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1

Rapid and sensitive determination of fluoride in toothpaste and water

samples using headspace single drop microextraction - gas chromatography

Massoud Kaykhaii and Maryam Hosseini Ghalehno

Department of Chemistry, Faculty of Sciences, University of Sistan and Baluchestan,

Zahedan 98135-674, Iran.

Phone: +98-541-2446413 // FAX: +98-541-2446888 // E-mail: [email protected]

Author for correspondence

Page 1 of 22 Analytical Methods

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View Article OnlineDOI: 10.1039/C3AY41004H


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Abstract

A fast and reliable method was developed for the determination of fluoride in toothpaste

based on the headspace single drop microextraction (HS-SDME) followed by gas

chromatography/flame ionization detection (GC/FID). The method is based on the

volatilization of fluoride using trimethylchlorosilane as the derivatization reagent to form

trimethylfluorosilane at acidic pH. The trimethylfluorosilane formed is preconcentrated at a

0.8 µL drop of mesitylene suspended from the tip of a common GC microsyringe to the

headspace of the sample. Parameters such as nature of extraction solvent, extraction time,

size of microdrop, sample volume, stirring rate, derivatization reaction time and pH of

sample solution were studied and optimized. The developed protocol was found to yield a

linear calibration curve in the concentration range from 5.0 - 39.0 mg.L−1 with a limit of

detection of 4.4 µg.L−1 with a good enrichment factor of 58.8 for the analyte. The

repeatability of the method was satisfactory (RSD≤ 5.41%). Total analysis time including

microextraction and gas chromatography analysis was less than 30 min. because

preconcentration and sampling is performed from headspace of the sample, this method is

suitable for the determination of trace amounts of fluoride in toothpastes and water

samples.

Page 2 of 22Analytical Methods

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Introduction

Fluoride is used in toothpaste formulations as caries preventive agents and in most

countries total fluoride concentration in toothpaste regulated to be between 0.50 to 1.50

mg.g−1 [1]. Therefore, its fast and accurate quantitative determination is important for

quality control and stability evaluation of these products [2]. Several techniques have been

used to analyze the water soluble fluoride species in toothpaste, of which, fluoride ion-

selective electrodes [3] and ion chromatography [4] are usually the first choice. Flow-

injection analysis [5], high performance liquid chromatography (HPLC) [6], gas

chromatography (GC) [7, 8], spectroscopic [9, 10] and electrochemical [11, 12] methods are

also widely used for F− determination. However, Fluorine needs to be separated from

complicated toothpaste samples before element determination. This can be done normally

by converting fluoride ions to a volatile derivative, such as trimethylfluorsilane (TMFS),

which can be subsequently extracted with an organic solvent and determined quantitatively

by gas chromatography. Wejnerowska et al [7] applied this reaction to volatilized F- before

its extraction by solid phase microextraction.

Although liquid–liquid extraction is efficient and precise and is probably the most widely

used sample extraction procedure for this purpose, it clearly has disadvantages such as high

consumption of time and solvents as well as its tedious application. Moreover, it is

hazardous to human health (as they use organic solvents) and extremely expensive with

respect to the disposal of solvents.

In the last few years, efforts have been directed toward miniaturizing the liquid–liquid

extraction procedure by greatly reducing the solvent to aqueous phase ratio, leading to the

development of solvent microextraction methodologies. One of the modes of this so called

“micro” extraction in which the extraction medium is in the form of a single drop is termed


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single drop microextraction (SDME). The technique is based on the distribution of analytes

between a microdrop of extraction solvent (usually few micro liters) at the tip of a

microsyringe needle and a liquid sample containing the analytes. After extraction, the

microdrop is retracted back into the microsyringe and injected into chromatographic or

electrophoretic systems for further analysis [13]. To the best of our knowledge, this method

has not been used for the determination of fluoride by GC.

