American Journal of Analytical Chemistry, 2012, 3, 613-621
doi:10.4236/ajac.2012.39080 Published Online September 2012 (
Copyright © 2012 SciRes. AJAC
Analysis of Urea in Petfood Matrices: Comparison of
Spectro-Colorimetric, Enzymatic and Liquid
Chromatography Electrospray Ionization High Resolution
Mass Spectrometry Methods
Patrick Pibarot
, Serge Pilard
Nestlé Research and Development Centers, Aubigny, France
Analytical Platform, University of Picardie Jules Verne, Amiens, France
Received July 4, 2012; revised August 12, 2012; accepted August 19, 2012
Adulteration may consist in non authorized source of nitrogen addition to increase the protein content of some raw ma-
terials. Urea which is authorized for feed is a non nutritional source of nitrogen in food and pet food. Adulteration of
food or pet food raw material by urea is thus monitored by manufacturer and governmental authorities with official
methods which are either enzymatic (Association of Official Agricultural Chemists, AOAC) or spectro-colorimetric
(European Community, EC). Each method gives results which are not comparable and spectro-colorimetric methods
may result in false-positive urea detection. Liquid chromatographic (LC/UV-DAD) analysis of extracts from spectro-
colorimetric method indicates that presence of free amino-acid may interfere with colorimetric detection of urea in the
EC method with pet food samples. Liquid chromatography electrospray ionization high resolution mass spectrometry
(LC/ESI-HRMS) has allowed to quantify low content (<0.01%) of urea in pet food water extracts for samples which
resulted in significant urea detection with colorimetric method and in content below the detection threshold with enzy-
matic method. This study demonstrates the EC colorimetric method is not applicable to pet food and also food samples
which have a complex composition with significant levels of free amino acids. On the other hand we clearly evidenced
by means of the LC/ESI-HRMS results that the AOC Enzymatic method is applicable to urea quantification in pet food
samples and gives reliable results.
Keywords: Pet Food; Food; Urea; Adulteration; Enzymatic Method; Spectro-Colorimetry; LC/UV-DAD;
1. Introduction
Non authorized food ingredient can be used to adulterate
ingredients, raw materials (e.g. Gluten) or finished prod-
uct (e.g. milk products) to artificially increase the protein
content. Melamine, for instance, has been used as the food
additive to increase the apparent protein content in food
products [1].
Ruminant can convert non protein nitrogen into protein
and urea may be used to increase the nutritional nitrogen
content of feed, hay or silage. Urea is authorized as a com-
ponent of animal feeds to provide nutritional source of
nitrogen. Urea is accepted as a “Generally Recognized as
Safe” food additive and is also used as a fermentation aid
in foods and beverages [2].
Urea is produced in the liver of mammals as an end-
product of protein metabolism and excreted in urine. Hu-
mans excrete up to 30 grams of urea per day in the urine [2].
Avian species (e.g. poultry) metabolize mainly uric acid
(55% - 72%) and some urea in lower proportion (2% -
11%), the rest of protein metabolism product is ammonia
(11% - 21%) [3,4].
Therefore, poultry meat and by products should not
present significant amount of urea while mammal meat
by-product may contain some urea in more significant
Kjeldahl or Dumas methods measure the protein con-
tent by analyzing total nitrogen content, without identi-
fying its sources. The research of non-protein nitrogen
source is complementary to protein analysis to check the
absence of adulteration in raw materials and finished pro-
Urea is one possible molecule which may be used to
adulterate (increase nitrogen content) food and petfood or
one of the ingredients used to manufacture food or pet-
Copyright © 2012 SciRes. AJAC
Therefore, even if toxicology is not an issue, the analy-
sis of urea in food ingredients and in finished products
may be of importance to prevent the use of non author-
ized nitrogen sources in food and petfood industries.
Various analytical methods including colorimetry, urea
enzymatic conversion to ammonia, liquid or gas chroma-
tography or near infrared spectrometry are described for
urea analysis in feed, silage, fertilizers, blood, serum, urine
and other matrices [5].
The two main analytical methods used in food and feed
areas are:
Spectro-colorimetric methods [6,7];
Enzymatic (urease) methods [8].
