Precise Time-Scaling of Gas Chromatographic

Methods Using Method Translation and

Retention Time Locking

Application

Gas Chromatography
May 1998

Authors Key Words the observed element content of the

peak. The combination of time and
B. D. Quimby, L. M. Blumberg, Pesticides, GC, GC-AED, retention element content narrows rapidly the
M. S. Klee, and P. L. Wylie time locking, RTL, method transla- possible compounds that could have
Agilent Technologies, Inc.
tion, scalable RT libraries produced the heteroatom response to
2850 Centerville Road
Wilmington, DE 19808-1610 a few pesticides.
USA Introduction The element-selective detection
Abstract Interest in the analysis of pesticide
is done with either gas
chromatography-atomic emission
residues has been increasing recently,
Complete development of a gas chro- detection (GC-AED), which can
in part due to the discovery that some
matographic method often involves a screen for all the individual elements
of these compounds act as endocrine
significant amount of effort. Once a found in pesticides, or with a combi-
disrupters. Agilent Technologies has
method is completed, retention time nation of other selective detectors
responded to the need for rapid, accu-
locking (RTL) can be used to implement like the electron capture detector
rate, and comprehensive screening
the method and to obtain the same (ECD), the nitrogen-phosphorus
retention times on multiple systems. analysis for pesticides by developing
detector (NPD), the flame photomet-
This application note describes how to a method to screen for 567 pesticides
ric detector (FPD), or the electrolytic
use method translation combined with and suspected endocrine disrupters.
conductivity detector (ELCD).
RTL to implement precise time-scaled The method uses element- selective
versions of a method on multiple instru- detection and a retention time locked The GC-AED technique can also be
ment types. This allows the original library of retention times to find and used to calculate element ratios and
method to be re-used with minimal identify pesticides in a sample.1 to quantitate unknown peaks that are
effort, while optimizing the method for detected because of its equimolar ele-
a given sample type or instrument In the method, sample extracts are
ment response factors. The measured
setup. In this way, the utility of the run with element-selective detection
element ratios can be used to further
original method is extended greatly, using a prescribed set of chromato-
distinguish between possible identi-
increasing the payback on the invest- graphic conditions and with the
ties of detected heteroatomic com-
ment in its development and optimizing column retention time locked to the
pounds, often resulting in a single
its use for specific analyses. In this retention times in a table. If any peaks
entry as the likely identity of a given
note, the Agilent RTL Pesticide Library containing heteroatoms are observed,
peak. With compound-independent
method is used as an example. The the section of the table corresponding
calibration, the amount of the
steps involved in precise time-scaling of to a small time window around the
the method to different speeds, detec- unknown can be calculated using ele-
observed peak is searched. The time
tors, and columns are presented. ment response factors generated with
search results are further sorted using
a different standard compound.
Once the element-selective screen is whenever the method is modified, for 2. Use the method translation soft-
completed, samples that contain any example, to increase its speed. ware4 to calculate the inlet pres-
suspect compounds are run on a GC sure and oven temperature
with mass spectral detection (GC-MS) Method translation2–4 is a calculation adjustments to obtain the desired
system that is retention time locked technique developed at Agilent scaling of the method. The scale
to the pesticide method, thus having Technologies that allows a capillary factor is the “speed gain” value
the same retention times as the column GC method to be translated reported in the method translation