In this paper, Headspace SDME extraction mode was applied for the determination of

fluoride in toothpaste after its volatilization using trimethylchlorosilane (TMCS) as the

derivatization reagent. In order fluoride ion become volatilized, TMCS was added to the

sample solution in which TMCS is converted to trimethylsilanol by hydrolysis. Under acidic

conditions, trimethylsilanol reacts further with free fluoride ion to form the volatile TMFS

with a boiling point of 16.4 oC [14].

Materials and Methods

Instrument

A Varian Star 3400 Cx gas chromatograph (Varian Inc., USA) equipped with a flame

ionization detector was used for all analyses. The GC was fitted with a PETROCOLTM DH

capillary column (50m × 0.53mm × 0.50μm). The gas chromatography conditions were as

follows: (1) the injector port was operated in split mode with a split ratio of 10:1 and it was

kept at 200 °C; (2) detector temperature 250 °C; (3) initial oven temperature 80 °C for 1 min,

and increased to 120 °C at 10 °C min−1 then raised to 220°C at 15°C min−1, stayed for 2

minutes at this final temperature; (4) usage of high-purity nitrogen as a carrier gas (1.2 ml

min−1). Hydrogen and air were used as detector gases at 40 and 400 ml min−1, respectively.

The ion chromatograph used was consisted of an IC 25 pump (Dionex, USA), equipped with


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an AS 50 chromatography compartment. An IONPAC® AS20 Dionex column was employed as

the analytical column.

Reagents

All reagents were of analytical grade and were purchased from Merck (Germany) and used

as received. Stock solution of sodium fluoride (500 mg.L-1 of F-) was prepared by dissolving

1.1162 g of the compound in triply deionized water. Reagent was dried in an oven (100 oC)

for 2.5 hr before dissolution. By dilution of this stock solution, working solutions were

prepared daily and stored in a polyethylene bottle at refrigerator.

HS-SDME procedure

The HS-SDME device is illustrated in one of our previous works [13]. A 40 mL vial with 2.0 mL

stock solution, 20 mL of deionized water, 1 mL of diluted hydrochloric acid and a stir bar

were placed on a magnetic stirrer. While this cocktail was stirred, 2 mL of TMCS was added.

After 20 min, SDME was performed with a commercially available 10 µL GC microsyringe.

The microsyringe was fixed above the extraction vial with a clamp. After the needle passed

through the septum, the needle tip was immersed into the sample solution and kept at the

same height in order to obtain a good reproduction. Then 0.8 µL extracting organic solvent

was extruded out of the needle to produce a microdrop at the needle tip. During the

extraction, the solution was stirred at a constant rate. After extracting for a prescribed

period of time, the solvent drop is retracted into the microsyringe, which was removed from

the sample vial. The extraction solvent with the extracted analytes was injected into the GC

for fluorine determination. Calibration was performed against different concentration of


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fluoride prepared in the same way. A blank submitted to the same procedure described

above was measured parallel to the samples and calibration solutions.

Results / Discussion

Selection of extraction solvent

The selection of an appropriate extraction solvent is a major challenge for the optimization

of the HS-SDME process. Beside the fact that the solvent must have a great extraction

capability, there are two other factors which should be considered: first it should have

excellent chromatographic behavior, i.e. solvent peaks must not mask the analyte peak, and

second, the extracting solvent should has a very low vapor pressure which results in minimal

loss of the microdrop during the extraction time [15]. On the basis of these considerations,

aniline, 1,4-dioxane, tetrahydrofuran, dimethylformamide, butyl acetate, isoamyl alcohol,

benzyl alcohol, 1-octanol, anisole, n-decane, un decane, n-dodecane, n-hexadecane,

propylene benzene, tert-butyl toluene, ethyl benzene, ethyl toluene, xylene, and

mesithylene were used as dissolving, extracting phase for the samples under analysis. The

results are given in Fig. 1. As is evident from the Figure, higher extraction efficiencies were

achieved with mesithylene. Accordingly, this compound was selected as the dissolving

solvent in this study.

