Quick-Start Guide to LPGC

Determining if LPGC is Right for You

Interested in trying low-pressure gas chromatography (LPGC) in your lab but aren’t sure if it will work for you? We can help with that! Restek recommends reviewing a few factors about your analysis to determine if it’s amenable to LPGC.

One factor to consider regarding your analysis is critical separations. If there are no critical separations, the analysis is generally amenable to LPGC. If there are critical separations in your conventional GC-MS analysis and those resolutions are important to maintain, then some additional considerations need to be weighed before switching to LPGC. Since the LPGC technique trades resolution for speed, applications requiring the same or better resolution of critical pairs may not be amenable. Applications that require a column longer than 30 m for a conventional GC-MS may not work for LPGC-MS because the length is most likely providing needed resolution of analytes. However, if your critical pairs are not isobars, the spectral resolving power of the MS can be used to separate them while speeding up your analysis times! This means LPGC can work for your analysis and result in significant speed gains (Figure 1).

Another factor to consider is the sample capacity and the increase in sensitivity (or S/N) that can result. If you’re already running a fast GC method (for example using a 10 m x 0.10 mm ID column), LPGC can provide more sample capacity than the smaller ID columns (short, narrow-bore columns) that are typically used. So, if you have a fast run but would like more sample capacity, LPGC is a good option!  Additionally, since LPGC allows for more sample capacity and narrower peaks, using LPGC can result in higher S/N, better sensitivity, and the ability to reach lower detection limits. 

	Peaks	tR (30 m)	tR (LPGC)	Conc. (ppm)	Quant Ion
1.	o-Toluidine	3.217	1.207	2	107
2.	o-Anisidine	4.42	1.589	2	123
3.	4-Chloroaniline	4.832	1.71	2	127
4.	p-Cresidine	5.519	1.92	2	137
5.	2,4,5-Trimethylaniline	5.582	1.935	2	135
6.	3-Chloro-o-toluidine	5.902	2.026	2	141
7.	4-Chloro-o-toluidine	6.025	2.061	2	141
8.	2,4-Diaminotoluene	7.637	2.523	2	122
9.	2,4-Diaminoanisole	8.89	2.872	2	138
10.	2-Naphthylamine	9.773	3.112	2	143
11.	2-Aminobiphenyl	9.892	3.148	2	169
12.	2-Amino-4-nitrotoluene	10.537	3.325	2	152

	Peaks	tR (30 m)	tR (LPGC)	Conc. (ppm)	Quant Ion
13.	4-Aminobiphenyl	12.154	3.767	2	169
14.	p-Aminoazobenzene	15.95	4.805	2	197
15.	4,4'-Oxydianiline	16.613	4.99	2	200
16.	4,4'-Diaminodiphenylmethane	16.714	5.016	2	198
17.	Benzidine	16.787	5.034	2	184
18.	o-Aminoazotoluene	17.53	5.24	2	225
19.	3,3'-Dimethyl-4,4'-diaminodiphenylmethane	18.23	5.436	2	226
20.	3,3'-Dimethylbenzidine	18.483	5.504	2	212
21.	4,4’-Thiodianiline	19.439	5.765	2	216
22.	3,3'-Dichlorobenzidine	19.795	5.864	2	252
23.	4,4'-Methylenebis(2-chloroaniline)	19.795	5.864	2	266
24.	3,3'-Dimethoxybenzidine	19.905	5.898	2	244

Column	See notes
Standard/Sample
	AccuStandard carcinogenic aryl amine mix (AE-00049-R1-10X)
	AccuStandard 2,4-diaminoanisole (ALR-070S-R2)
Diluent:	Ethyl acetate
Conc.:	20 µg/mL
Injection
Inj. Vol.:	1 µL split (split ratio 10:1)
Liner:	Topaz, splitless, single taper inlet liner, 4.0 mm x 6.5 x 78.5 (cat.# 23303)
Inj. Temp.:	280 °C
Carrier Gas	He