element-selective detectors. Using the to different chromatographic condi- software. Make sure that the new
possible identities generated from the tions. The technique calculates the method parameters are consistent
required changes in inlet pressure and with the hardware capabilities of
element screen, the GC-MS data is
oven temperature ramp rates and where the new method will be
evaluated to decide which (if any) of
hold times required to maintain peak used.
the possible identities for suspect
peaks is correct. The confirmation elution order identical to that of a ref- 3. Perform the RTL calibration runs
process is simplified greatly because erence method. In this way, the speed for the new method. Alternatively,
the element screen usually yields only of an analysis can be scaled pre- the method translation software
a few possibilities and because the dictably to accommodate the needs of can be used to calculate the RTL
retention time in the GC-MS run is a specific sample or instrument type. calibration points for the new
accurately known. In practice, method using those from the origi-
The inlet pressure calculated for the nal method.
extracted ion chromatograms for
new version of a method by the
characteristic ions of each possible 4. Retention time lock the new
method translation software is based
compound are used to determine the method using the locking refer-
on the assumed or nominal dimen-
identity of suspect compounds. ence standard from the original
sions of the column. As such, the cal-
method. The new method should
This screening method minimizes culated inlet pressure will provide a
be locked to the original reference
false negatives, even in dirty samples, close, but not exact, match to the
standard retention time divided by
by using element-selectivity and time desired scaled retention times. To
the scale factor.
in the initial screen. With element- match precisely the retention times of
selective detection, all compounds the scaled method to the desired 5. Export the retention time table as
scale factor, the new method must be a text file using the EXPORT func-
containing chlorine, phosphorus,
retention time locked. Retention time tion in the RTL SEARCH menu of
nitrogen, etc. are detected. Even if a
locking3 (RTL) is a technique devel- the RTL ChemStation software.
detected heteroatomic compound is
not in the table, its presence is oped by Agilent Technologies 6. Divide the retention times in the
known, and it can be marked for fur- whereby the inlet pressure required table by the scale factor in a
ther GC-MS evaluation. By using GC- to match retention times precisely is spreadsheet program like
MS for confirmation, false positives calculated from a calibration curve of Microsoft® Excel™.
are also minimized. inlet pressure versus retention time.
7. Re-import the new, scaled table.
The RTL Pesticide Library method is Using method translation followed by 8. Run a representative test mixture
a good example of a method in which RTL allows a method to be scaled by to validate the scaled method.
a substantial investment of time and a precisely known factor. Once the
material has been made. As with chromatography has been scaled, a Several examples of scaling the
many methods intended for use in retention time table, such as the RTL HP RTL Pesticide Library are
multiple laboratories, it would be Pesticide Library, can then be scaled presented below.
desirable to be able to scale the by the same factor, resulting in a new
method for use in different situations library whose retention times match
Experimental
of sample type and instrument setup. those of the scaled method precisely.
Because the method relies on the All data were collected on
The steps required to scale the
measured retention times of 567 com- Agilent 6890 Series GC systems. All
method are:
pounds, it would be impractical to re- systems were equipped with:
measure all the retention times 1. Determine the desired scale factor
• Electronic pneumatics control
for the new method.
(EPC)

