What is overlay injections?

[satyam_qa]

Dear all,

Can any one please tell about overlay injections? (Not overlay of Chromatograms)
Is there any technique that we can start second injection, while we first injection is still under process?


Thanks in advance
Satya,

[unmgvar]

why exactly do you want to do this?
there are several possibilities depending on what you are looking for
the simplest one these days is in order to shorten dead time of the injection for UHPLC systems
agilent and dionex allow this as far as i know
it is called bypass
the more complex settings require that you have 2 pumps and at least 1 switcher
what is your hardware configuration?

[AA]

In prep, what you describe is often called stacked injections. The next injection happens before the compounds of interest elute and you do this over and over having multiple injections on column. It is used to increase perp throughput, isocratic only though. Most analytical systems are not really set up to do this (because you get no retention time info) but you can often trick them into doing this by having the autosampler independant from the rest of the system.

You also might be talking about what I know as "loop offline" where you take the sample loop out of the flow path and fill it with the next sample. This would happen close to the end of the run after all of the compounds of interest have eluted, typically in the re-equlibration part of the run. Again this would be done to increase throughput as it removes the injector cycle time out of the equation.

Maybe you are talking about something else?

[Alexandre]

How old is your instrumnet?
Many latest LC instrumnets have this option - saves a lot of time great for production 24/7 labs.
Contact your nearest dealer.

[KDF]

As has been mentioned there are two types of overlapped injections [actually three]

In isocratic prep chromatography [e.g. chiral separations] a single injection may require 10 minutes to elute two peaks, but the peaks themselves elute in a 2 minute window. Injecting every two minutes gives a chromatogram with an initial 8 minute delay and then a pair of peaks every two minutes. Essentially you save 8 minutes for each injection after the first which is a five fold increase in processing. The work station must be synchronized to divert the frist peak of each pair to one container and the second peak to another to be effective. This is called stacked injection and when used effectively you can be collecting purified peaks about 90% of the time.

In the analytical world, it can take considerable time at the beginning of each method to prepare a sample for injection. If the chromatography is fast [e.g. five minutes and the system first waits for equillibration then injection processing, you can add 2-3 minutes between each run. Overlapped injection simply waits a specific amount after the injection of the current sample to begin loading the next sample. This generally requires the injection valve to be restored to the load position during data acquisition which can upset the baseline, so be careful where in this chromatogram this occurs. The injector then proceeds through all injection steps except actually switching the valve during the current method. At the end of the method, the system re-equilibrates and immediately switches the valve to perform the injection. In overlaped injection, each run is typically stored in a different data file as if it were completely independent. This method is suitable for both isocratic and gradient runs except if you start load sample late in a gradient run, some autosamplers may be filled with strong solvent that will also be injected.

Finally, some systems [typically LC/MS] support flow injection analysis [FIA] modes where you run isocratically and simply tell the system the injection interval and number of injections and start. All data are collected in the same file and deconvoluted by the software. The technique is generally used for direct injection of fractions to a mass spec, but has some applications for simple, fast isocratic methods.