These methods are either based:
On the reaction of urea with a specific molecule to
create a chromophore which content is analyzed by spe-
Or on the enzymatic degradation of urea in presence
of urease to ammonia and on ammonia analysis by the
Kjeldahl method (acidic titration) or other various met-
Their main advantage is rapid and easy to use protocol.
Other more sophisticated methods based on gas chro-
matography coupled to isotope ratio [9] or isotope dilu-
tion [10,11] mass spectrometry and liquid chromatogra-
phy hyphenated to tandem mass spectrometry [1] have been
Several cases of interference in colorimetric method or
enzyme activity disturbance are reported [5,11,12].
Co-elution problem leading to result variations in chr-
omatography methods are also reported [13].
The objective of the current study is:
To compare the results of spectro-colorimetric method
and enzymatic method reliability for the routine analy-
sis of urea dry petfood products;
To define which method is the most suitable for urea
analysis in dry petfood by comparing the results ob-
tained by the routine methods to the results obtained
with liquid chromatography electrospray ionization high
resolution mass spectrometry (LC/ESI-HRMS).
Finally the conclusion of the study should allow to rec-
ommend an official method for analyzing urea in dry pet-
food products and in pet food raw materials.
2. Experimental Sections
2.1. Spectro-Colorimetric Method (European
This method corresponds to the method recommended by
the European Union and is described in European Com-
mission Regulation (EC) No. 152/2009 laying down the
methods of sampling and analysis for the official control
of feed [6]. This method is also described as a AOAC
method [7].
The sample is suspended in water with a clarifying agent
(active carbon). The suspension is filtered. The urea con-
tent of the filtrate is determined after the addition of 4-
dimethylaminobenzaldehyde (4-DMAB) by measuring the
optical density at a wavelength of 420 nm. If the sample
contains simple nitrogenous compounds such as amino
acids, the optical density shall be measured at 435 nm.
2.2. Enzymatic Method
The enzymatic method is the official method of urea ana-
lysis in feed of AOAC international (No. 941.04) [8].
The method consists in digesting 1 g of samples in urease
enzyme solution at pH = 7 in order to convert the urea in
carbon dioxide and ammonia. After alkalization, ammo-
nia vapor is released and distillated in a known amount
of acid and the distillate titrated with standard base solu-
tion (Kjedhal method).
It should be noticed that ammonia is analyzed together
with urea and that some heavy metal may inhibit the ac-
tivity of urease.
2.3. Liquid Chromatography Ultraviolet-Visible
Diode Array Detection (LC/UV-DAD)
The analyses were run on AS DX 500 HPLC (Dionex,
Sunnyvale CA, USA) equipment with UV-Vis Diode Ar-
ray detector UVD 340 S.
After several trials with several columns and elution
conditions (ion exchange, pH adjustment) the following
chromatographic conditions were selected. The reverse
phase column was an Acclaim PA2 C18, 4.6 × 150 mm,
5 µm supplied by Dionex France.
The chromatographic conditions were:
Isocratic elution with 100% purified water at 1 mL/min
and 30˚C;
Sample injection 250 µL (auto-sampler AS 50);
Detection 200 - 595 nm (resolution 1nm). Urea quan-
tification is performed at 435 nm as indicated in the
EC protocol [6,7].
The samples of petfood were ground and extracted in
the same conditions than for the DMAB spectro-colori-
metric analysis (except active carbon treatment). The ex-
tract reacted with DMAB were directly injected on the
HPLC column. The separation and detection of the mole-
cules produced by reaction between urea and DMAB has
been chosen with two objectives:
To improve the selectivity and sensibility of the urea
detection by UV absorbance;
To check the presence of other molecules than urea in
dry pet food which may react with DMAB and absorb
at the wavelength used by the EC colorimetric method.
2.4. Liquid Chromatography Electrospray
Ionization High Resolution Mass
Spectrometry (LC/ESI-HRMS)
The LC/ESI-HRMS experiments were performed on a Q-
Copyright © 2012 SciRes. AJAC
TOF Ultima Global hybrid quadrupole time-of-flight inst-
rument (Waters-Micromass, Manchester, UK), coupled
with an Ultra-Fast Liquid Chromatograph (UFLC) system
(Shimadzu, Duisburg, Germany).