2
• Split/splitless inlet Results and Discussion Figure 1 shows the method transla-
tion software. The original method
• Automatic liquid sampler
Locking GC-MS with Other GC conditions for the GC-AED pesticide
The GC-AED system also included an Detectors method are entered in the column
Agilent G2350A atomic emission labeled “Original Method.” The
detector with GC-AED ChemStation When using selective GC detectors in column dimensions, carrier gas type,
software (rev B.00.00) for Microsoft® conjunction with GC-MS, one prob- inlet pressure, outlet pressure, ambi-
Windows NT®. lem that is encountered is knowing ent pressure, and oven temperature
the relationship between retention program are entered here. Note that
The GC-micro-ECD system was con- times on the selective detector and the inlet pressure is in psi (gauge),
trolled by Agilent GC ChemStation that of the GC-MS. In GC-MS, the while the outlet pressure and ambient
software (rev A.05.04). Both the outlet pressure of the column is pressure are psi (absolute). The origi-
GC-AED and the GC-micro-ECD vacuum, while with most other GC nal method here is being used on a
ChemStations contained RTL soft- detectors, the outlet pressure of the GC-AED system, so the outlet pres-
ware for GC ChemStation (G2080AA) column is at or near atmospheric sure is entered as atmospheric pres-
and the Retention Time Locking Pes- pressure. This difference in outlet sure plus 1.5 psi, the operating
ticide Library for GC ChemStation pressures results in large differences pressure of the GC-AED.
(G2081AA). in retention time between GC with
MS detection and GC with other The “Criterion” parameter is set to
The GC-MS system (G1723A) used detectors. Comparison of GC-FID, a “None,” which allows the user to
consisted of an 6890 Series GC general detector, with GC-MS is rea- select a specific value of “speed gain”
equipped with an Agilent 5973 mass sonably straightforward, because the by adjusting the value of hold-up time
selective detector (MSD). The total ion chromatogram (TIC) of the for the translated method (see
process for retention time locking the GC-MS system has similar response figure 1). In the column labeled
GC-MS system is described in to the FID. Retention times on the “Translated Method,” the parameters
reference 2. GC-MS system corresponding to of column dimensions, carrier gas
those on the GC-FID can be deter- type, outlet pressure, and ambient
All systems except the micro-ECD
mined by looking for similar patterns pressure for the GC-MS method are
instrument used 30 m ´ 0.25 mm id ´
of response. With selective detectors, entered. Note that the inlet pressure
0.25 mm HP-5MS columns (part no.
this is much more difficult because and oven program are not entered;
19091S-433). The Agilent micro-ECD
the response patterns from selective they are calculated by the program.
instrument used 10 m ´ 0.1 mm id ´
detectors usually do not resemble the To set the speed gain to a desired
0.1 mm HP-5 column (part no.
TIC. For this reason, matching the value, take the calculated value of
19091J-141).
retention times of selective detectors hold-up time in the first column
RTL measurements were made with a precisely with the GC-MS system sim- (0.996060 minute) and divide it by the
solution of dichlorvos, methyl chlor- plifies data analysis greatly. scale factor. Because in this case the
pyrifos, and mirex, each at 10-ppm desired scale factor (“speed gain”) is
In this first example of scaling the 1, the same hold-up time for both the
concentration in acetone. All injec-
RTL Pesticide Library, the method GC-AED and the GC-MS methods is
tions were 1-mL splitless, except for
will be scaled from the GC-AED required. Clicking the radio button
the micro-ECD experiments, which
method to the GC-MS method. In this next to the hold-up time in the “Trans-
were 1-mL split 100:1. In all methods,
case, the desired scale factor is lated Method” column will do this
inlets were operated at 250 °C and
exactly 1, that is, the GC-MS retention automatically.
detectors at 300 °C.
times are desired to be exactly the
Method translation requires inlets to same as those of the GC-AED. The The method translation indicates that
be run in constant pressure mode to first step is to use the method transla- to obtain the same retention times on
obtain precise scaling of retention tion software to determine the GC the GC-MS system as on the GC-AED,
times. Thus, all methods discussed in conditions to use for GC-MS. use all the same method parameters
the note were run in this mode.

