Lee Wah Lim

Rapid determination of bromide in seawater samples by capillary ion chromatography using monolithic silica columns modified with cetyltrimethylammonium ion.

Monolithic silica capillary columns dynamically modified with quaternary ammonium ions were evaluated for the determination of bromide in seawater samples. A quaternary ammonium ion such as cetyltrimethylammonium ion was dynamically introduced onto monolithic silica surfaces. The first layer of the modifier was introduced by electrostatic interaction, whereas the second layer was introduced by hydrophobic interaction. The latter layer worked as the anion-exchange sites. The modified monolithic silica capillary columns could be used for rapid separation of inorganic anions. Separation of authentic mixture of five anions was achieved within a few minutes. The addition of small amount of the modifier in the eluent improved the repeatability of the retention time. Seawater samples could be directly injected onto the prepared capillary columns, and bromide could be determined to be 63mg/L.

Simultaneous determination of antidepressants by non-aqueous capillary electrophoresis-time of flight mass spectrometry.

Simultaneous determination of 20 antidepressants in plasma samples was carried out by non-aqueous capillary electrophoresis with time of flight mass spectrometry via electrospray ionization, where a mixture of 60 mM ammonium acetate and 1M acetic acid in acetonitrile, and water, as well as methanol (100:1:0.5, v/v/v) was selected as the background electrolyte. By using time of flight mass spectrometry, accurate mass information was obtained and the background noise was dramatically decreased, thus causing a great improvement in qualitative ability. As for the plasma sample, solid phase extraction with Oasis HLB was used. The limits of detection and quantification were in the range of 0.5-1 and 1-5 ng/ml, respectively. The sensitivity of the present method was found better, i.e. approximately 10-60 folds compared to that using photo diode array detectors because the analyte peak could be clearly distinguished from the background derived from the plasma. The present method was found very useful and practical as regards to routine analysis of plasma samples.

Determination of iodide in seawater using C30 column modified with polyoxyethylene oleyl ether in ion chromatography.

An ion chromatographic method for rapid and direct determination of iodide in seawater samples is reported. Separation was achieved using a laboratory-made C30 packed column (100 mm x 0.32 mm i.d.) modified with polyoxyethylene oleyl ether, with an aqueous solution of 300 mM sodium chloride as eluent and using UV detection at 220 nm. Samples containing iodate, nitrate, iodide and thiocyanate were eluted within 8 min, and the relative standard deviations of the retention time, peak area and peak height were all smaller than 4.19% for all of the analyte anions. Effects of eluent composition on retention behavior of inorganic anions have been investigated. Both cation and anion of the eluent affected the retention time of analytes. When inorganic eluents, such as ammonium chloride, ammonium sulfate, lithium chloride, sodium chloride, sodium sulfate, magnesium chloride and magnesium sulfate were used, the retention time of analytes increased with increasing eluent concentration. The limit of detection of iodide was 19 microg l(-1) (S/N=3), while the limit of quantitation was 66 microg l(-1) (S/N=10). The present method was successfully applied to the rapid and direct determination of iodide in seawater samples.

On-line preconcentration of trace carcinogenic polycyclic aromatic hydrocarbons (PAHs) in microcolumn liquid chromatography via large volume injection.