The separation was obtained on a Nucleodur Pyramid
2.0 mm i.d. × 250 mm, 3 m C
reversed phase column
(Macherey-Nagel, Duren, Germany) at 25˚C, using an iso-
cratic elution with 100% water. The flow-rate and the
sample injection volume were set to 0.2 mL/min and 10
µL, respectively.
The effluent from the LC column was directly intro-
duced in the electrospray ion source (Z-spray) and the
ESI-HRMS data were recorded in the positive ion mode
with capillary and cone voltages of 3.7 kV and 184 V,
respectively. Nitrogen was used as a nebulizing (100 L/h)
and a drying (450 L/h) gas. The source and desolvatation
temperatures were kept at 120˚C and 250˚C, respectively.
The mass range was 100 - 200 Da and spectra were re-
corded in the profile mode at a scan rate of 0.1 s/scan
(interscan time: 0.1 s) and at a resolution of 5,000
(FWHM). Accurate mass calibration was achieved using
the cluster ions of an orthophosphoric acid solution (0.2%
in H
CN 50/50 v/v). Data acquisition and proc-
essing were performed with MassLynx 4.0 SP4 software.
The ground pet food samples were extracted by mix-
ing with water (2 g in 500 mL, 30˚ C, 30 min) and sam-
pling of the supernatant after decantation. Urea quantifi-
cation was performed using the integration of the recon-
structed ionic current (RIC) of the [di-Urea+Na]
Na, exact mass: 143.0545) selected with a 150
ppm (0.02 Da) mass window. For each set of samples,
urea calibration curves were obtained after injection of
standard solutions prepared in water.
3. Results and Discussion
3.1. Analyses of Dry Pet Food Finished Products
by EC and AOAC Methods
The DMAB spectro-colorimetric method is the European
official method for analyzing urea in feed materials while
in the American zone, for instance, the enzymatic method
is the official one. The comparison of the two methods
with a range of dry pet-food sample (from various man-
ufacturers) was therefore an interesting exercise.
The results of this comparison are given in Table 1.
The result demonstrate the two methods do not give
similar results and the spectro-colorimetric method gives
systematically urea concentration between 0.14 and 0.27
g/100g while the enzymatic method gives non detectable
amount of urea whatever is the analyzed sample (except
one sample at 0.01 g/100g).
Therefore either the enzymatic method is not able to
detect urea in petfood samples (e.g. urease inhibition) or
the spectro-colorimetric method results in false positive
by detecting other interfering molecules than urea.
Table 1. Comparison of urea content (g/100g) obtained in
dry pet food samples with spectro-colorimetric method and
enzymatic method.
Samples EC colorimetric method AOAC enzymatic method
Dog food 10.18 <0.01
Dog food 20.14 <0.01
Dog food 30.26 <0.01
Dog food 40.25 <0.01
Cat food 10.26 <0.01
Cat food 20.25 0.01
Cat food 30.27 <0.01
Cat food 40.17 <0.01
3.2. Analysis of Raw Materials by EC and
AOAC Methods
If the finished product contains some non acceptable level
of urea, some urea at higher level of concentration should
be found in the raw materials used in the formulation of
the finished products.
A wide range of raw materials have been analyzed.
Significant concentrations of urea are found by the spec-
tro-colorimetric method for animal and fish meals, vis-
cera and liver digest, colostrum and pure-amino acids.
All these colorimetric method positive samples are rich
in free amino acids.
The highest concentrations are found with pure amino-
acids which are already identified as molecules able to
react with DMAB and to be detected by UV absorption
even if the wavelength of 435 nm is selected by method
authors to minimize the interference of amino-acids.
A selection of the ingredients with significant urea con-
centration determined by spectro-colorimetry was con-
troled by enzymatic method. The results are shown in the
following tables.
Two batches of methionine, which may be added in
relatively significant amount in the recipe for palatability
purpose, were analyzed and the results of two different
samples are presented on Table 2.
Methionine is detected by the EC colorimetric method
as urea even if obviously the quantification is far from
the true value. Enzymatic method is not able to produce
ammonia with methionine as a substrate.
Analysis results comparison by both methods was also
performed with chicken viscera digest and pork liver digest
(See Table 3).