Effect of reaction time

The extraction of the fluoride into the organic drop was carried out after 15.0, 17.5, 20.0, 25.0

and 30.0 min of starting the derivitization reaction. The amount of fluoride by HS-LPME increased

with increasing exposure time up to 20 min and after that it almost remains constant. Hence the

sampling was performed 20 min after reaction time was started.


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Headspace volume

In order to further optimize the procedure, the effect of the aqueous sample and headspace

volume was investigated. The experiment was performed using 40 mL vials and the volume

of aqueous sample was increased from 1.0 to 8.0 mL. Fluoride working solutions with 0.3

g.mL−1 concentration was analyzed in duplicate with mesithylene extraction. The extraction

time was 20 min at 25 oC and stirring rate was 200 rpm. The relative peak areas obtained for

the analyte with different headspace volume are shown in Fig. 2. As can be seen, with

decrease of headspace volume, the relative peak area of the analyte increases for the first

2.5 millilitres and then constantly decreases. Therefore this water volume was selected

since this quantity provided acceptable results.

Volume of extracting micro-drop

The volume of extraction solvent has great affection on the extraction efficiency. The effects

of drop size on the extraction of derivitised fluorine were examined in the range of 0.2–0.8

µL while the total volume of aqueous fluorinated sample was kept at 32 mL.

Chromatography peak areas were increased with increasing the volume of solvent in this

range. It was not possible to increase the initial drop volume more than 0.8 µL in the tip of

the needle, because it is detached, especially in higher agitation rates. Hence, extraction

solvent volume of 0.8 µL was selected for subsequent experiments.

Extraction time

The extraction time profiles were examined by varying the exposure time of the extracting

microdrop in the headspace of the sample solution in the range of 1–15 min. Fig. 3 shows


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the response of the analytical instrument as a function of time. On the basis of the curve

obtained, for fluoride, the best extraction time is at about 4 min and with further increase in

the extraction time, the signal decreases. Therefore, 4 min was chosen throughout the

study. Long extraction time may result in evaporation of mesithylene drop and consequently

lead to poor sensitivity and precision.

Effect of sample temperature

The effect of temperature on the extraction efficiency is tested from 5 to 40 oC. As shown in

Fig. 4, the extracted amount of the analyte increase until the temperature is up to 30 oC and

then decrease. This can be ascribed to the double-faced effect of temperature on extraction

efficiency. Under the turning temperature, increase in temperature is favourable to the

evaporating of target compound and establishing of extraction equilibrium, resulting in the

increase in extraction efficiency. However, when the extraction temperature is beyond the

turning point, the decrease in distribution constant dominates, thus the extraction efficiency

decreases. Accordingly, in this work the extraction is performed at room temperature

(25oC).

Effect of volume of derivatizing reagent

The range of the TMCS volume was investigated in this study was between 5 and 35 µL. The

amount of extraction by HS-LPME increased with increasing of volume of TMCS from 5 to 30

µL and after that it most remains constant. Hence the optimum volume of derivatizing

reagent was 30 µL.


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Effect of pH

The solution pH is an important factor affecting the volatilization of the fluoride ion as well

as its extraction efficiency and recovery. As we mentioned earlier, in order to volatalize the

fluoride as TMFS, it is necessary to acidify the sample solution. Here, the effect of acidity on

the extraction efficiency is studied by changing the pH from 0.2 to 4.5 (Fig. 5). As can be

seen, around pH 0.4 the extraction efficiency achieves a maximum, and therefore, this pH is

considered the suitable extraction acidity.

Effect of sample stirring

Sampling stirring can increase extraction and reduces extraction time because the equilibrium

between the aqueous and vapor phases can be established more rapidly. Furthermore,

convection is induced in the headspace by the stirring of the aqueous phase [13, 16]. To evaluate

the effect of sample stirring, sets of stirring rates between 0 (static) and 1000 rpm were

considered. The peak area for the analyte increased up to 500 rpm, and then decreased slightly.

Therefore, a stirring speed of 500 rpm was used for subsequent experiments.