3
except inlet pressure. Instead of using
27.6 psi as is used on the GC-AED,
method translation calculates that
17.93 psi on the GC-MS system will
result in matching retention times. As
mentioned above, this inlet pressure
is calculated on the assumed dimen-
sions of the column in the GC-MS
system. To get the retention times to
match precisely, RTL3 is used.

To retention time lock the GC-MS

method to the GC-AED method in this
example, it is necessary to construct
an RTL calibration file for the GC-MS
system. Construction of this file only
needs to be done once. All subse-
quent users of the GC-MS method will
then be able to use this calibration
file for a similarly configured GC-MS
instrument.

The RTL calibration file is con-

structed by running five calibration Figure 1. Method translation software showing scaling HP RTL Pesticide Method from GC-
runs of the target compound, in this AED conditions to GC-MS with a scale factor of 1.
case methyl chlorpyrifos, at five dif-
ferent inlet pressures. The runs are the nominal method pressure, and the calibration data, method translation
made at conditions identical to the retention time is observed. The pres- can be used to calculate the new RTL
nominal method except that four of sure and resulting retention time are calibration points. This is useful when
the runs are made at different pres- then entered into the “(Re)Lock New you want to try a scaled method
sures. The pressures used are Column” menu item of the RTL soft- rapidly and save the time required in
typically: ware to calculate the correct pressure making the five runs. (Note: For
• Target pressure – 20% for obtaining locked retention times. methods that will be used exten-
sively, the five-runs approach may
• Target pressure – 10% Normally, the RTL calibration for a provide a somewhat better calibra-
• Target pressure (nominal method new method is determined by actually tion. It is recommended that for these
pressure) making the five calibration runs. In methods, the standard calibration be
the current example, methyl chlor- performed.)
• Target pressure + 10% pyrifos would be run at:
• Target pressure + 20% To calculate the five RTL calibration
• 17.93 psi – 20% = 14.34 psi
pairs of pressure and retention time
The retention time of the target com- • 17.93 psi – 10% = 16.14 psi for the GC-MS method from those of
pound is determined for each run. the GC-AED method:
• 17.93 psi (nominal method
The resulting set of five pressures and
pressure) • Take the inlet pressure used for
corresponding retention times is then
each original GC-AED RTL cali-
entered in the RTL calibration dialog • 17.93 psi + 10% = 19.72 psi
bration run, and enter it into the
box for the method and saved with • 17.93 psi + 20% = 21.56 psi method translation software for
the method.
the inlet pressure of the original
However, because the new GC-MS method. Make sure the hold-up
To lock the method on the GC-MS method is scaled from an existing times are locked, giving a “speed
setup, the target compound is run at GC-AED method that already has RTL gain” of 1.

4
• The inlet pressure calculated in Column 4 of table 2 shows the locking The RTL Pesticide Library contains
the “Translated Method” column pressures for the same set of runs but the retention times of the 567 pesti-
will now change to a new value, determined using the GC-MS RTL cal- cides measured with GC-FID. The
corresponding to the pressure ibration points calculated using values measured with the FID would
that would be obtained if the method translation. The calculated be the same observed with any detec-
calibration run were made on a data provide locking pressures that tor that is operated at or near atmos-
GC-MS system. This pressure is agree well with those based on mea- pheric pressure. Because retention
used with the retention time sured data. The range in locking pres- time matching is critical in this appli-
obtained for the corresponding sures pressure is only from 17.72 to cation, the retention times for all the
GC-AED calibration run as a cali-
17.75 psi. This range of 0.03 psi corre- compounds in the table were also
bration point for the GC-MS
sponds to only about a 0.006-minute measured on the GC-MS system after
method.
range in the retention time of methyl scaling as described here. Figure 3 is
When all five points have been chlorpyrifos. a plot of the difference between the
calculated in this way, they are retention times measured on the
Figure 2 shows the locked GC-FID and the GC-MS systems. The
entered into the RTL calibration
chromatograms from a three- plot shows the retention times match
dialog box for the GC-MS method and
component mixture run on GC-AED well within ± 0.1 minute out to 30
saved with the method. Table 1 lists
and GC-MS systems. As can be seen, minutes. A few compounds at the end
the original RTL calibration pressures
the retention times are well matched deviate outside this window, with one
and times with the calculated pres-
between the two methods. compound 0.2-minute different. The
sures and times for the GC-MS
method.

To test the accuracy of using a

predicted RTL calibration file for Table 1. RTL Calibration Points from Original GC-AED Method and
GC-MS, a real calibration set was Calculated Points for GC-MS
measured on the GC-MS system. The
GC-AED RTL Calibration GC-MS RTL Calibration
data is shown in the first two columns
Calculated Calculated
of table 2. (Note: The calibration Pressure Ret Time Pressure Ret Time
points are spaced ~ 5% apart in pres- (psi) (min) (psi) (min)
sure instead of the typical 10%.) A 33.1 15.346 24.27 15.346
GC-MS RTL calibration file was con- 30.4 15.919 21.18 15.919
structed with these measured points. 27.6 16.578 17.934 16.578
For each point, the locking pressure 24.8 17.338 14.654 17.338
required to lock the method was cal- 22.1 18.242 11.449 18.242
culated and is shown in column 3 of
table 2.
Table 2. Comparison of Locking Pressures Calculated Using
Measured and Predicted GC-MS RTL Calibration Data
The locking pressure is the pressure
determined by the RTL software that GC-MS Locking Runs Locking Pressures
would make methyl chlorpyifos have Measured GC-MS RTL Cal Points Using Measured Using Calculated
a retention time of 16.596 minutes. RTL Cal Points RTL Cal Points
This is determined by entering the Pressure Ret Time Pressure Pressure
( psi) (min) (psi) (psi)
pressure and retention time for each
20 16.127 17.73 17.75
point into the “(Re)Lock New
19 16.326 17.72 17.73
Column” menu item of the RTL soft-
18 16.536 17.72 17.72
ware. If the calibration is done cor-
17 16.760 17.74 17.74
rectly, the locking pressures
16 16.988 17.72 17.74
determined from each point should
be very similar, as they are in column
3 of table 2.