Chicken viscera digest contains free amino acid (because
of the enzymatic digestion) but is unlikely to contain sig-
nificant amount of urea as chicken metabolism produces
very limited amount of urea and mainly produces uric acid.
As matter of fact, the colorimetric method gives a pos-
itive answer while the enzymatic method does not detect
any urea.
Pork liver digest contain higher level of free amino acid
Copyright © 2012 SciRes. AJAC
Table 2. Comparison of urea contents (g/100g) obtained
with methionine ingredient samples by spectro-colorimetric
(measured at 435 nm) and enzymatic methods.
Colorimetric method
Enzymatic method
Methionin 1 5.56 <0.01
Methionin 2 4.67 <0.01
Table 3. Comparison of urea content (g/100g) obtained in
chicken viscera digest with spectro-colorimetric and enzy-
matic methods.
Colorimetric method
Enzymatic method
Chicken viscera digest 0.13 <0.01
Pork liver digest 0.95 0.12
and may contain urea as the pork metabolism (mammal-
ian) produce urea. Again as a matter of fact, the colori-
metric method results are higher than the enzymatic one
because of the amino acid interferences and the enzymatic
method is able to detect the natural low level of urea
which is expected.
To make a further proof of the adequacy of the enzy-
matic method for the analysis of urea in petfood, pet food
samples were spiked with 3 different levels of pure urea.
Results are given in Table 4.
For the non spiked sample, the enzymatic method gave
non detectable result. Sample spiked with known level of
urea were correctly analyzed by the enzymatic method and
this prove the enzymatic method is able to detect correctly
urea when it is present in the pet food products.
This comparison of urea content results obtained by
EC colorimetric method and AOAC enzymatic method,
gives first indication that some interfering amino com-
pound may result in false positive with the EC colorimet-
ric method. This should be confirmed by analyzing the
pet food samples with other analytical methods.
3.3. LC/UV-DAD Analyses of DMAB Product
The objective of the HPLC analysis of samples extracted
and treated with DMAB in similar way than with the EC
colorimetric method is to chromatographically separate
the interfering compounds from urea in order to demon-
strate that other molecules than urea are analyzed in pet
food samples when EC colorimetric method is used.
Urea molecule presents low retention time with reverse
or normal chromatography and is not easily separated from
the free amino-acid or bio-amines [14]. After several trials
the Acclaim PA2 C18 (Dionex) was selected as the most
able to separate urea and free amino-acids. Four dry prod-
ucts from the super-premium cat and dog products range
were selected for their significant contents in urea as ana-
lyzed with EC colorimetric method. These products were
analyzed by HPLC after water extraction and reaction with
DMAB as described in EC colorimetric protocol. Figure
1 gives an example of HPLC analysis with pet food sam-
ple spiked with 0.1% urea.
Analyses results of dog and cat pet food samples by
HPLC method are given in Table 5.
Free amino acid peaks detected after the peak of urea
(see Figure 1) were arbitrary quantified in order to rank
them. It should be noticed that the detection was at 435
nm and only molecules able to react with DMAB were
detected by the HPLC analysis.
The first fact is that the levels of urea quantified by the
HPLC method are lower of a magnitude of about 100 to
the content of urea quantified by the EC colorimetric
Nevertheless, no clear correlation is observed between
either the levels of urea quantified by both methods nei-
ther between amino-acid arbitrary ranking and the urea
Table 4. Results of analysis of spiked dry pet food by enzy-
matic method.
Dry PF with spiked Urea (g/100g,
spike levels)
Enzymatic method
0.53 –0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00 4.50 5.02
SEM 12 17 MARS 2009 #54 [modified by RDLab05AUR]
Ech 4 + 0.1 %+ DMAB
Ech 4 + 0.1%+ DMAB
WVL:435 nm
1 - 0.083
2 - DMAB - 1.775
3 - Urea - 2.150
Amino acids
Figure 1. Example of chromatogram obtained by LC/UV-
DAD analysis of pet food sample spiked with 0.1% of urea
(Acclaim C18, detection at 435 nm, urea peak at 2.1 min,
DMAB at 1.7 min)
Table 5. Results of analysis of dry pet food by LC/UV-DAD,
comparison with spectro colorimetric results.
