Gradients system use

For the red wines (82-84), which were injected directly into the HPLC without sample preparation, a ternary-gradient system using aqueous acetic acid (1% and 5% or 6%), and acidified acetonitrile (acetonitrile-acetic acid-water, 30 5 6) was used for cinnamic acid derivatives, catechins, flavonols, flavonol glycosides, and proanthocyanidins. Due to the large number of peaks, the gradient was extended to 150 min for the resolution of many peaks of important phenolics. This direct injection method was able to separate phenolic acids and esters, catechins, proanthocyanidins, flavonols, flavonol glycosides, and other compounds (such as tyrosol, and rrans-resveratrol) in wine in a single analysis. However, use of acetic acid did not permit the detector (PDA) to be used to record the UV spectra of phenolics below 240 nm (84). 

There are four basic system types. Type I are basic isocratic systems used for simple, routine analysis in a QA/QC environment often for fingerprinting mixtures or final product for impurity/yield checking. Type II systems are flexible research gradient systems used for methods development, complex gradients, and dial-mix isocratics for routine analysis and standards preparation. They fit the most common need for an HPLC system. Type III systems are fully automated, dedicated systems used for cost-per-test, round-the-clock analysis of a variety of gradient and isocratic samples typical of clinical and environmental analysis laboratories. Type TV systems are fully automated gra-... 

As previously mentioned, the RP-HPLC of phenolic compounds is carried out with polar eluents. Simple HPLC systems use one pump that pumps a single solvent or, more frequently, a solvent mixture through the column. This is known as isocratic HPLC. More complex gradient systems use more than one pump governed by a computer and pump changing mobile phases that can involve several solvents. Water is usually one of the components in the mixmre the other components most frequently include methanol, acetonitrile, and, more rarely, tetrahydrofuran, etc. The pH of the eluent is always acidic, and this acidity can be attained by the addition of acids such as acetic, formic, phosphoric, trifluoroacetic, and others. The selection of the acid used is dependent on the specific separation problems that need to be solved. 

The quantification of the metabolites of CYPs 2D6, 3A4, and 2C19 reactions is carried out using a SCIEX API 3000 mass spectrometer, running in the positive ion, selected reaction monitoring (SRM) mode. The HPLC gradient system used consists of two solvent mixtures. Solvent mixture A (SMA) consists of 94.9% H20, 5% MeOH, and 0.1% formic acid and solvent mixture B consists of 94.9% MeOH, 5% H20, and 0.1% formic acid. The analytical column used is a Develosil Combi-RP5, 5 pm, 3.5x20mm (Phenomenex, Inc., Torrance, CA), with a mobile phase flow rate of 1.1 mL/min. A short run time (1 min) HPLC method is used because of the specificity of the mass spectrometer and the lack of matrix effects (this was thoroughly... 

Packard 1080 gradient system uses two single-diaphragm reciprocating pumps. ... 

Fig. 8. Gradient system using a dual-head sinusoidal-drive counter-piston diaphragm reciprocating pump, a, a = containers of solvents A and B, b, b, = pump heads, c = pump motor, d, d = variable-speed or stepping motor to adjust the course of the counter-piston, e, e = pulse damping systems and flow-rate sensors, f = mixing chamber, g = to column, h = programmer.

The Orlita WKE 500-3 constant-flow gradient system uses a pumping assembly... 

The Sopares gradient system uses a similar device. A conducting chart on which the gradient curve 1s drawn with a compass point is displaced with a linear movement in front of an optoelectric device which follows the curve exactly. This gives two electric signals, which can control two tension-controlled pumps, A low-volume (less than 200 ul) mixing chamber is used. The response time of the system can be less than 1 sec. Two tapes drawn on the chart allow the control of various on-off devices. 

A typical chromatogram is shown in Fig. 1. Our method involves the use of a simple gradient system using a relatively non-toxic solvent and provides good separations in a reasonable analysis time. Unfortunately the isomers lutein and zeaxi.nthin are not unambigously resolved. 

Table 4 Gradient Systems Used for lEHPLC of Membrane Proteins...