5
deviation is clearly largest in the Gaining Speed in the Same the matrix. This approach can save a
isothermal hold region, which starts Instrument Setup significant amount of analysis time.
at 31.87 minutes. This effect is seen
with GC-MS, but not with scaling to In the analysis of pesticide residues in In this example of scaling the RTL
other atmospheric pressure detectors. food, there are usually only a few Pesticide Library, the method will be
While the cause is not yet clearly compounds encountered in any one increased in speed at the expense of
understood, it appears related to the sample. Because the screening chromatographic resolution. The first
vacuum outlet pressure of the GC-MS method uses selective detectors, it consideration is by what factor to
column. Although this level of match- makes sense to consider trading increase the speed. The method trans-
ing is very good, the table includes speed for chromatographic resolu- lation software is useful for determin-
both the GC-FID and GC-MS retention tion. Selective detectors respond to ing this. A candidate speed gain, in
times so that smaller time windows only those compounds containing a this example threefold, is entered into
can be used in searching unknowns. specific heteroatom(s), and the chro- the method translation software. The
matography only needs to resolve resulting inlet pressure and oven tem-
Locking GC-AED with Other GC those compounds from each other, perature ramp rates are then
Detectors not from every other compound in inspected to see if the instrument on

When the method translation step is 1

done to scale the GC-AED method to GC-MS 3
other atmospheric pressure detectors,
the only different parameter to enter
is the outlet pressure. The outlet pres- 2
sure for the GC-AED method is
16.2 psi and that for the others is
14.696 psi. The method translation
calculates that the nominal GC-AED 5 10 15 20 25 30 35
inlet pressure of 27.6 psi would be
changed to 26.29 psi for the other
GC-AED
atmospheric detectors. This differ-
ence (<5%) is so small that it can be
neglected, because corrections in this
range are compensated easily by the
retention time locking step. Thus, the
0 5 10 15 20 25 30 35 40 min
method conditions and RTL calibra-
tion points used with GC-AED are Figure 2. GC-AED chlorine and GC-MS TIC chromatograms of three-component locking mix-
interchangeable with FID, NPD, ECD, ture. Peak identifications: 1. dichlorvos, 2. methyl chlorpyrifos, 3. mirex.
FPD, and other atmospheric detector
methods. 0.300

Note that this would not always be 0.200

the case. If for example, a method is
Difference (mm)

being scaled that uses a very low inlet 0.100

pressure, the 1.5-psi difference in 0.000

outlet pressure could become signifi- 0 5 10 15 20 25 30 35 40 45
cant. It is best to check the method -0.100

with method translation and see if the -0.200

inlet pressure will change by >10%. If
it does, it would be advisable to col- -0.300
Retention Time (min)
lect (or translate) a new RTL calibra-
tion centered around the translated Figure 3. Difference plot of GC-MS and GC-FID retention times in RTL Pesticide Library.
nominal inlet pressure.