Colorimetric method LC/UV-DAD analysis
Samples Urea (g/100g) Urea (g/100g)
Amino acids
(arbitrary unit)
Dog food 5 0.32 0.004 3.61
Cat food 1 0.26 0.0058 5.99
Cat food 5 0.60 0.0062 2.75
Cat food 6 0.75 0.0038 3.99
Copyright © 2012 SciRes. AJAC
Therefore the LC/UV-DAD analyses have proven that
other molecules than urea can react with DMAB and in-
terfere in the colorimetric analysis to give overestimated
Even if some interesting facts are deduced from this
HPLC analysis, the proof of the overestimation and the
true level of urea in pet food samples is still to be done as
the urea and amino-acid peaks separation is not enough
to validate the quantification of urea. This lack of reten-
tion of urea and difficulty to separate urea from amino-acid
in HPLC has been already observed by some authors [14].
The selectivity and the effective separation of urea with
the other molecules derivatized by DMAB is not enough
to seriously quantify the true level of urea in dry pet food.
3.4. LC/ESI-HRMS of Pet Food Samples
The objective of the LC/ESI-HRMS analysis was to ac-
cess to urea content in pet food samples using a simple
water extraction without any further reaction (e.g. DMAB)
and a highly specific detection method. In order to sepa-
rate as far as possible urea from the interfering molecules
(e.g. free amino acids) we have used a liquid chromatog-
raphy method developed by Macherey Nagel on a Nu-
cleodur Pyramid C
column with water (100%) as the
eluent [15], in these conditions urea was detected at a
retention time of 2.86 min.
Electrospray and atmospheric pressure chemical ioni-
zations coupled to tandem mass spectrometry (ESI-MS/MS
and APCI-MS/MS) were applied to determine the con-
centration of urea present in the water of swimming pool
[16] but, to our knowledge, these methods were never
used for urea quantification in complex samples, such as
pet food. High resolution mass spectrometry, allowing the
determination of molecule elemental composition, is a
very selective and sensitive method to identify and quan-
tify a specific compound in mixtures. Consequently, in
these work we have investigated online LC/ESI-HRMS
using the high resolution capabilities of a quadrupole or-
thogonal time-of-flight (Q-TFOF) mass spectrometer [17].
The exact mass which was used for urea characteri-
zation and quantification was 143.0545, corresponding
to the elemental composition of the [di-Urea+Na]
Na). The mass selection window was set to
150 ppm (0.02 Da) for reconstructed ion chromatogram
(RIC of m/z 143.05 or 143.07 according to the mass spe-
ctrometer calibration).
Pure urea was diluted in water to prepare a range of
standard solutions. A stock solution was prepared by dis-
solution of 40 mg of pure urea in 1000 mL of de-ionized
water to obtain a urea solution at 4 mg/100mL. The stock
solution was diluted in water to prepare standard solutions
at 0.2, 0.4, 0.8, 1.2, 1.6 and 2 mg/100mL, in the range of
the urea concentration expected in pet food samples. These
standard solutions were injected in the LC/MS system to
build a calibration curve before each analyses session. The
chromatograms acquired at the exact mass of 143.05 (m/z)
and peak areas (retention time 2.86 min.) obtained are pre-
sented on Figure 2.
The corresponding calibration curve was obtained from
the least-squares linear regression presented with its cor-
relation coefficient (R
= 0.9927) on Figure 3.
The linearity of the calibration curve was demonstrated
for urea concentrations up to 4 mg/100mL. For each dry
pet food analysis 2 g of pellets were extracted in 500 mL
pure water during 30 minutes at 30˚C. For each series of
pet food samples new dilutions of urea stock solution at 4
mg/100mL were prepared to build a dedicated calibration
3.5. Pet Food Matrices Effect on LC/ESI-HRMS
To validate the eventual effect of the pellet matrix on the
quantification of urea by LC/ ESI-HRMS, the extraction
of a urea doped dry pet food sample, which was previ-
ously analyzed by EC spectro-colorimetric method (re-
sult: 0.46 g/100g), was performed. A sample of 4 grams
of ground pellets was doped with respectively 40 and 20
milligrams of pure urea powder and the resulting samples
analysis at 1% urea (Chromatogram A) and 0.5% (Chro-
matogram B) are shown Figure 4.
