000 Inductively Coupled Plasma Emission Spectrometer

(Shimadzu, Japan) was used for silicon determination by a refer-

ence method. AA-7000 Atomic Absorption Spectrophotometer

(Shimadzu, Japan) was used for phosphorus, aluminum and iron

determination by a reference method.

 2.5. The MCSWIA-CLS procedure

At the first stage of the measurement, sample portions of 0.1 mL 

each (a) were sequentially delivered to the thermostated under

60 C MC (1, 2) using valves (1) and (2). Then, for aluminum and

iron determination the 0.1 mL of 0.2 mmol L 1 chromazurol S solu-

tion (b) was delivered to the MC (1). To stir the reaction mixture, a

flow of air was passing through the port (d) at a rate of 5 mL min1

into the MC (1) for 30 s. To achieve the completeness of the color-

forming reaction the homogeneous solution was stay in the MC (1)

for 240 s.

At the second stage of measurement, while the colored com-

plexes were formed in MC (1), the components of reacting mixture

for determination of phosphorus and silicon were sequentially

delivered to the MC (2) through the port of valve (1) in following

order: 0.1 mL of 0.2 mol L 1 solution of ammonium molybdate

(e), 0.3 mL of 0.02 mol L 1 sulfuric acid ( f ) and 0.1 mL of  

0.01 mol L 1 oxalic acid solution ( g ). The reacting mixture was

stirred in the MC (2) by the flow of air ( d) for 60 s. After that the

0.1 mL of 0.05 mol L 1 ascorbic acid solution (h) was also delivered

to MC (2). Then, pause was kept to complete the color-forming

reactions for 120 s.

At the third stage of measurement, the reaction mixtures from

MC (1) and MC (2) using valve (1, 2) and peristaltic pump (2) were

sequentially delivered to the spectrophotometric detector where

the analytical signals were measured in the flow-stop conditionfor 20 s at the wavelength range from 510 to 570 nm (for alumi-

num and iron determination) and at the wavelength range from

820 to 910 nm (for phosphorus and silicon determination). Then,

the solutions from detector were discharged to waste. To avoid

the memory effect the manifold was washed out with deionized

water.

 2.6. Procedure for the determination silicon, phosphorus, iron and

aluminum in biodiesel by a reference method

For the determination of silicon   [7]   and phosphorus   [5]   in

biodiesel by a reference method the pre-treatment stage was per-

formed by dilution of the samples in ethanol or xylene and farther

analysis by microwave-induced plasma optical emission spectrom-

etry (MIP OES) and atomic absorption spectroscopy (AAS), respec-

tively. Aqueous solutions of silicon and phosphorus were used for

plotting the analytical curve.

The determination of iron and aluminum in biodiesel was per-

formed by high-resolution continuum source electrothermal

atomic absorption spectrometry [4]. Sample preparation includes

simply dilution the biodiesel samples by ethanol at room temper-

ature. Aqueous solutions of iron were used for plotting the analyt-

ical curve. For the calibration of aluminum the solutions were

prepared using ethanol.

3. Results and discussion

 3.1. Theoretical aspects

Color-forming reagent chromazurol S was chosen for simulta-

neous spectrophotometric determination of aluminum and iron

in biodiesel. This reagent forms stable intensively colored com-

plexes with aluminum (III) and iron (III) in acetate buffer medium

[21]. The absorbance spectra of color-forming reagent chromazurol

S and its complexes with aluminum (III) and iron (III), obtained in

MCSWIA manifold, are shown on Fig. 2. The absorbance spectra of 

iron (III) and aluminum (III) complexes with chromazurol S have

maxima at wavelengths 530 and 560 nm respectively and they

Fig. 1.   The MCSWIA manifold for determination of silicon, phosphorus, iron and aluminum in biodiesel.

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are strongly overlapped. So, the simultaneous spectrophotometric

determination of aluminum (III) and iron (III) by individual calibra-

tion curves is impossible.

The molybdenum blue reactions were chosen for determination

of silicon and phosphorus. The absorbance spectra of molybdo-

phosphoric and molybdosiliconheteropoly acids solutions mea-

sured in MSWA manifold (Fig. 2) are strongly overlapped. So, the

simultaneous spectrophotometric determination of silicon and

phosphorus by individual calibration curves is also impossible.

 3.2. The MCSWIA manifold

The MCSWIA manifold includes two similar eight-way solenoidvalves and two peristaltic single-channel pumps (Fig. 1). The first

valve is used for sequential injection of samples, reagent solutions

and a gas phase into the flow system. The gas phase is used for

mixing of solutions in the mixing chambers which are input to

the second valve.

