Pre-column Fluorescence Derivatization Research Articles

Highly selective and sensitive high performance liquid chromatography method for determination of fumonisins from feed samples with MIL-101 based dispersed micro-solid-phase extraction and precolumn fluorescence derivatization

A selective and rapid high-performance liquid chromatography with fluorescence detection (HPLC-FLD) method for determining fumonisins was established and validated, which based on dispersed microsolid-phase extraction (D-μ-SPE) and precolumn derivatization. The MIL-101 was prepared as sorbents to selectively enrich two fumonisins from feed samples. The obtained extracts were labeled by a novel derivatization reagent 2,5-dioxopyrrolidin-1-yl 4-(1-methyl-1H-phenanthro[9,10-d]imidazol-2-yl)benzoate (MPIBA-Su). There was no fluorescence from fumonisins itself, while their derivatives exhibited considerable fluorescence with a maximum excitation at λex 255 nm and a maximum emission at λem 457 nm. Key factors affecting extraction efficiency and derivatization efficiency were fully optimized. Excellent linearity was achieved with correlation coefficients > 0.9935 under optimal conditions. The detection limits (LODs) were between 2.4 and 3.5 ng·mL−1, and the quantitation limits (LOQs) were between 8.1 and 11.7 ng·mL−1. With recoveries in the range of 83.8% − 94.3%, the established method has successfully been applied to the quantitative analysis of fumonisins in feed samples.

Determination of clobetasol propionate in cosmetics by high performance liquid chromatography with pre-column fluorescence derivatization of 4-hydrazino-7-nitrobenzo [C] [1,2,5] oxadiazole

In this study, a fluorescent labeling reagent 4-hydrazino-7-nitrobenzo [c] [1,2,5] oxadiazole (NBD-NHNH2) was synthesized and used for fluorescence detection of clobetasol propionate in cosmetics by high performance liquid chromatography. In the presence of acidic catalyst (pH = 6.0), NBD-NHNH2 reacted at 45 °C for 30 min, and clobetasol propionate can be labeled quickly and efficiently. These derivatives have excellent stability and fluorescent properties. The maximum excitation wavelength was 450 nm, and the emission wavelength was 550 nm. The four cosmetic formulations were completely separated on a Waters e2695 HPLC C18 column by gradient elution. Under the optimal conditions, the standard curve showed a good linear response, and the correlation coefficient of r 2 = 0.9999. The LOD and LOQ were 0.01 ng/mL and 0.03 ng/mL, respectively. Therefore, NBD-NHNH2 was successfully applied in the detection of clobetasol propionate illegal added in cosmetics.

In-Needle Pre-Column Derivatization for Amino Acid Quantification (iPDAQ) Using HPLC.

Pre-column fluorescent derivatization has been used for the fast quantification of amino acids using high-performance liquid chromatography (HPLC) systems. However, it generally requires an offline in-vial derivatization process with multiple derivatization reagents. The offline derivatization requires the same number of reaction vials as the number of sample vials for use as a reaction chamber for the derivatization reaction in an autosampler. Therefore, the number of samples analyzed per batch using the pre-column derivatization method is halved. To benefit from the pre-column derivatization method, we transformed the derivatization process from an offline chamber process to an online in-needle process (in-needle Pre-column Derivatization for Amino acids Quantification; iPDAQ). Fluorescent derivatization in the injection needle obviated the need for vacant vials as reaction chambers. Consequently, the throughput per batch improved up to two times, and the consumption of derivatization reagents was reduced to less than one-tenth of that in the conventional vial method. We demonstrated to separate and quantify the amino acids in various biological samples. Herein, we presented a novel HPLC-based amino acid quantification method that enables the continuous analysis of a large number of samples. The iPDAQ facilitates accurate amino acid quantification due to the automation of derivatization and achieves improvement in the throughput and reduction of analysis labor.

Determination of trans-fatty acids in food samples based on the precolumn fluorescence derivatization by high performance liquid chromatography.