UREE-TEMOIN-005-070710-b Sm (SG, 3x20)TOF MS ES+
143.05 150PPM
UREE-TEMOIN-01-070710-b Sm (SG, 3x20)TOF MS ES+
143.05 150PPM
UREE-TEMOIN-02-070710-a Sm (SG, 3x20)TOF MS ES+
143.05 150PPM
UREE-TEMOIN-03-070710-a Sm (SG, 3x20)TOF MS ES+
143.05 150PPM
UREE-TEMOIN-04-070710-a Sm (SG, 3x20)TOF MS ES+
143.05 150PPM
UREE-TEMOIN-05-070710-b Sm (SG, 3x20)TOF MS ES+
143.05 150PPM
Figure 2. LC/ESI-HRMS analysis of urea standard solu-
tions at 0.2, 0.4, 0.8, 1.2, 1.6 and 2 mg/100mL (respectively
chromatogram from top to bottom; integration of urea
peak at 143.05 (m/z) and 2.86 min (Rt)).
Copyright © 2012 SciRes. AJAC
y = 1.6111x
R² = 0.9927
0 0.2 0.4 0.60.8 1 1.21.4
(mg/100 ml)
Peak area at m/z 143.05
Figure 3. LC/ESI-HRMS urea analysis calibration curve bet-
ween 0.2 and 2 mg/100mL.
UREE-046-20-b Sm (SG, 3x20)TOF MS ES+
143.055 150PPM
UREE-046-10-b Sm (SG, 3x20)TOF MS ES+
143.055 150PPM
Figure 4. LC/ESI-HRMS urea analysis of dry dog food ex-
tracted with a urea solution at 1% (chromatogram A) and
at 0.5% (chromatogram B).
The results of this doped samples analyses are shown
on Table 6.
The pellet matrix appears to suppress around 35% the
MS signal.
The EC spectro-colorimetric has determined a content
of 0.46 g/100g in this pet food sample which was doped
with urea. In order to take into account the matrix effect
on the LC/ESI-HRMS quantitative analysis, the correla-
tion between the urea content in the dry pellets concen-
tration of the doped samples and the LC/ESI-HRMS peak
area was established. The extrapolation of the curve to
zero added urea gives directly the urea concentration in
the analyzed sample (see Figure 5).
It can be determined from the curve the value of 0.05
g/100g for the real urea content of this sample. This value
is significantly below the EC spectro-colorimetric result
Table 6. Results of analysis by LC/ESI-HRMS (m/z 143.05)
of dry pet food pellets doped at 0.5% and 1% with pure
urea powder.
Area m/z
yields (%)
Doped at
0.82 1.32 2 66
Doped at
1.71 2.75 4 69
Figure 5. Extrapolation of the correlation between peak area
and urea concentration in doped pellet sample (g/100g) to
determine the real content of urea in the analyzed sample.
of 0.46 g/100g. Spiking method, being less accurate than
calibration method, further analyses are necessary to vali-
date this result.
3.6. LC/ESI-HRMS and Spectro-Colorimetric or
Enzymatic Methods Comparison
Further analyses were performed to compare the EC
spectro-colorimetric method, the enzymatic method and
LC/ESI-HRMS. The selected samples are described in
Table 7.
A new calibration file was established and results are
shown on Figures 6 and 7.
The chromatograms and peak areas obtained by analy-
sis of samples listed in Table 7 are presented on Figure 8
(the chromatogram at the top corresponds to a standard
solution at 0.2 mg/100mL urea).
The peaks at retention time between 2.70 and 3.05 min.
were taken as urea peak to keep the worst case hypothe-
sis. The results for the five analyzed samples are presented
on Table 8.
The results obtained by LC/EIS HRMS are significantly
below the results obtained by EC colorimetric method and
in the same magnitude than the results of the enzymatic
analysis (<0.01 g/100g, Table 7) even if the corrective
factor to take into account the 65% yield is not applied
(see Table 9).