To provide the effective mixing of solution in the MC, the cylin-

drical-shaped glass tube with a funnel-shaped inlet at the bottom

was preferred. The optimal volumes of samples and reagent solu-

tions were studied. A volume of 0.1 mL provides minimum con-

sumption of samples and reagent solutions as well as the

maximum reproducibility (RSD 0.25%). In order to prevent the mix-

ing of color-forming reactions at their consistent movement from

the MCs to the detector, these zones were segmented by gas bub-

bles, which were injected into the flow system for mixing of thesolutions in MCs. To eliminate the air bubbles from the flow

stream, the membrane separator was installed in front of the

detector.

 3.3. Optimization of chemical variables

 3.3.1. Optimization the MCSWIA spectrophotometric determination of 

iron (III) and aluminum (III)

To optimize the condition of MCSWIA spectrophotometric

determination of iron (III) and aluminum (III) the effect of concen-

tration of color-forming reagent has been studied. For this purpose,

a portion (0.1 mL) of 0.04 mmol L 1 aluminum (III) or iron (III)

solution, 0.1 mL of chromazurol S were delivered into the thermo-

static MC (1) (60C). The concentration of color-forming reagentwas varied from 0.05 to 0.5 mmol L 1. The reacting mixture was

stirred by the flow of air for 300 s, and then stopped for 300 s.

When the completeness of the coloring reaction was achieved

the reacting mixture was delivered to the spectrophotometric

detector. Absorbance of solution was measured in the stop-flow

conditions at 530 and 560 nm for iron (III) and aluminum (III) ions

respectively. It was found that the optimal concentration of color-

forming reagent for aluminum (III) and iron (III) determination is

0.08 mmol L 1 which ensures the formation of complexes in a ratio

of 2:1 (reagent:analyte). To provide the simultaneously determina-

tion of aluminum (III) and iron (III) with chromazurol S the concen-

tration of 0.2 mmol L 1 of color-forming reagent was chosen as

optimal.

Also the influence of temperature and time on the completeness

of color-forming reactions in MCSWIA was investigated. The tem-

perature in the MC (1) varied from 30  C to 60  C, as well as the

thermostating time in the MC (1) was varied from 1 to 10 min. It

was found that the completeness of complexes formation wereachieved by thermostating the reacting mixture during 4 min at

60 C for both aluminum and iron.

 3.3.2. Optimization the MCSWIA spectrophotometric determination of 

silicon and phosphorus

The molybdenum blue reactions are depended from the acidity

of medium and the concentration of molybdenum ions and oxalic

acid. To obtained the optimal acidity of MCSWIA-CLS spectropho-

tometric determination of silicon and phosphorus the effect of 

the concentration of sulfuric and oxalic acid has been studied.

For this purpose, 0.1 mL of 0.04 mmol L 1 phosphate ions or sili-

cate ions solution or water, 0.1 mL of 0.2 mol L 1 solution of 

molybdenum ions, 0.3 mL of sulfuric acid and 0.1 mL of solution

of oxalic acid were delivered into a MC (2). The concentration of sulfuric acid was varied from 0.005 to 0.05 mol L 1. The concentra-

tion of oxalic acid was varied from 0.005 to 0.05 mol L 1. The react-

ing mixture in the MC (2) was stirred by the flow of air for 60 s (f).

After that, the 0.1 mL of 0.05 mol L 1 of ascorbic acid solution was

delivered into the MC (2) and a pause was kept for 120 s for

Fig. 2.  Absorbance spectra of chromazurol S (a) and its complexes with aluminum

(III) (b), iron (III) (c) (CR  = 0.04 mmol L 1, C Al = 0.02 mmol L 1, C Fe = 0.02 mmol L 1);

the mixture of iron (III) and aluminum (III) (d) (C R  = 0.04 mmol L 1, CAl = 0.01 -

mmol L 1, CFe = 0.01 mmol L 1); phosphomolybdic (e), silicomolybdic (f) and

ammonium molybdic (g) heteropoly acids (CR  = 30 mmol L 1, CSi = 0.03 mmol L 1,

CP = 0.03 mmol L 1, pH = 2).

Fig. 3.   The RSD dependence on the number of set calibration solutions.

 Table 2

Concentrations of iron, aluminum, silicon and phosphorus in the calibration set

solutions (mg kg1) (matrix of concentrations C).