6
which the new method will be run is GC-AED method. This is done by the Table 4 compares the locking pres-
compatible with those parameters. same process as shown in the GC-MS sures determined with measured and
scaling above. In this case, when one with calculated RTL calibration
Figure 4 shows the method transla- of the original method RTL calibra- points. As in the above GC-MS exam-
tion software with the data entered tion pressures is entered, the result- ple, the range of the locking pressures
for a speed gain of 3. Note that ing holdup time must be divided by 3 from the calculated data is only
columns for “Original Method” and and entered for the holdup time in the 0.11 psi (87.88 to 87.99), which
“Translated Method” are set up as in “Translated Method” column. This corresponds to ~ 0.003 minute.
the previous example with two excep- will force the “speed gain” back to 3.
tions. Because the scaling is from The resulting inlet pressure is then Figure 5 compares the chro-
GC-AED to GC-AED, the outlet pres- paired with the retention time of the matograms of the RTL locking mix-
sure in both columns is entered as corresponding original GC-AED cali- ture from both the original and the 3´
16.2 psi. The second and most signifi- bration run, but divided by 3 as a cali- scaled methods. Note that while the
cant difference is the holdup time. bration point for the new method. chromatographic resolution is
The desired “speed gain” is 3. reduced, the speed is increased by a
Table 3 shows the RTL calibration factor of 3.
To set the speed gain, the calculated points from the original GC-AED
value of hold-up time in the first method and calculated points for the Figure 6 shows a plot of the differ-
column (0.996060 minute) is divided threefold speed gain (3´) method. ence between the RTL Pesticide
by exactly 3. This value Library retention times, divided by 3,
(0.33202 minute) is entered for the When the calibration data is entered and those of the 3´ method. The data
hold-up time in the second column. into the RTL calibration dialog box, were taken with a 36-component
This will force the speed gain to the target time for methyl chlorpyri- subset of the library. The plot shows
exactly 3. fos is entered as 5.532 minutes, which the retention times match well within
is 16.596 minutes divided by 3. ± 0.05 minute for all compounds, even
The inlet pressure and oven tempera-
ture ramp for the new threefold speed
method are now calculated. The cal-
culated inlet pressure is 87.862 psi,
which is compatible with the EPC
module on the current system (maxi-
mum 100 psi). Note that the helium
source supplying the GC must be
capable of reaching 100 psi of helium.
An optional 150-psi EPC module is
available for the HP 6890 GC to pro-
vide additional inlet pressure, if
necessary.

The oven temperature program calcu-

lated for the new method has the first
ramp listed as 75 °C/min. This ramp
rate is compatible with the 240-V
oven option on the current instru-
ment but would not work with a
120-V oven, which is limited to about
50 °C/min in this temperature range.
With a 120-V oven, the speed gain
would be limited to about 2.

The next step is to calculate the RTL Figure 4. Method translation software showing scaling RTL Pesticide method scaled to
calibration points from the original threefold faster method.

7
those in the 3.3-minute hold time at Table 3. RTL Calibration Points from Original GC-AED Method and
the end of the run. Calculated Points for Threefold Speed Gain (3´) Method

GC-AED RTL Calibration 3x GC-AED RTL Calibration

Gaining Speed with a Small-Bore Calculated Calculated
Pressure Ret Time Pressure Ret Time
Column (psi) (min) (psi) (min)
33.1 15.346 106.21 5.115
In the previous example, speed was
30.4 15.919 97.23 5.306
gained at the expense of resolution.
27.6 16.578 87.86 5.526
In this example, speed will be gained
24.8 17.338 78.44 5.779
while maintaining most of the resolu-
22.1 18.242 69.31 6.081
tion but sacrificing capacity. This is
done by scaling the original method
to a 0.1-mm id column.
Table 4. Comparison of Locking Pressures Calculated Using Mea-
In scaling to columns of a different
sured and Predicted 3´ GC-AED RTL Calibration Data
diameter, there are two important
considerations that must be obeyed 3x GC-AED Locking Runs Locking Pressures
to obtain precise matching to a Measured 3x GC-AED RTL Cal Points Using Measured Using Calculated
library or reference method. The first RTL Cal Points RTL Cal Points
is that the stationary phase composi- Pressure Ret Time Pressure Pressure
( psi) (min) (psi) (psi)
tion must be the same as that used in
97 5.319 87.99 87.99
the original method. The second is
92 5.433 87.94 87.95
that the phase ratio of the column
87 5.557 87.99 87.99
being scaled to must be the same as
82 5.689 87.99 87.96
that of the reference method.
77 5.832 87.97 87.88
Columns of the same phase ratio have
the same ratio of inner diameter to
film thickness. Because the reference
method was developed on a column 3
with 0.25 mm id ´ 0.25 mm film thick- 1 GC-AED (1x)
ness, scaling to a 0.1-mm id column
2
will require a 0.1-mm film thickness. A
10-m column of these dimensions was
chosen for this example.