Copyright © 2012 SciRes. AJAC
Table 7. Selection of a range of petfood samples for analysis
Pet food 1 0.69 <0.01
Pet food 2 0.31 <0.01
Pet food 3 0.17 <0.01
Pet food 4 1.09 <0.01
Pet food 5 0.26 <0.01
UREE-TEMOIN-01-070710-c Sm (SG, 3x20)TOF MS ES+
143.07 150PPM
UREE-TEMOIN-005-070710-c Sm (SG, 3x20)TOF MS ES+
143.07 150PPM
UREE-TEMOIN-0-070710-c Sm (SG, 3x20)TOF MS ES+
143.07 150PPM
Figure 6. LC/ESI-HRMS analysis of urea standard solution
at 0, 0.2 and 0.4 mg/100mL concentrations (respectively
chromatogram from top to bottom; integration of urea peak
at 143.07 (m/z) and 2.86 min (Rt)).
Figure 7. LC/ESI-HRMS urea analysis calibration curve at
0, 0.2 and 0.4 mg/100mL.
The concentrations analyzed by LC/ESI HRMS for this
set of dry pet food samples are in the magnitude order of
Figure 8. LC/ESI-HRMS urea analysis of dry dog food wa-
ter extracts. (Pet food 1: chromatogram at the bottom and
respectively pet food 2, 3, 4, 5 from bottom to top of the
figure, chromatogram at the top corresponds to a standard
solution at 0.2 mg/100mL urea).
Table 8. Results of analysis of dry pet food samples by
LC/ESI-HRMS (m/z 143.07) (2 g of samples extracted in 500
mL of water).
Peak area m/z
Water extract
concentration (mg/100mL)
Pellet content
Standard (0.2
1.64 0.223 na
Pet food 1 0.11 0.015 0.004
Pet food 2 0.03 0.004 0.001
Pet food 3 0.16 0.022 0.005
Pet food 4 0.08 0.011 0.003
Pet food 5 0.3 0.041 0.010
na: not applicable.
Table 9. Comparison of urea content in dry pet food pel-
lets obtained by EC colorimetric method, enzymatic method
Colorimetric methodEnzymatic method
(g/100g) (g/100g) (g/100g)
Pet food 10.69 <0.01 0.004
Pet food 20.31 <0.01 0.001
Pet food 30.17 <0.01 0.005
Pet food 41.09 <0.01 0.003
Pet food 50.26 <0.01 0.010
Copyright © 2012 SciRes. AJAC
the enzymatic method (<0.01%) when the EC colorimet-
ric method resulted in urea concentration between 0.1%
to 1%.
4. Conclusions
As a conclusion, the EC colorimetric method for urea
analysis is not applicable to dry petfood products as in-
terfering molecules result in significant urea concentra-
tion over dosing.
The interfering molecules may be free amino-acids as
they react with DMBA and absorb at 435 nm as shown
by the LC/UV-DAD analyses. As a matter of fact ingre-
dients known to contain significant level of free amino
acid present some content of urea when analyzed by the
EC colorimetric method.
The enzymatic method gives results <0.01 g/100g for
all petfood samples and is proven to be able to detect urea
in petfood samples when urea is spiked and also by analy-
sis of some ingredients like pork liver which may natu-
rally contain urea.
After several trials and studies, the analysis of a range
of dry petfood by LC/ESI-HRMS has allowed the valida-
tion of the results obtained using the enzymatic method.
The recommendation would be to propose a modifica-
tion of the EC Commission Regulation No. 152/2009 and
to recommend the enzymatic method for monitoring urea
in pet food products and in food product with complex
matrices and significant level of free amino acids. This
would also harmonize the regulation between Europe and
America zones as the AOAC already recommends the
enzymatic method for urea analysis in pet food.
The complete validation of the LC/ESI-HRMS would
require further work to validate the extraction method
and to study the stability of extracted petfood water solu-
tions doped with urea. This method is also quite sophis-
ticated and expensive for routine control in analytical labo-
ratories. A promising approach using a benchtop Orbi-
trap mass spectrometer for mass accuracy improvement
(mass selection window less than 3 ppm) [18] may also
be considered.
As a general comment the methods developed and vali-
dated for feed are often not suitable to petfood as the two
categories of products present very different compositions.
5. Acknowledgements
The authors are thankfull to the Eurofins laboratory who
have conducted the colorimetric and enzymatic analyses
of pet food samples.
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