Trans-fatty acids are unsaturated fatty acids that are considered to have health risks. 1,3,5,7-Tetramethyl-8-butyrethylenediamine-difluoroboradiaza-s-indacene is a highly sensitive fluorescent labeling reagent for carboxylic acids developed by our lab. In this study, using this precolumn fluorescent derivatization reagent, a rapid and accurate high-performance liquid chromatography with fluorescence detection method was developed for the determination of two trans-fatty acids in food samples. Under the optimized derivative conditions, two trans-fatty acids were tagged with the fluorescent labeling reagent in the presence of 1-ethyl-3-(3-dimethyl-aminopropyl) carbodiimide at 25°C for 30min. Then, the baseline separation of trans- and cis-fatty acids and their saturated fatty acid with similar structures was achieved with less interference using a reversed-phased C18 column with isocratic elution in 14min. With fluorescence detection at λex /λem =490 /510nm, the linear range of the TFAs was 1.0-200nM with low detection limits in the range of 0.1-0.2nM (signal-to-noise ratio=3). In addition, the proposed approach was successfully applied for the detection of trans-fatty acids in food samples, and the recoveries using this method ranged from 96.02 to 109.22% with low relative standard deviations of 1.2-4.3% (n=6).

Column-switching high-performance liquid chromatography-fluorescence detection method for malic acid enantiomers in commercial wines

Determination of malic acid (MA) in commercial drinks is imperative to assure the quality of taste and safety of the product to human health. In this study, a column-switching high-performance liquid chromatography (HPLC)-fluorescence detection method was employed to determine the content of d- and l-MA in wines. The developed method successfully detected both MA enantiomers in several wines with a pre-column fluorescence derivatization procedure. Additionally, the present HPLC system provided sufficient data with a small sample quantity. Moreover, the d-MA peak was detected in the chromatogram of wine samples without acidulants. Thus, we believe that the proposed method would benefit the wine-brewing process and facilitate quality certification in the wine industry.

Separation of vigabatrin enantiomers using mixed-mode chromatography and its application to determine the vigabatrin enantiomer levels in rat plasma

Mixed-mode chromatography—comprising a mixed phase with reversed and ionic phases, enabling hydrophobic and ion-exchange interactions simultaneously—was applied to identify vigabatrin enantiomers by HPLC with pre-column fluorescence derivatization with 2,5-dioxopyrrolidin-1-yl (4-(((2-nitrophenyl)sulfonyl)oxy)-6-(3-oxomorpholino)quinoline-2-carbonyl)prolinate (Ns-MOK-(S)-Pro-OSu). The MOK-(S)-Pro-vigabatrin enantiomers were efficiently separated within 12 min (total analysis time per sample: 28 min, including washing and equilibrium time for the column).The mobile phase was H2O/CH3OH/10 mM ammonium formate (pH 2.0) (20/20/60, v/v/v). Column temperature was maintained at 60℃. The proposed HPLC method could successfully monitor plasma vigabatrin enantiomer levels in rats administered (±)-vigabatrin (50 mg/kg, p.o.).

A sensitive HPLC-FLD method for the quantitative determination of 3-chloro-1,2-propanediol by pre-column fluorescence derivatization with 9-(2-Hydroxypropyl)adenine

In this paper, a sensitive method for the determination of 3-chloro-1,2-propanediol (3-MCPD) by high performance liquid chromatography-fluorescence detection (HPLC-FLD) was established, in which 9-(2-Hydroxypropyl) adenine (HP-Ade) was selected as the fluorescent derivatization reagent because of its good water solubility and hydrophobicity. The principle of the method lies in the origination of chloroacetaldehyde with the periodate oxidation of 3-MCPD and its' fluorescence derivatization by HP-Ade to produce a strong fluorescent substance, name as 1-N6 -vinylidene-9-(2-hydroxypropyl)adenine (ε-HP-Ade), used for quantitative determination of 3-MCPD by HPLC-FLD. Some mobile phase conditions and incubation parameters related with the method features were studied and optimized. Under the optimal conditions, the method shows high sensitivity, good linearity (r 2 = 0.999) and good repeatability (RSD% between 1.04% and 2.55%), the detection limit (LOD) and quantification limit (LOQ) were 0.02 ng/mL and 0.06 ng/mL respectively. It has been successfully applied for the determination of various edible vegetable oil samples, the chromatogram shows symmetrical peak shape, perfect separation and very “clean” profiles. In comparison with gas chromatography-mass spectrum (GC-MS), the pretreatment process of the proposed method was totally operated in aqueous phase, the instrument requirements and cost are relatively lower, thus it will provide another choice for the routine analysis of 3-MCPD.