Effect of ionic strength

Salting-out effect is widely applied in traditional liquid-liquid extraction, because it makes

the solubility of targets in aqueous phase decrease, thus more analytes enter into the

extracting phase. Here, the effect of salt on the extraction efficiency is studied by adding

different amounts of potassium chloride (KCl) ranging from 0 to 0.36 g.mL-1 to investigate

the effect of ionic strength on the extraction efficiency. It was found that the peak area of

fluoride decreased as potassium chloride concentration increased. Therefore, salt addition

was not used in further experiments.


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Linear range, limits of detection and precision

Under the optimized conditions, the linearity of the HS-SDME method was examined by

extracting the aqueous fluoride samples. The calibration curves were linear for ranging from

5 to 39 mg.L−1. The calibration equation is y = 25.767x + 33.021 with a correlation coefficient

of 0.997, where y is the peak area of TMFS in the chromatogram, and C is its concentration

of fluoride in the sample solution (mg.L-1). The limit of detection and quantification, of the

HS-SDME method defined as 3Sb/m and 10Sb/m (where Sb is the standard deviation of the

blank and m is the slope of the calibration graph), were 4.4 and 15.0 μg.L-1, respectively. The

relative standard deviation (RSD) for ten replicate measurements of 10.0 and 30.0 mg.L-1 of

fluoride were 4.26% and 5.41%, respectively. Triplicate injections were performed. An

enrichment factor of 58.8 was obtained when GC analysis of a standard 10 mg.L−1 solution

was performed by proposed method and compared to direct injection of 1.0 µL of the

headspace of it.

Table 1 compares the characteristic data of the present method with those using gas

chromatography for fluoride determination, reported in literature.

Determination of fluoride in real samples

In order to evaluate performance of the developed method, at first extraction and

determination of fluoride in tap water was tested out. Both external calibration and

standard addition protocols were used. To be sure of the accuracy of the results, an ion

chromatographic (IC) analysis was also performed according to EPA 300.1 Part A standard

method and results were compared to what was obtained from the proposed method.

Student’s t-test (error of the first kind α = 0.1) showed that there was no significant


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difference between the results obtained from the methods. Fluoride content in tap water

calculated to be 420.0 (±0.4) μg.L-1.

In order to study the wider applications of the proposed headspace SDME system, the

fluoride concentration in a toothpaste sample was tested and a sample chromatogram is

depicted in Fig. 6. Eight hundred milligrams of accurately weighed toothpaste, 30 mL of

deionized water and 1 mL of hydrochloric acid were introduced into a 40 mL vial and mixed

for few seconds. To be sure of dissolution of soluble matrix constituents, the sample was

placed in an ultrasonic bath for about 5 min. Then, without filtration, 30 µL of TMCS was

added and after 20 min, the fluorine content determined by HS-SDME/GC method. Results

compared favorably with a standard ion chromatographic procedure [19]. The fluoride

content in the toothpaste sample was determined from the standard curve and it was found

to be 0.143 wt %. Traditional IC method showed the fluoride content of 0.144 wt % and

manufacturer, reported Fluoride content in toothpaste as 0.15%. These results are

satisfactory along with an average relative standard deviation of 0.13%.

Conclusion

The results of this study show that proposed HS-SDME coupled to gas chromatography is a

simple and rapid extraction technique with good enrichment factor and low detection limit

to determine the fluoride in various, complicated matrices such as toothpaste which does

not require any significant sample preparation and can be easily automated. The total

analysis time is less than half an hour. Using headspace SDME system has the benefits of

high sensitivity, fast, easy, little organic solvent consumption, low cost, and has broad

prospects. The method compares favorably with a reference IC method.


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Notes and references

1- I. C. Guimarães, C. C. Rezende, J. A. Fracassi da-Silva and D. Pereira-de Jesus, Talanta,

2009, 78, 1436.

2- P. Wang, S.F.Y. Li and H.K. Lee, J. Chromatogr. A, 1997, 765, 353.

