Rugged and Reproducible Solid Phase Microextraction (SPME) Sampling

Elevate your fiber performance with nitinol-core
SPME fibers

SPME fibers with Carboxen^®/Polydimethylsiloxane (CAR/PDMS) and Polydimethylsiloxane/ Divinylbenzene (PDMS/DVB) coatings are now available on a nitinol core (NIT) in a SS assembly for improved fiber performance. Selectivity of the CAR/PDMS and PDMS/DVB SPME fibers has been maintained for seamless transition of methods from fused-silica core (FS) fibers to the improved NIT technology. NIT fibers offer the following benefits:

• Enhanced fiber reproducibility
• Greater fiber durability
• Maintained selectivity for method consistency
• Core inertness to ensure analyte stability

Improved Lot-to-Lot and Fiber-to-Fiber Reproducibility

The nitinol-core SPME manufacturing process has been optimized to enhance lot-to-lot and fiber-to-fiber reproducibility. This enhanced reproducibility stems from more stringent coating specifications reducing fiber variation significantly in particular for fiber diameter/coating thickness.

The improvements provide enhanced precision of chromatographic results. The NIT CAR/PDMS and NIT PDMS/DVB fibers were compared to FS versions of the same fiber chemistry following the methodology detailed in Table 1 and visually represented in Figure 1. The Figure 2 shows that both the NIT CAR/PDMS fibers and NIT PDMS/DVB fibers exhibit significant reduction in intra-lot variability and inter-lot variability as compared to the FS version.

SPME Conditions
Sample:	Isobutyl acetate 0.375 ug/mL, n-butanol 275 ug/mL, o-xylene 0.1825 ug/mL, and 0.175 ug/mL toluene in 25% NaCl water, pH 7 with 0.05 M phosphate buffer (double concentration for PDMS-DVB Fibers) in 10 mL screw capped vial
Fiber:	CAR/PDMS & PDMS/DVB on Nitinol and fused silica cores
Autosampler:	Gerstel MPS 2 with MFX feature for exchanging fibers
Sample Incubation:	2 min. at 40 °C
Extraction:	headspace 40 °C, 10 min with agitation at 200 rpm
Desorption:	3 min at 270 °C
Post Condition:	2 min at 300 °C in fiber conditioning Station

Table 1. Analytical Testing Method for Comparison of NIT and FS Fiber Inter-lot and Intra-lot Repeatability

GC Conditions
Column:	SUPELCOWAX® 10 30 m x 0.25 mm, 0.5 µm (24284)
Oven:	45 °C (hold 2 min) to 65 °C at 6 °C/min to 100 °C at 10 °C/min to 150 °C at 15 °C/min (hold 0.5 min)
Carrier Gas:	Helium, 1 mL/min constant flow
Injection port:	270 °C, with 0.75 mm ID liner
Injection:	Splitless for 0.5 min at 1 mL/min then vent at 50 mL/min
Detector:	FID at 300 °C

Figure 1. Representative Chromatogram of the Analytical Testing Method Depicted on a CAR/PDMS NIT Fiber

Figure 2. Inter-lot and Intra-lot Reproducibility Testing Comparing NIT and FS SPME Fibers

Greater SPME Fiber Durability

NIT provides improved durability over FS fibers. In this comparison, the NIT and FS fibers were subjected to a flexibility test to determine fiber strength/mechanical stability. Three FS core fiber and three NIT core fiber assemblies of each coating were flexed from 0° to 180° as indicated in Figure 3. NIT fibers were flexed to 180° without breaking or permanently kinking regardless of coating type.

Figure 4 shows a 90° flexion of a NIT fiber. FS fiber assemblies regardless of coating type broke off at the point of attachment when flexed between 40° and 50°. Despite the increase in mechanical stability it is still recommended to minimize the mechanical stress to the coated fibers as much as possible.

Figure 3. Flexion Points for Durability Test

Figure 4. 90° flexion of a NIT fiber

SPME Coating Selectivity and Core Inertness Maintained

To facilitate seamless transfer of methods from FS fibers to the NIT fibers of the same coating type, selectivity and inertness were compared to ensure consistency using two sets of indicator compounds: alkyl halides and small short chain amines. Comparisons were performed on four lots of CAR/PDMS and PDMS/DVB of each core type to confirm repeatability of results.