Highly Sensitive Fluorescence Detection of Daptomycin in Murine Samples through Derivatization with 2,3-Naphthalenedialdehyde.

A highly sensitive high-performance liquid chromatography method has been developed using the pre-column fluorescent derivatization of daptomycin (DAP) through cyclization of the amino group of ornithine with 2,3-naphthalenedialdehyde. With the proposed method, the limits of detection and quantification of DAP in murine serum were 8 and 3 nmol/L, respectively, and the calibration curve was linear across the examined dynamic range from 8 nmol/L to 1 μmol/L (n = 8, r = 0.9986). This method is suitable for animal experiments examining the side effects of DAP therapy using mice as a simple method with quantification to the order of 10 nmol/L.

Development of HPLC method for estimation of glyoxylic acid after pre-column fluorescence derivatization approach based on thiazine derivative formation: A new application in healthy and cardiovascular patients’ sera

Glyoxylic acid (GA) is the intermediate metabolite in various mammalian metabolic pathways. GA showed high reactivity towards formation of advanced glycation end-products (AGEs); the main cause of pathogenesis and complications of many diseases. The presented study aimed to detect GA in healthy and cardiovascular patients’ (CV) sera; however analysis of GA in biological fluid is a challenge and requires chemical derivatization. Hence, a new, highly sensitive, time saving and reproducible pre-column fluorescence derivatization procedure coupled with high performance liquid chromatography (HPLC) method was developed. The derivatization method was based on reaction of 2-aminobenzenthiol (2-ABT), a fluorogenic reagent, with GA in acidic medium to form highly fluorescent thiazine derivative (290 and 390 nm for excitation and emission wavelengths respectively). The fluorescent derivative was separated within 6 min on a reversed-phase ODS column using an isocratic elution with a mixture of methanol–water (70:30, v/v%). The proposed method parameters were optimized and the method was validated. A good linearity in the concentration range (0.05–5.0 µM) was obtained with detection limit (LOD) of 10 nM (200 fmol/injection), which is more sensitive than several previous methods. Moreover, the recovery results were within the range of 85.0–95.5 % and the intra- and inter-day precision results were ≤3.5%. It should be emphasized that this method is the first one for monitoring of GA in CV patients; to investigate its role for diagnosis and monitoring the severity and complications of this disease in clinical laboratory.

Sensitive determination of malondialdehyde in rat prostate by high performance liquid chromatography with fluorescence detection

An excellent pre-column fluorescent derivatization reagent N-acetylhydrazine acridone for the quantitative determination of malondialdehyde was synthesized. Malondialdehyde was derivatized at 80 °C for 30 min in the presence of trichloroacetic acid. The separation of the derivative was performed on an Agilent ZORBAX SB-C18 column in conjunction with gradient elution. The excitation and emission wavelengths were 370 nm and 420 nm, respectively. The developed method demonstrated good linear relationship in the range of 0.02 pmol to 2.5 pmol (r = 0.9998). The calculated limit of detection and limit of quantification were 2.5 fmol and 8.3 fmol, respectively. The analytical precisions of the method were in the range of 1.36–2.27% (intra-day) and 2.36–3.92% (inter-day) respectively. The method was sensitive, specific and simple. It was successfully implemented to analysis the malondialdehyde in rat prostate.

Determination of amino acids content in tea species using liquid chromatography via pre-column fluorescence derivatization

A sensitive method for the simultaneous detection of eight amino acids (AAs), glycine, alanine, serine, glutamic acid, arginine, tyrosine, phenylalanine, and tryptophan, in different tea species has been developed and validated. This method was based on separation and detection of AAs using high performance liquid chromatography (HPLC), with a fluorescence detector that permits ultra-sensitive detection. Pre-column derivatization was performed to convert the non-fluorescent AAs into fluorescent species, using a specific reagent, o-phthaldialdehyde (OPA) and 3-mercaptopropionic (3MPA), in an optimized ratio of 3:1. A reversed-phase HPLC-column with two gradient phases, acetate buffer (pH 7.6) containing 3% tetrahydrofuran and a mixture of acetate buffer (pH 7.6): acetonitrile: methanol (1:2:2,v/v/v), were used for the separation and detection of AAs over the linear ranges of 0.5–50 nmol/L. Further, the method was validated according to the official guidelines and successfully applied to determine the AAs content in tea products (green, black, oolong, and white). Considerable variations were observed based on the tea quality and type. In summary, this study could be used for the on-line monitoring of the tea production process to determine the grade quality, adulterations and detect the AAs intake.

