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Process
NMR – Initial Calibration and Validation
Practices
for NMR Multivariate Quantitative Analysis
(Ref:
ASTM Standard E1655-00)
The
following practices describe the development of the multivariate NMR
calibrations used to predict the chemical and physical properties of
materials. These practices are applicable to the development of
calibrations on any NMR instrument regardless of magnet field strength. The
practices stated are encompassed by standard E1655-00 which although centered on
Infrared spectroscopy is applicable to the NMR calibration development.
Procedures
for collecting and treating data for developing NMR calibrations will be
outlined. Also, the following
summary of the calibration process can be
given:
- Multivariate
mathematics is applied to correlate the relative intensity and position of
NMR resonance peaks measured for a set of calibration samples to validated
laboratory data of chemical and physical properties. The resulting
multivariate calibration models can be applied to NMR spectra of unknown
samples to provide an estimate of the chemical and/or property values for
the unknown sample.
- The
multivariate mathematics referes to techniques such as multilinear
regression (MLR), principal components regression (PCR), and partial least
squares (PLS) which are used to develop the calibration models.
- The
advatage of these techniques is that statistical tests can be applied to
detect outliers during the development of the calibration model. Outliers
include high leverage samples (samples whose spectra contribute a
statistically significant fraction of one or more of the spectral variables
used in the model), and samples whose reference values are inconsistent with
the model.
- Validation
of the calibration model is performed by using the model to analyse a set of
validation samples and statistically comparing the estimates for the
validation samples to reference values measured for these samples.
This allows test for bias in the model and for agreement of the model
with the reference method.
Referenced
Documents:
D
1265 Practice for Sampling Liquified Petroleum (LP) Gases (Manual Method)
D
4057 Practice for Manual Sampling of Petroleum and Petroleum Products
D
4177 Practice for Automatic Sampling of Petroleum and Petroleum Products
D
4855 Practices for Comparing Test Methods
D
6122 Practice for Validation of Multivariate Process Infrared Spectrophotometers
D
6299 Practice for Applying Statistical Quality Assurance Techniques to Evaluate
Analytical Measurement System Performance
D
6300 Practice for Determination of Precision and Bias Data for Use in Test
Methods for Petroleum Products and Lubricants
E
386 Standard
Practice for Data Presentation Relating to High-Resolution Nuclear Magnetic
Resonance (NMR) Spectroscopy
E
131 Terminology Relating to Molecular Spectroscopy
E
456 Terminology Relating to Quality and Statistics
E
691 Practice for Conducting an Interlaboratory Study to Determine the Precision
of a Test Method.
NMR
Multivariate Calibration:
1)
Selection
of a calibrations set: see 1655,
Section 17.
2)
Samples
from petrochemical processes can degrade with time unless careful handling and
sample storage procedures are followed. For successful calibration development
and calibration validation it is essential that the sample analysed in the
laboratory to generate the reference data be representative of the sample
analysed by the NMR to produce the spectrum to be correlated to the reference
value. Sampling should be done in accordance with methods such as D1265, D4057
or D4177, whichever is applicable. Sample storage for extended periods before
the test methods and NMR experiments are performed should be avoided.
3)
When
a calibration set has been determined all samples must be tested accurately to
determine their physical and chemical properties. For complex mixtures typical
to the petrochemical industry the physical and chemical property values must be
determined by a reference analytical method. The accuracy of a multivariate
model prediction will therefore be highly dependent on the accuracy and
precision of the primary reference test method used in building the calibration.
Biases and error in the test method will be present in the calibration by
definition. The expected agreement between the NMR predicted value and the value
from a single test method measurement is
limited to the precision (repeatability) of the reference values. The precision
of the reference data can be improved by using the average value of several
repeated test method measurements on the same sample.
4)
If
reference values come from a single laboratory the ASTM test methods should be
monitored for bias and precision by following quality assurance procedures
presented in ASTM procedure D6299.
5)
When
performing the primary test methods one should keep the following ideas taken
directly from E1655 Section 10 in mind:
“If
the reference method used to obtain reference values for the multivariate
calibration is an established ASTM method, then repeatability and
reproducibility data are included in the method. In this case, it is only
necessary to demonstrate that the reference measurement is being practiced in
accordance with the procedure described in the method, and that the
repeatability obtained is statistically comparable to that published in the
method. Data from established quality control procedures can be used to
demonstrate that the repeatability of the reference method is within ASTM
specifications. If such data is not available, then repeatability data should be
collected on at least three of the samples that are to be used in the
calibration. These samples should be chosen to span the range of values over
which the calibration is to be developed, one sample having a reference value in
the bottom third of the range, one sample having a value in the middle third of
the range, and one sample having a value in the upper third of the range. At
least six reference measurements should be made on each sample. The standard
deviation among the measurements should be calculated and compared to that
expected based on the published repeatability. If the reference method to be
used for the multivariate calibration is an established ASTM method, and the
samples to be used in the calibration have been analyzed by a cooperative
testing program (for example, octane values obtained from recognized exchange
groups), then the reference values obtained by the cooperative testing program
can be used directly, and the standard deviations established by the cooperative
testing program can be used as the estimate of the precision of
the reference data.
Reference
methods that are not ASTM methods can be used for the multivariate calibration
of infrared analyses, but in this case, it is the responsibility of the method
developer to establish the precision of the reference method using procedures
similar to those detailed in Practice E 691
and in Practice D
6300.
When
multiple reference measurements are made on an individual calibration or
validation sample, a Dixon’s
Test (see D3764 Appendix X1.1) should be applied to the values to determine if
all of the reference values came from the same population, or if one or more of
the values is suspect and should be rejected.