Solution 1 2 3 4 5 6 7 8 9 10

Al (III) 1 2 3 4 5 6 7 8 9 10

Fe (III) 20 18 16 14 12 10 8 6 4 2

Si (IV) 20 18 16 14 12 10 8 6 4 2

P (V) 2 4 6 8 10 12 14 16 18 20

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achieving the completeness of color-forming reactions. Then, the

colored solution was delivered from the MC (2) using the valve into

the spectrophotometric detector (5). The analytical signals were

measured in the stop-flow conditions at 882, 860 and 870 nm for

phosphate, silicate ions and water, respectively. It was found that

the optimal concentrations of sulfuric and oxalic acids for determi-

nation of phosphate and silicate ions are 0.02 mol L 1 and

0.01 mol L 1, respectively. These concentrations provide the lowest

absorbance of background solution and the greatest absorbance of 

phosphate and silicate ions solution.

 3.4. Multivariate calibration

For data analysis of silicon, phosphorus, iron and aluminum

determination the multivariate calibration CLS method have been

used. The procedures of multivariate calibration applied for deter-

mination of all analytes were similar. The 10 calibration set solu-

tions with different ratios of the concentrations of Al (III) and Fe

(III) or phosphate and silicate ions were prepared. According to

[22]   number of calibration set solutions must not be less than

the number of determined components. In order to find the opti-

mal number of calibration set solutions, RSD dependence on the

number of set calibration solutions (Fig. 3) has been studied. The

optimum number of calibration set solutions is 10.

To design a matrixes of concentrations (C-matrix) the concen-

trations of analytes are include into   Table 2. The calibration set

solutions were analyzed according to MCSWIA-CLS procedureincluding the stage of digestion. For determination of Al (III) and

Fe (III) following wavelengths were chosen as working: 510, 520,

530, 540, 550, 560 and 570 nm. For phosphate and silicate determi-

nation the chosen working wavelengths were 820, 830, 840, 850,

860, 870, 880, 890 900 and 910 nm.

The working wavelengths were chosen by the following crite-

ria: the number of wavelengths must be more than the amount

of analytes; the value of the absorbance at the chosen wavelength

should not be less than ½ the absorbance at the maximum of the

absorption spectra; with each selected wavelength linear depen-

dence of the absorption on the concentration of the analytes, must

be observed [22].

Absorbance, which were measured at the selected wavelengths

were placed into Table 3 with purpose to receive a matrixes A. The

experimental calibration matrixes (P -matrixes) (Table 4) were cal-

culated according to the formula   P  = CAT ( AAT )1. These matrixes

were used for the analysis of model and real samples. Computa-

tions of the P -matrixes were performed by Microsoft Excel.

The  P -matrixes were checked by MCSWIA determination of Al(III), Fe (III), phosphate and silicate ions in test samples. The absor-

bance of complexes of Al (III) and Fe (III) with chromazurol S and

molybdenum blue complexes in the test samples were placed in

table A*, and the concentrations of aluminum (III), iron (III), phos-

phate and silicate ions were found by the formula   C  = PA*. The

results are presented in Table 5.

 3.5. Interference study

The effect of potentially interfering ions on the determination of 

silicon, phosphorus, iron and aluminum in biodiesel was investi-

gated. It was performed by addition of known concentration of 

each ion to fixed concentrations of analytes. The tolerable concen-

tration of each taken foreign species is considered to be less than5% of relative error in the signal.

The influence of some inorganic compounds (Mn2+, Pb2+, Cd2+,

Zn2+, Co2+, Ca2+, Mg2+, Ni2+ and Cu2+ and organic compounds

(methanol, acetone, ethanol and glycerol) were studied. It was

investigated that interferences were not found for Cu2+ at 20-fold

 Table 3

Absorbance matrixes A.