The ability and efficiency of micro precolumns made of C30 particles, monolithic silica C18 stationary phase and quartz wool coated with C30, which act as novel solid phase absorbing materials, for the on-line enrichment of aqueous polycyclic aromatic hydrocarbons (PAHs) in microcolumn liquid chromatography (LC) was investigated. The enrichment unit was designed in such a way that micro precolumns were directly connected to a 6-port micro injection valve via fused-silica tubing (0.05 mm I.D.) in order to minimize band broadening of the samples, and the enrichment efficiency of the three materials was tested using 14 PAHs, which are selected by the US Environmental Protection Agency (US EPA), as the analytes. The separation of PAHs was evaluated by using laboratory-made C30 or ODS capillary columns and the results were compared. There were no significant differences showed from the separation of PAHs in terms of peak signal between the C30 and ODS capillary columns, but the C30 capillary column was chosen for the following experiment due to its ability to produce better repeatability than the ODS column. By using the three kinds of precolumn materials, results showed that the precolumn packed with C30 particles as well as the capillary monolithic C18 precolumns (0.1 or 0.2mm I.D.) provided better recovery than those of the quartz wools. As long as the recovery and separation of the PAHs were concerned, 0.1mm I.D. monolithic C18 precolumn showed the best results and the R.S.D.s (N=7) for the retention time, peak area and peak height were between 0.70-1.5, 2.3-5.8 and 2.4-6.6%, respectively. Large volume injection up to 0.5 mL, i.e. 2500-fold enrichment, was possible and no negative effect on the separation profile was found. The LOD (S/N=3) were between 0.10 and 4.6 pg mL(-1), while the LOQ (S/N=10) were in the range of 0.32-15 pg mL(-1), which showed that the system is comparable to many major analytical techniques and is sensitive enough for the trace analysis of PAHs in environmental samples. The system was then applied to the determination of trace PAHs present in soil sample which was randomly taken from a nearby highway.

Development of packed-column suppressor system for capillary ion chromatography and its application to environmental waters

A novel suppressor unit for capillary ion chromatography was designed to reduce the background conductivity and at the same time to increase the analyte signal. Regeneration of the suppressor was carried out on-line by passing an appropriate acidic solution through the column to displace the accumulated eluent cations. By using two 6-port microswitching valves and two packed capillary column suppressors, the background conductivity of sodium carbonate-bicarbonate mobile phase was maintained at low conductivity for continuous chromatographic runs, and the detection limits at low ppb levels were achieved. The relative standard deviations (RSDs) for the retention time, peak area and peak height of six common inorganic anions (0.05 mM each of F(-), Cl(-), NO(2)(-), Br(-), NO(3)(-) and SO(4)(2-)) were between 0.5-0.9, 1.1-4.6 and 0.7-4.9%, respectively. The present system was successfully applied to the determination of inorganic anions in river water and tap water samples.

Determination of common inorganic anions and cations by non-suppressed ion chromatography with column switching.

An ion chromatography (IC) method has been proposed for the determination of seven common inorganic anions (F(-), H(2)PO(4)(-), NO(2)(-), Cl(-), Br(-), NO(3)(-), and SO(4)(2-)) and/or five common inorganic cations (Na(+), NH(4)(+), K(+), Mg(2+), and Ca(2+)) using a single pump, a single eluent and a single detector. The present system used cation-exchange and anion-exchange columns connected in series via a single 10-port switching valve. The 10-port valve was switched for the separation of either cations or anions in a single chromatographic run. When 1.0mM trimellitic acid (pH 2.94) was used as the eluent, the seven anions and the five cations could be separated on the anion-exchange column and the cation-exchange column, respectively. The elution order was found to be F(-)<H(2)PO(4)(-)<NO(2)(-)<Cl(-)<Br(-)<NO(3)(-)<SO(4)(2-) for the anions and Na(+)<NH(4)(+)<K(+)<Mg(2+)<Ca(2+) for the cations. Complete separation of the above anions or cations was demonstrated within 35min each. Detection limits calculated at S/N=3 were 0.05-0.58ppm (mg/l) for the anions and 0.05-0.38ppm for the cations, whereas repeatability values were below 2.26, 2.76, and 2.90% for peak height, peak area and retention time, respectively. The method was successfully applied to the determination of inorganic anions and cations in river water samples.

Curcuminoid’s Content and Fingerprint Analysis for Authentication and Discrimination of Curcuma xanthorrhiza from Curcuma longa by High-Performance Liquid Chromatography-Diode Array Detector

An accurate and reliable method for authentication and discrimination of Curcuma xanthorrhiza (CX) from Curcuma longa (CL) by determining the curcuminoid’s content and analyzing the HPLC fingerprint combined with discriminant analysis (DA) was developed. By using the proposed method, it was found that CL had higher amount of all curcuminoid compounds compared to CX. Therefore, these two closely related species could be authenticated and discriminated by the amount of curcuminoids present in the samples. Authentication and discrimination of the two species were also achieved by comparing their HPLC fingerprint chromatograms using their typical marker peaks. To be more convincing, an aid from DA was also used. Combination of HPLC fingerprint analysis and DA gave excellent result that the two species were separated clearly, including CX samples adulterated with CL. The developed method was successfully used for quality control of the two plants.

