Anal. Chem. 1985, 57, 243-245
243
Trace Determination of Aldehydes in Water by High-Performance Liquid Chromatography Katsushige Takami,* Kazuhiro Kuwata, Akiyoshi Sugimae, and Masao Nakarnoto Environmental Pollution Control Center, 62-3, 1 Chome, Nakamichi, Higashinari-ku, Osaka City 537, J a p a n
A ilquld chromatographlc method Is presented to determine trace amounts of aliphatic and aromatic aldehydes in water by using an Innovative sampllng cartridge. The PTFE cartrldge used was packed with a a moderately sulfonated cation exchange resin charged wlth 2,4-dlnltrophenylhydrazlne. A 20-5OO-mL volume of water sample was passed through the cartridge. The 2,4-dlnitrophenylhydrazone derivatives of the aldehydes on the resin were selectively eluted wlth acetonitrlie. The eluate was analyzed by high-performance liquid chromatography wlth a 3-pm ODS column. With a 500-mL water sample, the detection limits were 0.6 pg/L for formaldehyde and 0.3 pg/L for acetaldehyde, propionaldehyde, and benzaldehyde. The aldehydes could be determined wlth 1.7-3.7% relative standard devlatlon in the range 1-10 pg/L.
Aldehydes are important substances in studies of environmental pollution. The method often used to determine aldehydes in the environment is to form their 2,4-dinitrophenylhydrazine derivatives and determine them by gas chromatography (1,2), high-performance liquid chromatography (HPLC) (3-7), or thin-layer chromatography (8). In studies of air pollution, a convenient silica column in which 2,4-dinitrophenylhydrazine(DNPH) and hydrochloric acid are coated is used to trap low parts-per-million (ppm) levels of formaldehyde (4). Glass cartridges packed with glass beads impregnated with DNPH in phosphoric acid saturated poly(ethy1ene glycol) were used to sample parts-per-billion (ppb) or low parts-per-billion levels of formaldehyde, acetaldehyde, benzaldehyde, and other carbonyl compounds, and the DNPH derivatives of carbonyl compounds are determined by HPLC (5). Recently, a rapid HPLC method using SepPAK CI8 cartridges impregnated with pure DNPH and phosphoric acid is reported to determine low molecular weight aliphatic aldehydes at low parts-per-billion levels with minimized background effects (7). However, these cartridges are unusable to trap aldehydes in water samples because the coated DNPH and the acids easily leave the cartridge with water. The extraction of aldehydes or their DNPH derivatives from water medium with organic solvent described elsewhere (3) encounters troublesome background peaks. So far, few reports have been published concerning effective enrichment and trace determination of aldehydes in water samples such as rainwater and river water. In this study, a rapid HPLC method using a convenient cartridge is presented to det9rmine formaldehyde, acetaldehyde, propionaldehyde, and benzaldehyde in water samples at the low micrograms-per-liter levels. The poly(tetrafluoroethylene) (PTFE) cartridge used was packed with a moderately sulfonated cation-exchange resin charged with DNPH. The aldehydes were trapped from water on the surface of the resin and derivatized by the catalytic action of the sulfonic groups. The resulting products were selectively eluted with acetonitrile. The use of the cartridge resulted in minimized background effects.