Rapid Determination of Amino Acids of Nitraria tangutorum Bobr. from the Qinghai-Tibet Plateau Using HPLC-FLD-MS/MS and a Highly Selective and Sensitive Pre-Column Derivatization Method.

Amino acids are indispensable components of living organisms. The high amino acid content in Nitraria tangutorum Bobr. fruit distinguishes it from other berry plants and is of great significance to its nutritional value. Herein, using 10-ethyl-acridine-3-sulfonyl chloride as a fluorescent pre-column labeling reagent, a method for the efficient and rapid determination of amino acid content in N. tangutorum by pre-column fluorescence derivatization and on-line mass spectrometry was established and further validated. The limits of detection (signal-to-noise ratio = 3) were between 0.13 and 1.13 nmol/L, with a linear coefficient greater than 0.997 and a relative standard deviation between 1.37% and 2.64%. In addition, the method required a short analysis time, separating 19 amino acids within 20 min. Subsequently, the method was used to analyze the amino acid content of Nitraria tangutorum Bobr. from tissues retrieved from seven regions of the Qinghai-Tibet Plateau. Nitraria tangutorum Bobr. was shown to contain a large amount of amino acids, with the total content and main amino acid varying between the different tissues. This research supports the nutritional evaluation, quality control, and development and utilization of Nitraria tangutorum Bobr.

High-performance liquid chromatographic determination of 2-aminoethylphosphonic acid and 2-amino-3-phosphonopropionic acid in seawater matrix using precolumn fluorescence derivatization with o-phthalaldehyde-ethanethiol

2-Aminoethylphosphonic acid (2-AEP) and 2-amino-3-phosphonopropionic acid (2-AP3) are two types of abundant and ubiquitous naturally occurring phosphonates used as sources of phosphorus by many prokaryotic lineages. The potential utilization mechanism of 2-AEP and 2-AP3 in eukaryotic phytoplankton is currently under investigation. However, the lack of suitable analytical methods in saline samples are the limitation of such researches. Herein, a high-performance liquid chromatography (HPLC) method for monitoring 2-AEP and 2-AP3 using precolumn fluorescence derivatization with o-phthalaldehyde-ethanethiol (OPA-ET) in seawater matrix was developed. The derivatization procedure and HPLC conditions were carefully examined, which included optimization of the fluorescence excitation and emission wavelengths, the ammonium acetate concentration and pH of the mobile phase, the OPA-ET reagent content and composition and derivatization time. Because increasing salinity was observed to lower the derivatization efficiency, working standards were freshly prepared in artificial seawater with the same salinity as that of the samples for the quantification of 2-AEP and 2-AP3. The developed HPLC method showed a wide linear response with high linearity (R2 > 0.999) and high repeatability at three concentration levels. The relative standard deviation was less than 4.1% for 2-AEP and less than 1.7% for 2-AP3 (n = 7). The limits of detection for 2-AEP and 2-AP3 in artificial seawater matrix were both 12.0 μg/L. The recoveries were 83.0–104% for 2-AEP and 72.6–98.6% for 2-AP3 in different aqueous samples, including algal culture medium prepared with filtered seawater. These results indicated the matrix effect of this method was insignificant.

Determination of d,l-Amino Acids in Collagen from Pig and Cod Skins by UPLC Using Pre-column Fluorescent Derivatization