3- B. Zhang, M. Wang and L, Wang, Anal. Lett., 2012, 45, 2455.

4- J. J. Potter, A. E. Hilliker and G. J. Breen, J. Chromatogr., 1986, 361, 423.

5- R. B. R. Mesquita, I. C. Santos, M. F. F. Pedrosa, A. F. Duque, P. M. L. Castro and A. O. S. S.

Rangel, Talanta, 2011, 84, 1291.

6- J. Musijowski, B. Szostek, M. Koc and M.Trojanowicz, J. Sep. Sci., 2010, 33, 2636.

7- G. Wejnerowska, A. Karczmarek and J. Gaca, J. Chromatogr. A, 2007, 1150, 173–177.

8- X. Zhang and R. H. Gongye, China Surfact. Det. Cosmet., 2010, 40, 311.

9- X. Gao, H. Zheng, G. Q. Shang and J. G. Xu, Talanta, 2007, 73, 770.

10- O. Sha, W. Ma, M. Lu, M. Sun, G. Xu, and R. H. Gongye, China Surfact. Det. Cosmet.,

2011, 41, 76.

11- J. R. Santos, R. A. S. Lapa and J. L. F. C. Lima, Anal. Chim. Acta, 2007, 583, 429.

12- M. Cernanska, P. Tomcik, Z. Janosikova, M. Rievaj and D. Miroslav, Talanta, 2010, 83,

1472.

13- M. Kaykhaii, S. Nazari and M. Chamsaz, Talanta, 2005, 65, 223.

14- M. Yuwono and S. Ebel, Arch. Phar., 1997, 330, 348.

15- M. Kaykhaii and M. Rahmani, J. Sep. Sci., 2007, 30, 573.

16- A. Krishna and K. Verma, Anal. Chim. Acta, 2011, 706, 37.

17- E. Pagliano, J. Meija, J. Ding, R. E. Sturgeon, A. D’Ulivo and Z. Mester, Anal. Chem., 2013,

85, 877.

18- A. M. Bouygues-de Ferran, J. Chromatogr. A, 1991, 585, 289.


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19- S. Burton and J. Erickson, Conc. Coll. J. Anal. Chem., 2012, 3, 13.


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Captions to Figurers and table

Fig. 1. Effect of sample solvent on extraction. Extraction conditions: extraction temperature,

ambient temperature (25 oC); sample volume, 31 mL; stirring speed, 200 min−1; extraction

time, 3 min; volume of derivatising agent, 25 µL; derivitization reaction time, 20 min; HCl

(conc.), 0.8 mL; solvent drop volume, 0.5 µL.

Fig. 2. Effect of sample headspace volume on the extraction efficiency. Extraction

conditions: extraction solvent, mesithylene; extraction temperature, ambient temperature

(25 oC); stirring speed, 200 min−1; extraction time, 3 min; volume of derivatising agent, 25

µL; derivitization reaction time, 20 min; HCl (conc.), 0.8 mL; solvent drop volume, 0.5 µL.

Fig. 3. Effect of extraction time on the SDME extraction. Extraction conditions: extraction

solvent, mesithylene; extraction temperature, ambient temperature (25 oC); sample

volume, 32 mL; stirring speed, 200 min−1; volume of derivatising agent, 25 µL; derivitization

reaction time, 20 min; HCl (conc.), 0.8 mL; solvent drop volume, 0.8 µL.

Fig. 4. Influence of sample temperature on extraction. Extraction conditions: extraction

solvent, mesithylene; sample volume, 32 mL; stirring speed, 200 min−1; extraction time, 4

min; volume of derivatising agent, 25 µL; derivitization reaction time, 20 min; HCl (conc.),

1.0 mL; solvent drop volume, 0.8 µL.

Fig. 5. The effect of pH of extracting phase (adjusted by addition of proper volume of

concentrated HCl) on SDME extraction. Extraction conditions: extraction solvent,

mesithylene; extraction temperature, ambient temperature (25 oC); sample volume, 32 mL;


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stirring speed, 200 min−1; extraction time, 4 min; volume of derivatising agent, 30 µL;

derivitization reaction time, 20 min; solvent drop volume, 0.8 µL.