EXPERIMENTAL SECTION Reagents and Materials. The acetonitrile used was of chromatographic grade from Wako (Osaka, Japan). The 2,4dinitrophenylhydrazine (DNPH) from Wako was recrystallized twice with acetonitrile. The aldehyde-free water was prepared by redistilling deionized distilled water by an earlier reported method (8). The formaldehyde and acetaldehyde used were of reagent grade from Wako, and their purity was determined by using their DNPH derivatives. The propionaldehyde and benzaldehyde used were of reagent grade from Tokyo Kasei (Tokyo, Japan). Other chemicals were of commercially available reagent grade. The standard solution for calibration was prepared prior to use by dissolving the aldehydes in redistilled water and appropriately diluting the solution with redistilled water. The heat-shrinkable PTFE tube (7.6 mm i.d. and 4.6 mm i.d.) which shrinks at 350-400 "C was from Norton Chemiplast, Inc. (Wein, NJ). Mitsubishi Kasei (Tokyo, Japan) HP-20 AG cross-linked polystyrene bead (50-100 mesh) was used for preparation of the cation exchange resin. Preparation of the Low-Capacity Cation Exchange Resin. Four grams of cross-linked polystyrene beads was mixed with 20 mL of sulfuric acid, and the mixture was stirred at 100 "C for 5 min. The mixture was poured into 300 mL of water and filtered through a glass-sinteredfilter. The sulfonated resin was washed with acetonitrile in a Soxhlet extractor for 8 h and dried at room temperature for 24 h. Cation exchange capacity was determined in the following way. A definite amount of the resin was charged with sodium ion in an alkaline medium. The exchanged resin was washed with redistilled water. The sodium that exchanged was removed with an acidic medium and determined by atomic absorption spectrometry. The ion exchange capacity of the resin was calculated from the amount of the sodium determined. Preparation of the Sampling Cartridge. A glass-sintered filter (pore size 50 bm, 2 mm thickness X 7.6 mm i.d.) was set 2 cm from one end of the PTFE tubing (7 cm X 7.6 mm id.) and fixed by heating from the position of the filter to the end at 350-400 "C. A 0.7-g amount of the resin was tightly packed into the tubing and plugged with another filter. The filter was fixed by heating in the same way. A joint was attached to the cartridge to sample water from a funnel. The cartridge was washed with 10 mL of 1 N sulfuric acid and 50 mL of redistilled water. A 30-mL volume of 0.08% DNPH in 1.2 N hydrochloric acid was passed at 2 mL/min through the cartridge. The cartridge was washed with 20 mL of redistilled water. The sampling cartridge in which the DNPH was adsorbed was plugged with glass stoppers and stored in a cool place (3-5 "C) in the dark. The sampling cartridge was conditioned prior to use by washing with 3 mL of acetonitrile and 10 mL of redistilled water and then by passing 5 mL of 0.08% DNPH in 1.2 N hydrochloric acid. Figure 1shows the cartridge for sampling aldehydes in water medium. Apparatus. A Waters Associates (Milford, MA) ALC/GPC 244 liquid chromatograph with a U6K injector and an ultraviolet absorbance detector adjusted to 365 nm was employed. The analytical column used was a 150 mm X 4 mm i.d. stainless steel tube packed with Develosile ODs-3 (3 bm) (Nomura Kagaku, Aichi, Japan). The mobile phase was acetonitrile/water (13/7 or 3/2, v/v), and the flow rate was 0.6 mL/min. Sampling and Analytical Procedure. A 20-500-mL volume of water sample was sampled by passing the sample at 2-5 mL/min through the cartridge. The water in the cartridge was purged with nitrogen gas at 30 mL/min for 20 s. The resulting products were eluted with acetonitrile, and an initial 2 mL of eluate was collected. A 2-5-wL aliquot of the eluate was analyzed
0003-2700/85/0357-0243$01.50/00 1984 Arnerlcan Chemical Soclety
244
ANALYTICAL CHEMISTRY, VOL. 57, NO. 1, JANUARY 1985
Table I. Collection Efficiency of the Aldehydes on the Cartridge collection efficiencv. % sample volUme: mL ~
amt spiked, pg
compound
formaldehyde acetaldehyde propionaldehyde benzaldehyde 'Synthetic samples were used.
5.00 4.85 6.23 5.81
50
100
200
500
av
hSDb
% RSDc
94.9 97.8 95.6 96.7
94.4 98.7 99.5 96.1
94.9 98.0 94.1 97.3
94.9 99.6 97.5 96.1
94.6 98.5 97.6 96.6
0.39 0.81 1.90 0.57
0.42 0.82 1.95 0.59
*Standard deviation. Relative standard deviation.
3
Figure 1. Sampling cartridge of aldehydes in water sample: (1) heat shrinkable PTFE tube (7.6 mm i.d.), (2) cation exchange resin, (3) glass-sintered filter (pore size, 50 pm), (4) glass wool, (5) glass tube, (6) heat shrinkable PTFE tube (4.5 mm Ld.), (7) joint for sampllng water, (8) sample flow direction.
by HPLC. The identification of the aldehyde was made by retention time, and the quantitation was performed by peak height. The cartridges were reused by washing them with 6 mL of redistilled water prior to the addition of another sample.
