Any chemist can tell you that among all the elements on the periodic table, Carbon is special. Carbon is the only element that can form a virtually unlimited number of compounds because each carbon atom can form four chemical bonds to other atoms, and because the carbon atom is just the right size to fit in parts of very large molecules.
Because of this ability, early 18th century chemists realized that the chemistry of non-living things (rocks, minerals, gases, etc.) was relatively simple; however the chemistry of living things was much more complex (see illustrations below of the simple inorganic molecules water, ammonia, and carbon dioxide compared to hemoglobin). Thus, the term “organic” chemistry was applied when referring to the chemistry derived from organisms. The chemistry of Carbon is so vast and complex, that within the academic discipline of Chemistry as a subject, the study of Carbon has its own branch known as Organic Chemistry.

Hemoglobin (C₂₉₅₂H₄₆₆₄O₈₃₂N₈₁₂S₈Fe₄)

The consequences of Carbon’s complexity abound. For example, when discussing nutrition we find it much more practical to talk about macronutrients (fats, proteins, and carbohydrates) as it would be an impossible task to consider all the different individual chemical compounds present in a meal. Similarly, petroleum fuel products such as gasoline, diesel, and kerosene are produced and defined by their range of boiling points, as listing each individual compound would be an enormous (and pointless) undertaking.

Organic Carbon vs Inorganic Carbon

There are, however, a class of Carbon compounds that are not very complex and are easy to list and describe: Inorganic Carbon. Inorganic Carbon refers to familiar mineral forms of Carbon such as limestone (Calcium Carbonate, CaCO₃), as well as the Carbon Dioxide (CO₂) exiting your lungs as you read this article. Carbon Dioxide readily dissolves into water, as we can all relate to carbonated beverages we have enjoyed either due to natural processes such as fermentation or to injection of CO₂ as is common in many soft drinks.

 There is not a universally accepted definition used by all chemists delineating the differences between Organic and Inorganic Carbon, but for the purposes of measuring Total Organic Carbon (TOC) the following definition is useful:

 Carbon that is covalently bound to only Oxygen is Inorganic Carbon, everything else is Organic Carbon.

 This is a very loose definition, and probably has some chemists cringing; but in the narrow context of TOC measurement, this definition works well (or well enough).

 Organic Carbon can be a food source for living organisms, but generally speaking Inorganic Carbon cannot. This is why pharmaceutical manufacturers must monitor and control the concentration of Organic Carbon in the water used to make drug products – especially those which are delivered by injection (“parenteral” drugs are those delivered non-orally. Controlling bacteria is particularly important when bypassing the body’s natural defences present throughout the digestive tract.). By limiting the concentration of Organic Carbon present in water, bacterial risk is significantly reduced.

 Measuring Total Organic Carbon

 Often TOC Analyzers determine both the Total Carbon, and the Inorganic Carbon concentrations. The difference between the two values gives us the Total Organic Carbon value:

 In environmental TOC measurement applications, it can sometimes be necessary to filter out suspended solids from the sample because these particles can cause fluid blockages in the TOC analyzer. In these cases, the sample is filtered through a 0.45 micron filter, and the filtrate is analyzed. This result is referred to as Dissolved Organic Carbon, or DOC. This approach of filtering out solids is not really desirable unless one needs to know only the immediately available or biologically available Organic Carbon to address a specific research question. Usually, for regulatory purposes the suspended organic matter that would otherwise be filtered out is an important part of the sample and should not be ignored. That this practice would be considered in order to accommodate design limitations of an instrument is not ideal; instrument capabilities should be chosen based on application requirements (more about this subject in future blog posts).