Simultaneous determination of gingerols and shogaol using capillary liquid chromatography and its application in discrimination of three ginger varieties from Indonesia

A new method using reversed phase capillary liquid chromatography was developed for simultaneous determination of four bioactive compounds found in ginger (Zingiber officinale) namely, 6-, 8-, 10-gingerol, and 6-shogaol. The separation of these four compounds was performed using C30 as the stationary phase and 60% acetonitrile as the mobile phase in isocratic elution mode with a flow rate of 5 μL/min. All four compounds were separated within 25 min with good resolution. As the evaluation of method validation, a linear regression of the four compounds was obtained within the tested range with correlation coefficients ≥ 0.9995. The limits of detection and quantitation were between 0.034-0.039 μg/mL and 0.112-0.129 μg/mL, respectively. Intra- and inter-day precision expressed as relative standard deviations (RSD) were less than 3.1%, and the accuracy based on recovery test was ranging from 97% to 105%. Stability of the analytes within 1 day was found in the range between 1.34% and 2.93% (RSD). In addition, based on the amount of these four compounds combining with the discriminant analysis, a reliable and accurate method was developed for discrimination of three ginger varieties found in Indonesia. The results indicated that the developed method could be used as quality control for ginger raw material and its related products.

Tunable separation of anions and cations by column switching in ion chromatography.

A convenient ion chromatography method has been proposed for the routine and simple determination of anions (Cl(-), SO(4)(2-) and NO(3)(-)) and/or cations (Na(+), NH(4)(+), K(+), Mg(2+) and Ca(2+)) using a single pump, a single eluent and a single detector. The present system used cation-exchange and anion-exchange columns connected in series via two 6-port switching valves or a single 10-port valve. The connection order of the ion-exchange columns could be varied by switching the valve(s). The present system therefore allowed the separation of either cations or anions in a single chromatographic run. While one ion-exchange column is being operated, the other ion-exchange column is being conditioned, i.e., the columns are always ready for analysis at any time. When 2.4mM 5-sulfosalicylic acid was used as the eluent, the three anions and the five cations could be separated on the anion-exchange column and cation-exchange column, respectively. In order to obtain the separations of the target ions, the injection valve was placed between the two columns. Complete separations of the above anions or cations were demonstrated within 10min each. The detection limits at S/N=3 were 19-50ppb (mug/l) for cations and 10-14ppb for anions. The relative standard deviations of the analyte ions were less than 1.1, 2.9 and 2.8% for retention time, peak area and peak height, respectively. This proposed technique was applied to the determination of common anions and cations in river water samples.

Polymer monolithic methacrylate base modified with tosylated-polyethylene glycol monomethyl ether as a stationary phase for capillary liquid chromatography

A polyethylene glycol (PEG) monolithic column was successfully prepared in situ for the separation of inorganic anions in ion exchange capillary chromatography. By attaching PEG-groups into the methacrylate-based polymer, the number of theoretical plates was improved from 1433 to 3346 plates/m (when nitrate was used as the analyte). The retention behavior of iodate, bromate, nitrite, bromide and nitrate was observed under various salt aqueous solutions. The retention was based on cations trapped among PEG chain and the positively charged pyridine that work as the anion exchange sites in the PEG monolith. The relative standard deviations (RSDs, for n=7) of retention time, peak height and peak area were less than 2.27% for all the analyte anions. The PEG monoliths showed satisfactory mechanical stability and did not swell or shrink significantly with swelling propensity value of 0.34 and 0.64 for methanol and THF, respectively. This stationary phase was successfully applied to the determination of these anions in seawater as well as public drinking water samples.