1. [Type text] [Type text] [Type text]
Fluoride Quantification Attempts in a Variety of Challenging Matrices,
Using Potentiometry and Ion Chromatography
Thomas J. Robshaw,* Karen E. Pardoe and John Wilson
Operations Services Science, British Sugar plc, Wissington SugarFactory, Norfolk, United Kingdom
S Supporting Information
ABSTRACT: Excessive intake of fluorine can lead to a
variety of health complications in animals, necessitating
quantification in animal feed additives and products to
allow quality standards to be met. A variety of analytical
methods are available for quantification of fluorine in its
ionic and acid-labile forms, but for challenging matrices,
the required sample preparation is often time and energy-
intensive. In this study an effective ion chromatography
method is reported, with a simple sample preparation
stage, capable of quantifying the fluoride content of
industrial gypsum samples in the range of 15 – 800 ppm with an error margin of 1.10 – 251 ppm. The natural
variability of the fluoride concentration within a small volume of sample, due to leaching and environmental
contamination was thought to greatly influence the error margin. The technique was verified by standard series
potentiometric measurements using a fluoride ion selective electrode (ISE) and reasonable agreement was attained (y =
1.009x + 1.7405, R2 = 0.7458). The agreement with external potentiometric measurements however, was poor. The ion
chromatography method was applied to a number of solid and liquid animal feeds derived from sugar beet, but
quantification was hindered by the presence of elevated levels of organic anions, with a similar retention time to fluoride.
Accurate potentiometric quantification in these matrices in the range of 0.5 – 10ppm was achieved by fixed volume
standard addition, as the standard series technique was found to be insufficiently robust. The proposed methodology is
suitable for certifying animal feeds and additives produced by the beet sugar industry as compliant with the Feed
Materials Assurance Scheme (FEMAS).
luoride is a common anionic form of the element
fluorine and is ubiquitous in the natural
environment.1,2 It is present in trace amounts in the
majority of human food products3 and in certain areas of
the world, both human and animal food and water sources
are known to be heavily polluted with fluoride.4 A
controlled fluoride intake is important to the health of
teeth and bones, but overconsumption can lead to dental
and skeletal fluorosis.5,6 Both fluorine and fluoride
quantification studies on human food products are
numerous7-10 but data on animal feed are minimal. This is
concerning, as fluorosis in livestock is prevalent in many
areas of the world, with cattle being the most badly
affected.6,11 Excess fluoride has also been shown to
adversely affect live weight gain in pigs12 and poultry13
Fluoride is commonly extracted from solid matrices
into aqueous solution for analysis to take place.14 If the
total fluorine content is to be determined, rather than
merely the ionic and acid-labile fraction, the sample
should generally be ashed,15 but in many food products,
the non-ionic fluorine fraction is insufficient to affect
results for unashed samples.7,16 Separation of the fluoride
is usually achieved by diffusion, using perchloric acid.
The use of hexamethyldisiloxane as a diffusion aid was
first reported in 1969 by Hall17 and this method, with
slight modifications, is still in regular use.8,10 Microwave
acid digestion has recently been investigated as an
alternative, less time-consuming form of fluoride
extraction, although the reported recovery was quite
variable (84-101%).9
Analysis of aqueous or otherwise solublised fluoride
is most commonly performed by photometry,
potentiometry or ion chromatography. Photometric
methods involve the reaction of fluoride ions with a
coloured metal complex, producing a ligand or counter-
ion exchange and a colour change, usually a bleaching
effect.18,19 A weakness of the technique is its
susceptibility to interference from other anions, often at
low relative concentrations.20 The fluoride concentration
range over which Beer’s Law is obeyed is also frequently
quire narrow.21 A study involving aluminium resorcin
blue reported linearity only over the range of 0-1 ppm.19
Consequently, there is little recent literature on
photometric determination.
The potentiometric method, using a fluoride ion
selective electrode (ISE) is the most frequently
encountered in quantification studies in a variety of
matrices.9,16,22,23 The advantage of the ISE is its tolerance
to all direct interferences, apart from the hydroxide ion.24
The fluoride ion may form complexes with Fe3+ or Al3+,
but this can be inhibited by the use of Total Ionic
F
Conductivity(µS)
t (min)
F-
British Sugar Gypsum HPIC Analysis
Received: August 17, 2015
Accepted: September 31, 2015
Article

2. [Type text] [Type text] [Type text]
Strength Adjustment Buffer (TISAB).25 Accordingly, the
majority of ISE analysis is conducted via standard series
measurements. Certain industrial standard methods
require variable volume standard addition to be used for
more complex matrices, such as wines.26 However, the
technique is relatively rare and standard series results are
not often verified by standard addition8-10. A recent
successful attempt was made to partially automate the
standard addition process and reduce the associated
longer analysis time.27 ISE results are frequently not as
reproducible as photometric techniques, as highlighted by
a recent review, which identified the need for industrial
standardisation.28 The methodical limit of quantification
is also variable, due to variances in the Nernstian
response concentration range.9,29,30 This may be affected
by the specific ISE model used, the type of TISAB and
the sample matrix.
