Samples, prepared in a suitable submission solvent, are usually introduced by an automated syringe drawing samples from vials and injecting into the LC column via a port consisting of an injection valve (e.g., six-port rotary valves) and sample loop.

Some consideration of injection technique (e.g., filled versus partial filling), loop dimensions, and dead volume is important as all influence analytical precision.

In addition, the composition and volume of injection solvent used can influence the chromatography and peak shape in particular.

A strong injection solvent, relative to the mobile-phase composition, can cause the sample to move quickly through the first portion of the column while it reequilibrates with the mobile phase, thereby broadening or even splitting peaks.

Where possible, it is best to inject the sample in the starting mobile phase or a weaker solvent to avoid this type of problem.

Alternatively, if injecting in a strong solvent is preferred, injection volumes must be kept small.

Increasing the volume injected beyond conventional volumes is possible if a weak submission solvent is used, but LVI has rarely been applied to pesticide residue analysis of matrices using LC-MS.

Although direct LVI of water samples has been shown to be a useful alternative to SPE of very polar pesticides, online SPE systems appear to be the preferred approach for introduction of larger volumes of sample.

In spite of the high selectivity of the LC-MS technique, sensitivity is rarely high enough to directly determine the trace amounts of pesticides in food commodities or water at the levels required by legislation; so preconcentration is usually required. Fortunately, the majority of LC-MS is conducted on MS/MS instruments with enhanced selectivity; so extensive cleanup is less crucial when compared with analysis by GC-MS and there is a growing trend toward LC-MS analysis without cleanup.

Any cleanup of food extracts is still usually performed off-line before LC-MS, whereas the use of online trace enrichment has been successfully used for the determination of pesticide residue in water. SPME, in-tube SPME, and SBSE have all been used as sample introduction devices for LC-MS.

p75, Analysis of Pesticides in Food and Environmental Samples, CRC Press, 2008, edited by José L. Tadeo