Detector volume

Mauriee H. Francombe and John L. Vossen, Homojunction and Quantum-Well Infrared Detectors, Volume 21, 1995. 

Extracolumn dispersion is a major problem for the packed fused silica capillary columns with internal diameters less than 0.35 mm. Peak standeunl deviations will be in the submicroliter range and extensive equipment modification is required for operation under optimum conditions. A reasonable compromise is to esploy injection voluMs of a few hundred nanoliters or less with detector volumes of a similar or preferably smaller size. This demands considerable ingenuity on behalf of the analyst since, as... 

Adequate dimensions of connecting capillaries and detector volume... 

Detector selectivity is much more important in LC than in GC since, in general, separations must be performed with a much smaller number of theoretical plates, and for complex mixtures both column separation and detector discrimination may be equally significant in obtaining an acceptable result. Sensitivity is important for trace analysis and for compatibility with the small sizes and miniaturised detector volumes associated with microcolumns in LC. The introduction of small bore packed columns in HPLC with reduced peak volume places an even greater strain on LC detector design. It is generally desirable to have a nondestructive detector this allows coupling several detectors in series (dual... 

How to best describe this broadening we expect to occur One way is by analogy to random error in measurements. We know or assume there is a truly correct answer to any measurement of quantity present and attempt to determine that number. In real measurements there are real, if random, sources of error. It is convenient to talk about standard deviation of the measurement but actually the error in measurement accumulates as the sum of the square of each error process or variance producing mechanism or total variance = sum of the individual variances. If we ignore effects outside the actual separation process (e.g. injection/spot size, connecting tubing, detector volume), this sum can be factored into three main influences ... 

To suppress the broadening, the flow-rate should be low, the connecting tubes short and narrow, the detector volume small and the response of the electronic circuitry fast. (For a more detailed discussion and references to the original literature, see [114, 136,150].)... 

It has been shown (20) that the maximum allowable detector volume Va.M can be expressed by... 

A band from a column eluted at a rate of 1.35 mL/min has a width at half-height of 16.3 s. The sample was applied as a sharp plug with a volume of 0.30 mL, and the detector volume is 0.20 mL. Find the variances introduced by injection and detection. What would be the width at half-height if broadening occurred only on the column ... 

The stochastic model applies to processes involving the stationary phase. To analyze the chromatogram, we need to subtract contributions to peak broadening from dispersion in the mobile phase and extra-column effects such as finite injection width and finite detector volume. These effects account for the width of the unretained peak. To subtract the unwanted effects, we write... 

An ideal detector of any type (Table 25-3) is sensitive to low concentrations of every analyte, provides linear response, and does not broaden the eluted peaks. It is also insensitive to changes in temperature and solvent composition. To prevent peak broadening, the detector volume should be less than 20% of the volume of the chromatographic band. Gas bubbles in the detector create noise, so back pressure may be applied to the detector to prevent bubble formation during depressurization of eluate. 

Figure 1.18. Types of chromatograms, (a) Differential chromatogram (b) integral chromatogram (c) peak resolution. O, injection point OX, injector volume OY, detector volume OA, holdup volume, VM OB, total retention volume, VR AB, adjusted retention volume,...

Detector volume. The volume of carrier gas (mobile phase) required to fill the detector at the operating temperature. 

Hold-up volume. V. Volume of mobile phase from the point of injection of the sample and the point of detection. In GC it is measured at the column outlet temperature and pressure and is a measure of the volume of carrier gas required to elute an unretained component (including injector and detector volumes). 

The disadvantage is the instrumental requirement to keep all dead volumes and detector volumes sufficiently small to prevent extracolumn zone broadening. It is also possible that special pumps, sample valves, and detectors may be required. The same requirements apply to the short columns used in fast LC. 

Take, for example, an LC peak whose 4ct width was measured to be 0.80 mm with a chart speed of 1.0 cm/min and a flow rate of 1.0 mL/min. By multiplying these values together, we find that this width corresponds to a volume of 80 jjlL. If the detector had a volume of 80 p.L or more, the entire analyte could be contained in it at one time and the peak would appear to be very broad due to dilution with mobile phase. A smaller detector volume—say 8 p,L—would have to be swept 10 times to accommodate all the analyte, and it would give a tall, sharp peak that more nearly represents the actual analyte distribution. As a general rule, the detector volume should be I or less of the volume of the smallest peak (usually the first peak unless there is a large solvent peak). 

If a data system were being used, it too would need to have a short sampling time in order to produce a sufficiently large number of digital signals to take advantage of the small detector volume. The smaller volume detector could produce at least 10 different signals as the analyte passed, and that would be a sufficient number to get a normal peak shape in most cases. 

The "plug-like velocity flow profile for electrokinetically pumped capillary columns (see Figure 1) is important in minimizing resistance to mass transfer within the mobile phase (4). Hydrostatically-pumped capillaries, have parabolic flow profiles which tend to severely disperse solute bands unless extreme narrow-bore (i.d.s less than 10 pm) capillaries are employed (12). Fortunately, larger capillaries, with less stringent detector volume requirements, can be efficiently used in MECC. 

Inter-detector volume detectors are placed at different physical positions and their signals must be aligned very precisely,... 

The variance due to the detection system is additive with the other variances contributing to peak broadening. The maximum effective volume of the detection system should be no greater than K/2>/n, a limitation identical to that for sample injection. Thus, in analogy to sample injection, the effects of detector volume are most significant when a peak with small retention volume is eluted. 

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