6)
NMR
Experiments and Pre-Processing of Data: NMR experiments are performed following
the practices describes in Practice E 386. All NMR settings such as optimum RF
pulse width, lock channel optimisation, gain settings are set upon installation
and unless there is catastrophic failure of an RF component they will not be
adjusted again for that application. An 8 pulse averaged NMR
free-induction-decay (FID) is acquired over a 4 kHz spectral window with an
appropriate relaxation delay between pulses. The following post processing
procedures are performed as the spectra are referenced and co-added to provide
the final NMR spectrum:
· Remove
dc
· Zero
fill to 16k (resolution enhancement)
· FFT
· Symmetric
autophase (proprietary algorithm)
· Remove
dc
· Reference
appropriate resonances
· Co-add
spectra together
· Integrate
spectrum every 0.1 ppm from 12 to –2 ppm and normalize sum of integrals to
100.
·
Save
“integral spectrum”
The
“integral spectrum” represents the NMR spectral data that is regressed
against the primary reference data to produce the calibrations. Before the data
is regressed the NMR data is mean-centered. This is a prodedure where the
average spectrum of the calibration set is subtracted from each of the
individual spectra.
7)
The multivariate mathematics that are used to generate the calibration
models are described in Practice E 1655 Section 12, and the references therein.
8)
The estimation of values from NMR spectra is described in Practice E 1655
Section 13, and the references therein.
9)
Statistics used in evaluating and optimising calibration models is
described in Practice E 1655, Section 15, and the references therein.
10)
Outlier diagnostics and their use in improving calibrations is described
in Practice
E
1655, Section 15, and the references therein.
11)
Validation of a multivariate model is described in Practice E 1655,
Section 18.
12)
Precision of NMR estimated
values can be calculated using the procedure described in E1655 Section 19.
Replace all references to “Infrared” with “NMR”.
Upon
initial installation laboratory NMR based models will be transferred to the
on-line NMR system. After initial start up and calibration of the NMR a total of
10-20 samples will be required to further train and expand the NMR model base.
After the original models have been updated with these new calibration samples
NMR models will be ready for validation.
Implementation
of On-Line Calibration Models and Model Validation for On-Line Process NMR
Systems (ref: D3764, D6122, D6299, E1655)
Preconditions:
Certain
conditions shall be met before the on-line system validation is begun:
Multivariate
NMR based calibration models (PLS) have been developed and validated
according to methods described in Practice E1655.
NMR
spectrometer calibration has been performed at installation (Toluene
Standard).
-
A
quality assurance program for the primary methods has been put in place in
order to determine the precision and bias of the primary method. This
quality assurance program should include following Practice D3764 to
etablish the precision and bias of the laboratory tests. This procedure
should be followed for 9 samples (3 from the bottom, 3 from the middle and
three from the upper end of the range of reference values). Once the
laboratory is shown to be performing to ASTM standards the procedure can be
applied periodically as a check of continued laboratory conformance.
Collection
of Line Samples: Withdraw
samples in accordance with accepted sampling methods as given by Practices, D
1265, D 4057, and D 4177, whichever is applicable. The intent of the practice is
to obtain a sample that corresponds directly to the spectrum generated by the
on-line NMR analyser at that time. Sample should be collected at a point close
to the NMR and at a time correlated to the NMR acquisition. For a more detailed
description of the influence of lag time between sample collection and NMR
measurement see Practice D3764. D 3764 also describes when samples should be
grabbed during steady state operation.
Procedure
for establishing reference sample value:
For samples to be analysed multiple times to determine laboratory test bias and
precision see D3764 Section 7.
Model
Verification – Outlier Response: Outlier
diagnostics should be applied to the initial process stream NMR spectra to
determine that the model is applicable to the process stream sample. See
Practice D 6122, Section 11 for details. If spectrum is found to be an outlier
one should perform a standard sample (toluene) test to determine if there is a
spectrometer problem. If the standard test is passed then the sample may
represent a composition that is not currently found in the models. The models
should be updated to include the sample in the calibration set. If
the sample is readily included without flagging as an outlier a second
sample should be predicted with the updated model to determine if the sample is
an outlier. If it is not one should proceed to the initial validation procedure.
Initial
Validation Procedure: The
validation test shall follow exactly the procedures described in Practice D6122,
Section 12.
Periodic
Validation by Plotting Control Charts of the Differences Between NMR Predictions
and Laboratory Methods: If
the analyser passes the initial validation the stability of the differences
between the NMR predictions and the lab results should be monitored using
control charts. Details of the control charts are given in D6299 and D6122
Section 13. See also D6122, Section 9.5 for a detailed description of periodic
validation tests.
Control
Limit Updates: Continued
monitoring of the agreement between NMR predictions and Lab values should be
performed after the production of 20 lab values. Significant biases and variance
should be monitored. If biases do appear then the root cause must be determined.
Both the NMR and the laboratory should be checked. The laboratory should at all
times be monitoring its conformance to ASTM repeatability/reproducibility
through an established quality assurance program.
Online
Analyzer Repeatability: can
be determined following D6122, Section 15.
Model
Updating: See
E 1655 Section 24. If models are updated with additional calibration samples the
outlier procedures described in E1655 must be applied to the updated models and
the models must be revalidated.
Performance
Tests: NMR
does not require regular performance tests. If validation procedures indicate
that outlier or “NMR-lab difference” limits are not being met then standard
performance test should be performed using toluene. Another possibility is the
use of a known process sample standard. The definition of the process sample
standard used is to be determined by the customer. The sample should be similar
in nature to the samples being analysed by the analyzer.
[FAT
Procedures ]
For
more information on this topic please contact:
John
Edwards
Manager,
Process and Analytical NMR Services
Process
NMR Associates LLC, 87A
Sand Pit Rd, Danbury,
CT 06810, USA, Tel:
(203) 744-5905
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