Fluoride detection via High Performance Ion
Chromatography (HPIC) was first reported by Small et
al. in 1975.31 The technique rapidly gained popularity,
due to its high degree of automation and is now in regular
analytical use.32-34 With the correct instrumental set-up,
HPIC has demonstrated equivalent accuracy and superior
precision and sensitivity to potentiometry.35,36 It is a
particularly convenient technique for water analysis, as a
large number of anions can be quantified in one
experimental run.14,31 HPIC has also quantified fluoride
in solid geological samples, such as mineral deposits and
coal, following combustion and/or acid extraction.32,34,35
Interference from metal species may be eliminated by
passing the sample through an ion exchanger, a dual
column pre-separation or using an internal suppressor.33,34
HPIC can be ineffective for samples with a high organic
content, as acids such as acetic and formic have a very
similar retention time to the fluoride ion and the peaks are
difficult to separate.36 As a result, HPIC is rarely used for
analysis of food products.
Other analytical methods for fluoride quantification
have included capillary electrophoresis, gas
chromatography and polarography.5,38,39 New
methodology is commonly verified by comparing results
to those attained by potentiometry. Notably, Van der
hoop et al.38 compared results for analysis of rainwater by
ISE, HPIC and electrophoresis and observed that the
matrix effects, caused by aluminium in the samples was
not completely negated by the ISE or electrophoresis
methods. There has been no comprehensive review of the
relative merits of different methods for fluoride analysis
since 1987.14
Animal Feed Products of The Beet Sugar
Industry. Sugar beet processing factories, of which the
remaining 4 in the United Kingdom are owned and
operated by British Sugar plc, produce table sugar by
diffusing the sucrose out of the finely sliced beet parent
material. The exhausted beet shreds are pressed and dried
to produce a nutritious animal feed, which is supplied, in
the main, to dairy cattle. Other animal feeds produced are
molasses, raffinate and vinasses, which are all derived
from the liquor that remains after the sugar is crystallised
out of solution and separated by centrifugation. They are
all viscous brown liquids with high concentrations of
salts, organic acids, sucrose and reducing
monosaccharides.40 A number of animal feed producers,
including British Sugar are accredited by the Feed
Materials Assurance Scheme (FEMAS). One of a large
number of requirements for FEMAS accreditation is that
animal feed products contain no more than 150 ppm
fluorine on an 88% dry matter basis. The requisite testing
is outsourced to an external laboratory, which uses a
fluoride ISE to analyse loose and pelleted beet shreds,
molasses, raffinate and vinasses. British Sugar also adds
industrial gypsum to the factory diffusers, which acts as a
pressing aid for the beet pulp. It can be proved, by mass
balancing, that the gypsum can theoretically contribute a
substantial quantity of fluorine to the final animal feed
products and therefore gypsum is also analysed for
fluoride concentration. The acceptable specification for
British Sugar gypsum is an in-house determined figure of
5800 ppm.
The present study therefore afforded an opportunity
to create in-house analytical methods for the analysis of
fluoride in a number of challenging and unusual matrices.
The ultimate objective was to be able to reliably perform
such analysis using an in-house HPIC system. However,
given the popularity and perceived reliability of
potentiometric determination, it was decided to first
develop an appropriate ISE method, which could then
provide the necessary verification for the
chromatographic results. A secondary goal therefore, was
to compare the accuracy of standard series ISE
measurements against fixed-volume standard addition, to
test their validity in the analysis of difficult matrices.
EXPERIMENTAL SECTION
Reagents, Chemicals and Equipment. Fluoride
standard solution, 1000 mg/L, Spectrosol® grade (SCP,
Marktoberdorf, Germany) was diluted to appropriate
concentrations with deionised water (>18 MΩ cm unless
otherwise stated) TISAB was prepared according to the
method of Frant and Ross25 (Supporting Information
pagES 8-9). Nitric acid, 69%, ACR grade (Merck,
Hertfordshire, UK) was diluted with deionised water as
required. 250mM KOH solution was prepared by
dissolving KOH pellets, ≥85% (Sigma Aldrich, Dorset,
UK) in deionised water (Elgastat UHQ). Filter papers
used were of grade 100/N (Sartorius, Surrey, UK)
Potentiometry data was collected using a fluoride
ISE (model 924305, Jenway, Staffordshire, UK) and
conductivity meter (model MP225, Mettler-Toledo,
Leicester, UK). HPIC data was attained with a Dionex
UCI-100 Universal Chromatography interface (Fisher,
Loughborough, UK). For the full system specification
and operating conditions, see the Supporting Information.