Fig 6. GC chromatogram obtained from HS-SDME extraction. Extraction conditions:

extraction solvent, mesithylene; extraction temperature, ambient temperature (25 oC);

sample volume, 32 mL; stirring speed, 500 min−1; extraction time, 4 min; volume of

derivatising agent, 30 µL; derivitization reaction time, 20 min; HCl (conc.), 1.0 mL; solvent

drop volume, 0.8 µL.

Table 1. Comparison of the published methods using gas chromatography for fluoride

determination with the proposed method in this work.


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Fig. 1. Effect of sample solvent on extraction. Extraction conditions: extraction temperature,

ambient temperature (25 oC); sample volume, 31 mL; stirring speed, 200 min−1; extraction

time, 3 min; volume of derivatising agent, 25 µL; derivitization reaction time, 20 min; HCl

(conc.), 0.8 mL; solvent drop volume, 0.5 µL.

0

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7 R

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Solvent


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Fig. 2. Effect of sample headspace volume on the extraction efficiency. Extraction

conditions: extraction solvent, mesithylene; extraction temperature, ambient temperature

(25 oC); stirring speed, 200 min−1; extraction time, 3 min; volume of derivatising agent, 25

µL; derivitization reaction time, 20 min; HCl (conc.), 0.8 mL; solvent drop volume, 0.5 µL.

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32 33 34 35 36 37 38 39 40

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Headspace Volume (mL)


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Fig. 3. Effect of extraction time on the RP-DISDME extraction. Extraction conditions:

extraction solvent, mesithylene; extraction temperature, ambient temperature (25 oC);

sample volume, 32 mL; stirring speed, 200 min−1; volume of derivatising agent, 25 µL;

derivitization reaction time, 20 min; HCl (conc.), 0.8 mL; solvent drop volume, 0.8 µL.

4

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14

0 2 4 6 8 10 12 14

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Extraction Time (min)


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Fig. 4. Influence of sample temperature on extraction. Extraction conditions: extraction

solvent, mesithylene; sample volume, 32 mL; stirring speed, 200 min−1; extraction time, 4



0

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5 10 15 20 25 30 35 40

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Temperature (°C)


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Fig. 5. The effect of pH of extracting phase (adjusted by addition of proper volume of

concentrated HCl) on SDME extraction. Extraction conditions: extraction solvent,

mesithylene; extraction temperature, ambient temperature (25 oC); sample volume, 32 mL;

stirring speed, 200 min−1; extraction time, 4 min; volume of derivatising agent, 30 µL;

derivitization reaction time, 20 min; solvent drop volume, 0.8 µL.

6

8

10

12

14

16

0.2 0.6 1.0 1.4 1.8 2.2 2.6 3.0 3.4 3.8 4.2 4.6

pH

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Fig. 6. GC chromatogram obtained from HS-SDME extraction of toothpaste sample.

Extraction conditions: extraction solvent, mesithylene; extraction temperature, ambient

temperature (25 oC); sample volume, 32 mL; stirring speed, 500 min−1; extraction time, 4




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Table 1. Comparison of the published methods using gas chromatography for fluoride

determination with the proposed method in this work

Matrix LOD Dynamic linear range RSD (%) Reference

tap water, seawater, urine 3.2 µg.L-1

up to 50 mg.L-1

not mentioned [17]

calcium ascorbate not mentioned 0.25 to 10.0 mg.L-1

7 [14]

raw materials for pharmaceuticals 10 µg.L-1

0.5-20 mg.L-1

0.8 [18]

toothpaste 6 µg.L-1

2.5-11.7 mg.L-1

12 [7]

toothpaste and water 4.4 µg.L-1

5.0-39.0 mg.L-1

5 this work


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rapid and sensitive determination of fluoride in toothpaste and water samples using headspace single...

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