k  (nm) Solution

1 2 3 4 5 6 7 8 9 10

 Absorbance of calibration solutions for aluminum and iron determination

510 0.706 0.697 0.678 0.697 0.712 0.577 0.656 0.646 0.593 0.546

520 0.78 0.779 0.771 0.802 0.824 0.687 0.788 0.78 0.749 0.704

530 0.833 0.842 0.85 0.898 0.936 0.8 0.938 0.94 0.946 0.919

540 0.849 0.879 0.915 0.984 1.043 0.927 1.096 1.108 1.169 1.162

550 0.86 0.892 0.931 1.003 1.06 0.938 1.106 1.106 1.166 1.138

560 0.835 0.848 0.845 0.885 0.879 0.721 0.798 0.731 0.725 0.599

570 0.742 0.729 0.698 0.718 0.689 0.554 0.605 0.544 0.534 0.44

 Absorbance of calibration solutions for silicon and phosphorus determination

820 0.388 0.440 0.532 0.284 0.320 0.488 0.484 0.422 0.440 0.409

830 0.432 0.492 0.588 0.320 0.356 0.532 0.524 0.465 0.472 0.453

840 0.464 0.532 0.636 0.352 0.388 0.568 0.552 0.499 0.500 0.490

850 0.488 0.564 0.672 0.372 0.416 0.588 0.576 0.524 0.516 0.517

860 0.496 0.576 0.688 0.384 0.424 0.596 0.592 0.535 0.520 0.527

870 0.496 0.576 0.684 0.384 0.424 0.592 0.596 0.534 0.516 0.526

880 0.484 0.560 0.668 0.376 0.412 0.580 0.592 0.522 0.504 0.513

890 0.464 0.536 0.644 0.360 0.396 0.564 0.576 0.504 0.488 0.494

900 0.440 0.508 0.608 0.340 0.372 0.548 0.556 0.480 0.464 0.469

910 0.412 0.476 0.572 0.320 0.348 0.528 0.532 0.454 0.444 0.443

 Table 4

Experimental calibration matrixes (P -matrixes).

Experimental calibration matrix (P-matrix) for aluminum and iron determination

155.91 313.45   199.13 86.27   14.47   1.42   35.22

7.53 291.82   425.29 96.58 74.96   107.59 114.11

Experimental calibration matrix (P-matrix) for silicon and phosphorus determination

106.00   1088.00 1213.00 498.00   3336.00 3104.00   1304.00 2288.00   3647.00 2231.14

1323.00 1385.00 31.00 886.00   994.00   1364.00 1328.00 308.00   53.00   270.47

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(at high copper content it is necessary to use thiourea), Mn2+, Pb2+,

Cd2+, Zn2+, Co  2+, Ni2+ at 100-fold excess and for Ca2+, Mg2+ at 2000-

fold excess for determination of aluminum (III) iron (III). In case of 

silicon and phosphorus determination the Mn2+, Pb2+, Cd2+, Zn2+,

Co2+, Ni2+, Ca2+, Mg2+ at 1000-fold excess have no interference to

the reaction.

Methanol, acetone, ethanol and glycerol do not interfere, as

their interfering effect is eliminated during sample preparation.

 3.6. Analytical performance

The limits of detection (LOD) obtained using the MCSWIA-CLS

system were 0.3 mg kg1 for aluminum and 0.6 mg kg1 for iron,

silicon and phosphorus (Table 6). The experimental calibration

matrixes for CLS were designed with 10 calibration set solutions

of 1–10 mg kg1 for aluminum and 2–20 mg kg1 for iron, silicon

and phosphorus. The weight of sample was 1 g.

 3.7. Application

The developed automated procedure was applied for determi-

nation of silicon, phosphorus, iron and aluminum in biodiesel sam-

ples (castor bean, sunflower, canola and corn). Samples were

digested by microwave and then analyzed by MCSWIA-CLS. The

correctness of the proposed method was carried out by the com-

parison with the reference methods. Analytical results measured

by MCSWIA and reference methods for each analyte are presented

in Table 7. The recoveries obtained for each of the samples was

within of 98–107% range which is considered acceptable for this

type of samples. The RSD does not exceed 10% for each element.

Analytical results measured for the samples by MCSWIA and refer-

ence methods were compared using the  F -test.  F -values  68.2 are

obtained indicating insignificant difference in precision between

both methods at the 95% confidence level (F -critical = 8.8).

 Table 5

Added and found concentrations of Fe, Al, Si, and P in model samples (n = 5,  P  = 0.95).

Analyte Added, mg kg1 Found, mg kg1 RSD (%)

Fe 5 5.1 ± 0.1 2

6 6.1 ± 0.2 2

2 2.1 ± 0.1 5

3 2.9 ± 0.1 3

1 1.0 ± 0.1 1

Al 1 1.1 ± 0.1 10

3 2.9 ± 0.2 4

8 8.1 ± 0.2 1

4 4.1 ± 0.1 2

7 7.2 ± 0.3 3

Si 4 4.1 ± 0.1 2

7 7.0 ± 0.1 1

1 0.9 ± 0.1 10

5 4.8 ± 0.2 4

9 9.2 ± 0.2 2

P 3 3.1 ± 0.1 3

9 9.1 ± 0.1 1

2 2.0 ± 0.2 1

8 8.2 ± 0.1 2

4 4.1 ± 0.1 2

 Table 6

Analytical performance data.