Detector	MS
Mode:	Scan
Transfer Line Temp.:	300 °C
Analyzer Type:	Quadrupole
Source Temp.:	230 °C
Quad Temp.:	150 °C
Electron Energy:	70 eV
Tune Type:	PFTBA
Ionization Mode:	EI
Instrument	Agilent 7890B GC & 5977A MSD
Sample Preparation	The standards were diluted with ethyl acetate to 20 ppm; analyzed in a 2 mL, short-cap, screw-thread vial (cat.# 21143); and capped with a short-cap, screw-vial closure (cat.# 24495).
Notes	Conventional (30 m) Analysis: Column: Rxi-35Sil MS, 30 m, 0.25 mm ID, 0.25 µm (cat.# 13823) Temp. program: 200 °C (hold 0.5 min) to 320 °C at 9.5 °C/min (hold 5 min) Flow: 2.0 mL/min Scan start time: 2 min Scan range: 30-300 amu Scan rate: 10 scans/sec LPGC-MS Analysis: Column: LPGC Rxi-35Sil MS column kit, includes 10 m x 0.32 mm ID x 0.25 μm Rxi-35Sil MS analytical column and 5 m x 0.15 mm ID Rxi restrictor factory connected via SilTite connector (cat.# 11806) Temp. program: 100 °C (hold 0.5 min) to 300 °C at 35 °C/min (hold 5 min) Flow: 0.9 mL/min Scan start time: 1 min Scan range: 35-300 amu Scan rate: 9.7 scans/sec Compound list based on EU legislation of Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH) Appendix 8 (https://reachonline.eu/reach/en/appendix-8.html). The 3-chloro-o-toluidine and 2-aminibiphenyl compounds are not part of the list.

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Installing and Optimizing Your LPGC Column Kit

Once you’ve determined if LPGC will work for you, you’re ready to select your LPGC kit from our broad catalog offering, install it, and optimize your run conditions! Here’s a quick rundown of what you’ll need to do: 

1. Select your LPGC kit from our website. To explore our offering of LPGC kits, visit www.restek.com, and search for "LPGC" with "Products" selected in the dropdown menu of the search bar.
2. Install your LPGC kit – it’s as easy as a column change! 
   1. Simply remove your conventional GC column, and then install the LPGC kit with the restrictor column at the inlet.
   2. Give the installation a leak check using our electronic leak detector (cat.# 28500). 
3. Optimize your run conditions for LPGC.
   1. Flow: To achieve the optimal flow for LPGC, set the flow rate using only the dimensions of the restrictor column at the oven’s initial temperature. For LPGC kits with a 0.53 mm ID analytical column, the optimal flow rate is 1.5-2.5 mL/min with helium. For LPGC kits with a 0.32 mm ID analytical column, the optimal flow rate is 0.9-1.1 mL/min with helium.
      1. The ideal flow rate is a balance between how fast the MS can work, how fast you want to perform the analysis, and the level of sensitivity needed. 
      2. For 0.53 mm ID columns, higher flows result in lower responses due to overloading the pump. Modern pumps may be able to handle a flow rate of up up to 6 mL/min. However, we recommend checking with your instrument manufacturer to determine the maximum flow rate it can handle.
      3. For 0.32 mm ID columns, flow rates higher than the ideal range (described in section 3a) may cause LPGC conditions to be lost.
   2. Temperature: The temperature program may need to be adjusted when converting from conventional GC to LPGC. A rapid heating program is required for LPGC with the optimal ramp set at 30-35 °C per minute. The start and end temperatures can also be lowered because the flow rate will be higher than a conventional GC column. LPGC takes advantage of a low-pressure system (using the MS to pull vacuum on the analytical column) to increase flow, making analytes elute faster at lower temperatures. 
      1. If your GC is having trouble keeping up with these oven ramp rates, try our GC Accelerator Oven Insert Kit (cat.# 23849). 
4. Start your run! 
   1. Be prepared to observe your analysis and optimize your conditions again based on initial data, if necessary. Don’t forget, coelution may occur as resolution is decreased with LPGC since it is a tradeoff for a faster run. If the coelutions are not isobars, use the spectral resolving power of the MS to analyze them!

With this simplified setup, you can start processing more samples per shift, have more time for instrument maintenance, or even put off that next big capital investment on a new instrument to accommodate your workload.

To learn more about the benefits of LPGC, visit www.restek.com/lpgc