Gradient detection limit

Screening methods for 44 nitro-, chloro-, and alkyl mono- and disubstituted phenolic compounds in water were developed on three columns cyanopropyl, C,g, and diphenyl (A = 220nm). A 60-min 50/50->90/10 methanol/water (lOmM acetate buffer at pH 4.8) gradient was used. Not all 44 compounds were baseline resolved, but a subset of 28 were resolved on the C g column using the above gradient. Detection limits were reported individually for all components and ranged from 0.5 to 1.7 ppm [105]. 

Phenolic acids (ferulic, syringic,/>-coumaric, vanillic, caffeic, protocatechuic, and p-hydroxybenzoic) were extracted fi-om soil and analyzed on a C g column X = 245 nm) using a 38-min 70/30 -> 30/70 (2/97.25/0.5/0.25 methanol/water/ acetic acid/ethyl acetate)/(80/17/2/1 methanol/water/acetic acid/ethyl acetate) gradient. Detection limits were reported as <1 pg/g for all compounds [113]. A working concentration curve of 1-lOpg/mL was also generated. 

Simple mixtures—like in alkyl sulfosuccinates—can be run using only one solvent. For more complex systems (e.g., ethoxylated fatty alcohol sulfosuccinates) a gradient technique is strongly recommended Technical mixtures of disodium laureth sulfosuccinate could be separated [68]. The separation was so effective that resolution of single homologs of ethoxylates was possible. The detection limit of this method lies at around 0.5 pg. Therefore reverse phase ion pair chromatography seems to be an excellent tool to analyze sulfosuccinates directly without the use of any kind of manipulation. 

The second most widely used detector in HPLC is the differential refractometer (RI). Being a bulk property detector, the RI responds to all substances. As noted in Table 3 the detection limits are several orders of magnitude higher than obtained with the UV detector. Thus, one turns to the RI detector in those cases in which substances are non-UV active, e.g. lipids, prostaglandins. In addition, the RI detector finds use in preparative scale operation. Finally, relative to the UV detector, the RI is significantly more temperature and flow sensitive and cannot be used in gradient elution. 

Use of 10 pm LiChrosorb RP18 column and binary eluent of methanol and aqueous 0.1 M phosphate buffer (pH 4.0) according to suitable gradient elution program in less than 20-min run time with satisfactory precision sensitivity of spectrophotometric detection optimized, achieving for all additives considered detection limits ranging from 0.1 to 3.0 mg/1, below maximum permitted levels Simultaneous separation (20 min) of 14 synthetic colors using uncoated fused silica capillary column operated at 25 kV and elution with 18% acetonitrile and 82% 0.05 M sodium deoxycholate in borate-phosphate buffer (pH 7.8), recovery of all colors better than 82%... 

Favaro and Fiorani [34] used an electrode, prepared by doping conductive C cement with 5% cobalt phthalocyanine, in LC systems to detect the pharmaceutical thiols, captopril, thiopronine, and penicillamine. FIA determinations were performed with pH 2 phosphate buffer as the carrier stream (1 mL/min), an injection volume of 20 pL, and an applied potential of 0.6 V versus Ag/AgCl (stainless steel counter electrode). Calibration curves were developed for 5-100 pM of each analyte, and the dynamic linear range was up to approximately 20 pM. The detection limits were 76, 73, and 88 nM for captopril, thiopronine, and penicillamine, respectively. LC determinations were performed using a 5-pm Bio-Sil C18 HL 90-5S column (15 cm x 4.6 mm i.d.) with 1 mM sodium 1-octanesulfonate in 0.01 M phosphate buffer/acetonitrile as the mobile phase (1 mL/min) and gradient elution from 9 1 (held for 5 min) to 7 3 (held for 10 min) in 5 min. The working electrode was maintained at 0.6 V versus Ag/AgCl, and the injection volume was 20 pL. For thiopronine, penicillamine, and captopril, the retention times were 3.1, 5.0, and 11.3 min, and the detection limits were 0.71, 1.0, and 2.5 pM, respectively. 

For the cationic surfactants, the available HPLC detection methods involve direct UV (for cationics with chromophores, such as benzylalkyl-dimethyl ammonium salts) or for compounds that lack UV absorbance, indirect photometry in conjunction with a post-column addition of bromophenol blue or other anionic dye [49], refractive index [50,51], conductivity detection [47,52] and fluorescence combined with postcolumn addition of the ion-pair [53] were used. These modes of detection, limited to isocratic elution, are not totally satisfactory for the separation of quaternary compounds with a wide range of molecular weights. Thus, to overcome the limitation of other detection systems, the ELS detector has been introduced as a universal detector compatible with gradient elution [45]. 

Boyd-Boland and Pawliszyn [77] pioneered the SPME analysis of APEOs by SPME-HPLC using normal-phase gradient elution with detection by UV absorbance at 220 nm. The Carbowax-template resin (CW-TR) and Carbowax-divinylbenzene (CW-DVB) fibres allowed the analysis of APEO with a linear range of 0.1-100 mg L 1. The former coating produced the best agreement between the distribution of ethoxymers before and after extraction. This CW-TR fibre provided a limit of detection for individual AP ethoxamers at the low ppb level. The determination of NP in water by SPME-GC (FID) was accomplished by Chee et al. [78] using a polydimethylsiloxane (PDMS) fibre. The linear range was between 1 and 15 mg L 1 with an estimated detection limit of 0.1 mg L-1. 

Another RP-HPLC technique has been applied for the determination of synthetic food dyes in soft drinks with a minimal clean-up. Separation of dyes was obtained in an ODS column (150 x 4 mm i.d. particle size 5 pm). Solvents A and B were methanol and 40 mM aqueous ammonium acetate (pH = 5), respectively. Gradient conditions were 0-3 min, 10 per cent A 3-5 min, to 25 per cent A 5-8 min, 25 per cent A 8-18 min, to 75 per cent A 18-20 min, 75 per cent A. The flow rate was 1 ml/min and dyes were detected at 414 nm. The separation of synthetic dyes achieved by the method is shown in Fig. 3.35. The concentrations of dyes found in commercial samples are compiled in Table 3.21. The quantification limit depended markedly on the type of dye, being the highest for E-104 (4.0 mg/1) and the lowest for E-102 and E-110 (1.0 mg/1). The detection limit ranged from 0.3 mg/1 (E-102 and E-110) to 1.0 mg/ml (E-104 and E-124). It was suggested that the method can be applied for the screening of food colourants in quality control laboratories [113]. 

---