Collection of Samples. Samples of beet pulp
shreds, beet pulp pellets, molasses, raffinate, vinasses and
gypsum were collected from all 4 British Sugar factories
(Bury St Edmunds, Cantley, Newark and Wissington,
UK). All samples were stored in airtight Nalgene bottles
at ambient temperature.
Analytical Chemistry Article

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Sample Preparation and Analysis. For full
methodology, see the Supporting Information. Solid
animal feed samples were coarsely milled, suspended in
1% nitric acid and mixed, using an orbital shaker, before
being filtered to produce an aqueous sample containing
the solublised fluoride. Liquid animal feed samples were
diluted with deionised water. Gypsumsamples were dried
in a 105ºC oven for a minimum of 8 hours, then
suspended in 1% nitric acid and prepared as per dry
animal feed.
For ISE experiments, samples contained 20%
TISAB. Standard series measurements were recorded by
measuring samples against a calibration curve produced
by analysing fluoride standards of 0.1-10 ppm. Fixed
volume standard addition measurements were recorded
by determining the electrode slope, as per the standard
series method, then analysing the sample, followed by at
least 3 additions.
For HPIC experiments, the Dionex system was set
up as per the Supporting Information. All standards and
samples contained 25 mM KOH.
RESULTS AND DISCUSSION
Comparison of ISE and HPIC Results for
Gypsum Samples. Fluoride was successfully quantified
in 15 gypsum samples from all 4 factories by both
standard series ISE and HPIC methods. Each result was
averaged from at least 2 sub-samples. The results are
compared in table 1, which includes the results returned
by the regular external laboratory.
No significant difference across the dataset could be
proved between the two in-house methods, verified by
both parametric (paired two-tailed t-test, p >0.1) and non-
parametric (Wilcoxon signed-rank test, p >0.05) testing.
A linear regression comparison (figure 1) yielded an
equation of y = 1.009x + 1.7405 and the R2 value was
0.7458. The 2 extreme differences between results, for
Ca-2 and Ca-6, were eliminated by Q test (p <0.01) for
this purpose. The overall percentage variance between
datasets was 0.81%.
The associated error values suggest the precision of
both in-house techniques was relatively poor. Michalski
and Mathews36 measured fluoride in a variety of
toothpastes, with concentrations of ~1.5 – 9.5 ppm using
HPIC and achieved error values of ≤ 0.2 pp m. This may
be explained by the nature of the sampled product, as
British Sugar gypsum stores are outdoors and uncovered
(Supporting Information pages 36-37). Fluorite, the main
form of ionic fluorine in gypsum, is generally poorly
soluble in water (17 mg/L at 25ºC).41 However, when
other trace metal contaminants are present, the Ksp of
fluorite may be modified, due to the formation of water-
soluble complexes.42 Masamba et al.43 demonstrated that
the presence of other ions could increase or decrease the
adsorption capacity of gypsum to extract fluoride from
water sources. British Sugar gypsum samples will be
subject to a varying amount of water through
precipitation, depending on their position in the bulk pile.
The leaching effect will also vary according to other
contaminating ions present, suggesting that the fluoride
concentration, even within a small sub-sample may vary
quite considerably, leading to the high error values
observed.Repeatability and reproducibility
studies were performed, using a smaller range of samples,
which are likely to be a more accurate estimation of
instrumental precision (Supporting Information page 25).
Although the measured concentrations suggest there
was no systematic bias inherent in either technique, there
were large discrepancies between results for individual
sub-samples. This may be due to the inability of the ISE
technique to completely negate the matrix effects caused
by aluminium cations.38 The pH of samples was
controlled somewhat by TISAB addition, but was not
adjusted to unity across all samples. Therefore there may
have been variation in the amount of HF and the HF·HF
complex, which is known to form at lower pHs.26,44 This
may also have caused erroneous ISE measurements.