Parameter Fe Al S i P

pH 5 5 2 2

Sample weight (g) 1

Time of MCSWIA analysis (min) 4

Linear range (mg kg1) 2–20 1–10 2–20 2–20

Detection limit (mg kg1) 0.6 0.3 0.6 0.6

Quantification limit (mg kg1) 2 1 2 2

RSD (%) (n = 10) 5 10 10 3

S ample freque nc y ( pe rhour) 6

 Table 7

The results of the determination of iron, aluminum, silicon and phosphorus in biodiesel samples (n = 10,  P  = 0.95).

Sample Analyte (mg kg1) Ad de d (mg kg1) Found by MCSWIA-CLS (mg kg1) Found by reference method (mg kg1) Recovery (%)   F -value

Sunflower Al 2.0 4.2 ± 0.4 4.1 ± 0.3 105 1.9

3.0 5.1 ± 0.5 5.3 ± 0.3 102 2.5

Fe 2.0 3.2 ± 0.2 3.1 ± 0.2 107 1.1

3.0 3.9 ± 0.3 4.1 ± 0.1 98 8.1

Si 2.0 3.2 ± 0.2 3.1 ± 0.2 106 1.1

3.0 4.1 ± 0.3 4.3 ± 0.1 103 8.2

P 2.0 10.2 ± 0.5 10.1 ± 0.2 102 6.4

3.0 11.0 ± 0.3 11.3 ± 0.4 100 2.0

Canola Al 2.0 5.2 ± 0.5 5.1 ± 0.5 104 1.1

3.0 5.9 ± 0.6 6.1 ± 0.4 98 2.1

Fe 2.0 3.1 ± 0.2 3.1 ± 0.2 103 1.0

3.0 4.0 ± 0.2 4.1 ± 0.1 100 3.8

Si 2.0 4.2 ± 0.2 4.1 ± 0.2 106 1.1

3.0 5.1 ± 0.3 5.2 ± 0.2 102 2.2

P 2.0 12.3 ± 0.5 12.1 ± 0.2 103 6.53.0 12.9 ± 0.3 13.0 ± 0.3 99 1.1

Corn Al 2.0 4.0 ± 0.4 4.1 ± 0.5 100 1.6

3.0 5.2 ± 0.5 5.1 ± 0.6 104 1.4

Fe 2.0 3.2 ± 0.2 3.1 ± 0.2 106 1.1

3.0 3.9 ± 0.3 4.1 ± 0.1 98 8.1

Si 2.0 3.0 ± 0.2 3.1 ± 0.2 100 1.0

3.0 4.1 ± 0.3 4.3 ± 0.1 103 8.2

P 2.0 13.2 ± 0.5 13.1 ± 0.2 102 6.3

3.0 14.0 ± 0.3 14.3 ± 0.1 100 2.2

Castor bean Al 2.0 5.0 ± 0.5 5.1 ± 0.5 100 1.0

3.0 6.1 ± 0.5 6.2 ± 0.5 102 1.0

Fe 2.0 5.2 ± 0.2 5.1 ± 0.2 106 1.0

3.0 6.0 ± 0.3 6.1 ± 0.2 100 2.2

Si 2.0 4.0 ± 0.2 4.1 ± 0.2 100 1.1

3.0 5.2 ± 0.3 5.3 ± 0.2 104 2.2

P 2.0 13.1 ± 0.5 13.1 ± 0.2 101 6.3

3.0 13.9 ± 0.3 14.3 ± 0.2 99 2.2

C.S. Vakh et al. / Fuel 135 (2014) 198–204   203

8/10/2019 multicommutated stepwise injection analysis of biodiesel with classical least squares method

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4. Conclusions

The MCSWIA-CLS provides the new possibility for multicompo-

nent analysis. The MCSWIA-CLS allows to achieve the completeness

of color-forming reaction for multivariate spectrophotometric

determination of several analytes. The CLS method was chosen for

multivariate calibration because of its simple mathematical tools

(EXCEL program is enough) and less time spent for calibration.Unlike FIA and SIA methods, MCSWIA-CLS simplifies greatly the cal-

ibration procedure, because it is not necessary to control the move-

ment of the sample in the carrier stream. Moreover, because of the

absence of dispersion of colored complex in the carrier stream

while its delivering to detector it is possible to receive the high sen-

sitivity of analysis.The MCSWIA-CLS determination of silicon, phos-

phorus, iron and aluminum in biodiesel is simple, fast and flexible.

It uses small quantities of samples and reagents, therefore generat-

ing less waste and especially can be provided at a lower cost.

 Acknowledgements

This work was supported by the Russian Foundation for Basic

Research (Project No. 13-03-00031-a). Scientific researches wereperformed at the Center for chemical analysis and materials

research of St. Petersburg State University.

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