RESULTS AND DISCUSSION The cation exchange resins with 0.4-1.0 mequiv/g of ion capacity gave acidity corresponding to 1-2 N hydrogen ion on the surface where the aldehydes optically reacted with DNPH. The recovery of the aldehydes was reduced from a water medium with resin with less than 0.2 mequiv/g, or more than 2 mequiv/g of ion capacity (35-77.2% of recovery with a 5 mequiv/g resin). The DNPH that could be charged on the resin was proportional to its ion capacity. The cartridges packed with the 0.4-1.0 mequiv/g resins were charged with 20-30 mg of DNPH, which was a suitable amount for sampling and determining the aldehydes in water medium at microgram per liter levels. Lower capacity resins resulted in an insufficient amount of DNPH in the cartridge while higher capacity resins gave excessive charge of DNPH in the cartridge (ca. 100 mg of DNPH with a 5 mequiv/g resin). From the results, the resin with 0.6 mequiv/g of ion capacity was synthesized and used for analysis. With this resin, ca. 25 mg of DNPH was charged in the cartridge. The DNPH that charged was eluted from the cartridge with sample water at a concentration of 5 pg/mL. However, after 500 mL of the redistilled water was passed, more than 20 mg of the DNPH was present in the cartridge. The aldehydes reacted with DNPH in the water medium to form their hydrazones in the cartridge. The DNPH derivatives were selectively eluted with acetonitrile from the cartridge. Most of the DNPH that unreacted remained in the cartridge. The DNPH derivatives were completely eluted with an initial 1 mL of the eluent regardless of ion capacity of the resin, and none of the DNPH derivatives was found in the later eluate. Therefore, the initial 2 mL of the eluate was collected for the analysis. The eluted samples were stable for more than a month in a dark and cool place (1-3 "C). On the resin preliminarily exchanged with cations such as sodium, potassium, and metallic ions, DNPH was insufficiently charged in the cartridge and did not react with the aldehydes in water medium. On the other hand, the DNPH that charged on the resin was easily replaced by such cations. The reaction of the aldehydes with DNPH was inhibited as
H-N-NII e R-C-H 0II
+
@NO,
H-N-N=C-R
('It),
@NO,
NO,
Aldehyde
t
H,O
NO,
IIy d r a z o n e
DNPII
Flgure 2. Suggested schematic of reaction mechanism of aldehydes and DNPH and interaction between the related compounds and the resin: (-) hydrophobic interaction: (- -) ionic interaction; (=) catalytic action.
-
the cations were charged in the cartridge with water sample at low millimole levels of the cations since the capacity of resin packed was ca. 0.4 mequiv. In the case of water sample containing more than 0.3 mM of total cations, the sample should be acidified with 1 N sulfuric acid to bring the pH lower than 3 to recover the reaction. These aspects suggest that the reaction depended upon catalytic action of the sulfonic groups on the resin. Figure 2 shows a suggested reaction mechanism of the aldehydes with DNPH and hypothetical interactions between the related substances and the cation exchange resin in water medium. The collection efficiency of the aldehydes was investigated by using synthetic samples at 0.01-0.12 pg/mL levels prepared from the redistilled water. Volumes of 50-500 mL of the samples were sampled at 5 mL/min. Table I demonstrates that satisfactory collection efficiency could be obtained in the sampling system. The sampling rate should not exceed 5 mL/min since the collection efficiency was decreased at a higher sampling rate. Figure 3 shows a typical chromatogram in the blank test with 500 mL of redistilled water. The blank peaks of formaldehyde and acetaldehyde corresponded to 0.3 and 0.1 Hg/L levels, respectively. No other aldehydes were detected. The background depended primarily upon the redistilled water. Minor background came from the reagent used and from decomposition of the resin. Since formaldehyde and acetaldehyde were produced at 1-2 pg/week in the conditioned cartridge, preconditioning of the cartridge was required before use. The detection limits, defined as twice the blank peaks, were 0.6 pg/L for formaldehyde and 0.3 pg/L for acetaldehyde, propionaldehyde, and benzaldehyde with 500 mL of water sample. The calibration range for the aldehydes was examined by using 20 mL of redistilled water containing 1-6 pg of the aldehydes. Excellent linearity was obtained for calibration
ANALYTICAL CHEMISTRY, VOL. 57, NO. 1, JANUARY 1985
245
Table IV. Aldehydes in Rainwater and River Water 1
sample rainwater
LL
A
3
B
N
C
2x
9
river water
4
I
concn of aldehyde," rg/L FA AA PA BA
sample vo1,mL
170
70 80 200 80 150 20
D E F
184 40 83 15 66
52 38 6.5 11 1.0
NDb
4.8 3.0 0.7 2.1
NDb ND
NDb 0.8
ND ND ND ND
'FA, formaldehyde; AA, acetaldehyde; PA, propionaldehyde; BA, benzaldehyde. Not detectable.
