Table 6. Calibration factors obtained at
four options, in order of increasing difficulty, for
different carrier gas linear velocities using
nonlinear calibration data: adjust the instrument or
a DB-1 column and 50-g/L solutions.
perform instrument maintenance; narrow the cali-
bration range until response is linear (<20% RSD for
Calibration factors
calibration factor); use a linear calibration model
(peak height/[g/L])
that does not pass through origin; use a nonlinear
Analyte
126 cm/s
76 cm/s
44 cm/s
calibration model.
TNT
104
106
109
From the shapes of the curves of peak height
2,6-DNT
91
90
92
data for TNT, 2-Am-DNT, and RDX over the range
2-Am-DNT
80
77
83
0.5 to 100 g/L (Fig. 4), which are representative of
RDX
79
57
52
calibration curves for the other analytes, we see that
HMX
75
59
32
4-Am-DNT
71
71
76
fitting the data to straight lines, whether through
DNA
69
65
69
the origin or not, is not at all appropriate. Narrow-
2,4-DNT
58
55
58
ing the concentration range does bring the average
Tetryl
46
48
59
calibration factors for most of the analytes within
TNB
45
48
50
20% RSD threshold for linearity prescribed in SW-
DNB
30
29
28
NB
18
21
9.9
846. This very limited linear range of the ECD is a
PETN
17
13
8.1
disadvantage compared to the HPLC-UV, which has
NG
12
4.9
4.5
a broad linear range. For GC-ECD, sample extracts
m-NT
7.5
7.0
7.4
would need to be diluted within the proper calibra-
o-NT
5.9
6.6
3.6
tion range. For samples with multiple analytes at
p-NT
2.5
5.8
4.5
varying concentrations, a single extract may require
several determinations at different dilution factors.
and Bonelli 1969). This narrow linear range is in-
Alternatively, nonlinear models in the form of sec-
convenient for quantitative analysis of samples
ond-order polynomials fit the data over broader
that can vary over three orders of magnitude in
concentration ranges (Fig. 4). Using nonlinear cali-
analyte concentrations. The SW-846 criterion for
bration models complicates computations, but re-
linearity is that the relative standard deviation
duces the number of reanalyses of multi-analyte
(RSD) of the mean calibration factor be less than or
samples.
equal to 20% for five standards at different concen-
trations. The calibration data we obtained show
Instability of low-concentration
linear models relating responses to concentrations
trinitroaromatic standards
that are not appropriate over the entire concentra-
The low-concentration calibration standards for
tion ranges we tested. SW-846 (USEPA 1997) lists
Table 7. Effect of injection-port temperature on GC response when the carrier gas
linear velocity was 133 cm/s.
Temperature (C)
Normalized
response*
200
210
220
230
240
250
260
270
280
290
300
NB
1
0.95
0.77
0.82
0.76
0.94
0.88
0.87
0.88
0.87
0.84
o-NT
1
0.94
0.79
0.80
0.73
0.88
0.82
0.80
0.81
0.79
0.76
m-NT
1
0.96
0.86
0.85
0.80
0.92
0.86
0.84
0.86
0.83
0.81
p-NT
1
0.96
0.87
0.84
0.79
0.89
0.83
0.82
0.82
0.80
0.78
DNB
0.98
0.98
1.00
0.93
0.93
1.00
0.99
0.95
0.99
0.99
0.87
2,6-DNT
0.95
0.96
1.00
0.95
0.91
0.99
0.98
0.94
0.97
0.98
0.94
2,4-DNT
0.99
0.95
1.00
0.95
0.90
0.98
0.97
0.95
0.97
0.97
0.97
TNB
1
0.98
0.95
0.95
0.89
0.92
0.93
0.94
0.90
0.87
0.86
TNT
1
0.99
0.90
0.98
0.85
0.89
0.92
0.93
0.90
0.83
0.82
RDX
0.95
0.94
1.00
0.95
0.95
1.00
0.97
0.99
1.00
0.96
0.97
4-Am-2,6-DNT
0.92
0.91
0.98
0.93
0.92
0.97
1.00
0.98
0.99
1.00
0.97
2-Am-4,6-DNT
0.92
0.91
0.97
0.94
0.92
0.96
1.00
0.96
0.97
0.99
0.96
HMX
0.71
0.77
0.87
0.83
0.90
0.97
0.95
1.00
1.00
0.96
0.95
NG
0.96
1.00
0.96
0.93
0.92
0.94
0.90
0.85
0.85
0.80
0.82
PETN
0.96
1.00
0.93
0.95
0.93
0.89
0.92
0.92
0.91
0.86
0.87
3,5-DNA
0.89
0.93
0.94
0.94
0.94
0.99
0.97
1.00
1.00
0.98
0.98
Tetryl
0.97
0.98
0.97
0.91
0.96
0.90
1.00
0.90
0.87
0.81
0.88
*([peak height]/[maximum peak height])
9
Previous Page