The micro-ECD for the 6890 GC is 0 5 10 15 20 25 30 35 40 min

extremely sensitive, with detection
limits in the low femtogram range for
polyhalogenated pesticides. These GC-AED (3x)
detection limits are so low that it is
reasonable to consider using split
mode for a rapid screening method.
Using split mode with a split ratio of
100 still gives a detection limits in the
0 2 4 6 8 10 12 min
range of a few picograms. The split is
also more compatible with the rela- Figure 5. Chlorine chromatograms from original and 3x GC-AED methods of three-component
locking mixture. Peak identifications: 1. dichlorvos, 2. methyl chlorpyrifos,
tively low capacity of the column.
3. mirex.

8
Figure 7 shows the method transla- 0.2
tion from the GC-AED method to the
0.1-mm id column with a scale factor 0.15
of 3. A speed gain of 3 was again
chosen based on oven and inlet limi- 0.1
tations as described above. The same
scaling process as used above is 0.05

Difference (min)
followed.
0
The RTL calibration points for 0 2 4 6 8 10 12 14
the new 3´ 0.1-mm micro-ECD -0.05
method were both calculated with
-0.1
method translation and measured.
Table 5 shows the calculated values.
-0.15
When the locking pressures from the
measured and calculated values were -0.2

examined, the calculated values pro- Retention Time (min)

vided much poorer predictions of Figure 6. Difference plot of RTL Pesticide Library (GC-FID) retention times divided by 3 minus
locking pressure than expected. The 3´ GC-AED retention times for 36-compound subset of the library.
pressure required to actually lock the
column was confirmed to be
65.95 psi, as predicted by the mea-
sured RTL calibration data. Method
translation had predicted the inlet
pressure would be 58.514 psi for an
assumed 10-m column length.
Because the actual locking pressure
was noticeably higher, this suggests
that the actual column length was
longer and/or the column diameter
was smaller and/or the film thickness
larger than the assumed values.

As an experiment, it was assumed

that the problem was in the assumed
length of the column used in calculat-
ing the RTL calibration points. The
column length entry for the 0.1-mm
column was iteratively adjusted until
the calculated inlet pressure matched
the actual locking pressure, 65.95 psi.
This resulted in a calculated column
length of 10.5622 m. A new set of cal-
culated RTL calibration points were
calculated using 10.5622 m as the
length of the 0.1-mm column. The
results are shown in table 6. Figure 7. Method translation software showing scaling RTL pesticide method scaled to a
threefold faster method on a 10-m ´ 0.1-mm id column.

