The Reston Groundwater Dating Laboratory

Low-Level VOCs Lab

Analytical Method

Detection of about 60 unique halogenated VOCs is possible. Of this 60 total, 33 compounds have been identified. Twenty-four of the 33 identified detections are quantifiable by standard calibration using a prepared gas standard of about 100ppb of each compound in nitrogen. An average response to the calibrated known compounds is used to quantify unknown detections. A water sample volume of approximately 34 cm3 is stripped of VOCs for a period of 4 minutes using ultra-pure N2 that has passed through a mole sieve. The stripping system is patterned after the CFC analysis system presented in Busenberg and Plummer (1992). A capillary column using helium as the carrier is used to separate the compounds which are detected by an Electron Capture Detector (ECD). Analysis of one sample takes about one hour to complete. Samples are usually analyzed in duplicate.

Advantages / Limitations

The primary advantage of purge and trap GC-ECD methods relative to the mass spectrometric analysis is the extremely low MDL. This allows contamination susceptibility to be detected earlier than would be using GC-MS. Because of the limitations of GC-ECD methods for analyzing a wide range of VOCs, mass-spectrometric methods are more commonly used in identifying and quantifying concentrations of VOCs in ground water. With reporting levels on the order of about 0.2 to 0.02 ug/L, the mass-spectrometric method is sufficient to meet USEPA regulatory requirements for drinking water.

The identification of compounds in ground water samples by GC-ECD is based on chromatographic retention time. For the compounds identified, concentrations are determined using calibration standards. Many purgeable halogenated VOCs are detected in GC-ECD chromatograms of untreated drinking water but have not been detected in any of the commercially available standards analyzed and are therefore unidentified. An average ECD response to known compounds plus retention time is used to quantify the unknown compounds. Using this method, the unknown compounds can be roughly quantified.

Although the MDLs can be as low as 1 picogram per liter (pg/L, 1x10-6 ug/L, or 1 part per quadrillion) for some halogenated VOCs using purge and trap GC-ECD methods, the ECD does not detect most non-halogenated hydrocarbons.

High concentrations (tens of parts per billion, ppb) of some halogenated VOCs can be difficult to quantify by GC-ECD because of non-linear response of ECDs, especially at higher concentrations. The sensitivity of the ECD also varies with the electron affinity of different halogenated VOCs.

The GC-ECD analytical procedure was not designed for quantitative analysis of contaminated samples. Concentrations greater than a few ug/L (depending on the compound) can be underestimated by GC-ECD analysis due to the relatively low maximum response of the ECD. Additionally, large contaminant detections can obscure detection and quantification of other VOCs.

GC-ECD peak identification

Compound retention times in the purge and trap GC-ECD analytical system are determined by (1) analyzing commercial gas mixtures containing concentrations of 100 ppb of specific VOCs in nitrogen, (2) analyzing commercially available single VOC compounds dissolved in methanol that are then diluted in ultra-pure nitrogen, (3) comparing chromatograms of commercial standards containing various combinations of the more common halogenated VOCs, and (4) comparing detections in GC-ECD chromatograms of ground-water samples with the corresponding GC-MS compound identifications and analyses for the same sample. About 60 VOCs, with retention times between 2.0 to 45 minutes, have been detected by GC-ECD in ground water. Thirty-three of these compounds are identified (Table 1) by retention time. Peak identification by GC-ECD may not be unique for a specific compound, and depending on instrumental conditions and peak area, compound identification can be obscured by overlapping peaks (multiple VOCs with similar retention times).

Table 1: Halogenated VOCs identified, approximate retention time in minutes, detection level in picograms per liter, and ability to quantify with standard calibration.

Compound Alternate Name
Retention
MDL
Calibrated
Time
pg/L
SF6 Sulfurhexafluoride
3.88
1
N
H2S Hydrogen Sulfide
3.90
na
N
CFC-12 Dichlorodifluoromethane
4.12
3
Y
Methyl Chloride Chloromethane
4.59
52
Y
Vinyl Chloride Chloroethene
4.79
2762
Y
Halon 1211 Bromochlorodifluoromethane
4.85
2
N
Bromomethane Methylbromide
5.60
1
N
CFC-114 1,2-Dichloro-1,1,2,2-tetrafluoroethane
5.60
2
N
Ethyl Chloride Chloroethane
5.75
92
Y
CFC-11 Trichlorofluoromethane
6.10
1
Y
CFC-113 1,1,2-Trichloro1,2,2-Trifluoroethane
6.82
3
Y
1,1-Dichloroethylene 1,1-Dichloroethene
7.35
52
Y
Methyl Iodide Iodomethane
7.95
1
Y
Methylene Chloride Dichloromethane
8.30
236
Y
Trans-1,2-dichloethylene  Trans-1,2-dichloethylene 
9.00
2
N
1,1-Dichloroethane 1,1-Dichloroethane
9.75
394
Y
Cis-1,2-Dichloroethylene Cis-1,2-Dichloroethene
10.99
1667
Y
Chloroform Trichloromethane
11.30
19
Y
Methylene Bromochloride Bromochloromethane
11.50
5
Y
Methyl Chloroform 1,1,1-Trichloroethane
12.13
4
Y
Carbon Tetrachloride Carbontetrachloride
12.60
1
Y
Ethylene Dichloride 1,2-Dichloroethane
13.08
97
Y
TCE Trichloroethylene
14.39
10
Y
1,2-Dichloropropane 1,2-Dichloropropane
15.10
10
N
Methylene Bromide Dibromomethane
15.45
2
Y
Cis-1,3-Dichloropropene Cis-1,3-Dichloropropene
16.60
894
Y
Trans-1,3-dichloropropene Trans-1,3-dichloropropene
16.70
681
Y
1,1,2-Trichloroethane 1,1,2-Trichloroethane
18.30
256
Y
PCE, Perchloroethylene Tetrachloroethylene
18.96
3
Y
Chlorodibromomethane Chlorodibromomethane
19.80
17
N
Ethylene Dibromide 1,2-Dibromoethane
20.20
14
N
1,1,1,2-Tetrachloroethane 1,1,1,2-Tetrachloroethane
21.00
12
N
R-130 1,1,2,2-Tetrachloroethane
23.75
2316
Y
Hexachloro-1,3-Butadiene Hexachloro-1,3-Butadiene
34.00
236
Y

*For sample volume of 34 cc.