Syringe injection precision

In practice, the injection of larger volumes (> ID pi) is possible only at lower inlet pressures. Further, the precision of syringe injection is usually not batter than W. 

Mercury concentrations in constructed and actual crude oil samples were measured using three analytical methods that were compared with respect to accuracy, precision, and detection limit. The combustion method (U.S. EPA 7473 hybrid) and a commercial extraction method (non-standard) were found adequate to provide a good combination of sensitivity and accuracy, while instrumental neutron activation analysis was found to suffer from interferences from elements other than mercury but typically in crude oil. In the combustion method, direct syringe injection of aliquots to the combustion chamber was found advantageous in that it minimized opportunities for loss of volatile mercury. 

Microsyringe with Chaney adapter (Hamilton Co.) for injection of liquid extracts 0.5 or 1.0 cm capacity gas-tight syringe for head-space sampling and direct injection (Precision Sampling Corp.)... 

Analytes are introduced into GC colmtms with several techniques. An ahquot of a relatively concentrated vapor or air sample— for example, from a plastic bag or a canister—can be introduced into a short section of tubing of known volume, called a sample loop, and subsequently pinged with carrier gas into the GC colurmt Volatile analytes in ambient air samples in a canister or trapped on a solid phase adsorbent are usually concentrated and focused in a cryogenic trap or a secondary adsorbent trap, then thermally vaporized into the GC carrier gas stream. However, in some analytical methods, volatiles trapped on an absorbent are thermally desorbed directly into the GC colunm. Aliquots of organic solvent extracts from various aqueous and solid samples are usually injected with a syringe into the carrier gas stream in a heated injection port. Both manual and antomated syringe injection systems (autoinjectors) are used and the latter are generally very reliable, precise, and have the capacity to process many samples unattended. 

Implementation of SFC has initially been hampered by instrumental problems, such as back-pressure regulation, need for syringe pumps, consistent flow-rates, pressure and density gradient control, modifier gradient elution, small volume injection (nL), poor reproducibility of injection, and miniaturised detection. These difficulties, which limited sensitivity, precision or reproducibility in industrial applications, were eventually overcome. Because instrumentation for SFC is quite complex and expensive, the technique is still not widely accepted. At the present time few SFC instrument manufacturers are active. Berger and Wilson [239] have described packed SFC instrumentation equipped with FID, UV/VIS and NPD, which can also be employed for open-tubular SFC in a pressure-control mode. Column technology has been largely borrowed from GC (for the open-tubular format) or from HPLC (for the packed format). Open-tubular coated capillaries (50-100 irn i.d.), packed capillaries (100-500 p,m i.d.), and packed columns (1 -4.6 mm i.d.) have been used for SFC (Table 4.27). 

An HPTC injector allows the introduction of a precise sample volume onto the column. A typical manual injector consists of a 6-port valve with a rotor, a sample loop and a needle port (Eigure 9). A sample solution is introduced into the sample loop using a 22-gauge blunt tip syringe in the TOAD position. The sample is then injected into the column by switching the valve to INJECT. The typical external sample loop size ranges from 6 pT to 2 mT. For many years, the Rheodyne 7125 injector was the industry-standard. In the early 1990s, it was replaced by the Rheodyne 7725 injector, which injects samples without momentary flow disruptions. ... 

An injector valve operates in two modes— the fixed-loop mode or the partial-loop mode. In the fixed-loop mode, a sample is overfilled into the loop at 2-4 times the loop volume and the entire loop content is injected. In the partial-loop fill mode, a variable sample aliquot, measured precisely by a syringe at <50% of the loop volume, is injected. Note that the sample slug is introduced into the end of the sample loop and is back flushed onto the column to minimize band dispersion by the sample loop (Figure 9). Due to the emphasis on productivity, manual injectors are seldom used in the pharmaceutical laboratory except for preparative applications. 

Figure 4 shows peak area precision vs. injection volume for a typical autosampler. Note that excellent peak area precision of 0.2% RSD was readily achievable for an injection volume >5 J,L. Precision levels are much poorer (0.5-1% RSD) for sampling volumes <5 J,L, attributable to the finite resolution of the sampling syringe and associated digital stepper motor. To obtain optimum peak area precision, the analyst must avoid potential problem situations such as an overly fast sample... 

FIGURE 4 Peak area precision study to evaluate the effect of injection volumes. The resolution of the sampling syringe was about 0.01 jlL as determined by the digital resolution of the stepper motor and the size of the sampling syringe. 

To calculate the response factor Kt of a compound t, it is essential, according to equation (4.7), to know the injected quantity. However, it is difficult to know precisely the injected volume, which depends on the injector or injection loop or the precision of the syringe. Moreover, the absolute response factor K, (not to be confused with the partition coefficient) depends on the tuning of the chromatograph. This factor is not an intrinsic property of the compound. This is why most chromatographic methods for quantitative analyses, whether they are pre-programmed into an integrating recorder or software, do not make use of the absolute response factor, Kj. 

When no peaks are observed upon injection of a sample, it is common to blame the column but there are other factors that could cause this. Is the sample actually getting into the column The syringe needle may become clogged with parts of the septum or insoluble material from the sample. Loose plungers can permit the sample to be blown back by the pressure of the carrier gas. The syringe is a precision device and should be handled with care. Those with burred or bent needles, or bent plungers should be repaired before use. 

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