Before purchasing, ensure that the unit you purchase meets the type of measurement that you'll be performing. When measuring TOC, typically we are converting carbon in the sample into CO2 and then measuring the CO2 via NDIR. Now besides ensuring that the overall functionality of the system meets your needs (autosampler, soil capability, etc) there are two main approaches to oxidize the available carbon, one being simple combustion and the other using persulfate-UV oxidation.
For most applications, the determining factor is sensitivity. Original TOC systems were combustion units, and to a large extent when the persulfate-UV instruments came out, due to significant improvements in sensitivity, a large portion of the instruments sold became the newer style. Recently though, the combustion units have become on par regarding sensitivity (though I suspect if ultimate sensitivity is your goal the persulfate gets the nod). However, if you are analyzing surface waters, which are likely to have a significant contribution of TOC from humic acids, the combustion unit will likely give you better analytical results. It seems that humic acid is either precipitating within the persulfate systems or is simply too stable to be have quantitative oxidation under the instrumental conditions. In this scenario, the combustion unit would be your better option.
We are using a Shimadzu 5000 (purchased from a used equipment reseller) with an autosampler as well as a solids module and have been satisfied with the systems performance.
Sincerely,
Greg Pronger