ADV06-15_1-412

398 / 413

Pharmaceutical
Assaying Local Anesthetics by GC/FID
Optimizing System Suitability, Using an Rxi™-Sms Column
By Rick Lake, Pharmaceutical Innovations Chemist
• Rxi™-Sms column assures excellent peak shapes for basic compounds.
Stable, repro ducible retention times.
Easy conformance to stringent system suitability criter ia.
Local anest hetics are biologica lly active compounds that reversibly inhibit the propagation , or broadcasting , of signals along nerve
cell pathways. Because of this action , they are widely used as drug compounds to produce temporary analgesia (loss of pain ) and
par alysis (loss of mu scle movement ). Anesthet ic compounds are formulated into a large number and wide variety of drug products,
rangi ng from over-the-counter topica l oin tme nts to clinical injecta bles, and they often are formulated in combination with other
active ingredien ts. There fore, many analyses of local anest hetics involve manufacturi ng assays, like potency and stability assays, which
require high-throu ghput and repro ducible results. These assays requ ire the fulfillment of system suitability criteria and, for this rea­
son, we investigated assaying local anesthetics by GC/FID, using common system suitability parameters as evaluatio n criteria.
By GC standards, a local anesthetic is a high molecular weight, weakly basic, active compound. We took these characteristics into
account when we chose the column and inlet liner for this applicatio n. Considering that these analytes are basic and active, the deac­
tivatio n of the inlet liner and capillary colum n is very important. For superior inertn ess, we chose to use an Rxin '-Sms colum n.
When analyzing high molecular weight com­
pounds - the no rmal case in pharmaceut ical assays
- discrimination and irreproducible injections
sometimes occur, prima rily due to incomplete
vapo rization of the analyt es. This can be especially
probl ematic for analysts who mu st meet stringent
system suitability criteria. Some liners, like the
laminar cup and cup splitter, were designed specif­
ically for samples containing high molecular
weight compounds. Th ese liner designs aid in
sample vaporization, but at a cost of reduced inter­
nal volumes and intricate flow paths that can cause
poo r reproducibility when such liners are used
with a solvent that has a large expansion volume,
like methanol. In this app lication, we used our
conventional, int erm ediate polarity deactivated,
split liners packed with intermediate polarity deac­
tivated wool. Wool in the liner provides a large sur­
face area, for rapid vapo rization, but the liner still
delivers a uniform vapor cloud to the split poin t.
Under these conditions, chro matog raphy from a
six-replicate system suitability analysis (Figure I)
was well within normal acceptance criteria (Table
I). USP tailing, approximately 1.00 for all analytes,
shows the exceptional inertness of the Rxi'l-Sms
column. In addition, retention times and area
respon ses were extremely stable. The Rxi" -Sms
column, coupled with an app ropri ate inlet liner,
provid es the stability and deactivation necessary to
afford easier conformance to system suitability cri­
teria. The lO-min ute anal ysis time for these com­
pounds ensures high sample throughput.
Rxi™-5ms Column (fused silica)
(Crossbond" S%diphenyl I 9S%dimethylpolysiloxane)
ill df (J1m) temp. limits length cal # price
0.53mm 1.00 -60 to 330/350°C 30-Meter 13455 $515
Forother dimensions, see page 5.
2006.03
Figure 1 An Rxi™-Sms column provides excellent peak shape and
stable retention times for basic compounds, for easier conformance
to system suitability criteria.
Excellent Peak Shape
and Retention Time!
1. benzocaine
2. prilocaine
7 3. lidocaine
2 3 4. procaine
5. tetracaine
6. bupivacaine
7. dibucaine
,
2 4 6
Time(min)
Column: Rxi· ·5ms30m, 0.53mm !D, 1.00jJm (cal.# 13455)
Sample: 50jJg/ mLeachcomponent inmethanol
Inj.: 1.0jJLsplit (10:1), 4mmsplit inlet linerwithwool (cat # 20781)
Inj. temp.: 250°C
carriergas: helium, constant flow
Flow rate: S.OmL/ min.
Oventemp.: 200°C(4min.) to320°C@ 30°C/ min.(hold 3 min.)
Det.: FID @300°C
Table 1 An Rxi™-Sms column provides exceptionally stable retention
times and area responses.
Peak Area
Com ound nRSD USPTailing EfflCien9'_
benzocaine 0.8S 1.00 5S8S8
prilocaine 1.36 0.02 1.00 (isothermal)
lidocaine 1.01 0.02 1.00
procaine 1.83 0.03 1.00
tetracaine 1.78 0.01 1.00
bupivacaine 1.64 0.02 1.02
dibucaine 1.17 0.06 1.00
Mean 1.38 0.03 1.00
six-replicate system suitability analysis
800-356-1688 • • 17 •
Website : E-mail : TelNo : 03 9762 2034 . . . in AUSTRALIA