I
I
0
5
I
15
10
20
Retention (mln)
Figure 3. Typical liquid chromatogram in the blank test (mobile phase, acetonitrile/water (13/7, v/v); sample size 2 pL): (1) DNPH, (2) formaldehyde, (3) acetaldehyde, (4) acetone. AUF is absorbance unit for full scale.
Table 11. Recovery of the Aldehydes from River Watern
compound formaldehyde acetaldehyde propionaldehyde benzaldehyde
amt spiked, amt found, recovery:
RSD,"
rg
rg
%
%
5.00 4.85 6.23 5.81
4.80 4.74 6.02 5.57
96.0 97.8 96.6 95.8
2.3 1.6 1.1 1.4
'A 200-mL volume of river water was used for the recovery test.
bAverage in seven runs. Relative standard deviation.
Table 111. Recovery of the Aldehydes from Seawater'
compound formaldehyde acetaldehyde propionaldehyde benzaldehyde
amt spiked, amt found,b recovery,b RSD," rg rg % % 5.00 4.85 6.23 5.81
4.94 4.80 6.25 5.88
98.7 99.0 100.4 101.2
1.9 0.8 0.9 1.1
'A 50-mL volume of seawater was used for the recovery test. To eliminate the effects of cations, sulfuric acid was added to bring the concentration to 0.05 M. bAverage in five runs. 'Relative standard deviation.
L
0
peak heights. The aldehydes were determined with 1.7-3.7% relative standard deviation in the range. Analyte recovery was investigated by using 200 mL of river water spiked with 5-6 pg of aldehydes and 50 mL of seawater spiked with 5-6 pg of the aldehydes. Table I1 reports that 94-98% of the aldehydes was recovered from the river water with 1.1-2.3% relative standard deviation. In the case of the seawater, sulfuric acid was added to bring the concentration to 0.05 M to avoid the effects of the cations on the determination. Table I11 reports that 98-101% of the aldehydes were recovered from seawater with 0.8-1.9% relative standard deviation.
20
30
40
Retention (rnin)
Figure 4. Typical liquid chromatogram for aldehydes in rainwater (mobile phase, acetonitrile/water (3/2, v/v)): (1) formaldehyde, 0.1 84 pg/mL; (2) acetaldehyde, 0.038 pg/mL; (3) propionaldehyde, 0.003 pg/mL; (4) benzaldehyde, 0.0008 pg/mL.
The sampling cartridges were easy to prepare and easy to handle in sampling and analyzing large numbers of water samples. A number of rainwater samples and river water samples were analyzed by use of proposed method, and traces of the aldehydes from these samples were successfully determined. Table IV reports typical analytical data for the aldehydes in rainwater and river water. Figure 4 shows a typical liquid chromatogram from a rainwater. As these features indicate, the proposed method may be useful in routinely analyzing a large number of water samples for studies of water pollution and acid rain. Registry No. DNPH, 119-26-6; FA, 50-00-0; AA, 75-07-0; PA, 123-38-6; BA, 100-52-7; water, 7732-18-5.
in this concentration range. The correlation coefficients were 0.996-0.999 between the concentrations of aldehydes and the
10
LITERATURE CITED (1) Kaliio, H.; Linko, R. R.; Kaltaranta, J. J . Chromatogr. 1972, 65,
355-360. (2) Hoshika, Y.; Takata, Y. J . Chromatogr. 1976, 720, 379-389. (3) Kuwata, K.; Ueborl, M.; Yamazaki, Y. J. Chromatogr. Sci. 1979, 77, 264-268. (4) Beasley, R. K.; Hoffman, C. E.; Rueppel, M. L.; Woriey, J. W. Anal. Chem. 1980, 52, 1110-1114. (5) GrosJean, D.; Fung, K. Anal. Chem. 1982, 54, 1221-1224. (6) Creech, G.; Johnson, R. T.; Stoffer, J. 0. J . Chromatogr. Sci. 1982, 2 0 , 67-72. (7) Kuwata, K.; Ueborl, M.; Yamasaki, H.; Kuge, Y.; Kiso, Y. Anal. Chem. 1983, 55, 2013-2019. (8) Hino, T.; Kaiho, S. Bunseki Kagaku 1977, 26, 451-455.
RECEIVED for review May 29,1984. Accepted September 10, 1984.
