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MS TIP No. 20 - As a detector for GC, mass spectrometry generally has a smaller linear dynamic range than FID. Is there any technique or method to extend the linear range?

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by: James B. Edwards
Affiliation: Enviroscan Corp.

As a principally environmental laboratory, my company does primarily target compound analysis using standard EPA methodologies. We use a Finnigan INCOS 50B with a CTC A200S autosampler for semi-volatiles, and a Finnigan ITS-40 with Tekmar LSC200/ALS2016 for volatiles. As such, the primary concern is low concentration sensitivity, and any concentrations above the calibrated linear range can be determined by simple dilution.

However, my company is also associated with a wastewater treatment technology firm that has a wide variety of analytical needs. In particular, we provide analyses which look for high concentrations of compounds in the raw waste to be treated, and (hopefully), low concentrations of the same compounds in the treated effluent samples. In order to avoid multiple dilutions for the varying treatment conditions attempted, one technique that we use involves the monitoring of a target compound using several different quantitation ions.

For example, in one study, we were monitoring for the presence and concentration of 1,4-dichlorobenzene. This compound gives a base peak of 146 m/z, with other chlorine cluster masses of 148 m/z (approximately 60% base peak) and 150 m/z (approximately 10% base peak). We set up our quantitation package to search for 1,4-dichlorobenzene in two entries. The first entry monitored m/z 146 and the second monitored m/z 150. By running the calibration curves were prepared, one using each m/z. the first entry was used for high sensitivity, as m/z 146 is the base peak. The second entry was used for extended calibrated range - since m/z 150 is approximately 10 percent of m/z 146, the calibrated range using m/z 150 was approximately one order of magnitude higher than when using m/z 146 (although it did not have the maximum low end sensitivity). The two m/z, when monitored in this fashion, will also provide a check on one another when analyzing mid-level concentration samples.

By using one mass for maximum sensitivity (normally the base peak) and one mass for the highest possible calibrated range, the effective calibrated range can be extended. This only requires the selection of a secondary m/z that is structurally/spectrally significant for the target compound, and one that is present in the mass spectrum at a high enough percentage as to avoid resolution problems.