Decreased Retention Time

When retention times of mixture components decrease, there may be problems with either the mobile or stationary phase. It may be that the mobile phase composition was not restored after a gradient elution, or it may be that the stationary phase was altered due to irreversed adsorption of mixture components, or simply chemical decomposition. Use of guard columns may avoid stationary phase problems. 

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Column temperature (T) Increased temperature decreased retention time Increased temperature sharper peaks effect on N difficult to define check maximum and minimum temperature before using column... 

Would higher carrier gas flow rates increase or decrease retention time ... 

In temperature programming, the temperature of a column is raised during the separation to increase solute vapor pressure and decrease retention times of late-eluting components. At a constant temperature of 150°C, the more volatile compounds in Figure 24-10 emerge close together, and less volatile compounds may not even be eluted from the column, [f the temperature is increased from 50° to 250°C at a rate of 8°/min, all compounds are eluted and the separation of peaks is fairly uniform. Do not raise the temperature so high that analytes and stationary phase decompose. 

Acetonitril is often recommended because of its separating power (cf. Fig. 5). Adding a small amount of trifluoroacetic acid (TFA) to both gradient components A and B, is state of the art in protein reversed-phase chromatography. It decreases retention time, improves resolution, and increases the recovery of the sample proteins. 

The increased retention time of the cyclic product is due to the formation of a large hydrophobic preferred binding domain as a consequence of the formation of (1-sheet structure in the peptide (Scheme 2). Removal of side-chain protecting formyl groups with MeOH/HCl for 16 hours results in the formation of a single product 2 with decreased retention time relative to the protected starting material 5 due to reduced hydrophobicity. It is clear that both the cyclization and deformylation reactions proceed smoothly with the quantitative... 

The modification of chromatographic properties, unlike in GC, is usually of secondary importance. Often, however, a lowering of the polarity of certain molecules (e.g., sugars) is observed which enables the separation on adsorption chromatographic systems with more convenient solvent systems. This in turn may decrease retention times and consequently improve detection limits. 

Temperature is the first of the variables affecting separation. Increased temperature decreases retention time on the column, sharpens peaks, and produces the change in relative peak retentions typical of an a effect. At first, this appears to be the ideal variable, similar to temperature programming for GLC. However, temperature changes have some drawbacks. 

Analytical Properties Separation of the drugs ibuprofen, ketoprofen, naproxen, 2-phenoxypropionic acid, bendroflumethiazide, ethotoin, hexobarbital, disopyramide, and RAC 109 retention and selectivity of the solutes can be regulated by addition of the tertiary amine /V,/V-dimethyloctylamine (DMOA) to the mobile phase DMOA decreases retention time and the enantioselectivity of the weaker acids but has opposite effects on the stronger acids Reference 3... 

The lipophilicities of ( )-3-benzylidenethiochroman 4-one 251 and its 1-oxide, 1,1-dioxide, and the 2-phenyl derivative have been determined by reverse phase HPLC and a good linear correlation with calculated values is observed. The stronger polarizability of the sulfinyl and sulfonyl compounds results in decreased retention time, but the thioflavanone has the greatest lipophilicity <2005JCH(819)283>. 

PTGC is the process of increasing the temperature of the column oven during a run. As we have just seen, the increasing temperature will cause the partition coefficients of the analytes still on the column to decrease, and they will move faster through the column, yielding decreased retention times. The effect can be seen in Figure 8.20. 

A sample is removed from the reaction mixture every 15 min, however, in some cases the chromatographic run time may be 20 min. If this is the case, the sample may be held in a syringe until the injection. Alternatively, the run time can be decreased by using a flow rate of 2.0 mU min. This increased flow decreases retention time and should not have a deleterious effect on the resolution. It is cautioned, however, that the backpressure of the column should not be exceeded, as suggested by the column care and use manual of the manufacturer. 

Litzen and Wahlund systematically studied error sources like temperature effects, sample overloading, sample adsorption to the accumulation wall membrane and influences of the carrier liquid composition, that occur with Fl-FFF [455] the latter has already been discussed above. It was shown that preservation of constant channel temperature is very important as repeated measurements of an identical sample resulted in gradually decreasing retention times due to increasing channel temperature caused by frictional heat, especially when using high flow rates. As constant channel temparature is usually not fulfilled with the standard Fl-FFF channels, which simply operate at room temperature without any temperature control, this is an important point to consider. 

The most common problem associated with analytical columns is column deterioration. Deterioration may appear as poor peak shapes, split peaks, shoulders, loss of resolution, decreased retention times, and high backpressure. These symptoms indicate contaminants that have accumulated on the frit or column inlet, or there are voids, channels, or a depression in the packing bed. Deterioration is more evident in higher efficiency columns. For example, a column with 3-pm packing is more susceptible to plugging than one with 5- or 10-pm packing. Proper column protection and sample preparation are essential to prolong a column s life and obtain its best performance. 

In addition to increasing the speed of separation by reducing capacity factors, an increase in column temperature will also decrease the viscosity of organic solvents in the mobile phase. This factor may be of considerable importance when the mobile phase has a relatively high viscosity (e.g. methanol-water mixtures) by allowing an increased mobile phase flow rate and a decreased retention time. 

Manymethods now specify the use of a column heater to run separations at higher temperatures, thus decreasing retention times and obtaining more rapid results. This can be very effective, but it is important to remember the following ... 

For HPLC, it has been decided that the preferred buffer is citrate it was necessary to establish the most suitable concentration. To study this influence, five diamidines were analyzed using a mobile phase consisting of 45.0% methanol, 4.00 mM octane sulphonate, and citrate buffer at various concentrations of 15.0, 25.0, and 35.0 mM, with T = 30.0°C and pH = 3.25 in all cases. In Fig. 2, we see the results obtained. A change from 25.0 to 35.0 mM barely affects retention times for any of the substances, but a drop from 25.0 to 15.0 mM decreases retention times by almost 30.0% in every case. 

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