The UNH Stable Isotope Laboratory measures stable isotope ratios of carbon and nitrogen in environmental samples. Two isotope ratio mass spectrometers, both Finnigan Delta XPs, are the workhorses of the laboratory.
The University of New Hampshire Stable Isotope Lab has two Thermo Electron DeltaPlus XP isotope ratio mass spectrometers for measuring the stable isotopes of C and N. The lab also houses two peripheral devices to introduce sample to the mass spectrometers.
Mass Spectrometer 1
The first DeltaPlus XP mass spectrometer is interfaced to a Costech ECS4010 Elemental Analyzer via a Conflo III and is dedicated to measuring 13C, 15N, C%, and N% of organic and inorganic samples. Samples are packed into small tin capsules and introduced into the Elemental Analyzer via a Costech Zero Blank Autosampler. Mass Spectrometer 1 has a universal triple collector which allows for the measurement of C and N stable isotopes. Additionally, we have outfitted the amplifier board on this mass spectrometer with two sets of gain resistors allowing us to measure a wider range of sample sizes. We can use a high amplification mode to measure C and N isotopes on samples containing as little as 5-10 ug of C or N.
Mass Spectrometer 2
The second DeltaPlus XP mass spectrometer is interfaced to a Thermo GC Trace Ultra. Mass Spectrometer 2 has a universal triple collector which allows for the measurement of C and N stable isotopes in inorganic and organic solid samples.
Costech ECS4010 Elemental Analyzer
In our lab the Costech ECS4010 Elemental Analyzer, or EA, is dedicated to measuring 13C, 15N, C%, and N% of organic and inorganic samples.
Solid samples that are ground to a fine powder are weighed to the thousandth of a milligram, tightly packed into small tin capsules, and stored in a dessicator prior to analysis. Samples are then placed into a Zero Blank autosampler where atmospheric gases containing C and N are purged by ultra high purity He carrier gas. Once the autosampler has been purged it is closed to the atmosphere and analysis begins.
Thermo Trace GC Ultra
The gas chromatograph (GC) in our lab is mainly used for compound specific isotope analysis on liquid or gaseous samples. For example, instead of measuring bulk fish tissue, more information can be obtained by measuring isotopes of individual amino acids or fatty acids in that fish tissue. Compound specific analyses require some preparative chemistry prior to analysis (speak with the lab manager about this). We have the ability to measure 13C and 15N, isotopes on compound specific analytes.
13C and 15N analysis on the Trace GC Ultra
Measurement of 13C and 15N analysis on the Trace GC Ultra, is similar to the elemental analyzer because the isotopes are measured on CO2 and N2 gas. However, the GC column on the Trace GC Ultra is more complex than the 3 meter column on the elemental analyzer. We have several different columns available to use on the Trace GC Ultra with a variety of active phases, polarities, and lengths (up to 60 meters long).
Samples are injected using an AS2000 autosampler into a heated inlet where the sample is vaporized (note that the sample has to be prepared in such a way that it is volatile at the inlet temperatures, 250-280o C). The vaporized sampled does not immediately undergo combustion as in the EA but instead passes through the GC column. Once the compound specific analytes are separated and elute from the column they pass through combustion and reduction reactors where they are individually combusted and reduced to yield CO2 and N2 gases. A water trap dries out the gas stream before it enters the mass spectrometer for isotope detection via an open split. The mass spectrometer can only measure isotope ratios of carbon and nitrogen on CO2 and N2. The mass spectrometer cannot not recognize for instance, one amino acid from another, or measure isotope ratios on amino acids. That is why efficient analyte separation on a GC column followed by combustion and reduction is crucial for compound specific isotope analyses.