2094 M. Beneito-Cambra et al.
by volatilization, the water content of the samples must be
reduced with particular care, which increases analysis time.41
Derivatization with cyclic anhydrides is, by contrast, tolerant
to the presence of small amounts of water in the samples.35–38
Chromium(VI) oxide (CrO3) in aqueous media containing
sulfuric or acetic acid (Jones’ reagent) is a well-known
reagent used in the oxidation of primary alcohols to car-
boxylic acids.45,46 The carboxylate group has a low molar
absorptivity and is therefore of little interest for UV-vis
detection, but can be detected with high sensitivity by MS,
particularly when the alkyl chain contains a large number
of carbon atoms.47 Thus, in this work, the feasibility of MS
determination of non-ethoxylated primary alcohols pre-
viously oxidized to carboxylic acids with the Jones’ reagent
was investigated. In the studied procedure, excess reagent
was removed with sodium sulfite and the oxidized alcohols
were separated from the resulting chromium(III) salts by
extraction in a mixture of ethyl acetate and acetone. After the
addition of an amine to increase the pH, and water to further
enhance ionization of the carboxylate groups, the extracts
were directly infused into the ESI source of the mass spect-
rometer. The study was also extended to ethoxylated
alcohols (cellosolves and FAEs), which yielded the corre-
sponding ethoxycarboxylic acids. The procedure can be
applied to the characterization and determination of fatty
alcohols in complex samples, such as cosmetics and body-
care products, and in environmental samples such as
seawater.
carbon atoms in the alkyl chain and m is the number of
ethoxy (EO) units (e.g., C10E0 and C10E2 stand for 1-decanol
and diethoxylated 1-decanol, respectively). Oxidation of
these compounds with chromium(VI) oxide yields the
corresponding carboxylic and ethoxycarboxylic acids, which
are designated below as CnEmA (e.g., C10E0A and C10E2A
stand for 1-decanoic and diethoxylated 1-decanoic acids,
respectively).
Acetic acid, methanol, acetone, ethyl acetate, acetonitrile
(ACN), hydrochloric and sulfuric acids (Scharlab, Barcelona,
Spain), butylamine (Fluka, Buchs, Switzerland), 3,4,5-trimet-
hoxybenzoic acid (TMBA, as internal standard, Sigma-
Aldrich, Steinheim, Germany), anhydrous sodium sulfite,
and chromium(VI) oxide (Panreac, Barcelona) were of
analytical grade. Non-ethoxylated alcohols C3E0 (Scharlab),
C8E0, C10E0, C12E0, C14E0, C16E0, and C18E0 (Fluka);
ethoxylated alcohols C2E1, C4E2, C8E1, C12E1, C12E2
(Fluka), C12E3, C12E4, C12E5, C12E6 (kindly donated by
C. Solans, CSIC, Barcelona), and C18E1 (Fluka); carboxylic
acids C2E0A, C3E0A, C8E0A, C10E0A, C12E0A (Fluka),
C14E0A, C16E0A, and C18E0A (Sigma-Aldrich); and ethox-
ycarboxylic acids C1E1A, C1E2A, C1E3A, and C2E1A
(Fluka) were used as standards. Deionized water (Barnstead
deionizer, Sybron, Boston, MA, USA), and the industrial raw
material FINDET 10/18 (a mixture of C10Em oligomers with
a nominal EO average of 6, also containing ca. 20% water,
Molins-Kao, Barcelona) were also used. Cosmetics and body-
care products were purchased at local shops. Seawater
samples were collected at a beach near the urban area of La
Pobla de Farnals (Valencia, Spain).
EXPERIMENTAL
Instrumentation and MS working conditions
The MS system consisted of a 1100 series VL ion trap and
was equipped with an ESI source (Agilent Technologies,
Waldbronn, Germany). Test solutions were infused into the
ESI-MS system using a syringe pump (kdScientific, Hollis-
ton, MA, USA) set at a constant flow of 0.3 mL hꢁ1 (5 mL
minꢁ1). MS was carried out within the m/z 50–800 range
in the negative-ion (NI) and positive-ion (PI) modes. The
capillary voltage was 4 kV, skimmer 2 was set at 6 V, and
the voltage of skimmer 1 was automatically selected as a
function of the target mass. Maximum loading of the ion
trap was 3 ꢂ 105 counts, and maximum collection time was
300 ms. Nitrogen (N2FLO generator, Claind, Lenno, Italy)
was used as both the nebulizing gas (15 psi) and the
drying gas (3 L minꢁ1 at 2508C), and helium (C-50, Carburos
Recommended procedures
Unless otherwise indicated, Jones’ reagent was prepared
with 6.7 g CrO3 and 6 mL sulfuric acid, completing the
volume to 50 mL with water.46 Stock solutions of standards
were prepared by dissolving the alcohols (50 mg each) in
acetone (25 mL). Aliquots of these solutions (250 mL) were
introduced into screw-capped centrifuge tubes, the volume
was completed to 3 mL with acetone, and 1 mL of Jones’
reagent was added dropwise under constant stirring at room
temperature. After 5 min, the precipitate of chromium(III)
salts was dissolved by the addition of 0.5 mL 2 M HCl. The
yellowish color indicated the presence of excess reagent,
which was removed by shaking the mixture with ca. 150 mg
Na2SO3. Then 4 mL of ethyl acetate were added to each of the
tubes, which were then vortex-stirred. Centrifugation was
used to accelerate separation of the mixture into two layers.
The non-colored and transparent upper layer (with a volume
of ca. 7 mL) contained the resulting carboxylic and ethoxy-
carboxylic acids in an ethyl acetate/acetone medium (4:3, v/
v). The chromium(III) salts remained in the water-rich lower
layer (with a volume of ca. 1 mL). A 2-mL aliquot of the
organic layer was mixed with 0.2 mL of a 0.33 M butylamine
aqueous solution containing 5.5 mM TMBA (internal stan-
dard). This mixture was directly infused in the MS system.
Portions of FINDET 10/18 (1 g), cosmetics, and body-care
products (2 g) were magnetically stirred with 10 mL acetone
for 10 min and then centrifuged, after which aliquots of
the supernatant were withdrawn. As shown later in this
work, reduction of the water concentration before the
´
Metalicos, Aranjuez, Spain) as the collision gas. In quanti-
tation studies, to minimize the influence of the ion-trap target
mass on the sensitivity, peak intensities (including those of
the analytes and the internal standard) were determined by
using the mass of the addressed peak as the target mass. All
measurements were averaged over a period of 2 min. The
data were analyzed using the Agilent LC/MSD version 4.2
software. The effect of temperature on the reaction yield was
studied using a water bath equipped with a temperature
probe (RTC þ ETS-D4, IKA, Staufen, Germany).
Reagents and samples
Linear non-ethoxylated and ethoxylated primary alcohols
are designated below as CnEm, where n is the number of
Copyright # 2010 John Wiley & Sons, Ltd.
Rapid Commun. Mass Spectrom. 2010; 24: 2093–2100
DOI: 10.1002/rcm