Injecting sample

A technique for injecting samples onto a capillary column in which only a small portion of the sample enters the column. 

A means for injecting samples in which the sample is loaded into a short section of tubing and injected onto the column by redirecting the mobile phase through the loop. 

Quantitative Calculations Quantitative analyses are often easier to conduct with HPLC than GC because injections are made with a fixed-volume injection loop instead of a syringe. As a result, variations in the amount of injected sample are minimized, and quantitative measurements can be made using external standards and a normal calibration curve. 

An injection technique in capillary electrophoresis in which pressure is used to inject sample into the capillary column. 

Making appropriate substitutions into equation 12.44 gives the volume of injected sample as... 

Since the injected sample plug is cylindrical, its length, /plug, is easily calculated using the equation for the volume of a cylinder. 

For GC, the injector is most frequently a small heated space attached to the start of the column. A sample of the mixture to be analyzed is injected into this space by use of a syringe, which pierces a rubber septum. The injector needs to be hot enough to immediately vaporize the sample, which is then swept onto the head of the column by the mobile gas phase. Generally, the injector is kept at a temperature 50 C higher than is the column oven. Variants on this principle are in use, in particular the split/splitless injector. This injector can be used in a splitless mode, in which the entire injected sample goes onto the column, or in a split mode, in which only part of the sample goes onto the column, the remainder vented to atmosphere. For other less usual forms of injector, a specialist book on GC should be consulted. 

Flow injection techniques can be used to inject sample volumes as small as 10 jiL into a flowing stream of water with little degradation of detection limits. Frit nebulizers have efficiencies as high as 94% and can be operated with as litde as 2 jiL of sample solution. 

It is seen that columns having diameters less than 2 mm will only tolerate a maximum sample volume of a fraction of a microliter. Although larger volume valves can be used to inject sample volumes of this size, the dispersion from the valve is still likely... 

Type of injection, sampling, and duct section L/D Maximum Maximum error 5 % error 10 % ... 

Gas chromatograph (GC) An analytical instrument with an internal tube or column that contains a solid sorbent, which allows some components of an injected sample to pass more quickly than others, separating the substances in the sample. 

Errors in the molecular weight data from HPSEC are usually due to improperly prepared samples, column dispersity, or flow rate variations. The sample to be analyzed should be completely dissolved in the mobile phase and filtered prior to injection onto the column. A plugged column inlet frit will invalidate results. In addition, do not load the column with excess sample. Column overloading affects the accuracy of data by broadening peaks, reducing resolution, and increasing elution volume. For best results, the concentration of the injected sample should be as low as possible while still providing adequate... 

The actual loading capacity always depends on the sample composition and the separation problem. As a rule the volume of the loaded sample should not exceed 5% of the column volume. However, this recommendation is valid only for preparative runs. For analytical applications when a high resolution is needed, the volume of the injected sample should be about 1% of the total column volume or even less. For a preparative run on a 1000 X 200-mm column (bed height 60 cm), two different sample volumes were injected. If the sample volume is 0.3% of the total bed volume, the separation is more efficient... 

We also have to count on variations of the main monomer unit as well. For example, HEMA materials always contain traces of unesterified free carboxylic groups. These acidic groups interact with polycations. As result, retardation is observed until total adsorption of the injected sample. Consequently, polycations can be analyzed on such a material only in acidic eluents, where the dissociation of the — COOH— group is suppressed. 

Split Injection2 In the split injector, the injected sample is vaporized into the stream of carrier gas, and a portion of the sample and solvent, if any, is directed onto the head of the GC column. The remainder of the sample is vented. Typical split ratios range from 10 1 to 100 1 and can be calculated from the equation ... 

Always adjust the carrier gas linear velocity to 20-40 cm/sec before heating the capillary column. Neglecting to supply carrier gas while heating a capillary column will destroy it. Avoid injecting samples containing inor-... 

GC assay of the organic layer showed no EIN(TMS)2 remaining after 15 min of stirring (GC conditions Restek RTX-1 column (30 m x 0.53 mm, 1 m film thickness), 2.53 mL/min, initial temperature 50°C, final temperature 300°C, rate 20 deg/min, injection temperature 200°C, detector temperature 350°C, injection volume 1 pL, inject sample neat retention times fert-butyl alcohol 1.4 min, THF 1.7 min, heptane 2.1 min, HN(TMS)2 2.6 min, ethylbenzene (present in commercial LHS) 3.1 min, te/ t-butyl acetate 4.0 min). Volume percents were determined based on standard solution counts. 

As already stated any sample placed on a column will have a finite volume, and the variance of the injected sample will contribute directly to the final peak variance. It follows that the maximum volume of sample that can be placed on the column must be limited, or the column efficiency will be seriously reduced. Consider a volume Vi, injected onto a column. Normal LC injections will start initially as a rectangular distribution and the variance of the eluted peak will be the sum of the variances of the injected sample plus the normal variance of the eluted peak. 

GPC chromatograms of AOT in benzene obtained with a 500 % + 100 A jjStyragel column set, and over a range of injected sample weights, are shown in Figure 1. The apparent peak molecular weight increases with the amount of sample applied to the columns. At higher sample loads, the value of Mn appears to ap-... 

The dependence of Mp" on sample size for the ethanol-soluble fraction is sumnarized by the solid lines in Figure 7. For both column sets, the apparent MW of the principal peak increases by nearly an order of magnitude as the mass of the injected sample is increased from one to four mg. In contrast, the ethanol-insoluble fraction exhibits a rather narrow chromatogram... 

The concentration of the analyte in the injected sample is determined based on the height or area of the analyte peak and interpolation of the internal or external standard linear calibration curve according to the following equation ... 

The resulting concentration value in micrograms per liter represents the concentration of the analyte in the injected sample. Using the concentration of analyte in the injected sample, the final extract volume, and the volume of water extracted, the concentration of analyte present in the water specimen can be calculated. The concentration (qgL ) of herbicide in the water specimen is calculated by multiplying the analyte concentration (qgL ) by the final volume (mL) and dividing by the water specimen volume (mL) ... 

Once the concentration of the analyte in micrograms per liter has been determined in the injected sample, the remainder of the calculations is the same as for the linear calibration curve. 

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