Table 1. Determined fluoride concentrations in gypsum samples by ion chromatography and
potentiometry
Sample Reference ISE fluoride
conc. (ppm)
ISE error (2 x
standard deviation)
HPIC fluoride
conc. (ppm)
HPIC error (2 x
standard deviation)
External result
(ppm)
Bu-1 80.64 6.24 53.81 4.12 75.8
Bu-2 62.57 8.78 76.29 29.7 40.2
Bu-3 14.41 2.83 11.14 2.11 12.0
Bu-4 47.40 5.45 48.63 6.98 64.0
Ca-1 37.62 2.98 25.78 13.0 58.9
Ca-2 803.5 46.1 882.7 61.7 854
Ca-3 36.10 5.78 36.82 7.11 46.8
Ca-4 34.04 2.85 47.56 5.45 124
Ca-5 26.89 1.17 28.26 3.91 41.5
Ca-6 514.4 59.5 433.4 251 N/A
Ne-1 85.88 22.0 65.76 35.4 34.3
Ne-2 29.59 0.06 33.15 10.1 51.5
Ne-3 25.31 4.40 29.35 4.01 42.8
Wi-1 82.78 21.9 82.89 6.44 106
Wi-2 42.80 6.03 45.01 1.10 N/A
Average 13.1 29.5
Bu = Bury, Ca = Cantley, Ne = Newark, Wi = Wissington
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Finally, reproducibility studies (Supporting Information
pages 24-25) showed that both techniques could be
susceptible to a high degree of experimental bias that
could vary over several days.
No significant difference across the dataset could be
proved between an average of the two in-house methods
and the external results (t-test p >0.1, Wilcoxon test p
>0.05). However, the external laboratory results were on
average ~40% higher than in-house. The linear regression
equation was y = 0.9939x + 12.75 (Supporting
Information, fig 2.5) suggesting a clear systematic bias. A
number of quality control measures were employed for
in-house testing. A CRM (P.E. check – minerals, level 1,
SCP, Marktoberdorf, Germany) was used for every HPIC
run, the results for which were always within ± 5%. 8 of
the 15 samples were analysed by fixed volume standard
addition. There was no significant difference across the
dataset between standard addition and standard series
measurements (t-test p >0.1, Wilcoxon test p >0.05) and
the average difference between measurements was 2.35
ppm (Supporting Information pages 22-23).
Sample Preparation. 7 of the 15 samples were
also prepared by aqua regia acid digestion. No significant
difference could be proved across the dataset between
these results and those attained by the standard sample
preparation (t-test p >0.1, Wilcoxon test p >0.05). The
standard sample preparation results were on average 6.46
ppm higher, suggesting the fluoride extraction or
retention was superior. Mineral acids, even at low
concentrations can be highly effective at leaching
fluoride-containing species from phosphogypsum.45
Furthermore, even in closed-container acid digestion,
there is inevitably some loss of halides as HX species.46
The evidence from this study suggests that the simpler
sample preparation was both more effective and
economical.
ISE Results for Animal Feed Samples. Fluoride
was quantified in 19 solid feed and 18 liquid feed
samples by standard series and standard addition methods
in the range of 0.5 to 10 ppm (Supporting Information
tables 3.1 and 4.1). In solid feed no significant difference
across the dataset could be proved between the two
techniques (t-test p >0.1, Wilcoxon test p >0.05).
However, the agreement between the two techniques was
poor (linear regression: y =0.6556x + 2.090 R2 value =
0.667). In liquid feed, the reported fluoride concentration
was significantly lower for the standard addition method
(t-test p <0.1, Wilcoxon test p <0.05). The quantity of
fluoride in liquid feed derived from sugar beet is
substantially less than in solid feed, as only a small
amount of gypsum is solublised during diffusion, the
majority being separated with the exhausted beet pulp.47
As a result, any interference from the matrix will be more
pronounced for standard series measurements of liquid
feed, due to the reduced signal to noise ratio. Spadaro et
al.48 compared standard series and standard addition
results with an aqueous matrix and discovered that a
variety of organic species at a concentration of 5%
increased the electrode response, giving artificially high
results for the standard series method. Conversely,
Yuwano49 reported that standard addition potentiometry
was able to eliminate an artificially low electrode
response in analysis of teas. The evidence from this study
suggests that standard addition was necessary for the
animal feed matrices analysed.
No precise external laboratory results were available
for comparison, as their limit of detection is 5ppm. No
samples submitted in the last 12 months returned a
fluoride concentration of <5 ppm, which was in
agreement with in-house results. Overall, the method was
more than sufficient to demonstrate the adherence of
animal feed products to FEMAS specification.
Attempts to Analyse Animal Feed by HPIC.
Attempts to quantify fluoride in solid and liquid animal
feed by HPIC were unsuccessful, due to the presence of
several organic anions, which exhibited similar retention
times to the fluoride ion and were present in much greater
quantities, thereby completely engulfing the fluoride
peak. Figure 2 shows an example of the interfering peaks
on an animal feed chromatogram, in contrast with the
gypsum chromatogram, in which the fluoride peak is
adequately separated.