Because RPSA is appHed to gain maximum product rate from minimum adsorbent, single beds are the norm. In such cycles where the steps take only a few seconds, flows to and from the bed are discontinuous. Therefore, surge vessels are usuaHy used on feed and product streams to provide unintermpted flow. Some RPSA cycles incorporate delay steps unique to these processes. During these steps, the adsorbent bed is completely isolated and any pressure gradient is aHowed to dissipate (68). The UOP Polybed PSA system uses five to ten beds to maximize the recovery of the less selectively adsorbed component and to extend the process to larger capacities (69). 

The gradients of H, Na, and other cations and anions established by ATPases and other energy sources can be used for secondary active transport of various substrates. The best-understood systems use Na or gradients to transport amino acids and sugars in certain cells. Many of these systems operate as symports, with the ion and the transported amino acid or sugar moving in the same direction (that is, into the cell). In antiport processes, the ion and the other transported species move in opposite directions. (For example, the anion transporter of erythrocytes is an antiport.) Proton symport proteins are used by E. coU and other bacteria to accumulate lactose, arabinose, ribose, and a variety of amino acids. E. coli also possesses Na -symport systems for melibiose as well as for glutamate and other amino acids. 

Johnson et al. (34) eoupled SEC in the non-aqueous mode (Mieropak TSK gel eluted with tetrahydrofuran) to a gradient RP LC system using aeetonitrile/water for the determination of malathion in tomato plants and lemonin in grapefruit peel. 

Figure 12-141. Typical gradient curves of pressure through a fan system. (Used by permission The Howden Fan Company.)...

Catastrophic corrosion damage to buried structures may occasionally occur as a result of earth leakage faults on d.c. equipment or on traction systems using metallic posts to support overhead catenary wires. Care should be taken when making earth potential measurements because of the high potential gradients that may be present. 

Chemical reduction of the antibody results iu the productiou of both light aud heavy chaius, with the heavy chaius showiug the differeut levels of glycosyla-tion that are of iuterest. The HPLC system used to separate the fight and heavy chains consisted of a Poros Rl/H 100 x 2.1 nun column maintained at 60°C. Gradient elution was used from 90% of a 2% acetic acid solution (solvent A) 10% acetonitrile/2-propanol (70 30 vol/vol) (solvent B) to 25% solvent A 75% solvent B over 30 min at a flow rate of 0.5 mimin. ... 

The HPLC system used consisted of a 30 x 2 mm Luna CN column with linear gradient elution employing two mobile phases A and B (A, 90% H2O 10% acetonitrile B, 10% H2O 90% acetonittile) with both phases containing 5 mM ammonium acetate and 0.2% formic acid. The hnear gradient commenced with 50 50 A B increasing to 100% B after 1 min of the analysis this composition was maintained for 1 min before returning to 50 50 A B after 4 min. Positive-ion ionspray (pneumatically assisted electrospray) was used to obtain mass spectra, with the spectrometer operating at a resolution of 5000. 

The Dionex system uses a Garbo Pac PA-1 anion exchange column and a CarboPac PA-1 Guard. The column was loaded with 25 pi of the RG solution and eluted with a linear gradient of 0 - 0.5 M NaOAc in 0.1 N NaOH during 50 minutes. The flow rate was 1.0 ml/min and the process was monitored using a PE detector. 

In case of fast gradient (below 15 min), S could be considered constant for all the investigated molecules and wiU only have a small influence on the retention time of the compounds. Thus, the gradient retention times, of a calibration set of compounds are linearly related to the ( )o values [39]. Moreover, Valko et al. also demonstrated that the faster the gradient was, the better the correlation between t, and < )o [40]. Once the regression model was established for the calibration standards, Eq. 8 allowed the conversion of gradient retention times to CHI values for any compound in the same gradient system. Results are then suitable for interlaboratory comparison and database construction. The CH I scale (between 0 and 100) can be used as an independent measure of lipophilicity or also easily converted to a log P scale. 

Supercritical fluid chromatography (SEC) was first reported in 1962, and applications of the technique rapidly increased following the introduction of commercially available instrumentation in the early 1980s due to the ability to determine thermally labile compounds using detection systems more commonly employed with GC. However, few applications of SEC have been published with regard to the determination of triazines. Recently, a chemiluminescence nitrogen detector was used with packed-column SEC and a methanol-modified CO2 mobile phase for the determination of atrazine, simazine, and propazine. Pressure and mobile phase gradients were used to demonstrate the efficacy of fhe fechnique. 

---