A simple and sensitive analytical method for the quantification of d,l-amino acids by ultra-performance liquid chromatography (UPLC) with fluorescence (FL) detection is described. The reaction of the R(-)-4-(3-isothiocyanatopyrrolidin-1-yl)-7-(N,N-dimethylaminosulfonyl)-2,1,3-benzoxadiazole [R(-)-DBD-PyNCS] with d,l-hydroxyproline (Hyp), glycine (Gly), d,l-aspartic acid (Asp), and d,l-proline (Pro) effectively proceeds at 55 °C for 20 min in the presence of 3% TEA to produce the corresponding fluorescent diastereomers (excitation at 460 nm, emission at 550 nm). The mixture was simultaneously separated within 20 min on an ACQUITY UPLCTM BEH C18 (1.7 μm, 100 mm × 2.1 mm i.d.) by gradient elutions using water–acetonitrile containing 0.2% formic acid as the mobile phase. Peak resolution was in the range of 1.62 (d,l-Asp), 2.99 (d,l-Pro), and 6.74 (d,l-Hyp). A good linearity was achieved from the calibration curves (r2 > 0.9984) in the range of 1.0~100 pmol; the limit of detection (S/N = 3) was 42.0–250 fmol, the inter-day and intra-day assay precisions were all less than 6.23%, and the mean recoveries (%) of the d,l-amino acids spiked in the collagen from pig and dried cod skins were 87.58–107.41%. The derivatives of the free d,l-amino acids in the collagen were successfully identified by the proposed procedure. To the best of our knowledge, for the first time, these three kinds of d-amino acids, which were d-Asp, d-Pro, and d-Hyp, were detected in the collagen samples.

Highly sensitive method for aldehydes detection: Application to furfurals analysis in raisin and bovine milk powder

A pre-column fluorescent derivatization method based on nitrone formation has been applied to determine furfurals (e.g. furfural (F), 5-methyfurfural (5-MF) and 5-hydroxymethylfurfural (5-HMF)) in food samples for the first time. An N-substituted hydroxylamine reagent 4-((hydroxyamino)butyl)-7-hydroxycoumarin (HAHC) was used to react with the aldehyde group of furfurals to form stable nitrone derivatives with high fluorescence intensities. The reactions proceeded under mild conditions in 30 min with high derivatization yields (>93%). A baseline-separation of three furfurals derivatives was subsequently achieved within 25 min on a reversed-phase column. The detection limits were at the low femtomol level (S/N = 3, 20 μL per injection). The linear range of the calibration curve was 0.4–4000 nM with good correlation coefficients (R2 ≥ 0.9991). The proposed method was further applied for food sample analysis, such as bovine milk powder and raisin. Satisfactory recoveries were obtained in the range of 94.7%–103.5%. Above all, this pre-column derivatization method is simple, fast and highly sensitive, providing an effective and promising way for future studies of aldehydes in different matrices.

Elevated urinary d-lactate levels in patients with diabetes and microalbuminuria

Diabetic nephropathy (DN) has become the major cause of end-stage renal disease. Early detection of disease risk, to enable intervention before advanced renal damage occurs, is an important goal. Microalbuminuria has been used to monitor renal damage in clinical settings for years. In this study, we divided patients with diabetes into different groups based on their microalbumin values to elucidate the relationship between urinary d-lactate and corresponding microalbumin values. Group DM1 comprised of patients with an albumin-to-creatinine ratio (ACR) of less than 30μg albumin/mg creatinine (normal range); Group DM2 comprised of patients with an ACR of 30–299μg albumin/mg creatinine (microalbuminuria); and Group DM3 comprised of patients with an ACR of ≥300μg albumin/mg creatinine (macroalbuminuria). The urinary d-lactate concentration of patients with diabetes was determined by pre-column fluorescence derivatization with 4-nitro-7-piperazino-2,1,3-benzoxadiazole (NBD-PZ), and the accuracy (recovery) and precision (relative standard deviation; RSD) were validated. The measured values showed an accuracy that was in the acceptable range (91.59–112.96%), with an RSD in the range of 3.13–13.21%. The urinary d-lactate levels of the 3 diabetic groups (groups DM1, DM2, and DM3) were significantly higher than those of healthy subjects (78.31±22.13, 92.47±21.98, and 47.29±17.51 vs. 6.28±2.39 nmol/mg creatinine, respectively; p<0.05), with urinary d-lactate levels in the DM2 group being the highest. This modified fluorescence-based, high-performance liquid chromatography method to quantify d-lactate concentrations in the urine of patients with diabetes was established. Also, measuring the new risk marker identified in this study (d-lactate) in combination with microalbumin may facilitate the prevention of DN.

