Question

1. What are the advantages and disadvantages to the method of internal standards?

2. If someone made a mistake while preparing the unknown by adding too much solvent and going over the mark on the volumetric flask, how would the results be affected?

3. What is the purpose of the temperature ramp in the GC program? Based on your chromatogram, was the use of a ramp successful? Why/why not?

4. The internal standard method accounted for variation in injection volume. However, in some instances it would be useful to know how much variability there was in injection volume. How could you estimate the variability in the manually injected volume?

Answer 1

2) if excess solvent is added in volumetric flask thar result to change in the concentration of solution but there is no change in final resolution.

3) There are two ways to affect the selectivity (?) of a GC separation. Changing the chemical nature of the stationary phase and altering the temperature - either the isothermal temperature or the ramp rate (^oC/min.) in a temperature programmed separation.

Each component is identified by comparing its "retention time", the length of time that it remains in the column, to that of a standard. The retention time of a vapor depends on the column temperature limits and ramp rate, the column length, type of stationary phase, and carrier gas velocity. If these variables are kept constant, the retention time of a component may be tentatively identified by comparison to the retention time of a known standard run under identical operating conditions. As a result, they move down as well defined bands, eluting at characteristic temperatures. By careful choice of the temperature ramp rate and carrier gas flow rate, each component can be eluted at a temperature approximating the optimum for separation from adjacent solutes. Although the resolution of closely spaced peaks cannot be improved over that at a single optimum temperature, the resolution of widely spaced peaks can be improved considerably.

Retention indices are independent from

• delay of acquisition (absolute shift of time axis) has no influence on RI
• unit of time measurement (min, seconds, scans) has no influence on RI
• carrier gas pressure and flow rates have no influence if held constant during one measurement
• column length, column diameter and stationary film thickness have no influence on RI
• pre-columns have no or neglectible influence on the RI
• different isothermal temperatures have neglectible on RI
• different linear temperature ramps have neglectible on RI

Typical temperature programs composed of isothermal sections and linear temperature ramps give only slight to medium RI variations, particularly near the inflection points of the temperature profile. Best reproducibility is obtained if the temperature program consists of only one continuous temperature ramp. An inital isothermal period does no harm if the start of the temperature ramp is earlier then the retention time of the first actually required n-alkane standard.

4) As with any analytical instrument, there is some variability. So let us say we have a ±10 µL swing from injection to injection on a 1.0 mL sample loop (remember the pressure effects I alluded to earlier). Well that 10 µL variability would represent 1 %; however, the same 10 µL fluctuation on a 5.0 mL sample loop would only represent 0.2% inconsistency. So in theory, the 5.0 mL sample loop minimizes the RSDs across ISTD injections, and in practice it does. Obviously, I acknowledge we are not talking about orders of magnitude here, so you will be perfectly fine with a 1.0 mL loop.