Organic Carbon Analysis Methods
There are several methods which can be employed to measure DOC concentrations. These include high temperature combustion, high temperature catalytic oxidation, persulfate oxidation, ultra-violet promoted persulfate oxidation, acid digestion (HCl), and dichromate digestion (Wlakley-Black). For this experiment, the samples will be analyzed on a Dohrman DC-180 Carbon Analyzer which utilizes the ultra-violet promoted persulfate oxidation method.
Dohrman DC-180 Carbon Analyzer
The Dohrman DC-180 Carbon Analyzer is capable of handling a wide range of concentrations in a variety of water samples with little user intervention during the analysis. It can determine total organic carbon (TOC), non-purgeable organic carbon (NPOC), total carbon (TC), purgeable organic carbon (POC), and inorganic carbon (IC) with the appropriate add-ons. The machine, when working properly, can detect samples in the range of 10 ppb (parts per billion) to 30,000 ppm (parts per million) with a precision of ± 2% of the measured value or ± 5 ppb whichever is greater. For the 10 mL injection volume, it is ± 5 ppb.
Analysis of Operation
When a NPOC analysis is started a peristaltic pump fills the pickup loop with the sample, then the analyzer sends a pre-set amount of acid and sample (5 mL - 10 mL) from the pickup loop to the liquid sparger. As a result of acidification with 20% phosphoric acid, all of the IC is stripped out along with the POC present. During sparger, the sample is bubbled with O2 and strips the liquid of all POC (via volatilization) and IC (converted to CO2 due to acid volatilization). The IC can be measured or vented to the atmosphere as it was in this case. If the POC fraction is to be measured, the IC derived CO2 is trapped on a LiOH scrubber allowing only the POC portion to pass on to the UV reactor for oxidation. When the POC trap and release option is employed, the POC is pre-concentrated on Tenax GC and then thermally desorbed into the POC furnace for combustion. This POC ovenč method is recommended for samples which contain halogenated hydrocarbons or compounds with any appreciable vapor pressure. Since this option was not available for this experiment, the POC concentration was not quantified. Plus, with these samples it was assumed that the POC concentration was insignificant; therefore, NPOC equaled TOC.The sparged liquid remaining in the sparger is now analyzed in the NPOC. In this mode, an aliquot of the liquid in the sparger is delivered directly into the reactor via the injection loop where UV and oxygen-rich persulfate oxidize all of the organics present in the sample. After being exposed to the UV and persulfate reagent, the gaseous products (CO2 and OH) of oxidation are carried to the G/L separator where they are bubbled acidified water. A pH of three is maintained here to optimize the elimination of water vapor from the CO2 , the product of interest. A permeation dryer removes the moisture from the CO2 analyte by osmotic pressure. From here, the sample is transferred to the NDIR for the measurement of the CO2 product by the absorbance of infrared radiation at 667 and 2330 cm-1. The Electronic/Control module intergrates the detector signal and displays the result in ppm C. The data acquisition commences once the sample is injected into the UV reactor. The integration process ends once the detector signal comes back to the memorized baseline concentration.
Insert Flow Diagram
Operating Parameters
| Parameter | Value | Parameter | Value |
| Empty Sparger | 45 | Pick-up Loop Fill | 210 |
| Loop to Sparger Transfer | 45 | Acid Addition | 10 |
| Sparging Time | 150 | Injection Loop Fill Time | 5 |
| Injection Time | 120 | POC Trap Deorb Time | 60 |
| Delay Between Runs | 5 | Delay Between Modes | 60 |
| Integration Time-out (min) | 10 | Number of Repeats | 1 |
| Number of NPOC Injections | 3 | Analysis Mode | NPOC w/ Injection Loop |
| Calibration Mode | NPOC w/ Injection Loop | Calibration Factor | 1* |
| System Blank | 0 | Pick-up Loop Volume (mL) | 10 |
| Injection Loop Volume (mL> | 1 | Multi-Sampler Used | yes |
| Flush Between Samples | 1 | Rinse with Sample | no |