The selectivity of CAR/PDMS and PDMS/DVB fibers for the alkyl halides on each core type was examined through a comparison of analyte response rates using the method outlined in Table 2. Figure 5 shows that coating selectivity is maintained for both the CAR/PDMS and PDMS/DVB chemistries for the alkyl halides regardless of core type. The difference between responses of halogenated alkanes extracted using coatings on NIT and FS cores was less than 3% for all coatings. Because alkyl halides are known to breakdown at elevated temperatures upon interaction with exposed active sites, the inertness of the two fiber types was then investigated to ensure that no increase in degradation was taking place on the NIT fibers. Percent conversion of the studied alkyl halide to their corresponding alkene is displayed in Figure 6. For all analytes, which are known to be prone to degradation, NIT fibers exhibit equal or better preservation of sample for both coating types. Percent breakdown was also found to be correlated with temperature, as anticipated.

Inertness and selectivity of the fibers was then compared for more polar compounds using the series of short chain amines. Short chain amines also tend to be problematic analytes due to their reactivity with metal which results in oxidative degradation that intensifies at elevated temperatures. The analyte response of each amine was divided by the internal standard, n-propanol, which is stable under these conditions, to examine amine breakdown or irreversible adsorption. If breakdown were to occur, the ratio would be lower on a more active core. The chromatogram and ratios depicted in Figure 7 indicate that the selectivity and core inertness is maintained between the FS and NIT fibers for the amine series. The values are statistically the same at 5% RSD.

SPME Conditions
Sample:	Halogenated alkanes at 150 ug/mL in 4 mL water, 25% NaCl, pH 7 in 10 mL vial
Preincubation:	2 min with agitation at 40 °C
Extraction:	Headspace, 10 min at 40 °C with agitation at 250 rpm using MPS II
Fibers:	CAR/PDMS & PDMS/DVB on Nitinol and fused silica cores
Desorption:	3 min at 280 °C for CAR/PDMS and 230 °C for PDMS/DVB
Post Bake:	2 min at 250 °C

Table 2. Analytical Method for the GC Analysis of Alkyl Halides on NIT and FS Fibers

GC-MS Conditions
Oven program:	40 °C hold 2 min to 70 °C at 8 °C /min to 150 °C hold 3 min
Column:	VOCOL® 30m x 0.25mm ID, 1.5 µm df (24205-U)
Injector:	280 °C for CAR/PDMS and 230 °C for PDMS/DVB, splitless open after 0.75 min at 30 mL/min
Desorption:	3 min at set injection temperatures
Carrier Gas:	Helium, 1 mL/min constant flow
Detector:	MSD-m/z 40-300 full scan

Figure 5. Alkyl Halide Analyte Responses by Core Type and Coating

Figure 6. Core Inertness Study on the Breakdown of Alkyl Halides

Column:	SPB®-1 Sulfur (base deactivated), 30 m x 0.32 mm x 4.0 µm (24158)
Sample:	1 ug/mL each in water with 25% NaCl, 0.05 M phosphate buffer pH 11
Extraction:	10 min immersion
Fibers:	PDMS/DVB Fused Silica & NIT cores
Desorb:	3 min @ 250 °C
Oven:	45 °C (1.5 min) to 80 °C at 8 °C/min to 200 °C at 20 °C/min
Injector:	250 °C, splitless closed initial 0.75 min
Detector:	FID at 300 °C

Figure 7. Core Inertness and Analyte Response Comparison of Amines in Relation to n-Propanol on NIT and FS Fibers

Benefits of nitinol-core SPME fibers

The nitinol-core SPME fibers show enhanced lot-to-lot as well as fiber-to-fiber reproducibility. The coating consistency has been optimized lending to an improvement in repeatability of analytical results. Durability of the fibers has been vastly increased by the NIT core allowing the fibers to handle 180° distortion. With these improvements, the selectivity and inertness of the CAR/PDMS or PDMS/DVB SPME FS fibers is maintained allowing simple transfer of methods from the FS version to the new NIT variety.

Sampling Mode and Needle Size
Fiber Coating and Thickness	Fiber Core/ Assembly Type	Hub Desciption	Manual Holder/ (w/spring)		Autosampler
			23 Ga*	24 Ga*	23 Ga*	24 Ga*
Carboxen®/Polydimethylsiloxane (CAR/PDMS)
75 µm CAR/PDMS	NIT/SS	Black Metallic	57901-U	57904-U	57907-U	57908-U
Polydimethylsiloxane/Divinylbenzene (PDMS/DVB)
65 µm PDMS/DVB	NIT/SS	Blue Metallic	57916-U	57921-U	57923-U	57931-U