By

I just read the latest LCGC article, Feb.

2000- GC Connections.

Any hints for calculating S/N and MDL's

for GC/MS.

I have read several methods, Peak heights

being the most accurate, however we have

an older HP GCD, with GCD Chemstation

Software- Peak heights are not available

without considerable effort writing Macros.

The quick and dirty way

Peak Area= Peak Height x 2 Width at half height.

The problem also with the software is the

RTE integrator, peak width is automatically

calcuated, Chemstation Integrator is more

flexible.

Any suggestions in streamlining the S/N

dilemma ?

Thanks.

By

With an older system like this, the best procedure is to record the mass chromatogram (SIM or full scan) of your test peak then determine the noise level--adjacent to the peak of interest--and the peak area and height manually, especially if writing macros is difficult in this system.

If you want to work from the area and width report you have, the triangular approximation of peak area for computing the height works fairly well. The peak area is about 106% of the triangle area. I use these formulas:

A = 1.06 * (w_{b} * m_{max}) / 2 this is eq.2 from the article in LC/GC

or

A = 1.06 * (1.7 * w_{h} * m_{max}) / 2

or

A = 0.90 * w_{h} * m_{max}

where m_{max} is the peak height. Note that 1.7 x the width at half-height equals the width at base. Derived formulas relate the height to the area.

I address detectability in the LC/GC article, and not MDL so much. Most analysts consider MS a mass-flow type detector, so the detectability should be expressed in grams/second. However, most manufacturers specify the test column and conditions and simply express it in terms of grams, using peak height in the calculation.

For MDL determination, most users choose a S/N ratio of up to 20:1 to incorporate a safety factor. This increases the MDL by about a factor of 10 over the minimum detectability.

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