Rapid, Selective, and Sensitive Analysis of Triterpenic Acids in Hippophae rhamnoides L. Using HPLC with Pre-Column Fluorescent Derivatization and Identification with Post-Column APCI-MS

Hippophae rhamnoides L., a fruit which is rich in natural chemical ingredients, has become an important raw material of health food, medicine, and cosmetics. In the present work, a method using 2-(5-benzoacridine)ethyl-p-toluenesulfonate as pre-column derivatization reagent with high sensitivity, selectivity, and short separation time was developed to analyze triterpenic acids (TTAs) in H. rhamnoides L. by HPLC with fluorescence detection and simultaneously confirmed by post-column atmospheric pressure chemical ionization-mass spectrometry. The method was validated by linearity, limits of detection (LODs) and lower limits of quantification (LLOQs), precision, and accuracy. Good linear correlations were observed for all TTAs with correlation coefficients greater than 0.9996. LODs and LLOQs were in the range of 1.71–2.14 and 7.45–8.23 ng mL−1, respectively. The method was successfully employed to analyze TTAs in H. rhamnoides L. from various origins in the Qinghai–Tibetan plateau with trace amount of samples within 15 min. A systemic mapping of the TTAs in H. rhamnoides L. of different origins was established. The results indicated that H. rhamnoides L. is rich in TTAs, especially in oleanolic and ursolic acids, and the content of TTAs varies across origins. This work will support the research on pharmaceutical applications of H. rhamnoides L. and provide information for quality control.

Separation and identification of an oligomeric light stabilizer Chimassorb 944 by gradient elution chromatography coupled with matrix-assisted laser desorption/ionization mass spectrometry

A non-aqueous reversed-phase high-performance liquid chromatographic (HPLC) method has been developed to separate a light stabilizer Chimassorb 944 into individual oligomers, which are further identified using pre-column fluorescent derivatization and matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS). Consistent with previous studies, we find that the Chimassorb 944 product is a complex mixture consisting of a homologous series with the amine end groups and the number of repeat units (n) span from 1 to 26. In addition to the dominant linear species, cyclic oligomers are present at relatively high levels in the low-mass range. Their concentration decreases rapidly with the length of the oligomer backbone and becomes undetectable when n>7. Moreover, comparison of the HPLC and MALDI-MS molar mass distributions of Chimassorb 944 shows that the HPLC analysis produces greater molar mass averages and thus offers an effective means for accurate measure of the relative abundances of the oligomers.

Fluorescence derivatization method for sensitive chromatographic determination of zidovudine based on the Huisgen reaction

A novel pre-column fluorescence derivatization method for chromatographic analysis of azide compounds was developed based on the Huisgen reaction, which is a specific cycloaddition reaction between an alkyne and an azide. We designed and synthesized a fluorescent alkyne, 2-(4-ethynylphenyl)-4,5-diphenyl-1H-imidazole (DIB-ET) as a reagent. DIB-ET has a lophine skeleton carrying an alkyne acting as fluorophore and reactive center, respectively. In order to evaluate the practicality of DIB-ET, a high-performance liquid chromatography with fluorescence detection method was developed for the determination of zidovudine as a model of azide compound. Zidovudine could be reacted with DIB-ET in the presence of copper(II) sulfate and L-ascorbic acid as catalysts, and the formed derivative was detected fluorometrically. The proposed method allows sensitive and selective determination of zidovudine in plasma samples with the detection limit of 0.28ngmL−1 at a S/N=3. Finally, the proposed method could be applied to determine the zidovudine concentration in rat plasma after administration of zidovudine without interference from biological components.

Determination of Fatty Acids in Saliva of Smokers and Nonsmokers by HPLC with Fluorescence Detection Using a Hydrazine-Based Difluoro-boraindacene Reagent

1,3,5,7-Tetramethyl-8-daminozide-difluoroboradiaza-s-indacence (TMBODIPY-H), a pre-column fluorescent derivatization reagent, was applied to the analysis of fatty acid by high-performance liquid chromatography with fluorescence detection. Using this reagent, 12 fatty acids from propionic acid (C3) to stearic acid (C18) can be derivatized at room temperature in the presence of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide within 2 h. The baseline separation of these derivatives can be achieved within 46 min by gradient elution. The detection limits of these fatty acids are in the range of 0.1–1 nM and the linear ranges of most of them are 0.5–100 nM. This was the first application of hydrazine-based difluoro-boraindacene reagent for the analysis of fatty acids, and the proposed method has been successfully used for the determination of saliva samples of smokers and nonsmokers with recoveries of 88–110 %.