One of the interfering peaks is likely to be acetate,
as acetic acid is present at high levels in the matrix and its
retention time is known to be similar to fluoride.14,37
Other possible interferences are hydrogencarbonate,
lactate, propionate and formate.21,50,51 Switching from
isocratic to gradient elution produced no beneficial effect.
A number of ion exchange cartridges were tried,
including DigiSEP Orange Label, DigiSEP Red Label,
(SCP, Marktoberdorf, Germany) styrene-divinylbenzene
and C18-U (Phenomenex, Cheshire, UK). None separated
the organic ions to the extent that the fluoride peak was
detectable on the chromatogram (Supporting Information
page 35). The DigiSEP Red Label was the most
successful in this regard. However, potentiometric
0
20
40
60
80
100
120
140
0 20 40 60 80 100 120 140
ISE
HPIC
Figure 1. Linear regression comparison between ISE and HPIC
measurements for fluoride concentration (ppm) in British Sugar
gypsum samples. Results are averaged from at least 2 sub-samples.
Error bars represent standard deviation x 2. Regression equation: y
= 1.009x+ 1.7405.R2
= 0.7458
Analytical Chemistry Article

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analysis indicated that this cartridge also affected the
fluoride recovery by >40%. Acid digestion sample
preparation was also trialled, but had a negligible effect
on removal of the interference.
No literature examples of removal of organic
interference via anion-exchange pre-treatment were
found. Fluoride has been quantified in solid calcium
carbonate by dissolving the matrix in HCl, then removing
the excess chloride, which otherwise engulfs the fluoride
peak, using silver-embedded resin cartridges.44 However,
no organic component was present in the matrix. An
alternative column type may have been more successful
in separating the fluoride peak from the interference.
Michigami et al.50 quantified fluoride in urine using an
octadecylsilyl (ODS) column, which successfully
separated the peak from organic interferences present at
~40 times the fluoride concentration. Zhong et al.51
separated fluoride and acetate peaks in Bayer liquors,
using an IonPac AS11-HC column and gradient elution.
Müller et al.46 measured fluoride in cigarette
tobacco via microwave-induced combustion (MIC) prior
to HPIC analysis. All organic interference was removed
and the quantification limit was 0.1ppm.
Sample Preparation. FEMAS accreditation of
animal feeds requires quantification of total fluorine
content, rather than ionised or acid-labile fluorine
content. Some national standards for food analysis
require ashing of samples by alkali fusion, then
distillation, before potentiometric measurements.39
However, in comparisons between ashed and unashed
food samples, both sugar products7 and root vegetables3
were shown to contain a negligible amount of non acid-
labile fluorine, suggesting that the sample preparation
described in this study was sufficient to liberate close to
the entire fluorine content.
CONCLUSIONS
Analysis of fluoride content by ion chromatography is a
convenient and increasingly viable alternative to the
established potentiometric technique. It is well suited to
analysis of water, but is more rarely used for solid
samples or those with a high organic content, due to the
intensive sample preparation and high probability of peak
interference on the chromatogram. In this study,
successful quantification of fluoride in gypsum is
reported, with rapid, simple and low-energy sample
preparation. Results were broadly in agreement with in-
house standard series potentiometric measurements and,
although the precision of the HPIC appeared poor, this
was believed to be due to the higher sensitivity of the
technique. Agreement with external results was poor, but
internal QC checks suggested that the proposed method
was more than sufficiently accurate.
No reliable HPIC results could be attained for the
analysis of solid or liquid animal feeds, due to
chromatographic interference by organic anions, which
could not be fully removed by ion-exchange methods. An
in-house potentiometric method was developed, the
results of which suggested that the matrix was
sufficiently challenging to necessitate fixed-volume
standard addition measurements for accurate
quantification.
The described chromatographic and potentiometric
methods are postulated as being sufficiently robust to
analyse British Sugar samples according to FEMAS
specification.
ASSOCIATED CONTENT
Supporting Information
Full methodology, including parameters for setup and
optimisation of the Dionex system, selected statistical
analysis, selected chromatograms, repeatability studies
and illustrations of British Sugar gypsum storage
facilities. The Supporting Information is available free of
charge on the ACS Publications website.
AUTHOR INFORMATION
CorrespondingAuthors
*E-mail: t-j-robshaw@hotmail.co.uk
Notes
The authors declare no competing financial interest.
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0
1
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4
0 1 2 3 4 5
Conductivity(µS)
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b
-1
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~2.8 min] (c)typical solidanimal feedsample with ~1/100 dilution[F-
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