9
Table 7 shows a comparison of lock- Table 5. RTL Calibration Points from Original GC-AED Method and
ing pressures calculated using mea- Calculated Points for 3´ 0.1-mm id Micro-ECD Method
sured and predicted 3´ 0.1-mm id Assuming 10-m Column Length
micro-ECD calibration data. The
GC-AED RTL Calibration 3x Micro-ECD RTL Calibration
range of locking pressures from the
Calculated Calculated
measured data (66.03 to 65.93) only Pressure Ret Time Pressure Ret Time
corresponds to a spread in retention (psi) (min) (psi) (min)
times of about 0.004 minute. How- 33.1 15.346 71.03 5.115
ever, with the data calculated based 30.4 15.919 64.90 5.306
on a 10-m assumed length, the spread 27.6 16.578 58.51 5.526
(66.38 to 63.18) is much larger and 24.8 17.338 52.11 5.779
would correspond to a time range of 22.1 18.242 45.91 6.081
0.14 minute. The locking pressures
calculated using the 10.5622 value are
much more consistent with the mea-
sured values. The range in retention Table 6. RTL Calibration Points from Original GC-AED Method and
Calculated Points for 3´ 0.1-mm id Micro-ECD Method
times would be ~ 0.03 minute if all the
Assuming 10.5622-m Column Length
calculated points are used, and if the
first value in column 5 is ignored, the GC-AED RTL Calibration 3x Micro-ECD RTL Calibration
range drops to ~ 0.005 minute. Calculated Calculated
Pressure Ret Time Pressure Ret Time
(psi) (min) (psi) (min)
The fact that the agreement in locking
33.1 15.346 80.03 5.115
pressures is much improved by using
30.4 15.919 73.13 5.306
10.56 m instead of 10 m suggests that
27.6 16.578 65.95 5.526
length is probably the largest contrib-
24.8 17.338 58.74 5.779
utor to the discrepancy. These results
22.1 18.242 51.75 6.081
should reinforce the recommendation
that if a method is to be used exten-
sively, it is prudent to obtain mea-
sured RTL calibration data. It should Table 7. Comparison of Locking Pressures Calculated Using Measured and
be noted, however, that even with the Predicted 3´ 0.1-mm id Micro-ECD Calibration Data
RTL calibration from the 10-m 3x Micro-ECD Locking Runs Locking Pressures
assumed length, the worst conse- Measured 3x Micro-ECD RTL Using Measured Using 10-m Calculated Using 10.56-m Calculated
quence would be that the RT locking Cal Points RTL Cal Points RTL Cal Points RTL Cal Points
step would need to be repeated an Pressure Ret Time Pressure Pressure Pressure
extra time to get a more precise (psi) (min) (psi) (psi) (psi)
match. 48.81 6.323 65.95 66.38 65.30
52.66 6.041 66.03 65.77 65.85
Figure 8 compares the chromato- 58.51 5.797 65.95 65.12 65.96
grams of the RTL locking mixture 64.36 5.585 65.93 64.36 65.95
from both the original and the 70.22 5.396 66.00 63.18 65.90
3 ´ 0.1-mm id micro-ECD methods.

10
Note that while the most of the References
chromatographic resolution is pre- 1. P. L. Wylie and B. D. Quimby, “A
served, the speed is increased by a Method Used to Screen for 567
factor of 3. Pesticides and Suspected
Endocrine Disrupters,”
After being locked, the three peaks in Hewlett-Packard Company, Appli-
the 3´ micro-ECD method had reten- cation Note 228-402, Publication
tion times of 1.924, 5.533, and 9.963 5967-5860E, April 1998.
minutes, respectively. These values
2. M. Klee and V. Giarrocco, “Pre-
are very close to the RTL Pesticide
dictable Translation of Capillary
Library retention times for the three
GC Methods for Fast GC,”
compounds divided by 3: 1.932, 5.532,
Hewlett-Packard Company, Appli-
and 9.949. The fact that the largest
cation Note 228-373, Publication
difference between the scaled table
5965-7673E, March 1997.
and the 3´ micro-ECD method is only
0.014 minute again demonstrates the 3. V. Giarrocco, B. D. Quimby, and
precision of retention time matching M. S. Klee,“Retention Time Lock-
achievable with the scaling technique ing: Concepts and Applications,”
described here. Hewlett-Packard Company, Appli-
cation Note 228-392, Publication
5966-2469E, December 1997.
Conclusions
4. Capillary Column Method Transla-
Using method translation combined tor, user contributed software,
with retention time locking provides a free download from:
means of extending the usefulness of www.hp.com/go/mts.
existing capillary GC methods. The
ability to precisely scale a method to
meet the needs of different samples
and instrument types greatly reduces
the effort required to re-use methods,
thus saving time and money.

3
1 GC-AED (1x)
2

0 5 10 15 20 25 30 35 40 min

GC-micro-ECD (3x)

0 2 4 6 8 10 12 min

Figure 8. Chlorine chromatogram from 1´ GC-AED method (top) and 3´ micro-ECD method
(bottom) of three-component locking mixture. Peak identifications: 1. dichlorvos, 2.
methyl chlorpyrifos, 3. mirex.

11
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